ISSN:0265-0568    

DOI:10.1039/a805hopy

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids John R. Lewis Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, UK, AB24 3UE Covering: July 1995 to June 1996 Previous review: 1998, 15, 371 1 Muscarine, imidazole, oxazole and thiazole alkaloids 2 Peptide alkaloids 3 Miscellaneous alkaloids 4 References 1 Muscarine, imidazole, oxazole and thiazole alkaloids A synthesis of (+)-epiallomuscarine 1 starting with D-glucose utilises a ‘chiron’ approach, each step being fully regio- and stereo-specifically controlled.1 Isobromotopsentin 2 is a new bisindole imidazole alkaloid isolated from a deep water marine sponge of the Spongosorite family.This bright yellow sponge was collected by a deep water dredging process and frozen prior to its ethanol extraction. Structural elucidation was achieved by NMR and MS measurements.2 Oxytriphine 3 is a component of the extract of the aerial parts of Oxytropus trichophysa.The presence of benzoic acid and (")-N-benzoyl-2-phenyl-2-hydroxyethylamine in the extract leads to a possible biogenetic relationship.3 Melanoxadin 4 is a fungal metabolite obtained from Trichoderma species. It possesses melanin biosynthesis inhibitor properties.4 An oxazole–indole combination accounts for the structure of almazole C 5, an alkaloid obtained from a red seaweed probably of the genus Haraltiophyllum and belonging to the Delerseriaceae family.Since indole dipeptides 6 and 7 were also isolated from this extract a biogenesis for the almazole alkaloids can be proposed.5 The isolation of five new bengazoles from the sponge Jaspis has enabled a complete relative and absolute configurational relationship to be made to these fatty acid esters.6 The parent bis-oxazole 8 is coupled to homologous, n, iso and anteiso-fatty acids (C13–C16), i.e. bengazole C is 8, R=CO(CH2)11Me; D is 8; R=CO(CH2)10CHME2; E is 8, R=CO(CH2)13Me; F is 8, R=CO(CH2)11CHMeCH2Me and G is 8, R=CO(CH2)14Me.Madumycin 11 32 is a member of the streptogramin A group, which comprises a number of oxazoles with antibiotic properties with the potential to combat resistant bacterial pathogens. It has been synthesised (Scheme 1) from two fragments A and B (themselves designed by a disconnection evaluation of the parent antibiotic).7 A represents the South and West quadrants and B the North and East. Fragment A 21 was prepared by treating the Weinreb amide 9 with allylmagnesium bromide to furnish a ‚,„-unsaturated ketone which was reduced, with high stereoselectivity (>99%), to a single diastereomeric alcohol 10 which was protected as 11 so as to allow ozonolysis to produce the aldehyde 12, thence to the acid 13 so as to give the hydroxyamide 14 using S-serine ethyl ester.Cyclisation, using the Burgess reagent, gave oxazoline 15 which upon oxidation with a CuI–CuII peroxide reagent gave oxazole 16.Deprotection was followed by equilibration with 1,3,5-mesitylformaldehyde in the presence of a catalytic amount of camphorsulfonic acid to produce stereochemically pure 1,3-dioxane 18. Oxidation to aldehyde 19 allowed a Wittig olefination to give 20 which could be chain extended to 21. Ester hydrolysis thus gave A 22. Fragment B 27 was accessed starting with the syn-adduct 23 which, by removing the chiral auxiliary via the Weinreb amide followed by DIBAL-H reduction, gave the unstable aldehyde 24 which was immediately subjected to a Horner–Emmons– Wadsworth olefination giving 25.This unsaturated silyl ester was esterified with N-Boc-D-alanine to give depsipeptide 26. Removal of the Boc protecting group gave the primary amino ester 27, synthon B. Connection of 22 and 27 with DCC gave amide 28 which upon treatment with methylamine in ethanol– benzene released the free primary amine 29. Hydrolysis of the silyl ester with Bu4NF created a crude amino acid 30 which was treated with BEPCl in the presence of Hünig’s base to give 31.Hydrolysis of the dioxane moiety thus produced the natural product 32. An isoxatidone with siderophoric activity has been isolated from the culture broth of Psudomonas fluorescens AH2 when it was grown under iron-deficient conditions. The metabolite, called pseudomorine 33, was accompanied by sodium salicylate.8 WS75624 A 34 and B 35 are enzyme inhibitors to endothelin conversion. Both were obtained from the culture broth of Saccharothrix species No. 75624.9 Barbamide 36 is a chlorinated metabolite obtained from the Caribbean cyanobacterium Lyngbya majuscula. The trichloromethyl group is unusual as is the methyl enol ether of a O HO NMe3 NH N O Ph Ph N O OH OH NH O OH N NH Br 1 2 3 4 + I– NH NH NH N O O N N NMe2 Ph H H N Ph O H N Ph OH OH OR H NMe2 NMe2 H OH OH CO2H H O O O 5 6 7 8 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 417‚-keto amide.10 Another enantioselective synthesis of curacin A 40 has been reported.The first paper11 describes the synthesis of three segments 37, 38 and 39. The second paper describes the combination of 37 and 38 by a Julia coupling and then 39 was added via an imino ether condensation.12 O O O O O N O N O N O N O N NMeCHO R2 O CO2Me TBDMSO O O O CO2Me O OR1 MgBr O O NPhth HO Me3Si O O HO Me3Si O O O RHN O OH O O CO2R O O CO2Me R Mes Mes R PBu3 9 15 23 24 25 10 R1 = H; R2 = allyl 11 R1 = TBDMS; R2 = allyl 12 R1 = TBDMS; R2 = CHO 13 R1 = TBDMS; R2 = COOH 14 R1 = TBDMS; R2 = NHCH(CO2H)CH2OH 16 R = TBDMS 17 R = H 18 R = CH2OH 19 R = CHO 20 R = CH=C(Me)CHO 21 R = Me 22 R = H º A 26 R = Boc 27 R =H º B iii iv v vi vii ix x xii xi xiii xiv xv xvi xvii xviii xix i, ii viii + Br– OHC Scheme 1 Reagents and conditions: i, Allylmagnesium bromide, "78 )C, THF, 2 h; ii, LiAlH4, Et2O, "100 )C, 45 min; iii, TBDMSCl, imidazole, DMF; iv, O3, DMS; v, NaClO2, H2O, MeCN · H2O; vi, Isobutyl chloroformate, N-methylmorpholine, serine methyl ester, HCl; vii, MeCO2NSO2NEt3; viii, tert-butyl perbenzoate, CuBr,Cu(OAc)2, benzene, reflux 7.5 h; ix, TBAF, THF rt, 4 h; x, 1,3,5-mesitylformaldehyde dimethyl acetal, camphorsulfonic acid, CH2Cl2, 0 )C, 48 h; xi, SO3 · py, DMSO, Et3N; xii, PhP=C(Me)CHO, benzene, reflux, 23 h; xiii, CH2=CHPBu3Br, potassium phthalimide, THF, 65 )C, 48 h; xiv, reflux, pyridine; xv, Me3Al, CH2Cl2, 0 )C, 2 h then MeNH(OMe)HCl; xvi, DIBAL-H, toluene, "78 )C, 30 min; xvii, (EtO)2P(O)CH2CO2CH2SiMe3, LiCl, CH3CN, PriEt2N; xviii, DCC, D-Boc-alanine, CH2Cl2 · 0 )C]rt, 11 h; xix, TsOH · H2O, 23 )C, 16 h; xx, DCC, DMAP, CH2Cl2, 0]25 )C, 9 h; xxi, MeNH2, ethanol, benzene, 50 )C, 48 h; xxii, Bu4NF, THF, 0]25 )C, 9 h; xxiii, Hunig’s base, Pr2 iEtN, BOPCl, CH2Cl2, "10]23 )C, 18 h; xxiv, TFA, "5 )C O N N NH N HN MeO OMe OH O O CO2H S N R 33 34 R = CH2(CH2)3CMe2OH 35 R = CH2(CH2)3CH(Me)OH Ph Me N O N S SO2Tol H2N BocN S OHC OMe O H H OMe CCl3 36 37 38 39 418 Natural Product Reports, 1998Cyanobacterium Lyngbya majuscula collected oV Curacao contains three curacins, namely A 40, B and C.The latter two are geometric isomers of A. Thus A is as depicted, B is 41 and C is 42. All possess activity against various tumour cell lines.13 Melittangium lichenicolum produces two thiazoline metabolites, namely melithiazole A 43 (ab saturated) and melithiazole B 43 (ab unsaturated).Both possess antimycotic activity.14 The synthesis of a number of epimeric 4-methylthiazolines has unambiguously established the stereochemistry of didehydromirabazole A 4415 and thiangazole 45.16 All have the (R)-configuration at the quaternary centre A. Dolastin E is a hexapeptide isolated from the sea hare Dolabella auricularia.17 Its stereochemistry has been confirmed by a total synthesis of all eight stereoisomers. Structure 46 has the correct arrangement.18 Allelochemical nostocyclamide A is a new macrocyclic thiazole-bearing alkaloid from the cyanobacterium Nostoc 31.19 The eighteen membered ring includes two thiazole and one oxazole moieties: 47.Dendromides are heptapeptides found in the terrestrial blue-green alga Btigonema dendroideum.20 Three, which have been isolated, have had their structures determined by acidic hydrolysis of the intact and ozonized metabolites coupled with spectroscopic measurements. Thus dendromide A is 48, R1=Me, R2=CHMe2; B is 48, R1=(CH2)2SMe, R2=Me and C is 48, R1=(CH2)2SMe, R2=Me.The total synthesis of lissoclinamide 4 58 is claimed to be the first for a chiral thiazoline-containing macrocyclic peptide. The key step lies in the synthesis of a chiral thiazoline ring which thitherto was too sensitive to elaborate. Thiazoline rings have been synthesised by the dehydration of serine-derived hydroxyamides and since oxazoline rings are created by the same procedure this synthetic strategy used a double cyclodehydration to create the natural product.21 The two synthons were 53 and 54.Compound 54 was made by standard methods while the synthesis of 53 was carried out in an essentially linear manner starting with substituted thiazole 49. Removal of the Boc protecting group allowed coupling to protected serine by the symmetrical anhydride method to give tripeptide 50. This was hydrolysed and the product was coupled to the thioacylating agent 51 to give endomonothiotetrapeptide 52.Boc deprotection allowed coupling with Boc-allo-threonine to give synthon 53. Deprotection of 53 with TFA allowed condensation with acid 54 and the resulting heptapeptide ester 55 was hydrolysed to its parent acid 56 which allowed Boc deprotection to enable intramolecular cyclisation to give macrocycle 57. This yield was 32% and indicated that macrocyclisation was not influenced by the ‘opening’ of the hetero-rings. Ring closure to the heterocyclic five-membered rings was accomplished, in 61% yield, using the Burgess reagent (Scheme 2) thereby completing the synthesis of lissoclinamide 4 58.Antibiotic GE 37468 is a new inhibitor of bacterial synthesis. This tetrathiazolyl-oxazol macrolide 59 was found in the culture broth of Streptomyces ATCC 55365. It has a broad spectrum of biological activity.22 2 Peptide alkaloids The synthesis of (")-LL-C10037· 74 starts with 2,5- dimethoxyaniline 60 which is first N-protected by the allyloxycarbonyl group to give 61.Hypervalent iodine oxidation gave ketone 62 which upon borohydride reduction produced methoxyphenol 63. A second hypervalent iodine oxidation in the presence of (R,R)-pentane-2,5-diol 64 gave ether 65. Demethylation gave 66 which then led to diacetal 67. The stereochemically more stable diacetal 67a was also the N S N S N S H H H H H H H H OMe OMe OMe 40 41 42 S N S N S N S N OMe MeO CO2Me N S N S S N N S S N Ph O N CONHMe 43 44 45 a b A A S N NH N O HN S N NH O O N NH N S HN S N O NH O O O O 46 47 S N NH N O HN S N R1 NH O R2 O O 48 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 419sterically desired product since its methyl group hindered nucleophilic approach on the ‚-face of the dienone. Consequently epoxidation gave a 4.5:1 predominance of the ·-epoxide 68.Reduction gave mainly the ·-enol 69 which upon tert-butyldiphenylsiloxy protection and chromatographic separation gave 70.Removal of the diacetyl group occurred with TsOH and PPTS in acetone and the subsequent ketone 71 was N-acetylated to give 72 prior to allyloxy group removal to give 73. Finally O-deprotection gave the chiral natural product 74 (Scheme 3).23 A second reported synthesis of this important antitumour antibiotic 74 gave it in the racemic form. Starting also from 2,5-dimethoxyaniline 60 the N-protecting group was tert-butoxycarbonyl (Boc), the other steps being essentially similar.24 Extracts of the Okinawan sea sponge Psammaplysilla purea have been examined in more detail and nine new pure alidin alkaloids have been identified.These are designated J 75; K 76; L 77; M 78, R1=OH, R2=NH2; N 78, R1=OH, R2=H; O 79; P 80 and Q 81. All these structures were elucidated by spectroscopic analysis.25 Four new bromotyrosine derivatives have been isolated from the styelid ascidians of the genus Botryllus. Called botryllamides A, C, B and D, their structures were determined by one- and two-dimensional NMR as 82, R=Br; 82, R=H; 83, R1=Br, R2=H and 83, R1=R2=H respectively.26 Thiazenotrienomycins 84 are antitumour metabolites produced by Streptomyces species MJ 672-m3.Five have been characterised.27 Volutamides are alkaloids possessing antifeeding properties produced by the Atlantic bryozoan Amathia convoluta. Studies have indicated that this species of coral is not predated upon by fish and sea-urchins.Five related metabolites have been identified and designated A–E, having structures 85, 86 (R=H), 86 (R=Me), 87 and 88 respectively.28 Antifouling agents are normally based on organometallic compounds but their toxicity to the environment has currently limited their use. In a study of soft body benthic invertebrates that have chemical defences against predators, a methanol extract of the marine sponge Pseudoceratina purpurea was found to be active against cyprids of the barnacle Balanus amphritrite. A bioassay guided fractionation produced an active compound creatinamine 89.The cyanoformamide grouping is certainly unusual for a natural product.29 A potent inhibitor of cysteine proteases has been isolated from the marine fungus Microascus longirostris. Cathestatin C 90, like its contemporaries A and B, was located in the fermentation broth of this fungus.30 Epoxyguinomicin A 91, R=Cl and its dechlorinated derivative B 91, R=H have been obtained from the culture broth of an Amycolatopsis species.Both metabolites showed activity against gram-positive and gram-negative bacteria.31 Caledonin 92 is a peptide isolated from the deep pink, red or yellow encrusting ascidian Didemnun rodriguese. It is able to complex both ZnII and CuI cations although the latter complex N S NHBoc MeO2C Ph N S NH MeO2C Ph OH O BocHN S N Ph N S MeO2C Ph NH NH OH HN O BocHN OH O BocHN S O N S MeO2C Ph NH NH OH S O NHBoc CO2H S N Ph NH NH OH HN O NH OH S O RO2C N NH Boc Ph O O NH HN O S OH HN O Ph N S HN O N Ph O NH O OH NH O HN O Ph N S HN O N Ph O N S N O H H BocHN N N O 49 50 51 52 53 54 55 R = Me 56 R = H 57 58 + i i, iii, iv vi vii viii x + i, ix ii i, v Scheme 2 Reagents and conditions: i, 50% TFA, CH2Cl2, 25 )C; ii, Boc-Ser-OH (2.5 equiv.), DCC (1.25 equiv.), Et3N (1 equiv.), CH2Cl2, 0 )C; iii, NaHCO3, CH2 Cl2–H2O; iv, 51, DMF, 0]5 )C; v, Boc-Thr-OH, DCC–HOBt, Pri 2 NEt, DMF, 0]25 )C; vi, TFA, CH2Cl2; vii, 54, DCC–HOBt, Pri 2 NEt, DMF, 25 )C; viii, NaOH, MeOH–H2O; ix, Pri 2 NEt, FDPP, DMF, 25 )C, 72 h; x, MeO2CN–SO2NEt3 internal salt, THF, 65 )C, 30 min S N CONH N S N N S N OH O NH O OH S N NH S N H2N O O Ph O N CCONH CH2 CCO2H CH2 HN 59 420 Natural Product Reports, 1998breaks down due to copper(I) oxidation.The former is a stable complex with a 2:1 substrate:metal ratio. The bolaphilic properties of caledonin 92 are attributed to the guanidine and keto amine groupings being polarophiles while the C8 hydrocarbon side chain is hydrophobic; this combination enables ions to be transported through membranes.32 Several Caribbean specimens of a sponge possessing the morphological characters of Pseudoceratina crassa have been analysed and a number of known brominated metabolites identified together with a new one, 93.Also present were the simple metabolites 94 and 95 which were isolated from this source for the first time. No definite taxonomical evidence was forthcoming from this investigation.33 Monamidocin 96 is a novel fibrinogen receptor antagonist which has been isolated from a culture broth of the Streptomyces species NR 0637.34 Malyugamide HY is an ichthytoxic amide isolated from the Caribbean cyanobacterium Lyngbya mauscula.It possesses a novel carbon skeleton 97 and accompanying it is (E)-7- methoxytetradec-4-enoic acid.35 Two new aminopeptidase- N-inhibitors have been isolated from Streptomyces megagawaensis.36 Designated MR-387A and B they diVer only in the ester terminus, A being 98 and B 99.O OMe NHR OMe MeO OMe O OH NHAllox OMe O OR OR O O O O O O O O O O O NHAllox NHAllox NHAllox O O NAllox O O NHAc O O NHAllox NHAllox OH OTBDPS OR OH OH ii i iii iv v vi vii viii ix x + 60 R = H 61 R = Allox 62 63 64 65 R = Me 66 R = H 68 67a 67b 69 R = H 70 R = TBDPS 71 R = H 72 R = Ac 73 R = TBDPS 74 R = H R x xi NHAllox Scheme 3 Reagents and conditions: i, AllOCOCl, NEt3, THF–H2O; ii, PhI(OAc)2, MeOH; iii, NaBH4, 0 )C, MeOH, 10 min; iv, 64, PhI(OAc)2, CHCl2; v, PPTs, PhH–THF, 65 )C, 3 h; vi, H2O2, K2CO3, THF–H2O (3:1); vii, NaBH4, MeOH, "20 )C, 30 min; viii, TBDPSCl, imidazole, CH2Cl2, 4 h; ix, pTsOH, PPTS, acetone–H2O, pH 3, 38 )C; x, PdCl2(PPh3)2, Bu3 SnH, AcOH, CH2Cl2, "20 )C; xi, HF, MeCN–H2O, 0 )C.Allox=allyloxycarbamyl O N HN (CH2)3 O HO Br OMe Br O HO Br OMe Br HN (CH2)4NH C Br MeO Br OH N HO O NH NH2 Br Br NMe2 O N HN O O NMe2 Br Br 79 80 81 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 421A synthesis of sarmentosine 104, the amide alkaloid found in the fruits of Piper sarmentosum, in a short eYcient pathway (21% yield) utilised as a key step an aldol–Grob fragmentation sequence37 (cheme 4).Starting with piperonal 100 a BF3 catalysed condensation with cyclohexanone 101 followed by addition of propane-1,3-diol produced ester 102. Treatment with bis[N,N-bis(trimethylsilyl)amino]tin(II) and pyrrolidine gave, after the appropriate work up, amide 103.Introduction of the 2,3-double bond was accomplished by first ·-selenenylation whence oxidation to the selenoxide enabled Ei elimination to give the natural product sarmentosine 104. The control of beetroot rotting by Botrytis cinera can be achieved by applying the microorganism Pseudomonas cepacia D 202. In an examination of the metabolites produced by this microorganism, monitored by a Botrytis assay, fungitoxic R1 MeO Br N H O OMe R MeO Br N H O OMe OR2 OH 82 83 R1 NH O O O OMe NH O HO R2 R3 R4 84 R1 = cyclohexyl, cyclohexenyl, isobutyl R2 = H when R3,R4 = –NHCOCH2S– or R4 = H when R2,R3 = –NHCOCH2S– MeO Br Br Me N H N Me O MeO Br Br Me N HN O MeO Br Br Me N NMeR MeO MeO Br Me N HN O NMe2 OMe Br O NMe2 OMe Br 85 86 87 88 NH2 O NH NC NH (CH2)5 NH2 NH O Br Br O HO2C O H O OH 89 90 HN N HN HO N NH SH O O O O HO R O Ph O NH2 91 92 N Me2 CN HN H2N N H NH CO2H Br MeO Br HO N HO O N NH MeO HO CO2 – NH OH O Ph + 93 94 95 96 NH (CH2)6 Me O CH2 O OH O 97 N HN Ph NH2 OH O O CO R N OH HO2C 98 R = N CO2H 99 R = 422 Natural Product Reports, 1998agent cepaciamide A 105 has been identified.Interestingly its acid component showed a much higher fungicidal activity.38 Bistramides A, B, C, D and K are cyclic polyethers possessing in vitro cytotoxicity to several tumour lines including human non-small cell lung carcinoma.39 All have been obtained from an extract of Lissoclinum bistratum.Whereas A has been previously reported40 the others are new. Bistramide B is 106, R1=C3H7, R2=CH3CHOHY; bistramide C is 106, R1=CH3CH=CH, R2=CH3CO; bistramide D 107 and bistramide K 108. A new aromatase inhibitor FR 901537 is produced by a Bascillus designated species No. 3072. A detailed examination of its chemical and physical properties established it to be a naphthol derivative 109.41 Ningnanmycin 110 is a new agricultural antibiotic isolated from the culture broth of Streptomyces noursei var xichangensis.42 Thiomarinols B and C are antimicrobial antibiotics produced by the marine bacterium Alteromonas rava.Both metabolites B 111 (R=H, n=0) and C 111 (R=OH, n=2) showed gram-positive and gram-negative activity. B is slightly more eVective than C.43 O O CHO O O O OH O O O N O O O N O 100 101 102 103 104 O + ii, iii iv, v HO(CH2)3OH i Scheme 4 Reagents and conditions: i, BF3, Et2O, rt; ii, Sn[N(TMS)2]2, argon, hexane, then pyrrolidine, 20 )C, 3 h; iii, work up MeOH, H2O, KF; iv, LDA, THF, argon, 35 min, then PhSeCl, THF, 1 h, "78 )C; v, Oxone (K2S2O7), THF, H2O, 0]20 )C, 15 min HN HN O C12H25 O H O C13H27 H O H OH H H 105 O R1 HN O HN O O R2 O O OH O HN O HN O O OH O OH OH HN O HN O O O OH OH OH OH 106 107 108 O S HN HN OH NH HN MeHN S HN O OH O O N O CH2OH O HO CONH2 OH N O NH2 OH 109 110 O O(CH2)7CON OH HO OH R O On S S N O H 111 N O NH Ph HN HN Br RO Br N HO O NH N NH2 HO Ph O OH O NH H N NH 112 R = CO(CH2)7CH=CH(CH2)4CHMe2 113 R = CO(CH2)17Me 114 R = H 115 Z Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 423Lipopurealins D, E and H are new bromotyrosine alkaloids obtained from the Okinawan marine sponge Psaminaplysilla purea.44 Structure variation occurred by diVering substitution on the phenolic grouping: D was 112, E 113 and H 114.A dipeptide 115 isolated from the cyanobacterium Oscillatoria agardhi possesses thrombin and trypsin inhibitor proprties.45 The culturing of Azotobacter vinelandii in an iron limited medium produces siderophores azotochelin 116, aminochein i.e.bis(2,3-dihydroxybenzoyl)lysine and 2,3-dihydroxybenzoyl putrescine. If the iron is replaced by molybdate a new siderophore called protochelin 117, is produced.46 More antitumour metabolites have been found in the sea hare Dolabella auricularion. Designated dolastin H 118 and isodolastin H 119 these minor constituents have contributed to the understanding of the activity relationships of these peptide alkaloids.47 A Patagonia sponge belonging to the Cliona family has been found to contain four peptide alkaloids.Named storniamides A, B, C and D their structures, as determined by spectroscopic methods, are 120, R1=OH, R2=R3=H; 120, R1=R3=OH, R2=H; 120, R1=H, R2=R3=OH; and 120, R1=R2=R3=OH respectively.48 An antifouling alkaloid has been obtained from the marine sponge Agelas mauritania. Identified by its inhibitor activity against the barnacle larvae of Balanus amphitrite it was characterised as 121; a new oroidin dimer.49 N-Methylated agelferins have been isolated from the sponge Astrosclera willeyana.Seven in all, they are also dimers of oroidin with variation in structure associated with or without bromine substitution at R1 to R4 122.50 Two sponges, a Xestospongria species and Agelas novaecaledoniae, are sources for two other oroidin related metabolites, namely sceptrin 123 and ageliferine 124.They are intestinal peptide neurotransmitter inhibitors.51 An interesting series of fungicides can be produced from a Streptoverticillin species designated SAM 2084. Based on structure 125 the variation includes linear branched, saturated or unsaturated aliphatic acyl groups.52 An enantioselective total synthesis of the macrocyclic spermidine alkaloid (")-oncinotine 145 has been achieved starting with L-valine (Scheme 5).53 By standard procedure the NH HN O N H HN NH O NH O OH OH O CO2H H O OH OH HO HO O OH OH OH HO 116 117 HN Me2N O HN Me N N CO2R Ph OH Ph R2 HO O N HN OMe OMe OH O O HO HO OH OH OH OH R1 O R3 OH OH 120 118 R = 119 R = NH HN NH N Br O NH N NH2 NH2 O HN NH Br O Br Br NR 1 HN R2 O HN NR 1 R5 O R3 R4 NH NH N NH2 NH2 121 122 NH HN Br O HN N N HN NH2 NH2 123 NH HN Br O NH NH N NH N NH2 NH2 124 NH Br HN O NH Br O N MeO OH HN O Ph O O O O OR 125 424 Natural Product Reports, 1998protected pentenol 126 was made and reacted with cyclic nitrone 127 in boiling toluene to produce mainly the erythroproduct 128.O-Deprotection and reduction gave trans-diol 129 which was subsequently N-protected to give 130. Glycol cleavage with periodic acid gave aldehyde 131 whence Wittig reaction with phosphonium ester 132 produced cis-alkene 133 which was hydrolysed to acid 134. The remaining diamino portion 139 of the macrocycle was prepared from 3-aminopropan-1-ol 135 and N-protected 4-bromobutylamine 136 to give diamine 137 whence N-protection via carbamate (Z) allowed the alcohol terminus to be silylated 138 which was followed by decarbamoylation to give amine 139.Condensation of amine 139 with acid 134 was achieved using diethylphosphoryl cyanide with triethylamine in DMF and the resulting amide 140 was O-deprotected to give 141 and oxidised to aldehyde 142. Hydrogenolysis released the appropriate amino group which then led to 143, reduction of which gave 144.Deprotection aVorded the natural product 145. The spermidine alkaloid albizzine was isolated from the bark of Albizzia myriophylla.54 By spectroscopic analysis its structure was determined and the novel lactam arrangement 146 was assigned to it. Viburnine 147 is a new macrocyclic spermidine alkaloid isolated from the flowers of Verburnium rhytidophyllum cultivated in Egypt.55 A novel approach to the synthesis of bouvardin 148 and that of piperazinomycin 149 involves an intramolecular SNAr reaction to create the biaryl ether.Thus dipeptide 150 when treated with K2CO3 in DMF gave in 62% yield a 1:1 mixture of atropisomers 151 and 152 at room temperature. The rationale for this unprecedented ease of cyclisation was obtained by a computational simulation of the conformation of the dipeptide 150 in aqueous solution which indicated an edge to face proximity of the two aromatic rings where the reacting groups N N O O– NR NR CHO N Cbz (CH2)8CO2R HO NH2 Br NHBoc R1O N R2 NHBoc NCbz N O R NHBoc N N O NHBoc N N O RHN H OTBDPS H H OH H H H H OH H OTBDPS 126 127 128 erythro : threo 37 : 7 131 132 135 136 143 129 R = H 130 R = Cbz 133 R = Me 134 R = H 137 R1 = R2 = H 138 R1 = SiMe2But; R2 = Z 139 R1 = SiMe2But; R2 = H 140 R = CH2OSiMe2But 141 R = CH2OH 142 R = CHO 144 R = Boc 145 R = H + + ii, iii i iv v vi vii viii x Ph3P(CH2)8CO2Me + + + ix xi xiii xii xv xiv + Scheme 5 Reagents and conditions: i, toluene, reflux, Ar, 14 h; ii, Bu4NF, THF, Ar, 45 )C, 1.5 h; iii, H2, PdCl2, MeOH, 6.5 atm, 37 h; iv, CbzCl (2.5 equiv.), ac Na2CO3, CH2Cl2, 16 h, rt; v, HIO4, THF, H2O, 0 )C, 2 h; vi, KOBut, THF, Ar, 0 )C, 20 min; vii, KOH aq, MeOH, rt, 5 h; viii, K2CO3, DMF, 90 )C; ix, ZCl, aq Na2CO3, CH2Cl2, 0 )C; x, ButMe2SiCl, imidazole, DMF, rt; xi, (EtO)2P(O)CN, Et3, DMF, 14 h, rt; xii, Bun 4NF, THF, rt; xiii, (COCl)2, MeSO, Et3N, CH2Cl2, "78 )C]rt; xiv, H2, Pd(OH)2, MeOH 2 days, rt; xv, HCl (3 M) MeOH, then aq K2CO3, rt N HN HN O COEt MeOCN N H N O Ph O Ph 146 147 O OH Me N O OH NMe HN O HN OMe O HN O O NH O OH NH HN 148 149 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 425OH and C–F are within 5.129 A.This results in a low activation energy and favourable entropy for cyclisation.56 New cyclopeptide alkaloids containing 14-membered rings have been identified by spectroscopic analysis.Lotusanines A 153 and B 154 were located in the aerial parts of Ziziphus lotus.57 A regioselective synthesis of 14-membered biaryl ethers has resulted in a total synthesis of RA-VII 155 and deoxybouvardin 156. The key intermediate is 160 and hitherto the yield determining step has been biphenyl ether formation. In this publication coupling has been achieved by thallium trinitrate oxidation (Scheme 6).58 Thus peptide 157 on treatment with TTN in methanol gave dienone ether 158 which could be reduced to biphenyl 159 with zinc dust in acetic acid.Finally dehalogenation and N-deprotection could be achieved by hydrogenation with palladium/charcoal catalyst giving 160. Discaria longispina contains four cyclopeptide alkaloids. One is new and called discarine X 161, the others were adoutine-Y, discarine-B and discarine-E.59 Five cytotoxic peptides have been isolated from the marine sponge of the Cymbastela family.60 A bioassay-directed fractionation yielded known geodiamolides A to F and hemiasterlin 162.The new ones were geodiamolide G 163, hemiasterlin A 164, hemiasterlin B 165, criamide A 166 and cricumie B 167. Depsipeptide arenastatin A 168 has been resynthesised by an improved procedure thus making suYcient quantities of this metabolite available for screening purposes.61 Through a disconnection approach four segments A 169, B 170, C 171 and D NO2 F MeO2C HN O NHBoc OH O NO2 HN O NHBoc MeO2C O NO2 HN O NHBoc MeO2C 150 151 (31%) 152 (31%) O Pri NH O NH O Pri NH O NH HN Ph Me2N O O Ph O HN O N Ph O 153 154 O OR Me N O NH HN Me N O O O OMe HN NMe O O 155 R = Me 156 R = H Br OH Br MeO2C N Me O NMeCbz Cl OH Cl OH O MeO2C N Me O NMeCbz O O MeO2C N Me O NMeCbz MeO Cl OH O MeO2C N Me O NHMe Br Cl Cl Br Cl 157 158 159 160 TTN MeOH Zn, HOAc H2, Pd/C MeOH Scheme 6 NH NH Me2N O O O NH NH O 161 NR 1 NH Me N OH NH O O N Me NH NH Me O R2 O O I HO O O O O CH2 163 162 R1 = R2 = Me 164 R1 = H; R2 = Me 165 R1 = R2 = H 426 Natural Product Reports, 1998172 were targeted and synthesised. Combination of 169 and 171 followed by 170 and then 172 gave peptide 173.Macrolactamisation using DDPA occurred in 90% yield to give deoxyarenastatin 174. Epoxidation gave the highly cytotoxic metabolite 168. Other isomers of 168, ·-epoxides, ·-methyl grouping, or a ‚-homotyrosyl functionality caused total loss of cytotoxicity.61 An ·-carboline system is incorporated into the cyclic hexapeptide kapakaline B.This unprecedented fused tetracycle 175 was isolated from the marine sponge Cribrochalina olemda and it showed moderate cytotoxicity against P388 murine leukemia cells.62 Aplidites are macrocyclic orthonitrites found in an Australian tunicate of the Aplidium family.63 Seven have been characterised by spectroscopic analysis, degradation and derivatisation. These unusual metabolites were designated aplidite A 176, B 177, C 178, D 179, E 180, F 181 and G 182.The absolute stereochemistry of bastadins 8, 10 and 12 (183) has been determined by the Mosher–Trost method. In all the NR NH Me N NH OH H N NH2 NH Me O O O O NH 166 R = H 167 R = Me Ph OTBDPS O TMS HN P OEt O HN O O HN O Ph O O OMe O OH MeO O O O OEt 168 169 170 OMPM HO C NHBoc O Cl O Ph O O O HN O HN O OMe Ph O O O HN O HO O OMe O NH2HCl O 171 172 173 174 168 N N O Ph O N NH NH NH O NH2 Ph HN O O O 175 a b O N O OR2 O R1 O O O NHR3 OMe 176 R1 = R2 = R3 = H; ab E or Z 177 R1 = R2 = R3 = H; ab Z or E 178 R1 = OH; R2 = R3 = H; ab E or Z 179 R1 = OH; R2 = R3 = H; ab Z or E 180 R1 = OH; R2 = Ac; R3 = H; ab E or Z 181 R1 = OH; R2 = Ac; R3 = H; ab Z or E 182 R1 = OAc; R2 = R3 = H; ab Z or E O Br HO Br Br OH HN O N OR Br OH O Br Br(H) N OH NH O 6 183 N NH (NHCOCH=CH)2–CH=CH(CH2)3CH=CH(Me)OH N N NH O O Ph O O 184 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 427orientation at position 6 is ·.64 Streptomyces species 1-75 in shake culture produces RK-75 184.Interest in this metabolite stems from its antitumour behaviour.65 Neoplasm inhibitors can be produced from Verticillium chlamydosporium by growing this microorganism under shake culture conditions. Structure 185 is assigned to one of these inhibitors (TAN-1746).66 Aburatsubolactams are both neoplasm inhibitors and antiinflammatory agents. They are manufactured by shake culturing of a marine Streptomyces species SCRC-A20.67 Aburatsubolactam A is 186, B 187, R=OH and C 187, R=H.Neosephoniamolide A 188 is a novel cyclodepsipeptide with antifungal activity.68 It was isolated from the marine sponge Neosiphonia superstes. HN HN HN N Ph O O O OH O O HN O OH NMe O O HO NMe O O HO NH O O R 185 186 187 O BocN O NHCbz NH BocN O NHCbz HN OMe Ph O OBzCl2 O O BocHN HN OMe Ph O OBzCl2 O AcHN N O Cbz HN CO2TMSe CbzHN BocN O NH HN OBzCl2 O O HN Ph O NHAc O N TMSeO2C NH HN OR2 O O HN Ph O NHAc O N NH O HO H NH HN OH O O HN Ph O NHAc O N NH O HO H NH NHR RN 189 190 191 192 193 194 R1 = Cbz; R2 = BzCl2 195 R1 = R2 = H 197 R = Boc 198 R = H ii i iii iv v vi vii ix, x NHR1 BocN NH2 NHBoc i–iii viii 196 Scheme 7 Reagents and conditions: i, Bu4NF, THF, 0 )C, 1 h, rt, 2 h; ii, TsOH, H2O (7 equiv.), CH2Cl2, rt, 15 h; iii, FDPP, Pri 2NEt, DMF, rt, 14 h; iv, H2, 5% Pd/C, MeOH, rt, 45 min; v, LiOH, THF, H2O (3:1), 0 )C, 20 min, rt, 4 h; vi, DEPC (1.5 equiv.), Pri 2NEt (2 equiv.), DMF, 0 )C, overnight; vii, 31% HBr–HOAc, rt, 23 min; viii, N,N*-Di(tert-butoxycarbonyl)thiourea (196), HgCl2, Et3N, DMF, 0 )C, 10 min, then rt, 0.5 h; ix, Dess–Martin periodinane, MeCN–CH2Cl2 (1:2), 55 )C, 7 h; x, CF3CO2H, thioanisole, rt, 13 h, then HPLC purification HO I NH O MeN NH O O O O 188 428 Natural Product Reports, 1998Cyclotheonamides, e.g. 198, are metabolites of the marine sponge Theonella.69 They possess inhibitor properties towards proteases, particularly thrombin.These arginine containing peptides present synthetic problems due to the tendency of arginine to be converted into ornithine. A new synthetic approach uses guanidination of the ‰-amino group of ornithine so as to produce an arginine moiety.70,71 Thus (Scheme 7) assembly of the appropriate component sections of 189 and 190 gave 191 which was then combined with 192, each requiring the necessary deprotection so as to create 193. Cbz deprotection of 193 allowed macrocyclisation with penta- fluorophenyl diphenylphosphinate (FDPP) in a better yield (57%) than other procedures.The resulting macrolide 194 upon treatment with HBr in acetic acid resulted in both N-deprotection and O-benzyl deprotection to give 195. Guanidination with thiourea 196 aVorded the arginine product 197. Finally Dess–Martin oxidation and deprotection gave cyclotheonamide B 198. A 90720A is a serine protease inhibitor isolated from a terrestrial blue-green alga. Microchazete loktakensis.This depsipeptide 199 was located on the basis of its thrombin inhibitory activity.72 Beauvericin 200 is a toxic cyclodepsipeptide which has been isolated from a toxigenic strain of Fusarium proliferatuus. This is not the normal source for this toxin.73 Cyclic octapeptide pseudostellarin H 201 has been obtained from the roots of Pseudostellaria heterophylla.74 It is a minor component of the extract and like its predecessor the pseudostellarins A–G it possesses tyrosinase inhibitor activity.Oscillamide Y 202 is a chromotrypsin inhibitor isolated from the fresh water toxic cyanobacterium Oscillatoria ogardhii, one of the alga that form water blooms in fresh water lakes and reservoirs.75 Pholipeptin is a novel lipoundecapeptide isolated from a Pseudomonas species. Its structure 203 was determined by first amino acid analysis followed by NMR measuremets.76 It is an inhibitor of phosphatidylinositol specific phospholipase C. 3 Miscellaneous alkaloids The synthesis of azetidines with a straight chain addendum has been achieved thus opening the way to the synthesis of two alkaloids containing this functionality namely penaresidin A (204, R1=H, R2=OH) and penaresidin B (204, R1=OH, R2=H).Both are constituents of the Okinawan marine sponge belonging to the Penares family.40 The key step in the synthesis77 is the biomimetic type ring closure of phytophingosines, precursors thought to be involved in the metabolites’ biosynthesis.Thus 205 on treatment with mesyl chloride followed by sodium hydride gave azetidine 206. Later NH2 HN NH NH N NH O NH O HN OH O SO3H O MeN OH HN O O O OH O O 199 O O N Me O Pri Ph Ph N Me O N Me O O O Pri O Pri O Ph 200 NH N HN NH NH HN NH N O OH O O Ph O Ph O O O OH O 201 HO HN NH HN HN NMe HN NH O O O Ph O O O CO2 H OH 202 HN HN NH HN HO O O O O O HN NH O NH HN O O O CO2H O HN O HN HO2C NH HO O O OH 203 HN HOCH2 BnO C12H25 BnO Ms N H C12H25 H H OBn OBn NH2 OH OH H R2 (CH2)9C R1 204 205 206 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 429in 1995 another group published the synthesis of penaresidin A (204, R1=H, R2=OH) itself.Since the absolute stereochemistry of the metabolite is unknown this synthesis limits the variation to either 15R, 16R or 15S, 16S. The synthesis began with L-isoleucine 207 which was converted in 6 steps (Scheme 8) to the alkynol 208 and thence to the protected compound 209.Coupling with Garner’s aldehyde 210 gave 211 which was converted into the tosyl derivative 212 whereby epoxidation gave 213. Reduction with DIBAL followed by mesylation gave 214 regioselectively. Reaction with sodium hydride gave ring closure to azetidine 215 whence deprotection gave (2S,3R,4S,15R,16S)-penaresidin A 204. If intermediate 208 was inverted under Mitsunobu conditions enantiomeric 208 was produced which by the same procedures as before gave the (2S,3R,4S,15S,16S)-isomer. The (2S,3R,4S,15R,16R)- isomer was also synthesised using epoxide 207b to create the (15R,16R) terminus.Comparison of the 500 Hz NMR spectra of naturally occurring penaresidin A tetraacetate with all four synthetic tetraacetates led to the conclusion that (15R:16R) or its antipode are the only possible arrangements. An oxirane carboxamide designated SB-20490 has been isolated from the leaves of Clausena lansium. Its structure 216 was elucidated by spectroscopic measurements and confirmed by X-ray crystallography.79 A total synthesis of (+)-trichoviridin 225 was achieved utilising a precursor 218 first synthesised in 1989 from silyloxyfulvene 217 (Scheme 9).80 When 218 was treated with polymer bound triphenylphosphine (3 equiv.) and pyruvic aldehyde (3 equiv.) two products 219 and 220 were formed in variable yields, however 219 could be converted into 220 using DBU in methylene chloride.The dehydration step for converting 220 into isonitrile 221 was finally achieved by successively using dimethyldioxirane at "40)C for 10 min then adding diisopropylethylamine for 2 min followed by trifluoroacetic anhydride, thereby producing isonitrile 221 with minimal decomposition.Protection of the isonitrile group by forming its dibromide 222 allowed epoxidation to proceed to 223 which when followed by debromination with triphenyl phosphite to give 224 allowed deprotection to give (+)- trichoviridin 225.Phevalin 226 is a bioactive metabolite obtained from the Streptomyces species strain SC433. It is able to inhibit calpain formation.81 CO2H (CH2)9CºCH NBoc O O NBoc (CH2)8 TBSO TBSO (CH2)8 TBSO (CH2)8 TBSO NH (CH2)7 CHO OTBS O O OH OTBS OTBS OTBS OTBS OMs NHTs OTBS OTBS NHTs OTBS NHTs OTBS OR NH2 ii i ix x, xi 207a 211 210 207b 212 213 214 215 208 R = H 209 R = TBS 6 steps vi vii, viii 204 (15 R,16 S) iii–v Scheme 8 Reagents and conditions: i, TBSCl, imidazole, DMF; ii, BunLi, THF then 210; iii, Li, EtNH2; iv, TBSOTf, 2,6-lutidine, CH2Cl2; v, TsCL, py; vi, M-CPBA, NaHCO3, hexane; vii, DIBAL, toluene; viii, MsCl, py; ix, NaH, THF; x, Na, naphthalene, DME; xi, HF, MeCN O Me N Ph O Ph H H 216 iii iv v vi O OH OTBDMS OTMS O OH OTBDMS O OH OTBDMS O OH OTBDMS O OH OTBDMS O OTBDMS O O OH OR O NSTol OAc NHCHO N Br Br NC NHCHO N Br Br NC 7 steps i 217 218 219 220 221 222 223 224 R = TBDMS 225 R = H ii vii Scheme 9 Reagents and conditions: i, polymer bound PPh3 (3 equiv.), AcOCHO (3 equiv.), polylene oxide, CH2Cl2, rt, 24 h; ii, DBU (5 equiv.), CH2Cl2, rt, 3 h; iii, dimethyldioxirane (3 equiv.), CH2Cl2, "40 )C, 10 min then Pri 2 NEt (10 equiv.), 2 min, then Tf2O (1.5 equiv.), "78 )C, 20 min; iv, Amberlyst A-26 Br3 " form (1 equiv.), propylene oxide (10 equiv.), CH2Cl2, rt, 2 h in the dark; v, methyl(tri- fluoromethyl)dioxirane, propylene oxide, "20 )C, CH2Cl2; vi, P(OEt)3 (10 equiv.), rt, 1 h; vii, Bu4NF (1.15 equiv.), 0 )C, THF 430 Natural Product Reports, 1998Lacarin 227 is a new alkaloid isolated from the mushroom Lacaria vinaceoavellanea.It showed phosphodiesterase inhibiting activity.82 Sibyllimycine 230 is a novel metabolite from a Thermoactinomyces species. This hot spring stable microorganism was incubated and after three days this metabolite appeared.83 Its structure was confirmed by synthesis from 4-methylimidazole 228 by acetylation followed by alkylation with 4-bromobutyronitrile at 160)C to give a mixture of N-alkylated products.Separation gave 229 which cyclised to give the natural product 230. The thiohydrazine metabolites 231, R=SOMe or R=SO2Me found in an extract of Calvatia cramiformis,84 have been reported as useful neoplasm inhibitors. HN N NH HN CHMe2 O Ph O O 226 227 HN N N C N N N O 228 229 230 Br(CH2)3CN OH SMe R N N CONH2 HN HO2C OH OMe NMe HO CH2OH 231 232 HN N O Me N N N N N+ O– O– R O O O R O O + 233 234 235 236 N N HO OMe Ph OH H 237 N HN N N NH NH HN O OMe OMe Ph OH H OH OMe O OMe OH Ph OH H O O 238 239 240 N N CO2Me N N CO2H N N CO2H N N CO2H CO2H OMe CH2 OH CH2 R HO CO2Me O CH2 H H2N R O O H2N 241 242 243 244 N HN R MeN N Me N N N N N Me NMe O O COPh O O O O O O O OH OH HO OMe 245 246 O N N O NH OH HO C12H25 H O OH O CHO 247 248 249 O NH O Pr 250 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 431A microsporine like amino acid has been obtained from the reef building corals Pocillopora damicomis and Stylophora pistillata.85 This metabolite is novel in that it absorbs light at 330 nm and it is thought to be a protective agent to minimise UV damage. Its structure 232 led to its name – microsporine-methylamine.Two investigations into the alkaloidal constituent of Nitraria komarovii have identified firstly 23386 and then 234, R=OH, together with known 234, R=H, i.e. the N-oxides of vascicinone and deoxyvascicinone.87 Bassiatin is a new platelet aggregation inhibitor produced by the fungus Beauveria bassiana strain K-717 found in a soil sample obtained in Yunnan Province.Its structure 235 was determined by X-ray crystallography and confirmed by synthesis.88 The first reported isolation of maculisin type compounds from a Streptomyces has been reported.89 Three were identified as maculosin-1 (236, R=CH2C6H4OH-4), maculosin-5 (236, R=CHMe2) and maculosin-6 (236, R=CH2CHMe2).Terezines A to D are new bioactive metabolites obtained from the coprophilous fungus Sporormiella teretispora.90 Their structures are based on pyrazine itself thus A is 237, B 238, C 239 and D 240. New antibiotic phenocomycin 241 has been extracted from the fungus Streptomyces species HIL-Y-9031725.91 A series of new anticancer antibiotics, the pelagiomicins, have been isolated from a new marine bacterium designated Pelagiobacter variabilis. These metabolites were red or orange in colour and unstable in water and alcohol.A total of twelve in all were isolated but most were too unstable or in too small a quantity to be identified. Three were identified as 242, R=H, 242, R=OH and 243.92 Two novel phenazines obtained from a Streptomyces species have been found to possess inhibitor properties against metalloenzymes. Structure 244, R=H and structure 244, R=SCH2CH(NHCOMe)CO2H are attributed to the metabolites.93 Two metabolites produced by Streptomyces purpeofucus are reported to protect neurons from L-glutamate toxicity.Called aestivophoenine A (245, R=H) and aestivophoenine B (245, R=CH2CH=CMe2) they also suppressed the toxicity of L-glutamate in N18-RE105 cells.94 Odontosyllis undecimdonta is a marine polychaete which produces the metabolite 246. It is a dimer of 1,3-dimethyl-6- acyllumazine derivatives.95 An enantioselective synthesis of (")-desoxoprosopinine 249 has been carried out utilising a radical cyclisation procedure to convert aldehyde 247 into piperidine 248.In this tributyltin HN NH NH HN Ph H Ph O 253 HN HN H H H H H H HN HN H H H H H H H H Sn Bu3 Sn Bu3 a or b 252 251 OBn I R MeO2C OMOM HO MeO2C OBn I N O OMOM R MeO2C OBn I N O OMOM OAc MeO2C OBn I N O OMOM OR MeO2C OR1 I N O OMOM CH2 NCO2Me MeO2C OBn N O OMOM NCO2Me CH2 MeO2C OR1 N O OMOM NCO2Me MeO2C OBn N O OMOM NCO2Me OHC OH N O OH NH OCONH2 NCO2Me R OH O OR2 254 R = NO2 255 R = NO 257 R = OH 258 R = OAc 262 R = Ac 263 R = H 259 R = OH a 260 R = OTf a 261 R = N3 b 256 264 265 266 267 ii i iii iv v vi vii viii ix, x xI + 268 xIv xIi, xiii several steps Scheme 10 Reagents and conditions: i, SmI2 (4 equiv.), THF, "78 )C then Oxone (3 equiv.) and H2O, 0 )C; ii, Diene 251, PhH, 80 )C; iii, Ac2O, py, CH2Cl2, 22 )C; iv, OsO4 (1 equiv.), Me3NO · H2O, CH2Cl2, PhH, 22 )C; v, Tf2O (1 equiv.), py, CH2Cl2, 0 )C; vi, Bu4NN3, DMF, 22 )C; vii, Tf2O (1.5 equiv.), py, CH2Cl2, 0 )C then Ph3P, then NH4OH, then methyl chloroformate (1.5 equiv.), py, CH2Cl2, 0 )C; viii, K2CO3, MeOH, 22 )C; ix, DMSO, oxalyl chloride, CH2Cl2, "78 )C then Et3N; x, Ph3PCH3Br, NaHMDS, THF; xi, (Ph3P)4Pd, Et3N, CH3CN, 90 )C, 18 h; xii, OsO4, NMO, acetone–H2O, 22 )C; xiii, DIAD, Ph3P, THF, 22 )C; xiv, SmI2 (2 equiv.), N,N-dimethylethanolamine (10 equiv.), THF, "78 )C 432 Natural Product Reports, 1998hydride–AlBN induced reaction complete facial selectivity was observed resulting in high trans-selectivity.Thereafter side chain modification, lactone ring opening and reduction gave the natural product.96 Two syntheses of 9-propyl-10-azacyclododecan-12-olide 250, which is a minor component of the defensive secretion of the Mexican bean beetle Epilachna varivestis, have confirmed its structure.97,98 Papuamine 252· is the major metabolite of the bright red encrusting sponge Haliclona.Haliclonadamine is its isomer 252‚. Both have been synthesised utilising as a cyclisation step a PdII catalysed intramolecular cyclisation of a bis[(E)- vinylstannane] 251.99 The other synthesis100 also employs this same procedure.Verbacine 253 (E) and verballocine 253 (Z) are two novel macrocyclic spermine alkaloids containing a 17-membered lactam ring. The two alkaloids diVer only in the stereochemistry of the cinnamoyl double bond.101 The nitromycins and related compounds have presented a challenge for synthesis due to their complex functionality. The metabolite obtained from Streptomyces sandaensis has a somewhat similar structure 268 and its synthesis has been accomplished102 by employing a hetero-Diels–Alder reaction of a heavily functionalised nitroso aromatic system (Scheme 10).Thus iodobenzene 254, R=NO2 was reduced with samarium iodide to the nitroso compound 255, R=NO which underwent the Diels–Alder reaction with diene 256 to give adduct 257. Acetylation to 258 followed by osmylation gave diol 259 which was triflated to give 260 to enable azide introduction as 261 with the required ‚-stereochemistry.Azirane formation followed to 262 whence deacetylation to 263 and dehydration gave olefin 264 which was cyclised to 265. Epoxidation to 266 followed by its hydrogenolysis gave alcohol 267. O-Deprotection and TIPS protection set the scene for functional group manipulation, ester to aldehyde, N-formate to NH, and OTIPS to OH and OCONH2 thus giving the natural product 268. Azafluorenone alkaloid cyathocaline 269 has been isolated from the stem bark of Cyathocalyx zeylanica.Bioassay guided fractionation using a brine shrimp toxicity test led to the active component.103 A new alkaloid called isocryptolepine 270 has been obtained from the roots of Cryptolepis sanguinolenta.104 Amphimedine 275 is a cytotoxic alkaloid isolated from the sponge collected at Guam Island and identified as an Amphimedon species. It has been synthesised;105 this new procedure is convenient since it comprises only two steps from a conveniently available quinoline 271. Thus its cross coupling reaction with 4-(trimethylstannyl)pyridine 272 gave biaryl 273 which with dimethylformamide diethyl acetal (DMFDEA) gave enamine 274 which on acid treatment ring-closed to amphimedine 275.The sponge Zyzzya fuliginosa now contains fourteen alkaloids of the pyrroloiminoquinone family. In a new investigation106 six new ones, namely makaluvamines H to M were identified together with the previously known C 276, D and G.69 The structures of these alkaloids were determined by spectroscopic analysis and are 277 to 282 for H to M respectively.In addition a seventh alkaloid damirone 283 was isolated from this extract. A new discorhabidin alkaloid has been isolated from the Antarctic sponge Latrunculia apicalis.107 After extraction of the freeze dried sponge Sephadex chromatography showed two coloured bands. A red pigment was identified as discorhabdin C108 while a green pigment was not correlated with any known alkaloid.Structurally it was characterised as a discorhabdin and designated G 284. An asymmetric total synthesis of (R)-(")-julandine 299 and (R)-(")-cryptopleurine 303 has been carried out starting with N N O NHR3 HN NH O O R1 R2 (CH2)2 OH CH OH CH2CH R1 = R2 = H; R3 = R1 = H; R2 = Me; R3 = R1 = Me; R2 = H; R3 = R1 = H; R2 = Me; R3 = R1 = R2 = H; R3 = (CH2)2 OH (CH2)2 OH CH OH CH2CH R1 = R2 = Me; R3 = H R1 = R2 = R3 = H 276 C 279 J 280 K 281 L 282 M 277 H 278 I 283 + 269 N OH MeO HO O N NH N CN Cl N N SnMe3 CN N N N NH O N N N 270 271 272 273 274 + Pd(Ph3P)4 dioxane D, N2 275 DMF, DEA 100 °C H2SO4 HOAc D N NH O HN O Br + 284 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 433(S)-N-nitrosoprolinol 285 (Scheme 11).109 O-Protection to 286 was followed by reduction to the aminopyrrolidine 287 which was converted by glutaric anhydride into 288.Reduction with LiBEt3H gave alcohol 289 as a diastereomeric mixture which was methylated to aVord 290.In the presence of BF3–diethyl ether and silyl enol ether 291 an in situ asymmetric addition occurred to give, with excellent diastereoselectivity, the C-6 keto lactam 293. Reduction resulted in removal of the pyrrolidine grouping and conversion into the alcohol 295. These intermediates from 291 (X=H) or 292 (X=Br) could now be used to give julandine 299 (path A) or cryptopleurine 303 (path B).Condensation with 4-methoxyphenylacetyl chloride gave amide 296 which was oxidised to ketone 297 whence cyclisation led to oxojulandine 298; deoxygenation gave (R)-(")-julandine 299. In path B a similar procedure resulted in alcohol 300 being converted into ketone 301 allowing ring closure to 302. Binary formation was induced with either photocyclisation (54%) or, better, with radical involvement using AlBN (87%) – both giving (R)-(")- cryptopleurine 303.Ten phenanthroindolizidine alkaloids have been found in an extract of the fresh leaves of Tylophora tanakae.110 Two, isotylocebrine 304 and tylophorine 311, are known but the other eight, 306 to 313, are new. A concise and eYcient total synthesis of three cytotoxic marine alkaloids, namely cystodytin J 320, diplamine 321 and shermilamine B 328, involved a pyridine forming reaction coupled with a triplet sensitised thermophotolysis of an aryl azide which established a C–N bond to an unactivated benzylic site111 (Scheme 12).Known dienone 314 was mesylated to 315 and reacted with ethyl vinyl ether under YbIII catalysis to provide pyran 316 which could be converted into amide 317. This underwent a pyridine forming reaction using hydroxylamine hydrochloride in moist acetonitrile giving 318. Ozonolysis gave ketone 319 which upon irradiation followed by careful DDQ oxidation furnished cystodytin J 320. This in turn reacted with methanethiol to give diplamine 321.If the original dienone 314 was converted into its acetate 322 and thence into ketone 323, by the same procedures as converted 315 into 319, the bromo-substituted ketone 324 then enabled introduction of a thiomethyl acetate grouping as 325 which could be cyclised to azide acetate 326. Hydrolysis of the acetate grouping and conversion into the amide 327 thus enabled photolysis to create shermilamine B 328. An extension of synthetic Scheme 12 has been used to synthesise other sulfur containing pyridoacridine alkaloids.112 Thus dercitin 329, and kuanoniamines 330, R=Ac or Me have been made.What is thought to be another defense alkaloid secreted by the coccinellid beetle Chilocorus cacti has been identified as a N H R2 OR1 OMe OMe OR3 N+ H R2 OR1 OMe OMe OMe O– 304 R1 = R2 = Me; R3 = H 305 R1 = H; R2 = R3 = Me 306 R1 = R2 = H; R3 = Me 307 R1 = Me; R2 = H 308 R1 = Me; R2 = OH 309 R1 = H; R2 = OH N N N R2 OR1 OBn O O N N OBn OR O NH N N BnO O OMe OMe X OMe OMe X O OMe OMe X OSiMe3 CO N R OMe OMe CH2 MeO MeO MeO N R OH OMe CO N R OMe OMe CH2 OMe H Br 285 R1 = H; R2 = NO 286 R1 = Bn; R2 = NO 287 R1 = Bn; R2 = NH2 288 295 289 R = H 290 R = Me 291 X = H 292 X = Br 293 X = H 294 X = Br 296 R = OH 297 R = O 298 R = O 299 R = 2H 300 R = H,OH ii i iii iv v vi ix + A viii vii xi ix x N MeO MeO N O H MeO MeO N H OMe Br OMe OMe MeO Br MeO O R H xii 301 R = O 302 303 B viii x Scheme 11 Reagents and conditions: i, BnBr, NaH, DMF, THF; ii, LiAlH4, THF; iii, glutaric anhydride, CH2Cl2, 1 h rt then Ac2O, trace NaOAc, reflux, 1 h; iv, LiEt3BH, THF; v, PPTS, MeOH; vi, 191 or 192, BF3 · Et2O, CH2Cl2; vii, BF3, THF; viii, 4-methoxyphenylacetyl chloride, K2CO3, MeOH, H2O, reflux; ix, PDC, CH2Cl2; x, 5% KOH, EtOH; xi, LiAlH4–AlCl3 (3:1), THF; xii, hÌ or Bu3SnH, AlBN, PhH, reflux 3 h 434 Natural Product Reports, 1998‘dimeric’ arrangement of a hippodamine moiety associated with an azacenaphthylene system.113 In this case chilocorine B has also two bridges but they are spiro in their connection as 331 rather than as in chilocorine A 332.On the basis of biogenetic hypothesis the biogenetic precursor to nitraramine 340 is the bispiperidine 339 which cyclises to the alkaloid 340 upon heating in a buVered aqueous solution at pH 7; only one isomer, that of the natural product, is produced. Synthesis (Scheme 13) of this precursor 339 started with piperidone 333 whereby condensation with glutaraldehyde monoacetal 334 gave alcohol 335 which upon dehydration and release of the aldehyde functionality as 336 allowed a second piperidone to be added creating 337.Careful reduction led to unstable 338 which upon N-deprotection gave 339 and followed by heating gave nitraramine 340. As a result of this synthesis the authors suggest that this alkaloid and related ones are formed by non-enzymic catalysed cyclisation.114 N H OMe OMe OMe N H OMe OMe OR N+ OH OMe OMe OMe O– OMe OMe 310 311 R = Me 312 R = H 313 ii i iii iv v vi vii viii vi xi O N N N N N N S NH HN N S NH N3 RO R AcHN N O O OEt O AcHN AcHN MeS AcHN N O RO O R O AcO O AcHN O R N3 N3 N3 N3 N3 N3 N3 N3 N3 N3 H H 314 R = H 315 R = Ms 316 R = OMs 317 R = NHAc 320 319 318 321 314 R = H 322 R = Ac 328 326 R = OAc 327 R = NHAc 323 R = H 324 R = Br 325 R = SCH2CO2Me ii, iv, v ix x xii Scheme 12 Reagents and conditions: i, MsCl, CH2Cl2, NEt3, 0 )C to rt; ii, EtOCH=CH2, cat.Yb(fod)3, DCE, reflux; iii, AcNH2, NaH, DMF, 0 )C to rt; iv, moist HONH2HCl, MeCN, reflux; v, O3, CH2Cl2, MeOH (4:1), "78 )C then Me2S, "78 )C to rt; vi, hÌ, PhCl, 110 )C cool to rt then titrate DDQ; vii, MeSH, CH2Cl2, AcOH (4:1), rt then evap.volatiles, titrate DDQ in CH2Cl2; viii, Ac2O, py, rt; ix, pyridinium tribromide; x, HSCH2CO2Me, Pri 2NEt, EtOH, rt; xi, BF3Et2O, CHCl3, rt; xii, standard OAc]OMs]NHAc procedures N N N S Me2N NH N N S NH R N O H CH2 N N O H N 329 330 R = Ac or Me 331 332 Lewis: Muscarine, imidazole, oxazole, thiazole and peptide alkaloids, and other miscellaneous alkaloids 435A hydrooxadiazine associated with a four-ring fused structure 341 has been established by X-ray studies for the alkaloid fissoldhimine obtained from the plant Fissiptigma oldhamii.115 Martinelline 342 and martinellinic acid 343 are found in the ethanol extract of the roots of the ethnobotanical plant Martinella iquitosensis.The juice of the crushed roots of this plant is used in South America as an eye medication and these metabolites have been found to react with the bradykinin receptor and to have mild antibacterial activity.116 4 References 1 V.Popsavin, O. 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ISSN:0265-0568    

DOI:10.1039/a815417y

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Monoterpenoids David H. Grayson University Chemical Laboratory, Trinity College, Dublin 2, Ireland Covering: part of 1995, all of 1996, and part of 1997 Previous review: 1997, 14, 477 1 Introduction 2 2,6-Dimethyloctanes 3 Artemisyl, santolinyl and chrysanthemyl systems 4 Cineol derivatives 5 Menthanes 6 Pinanes 7 Camphanes and isocamphanes 8 Caranes 9 Fenchanes 10 Thujanes 11 Ionone derivatives 12 Iridanes 13 Cannabinoids 14 References 1 Introduction This review covers developments in monoterpenoid chemistry which were published between mid-1995 and mid-1997.Its format closely follows that of the previous article in the series.1 The novel glycoside 1 has been obtained2 from Ferula sinaica, and the benzoate 2 is a major component of ‘Re Gung’, a Vietnamese Cinnamomum species.3 The glycoside marounoside 3, which possesses an unusual carbon skeleton, has been isolated4 from Nauclea diderrichii. The furanoid rabdoketones A 4 and B 5 have been obtained5 from Rabdosia eriocalyx.Regio- and stereo-controlled reactions of isoprene 6 which provide functionalised building blocks for terpenoid synthesis have been reviewed.6 Two new syntheses of rosefuran 7 have been described.7,8 (")-cis-Rose oxide 8 is biotransformed by Aspergillus niger to give, sequentially, the alcohol 9 and the carboxylic acid 10. The trans-oxide undergoes analogous transformations.9 The chemistry of the monoterpenoid indole alkaloids has been reviewed.10 Seven new monoterpenoid isoquinoline glycosides have been isolated11 from Alangium lamarckii.The novel composite alkaloid incarvine 11 has been obtained12 from Incarvillea sinensis. The absolute configuration of incarvilline 12 from the same source has been determined by X-ray methods, and the structure of hydroxyincarvilline 13 has been elucidated.13 X-Ray techniques have also been utilised14 in a determination of the structure of the novel kinabalurine 14, an epimer of incarvilline 12, which has been isolated from Kopsia pauciflora.The new compound kopsilactone 15 has been found15 in Kopsia macrophylla. Biomimetic syntheses of some pyridine monoterpenoid alkaloids have been carried out by treating a number of iridoid glycosides with ‚-glucosidase and aqueous ammonium acetate.16 The role and occurrence of plant monoterpenols and of their glycosides have been reviewed in articles17,18 which also discuss the use of enzyme-mediated hydrolytic procedures for the release of aroma and flavour compounds from them.Various monoterpenoid alcohols have been converted into the corresponding O-benzyl-·-D-glucopyranosides.19 The possibilities inherent in the bioengineering of isoprenoid biosynthesis have been reviewed.20 The production of aroma compounds by plant cell cultures has been discussed,21 as have industrial applications of plant cell cultures which are increasingly valuable tools for eVecting biotransformations of monoterpenoids.22–24 Specific systems which have attracted attention include the use of in vitro cultures of Achillea millefolium ssp.millefolium25 and of Eucalyptus spp.26 for the production of essential oils. Stable five-year-old transformed shoot cultures of Mentha citrata that have been maintained by regular sub-culturing aVord an oil which closely resembles that produced by the parent plant and which contains mainly linalool and bornyl acetate. However, a similarly treated culture of Mentha piperita which originally produced menthol, menthone and menthofuran was found to produce pulegone and menthofuran after five years.27 Attempts to restore the original pattern of monoterpenoid output by altering various conditions met with failure.The industrial development of essential oils has been reviewed.28 The potential of common monoterpenoids as feedstocks for the future biocatalytic production of flavours has been critically discussed,29 as has the application of lipases for the synthesis of esterified flavouring components from monoterpenoid alcohols.30 O-b-D-Glc R O OH OH HO O O O OH HO OH OH CO2H O O O O O O R 8 R = Me 9 R = CH2OH 10 R = CO2H 7 4 5 3 2 R = OCOPh 6 R = H 1 N Me O CO2 O N Me N Me HO R N Me HO N Me O O 15 14 12 R = a-H 13 R = a-OH 11 •• • • Grayson: Monoterpenoids 439Methods for the isolation of essential oils from plant sources have been described.31 The extraction of essential oils using supercritical CO2 has been reviewed, and calculations of phase equilibria for pure components with their mixed source oils have been carried out.32 The essential oils of the genus Lippia33 and the phytochemistry of Rosa rugosa34 have each been discussed.The biosynthetic pathways which lead to terpenoids including monoterpenoids have been reviewed.35 Remarkably, it has been demonstrated that 1-deoxy-D-xylulose is incorporated into isoprene which is biosynthesised by higher plants.36 The enantiomeric compositions of monoterpene hydrocarbons in diVerent tissues of Pinus sylvestris37 and in the xylem and needles of Picea abies38 have been investigated.The release of monoterpenoids by plants in response to their attack by feeding insects has been reviewed.39 It has been shown that the oral secretions of insects may contain elicitors which stimulate plant defence mechanisms, triggering monoterpenoid output, and that simple mechanical wounding of the plant does not evoke the same response.The production by coniferous trees of terpenoids which are involved in interactions with insects and with fungal pathogens has been discussed.40 Slash and loblolly pines having high concentrations of ‚-phellandrene in their cortical tissues are much more resistant to fusiform rust disease than those with lower concentrations of the monoterpene,41 whilst slash and loblolly pine seedlings of various ages emit monoterpenoids at diVerent levels which can be correlated with their resistances to attack by the Nantucket tipmoth Rhyacionia frustrana.42 Limonene, which is present in the oleorosin of Picea glauca, is toxic to adults of the spruce beetle Dendroctonus rufipennis, causing 100% mortality after 24 h exposure at 80 ppm,43 and limonene, myrcene and ‚-phellandrene are all toxic to adults of the Eastern larch beetle Dendroctonus simplex. These monoterpenes are found in enhanced amounts in wound tissues of Picea glauca, suggesting the existence of an induced defensive system.Some aspects of the use of monoterpenoids as mosquito repellents have been discussed.44 The essential oil from Lippia multiflora inhibits the growth of in vitro cultures of the chloroquine-resistant FcB1-Columbia and F32-Tanzania strains of Plasmodium falciparum.45 The oil from Mentha piperita inhibits the growth of Salmonella enteritidis and Listeria monocytogenes in culture media and in slightly acidic foods such as tzatziki and taramosalata, but is much less eVective in more alkaline foods such as pâté.46 It has been suggested that the flowers of Brugmansia candida exert intoxicating eVects.An investigation47 has revealed the presence of trans-ocimene and of 1,8-cineol but, not surprisingly, showed that volatile tropane alkaloids were not emitted. Monoterpenoids, including limonene and camphene, have been detected in various mushrooms of the family Boletales, 48,49 and cis- and trans-sabinene hydrates have been found in a number of Basidiomycete species.50 The use of enantioselective GC employing capillary columns with modified cyclodextrins as stationary phases for determination of the authenticity of essential oil samples has been reviewed,51–53 as has the measurement of their diagnostic stable isotope ratios by mass spectrometry.53 The simultaneous analysis of the enantiomeric compositions of many monoterpenoid hydrocarbons, alcohols and acetates by chiral GC using heptakis-(2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl)-‚- cyclodextrin columns has been described.54 The enantiomeric ratios of monoterpene hydrocarbons in the needle oils of Pinus sylvestris and of Juniperus communis have been examined55 and, in an ominous new development, pesticide residues in essential oils have been quantified by GC methods.56 A method for the enantiodiVerentiation of monoterpenoids via examination of their 13C NMR spectra in the presence of the chiral shift reagent [Yb(hfc)3] has been described.57 Terpenoid alcohols such as (S)-citronellol, (")-menthol and borneol have been attached to cellulose acetate membranes by plasma polymerisation, and the treated membranes have been shown to resolve rac-tryptophan by dialysis or by ultra- filtration.58 D-Tryptophan preferentially permeates through the prepared membranes.The asymmetric reduction of karahanaenone 16 by various microorganisms has been studied.59 Glomerella cingulata and Aspergillus niger provide the (S)-alcohol 17, and the (R)- enantiomer is produced by Fusarium solani.A synthesis of (")-(1S,5R)-karahana ether 18 has been described,60 and (R)-mevalonolactone 19 has been synthesised using a carbohydrate template.61 A number of enantioselective syntheses of (+)- and of (")-grandisol 20 have been reported.62–65 Applications of the Schenck oxo-ene reaction have been reviewed.66 Experimental evidence that the Karplus relationship does not satisfactorily describe 3J (13C,1H) in the 13C=C–C–1H system which is commonly found in monoterpenoids has been adduced.67 Plant essential oils continue to be scrutinised for their components, and Table 1 lists the principal species of actual or potential economic interest which have been examined during the period under review and for which monoterpenoid constituents have been identified and quantified. 2 2,6-Dimethyloctanes The novel concinnamide 21, isolated from seeds of the medicinal plant Acacia concinna, has been assigned the revised structure shown, and a synthesis of (–)-21 from (–)-linalool 22 has been described.248 The amide 21 had previously been assigned a seven-membered lactam structure by the same authors.The new geranyloxycoumarins chloromarmin 23 and aeglin 24 have been obtained249 from the bark of Aegle marmellos. Aeglin 24 has been synthesised249 by a route which confirms its absolute configuration. The new green tea aroma precursor geranyl 6-O-·-L-arabinopyranosyl-‚-Dglucopyranoside 25 has been isolated from Camellia sinensis var.sinensis cv. ‘Yabukita’,250 and four new geranyl glycosides have been obtained from Ligustrum robustum.251 The four novel glycosides 26–29 have been isolated from Cunila spicata together with the triol 30 which was obtained as a 10:1 mixture of diastereoisomers.252 (R)-Linalyl ‚-D-glucoside 31 has been isolated from Thujopsis dolabrata Seib. et Zucc,253 and the new oxygenated linalyl glycoside portuloside A 32 has been found254 in Portulaca oleracea.A synthesis of 32 from linalool 22 has been described.254 The novel glycosides kudingoside A R O O O OH 19 18 16 R = O 17 R = H, a-OH OH (+)-20 HO R O O O R1 R3 R2 23 R1 = H; R2 = OH; R3 = Cl 24 R1 = R3 = OH; R2 = O-b-D-Glc 21 R = CONH2 22 R = H 440 Natural Product Reports, 1998Table 1 Sources of monoterpenoids Species Principal constituents Reference Abies balsamea Mill ·- and ‚-pinene, ‚-phellandrene, limonene 68 Achillea biebersteinii Afan piperitone, or cineol and camphor 69 Aloysia triphylla (L’Herit.) Britton citral, limonene 70 Alpinia conchigera GriV. cineol 71 Alseuosmia macrophylla A.Cunn. flowers: linalool 72 Ambrosia microcephala DC. camphor, bornyl acetate 73 Amomum subulatum Roxb. cineol, ·-terpineol, ·- and ‚-pinene 74 Artemisia afra yomogi alcohol, artemisyl acetate 75 Artemisia afra Willd. cineol, terpinen-4-ol, borneol 76 Artemisia ordosica ·- and ‚-pinene, sabinene 77 Austromyrtus dulcis (C.T. White) L. S. Smith (chemotype) ·-pinene, cineol 78 Boronia megastigma Nees. ‚-ionone, 8-hydroxylinalyl esters, geranyloxy cinnamate 79 Bouea macrophylla GriV. (E)-‚-ocimene, ·-pinene 80 Calamintha cretica piperitenone, piperitenone oxide 81 Calamintha incana (Sm.) Boiss. piperitenone oxide 82 Callistemon speciosus Anthor. cineol, ·-pinene, ‚-terpineol 83 Canella winteriana Gaertn. stem and bark: cineol, terpinen-4-ol, ·-terpinyl acetate, ·-terpineol; leaves: myrcene 84 Cedronella canariensis Webb et Berth.ssp. canariensis pinocarvone 85 Chamelaucium uncinatum genotypes ·-pinene, citronellal, limonene, linalool, ·-terpinyl acetate 86 Chenopodium ambrosioides limonene, trans-pinocarveol 87 Chenopodium botrys — 88 Chrysanthemum balsamata carvone, camphor 89 Chrysanthemum lavandulaefolium menthol 90 Cinnamomum camphora Nees et Ebermaier camphor, cineol, terpinen-4-ol, limonene 91 Cinnamomum puciflorum — 92 Citrus grandis limonene 93 Clausena anisata (Willd.) J.D. Hook limonene, myrcene 94 Coleus aromaticus carvacrol, camphor 95 Coreopsis barteri Oliv. & Hiern limonene, ·-phellandrene 96 Coreopsis grandis Hogg ex Sweet myrcene 97 Coriandrum sativum seeds: linalool 97 Coridothymus capitatus Reich. f. carvacrol 98, 99 Cunila fasiculata menthofuran 100 Cunila microcephala menthofuran, limonene 100 Curcuma aeruginosa Roxb. cineol, camphor 101 Curcuma amada Roxb. myrcene 102 Curcuma aromatica Salisb.cineol, camphor, isoborneol, camphene 103 Curcuma cochinchinensis Gagnep. cineol 104 Curcuma domestica ·-phellandrene, cineol, p-cymene, ‚-pinene 105 Curcuma pierreana Gagnep. Rhizome, stem: isoborneol, isobornyl acetate leaf: isoborneol, camphor 106 Croton aubrevillei J. Leonard linalool 107 Croton zambesicus Muell. Arg. linalool 107 Cyclotrichum origanifolium (Labill.) Manden. et Scheng. cis-isopulegone, pulegone, isomenthone, menthol 108 Cymbopogon caesius (Nees et Hook. et Arn.) Stapf.perillyl alcohol, geraniol, limonene 109 Cymbopogon citratus (DC.) Stapf. citral, geraniol, myrcene, ·- and ‚-pinene 110 Cymbopogon khasianus (Munro ex Hack.) Bor. geraniol, geranyl acetate, linalool 111 Cymbopogon schoenanthus piperitone, limonene 112 Daphne mezereum flowers: linalool, linalyl oxides 113 Echinophora chryantha Freyn et Sint. ·-phellandrene 114 Egletes viscosa various terpenol acetates 115 Elsholtzia ciliata (Thunb.) Hyland chemotype (a): elsholtzia ketone, ‚-thujone chemotype (b): neral, geranial, limonene 116 117 Elsholtzia patrini Garcke.elsholtzia ketone, carvone 118 Elsholtzia stauntonii Benth. rosefuran, rosefuran epoxide 119 Eremocitrus glauca (Lindl.) Swing. ·- and ‚-pinene 120 Eryngium foetidum seeds: ·-pinene 121 Eucalyptus bicostata cineol (27%) 122 Eucalyptus citriodora citronellal 123 Eucalyptus deanei cineol (70%) 122 Eucalyptus dives piperitone 124 Eucalyptus dunnii cineol (63%) 122 Eucalyptus kirtoniana F.Muell. camphene 95 Eucalyptus macarthurii geranyl acetate 124 Eucalyptus torelliana F. Muell. ·-pinene, p-cymene 123 Eucalyptus viminalis cineol (23%) 122 Eucalyptus spp. — 125–134 Eupatorium argentinum Ariza ·-pinene, p-cymene 135 Eupatorium arnottianum Griseb. p-cymene, ·-pinene, thymyl acetate 136 Eupatorium hecatanthum (DC.) Baker ·-pinene, p-cymene, thymyl acetate 135 Eupatorium subhastatum Hook. ·-pinene, p-cymene 135 Fagraea berteriana flowers: (E)-‚-ocimene 137 Grayson: Monoterpenoids 441Table 1 Continued.Species Principal constituents Reference Geleznowia verrucosa Turcz. ·-pinene 138 Geum montanum ·-phellandrene 139 Gouroupita guianensis Aubl. linalool, nerol 140 Hedychium acuminatum Roscoe rhizomes: cineol 141 Hedychium coronarium ·- and ‚-pinene, linalool 142 Hedyotis diVusa Willd. linalool 143 Heterotheca inuloides Cass. borneol, myrcene, bornyl acetate, camphor, trans-‚-ocimene 144 Hexalobus monopetalus (A.Rich) Engl. geranial, neral, linalool 145 Hyptis capitata Jacq. citronellyl acetate, piperitone oxide, geranyl acetate 146 Hyptis suaveolens Poit. chemotype (a): as Hyptis capitata above chemotype (b): limonene, fenchone, linalool, menthol 146 147 Ichthyothere cunabi ·- and ‚-pinene 148 Ichthyothere terminalis ·-pinene, sabinene, limonene 149 Illicium brevistylum cineol, terpinen-4-ol, linalool 149 Illicium griYthii Hook. f. et Thoms. ·-pinene, linalool, limonene, cineol 150 Illicium jiadifengpi linalool 149 Illicium majus Hook.f. et Thoms linalool, limonene 151 Illicium microanthum Dunn. linalool, limonene 151 Illicium simonsii Maxim. (")-limonene 152 Inula viscosa Aiton borneol, bornyl acetate, isobornyl acetate 153 Juniperus indica Bertol. sabinene, ‚-thujone, terpinen-4-ol, trans-sabinyl acetate 154 Juniperus squamata D. Don var. fargesii Redh. & Wils. ·-pinene, sabinene, ‚-thujone 155 Lavandula pinnata var. pinnata ·- and ‚-phellandrene 156 Leptospermum javanicum Blume chemotype (a): ·- and ‚-pinene chemotype (b): terpinen-4-ol 157 Ligustrum lucidum Aiton ·- and ‚-pinene, limonene, terpinen-4-ol 158 Lippia alba (Mill.) N.E. Brown limonene, carvone, piperitenone 159 Lippia integrifolia camphor, lippifoli-1(6)-en-5-one, limonene 160 Lippia junelliana myrcenone 160 Lippia multiflora neral, geranial, cineol, ·-pinene, sabinene 112 Lippia multiflora moldenke myrtenol, linalool, cineol 161 Lippia polystacha ·-thujone 160 Lippia turbinata ·-thujone 160 Liquidambar formosana ·- and ‚-pinene, camphor, terpinolene, bornyl acetate 162 Litsea glutinosa (Lour.) C.B. Rob. (E)-‚-ocimene 163 Magnolia poasana trans-‚-ocimene, ‚-pinene, linalool 164 Mediasia macrophylla (Regel et Schmalh.) Pimen. p-cymene, thymol, carvacrol 165 Mentha cordifolia carvone, limonene, trans-carveol, dihydrocarvone, trans-carveyl acetate 166 Micromeria biflora neral, geranial 167 Micromeria carminea P. H. Davis borneol 168 Micromeria fruticosa ssp.giresunica pulegone, menthol, menthone 169 Minthostachys mollis Griseb. pulegone 170 Minthostachys mollis menthone, pulegone, menthols, isopulegols 171 Monodora brevipes Benth. (Z)-‚-ocimene 172 Murraya koenegii Spreng. (E)-‚-ocimene, linalool 173 Myrcia acuminatissima linalool 174 Myrcia bombycina ·- and ‚-pinene 174 Myrcianthes cisplatensis (Camb.) Berg. cineol, limonene 175 Myrcianthes pungens (Berg.) Legrand cineol, limonene 175 Nepeta viscida Boiss.·-terpineol 176 Ocimum americanum camphor, linalool 177 Ocimum basilicum var. glabratum linalool 178 Origanum acutidens (Hand.-Mazz.) letswaart carvacrol, p-cymene 179 Origanum bilger P. H. Davis carvacrol 180 Origanum leptocladum Boiss. p-cymene, carvacrol, borneol 181 Origanum micranthum Vogel linalyl acetate, cis-sabinene hydrate, ·-terpineol, linalool, terpinen-4-ol 182 Origanum saccatum P. H. Davis p-cymene 183 Pandanus tectorius isopentenyl acetate, dimethylallyl acetate 184 Pectis prostrata Cav.limonene, perillaldehyde 185 Perovskia atriplicifolia Benth. cineol, limonene, ·- and ‚-pinene, 3-carene, camphene, camphor 186 Persea bombycina (King ex Hook. f.) Kost. linalool oxides 187 Persea borbonica Spreng. camphor, cineol 188 Persea humilis Nash camphor, cineol 188 Persea palustris (Raf.) Sarg. cineol, p-cymene, camphor 188 Persea podadenia Blake ·-pinene, 3-carene, limonene, myrcene, ·-terpinene, camphene, ‚-pinene, ·-phellandrene 189 Peucedanum mashanense Shan et Sheh ·-pinene, thujene, terpinene 190 Peucedanum quangxiense ·-pinene, thujene 191 Peumis boldus ascaridole, cineol 192 Picris radicata Forssk.ionone derivatives 193 Pinus heldreichii Christ. limonene 194 442 Natural Product Reports, 1998Table 1 Continued. Species Principal constituents Reference Pinus nigra Arn. ·-pinene 195 Piper acutifolium Ruiz and Pav. (E)-‚-ocimene 196 Piper auritum „-terpinene, terpinolene, camphor, ·-terpinene, camphene 197, 198 Pistacia lentiscus ·-pinene, terpinen-4-ol 199 Plecostachys serpyllifolia ‚-pinene, sabinene 200 Plectranthes tenuifolius thymol 201 Porophyllum tagetoides citronellal 202 Prangos ferulacea Lindl.fruits: „-terpinene, ·-pinene 203 Protium heptaphyllum (Aubl.) March terpinolene 204 Psidium friedrichsthalianum (Berg.) Niedenzu ·-pinene 205 Psidium guineense Sw. ·-pinene 205 Psidium sartorianum (Berg.) Niedenzu ·-pinene, ·-phellandrene 205 Ravensara aromatica cineol, sabinene, ·-terpineol 206 Rhododendron mucronulatum limonene 207 Rhododendron spp.limonene, cineol, borneol, bornyl acetate, geranyl acetate 208 Salvia caespitosa Montbret. et Aucher ex Benth. camphor, cineol, ‚-pinene 209 Salvia clevelandii (Gray) cv. ‘Winifred Gilman’ camphor, cineol 210 Sassafras albidum (Nuttall) Nees camphor 211 Satureja montana spp. carvacrol 212 Satureja odora (Gris.) Epl. pulegone 213 Satureja parvifolia (Phil.) Epl. carvacrol, carvacryl acetate, p-cymene, „-terpinene 213 Satureja spicigera (C.Koch) Boiss. thymol, p-cymene, carvacrol, „-terpinene 214 Schizonepeta tenuifolia (Benth.) Briq. menthone, carvomenthone 215 Sideritis arguta cineol 169 Sideritis argyrea ·- and ‚-pinene 216 Sideritis amasiaca Bornm. ‚-pinene 217 Sideritis condensata ·-pinene 216 Sideritis congesta ·- and ‚-pinene 216 Sideritis lycia ·- and ‚-pinene 169 Sideritis lycia Boiss. et Heldr. ·- and ‚-pinene 218 Sideritis perfoliata limonene 216 Sideritis raeseri Boiss.et Heldr. camphor, cineol 219 Sideritis scardica Griseb. ssp. scardica ‚-pinene, carvacrol, ·-pinene 220 Smyrnum perfoliatum ·-pinene, neryl isovalerate, ·-terpinyl isovalerate 221 Solanum incisum Griseb. ‚-pinene, limonene, cineol, myrcene 222 Solanum stuckertii Bitter myrcene, ‚-pinene, p-cymene, (Z)-‚-ocimene 222 Sphaeranthus cyathuloides O. HoVm. trans- and cis-dihydrocarvone 223 Spondias cythera ·-terpineol 224 Stachys glutinosa ·-pinene, ‚-phellandrene, „-terpinene, terpinen-4-ol, ·-terpineol. ·-terpinyl acetate 225 Steganotenia aralicea Hochst.limonene, ‚-phellandrene, ·-pinene, sabinene 226 Stilpnolepsis centiflora (Maxim.) Krasch ‚-myrcene 227 Syncarpia spp. ·-pinene 228 Tagetes lemmonii Gray dihydrotagetone, (E)-tagetone, (E)-ocimenone 229 Tagetes minuta dihydrotagetone, (Z)-‚-ocimene, (E)-tagetenone 230 Tagetes riojana Ferraro (E)-tagetone, (Z)-ocimenone, dihydrotagetone 231 Talauma gioi Aug. Chev. camphor 232 Teucrium heterophyllum L’Herit ·-pinene 233 Thuja koraiensis Nakai carvyl acetate, fenchone, thujone, sabinene, terpinen-4-ol 234 Thymbra sintenisii Bornm.et Aznav. ssp. isaurica P. H. Davis carvacrol, p-cymene, „-terpinene 181 Thymus eigii (M. Zohary et P. H. Davis) carvacrol 235 Thymus leucostomus Hausskn. et Velen. var. leucostomus chemotype (a): carvacrol, p-cymene, thymol chemotype (b): ·-terpinyl acetate, borneol, linalool, thymol 236 Thymus praecox ssp. grossheimii var.grossheimii thymol 237 Thymus praecox ssp. skorpilii var. laniger thymol 237 Thymus praecox ssp. skorpilii var. skorpilii geraniol, ·-terpinyl acetate, geranyl acetate 237 Thymus thracicus Velen var. longidens (Velen) Jalas. chemotype (a): geraniol, thymol, p-cymene, „-terpinene, carvacrol chemotype (b): geraniol, geranyl acetate 238 Thymus tosevii ssp. Tosevii var. longifrons thymol, carvacrol 239 Thymus zygioides Griseb. var. lycaonicus (Celak) Ronni geraniol, carvacrol, ·-terpinyl acetate, thymol 240 Trifolium pratense flowers: linalool 241 Trifolium repens flowers: linalool 241 Triphasia trifolia terpinen-4-ol, carvacrol 95 Uromyrtus metrosideros ·- and ‚-pinene, ·-terpineol 242 Valeriana oYcinalis var.sambucifolia roots (both normal and Agrobacterium-mediated): bornyl acetate 243 Vepris elliotii ·-pinene, limonene, terpinolene 244 Welchiodendron longivalve (F. Muell.) Peter G. Wilson & J. T. Waterh. trans-‚-ocimene 245 Xylopia sericea St.Hill leaf: (Z)-‚-ocimene fruit: myrcene, ‚-pinene root and trunk: ‚-pinene 246 Zingiber zerumbet Sm. ·-pinene, linalool 247 Grayson: Monoterpenoids 44333 and kudingoside B 34, both of which inhibit acyl-CoA: cholesterol acyltransferase, have been obtained from Ligustrum pedunculare Rehd.255 The geranyl-derived ether 35 has been isolated from the liverwort Trichocolea mollissima,256 and the new nor-monoterpenoid 36 has been obtained from Achillea fragrantissima.257 Several options (direct esterification of the alcohol, transesterification of butyl acetate or transesterification of citronellyl butyrate using butyl acetate as donor) for the eVective lipasecatalysed synthesis from citronellol 37 of citronellyl acetate 38 have been critically evaluated, and alcoholysis has been found to provide the best yields.258 A study of the various parameters aVecting the esterification of citronellol 37 with butanoic acid in heptane using an immobilised lipase has been carried out259 and, in another systematic investigation, the enantioselective lipase-catalysed synthesis of citronellyl butyrate 39 has been found to proceed best with Candida rugosa lipase in 88% aqueous butanoic acid.260 The lipase-catalysed esterification of citronellol 37 with valeric acid in supercritical CO2 has been examined, and the reaction rate reaches a sharp maximum in the near-critical region.261 A lipase obtained from Mucor miehei gave the best results of a number of these enzymes which were studied as catalysts for the esterification of geraniol 40 with butanoic or valeric acids in the absence of any additional solvents.262 The microbial oxidation of (&)-citronellol rac-37 in an organic–aqueous interface reactor by various organisms has been examined.263 (&)-Citronellol rac-37 has been enantioselectively oxidised by Hansenula saturnus IFO-0809 at the hydrophobic–hydrophilic interface in a specially designed bioreactor to give (R)-37 of 70–85% ee which accumulates in the organic phase and (S)-citronellic acid 41 of 68–70% ee which accumulates in the aqueous phase.264 A newly-isolated denitrifying bacterium, strain 47Lol, which contains a novel 3,1-hydroxy-ƒ1-ƒ2-mutase enzyme stereospecifically isomerises linalool 22 to geraniol 40.265 Spores of Penicillium italicum germinate on a solid medium to produce a mycelial mat which slowly (2 months) oxidises both geraniol 40 and nerol 42 to give methylheptenone 43.266,267 The biotransformations of various monoterpenoids, including geraniol 40, by cell suspension cultures of Achillea millefolium ssp. millefolium have been studied.268 Citronellal 44 is biotransformed by Glomerella cingulata to give, sequentially, citronellol 37 and the triol 45269 The same microorganism has been utilised to eVect the resolution of the (&)-cis- and (&)-trans-pyranoid linalyl oxides 46 via their enantioselective esterification with malonic acid.270 An epoxide hydrolase from a Rhodococcus sp.has been used to carry out a kinetic resolution of the diastereoisomeric 6,7-epoxylinalyl acetates 47, aVording the (3R,6R)-epoxide and the (6R)-diol 48.271 Two-dimensional DEPT C–C relay NMR experiments have been employed to obtain a 2-D C–C COSY spectrum of linalool 22 without the need for double quantum filtration.272 Pyrolysis of 3(10)-dihydromyrcene 49 aVorded all four diastereoisomers of the cyclopentane 50, and three of these have been purified and characterised by their NMR and mass spectra.273 The imino allene 51 cyclises to yield the cyclopentene 52 when it is heated at 250 )C, and this then undergoes further cyclisation to give the bicyclic amine 53.274 O O O O OH HO OH OH HO OH OH O-b-D-Glc O-b-D-Glc OH O-b-D-Glc OH OH OH OR 25 26 27 28 29 R = b-D-Glc 30 R = H O-b-D-Glc O-b-D-Glc O O OH O OR O O HO O Me OH OH OH OH 33 R = 32 31 34 R = O O OMe CO2Me OH OH 36 35 OR OH 40 37 R = H 38 R = Ac 39 R = COPr CO2H OH 42 41 O 43 CHO OH OH OH OH OH OAc O OH O OAc 47 46 48 45 44 444 Natural Product Reports, 1998A practical route for the conversion of (S)-citronellol ent-37 into (3S)-methylheptanoic acid 54 has been described.275 The gas-phase oxidation of linalool 22 has been investigated with a view to determining its possible contribution to the formation of atmospheric ozone and of photooxidation products such as formaldehyde.276 The acylation of linalool 22 by acetic anhydride in a reaction which is catalysed by polymeric pyrrolidinopyridines has been described.277 Linalyl acetate 55 is isomerised in the presence of [PdCl2(PhCN)2] to give278 an 86:14 mixture of geranyl acetate 56 and neryl acetate 57, and linalyl 3-oxobutanoate 58 reacts in the presence of [Pd(PPh3)4] to yield mainly geranylacetone 59.In an important industrial process, geraniol 40 can be hydrogenated over chiral RuII catalysts to give optically active citronellol 37. The same catalysts have now been shown279 to cause a competing isomerisation of 40 to „-geraniol 60 which is then hydrogenated more rapidly than geraniol itself to give citronellol 37 of opposite absolute configuration to that obtained directly from geraniol, thus reducing the overall ee of the reaction product. The mixed acetal obtained via reaction of geraniol 40 with 2-methoxypropene in the presence of POCl3 is converted into the alcohol 61 in a regioselective ketal Claisen rearrangement which is catalysed by Bui 3Al.280 The anodic oxidation of geraniol 40 in methanolic solution to give geranial 62 has been reported.281 A number of monoterpenes have been successfully epoxidised at ambient temperatures using Ni(acac)2–O2 in the presence of branchedchain aliphatic aldehydes.282 The Sharpless asymmetric epoxidation of geraniol 40 has been used283 as a key step in a synthesis of the (3S)- and (3R)-3-hydroxycitronellic acids 63.The (3R)- acid is a natural product which occurs in Ceratocystis fimbriata sp. platani. The geranyl ether 64 undergoes a smooth [2,3]-Wittig rearrangement to yield 65 when it is treated with BuLi–THF.284 Reaction of diethylgeranyl phosphate 66 with AlMe3 in the presence of CuCN aVords the SN* product 67 and the SN product 68 in 36:1 ratio.285 If the CuCN is omitted the reaction gives mainly 68. Treatment of either of the carbonate 69 or its (Z)-isomer with Pd0 catalysts gives286 linalool oxide 70 and the three diols 71–73.Geranyl acetate 56 reacts287 with diphenyl diselenide to give the diasteroisomeric cyclic ethers 74, one of which has been converted into the ether 75. The lowtemperature superacid-mediated cyclisation reactions of a variety of terpenols and of their acetates have been reviewed.288 The terpenols aVord cyclic homoallylic alcohols, and their acetates yield monoacetates of 1),3)-cyclic-„-diols. Sharpless asymmetric epoxidation of geraniol 40 followed by Swern oxidation aVords the epoxyaldehyde 76 which reacts diastereoselectively with the lithium enolate of tert-butyl acetate to give mainly the anti-product 77.289 The enantiomer of 76, obtained by a similar route, undergoes an unusual intramolecular Prins reaction to give, predominantly, the cyclisation product 78 when it is treated with Zn – TMSCl.290 Citral 62 has been converted into (&)-loliolide 79 which is a potent cytotoxic agent.291 Syntheses of (E)-tagetone 80,292 and of (S)-(")-ipsenol 81 and (S)-(+)-ipsdienol 82 have been reported.293 Another synthesis of (S)-(")-ipsenol 81 involves application of the useful allyltin derivative 83 which has been C NPh NPh NHPh 53 52 51 50 49 CO2H OR O OH 60 59 55 R = Ac 58 R = COCH2COCH3 54 OAc OAc 57 56 OH CHO OH CO2H 63 62 61 R 64 R = OCH2TMS 66 R = OP(OEt)2 68 R = Me R O HO OH OH OH OH SePh OH O PhSe SePh O OH 75 74 73 71 ( Z)-isomer 72 ( E)-isomer 70 65 R = OH 67 R = H OH OCO2Et 69 OH O CHO O CO2But OH 77 76 Cl OSiMe3 OSiMe3 O O HO 79 78 Grayson: Monoterpenoids 445obtained in good ee.Its methyl ether reacts diastereoselectively with 3-methylbutanal to give 84 which is then converted294 into (")-81. The (")-enantiomer of ipsdienol 82 has been obtained via a key ene-reaction catalysed by a chiral BINOL–Ti complex.295 The functionalisation of ‰-pyronene 85, obtained from myrcene 86, has been studied,296 and synthetic routes to ‚-cyclolavandulal 87 and to ‚-cyclolavandulol 88 have been published.297 3 Artemisyl, santolinyl and chrysanthemyl systems The GC–MS analysis of several essential oils from Artemisia herba alba grown in Algeria has been reviewed, and has revealed the presence of a number of novel chrysanthemyl esters.298 Tetrahydrolavandulol 89 is metabolised by the plant pathogenic fungus Glomerella cingulata to give the 5-hydroxy derivative 90.299 Syntheses of (R)-(")-lavandulol 91300 and of (&)- lavandulol301,302 have been described.One of the latter301 utilises aqueous CuCl2 to eVect the clean hydrolysis of the sensitive ‚,„-unsaturated aldehyde dimethylhydrazone 92. Reduction of the liberated aldehyde with sodium borohydride aVords lavandulol in good yield. A new route to caronaldehyde 93 from (+)-car-3-ene 94 has been described,303 and the aldehydo ester 95, a synthon for (1R)-cis-pyrethroids, has been obtained304 via transformation of the microbially-produced diol 96.A new stereoselective synthesis of trans-chrysanthemic acid 97 has been reported.305 Irradiation of the chromium pentacarbonyl derivative 98 leads to the cyclopentenone 99 through a vinylcyclopropyl rearrangement which proceeds via a photogenerated ketene.306 4 Cineol derivatives The new glycoside 100 has been obtained251 from Cunila spicata, and the five monoglucosides 101–105 have been isolated307 from the fruits of Foeniculum vulgare. The chemistry of Eucalyptus oils has been reviewed,308 as have methods for their distillation.309 Syntheses of the diols 106 and 107 which are urinary metabolites produced by brushtail possums fed, respectively, cineol 108 or 2·-hydroxycineol 109 have been accomplished.310 Cineol 108 has been shown to be an attractant for the banana weevil Cosmopolites sordidus.311 A comprehensive study of the formation of cineol 108 via oxymercuration reactions of the various isomeric terpineols has been published.312 5 Menthanes The new saturated p-menthane-cis-1,4-diol 110 has been found in Senecia phylolleptus Cuatr., S.viridis Phil. and S. nutans Sch. Bip.313 Enantiomerically pure (4R)-(+)-·-terpineol 111 has been obtained from the essential oil of Micromeria fruticosa Druce., and the (4S)-(")-enantiomer of 111 of ca. 80% ee has been shown to be present in cinnamon and in Laurus nobilis oils by chiral capillary GC analysis using permethylated ‚-cyclodextrin as the stationary phase.314 The novel unsaturated diol 112 has been claimed from the leaf oil of Aegle marmelos,315 and the ketol 113 has been reported from Senecio chrysanthemoides.316 The hydroxy aldehyde 114, an oxidative metabolite of perillaldehyde, has been found in Perilla frutescens,317 and isorobinal 115 has been discovered in Rhizoglyphus mites.318 A glucoside of carvacrol 116 has been isolated from Rabdosia eVusa Hara.319 O HO HO SnBu3 OH HO OMe 85 84 83 82 81 80 R 87 R = CHO 88 R = CH2OH 86 R OH OH NNMe2 92 91 89 R = H 90 R = OH O O HO CO2Me OHC 95 94 93 Br OH OH HO2C (CO)5Cr O OMe 99 98 97 96 O R2 R1 Glc-D-b-O O Glc-D-b-O OH 103 100 R1 = R2 = H 101 R1 = OH; R2 = H 102 R1 = H; R2 = OH O OR1 OR2 O OH HO O R1 R2 107 R1 = R2 = OH 108 R1 = R2 = H 109 R1 = H; R2 = OH 106 104 R1 = R2 = -b-D-Glc 105 R1 = -b-D-Glc; R2 = H OH OH OH HO OH O HO CHO OH 114 113 112 111 110 446 Natural Product Reports, 1998The configuration of ‘wine lactone’ 117 has now been determined.320 This compound has not yet been detected in wine but appears to be present in the urine of koala bears which have been feeding on the leaves of Eucalyptus punctata. The plant Chenopodium ambrosiodes is only permitted to grow in the chinampas of San Andreas Mixquic, Mexico, wherever cultivated crops are absent since it stunts the growth and development of many agriculturally important species. It has now been found321 that ascaridole 118 is the principal allelopathic compound present in C.ambrosiodes, and that this peroxide is an inhibitor of germination and of hypocotyl growth. Exposure of Saccharomyces cerevisiae to ·-terpinene 119 at a concentration of 1% causes cell death, but 0.1% levels of 119 provoke the production of a new 26 kDa protein together with the development of marked heat resistance, suggesting an adaptive response by the yeast.322 The inhibitory eVects of citronellol 37, ·-terpineol 111 and menthol 120 upon the growth of a number of Aspergillus spp.have been studied, and citronellol has been found to exert the greatest eVect.323 The antimicrobial and cytotoxic eVects of Origanum essential oils have also been investigated.324 Their major components carvacrol 116 and thymol 121 are mainly responsible for their actions against Gram-positive and Gram-negative bacteria. The oil obtained from Origanum vulgare ssp. hirsutum has been shown to be very bactericidal at a dilution of 1:4000, and is also highly toxic towards a number of animal cell lines including two derived from human cancers.Potential uses for (+)-limonene 122 as a solvent in products such as multi-purpose cleaning formulations, hand-washing pastes, etc. have been reviewed.325 The solubilities of limonene 122 and of linalool 22 in supercritical CO2 have been studied, and the experimental data obtained have been satisfactorily correlated with a theoretical model.326 The selective oxidations of limonene 122 and of a number of other monoterpene dienes and terpenols by hydrogen peroxide catalysed by peroxotungstaphosphate species under biphasic conditions (CHCl3–H2O) have been investigated.327 Limonene 122 aVords the 1,2-epoxides 123 with 97% selectivity under these conditions. Silica gel that has been treated with Ti(PriO)4, and then heated to 140 )C, is an eYcient catalyst for the epoxidation of limonene 122 with ButOOH, leading to formation of the 1,2-epoxide 123 (88%) and the 7,8-epoxide 124 (12%).328 The mixture of diastereoisomeric limonene 1,2-epoxides 123, normally obtained by epoxidation of the terpene, reacts with 1 M NaHSO3 in CH2Cl2–H2O to eVect rapid destruction of the cis-epoxide, leaving pure trans-epoxide to be recovered.329 A 1:1 alumina–yttria catalyst prepared from the nitrate by precipitation with urea has been characterised morphologically by scanning electron microscopy and has been shown to have a lower surface area and to be more basic than a catalyst precipitated using ammonia.330 Its enhanced basicity makes it useful for isomerisation reactions of limonene epoxide 123.The epoxide 123 has been reacted with diphenylphosphine or lithium diphenylphosphide to aVord the derived ‚-hydroxyphosphines.331 Limonene 122 is converted into the lactones 125 on reaction with aliphatic carboxylic acids in the presence of ceric ammonium nitrate.332 The allylic oxidation by ButOOH of monoterpene alkenes including limonene 122 to give derived ·,‚-unsaturated ketones proceeds in good yield and with high selectivity under catalysis by chromium-doped aluminophosphate species.333 A study of the kinetics of limonene autoxidation has been published,334 and the cytochrome P450 limonene hydroxylases of Mentha spp.which catalyse formation of the characteristic oxygenated monoterpenoids of this plant family have been reviewed.335 The regioselective alkoxylation of limonene 122 over heterogeneous ‚-zeolite catalysts to give 7-ethers 126 has been reported.336 Ozonolysis of limonene 122 aVords337 a mixture of the two ozonides 127 and 128, and ozonolysis of 122 followed by a reductive work-up and then further degradative steps leads eVectively to (S)-3-ethylheptanoic acid 129.338 The hydrosilylation of limonene 122 with chlorodimethylsilane leads regioselectively to 130 which can be reacted with alkynylmagnesium halides to aVord 131.339 Reaction of limonene 122 with dimethyl disulfide in the presence of a Lewis acid aVords a mixture of the three products 132–134.340 Limonene is converted into a mixture of both diastereoisomers of the bicyclic alcohol 135, each of which exhibits good perfumery characteristics, when it is treated with CO–H2– [Pt(Cl)2DPPB]–PPh3–SnCl2.341 Reaction of limonene 122 with 4-hydroxybenzaldehyde in the presence of askanite–bentonite clays aVords the ether 136.342 A synthesis of (+)-1-acetyllimonene 137 has been described,343 and (+)-limonene has been converted via the alcohol 138 into (S)-1-methylcyclohex-2-en-1-ol 139, an aggregation pheromone of the female Douglas fir beetle Dendroctonus pseudotsugae.344 The gas-phase reactions of NO3 radicals with ·-terpinene 119 and with ·-phellandrene 140 have been investigated.345 Both monoterpenes yield p-cymene 141, whereas the model CHO O OH O O O O 118 117 116 115 OH OH 122 121 120 119 O O O O R OR 126 125 124 123 O O O O O O O O O 128 127 CO2H 129 Grayson: Monoterpenoids 447compound cyclohexa-1,3-diene contrastingly aVords its monoepoxide together with a little benzene under the same conditions.„-Terpinene 142 reacts with phenol under catalysis by an acidic ion-exchange resin to yield mainly the doubly-arylated compound 143 and some 144 together with seven other minor products.346 The bis(sulfonamide) 145, derived from ·-phellandrene 140, is reductively cleaved by Na–NH3 to give the cis-diamine 146.347 Oxidative hydroboronation of 145 followed by reduction using Na–NH3 aVords the diamino alcohol 147. Optically active ·-terpineol 111 has been synthesised via the product of an asymmetric Diels–Alder reaction involving isoprene and the keto-sulfone 148.348 The hydrogenation at 1 atm of ·-terpineol 111 over Raney nickel at 100 )C aVords the saturated alcohol 149 with 98% selectivity.349 (R)-(+)-·- Terpineol 111 has been converted into the crystalline dibromo epoxide 150 whose stereochemistry has been determined by X-ray crystallographic analysis.350 Reaction of ·-terpineol 111 with dimethyl disulfide in the presence of BF3 · Et2O yields the disulfide 151 together with both diastereoisomers of the cineol derivative 152.351 A new reductive transposition reaction of allylic alcohols is exemplified by the conversion of perilla alcohol 153 into the diene 154 by treatment with PPh3–DEAD and o-nitrobenzenesulfonylhydrazine.352 Mitsunobu SN2 displacement of the allylic hydroxy group takes place at "30 )C and, on warming to "23 )C, the derived hydrazine 155 then eliminates benzenesulfinic acid to give 156 which rearranges to 154 with loss of dinitrogen.The allylic ether 157 undergoes a [2,3]-Wittig rearrangement to give 158 when it is treated with BuLi–THF.282 A route to the diol 159 has been described,353 and the thioacetal 160 has been successfully hydrolysed to the derived acid-sensitive allylic aldehyde by treatment with H4IO6 in ether–THF.354 Commercial routes to the important alcohol (")-menthol 120 have been reviewed,355 as have asymmetric syntheses of menthol which employ transition metal catalysis.356 The eY- cient kinetic resolution of (&)-menthol via lipase-catalysed enantioselective esterification reactions with propanoic anhydride has been studied.357 Best results were obtained using Candida cylindracea OF360 lipase which aVorded the (")- ester of 98% ee. An unusual, and potentially very useful remote functionalisation of menthol 122 has been achieved by reaction with the Selectfluor= reagent F-TEDA-BF4 in acetonitrile which leads to the salt 161, the structure of which has been verified by X-ray analysis.Hydrolysis of 161 using aqueous base aVords the 7-amide 162.358 A cleavage reaction of allylic ethers which is generally applicable can be eVected by reaction of, for example, 163 with the reagent combination ClTi(PriO)3–BuMgCl.359 (")- Menthol 122 reacts with 1,1,2-trichloroethene to give, after conversion to the alkyne and alkylation, the ether 164.This can be reduced using LiAlH4 to yield the but-1-enyl ether 165.360 The tert-butyldimethylsilyl ether 166 is cleaved to give menthol 122 when it is treated with ceric ammonium nitrate in methanol at 0 )C.361 Tetrahydropyranyl ethers are also cleaved under the same conditions. SiMe2R SMe SMe SMe SMe SMe SMe OH 135 134 133 132 130 R = Cl 131 R = CºC–R1 O HO O 137 136 OH OH 139 138 141 140 OH OH OH OH 144 143 142 RHN RHN H2N H2N 147 145 R = TolSO2 146 R = H OH SO2Ph O OH Br Br O SMe SMe O SMe 152 151 150 149 148 R 153 R = OH 155 R = ArSO2N(NH2)– 156 R = –N=NH OH O 157 158 154 TMS 448 Natural Product Reports, 1998Menthyllithium 167 lithiates (arene)tricarbonylchromium acetal complexes at their pro-(R) sites in up to 70% ee, but the 8-phenylmenthyl analogue 168 exhibits the reverse enantioselectivity, abstracting pro-(S) protons in up to 80% ee.362 A number of asymmetric conjugate addition reactions of organolithium reagents to chromium[(")-menthyloxy]- (aryl)carbene complexes have been examined.Thus, for example, sequential treatment of 169 with sec-butyllithium and then with methyl triflate aVords 170 which can be converted into the alcohol 171.363 The lithiated enol ether 172, obtained from the corresponding (Z)-bromo ether, reacts with alkyl iodides to give the (Z)-configured higher ethers 173.364 (")-Menthol 122 has been converted into the two diastereoisomeric pyridyl alcohols 174 via reaction of the derived neomenthyl nitrile 175 with 2-lithiopyridine.365 Reaction of, for example, (")-menthol 122 with phosgene aVords the chloroformate 176 which, after reaction with hydrazine and then NBS, is converted into the chiral diazanedicarboxylate 177.366 This reacts with achiral ester enolates to give ·-hydrazino-acid derivatives, but these reactions proceed with little or no diastereoselectivity.The same diaza compound 177 undergoes367 azo-ene reactions with alkenes to yield adducts of varying de.However, it has proved diYcult to cleave the menthyl ester functions from the products which are formed. (")-Menthol 122 has been converted into the chiral alkoxycyclopentadienyl ligand precursor 178.368 The menthyl sulfinate 179 reacts with Grignard reagents or with R2NMgX at low temperatures in a highly stereoselective manner to give, respectively, chiral sulfoxides or sulfinamides. 369 The presence of the 4-bromo group in the aryl ring greatly enhances the reactivity of 179 by comparison with its phenyl analogue. The related toluene-p-sulfinate 180 reacts with LiHMDS to give 181 which can then be converted into chiral sulfinimines 182 which are exclusively of (E)- configuration and of greater than 96% ee.370 The expanded phosphines 183 have been synthesised, and have been tested for their usefulness as chiral ligands in a number of catalytic enantioselective processes, including hydrogenation reactions.371 The levels of asymmetric induction which were achieved were disappointingly low.Menthylphosphine 184 has been converted into the P-3-(chloropropyl) derivative 185 which reacts with Me2S · BH3 and then ButOK to give, in 30% yield, the phosphetane 186. Treatment of 186 with S8–Et3N leads to the 1-menthylphosphetane sulfide 187 which can be deprotonated using BusLi and then reacted with benzyl bromide to give a 1:1 mixture of the alkylated products 188 and 189.These can be separated by chromatography over alumina, and X-ray analysis has revealed the configuration of one of them to be R at P and S at C*.372 The diastereoisomerically pure chiral phosphine–borane complex 190 has been obtained via fractional crystallisation of a P-(R,S) mixture, and treatment of 190 with Et2NH aVorded 191, the first secondary phosphine that is homochiral at phosphorus to be successfully isolated.373 The diastereoisomeric telluronium ylides 192 have been synthesised as a mixture and, in an attempt to separate them, five-fold recrystallisation led to a product of constant [·]D.However, the optical purity was low when judged by 1H NMR.374 The synthesis and properties of di-[(")-menthyl]methyltin hydride 193 have been described.375 The reagent gave a very low degree of asymmetric induction when it was applied to the reduction of acetophenone. Bromomagnesium diisopropylamide (Pri 2NMgBr) was the best of a number of bases surveyed for their abilities to convert OH OH S S N O H BF4 – + 161 160 159 NHAc OH OR 163 R = CH2CH=CH2 164 R = CºCCH2CH3 165 R = (E)–CH=CHCH2CH3 166 R = SiMe2But 162 R Li O Ph Cr(CO)5 O Cr(CO)5 Ph OH Ph 171 170 169 167 R = H 168 R = Ph O R OH N CN OCOCl OCON=)2 O 178 177 176 175 174 172 R = Li 173 R = alkyl O S O R Me S O N(SiMe3)2 Me S O N R PR2 PR2 O CH2 O 183 R = 182 181 179 R = Br 180 R = Me : : Grayson: Monoterpenoids 449menthone 194 into its enolate.376 Menthone 194 reacts with 2-lithiofuran to give the alcohol 195 which can be converted into the unsaturated cyclic keto ester 196.Diels–Alder reaction of 196 with cyclopentadiene at 0 )C aVords the adduct 197 in good de.377 The acylated menthopyrazole 198 can be deprotonated and then diastereoselectively ·-acylated to yield products 199 which are cleavable with retention of configuration to give the ‚-keto-amides 200.378 The diastereoselectivities of 1,3-dipolar cycloaddition reactions of the ·,‚-unsaturated acylpyrazole 201 have been investigated.379 Reaction of (")-menthone 194 with mercaptoacetic acid and benzylamines PhCH2NHR aVords products 202 as single diastereoisomers.380 The crystalline aldol product 203 derived from (")- menthone has been examined by X-ray methods and has been shown to possess the configuration indicated where the 2-substituent is axial,381 but an NMR study has revealed that 203 exists in CFCl3 solution in a chair form with an all-equatorial configuration which is stabilised via an intramolecular hydrogen bond.382 Vapour–liquid equilibria for the binary system (+)-limonene 122–(+)-carvone 204 have been measured for the temperature range 365–411 K.383 The electrocatalytic hydrogenations of limonene 122 and of carvone 204 at Raney nickel cathodes have been examined in aqueous media and for micellar systems where a cationic surfactant is present.384 Limonene 122 gives p-menthene 205 in water, but is converted into p-menthane 206 in the micellar system, and carvone 204 yields all four of the possible carvomenthols 207 under both sets of conditions.The utilisation of (S)-(+)-carvone 204, which can be obtained from the oil extracted from seeds of caraway Carum carvi, as a chiral starting material for the synthesis of biologically active natural products has been reviewed.385 The application of carvone as a potato sprouting inhibitor has led to the development of a plant breeding programme which seeks to obtain C.carvi varieties with enhanced seed oil yields.386 The lipase from Candida antarctica catalyses the esterification of (R)-ketoprofen 208 when the reaction is conducted in a mixture of isobutylmethylketone and (S)-(+)-carvone 204, but (S)-ketoprofen is esterified more rapidly when (R)-(")- carvone is employed as co-solvent.387 This eVect may possibly be explained by invoking the formation of diastereoisomeric solvent:substrate complexes.Carvone oxime 209 reacts with NaNO2–AcOH to yield388 the nitro compound 210. Epoxidation of the 7,8-bond of (")-carvone ent-204 using the chiral perfluoro-cis-2,3-dialkyloxaziridine 211 aVords a 50:50 mixture of the diastereoisomeric epoxides 212.389 In a reaction which is generally applicable, the 1,2-epoxide 213 can be reduced to (")-carvone ent-204 by reaction with thiourea dioxide H2N(C=NH)SO2H in THF under alkaline conditions and in the presence of Bu4NBr as phase-transfer catalyst.390 A series of cationic PdII allyl complexes derived from carvone have been prepared, and the structure of the example 214 has been determined by X-ray crystallography.391 The history and use of thymol 121 and its derivatives in aroma chemistry has been reviewed.392 A bakers’ yeast-mediated reduction of a ‚-keto ester has been employed as a key chirality-inducing step in a synthesis of the unnatural (+)-mintlactone 215,393 and a series of m-menthane lactones 216 and 217 have been prepared.394 The )2SnMeH CO2 Te+ O O – 192 193 O OH O O O O O O 197 196 195 O 194 PHR P BH3 186 184 R = H 185 R = CH2CH2CH2Cl P S R1 R2 P H R Me Me Me 190 R = BH3 191 R = : 187 R1 = R2 = H 188 R1 = H; R2 = CH2Ph 189 R1 = CH2Ph; R2 = H N NCOR Ph RNH R2 O R1 O N S O R 202 200 198 R = CH2R1 199 R = CHR1COR2 201 R = ( E)-CH=CHR1 O OH Ph O OH O CO2H 208 207 206 205 204 203 NOH OAc O2N 210 209 450 Natural Product Reports, 1998intermediate 218, obtained from isopulegone 219, has been used in syntheses of both enantiomers of linden ether 220,395 and (")-acetylsaturejol 221 and (+)-isoacetylsaturejol 222 have both been synthesised from (")-cis-4-acetoxypulegone 223.396 The latter was obtained from cis-carvyl acetate 224 which was oxidised using di-tert-butyl chromate to give the enone 225 which was then reduced and isomerised using [(PPh3)CuH]6 in THF–H2O.The pulegone derivative 223 was finally photooxygenated using hÌ (vis)–O2–rose Bengal to give 221, or using hÌ (350 nm)–O2–CuSO4 to give 222.(")- Isopulegol 226 has been converted into the antimalarial compound (+)-artemisinin,397 and a practical route to (R)-(+)-4- methylcyclohex-2-enone 227 from (R)-(+)-pulegone 228 has been described.398 A gram-scale synthesis of the leishmanicidal and trypanocidal phenol espintanol 229 which occurs naturally in Oxandra espintana has been reported,399 and another route to 229 which utilises thymol 121 as starting material has been published.400 The menthone-derived acetoacetate 230 undergoes a biomimetic cascade cyclisation of its polyene system via a photoinduced electron-transfer process when it is irradiated at 300 nm in the presence of biphenyl and 1,4-dicyano-2,3,5,6- tetramethylbenzene.401 This represents a remarkable example of remote asymmetric induction by a chiral auxiliary since the product 231 can be hydrolysed by aqueous base to yield 232 of >99% ee. 6 Pinanes The new mudanpiosides A–F 233–238 have been isolated402 from Paeonia suVruticosa, and the glycoside 238 has also been obtained403 from Cnidium silaifolium together with the known compound paeoniflorin. It has been demonstrated by means of 14CO2-labelling experiments that cotton plants which have been damaged by feeding beet army-worms are induced to engage in the de novo biosynthesis of ·-pinene 239 presumably as a defence strategy. 404 Volatile metabolites of the fungus Cryptoporus volvatus which aVects ponderosa pine trees have now been shown to attract various pine beetles.405 These metabolites include (+)- trans-pinocarveol 240 and (+)-isopinocamphone 241. Males of the North American beaver Castor canadensis mark their territories with a paste known as castoreum which is exuded from their castor glands, and this has been found to include N O F C3F7 C4F9 O O O O O Pd N N Ph4B– + 214 213 212 211 O O O O O O O O HO 218 217 216 215 O O O O AcO OH O O AcO OH 222 221 220 219 AcO O AcO O AcO O OH 226 224 225 223 O O 228 227 OH OMe MeO O O O HO O O O 231 230 229 HO OH CO2H 232 OH O O O O R3 R2 O OH OH OH R1O O O Glc-D-b-O OH 238 233 R1 = 4-MeO–C6H4–CO; R2 = R3 = H 234 R1 = 4-MeO–C6H4–CO; R2 = OH; R3 = H 235 R1 = 4-HO–C6H4–CO; R2 = R3 = H 236 R1 = R3 = H; R2 = OMe 237 R1 = R3 = H; R2 = OH Grayson: Monoterpenoids 451many oxgenated monoterpenoids including isopinocamphone 241, pinocamphone 242, linalool oxides and their acetates.406 The microbial oxidations by Cephalosporium aphidicola of myrtenol 243 and of (")-nopol 244 aVord, respectively, the diol 245 and the hydroxy ether 246.407 (")-Nopol 244 is converted by Glomerella cingulata into the diols 247 and 248 and the ketone 249.408 Vibrational CD spectra of the cis- 250 and trans-pinanes 251,409 and of ·-pinene 239 and ‚-pinene 252409–411 have been recorded, although the validity of some of the data may be questionable.The 13C NMR spectrum of the acetate 253 has been carefully analysed412 and some of the extracted 3JC,H values have been found to be 0 Hz or thereabouts.Stoichiometric coeYcients and apparent formation constants for complexes of ·- and ‚-cyclodextrins with, for example, (&)-·-pinene, (&)-‚-pinene and (&)-limonene have been determined using reversed-phase liquid chromatography. 413 ·-Cyclodextrin forms 1:2 guest:host complexes with the pinenes and 1:1 complexes with limonene, and ‚-cyclodextrin forms 1:1 complexes with all three monoterpenes. NMR spectroscopy has been utilised to study the binding in aqueous solution of (+)- and (")-·-pinenes to ·-, ‚- and „-cyclodextrins and to their permethylated derivatives. 414 All of the cyclodextrins studied bind preferentially to (")-·-pinene, but only ·-cyclodextrin shows strong enantioselectivity. These results indicate that the presence of a benzene ring or the intervention of hydrogen-bonding is not a prerequisite for enantioselective binding as had been assumed previously.The reactions of ·-pinene 239 and of a number of other monoterpenes with pillared alumina clays and with an analogous layered ·-tin phosphate indicate that this provides a means of obtaining the derived carbocations in a solid state environment.415 The gas-phase rearrangements of ·- 239 and of ‚-pinene 252 over metal(IV) phosphate polymers lead mainly to products which are derived via cationic rearrangements, but a modified radical rearrangement of ·-pinene 239 to ·-pyronene 254 (rather than to a mixture of ocimene and alloocimene) is also observed.416 Reducing the acidity of the catalysts suppresses the cationic pathway but not the radical reaction.·-Pinene 239 reacts in the presence of a H-mordenite which has been pre-treated with a tetraalkylammonium chloride to give predominantly monocyclic products.417 An improved method for the conversion of ·-pinene 239 to terpin hydrate 255 and thence to ·-terpineol 111 has been described.418 Electron transfer from ·-pinene 239 or from ‚-pinene 252 to 2,3,5,6-tetrachlorobenzoquinone on sensitised irradiation leads to a radical cation which rapidly deprotonates to give verbenene 256 with retention of chirality.419 The thermolyses of a range of monoterpenoids including many pinane derivatives have been re-examined under flash vacuum pyrolysis conditions.420 The relative rates of metallation of a number of endo- and exo-cyclic alkenes by BunLi–TMEDA or by BunLi–ButOK have been measured, and ‚-pinene 252 has been found to possess anomalously high reactivity in this respect.421 This may be accounted for by assuming that an unfavourable interaction between the syn allylic proton and the gemdimethyl group is relieved by metallation. The catalytic hydroformylation reactions of (+)-·-pinene 239 and of (")-‚-pinene 252 have been studied and the products characterised.422 Yields from the latter are moderate due to competing isomerisation to ·-pinene.The liquid-phase oxidation of cis-pinane 250 by molecular oxygen has been examined and, remarkably, it has been found that the C-2 methine proton of 250 is more reactive than the tertiary methine proton of cumene423 (cf. reference 421 above). The autoxidation of cis-pinane 250 and of trans-pinane 251 by the Co(OAc)2–Mn(OAc)2–NH4Br–O2 system aVords mixtures of diastereoisomeric hydroperoxides which can be reduced using triphenylphosphine.424 The pinanols so produced can also be pyrolysed at 600 )C to yield linalool 22.An MgO-supported poly(titanazane) cobalt complex which incorporates CoIII species has been demonstrated to be a good catalyst for the epoxidation of ·-pinene 239 by molecular oxygen at 1 atm in the presence of an aliphatic aldehyde as reductant.425 A poly(vinylbenzyl)acetylacetonate Co complex also works well, and may be recycled many times.426 The photocatalytic oxygenation of ·-pinene 239 in water which is catalysed by water-soluble metalloporphyrins has been reported.427 ·-Pinene 239 failed to yield an ozonide under conditions which were successful with several other monoterpenes.337 (")-·-Pinene ent-239 can be cleaved to give the keto aldehyde 257 via the ruthenium diester 258 by reaction with RuO4 in aqueous acetone or in tetrachloromethane.428 (+)-·-Pinene 239 has been eYciently converted into (+)-cis-pinononic acid 259, and reactions of the alkyl radical produced upon decarboxylation of the acid 259 have been explored.429 ·-Pinene 239 reacts in the presence of the reagent combination trifluoroacetic anhydride–Me2S · BF3 to yield the bicyclic hemiacetal 260,430 and with iodosyl fluorosulfate to give the rearranged tricyclic sulfonate 261.431 ‚-Pinene 252 of 87% ee reacts with silver perchlorate to give a solid complex which crystallises from the reaction mixture. When this is decomposed by water enantioenriched ‚-pinene is liberated which can be ozonised to yield nopinone 262 of high ee.432 Photoreaction of ‚-pinene 252 with the thioketone 263 leads to a 75:25 mixture of 264 and of its diastereoisomer.433 The structure of the thioether 264 was determined by X-ray crystallography.OH 240 239 O O 242 241 OH R OH R OH OH OH O 249 248 244 R = H 246 R = MeO 247 R = OH 243 R = H 245 R = OH 252 250 b-Me 251 a-Me AcO OH OH 256 255 254 253 452 Natural Product Reports, 1998(")-‚-Pinene ent-252 has been converted into (")- qinghaosu IV (artemisinin D),434 and the pinanol 265 aVords ·-fenchol when it is heated over an aluminium phosphate catalyst.435 A theoretical study of the allylic rearrangements of transpinocarveyl esters to give myrtenyl esters has been carried out.436 (")-Myrtenol 243 has been converted into the ligand 266.437 This reacts with lithium aluminium hydride to give a modified reagent which reduces aryl methyl ketones to secondary alcohols of 50–91% ee.Myrtenal 267 reacts with N,Ndimethyl- 2-chloroacetamide to give the trapped intermediate 268 (14%) together with the expected Darzens epoxide 269 (40%).438 Myrtenal 267, obtained from ·-pinene 239 via selenium dioxide oxidation, can be ring-opened over a Cu–Zn catalyst system and then oximated to give perillaldehyde oxime 270 which has found practical application as a low-calorie sweetener.439 Pinocarvone 271 reacts with cycloocta-1,3,5-triene in the presence of catalytic dicarbonylbis(Á4-pinocarvone)- molybdenum to aVord the adduct 272.440 Verbenone 273 can be deprotonated using LDA to give the dienolate 274 which reacts under kinetic conditions with aromatic aldehydes to give „-aldol products 275.441 When verbenone 273 is deprotonated using LDA in THF–HMPA at "78 )C, the derived enolate undergoes oxidative coupling in the presence of CuCl2.The dimer 276 is formed if the temperature of reaction with CuCl2 is controlled to "40 )C, but if reaction with CuCl2 is allowed to proceed at nearambient temperatures the (E)-configured ethene derivative 277 is produced.442 The ethane 276 can be converted into the ethene 277 via deprotonation using LDA followed by treatment with FeCl3.The C2-symmetric diketone 278 has been prepared from 277. (")-Verbenone ent-273 has been converted into the verbanone derivative 279 and then via 280 into a number of chiral annulated indenes 281 which may find application as chiral ligands.443 The chiral host 282 enantioselectively forms a 1:1 inclusion complex with (")-verbenone ent-273 which, after crystallisation, aVords 273 of up to 99% ee.444 The related host 283 similarly forms a 1:1 complex with (+)-apoverbenone 284 which can thus be obtained in up to 98% ee.(R)-(+)-Verbenone 273 has been converted into the lactone 285, a synthon for members of the taxane series,445 and another taxane synthon 286 has been accessed from (+)-·- pinene 239.446 Verbenone oxime 287 reacts with sulfuric acid in CH3CN to yield the unusual lactam 288 whose structure was determined by X-ray analysis.447 Applications of derivatives of the ketol 289 in asymmetric synthesis have been discussed,448 and comprehensive studies of asymmetric alkylation reactions of derived SchiV bases have been reported.449,450 An erratum to one of these papers has appeared.451 A large-scale synthesis of 289 from ·-pinene 239 has been described which proceeds via diastereoselective dihydroxylation of 239 to give the cis-diol which is then oxidised to 289.452 The hydroxyamino oxime 290 reacts with NaBH4 in acetonitrile (this solvent is essential) to yield the lactam oxime O CHO O O Ru=O O CO2H O CF3 OH 260 259 258 257 2 O SO2 O 262 261 SO2 Ph S S SO2Tol H C6H5 OH OEt OH NHPh 266 265 264 263 CHO Cl CONMe2 O CONMe2 268 267 CONMe2 O NOH O O 272 271 270 269 O OLi O Ar OH O )2 276 275 274 273 O )2 O O 278 ( E)-277 O )2 HO R R O O HO OH R R R R 282 R = Me 283 R = H 281 280 279 Grayson: Monoterpenoids 453291 whose structure was elucidated by X-ray crystallography. 453 The mechanism by which this reaction takes place is not clear. ·-Pinene 239 reacts with PH3 to yield the phosphines 292 and 293.454 The P,P-dichlorophosphines 294 and 295 have been synthesised via reaction of the corresponding alkylmagnesium chlorides with PCl3 or with (Et2N)2PCl, and the dichlorophosphine 294 reacts with 2,2,3-trimethylbut-3-ene in the presence of AlCl3 to give, after hydrolysis, the chiral phosphetane oxide 296.455 The phosphine 297 has been synthesised by Michael addition of Ph2PLi to myrtenoic acid.456 It is sensitive to both air and moisture, but its crystalline adduct with BH3 is stable under these conditions.The phosphine 297 is a good ligand for a number of Pd-catalysed allylic substitution reactions. The myrtenylstannane 298 undergoes photoreaction with C60 to give 299 in 15% yield.457 (")-‚-Pinene 252 has been converted into a series of alcohols 300 which may find use as chiral auxiliaries.458 A number of chiral bipyridyl derivatives 301 have been synthesised, and the structure of 301 (R=CHOH) has been determined by X-ray analysis.459 The related platinated thienylpyridine derivative 302 has also been described.460 Nopadiene 303 has been converted into the Á3-allyltitanium complex 304, and this reacts with aldehydes to yield products of which the major diastereoisomers are represented by 305,461 and with myrtenal 267 to give 306.462 A number of chiral titanocene and zirconocene complexes derived from cyclopentadiene-substituted or cyclopentadiene-annulated monoterpenes have been evaluated for their abilities to catalyse the enantioselective hydrogenation of 1,1-disubstituted alkenes.463 The syntheses of a number of novel monoterpenoid 1,2- azoles including the pinane derivative 307 have been described.464 All four possible diastereoisomeric NADH model compounds 308 have been synthesised and characterised, and one of them reduces methyl phenylglyoxylate to give methyl (S)- (+)-mandelate of 72% ee.465 The applications of chiral pinene-derived organoboranes in asymmetric synthesis have been reviewed,466,467 as have recent advances in the utilisation of B-chlorodiisopinocampheylborane (DIP-Cl) 309.468 A convenient and economical preparation of 309 from ·-pinene 239 by reaction with NaBH4–BCl3 in 1,2-dimethoxyethane has been described,469 as have generally applicable syntheses of various [monoterpene]2BX derivatives where X is Cl, Br or I.470 The iodoborane 310 is pyrophoric and hygroscopic, fuming in moist air.471 Apopinene derivatives have also attracted attention,470 and the formation of a number of B-(2- organylapoisopinocampheyl)-9-borabicyclo[3.3.1]nonanes via the hydroboronation of a series of 2-organylapopinenes with 9-BBN has been described.472 The interesting allylic–vinylic borane derivative 311 reacts at "78 )C with aldehydes to give products 312 which, after oxidation using H2O2–NaOH, aVord unsaturated diols 313 in greater than 95% de and of 90–95% ee.473 An eYcient route to the 1,3,2-dioxaboroles 314 via reactions of 1,2-diketones with DIP-Cl 309 or of ·-hydroxy ketones with isopinocampheyldichloroborane 315 has been described.474 O O O O Li NOH N O H O SO2 OH O 289 288 287 286 285 284 NHOH NOH N NOH 291 290 H PH2 P H PCl2 PCl2 P O CO2H PPh2 297 296 295 294 293 292 SnBu3 H C60 299 298 OH Ar N N )2R H N S Pt 302 2 301 R = bond, CHOH or SiMe2 300 TiCp2 R OH 305 304 303 OH N NH N MeO2C Pr N Ph O H )2BR 309 R = Cl 310 R = I 308 307 306 454 Natural Product Reports, 1998In a sequence related to Brown’s synthesis of 1,2-diols above, the borane 316 reacts with aldehydes RCHO to give 317 which can be hydrolysed to yield amino alcohols 318.475 The boranes 319 react with benzaldehyde to give, after oxidative work-up, the (S)-alcohols 320 of 75% ee.476 The silylated imine 321 reacts at "78 )C with B-allyldiisopinocampheylborane to give, after hydrolysis, the homoallylic amine 322 of 73% ee.477 The kinetic resolution of ·-tertiary ketones using (")-DIPCl 309 has been studied.478 For example, (&)-camphor is enantioselectively reduced by 0.5 equiv.of 309 to give (1S,2S)- isoborneol 323 of 94% ee together with residual (R)-(+)- camphor 324 of 66% ee. If, alternatively, 0.6 equiv. of the reagent 309 is employed, the unreduced camphor 324 is obtained in 98% ee.A further study utilising both enantiomers of the reagent 309 has revealed479 that, for all cases examined, (+)-DIP-Cl ent-309 reduces ·-tertiary ketones of the (R)- configuration to give alcohols of at least 100:1 de, and (")- DIP-Cl 309 exhibits similar selectivity when (S)-·-tertiary ketones are the substrates. However, if (")-DIP-Cl 309 is employed to reduce ·-tertiary ketones of (R)-configuration (a ‘mismatched pair’) then the de values of the derived alcohols range from 4:1 to 15:1.A good practical method for the conversion of (+)-·-pinene 239 and of its enantiomer into the optically pure amino alcohols 325 and ent-325, and then into the oxazaborolidines 326 has been described.480 These act as eVective catalysts for the reduction of prochiral ketones using borane, aVording alcohols of 37–96% ee.481 A convenient procedure for the eYcient oxidation using sodium perborate of the diorganoborane derived from (")-·- pinene ent-239 to give (+)-isopinocampheol 327 has been described.482 1,2- and 1,3-diols 328 of 16–99% ee have been obtained via the intramolecular asymmetric reduction of the diisopinocampheylborinates 329 formed by reaction of ·- or ‚-hydroxy ketones with (+)-DIP-Cl ent-309.The intermediate esters 330 are hydrolysed using aqueous NaBO3.483 The generality of an asymmetric hydroboronation– amination sequence which proceeds via formation of a B-chlorobismonoterpenylborane, reaction with AlMe3 to give the B-Me species, and then reaction with hydroxylamine-Osulfonic acid followed by HCl to aVord the amine has been studied.484 (")-Nopol 244 has been converted into the unsaturated acid 331 which has been used as a synthon for robustadial A 332 and some analogues.485 7 Camphanes and isocamphanes The novel bornyl ester 333 has been obtained from the roots of Heliopsis longpipes,486 and the esters 334 and 335 have been isolated from the liverwort Conocephalum conicum.487 The isobornyl esters fersin 336 and fersinin 337 have been found in the roots of Ferula soongarica.488 The predominant chiral monoterpenoids which are found in fossilised amber are borneol 338, isoborneol 339 and camphene 340.The alcohols 338 and 339 obtained from such sources are of good ee and, for any one amber sample, have the same absolute configurations.489 The monoterpenoid content O B O R OB(IPC)2 B O O R OH OH O B O R R IPC BCl2 )2B N Ph Ph 312 311 316 315 314 313 B(IPC)2 R OH N Ph Ph R OH NH2 )2B SiMe2R Ph SiMe2R OH 320 319 R = Me or Ph 318 317 NSiMe3 H NH2 HO O 324 323 322 321 OH NH2 OH NH B O R 326 R = Me, Bu, Ph 327 325 OH R (C OH H2) n R1 R1 328 n = O or 1 B IPC O (CH2) n R1 R1 R2 O B O O (CH2) n IPC R2 R1 R1 CO2H O OH CHO HO OHC 332 331 330 329 OH O O OH OMe O OR OAc CO OAc CO OMe OR 339 R = H 337 R = 336 R = 338 R = H 335 R = 334 R = 333 R = Grayson: Monoterpenoids 455of fossilised amber contrasts with that of living conifers where limonene, the pinenes and camphene are the main constituents. (")-Camphorquinone 341 and its enantiomer are biotransformed by a variety of microorganisms to give derived ·-ketols of all possible regio- and stereo-chemistries.490 An unsuccessful search for electron circular dichroism, the preferential transmission of longitudinally polarised electrons through a chiral medium, has been carried out using both (R)-(+)-camphor 324 and its enantiomer.491 Ab initio calculations of the vibrational CD spectrum of camphor 324 have been carried out using new methodology, and the results obtained are in good agreement with experiment.492 The EI mass spectra of a series of 3-substituted camphor derivatives have been successfully analysed and correlated with the relative stereochemistries of the substituents.493 Resolution of the enantiomers of borneol 338 by capillary GC using a permethylated ‚-cyclodextrin stationary phase has been accomplished.494 The addition reactions of camphene 340 and of tricyclene 342 with HCO2H and with DCO2H have been reviewed.495 Camphene 340 reacts with benzene in the presence of ‚-zeolites to yield the arylated product (&)-343.496 Under similar conditions, camphene reacts with benzyl alcohol to give rearranged addition products, and with benzhydrol to yield 344 and 345.497 Camphene 340 has been converted into the chiral alkoxycyclopentadiene 346.368 The reaction of (+)-camphene 340 with N2O4 leads to nitration products together with the novel heterocycle 347.498 Both enantiomers of the chiral nitroxyl radical 348 have been synthesised from camphene,499 and the related nitroxyl radicals 349 and 350 have been prepared from camphorsulfonic acid 351. Bromination of camphene 340 aVords a mixture of 352 (10%), 353 (59%) and 354 (5%).500 The formyl derivative 355 can be hydrogenated at high pressures to yield the derived saturated alcohol.501 Remote functionalisation reactions (mainly bromination) of camphor 324 have been reviewed in the context of applications of the products to the synthesis of naturally-occurring compounds, 502 and the more general use of cyclic monoterpenoids as enantiopure starting materials for natural product synthesis has also been discussed.503 The lithium enolate 356 and the chlorotitanocene enolate 357 react with electrophilic oxidising agents to give the exo-ketol 358 of 12–60% de (Li) or 88–90% de (Ti).504 Camphor 324 reacts with lithiated ferrocene to yield 359 together with the product of alkylation of both cyclopentadienyl rings.505 X-Ray analysis of the latter has revealed that the ferrocenyl moiety exists in the unusual eclipsed conformation. Camphor 324 has been converted into the acetylenic peroxide 360 and into the derived methacrylate ester.506 Reaction of camphor 324 with acetylene anion aVords both of the epimeric alcohols 361.The acetylenic anion derived from the exo-alcohol 361 is reported to undergo further reaction with ketones R1R2C=O to give diols 362 as single diastereoisomers.507 Reaction of (&)-camphor rac-324 with triflic anhydride aVords the triflate 363 which, in the presence of triflic acid, rearranges to give 364.Ritter reaction of 364 with acetonitrile followed by LiAlH4 reduction of the resulting amide leads to the amine 365 which exhibits a minimum inhibitory concentration of 0.1 g ml"1 against influenza A virus present in Vero cells.508 R O O 341 340 R = H 344 R = CHPh2 Ph Ph O 346 345 343 342 O N O N •O 348 347 O N Bu t O• N O •O O SO3H 351 350 349 CHBr Br Br Br2HC Br 354 353 352 CHO 355 OR OH O 358 356 R = Li 357 R = TiCp2Cl OH Cp Fe Cp OH O O OH H OH R1 R2 OH 362 361 360 359 OSO2CF3 OSO2CF3 NHEt 365 364 363 456 Natural Product Reports, 1998The application of camphor derivatives as chiral auxiliaries continues to attract widespread interest.A molecular modelling study and semi-empirical calculations have been made of the diastereoselectivity of some alkylation reactions of the deprotonated phosphonate 366, and a good correlation between theory and experiment has been found.509 The C2-symmetric SchiV base 367 aVords a copper(II) complex which catalyses the cyclopropanation of styrene to give products of low to medium ee.510 The acetylenic host molecules 368–370 have been synthesised from camphor 324.511 Crystallisation of these hosts from solutions of appropriate racemic compounds leads to inclusion compounds which exhibit chiral separation of their guests. Thus, 368 shows complete enantioselectivity for the (R)-form of 4-pentanolide, 369 exhibits 70% enantioselectivity for the (S)-form of 3-methyltetrahydropyran and 370 is 100% enantioselective for the (S)-form of styrene oxide.(")-Camphorquinone 341 has been converted into the crown ethers 371–373, and the interactions between these and some racemic phenylglycine salts have been investigated.512 No enantiomeric enrichments were observed.Bi-thiocamphor 374 can be selectively reduced using Me2S · BH3 to give either the exo,exo-dithiol or its exo,endoisomer depending upon whether the reagent is added to the thioketone or vice versa.513 Oxidation of 374 with Hg(OAc)2 or with K3[Fe(CN)6] leads, respectively, to the thiophene 375 or to the disulfide 376. The diketone 377 has been accessed via reaction of 374 with hydrazine followed by hydrolysis of the dihydropyrazine which is formed.514 Some ring-closure and prototropic rearrangement reactions of 374 have been studied.515 Irradiation of 374 at 254 nm has been found to give 378.516 The ligands 379 have been synthesised and catalyse the enantioselective addition of diethylzinc to benzaldehyde to give (S)-1-phenylpropanol of 85–88% ee.517 The ligand 380 has been converted into 381 which reacts with organometallic reagents R1Msuch as BuLi, Et2Zn or allylmagnesium bromide to give products 382 of modest de.518 These can be hydrolysed under acidic conditions to give optically active amines 383 together with the disulfide 384 which can be recycled to 380.The titanium complex 385 has been synthesised and characterised.519 The synthesis of a series of chiral diols 386, potentially useful as ligands or as auxiliaries, has been simultaneously reported by two independent groups.520,521 The homochiral acetal 387, prepared from a camphor-derived diol, can be deprotonated using LDA to give an anion which is alkylated by MeI in good yield but with poor de.522 The diastereoisomeric acetals 388 of, for example, aromatic cyanohydrins which are prepared from the corresponding camphor-derived lactol exhibit 1H NMR spectra which can be used to assign absolute configurations at their nitrile-bearing carbon atoms.523 Camphor-10-thiol 389 has been converted into its S-methyl thioether and then into the derived thioketone.This has been reduced using DIBAL and then further methylated to give the dithioether 390,524 but no applications of this potentially useful auxiliary appear to have been reported during the period under review. Aldol condensation of the sulfoxide 391 with RCHO followed by elimination of isoborneol-10-sulfenic acid leads to „-hydroxy-·,‚-unsaturated esters 392 of moderate ee.525 The sulfoxides 393 can be obtained as single diastereoisomers by reaction of 394 with RMgBr.526 The structure of 393 (R=Me) has been determined by X-ray crystallography.The optically pure alkoxychlorosulfuranes 395 have been prepared, and react with water to yield inverted sulfoxides or with the sodium salt of toluene-p-sulfonamide to yield compounds 396.527 An X-ray analysis has been carried out on crystals of 396 (R=Ph). The chlorosulfurane 395 (R=Me) reacts with dimethyl malonate anion to give the sulfonium ylide 397 as a mixture of diastereoisomers. 527 Asymmetric Diels–Alder reactions based upon chiral ·,‚- unsaturated sulfoxides derived from 10-thioisoborneol have been reviewed.528 The vinylic sulfoxide 398 (R=H) undergoes Diels–Alder cycloaddition with cyclopentadiene at "78 )C in N PO(OEt)2 N )2 OH ) R R O O O O O O 371 R = H 372 R = Me 373 R = Ph 370 L = 369 L = 368 L = 367 366 2L R R (S) n 375 n = 1 376 n = 2 374 R = S 377 R = O SH S 378 OMe SN R OMe SN R M R1 H2N R R1 OMe S )2 OR2 SR1 379 R1 = Me, Pri or Bz; R2 = H 380 R1 = H; R2 = Me 381 382 383 384 * O S Ti O Cl Cp(Me)5 OH OH Ar Ph 385 386 Ar = Ph, 1-naphthyl or 9-anthryl O O CO2Et H H O O H CN R 388 387 Grayson: Monoterpenoids 457the presence of ZnCl2 to give an adduct 399 of 99.4% de.529 1,4-Addition of PhMgX to the crotonate 400 proceeds with only modest diastereoselectivity.530 Reaction of the silver salt of camphorsulfonic acid 351 with ButPhP(=O)Cl in acetonitrile leads to formation of the mixed anhydride 401 whose diastereoisomers have been separated.531 The sulfonimides 402 are reduced using sodium borohydride to yield the derived sultams 403 of good de.532 The reaction is not useful for 402 (R=Br) since reductive loss of bromine takes place prior to saturation of the C=N bond.A two-step, 100 g scale route to the sultam 403 (R=H) which proceeds in 66% yield has been described.533 The molecular and crystal structures of the Npropionylsultam 404 have been determined.534 The bromoacetyl derivative 405 undergoes aza-Darzens reactions with N-(diphenylphosphinyl)arylmethanamines to give 406 (X-ray) of high de.Cleavage of 406 gives the chiral aziridines 407.535 The phthaloyl sultam 408 has been found useful for the derivatisation of alcohols to give diastereoisomeric esters which are resolvable by HPLC, and which can be used as an aid for the X-ray determination of the absolute configurations of alcohols. 536 The crotonoylsultam 409 reacts with dibutylboron triflate to give a chiral dienolate which aVords 410 of 100% de when it is treated with (E)-crotonaldehyde.Oxy-Cope rearrangement of 410 leads to the aldehyde 411 which has been oxidised to the acid 412.537 A series of camphor-10-sulfonamide derivatives have been prepared of which the example 413 has been found to be a potent growth hormone secretagogue.538 The camphorsulfonylbenzimidazole 414 and some analogues have been synthesised.539 The sulfonamide 415 undergoes acid-catalysed rearrangement to yield the enantiomerically pure primary sulfonamide 416.540 Full details of preparations of the camphorsulfonyloxaziridines 417, and protocols for their application in enantioselective oxidation reactions of some tetralone derivatives have been reported.541 A number of optically pure Te-chiral alkoxytelluranes 418 have been prepared, and an X-ray crystal structure of one of them 418 (R=Ph) has been obtained.542 The trimethylsilyl enol ether 419 of ·-bromocamphor can be lithiated using ButLi to yield a derived vinyllithium which then reacts sequentially with tellurium and with cyanogen bromide to give 420.Hydrolysis of 420 aVords the transient ·-telluroketone 421 which dimerises to give the syn- and anti-1,3-ditelluranes 422 and 423.543 The structure of 422 was determined by X-ray methods. The optically pure selenoxides 424 have been found to act as chiral proton sources, and 424 (R=4-MeO-C6H4) converts the zinc enolate 425 into (S)-2-benzylcyclohexanone of high ee.544 SH O SMe SMe OH S O CO2Et R CO2 Et OH OH S O R S O O 394 393 392 391 390 389 : : O S Cl R OH S Ts R OH S+ Me CO2Me MeO2C OR S O CO2Me OH S O CO2Me SO2Ph O O 400 399 398 397 – 396 395 : : O SO2 O P O Ph SO2 N R R R R NH SO2 403 402 R = H, Cl, Br or OMe 401 NR SO2 CO HO2C N SO2 N O R POPh2 N R HO2C POPh2 407 406 409 R = (E)-crotonoyl 404 R = –COPr 405 R = –COCH2Br 408 R = N SO2 O HO N SO2 O R 411 R = CHO 412 R = CO2H 410 NHCO N OH SO2 N O SO2 N OH SO2NH2 SO2NH2 416 415 414 413 458 Natural Product Reports, 1998The chloroselenuranes 426 undergo diastereoselective imination to give the selenimides 427 when they are reacted with RNHLi.[2,3]-Sigmatropic rearrangement of the selenimides 427 leads to 428 which can be cleaved to give allylic amines 429 of 24–93% ee.545 Reaction of the (RSe)-chloroselenurane 430 with, for example, acetylacetonate anion, leads to formation of the (SSe)-selenonium ylide 431.546 The selenamide 432 can act as a glutathione mimic, promoting the oxidation by ButOOH of benzyl thiol to the derived disulfide.547 The diselenide 433 has been converted into 434 which reacts with sulfuryl chloride to give the chiral selenenyl chloride 435.This promotes the electrophilic cyclisation of pent-4-en-1-ol to give the tetrahydrofuran 436 of 80% de, and of pent-4-enoic acid to give the butanolide 437 of 86% de.548 A number of camphor-derived cyclopentadienes 438 have been synthesised via Nazarov cyclisation reactions and converted into their bis(cyclopentadienyl)zirconium and bis(cyclopentadienyl)titanium dichlorides.549 A one-step synthesis of the valuable ligand 439 can be achieved by the reduction of camphorquinone monooxime using sodium borohydride in the presence of NiCl2 · 6H2O.550 (+)-Camphor 324 has been converted via a six-step sequence into the oxazolidinone 440 which is a ‘transfigomer’ of the previously-described auxiliary 441.551 The new auxiliary 440 is superior to 441 in Lewis acid-catalysed Diels–Alder reactions of its derived ·,‚-unsaturated amides with cyclopentadiene, and the homologue 442 exhibits even better performance.The tributylstannyloxazolidine 443, which was synthesised from the corresponding amino alcohol, has been converted into the ketones 444 of 100% de. These react with R1MgX to give tertiary alcohols 445 which are lactonised to 446 on treatment with ButOK. Acid-catalysed hydrolysis of 446 followed by borohydride reduction of the derived ·-hydroxy aldehydes leads to diols 447 of >96% ee.552 The new auxiliary 448 has been prepared, and its N-acyl derivatives 449 are deprotonated by NaHMDS to form enolates which are alkylated to 450 in good de.Reductive cleavage using lithium aluminium hydride leads to optically active alcohols 451.553 The acetyl derivative 449 (R1=H) has been converted into its lithium and titanium enolates, and these react with aldehydes RCHO to yield aldols which can be R R N SO2 O O Te F Ar R OSiMe3 Te O O Te O Te O Te Te O 423 422 421 419 R = Br 420 R = TeCN 418 417 : OH Se O R OZnBr Ph 425 424 R = Ar : O Se Cl R OH Se R R1HN OH Se N R1 R R NHR1 429 428 427 426 R = Ph or Pri : : O Se Cl Ph O O OH Se Ph – 431 430 : : + Se OH NCOCH3 O Se )2 Se )2 O N O H SeCl O N O H Se O N O O R H R 438 R = Me, Ph or Bu t 436 R = H,H 437 R = O 435 434 433 432 NH2 OH O N R H O 440 R = H 442 R = Me 439 O N H O O N CO2But SnBu3 O N CO2But COR O N CO2But OH R R1 O N O O R R1 OH OH R1 R 447 446 445 444 443 441 N N O Ph R N N O Ph O R E 450 448 R = H 449 R = COCH2R1 HO E R HO2C R OH 452 451 Grayson: Monoterpenoids 459hydrolytically cleaved using LiOH–H2O2 to give ‚-hydroxy acids 452 of moderate ee.554 The potentially useful camphor-derived pyrroles 453 and 454 have been synthesised.555 The novel hydroxy pyrazole ligands 455 and 456 have been obtained via reaction of the parent compound 457 with homochiral styrene oxide under highpressure conditions,556 and the pyrazole 455 catalyses the enantioselective addition of diethylzinc to benzaldehyde to give (R)-1-phenylpropanol of 93% ee.557 The camphor-based boryl enolates 458 have been obtained, and react with aldehydes in the presence of Lewis acids such as TiCl4 or SnCl4 to give aldols 459 with very good diastereoselectivity (often>98% de).Cleavage of 459 using hydroperoxide anion gives the ‚-hydroxy acids 460 and the oxazolidinone 461 which can be recovered and reused.558 The related N-acetyloxazolidinethione 462 forms the titanium enolate 463 which also undergoes diastereoselective aldol reactions.559 The isocyanides 464, which have been synthesised from (+)-ketopinic acid, react with [Pd(acac)2] to form catalysts which eVectively promote the intramolecular bis-silylation of 465 to give 466 of 7–64% ee.560 The homochiral, acyl-activated isocyanate 467 has been reported.561 (+)-8-Bromocamphor 468 has been converted into the phosphine oxide 469, and 8,10-dibromocamphor has been converted into the bis-oxide 470.562 Both epimers of the dichlorophosphine 471 have been prepared.455 Oxidation of 472 with K3[Fe(CN)6] had been reported to yield the nitroxyl radical 473, but a reinvestigation has shown that the blue solid which is obtained exhibits a sharp 1H NMR spectrum inconsistent with this free radical structure, and that the product is actually an equilibrium mixture of 473 with its more favoured dimer 474.563 8 Caranes (+)-Car-2-ene 475 has been converted into its radical cation in methanol solution via photoinduced single electron transfer to the first excited singlet state of 1,4-dicyanobenzene when the products 476 and 477 are formed.564 ESR spectroscopy has been used to study the free radical species which are obtained by reacting photogenerated tert-butoxy radicals with various monoterpenes including (+)-car-3-ene 94.565 When (+)-car-3-ene 94 is treated with a Bu4Sn-modified Ni-SiO2 catalyst system,566 it is isomerised with 91% selectivity at 48% conversion to its lower-boiling ‘conjugated’ 2-isomer 475, a starting material for a synthesis of (")-menthol.(+)-Car-3-ene 94 forms adducts with N-sulfinyl arenesulfonamides which, when heated with HMDS at 80 )C, are converted into the cycloheptatriene 478.567 The isomerisation of the 3,4-epoxycarane 479 over binary oxide catalysts such as Al2O3–M2O3 (M=Y, Sm, Eu or Nd) and Al2O3-Pr6O11 has been systematically examined.568 At reaction temperatures of ca. 80)C the major products are ring-contracted aldehydes and the caranone 480. (+)-Car-3-ene 94 is oxidised at 333 K in the presence of a mixed Co–Ce-naphthenate catalyst to yield a product mixture containing 38% of the carenone 481.569 N R H N N Ph HO N N Ph OH N N H 457 456 455 453 R = H 454 R = SMe N O O O BL N O O R O OH R CO2H OH N O X R N O S O Ti Cl Cl Cl 463 461 R = H; X = O 462 R = Ac; X = S 460 459 458 L = Bu2 or 9-BBN N C OR 464 R = Me or Et O Si PhMe2Si Ar Ar O Si Ar Ar SiMe2Ph O N C O O Br O R POPh2 O Cl2P 471 469 R = H 470 R = POPh2 468 467 466 465 N R O N O O O N O 474 472 R = OH 473 R = O• NC OMe OMe CN 94 477 476 475 460 Natural Product Reports, 1998The amino oxime 482 can be eVectively hydrolysed to give the derived amino ketone 483 in the presence of titanium(III) salts.570 In an unusual reaction already referred to for a pinane-based example above, the hydroxyamino oxime 484 is converted into the lactam system 485 when it is treated with NaBH4–MeCN.453 The unsaturated oxime ester 486 aVords the ring-contracted nitro compound 487 when it is reacted with NaNO2–AcOH.388 The chiral thienylpyridine derivative 488 has been synthesised from (+)-car-3-ene 94 via the derived aldehyde 489, and the thienylpyridine 490 has been obtained from (+)-car-2-ene 475 via the enone 491.571 Photolysis of 491 during seven days leads to the known ·,‚-unsaturated ketone 492.(+)-Car-3-ene 94 has been converted into the pyrazole 493 and into the vinylogous amide 494. The pyrazole 493 forms easily recrystallisable diastereoisomeric amides with pyrethroid acyl chlorides.572 The diastereoselectivities of the reactions with benzaldehyde of Li, Ti and B enolates derived from the acetates 495 and 496 have been examined.573 Basecatalysed hydrolysis of the products leads to optically active 3-hydroxy-3-phenylpropanoic acid.Ozonolysis in CH2Cl2–pyridine of the enol trimethylsilyl ether 497 aVords a mixture of 498–500, but ozonolysis in CH2Cl2–methanol followed by reductive work up using Me2S gives the unsaturated ketone 501.574 The sulfide 502 has been synthesised.575 9 Fenchanes There has been little or no activity in this area during the period under review. (")-Fenchone 503 is reported to react with crotylmagnesium chloride to yield only the (Z)-configured endo-alcohol 504,505 and has been converted into the ferrocenyl derivative 505.576 The C2-symmetric SchiV base 506 has been synthesised, but its Cu complex is not a good catalyst for the asymmetric cyclopropanation of styrene.510 10 Thujanes Thujone 507 has been regioselectively converted into homothujone 508,577 and a new synthesis of trans-sabinene hydrate 509 has been reported.578 11 Ionone derivatives The flowers of Lawsonia inermis can be either red or yellow.Flowers of both colours contain ‚-ionone 510, but red flowers contain only 2.5% of the ketone whereas yellow flowers provide up to 48.6%.579 The ionone derivative 511 has been obtained from Viburnum dilatatum,580 and the related glycoside byzantionoside A 512 has been found together with byzantionoside B 513 in Stachys byzantina.581 The novel triols 514 and 515 have been isolated from the leaves of Apollonias barbujana,582 and the histamine-release inhibitors corchionosides A–C 516–518 have been found in Vietnamese Corchorus olitorius.583 The ionol glycoside 519 occurs in Maerua crassifolia, 584 some ‚-D-glucosides of the diastereoisomeric 3-hydroxy-·-ionols have been isolated from Urtica dioica,585 and five glycosylated ionone derivatives have been found in the O O O 481 480 479 478 R NOH NMe2 O N H N OH NOH 486 485 483 482 R = NMe2 484 R = NHOH OAc O2N N S CHO N S O O 492 491 490 489 488 487 N N H NH2 O R1 R2 495 R1 = H; R2 = OAc 496 R1 = OAc; R2 = H 494 493 OTMS CHO R O CO2H CHO S S S 502 500 498 R = O 499 R = H, CO2H 497 O 501 O OH OH Cp Fe Cp N )2 506 505 504 503 (CH2) n O OH 509 507 n = 1 508 n = 2 Grayson: Monoterpenoids 461leaves of the tomato Lycopersicon esculentum.586 The leaves of Anthriscus nitida contain 5,6-epoxy-3-hydroxy-‚-ionol.587 A synthesis of the spiro-tetrahydrofuran 520, an odoriferous principle of black tea, has been described.588 ‚-Ionone 510 is metabolised by Aspergillus niger IFO 8541 to give a mixture of hydroxy- and oxo-derivatives in quantitative yield.589 The biotransformations of ‚-ionone 510 by A.niger which has been immobilised on calcium alginate beads have been reviewed,590 as has the hydroxylation of 510 by the same organism.591 The absolute stereochemistry of stratioside-1 521 from Pistia stratiotes has been determined by NMR via its Mosher ester.592 O O RO 511 R = H 512 R = b-D-Glc 510 O O-b-D-Glc OH HO OH OH HO OH OH Glc-D-b-O O OH O O O-b-D-Glc O-b-D-Glc OH O O-b-D-Glc HO O O 520 519 518 517 516 b-bond 521 a-bond 515 514 513 O O O O O O 525 524 523 522 OLi O O O O 529 528 527 526 CHO O 530 O HO CO2 O O HO OH OH O O MeO O OH OH O O HO HO OH O CN O CO2Me HO HO2C O-b-D-Glc O O OH HO 533 532 531 O CO2Me HO O-b-D-Glc O O R O CO2H Glc-D-b-O O O O O O HO2C O-b-D-Glc O CO2 Glc-D-b-O O O O O O O2C O-b-D-Glc O CO2H Glc-D-b-O O HO2C O-b-D-Glc 536 535 534 R = cis- or trans- p-coumaryl 542 541 540 539 538 537 • • • • O O OH HO O O-b-D-Glc HO H H O O-b-D-Glc HO BuO O O-b-D-Glc OBu HO HO O CO2Me O-b-D-Glc HO Me H H CHO OAc 544 543 • • O O O O OH HO HO O O OH HO HO O O O-b-D-Glc OH HO CO2Me 462 Natural Product Reports, 1998Table 2 Sources of iridoids Species Principal constituents Reference Anthocephalus chinensis 3*-O-caVeoylsweroside 545 614 Anthocleista djalonensis djalonenol 546 615 Avicennia germinans 2*-caVeoylmussaenosidic acid 547 616 Cajophora pentlandii pentlandioside 548 617 Castilleja tennuiflora iridoid glycosides 618 Catalpa bignonoides iridoid glycosides 619 Catalpae fructus kisasagenol A 549, kisasagenol B 550, epicatalpin 551 620 Cistanche phelypaea (L.) Cout.phelypaeside 552 621 Clerodendrum colebrookianum iridoid glycosides 622 Crucianella graeca iridoid glycosides 623 Cruciata glabra iridoid glycosides 623 Cruciata leavipes iridoid glycosides 623 Cruciata pedmontana iridoid glycosides 623 Cymbalaria muralis muralioside 553 624 Dipsacus asperoides loganic acid 6*-O-‚-D-glucoside 554 625 Duranta erecta duranterectoside A 555, duranterectoside B 556, duranterectoside C 557, duranterectoside D 558 626 Fraxinus angustifolia angustifolioside C 559 627 Fraxinus ornus 2+-hydroxyornoside 560 628 Fraxinus uhdei uhdenoside 561 629 Galium aegeum iridoid glycosides 630 Galium lovcense 562 and 563 631 Galium macedonicum iridoid glycosides 630 Galium mirum iridoid glycosides 630 Galium rhodopeum iridoid glycosides 630 Gentiana algeda 564 632 Gentiana depressa 3+-glucosyldepresteroside 565, depressine 566 633 Gentiana macrophylla 567 and 568 634 Gonocaryum calleryanum gonocaryoside A 569, gonocaryoside B 570, gonocaryoside C 571, gonocaryoside D 572 635 Hemiphragma heterophyllum hemiphrosides A and B 636 Incarvillea olgae 7-O-benzoyltecomoside 573, stansioside 637 638 Jasminum hemsleyi jashemsloside 574 639 Jasminum lanceolarium 575–577 640 Jasminum odoratissimum 10-acetoxyoleoside dimethyl ester 578 641 Jasminum polyanthum jaspolyoside 579, jaspolyanthoside 580; new secoiridoids; jaspolyside 581, jaspofoliamoside A 582, jaspofoliamoside B 583, jaspolinaloside III 584, neopolyanoside IV 585 642 643 644 645 Jasminum polyanthum Franch.secoiridoid glycosides 646 Jasminum sambac oligomeric iridodial glycosides 647 Kickxia spp. 5-O-menthiafoloylkickxioside 586, kickxin 587 648 Lamiophlomis rotata lamiophlomiside 588 649 Ligustrum lucidum Ait. nuezhenidic acid 589 650 Linaria arcusangeli 8-epi-muraloside 590, arcusangeloside 591 651 652 Linaria flava ssp. sardoa 6*-O-acetylantirrhinoside 592 653 Linaria japonica iridolinaroside A 593 654 Liquidambar formasans iridoid glycosides 655 Lonicera caerulea caeruloside A 594, caeruloside B 595, caeruloside C 596 656 657 Lonicera gracilipes var.glandulosa Maxim. loganin, sweroside, grandifloroside 658 Nepeta cilicia nepetacilicioside 597 659 Grayson: Monoterpenoids 463‚-Ionone 510 is selectively hydrogenated to cisdihydroionone 522 over Pd/AlPO4 in methanol.593 Photooxygenation of ‚-ionone 510 leads to the 1,2,4-trioxane 523.594 New routes to ‚-damascone 524,595 and to (+)-(2R,6R)- trans-„-irone 525596 have been described. The lithium enolate 526 is protonated by (")-Nisopropylephedrine to give the (S)-configured ketone 527 which can then be isomerised to yield (S)-·-damascone 528.597 ·-Ionone epoxide 529 undergoes rearrangement in the presence of catalytic amounts of aminium salts to give the ring-contracted aldehyde 530.598 12 Iridanes The plant iridoids have been reviewed,599 and the distribution of iridoids in various populations of Physostegia virginiana has been examined.600 The biosynthesis of iridoids in Syringa and Fraxinus spp.has been studied using [2H]-labelled secoiridoid precursors.601,602 The complex cyanogenic glycoside 531 has been found in Canthium schimperianum,603 and 10-carboxyloganin 532 has been obtained together with several nor-monoterpenoid glucosides from the leaves of Cerbera manghas.604 1,2- Dihydroxymintlactone 533, which exhibits useful nematicidal activity, is a new minor metabolite from culture filtrates of the basidiomycete Cheimonophyllum candidissimum.605 The novel secologanol derivative 534 has been obtained from Fontanesia fortunei as a light- and acid-sensitive mixture of (E)- and (Z)-isomers and its biosynthesis has been investigated.606 The complex monoterpenoid-bridged secoiridoids rhodanthoside A 535 and rhodanthoside B 536 have been isolated from Gentiana rhodantha.607 7-O-Butylsecologanic acid and secologanin dibutyl acetal have both been isolated from butanol extracts of Lonicera japonica but have been shown to be artefacts produced by the extraction process.608 Iridodial ‚-monoenol acetate 537 has been found in Nepeta leucophylla Benth.and is an eVective agent against the fungus Sclerotium rolfsii.609 Nepetanudoside 538, which has been obtained from Nepeta nuda ssp.albiflora, has ring-fusion stereochemistry which is opposite to that of the known mussaenoside.610 The three new aucubin derivatives 539–541 and pedicularis lactone 542 have been isolated from the roots of Pedicularis chinensis.611 The extraction was carried out using methanol, but it seems possible that a later step involving butanol as solvent may have given rise to the ethers 539 and 540. Phlomoside A 543 has been obtained from Phlomis thapsoides, and is identical with the product which is obtained via the hydrogenation of strictoloside.612 The major bitter principle of Swertia chirata is the sweroside derivative 544 whose structure has been determined by NMR methods.613 This structure had already been reported, but for a compound having quite diVerent chemical properties and for which incomplete spectroscopic data was provided. Many plants have been screened for the presence of new and previously described iridoids during the period under review, and Table 2 incorporates the principal results which have been obtained, with structural formulae attaching to the names of new compounds.Progress in the synthesis of iridoids has been reviewed,682 and enantioselective syntheses of (+)-mitsugashiwalactone 614 and of (")-dolichodial 615 have been reported.683 SAMP methodology has been applied in syntheses of (&)- dehydroiridodial 616 and of (&)-dehydroiridodiol 617.684 A synthesis of (&)-isoiridomyrmecin 618 has been achieved via Pd0-mediated cyclocarboxylation of the allene 619 to give 620 which was then further processed to aVord 618.685 Formal total syntheses of (")-isoiridomyrmecin 618, (+)- iridomyrmecin 621 and (+)-teucrium lactone 622 have been accomplished via the common intermediate (+)-623.686 Other syntheses which have been reported include routes to Table 2 Continued.Species Principal constituents Reference Nepeta nuda ssp. albiflora nepetanudoside B 598, nepetanudoside C 599, nepetanudoside D 600; nepetalactones 660 661 Nyctanthes arbor-tristis arborside D 601 662 Mussaenda pubescens mussaenin A 602, mussaenin B 603, mussaenin C 604 663 Patrina scabra patrinioside 605 664 Pedicularis chinensis iridoid glycosides 665 Pedicularis longiflora var.tubiformis iridoid glycosides 666 Pedicularis spp. iridoid glycosides 667 Penstemon crandallii verbascoside derivatives 668 Phlomis brachyodon iridoid glycosides 669 Phlomis regelii phlomoside B 606 670 Phlomis umbrosa sesamoside 671 Plantago alpina 607 and 608 672 Plantago altissima 609 672 Psychotria mariniana asperuloside 673 Scrophularia scorodonia scorodioside 610 674 Sideritis libanotica Labill.ssp. linearis (Benth.) Bornm. ajugoside 675 Thunbergia grandiflora isounedoside 611, grandifloric acid 612 676 Verbascum undulatum unduloside 613 677 Verbena tenera pulchellosides I and II 678 Verbenoxylum reitzii viridoside 679 Veronica triphyllos 8-epi-loganin, 6-O-vanilloylcatalpol 680 Westringia fruticosa catalpol cinnamates 681 Westringia viminalis catalpol cinnamates 681 464 Natural Product Reports, 1998(&)-patriscabrol 624 and to (&)-boschnialactone 625.687 A synthesis of (+)-villosol 626 has led to a revision of its previously-assigned absolute configuration,688 and (")- linalool 22 has also been converted into villosol 626.689 13 Cannabinoids The new cannabinorolic acid 627 has been obtained from Cannabis sativa,690 and the phytochemistry of this species has been reviewed in an article which emphasises compounds O O O O O HO HO O OH O OH OH O O OH OH O CO2H HO O O HO HO OH O O OH OH O CO2Me O-b-D-Glc O O CO2Me O-b-D-Glc HO O O R OH HO CO2 OH O OH O HO CO2 HO O O-b-D-Glc HO HO O O OH HO HO HO O-b-D-Glc O HO CO2H O-b-D-Glc-(6¢-O-b-D-Glc) 547 546 545 554 553 552 O RO HO OH CO2Me O-b-D-Glc O OMe OH O CO2Me AcO AcO HO O-b-D-Glc O O-b-D-Glc CO2 O O HO Glc-D-b-O OH OH O O-b-D-Glc O O OH O O O HO 551 549 R = O 550 R = H,H 548 560 559 O HO O Glc-D-b-O HO CO2Me O O CO2Me O-b-D-Glc R1 R2 O O O O-2¢-(2�,3�-dihydroxybenzoyl)-b-D-Glc O CO2 O-b-D-Glc O O-b-D-Glc CO2Me CO2 OH Glc-D-b-O 558 556 R = ( Z)-cinnamoyl 557 R = 4'-OH-( Z)-cinnamoyl 555 R = 565 564 562 R1 = OAc; R2 = CH2OAc 563 R1 = OH; R2 = CO2H O CO2Me O-b-D-Glc CO2 OH O-b-D-Glc O O O R O O AcO OAc O2 O O O CO2R1 O HO R2 O O-b-D-Glc O CO2Me CO2H O O-b-D-Glc O OH HO O CHO O-b-D-Glc PhCO2 561 • 573 572 O CO2Me O-b-D-Glc O O O OH OH OH O OH OH HO O O CO2H CO2Me O-b-D-Glc HO R CO2 O CO2Me CO2Me AcO O-b-D-Glc 569 R1 = R2 = Me 570 R1 = H; R2 = Me 571 R1 = Me; R2 = CH2OH 567 R = H 568 R = OH 566 578 575 ( Z); R = H 576 ( Z); R = OMe 577 ( E); R = OH 574 • • • • • • • • • • • •• •• • • • • • C OH O-b-D-Glc • Grayson: Monoterpenoids 465O CO2Me O-b-D-Glc O CO2Me O2 O O OH OH OH O2C C O CO2Me O-b-D-Glc CO2 O CO2Me HO2C O-b-D-Glc O CO2Me CO2R O OH OH OH OH O O-b-D-Glc O O-b-D-Glc O OH O CO2Me O-b-D-Glc O CO2Me O-b-D-Glc O OH OH OH O2C O O RO RO O-b-D-Glc CO HO O HO CO O-b-D-Glc O O-b-D-Glc O OH O CO2 581 580 579 587 O CO2Me OH HO O-b-D-Glc O CO2Me OH O-b-D-Glc HO2C HO2C O OH HO HO HO O-b-D-Glc O O O O-b-D-Glc O O CO O O OH HO O-b-D-(6¢-OAc)-Glc O O CO2-b-D-Glc 586 R = 585 584 R = 583 R = 582 R = 593 O CO2Me HO OR O O OR O O O OH OH O O H O CO2Me O-b-D-Glc H O CO2Me O-b-D-Glc O OH OH OH O O O CHO HO O-b-D-Glc 592 591 590 589 588 597 596 594 and 595 R = 595 594 O R O-b-D-Glc O CHO O-b-D-Glc O CO2Me HO HO PhCO2 O-b-D-Glc O HO O OH O CO2Et HO O O HO O OEt • • • • • 604 OH O-b-D-Glc OH O O O CO2Me OH AcO AcO O-b-D-Glc O CO2H AcO O-b-D-Glc O CO2H AcO O-b-D-Glc O CO AcO O-b-D-Glc O-b-D-Glc O O HO O-b-D-Glc O O Me HO AcO O O 603 602 601 600 598 R = CO2H 599 R = CHO 610 O O HO O-b-D-Glc O HO2C O-b-D-Glc O O-b-D-Glc HO O O OH OH HO CO2 HO MeO 609 608 607 606 605 611 612 613 C HO HO O CO2Me O-b-D-Glc O2 &; 466 Natural Product Reports, 1998which have been isolated and identified between 1980 and 1994.691 ƒ6-Tetrahydrocannabinol 628 is converted into its ‚-Dglucoside by tissue segments from Pinellia ternata tubers.692 Syntheses of 4+,5+-bisnor-ƒ1-tetrahydrocannabinol-7,3+- dioic acid 629 and of some [2H]-labelled analogues have been reported.693 The acid 629 is a major human urinary metabolite of ƒ1-tetrahydrocannabinol. 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ISSN:0265-0568    

DOI:10.1039/a815439y

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Marine polypropionates Michael T. Davies-Colemana and Mary J. Garsonb aDepartment of Chemistry, Rhodes University, Grahamstown 6140, South Africa† bDepartment of Chemistry, The University of Queensland, Brisbane 4072 QLD, Australia‡ Covering: up to the end of 1997 1 Introduction 2 Marine polypropionates from molluscs: isolation, stereochemistry and synthesis 2.1 Acyclic metabolites 2.2 Metabolites containing a furanone ring 2.3 Metabolites containing a 2-pyrone ring 2.4 Metabolites containing a 4-pyrone ring 2.5 Metabolites containing a hemiacetal ring 3 Polypropionate metabolites from other marine sources 4 Biosynthesis of marine polypropionates 4.1 Overview of biosynthetic processes in molluscs 4.2 Polypropionate biosynthesis in marine molluscs 4.3 The biosynthetic status of marine polypropionate metabolites 4.4 A comparison of polypropionate biosynthesis in marine molluscs and in terrestrial bacteria 5 Ecology and biological activity of marine polypropionates 6 Acknowledgements 7 References 1 Introduction Macrolides from bacteria and fungi are among the most important commercial antibiotics used as therapies against respiratory infections, against legionnaires disease, and for minor infections in patients sensitive to penicillin antibiotics, while polyether antibiotics are used to prevent poultry infections and in other agrochemical applications.An important structural and biosynthetic connection between these two classes of antibiotics is the use of propionate as a biosynthetic building block.Details of the paths of biosynthesis leading to bioactive polypropionates such as erythromycin, methymycin, tylosin, monensin, nonactic acid, and other key metabolites, have been probed in a number of biosynthetic laboratories worldwide, and parallel the exciting advances in our understanding of the biosynthesis of polyketide metabolites.1 In recent years there have been significant advances in our understanding of the molecular basis of polyketide and polypropionate biosynthesis, resulting in ‘engineered’ modification of genomic material and the production of ‘non-natural’ metabolites.2–6 It is not surprising that studies on polyketide/ polypropionate biosynthesis, as cutting-edge, interdisciplinary research, are frequently topics for review in the chemical literature.2–8 Over the last thirty years, many structurally-novel polypropionate metabolites have been isolated from marine systems as phylogenetically diverse as marine bacteria and sponges.The most important source of marine polypropionate compounds are the Mollusca. Some of the marine polypropionates isolated are biologically active while other polypropionate metabolites clearly exert an ecological influence in the organism which manufactures them even though no pharmacological activity per se has yet been demonstrated in the laboratory. Our knowledge of polypropionate and polyketide biosynthetic processes in marine systems is in its infancy.Inevitably one important question surfaces: are these metabolites biosynthesised by identical pathways to those operating in terrestrial bacteria, or are there distinctive biosynthetic processes involved? In this review, we have set out to document recent literature on the structures, biosynthesis and ecology of marine polypropionates, notably those from the Mollusca. A brief overview is given of selected polypropionate examples from other marine phyla.We then compare and contrast the limited knowledge of polypropionate biosynthesis in marine systems with terrestrial polypropionate biosynthesis, and overview the ecological roles of marine polypropionates. Some aspects of marine polypropionate research have been reviewed previously in this journal; the excellent series of articles by Faulkner should be consulted for structural information,9 while biosynthetic studies on marine polypropionates have previously been covered in articles on polyketide biosynthesis.10 2 Marine polypropionate compounds from molluscs: isolation, stereochemistry and synthesis Metabolites which appear to derive from the condensation of propionate units are characteristic metabolites of molluscs. As shown in Section 3, there is now experimental evidence to support the use of propionate in their biosynthesis.The following sections provide a summary by structural class. 2.1 Acyclic metabolites The opisthobranch mollusc Siphonaria grisea, collected from Senegal, yielded the simple aldehyde, siphonarienal 1, as a minor metabolite.11 Hampered by the instability of 1, Norte et al. used a biomimetic, combined aldol-reduction synthetic sequence to synthesize this compound and confirm the expected (4S,6S,8S)-configuration.11 Three further acyclic polypropionate compounds siphonarienone 2, siphonarienedione 3 and siphonarienolone 4 were isolated from Canary Island specimens of S.grisea.12,13 Oxidative cleavage of 2–4 with HIO4–RuCl3 followed by esterification gave methyl (2S,4S,6S)-trimethylnonanoate as the major product and established the absolute configuration of these three compounds. 12 A recent diastereoselective synthesis of 2 further confirmed the absolute stereochemistry of this compound.14 The relative stereochemistry of the chiral secondary alcohol functionality in 4 was tentatively proposed from selected 1H NMR coupling constants which were similar to those of the acyclic ester 5 isolated from S.australis.15 The latter compound is the decomposition product of the hemiacetal 6 also found in this New Zealand species. Application of an exciton chirality CD method to p-bromobenzoate ester derivatives of the alcohol from 5 and two synthetic diastereomers of this compound,16 indicated that the C-4 and C-5 absolute stereochemistry was opposite to that originally proposed for 5 by Faulkner et al. Compounds 2, 3 (isolated as a C-4 epimeric mixture), 4, norsiphonarienone 7 and isosiphonarienolone 8 were recently obtained from S.pectinata collected near Cadiz, Spain.17 A paucity of 7 and 8 led to an assignment of absolute †E-mail: chdc@warthog.ru.ac.za ‡E-mail: garson@chemistry.uq.edu.au Davies-Coleman and Garson: Marine polypropionates 477stereochemistry based on biosynthetic arguments while the (3S)-stereochemistry in 8 was tenuously proposed from the H3–H4 coupling constant.Chilean specimens of S. lessoni yielded 9 and norsiphonariendione 10.18 The stereochemistry at C-6 in 9 and C-4 and C-8 in 10 was not established. Acyclic polypropionate metabolites are also found in other opisthobranch molluscs. Aglajne-1 11 was first isolated from the predatory Mediterranean mollusc Aglaja depicta19 and later from another opisthobranch, Bulla striata which forms part of the diet of A. depicta.20 The E-geometries of the olefinic bonds in 11 were inferred from the 13C NMR chemical shifts of the vinylic methyl groups while the (14S,16S)-configuration was assigned by oxidative degradation (ozonolysis followed by periodate oxidation) of 11 to (2S,4S)-dimethylhexanoic acid.20 The stereochemistry at C-4 and C-10 in 11 remains unassigned. Another related predatory mollusc, Philinopsis speciosa from Hawaii yielded two structural analogs of 11, niuhinone-A 12 and niuhinone B 13.21 The Z configuration of the ƒ14 olefin in 12 was proposed from the irreversible isomerization of this vinylic functionality to the less strained E-configuration on treatment of 12 with base followed by acid.Compound 13 was also isolated from the Californian opisthobranchs Navanax inermis and its prey Bulla gouldiana.22 Ozonolysis of the niuhinone B isolated from these two organisms, followed by NaBH4 reduction, yielded a 2-methylbutan-1-ol fragment containing the C-16 chiral centre of 13. Comparison of the 1H NMR spectrum of the (R)-MTPA derivative of this fragment with the 1H NMR spectrum of the analogous MTPA derivative of (S)-2-methylbutan-1-ol unequivocally established the (16S)-stereochemistry in 11. The cytotoxic nalodionol 14, which is isolated as an inseparable pair of epimers, was obtained from a 1995 collection of the cephalaspidean mollusc Smaragdinella calyculata; curiously, earlier collections of the mollusc had only yielded an aminoalkylpyridine, naloamine.23 2.2 Metabolites containing a furanone ring An NMR examination of an inseparable mixture of furanones from S.lessoni suggested that the mixture comprised siphonarienfuranone 15 and its Z-isomer 16.18,24 The stereochemistry of the acyclic side chain in these compounds was erroneously proposed as (8S,10R,12S) from oxidative ozonolysis studies of the mixture.24 Norte et al. also obtained 15 as an inseparable mixture from S. grisea and after establishing that 15 isomerised to 16 on standing, suggested that the duplication of signals in the NMR spectra of the mixture possibly reflected epimerization at C-2 and not isomerization of the ƒ6 olefin.12 Oxidative ozonolysis of the furanone mixture from S.grisea established the biosynthetically expected (S)-stereochemistry for each of the three chiral centres in the side chain of 15.12 Two separate mixtures of the C-2 epimers of 15 and 16 were recently isolated from S. pectinata.17 A mixture of aglajne-2 17 and two isolation artifacts, the C-10 epimers 18 and 19 of 6Z-aglajne-2, isolated from the molluscs Aglaja depicta and Bulla striata, were separated and identified as their acetate derivatives 20–22.20 The co-occurence of 17 and 12 in the molluscan extracts suggested a similar biosynthetic origin for these two compounds and a (14S,16S)-stereochemistry was thus tentatively proposed for 17.R3 R1 O R2 R4 R1 O R2 R3 O O O O H OH O O OH 6 5 1 R1 = H; R2 = a-Me; R3 = Me 2 R1 = Et; R2 = a-Me; R3 = Me 7 R1 = Me; R2 = a-Me; R3 = Me 9 R1 = Et; R2 = Me; R3 = H 8 3 R1 = O; R2 = b-Me; R3 = a-Me; R4 = Me 4 R1 = a-OH; R2 = b-Me; R3 = a-Me; R4 = Me 10 R1 = O; R2 = R3 = Me; R4 = H O O O R O O O O O OH 14 4 1 10 11 R = b-Me 13 D14, R = Me 12 14 16 14 O O O RO O R2 O O R1O HO O O HO O O 16 10 15 10 2 18 R1 = H; R2 = b-Me 19 R1 = H; R2 = a-Me 21 R1 = Ac; R2 = b-Me 22 R1 = Ac; R2 = a-Me 14 17 R = H 20 R = Ac 6 478 Natural Product Reports, 19982.3 Metabolites containing a 2-pyrone ring Ascoglossan molluscs from two families (Polybranchiidae and Stiligeridae) are a good source of polypropionate compounds constructed from five propionic units and containing either a 2-pyrone or 4-pyrone ring.The Australian species Cyerce nigricans (Polybranchiidae) yielded two polypropionate metabolites, one of which, 23 (containing a substituted 2-pyrone ring),25,26 was poorly characterised by NMR spectroscopy. An analogous series of compounds, the cyercenes 1–5 (24–28), were isolated from C.cristallina27,28 collected from the Bay of Naples, while a second Mediterranean species, Ercolania funeria (Stiligeridae) yielded two compounds, 29 and 30, isomeric with 27 and 28.29 Standard spectroscopic methods were used to determine the structures of 23–30 with the 13C NMR chemical shifts of the vinylic methyl carbons defining the geometry of the trisubstituted olefins in the side chains of these compounds. Two geometric isomers, diemenensin A 31 and B 32, were isolated from an Australian siphonariid mollusc, S.diemenensis. 30 The (10S,12S) absolute configuration in 31 was established in the usual manner. Ozonolysis of 31 followed by methylation of the ozonolysis products gave methyl (2S,4S)- dimethylheptanoate. Conversion of 32 to 31 via isomerization on standing determined the stereochemistry of the former compound. Pectinatone 33, the 8,9-dihydro analog of 31, is an ubiquitous polypropionate metabolite, occurring in four different Siphonaria species; S.pectinata,17,31 S. grisea,12 S. virgulata32 and S. concinna.33 The initial stereochemical studies on 33 suggested a (8S,10R,12S)-configuration for this compound. 31 An X-ray analysis of 33 however, showed that the relative stereochemistry of the three chiral centres were all the same, either all (S) or all (R).32 A simultaneous re-examination of the oxidative degradation products of 33 coupled with a second independent X-ray analysis unequivocally confirmed the biosynthetically favoured (8S,10S,12S)-stereochemistry of this compound.12 An (8S,10R,12S)-stereochemistry was originally proposed for norpectinatone 34,24 a co-metabolite of 33 in several siphonariid molluscs, but this was subsequently queried by Oppolzer who unambiguously synthesised the (8S,10R,12S)-isomer with diVerent chiroptic and proton NMR properties from the natural product.34 In view of the pectinatone studies, norpectinatone should be revised to the structure given here.32 Recently the 6Z-isomer of 33, isopectinatone 35, was also isolated from S.pectinata.17 2-Pyrone polypropionate compounds are also found in other opisthobranch molluscs. Aglajne-3 36 and 5,6- dehydroaglajne-3 37 were isolated from the predator/prey combinations of Aglaja depicta/Bulla striata and Navanax inermis/Bulla gouldiana respectively.20,22 Oxidative degradation studies confirmed the (S)-configuration at C-3 and C-5 in 36 and at C-5 in 37. The stereochemistry of the remaining chiral carbon in both these compounds is unassigned. 2.4 Metabolites containing a 4-pyrone ring The ascoglossan molluscs Cyerce nigricans, C. cristallina and Ercolania funera have yielded several polypropionate compounds containing a 4-pyrone ring, isomeric with the 2-pyrone series 23–30 also isolated from these three species. Compound 38 was found in extracts of C. nigricans25,26 while cyercenes A and B 39 and 40 were isolated from C. cristallina.28 E.funera yielded 40, 12-norcyercene B 41, 7-methylcyercene B 42, and 7-methyl-12-norcyercene B 43.29 The methodology used to establish the structures of 23–30 also proved eVective for the structural elucidation of 38–40. Four geometric isomers of cyercene A and 12-norcyercene A, placidenes A and B 44 and 45 and isoplacidenes A and B 46 and 47, were isolated from the Mediterranean ascoglossan species, Placida dendritica.35 NOE experiments confirmed the Z-configuration of the ƒ9 double bond in the side chains of 44–47 and the Z-configuration of the ƒ7 double bond in 46 and 47.Three polypropionate metabolites containing a dihydro-4- pyrone ring, the membrenones A–C 48–50, were obtained from the Mediterranean opisthobranch mollusc, Pleurobranchus membranaceus.36 The trans-stereochemistry of the two ring substituents in 48–50 was assigned from the large H-6–H-7 O O OMe R1 R4 R2 R3 O O OMe 23 24 R1 = R4 = Me; R2 = R3 = H 25 R1 = Me; R2 = R3 = H; R4 = Et 26 R1 = Me; R2 = R3 = H; R4 = PrI 27 R1 = H; R2 = R3 = R4 = Me 28 R1 = H; R2 = R3 = Me; R4 = Et 29 R1 = R3 = R4 = Me; R2 = H 30 R1 = R3 = Me; R2 = H; R4 = Et R2 R1 OH O O R OH O O R O O O OH 32 D8; R = Me 35 R = a-Me 10 8 31 D8; R1 = R2 = Me 33 R1 = a-Me; R2 = Me 34 R1 = a-Me; R2 = H 8 36 R = a-Me 37 D4; R = Me 5 3 O O R2 OMe R1 O O OMe R O R1 R4 R2 R3 O OMe 1 46 R = Et 47 R = Me 38 R1 = H; R2 = Et 44 R1 = Me; R2 = Et 45 R1 = Me; R2 = Me 7 9 39 R1 = R2 = R3 = Me; R4 = Et 40 R1 = Me; R2 = R3 = H; R4 = Et 41 R1 = Me; R2 = R3 = H; R4 = Me 42 R1 = R3 = Me; R2 = H; R4 = Et 43 R1 = R3 = R4 = Me; R2 = H Davies-Coleman and Garson: Marine polypropionates 479coupling constant (13.4 Hz) while the (R)-configuration of the acyl moiety followed from the LiAlH4 reduction of 48 which gave (R)-2-methylbutanol as one of the reduction products.The localization of 48–50 in the skin of this soft-bodied mollusc would suggest a chemical defence role for these compounds.Interestingly, membrenone C is structurally similar to vallartanones A 51 and B 52 obtained from another opisthobranch mollusc, Siphonaria maura, collected from Puerto Vallarta, Mexico.37 Hydrogenation of 51 gave 6,7- dihydrovallartanone A with the 4-pyrone ring still intact. The small H-7–H-8 coupling constant (4 Hz) in the 1H NMR spectrum of the latter compound was consistent with a dihedral angle between these two protons of 60o and suggested a (7S*,8R*)-relative stereochemistry for 6,7-dihydrovallartanone A.Although the negative split Cotton eVect in the CD spectrum of 51 was originally interpreted as supporting an (8R)-absolute stereochemistry for this compound,37 a recent synthesis of both the (8R)- and (8S)-epimers of 51 from (R)- and (S)-3-hydroxy-2-methylpropionate respectively, indicated that the original assignment was incorrect and should be reversed.38,39 The syntheses of 4-pyrone-containing polypropionate metabolites from marine molluscs have recently been comprehensively reviewed by Yamamura and Nishiyama.40 Specimens of S.maura from Costa Rica yielded four diastereomerically related bis-4-pyrone compounds, the maurapyrones A–D 53–56, as racemic mixtures.41 An X-ray analysis of a racemate of 53 confirmed the structure and relative stereochemistry of this compound while NMR data indicated that 54 was a racemic diastereomer of 53. Compounds 55 and 56 were similarly shown to be a pair of racemic diastereomers. A fifth polypropionate metabolite, maurenone 57, although isolated with 53–56, appeared to be unrelated to these four compounds.41 The baconipyrones A–D 58–61 from S.baconi collected from Melbourne, Australia are unusual because they do not contain the contiguous carbon skeleton normally associated with polypropionate metabolites.42 The structure of 59, and hence the structures of the remaining members of this series, was established from X-ray data which also provided the relative stereochemistry at the nine chiral centres in this compound. Faulkner and coworkers suggested that baconipyrone B could be derived biosynthetically from siphonarin A 62, also found in the S.baconi extract, by a retro-aldol mechanism.42 However a recent careful re-examination of S. baconi43 has failed to detect baconipyrones, which may therefore be artifacts of extraction rather than biosyntheticallyderived metabolites. The structure and relative stereochemistry of 62, previously isolated together with its higher homologue siphonarin B 63 from the Australian species S.zelandica and S. atra,44 was also determined by X-ray analysis. Two further closely related compounds, dihydrosiphonarin A and B 64 and 65, were obtained from S. normalis and S. lacinosa.44 Chromatography of the former extract yielded artifacts 66 and 67, probably formed via the retro-Claisen rearrangement of the ‚-diketones produced on the opening of the ketal rings of 64 and 65.44 An X-ray analysis45 of the p-bromophenylboronate derivative of 62 and the synthesis46 of the enantiomer of 67 confirmed that the absolute configuration for compounds 58–65 is as shown here and reversed the stereochemistry arbitrarily chosen in earlier studies. O O O O R1 R2 O O O O O O R1 R2 O O O O R O O O O H H O O OH H 1 6 48 R1 = a-Me; R2 = Me 49 R1 = R2 = H 51 R = Me 52 R = H 53 R1 = a-Me; R2 = Bu n 54 R1 = b-Me; R2 = Bu n 55 R1 = a-Me; R2 = Pri 56 R1 = b-Me; R2 = Pri 10 1 8 1 8 50 57 O O R O O OH O O O O O R O O OH O O HO 58 R = Et 59 R = Me 60 R = Et 61 R = Me O O O O R1 OH HO O O R O HO O OH O O O O OH O O O O O OH O O OH O HO2C O O HO R2 8 68 62 R1 = Me; R2 = O 63 R1 = Et; R2 = O 64 R1 = Me; R2 = b-OH 65 R1 = Et; R2 = b-OH 66 R = Me 67 R = Et 69 480 Natural Product Reports, 1998A collection of S.zelandica from Queensland in Australia yielded 62 and 63 and the tetracyclic metabolite caloundrin B 68.47 The structure of 68 followed from NMR, UV and MS data with NOE correlations defining the relative stereochemistry of the tricyclic acetal functionality.The configuration at the four acyclic, chiral centres in 68 were proposed from the following biosynthetic arguments.47,48 Caloundrin B is isomeric with 63 and the co-occurence of these two compounds suggested a common biosynthetic precursor 69 for 63 and 68. Although loss of the terminal carboxylate moiety in 69 followed by cyclization would theoretically give the C-8 epimer of 68, the epimerization prone position, between two carbonyl groups, of the corresponding stereogenic centre in 69 could explain this C-8 epimerization.47 Saponification of a mixture of bis-4-pyrone propionate esters isolated from the Hawaiian mollusc, Onchidium verrucalatum gave the triol ilikonapyrone 70.49 X-Ray analysis of the acetonide derivative of 70 established the structure of this compound and provided the relative stereochemistry of the seven asymmetric centres.Oxidative degradation of 70 with OsO4–NaIO4 followed by NaBH4 reduction gave the monopyrones 71 and 72. The single chiral centre in the former degradation product made this an attractive synthetic target to establish the absolute stereochemistry of 70. Accordingly, the synthesis of the enantiomer of 72 from commercially available methyl (S)-3-hydroxy-2-methylpropanoate confirmed an (R)- configuration for this compound and consequently a (3S,4S,10R,11R,12R,13R,16R)-stereochemistry for 70.50 The two diastereomers peroniatriols I and II 73 and 74 were obtained from the saponified extracts of another onchid mollusc Peronia peronii collected from Guam.51 The close structural relationship between 73 and 74 and ilikonapyrone facilitated interpretation of their spectral data.Ozonolysis followed by reductive work-up of both 73 and 74 gave monopyrones 75 and 76 from the former compound and 76 and 77 from the latter.From an examination of 1H NMR coupling constants Biskupiak and Ireland suggested that the peroniatriols diVered in their configurations at C-10 and C-13. While stereochemical relationships to the onchitriols 78 and 79 suggest that the latter assumption is correct,40 synthesis of both C-10 epimers of the degradation products 75 and 77 by Yamamura et al. unambiguously established that 73 and 74 diVer in the configuration at C-4 and not C-10 as originally proposed.52 The (14R,15R,16R)-absolute stereochemistry of the peroniatriols was determined from the synthesis of both enantiomers of 76, the common oxidative degradation product from both compounds.50 A New Caledonian pulmonate mollusc, Onchidium sp., yielded eight polypropionate derived esters 80–87 which were found to have in vitro activity against KB human epidermoid carcinoma cells.53 Saponification of these esters gave onchitriol I 78 and onchitriol II 79.Standard spectroscopic methods and similarities in NMR data between 78 and 79 and the closely related peroniatriols, established the structures of the former compounds and their naturally occurring esters.The relative stereochemistry of the 1,3-diol moieties in 78 and 79 was determined from comparison of the NMR coupling constants of the acetonide derivatives of these compounds with those obtained from molecular modelling studies of known compounds. Preparation of the mono-, tri- and di- (R)- and (S)-O-mandelate esters of 80, 78 and 84 respectively, in accordance with the Mosher–Trost method,54 suggested (3S,4S,10S,13R,14R,15R,16R)- and (3S,4S,10R,13S,14R,15R, 16R)-absolute configurations for 78 and 79 respectively.53,55 The proposed absolute stereochemistry of onchitriol II was confirmed by total synthesis.56 The concomitant synthesis of three diastereomers of onchiotriol II, one of which possessed the stereochemistry initially proposed for onchitriol I, indicated that the stereochemistry of 78 was incorrect and in need of revision.56 A subsequent total synthesis of 78 unequivocally showed that the configurations at C-4 and C-10 should be revised to (4R) and (10R) as shown here.57 Two cytotoxic polypropionates, the auripyrones A and B 88 and 89, were recently isolated from Japanese specimens of the sea hare Dolabella auricularia.58 HRFABMS and 2D NMR data were used to establish the structures of these compounds with NOE correlations and proton coupling constants defining the relative stereochemistry of the substituents around the bicyclic spiroacetal and the two adjoining acyclic chiral centres. The sacoglossan molluscs Tridachiella diomedea, commonly known as the Mexican Dancer, and Tridachia crispata, similar in appearance to T.diomedea, have proved to be a good source of polypropionate metabolites derived from the condensation of seven and eight propionate units respectively. The structure O O OH OH O O OH OH O O OH OH O O OH 3 70 71 72 O O OH OH O O OH R OH O O O O R2 OH R1 OH O O O O OR3 OR1 OR2 O O O O OR3 OR1 OR2 76 79 R1 = R2 = R3 = H 84 R1 = R2 = H; R3 = Ac 85 R1 = R2 = H; R3 = COEt 86 R1 = R2 = R3 = Ac 87 R1 = R3 = Ac; R2 = H 75 R = a-Me 77 R = b-Me 10 78 R1 = R2 = R3 = H 80 R1 = H; R2 = R3 = Ac 81 R1 = H; R2 = COEt; R3 = Ac 82 R1 = H; R2 = Ac; R3 = COEt 83 R1 = Ac; R2 = R3 = COEt 73 R1 = a-Me; R2 = b-OH 74 R1 = b-Me; R2 = a-OH 4 13 10 10 4 Davies-Coleman and Garson: Marine polypropionates 481and stereochemistry of tridachione 90, the major metabolite from T.diomedea, followed from an X-ray analysis of the isomeric ketone 91 formed from treating 90 with BF3–diethyl ether.59 The high stereospecificity of the cyclic epoxide rearrangement which yielded 91 suggested a 9,10-epoxy moiety in 90. An analogue of 90, 9,10-deoxytridachione 92, was subsequently isolated from specimens of T. diomedea collected in the Gulf of California.60 The structure of this latter compound followed from chemical interconversion with 90.Two major metabolites obtained from T. crispata were crispatone 93, whose structure and relative stereochemistry were determined from X-ray data,61 and the related unsaturated, dideoxy-compound crispatene 94.60 Both T. crispata and T. diomedea belong to a small group of molluscs containing functional chloroplasts and although diVerent polypropionates are found in each species, these compounds are thought to be biosynthetically related. Therefore, recognising the photosynthetic capabilities of T.diomedea and T. crispata, Faulkner et al. were able to show a photochemical relationship between the cyclohexa-1,3-diene ring systems of 92 and the bicyclo[3.1.0]hex-2-ene ring of 94 by obtaining photodeoxytridachione 95 from photolysis of 92 in benzene.60 The in vivo photochemical conversion of 92 to 95 in the Hawaiian sacoglossan, Placobranchus ocellatus, was confirmed by Ireland and Scheuer.62 Reflecting the biogeographical variation in polypropionate metabolites from T. crispata, 93 and 94 plus eight minor compounds were obtained from Jamaican specimens of this species.63 Tridachiapyrone-A 96 and isotridachiapyrone-A 97 diVer from 92 by possessing an additional propionate unit in their side chains.The diVerent 1H NMR chemical shifts of the C-14 methyl group in 96 and 97 suggested that these two compounds were epimeric at this position. A similar argument was used to confirm the C-14 epimeric relationship between tridachiapyrone B 98 and isotridachiapyrone B 99.The C-14 stereochemistry of tridachiapyrone C 106, the 9,10-epoxy-11- epi-analogue of 96, was proposed from the biosynthetic relationship between this compound and 94. Tridachiapyrone D 101 is the ƒ12,23, 13-hydroxy derivative of 96 whereas tridachiapyrones E and F 102 and 103 are structurally similar to crispatene and crispatone respectively with one propionate O O O O RO O O O 88 R = 89 R = O O OMe O O O OMe O O O OMe O O O O OMe R2 R1 OMe O O R O O O O OMe R O 98 R = b-Me 99 R = a-Me 14 93 9 91 90 92 D9 96 R = b-Me 97 R = a-Me 94 R1 = C2H5CO; R2 = Me 95 R1 = H; R2 = Me 100 R1 = R2 = H O O O OMe HO O O O O OMe O OMe O O O O OMe O O O O OMe O O O OMe O O O OMe 12 102 106 101 13 23 103 13 104 107 105 6 482 Natural Product Reports, 1998unit less in their side chains.The opposite stereochemistry at C-10 in 103 with respect to 93 followed from the large H-10–H-11 coupling constant observed in the 1H NMR spectrum of 101.Compounds 93, 94, 96, 98 and 101 were found to be active against lymphocytic leukemia.63 Specimens of T. crispata collected oV the coast of Venezuela and Mexico have yielded 93–95 and tridachiahydropyrone 104.64,65 While standard spectroscopic methods were used to establish the structure and stereochemistry of 104, the position of the methyl group at C-13 in this compound is unusual and contradicts the accepted biogenetic propionate rule; an alternative chain starter unit may thus be involved.Surprisingly, the Canadian mollusc, Elysia chlorotica, yielded the enantiomer of 92 and elysione 105 which is a stereoisomer of tridachiapyrone A and isotridachiapyrone A.66 Elysione has also been isolated from the Mediterranean E. viridia. and has been suggested to have (14S)-stereochemistry. 67 While the occurrence of enantiomers in Nature is not unknown, it is certainly unusual. 15-Norphotodeoxytridachione 100, 9,10-deoxyisotridachione 107, 92 and 95 were isolated from the Mediterranean sacoglossan E. timida.68 The presence of these compounds in the mucous secretions of E. timida and E. viridens, coupled with their ichthyotoxic properties, suggest that these metabolites protect the molluscs against predation. 2.5 Metabolites containing a hemiacetal ring One of the simplest examples of a marine polypropionate metabolite containing a hemiacetal ring is compound 7 isolated from Siphonaria australis.15,16 Two related compounds, denticulatins A and B 108 and 109 were obtained from another Australian Siphonariid species, S.denticulata.69 The structure of the denticulatins was established from an X-ray analysis of 109. The C-10 epimeric relationship between these two compounds was confirmed by chemical conversion, via the proposed intermediate 110, of a basic solution of 109 into a solution containing a 1:1 mixture of 108 and 109.Prolonged treatment of the denticulatins with DBU gave the known ketone 111 which established an (R)-stereochemistry at C-12 in 108 and 109 and, together with the X-ray data, defined the absolute configuration of both compounds. The contiguous chiral centres in the denticulatins have made these compounds popular synthetic targets with four independent groups tackling their total syntheses. Ziegler and Becker prepared the protected open chain precursor 112 of the denticulatins via an aldol condensation of the ketone 111 and the keto aldehyde 113.Cyclization upon mild acid hydrolysis of 112 yielded a mixture of 108 and 109 in low yield.70 HoVmann and co-workers intuitively predicted that the protected denticulatin precursor 114, synthesised from two key components 115 and 111, would spontaneously isomerise to give the more thermodynamically favoured denticulatins upon deprotection. 71,72 The former aldehyde component was prepared in three stereoselective consecutive steps using chiral allyl boronates while the latter ketone was obtained via a kinetic Sharpless resolution of the allylic alcohol, (3R)-2-methylpent- 1-en-3-ol, followed by an Ireland–Claisen rearrangement.Unfortunately, while deprotection of 114 resulted in the anticipated isomerisation, unwanted epimerization at C-10 also occurred to give a mixture of the denticulatins. Conversely, Paterson and Perkins were able to achieve eYcient cyclization following deprotection of the di-tertbutylsilyl derivative 116 to give 109 without compromising the C-10 stereochemistry through epimerization.73,74 Novel boron mediated aldol/reduction and titanium mediated aldol coupling methodology were instrumental in achieving Paterson and Perkins’ other goal, namely to eVect an eYcient substrate based controlled synthesis of 116.Epimerization of 116 to yield 117, prior to deprotection, provided a convenient route to denticulatin A. While the final step in de Brabander and Oppolzer’s synthesis of denticulatin A and B involved a similar deprotection (with improved yields) of 116 and 117, their approach to these precursors, from the meso-dialdehyde 118, diVered from that adopted previously.75,76 The coupling of 118 with the chiral auxiliary, bornane sultam 119, to give 120 proved to be the key step in their synthesis and involved a novel group selective aldolization of 118 in which the inherent chirality present in 119 was successfully transmitted to five asymmetric centres in 120.Muamvatin 121, isolated from the Fijian mollusc, Siphonaria normalis, was the first reported natural product containing an unusual 2,4,6-trioxaadamantane ring skeleton.77 Standard spectroscopic methods were used to establish the structure of this compound with proton coupling constants, in tandem with a 2D NOE experiment, defining the relative stereochemistry of the trioxaadamantane ring. PCC Oxidation of 121 gave the optically active product 122 which was later synthesised by HoVmann and Dahmann.78,79 Although the absolute configuration at C-10 and of the trioxaadamantane O OH H OH R O O OH O O O– OH OHC O O O O MeO- p-C6H4 R1 O OR2 OR3 O O 108 R = a-Me 109 R = b-Me 1 10 12 111 113 110 1 10 p-C6H4OMe Si But But Si But But O H OH O- p-C6H4OMe O O O H OH O- p-C6H4OMe OHC OHC CHO OTBS O2 S N SO2 N O O OTBS OH O 112 R1 = Me; R2 + R3 = 116 R1 = b-Me; R2 + R3 = 117 R1 = a-Me; R2 + R3 = 114 10 115 118 119 120 Davies-Coleman and Garson: Marine polypropionates 483ring was determined by this synthesis, the stereochemistry at C-11 proved elusive.However, a concurrent independent synthesis of a 17:1 mixture of 123 and 121 by addition of the dienyllithium derivative of the iodide 124 to 122 established an (11S)-configuration for 121.80 Catalytic oxidation of 123 followed by stereocontrolled reduction of the resultant dienone with DIBAL quantitatively converted 123 to 121.Specimens of S. funiculata, collected oV the coast of Queensland, Australia yielded funiculatin A 125 and trace amounts of the C-10 epimer of this compound, funiculatin B.47 Spectroscopic methods were used to establish the structures of these compounds with proton coupling data, NOE diVerence and NOESY experiments providing the stereochemistry of the bicyclic acetal. The absolute configurations at all of the stereocentres with the exception of C-10 followed from the serendipitous conversion of denticulatin A to 125 via an attempted dihydroxylation of the former compound using the Sharpless procedure.The rearrangement was triggered by the K2CO3 present in the reaction mixture. The uncertainty of the C-10 stereochemistry arises from its labile position in the proposed ‚-diketone precursor 126 of funiculatin A. The structure and relative stereochemistry of dolabriferol 127 isolated from parapodia of the Cuban mollusc Dolabrifera dolabrifera was established from X-ray data.81 The noncontiguous carbon skeleton of 127 is unusual in polypropionate metabolites.42 This is the first reported natural product investigation of the opisthobranch mollusc family Dolabriferidae and suggests that this family may also prove to be a good source of new polypropionate metabolites. In summary, polypropionate metabolites have been isolated from four of the eight orders of opisthobranch molluscs (Anaspidea, Cephalaspidea, Notaspidea and Sacoglossa) and in two orders from the Pulmonata (families Siphonariacea and Onchidiacea). 3 Polypropionate metabolites from other marine sources The metabolites covered in this section are chosen to illustrate structural diversity and biological activity and because their biosyntheses may involve propionate. The octalins A and B, 128 and 129, are cytotoxic acetogenins isolated from a Streptomyces sp.82 and maduralide 130 is a complex glycoside macrolide isolated from an actinomycete,83 while unidentified marine sediment bacteria produce the cytotoxic lagunapyrones A–C 131–133,84 and the elijopyrones A–D 134–137.85 Salt water culture of an unidentified fungus isolated from the sponge Stylotella sp.has yielded the nectriapyrones A and B 138 and 139 which have close structural similarity to products from terrestrial fungi.86 A key biosynthetic diVerence between terrestrial bacteria and fungi is in their use of propionate and acetate-methionine respectively as precursors for C3 units.Although bacteria use propionate for macrolide and polyether synthesis, there are very few documented examples of propionate-based biosynthesis in fungal metabolites;87,88 whether such a distinction also separates marine fungi and bacteria remains to be established. Although many products of dinoflagellate and cyanobacterial metabolism have highly methylated carbon skeletons, biosynthetic studies have so far indicated a polyacetate rather than a polypropionate pathway.Examples of metabolites include okadaic acid 140, the amphidinolides e.g. 141, tolytoxin 142 and microcystin-LR 143.9 Stable isotope labelling of DTX-4 toxin 144 produced by Prorocentrum lima establishes that the five pendant methyls all derive from the methyl group of acetate (perhaps via an aldol-type condensation of malonate on to a keto functionality);89 labelling studies with the amphidinolides are also consistent with a similar biosynthetic pathway.90 In tolytoxin, the seven methyl-derived substituents all originate from methionine,91 while the unusual O O O HO R O O O HO CHO I O O H O HO O O OH O O O OH O O OH O 124 10 125 122 11 121 R = b-OH 123 R = a-OH 126 127 O O HO O OH O O HO OH O O R OH OH OH O O MeO R1 R2 O O R O OH O OH O O O O OH O OH OH OMe O O 131 R = Me 132 R = Pr n 133 R = Bu n 138 R1 = H; R2 = CMe=CHMe ( Z) 139 R1 = Me; R2 = Et 134 R = CH(OH)Me 135 R = Et 136 R = COMe 137 R = CH=CH2 128 130 129 484 Natural Product Reports, 19982,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (Adda) moiety found in 143 and related cyanobacterial products is built up from acetate, methionine and phenylalanine.92 The Porifera are an important source of many interesting and bioactive natural products, many of which contain some circumstantial evidence of an acetate-methionine or propionate motif in their structures. Metabolites such as orbiculamide A, halichondramide, pateamine, theonezolide, tedanolide, misakinolide, the spongistatins, tedanolide, the calyculins, calyculinamides, and the halichondrins demonstrate between them quite varied levels of methylation along the carbon backbone.9 Swinholide A 145 is a highly methylated macrolide from Theonella swinhoei93 which has recently been synthesised using propionate-based methodology,94 and which is known to be produced by bacterial symbionts within the sponge tissue.95 If the bacterial pathway is followed, then swinholide A may perhaps be biosynthesised via a mixed propionate–acetate pathway.Callystatin A 146 is a highly methylated cytotoxic pyrone produced by the sponge Callyspongia truncata.96 Pteroenone 147, the defensive metabolite of the Antarctic pteropod Clione antarctica, may be an unusual example of propionate metabolism; its biosynthesis is proposed to involve either (i) an acetate chain starter plus three propionates plus two acetates (not butyrate as stated by the authors) as terminating group or (ii) a butyrate starter unit plus three propionates and an acetate terminating group.97 Doliculide 148 is a cytotoxic cyclodepsipeptide isolated from the sea hare Dolabella auricularia. 98 It appears to be a product of mixed peptide-polyketide origin; in view of the isolation from D.auricularia of the auripyrones 88 and 89,58 it is interesting to speculate whether the polyketide component is in part propionate-derived. 4 Biosynthesis of marine polypropionates 4.1 Overview of biosynthetic processes in molluscs This topic was last reviewed in 1993 as part of a survey of biosynthetic processes in marine molluscs.99 From a chemical O O RO O OH OH H O O O OH O O OH H OH HO H2C O OH O OSO3H OSO3H OH OH OSO3H O OH OMe Me OMe O O O OMe OH OMe O N Me O H H O O OMe HN CO2H O N NH NH NH NH2 O HN HN O O O CO2 H O HN O 140 R = H 144 R = 141 142 143 O O O OMe OH OH OH O OMe O O MeO OH OH OH O O HO OMe 145 O O OH O O OH NH O O O MeN OH O I HO 148 146 147 Davies-Coleman and Garson: Marine polypropionates 485perspective, the most intensively studied molluscs have been the opisthobranchs which show a rich variety of secondary metabolites, many of which appear to be terpenoid in origin. Given their pronounced biological activity, it is not surprising that experimental attention has focused primarily on the biosynthesis of terpenes in nudibranch molluscs, and on whether the metabolites derive from dietary sources or are products of de novo biosynthesis.99–101 It is useful to summarise the current status of marine polypropionate formation in the context of the better understood nudibranch biosynthesis.In an early study using 14C-labelled mevalonate, Cimino and co-workers demonstrated that some nudibranchs can synthesise terpenoid metabolites,102,103 but further research has indicated limited success in demonstrating de novo biosynthesis in nudibranchs.104–107 More recently, 13C-labelling studies have provided the mechanistic basis of de novo biosynthesis of sesquiterpenoid,108,109 diterpenoid,109 and polyketide110, metabolites in nudibranchs.Other nudibranch species, notably the Phyllidiidae from predominantly tropical waters, acquire terpenes from dietary sponge sources.99,107,112,113 The phyllidids, for example, frequently uptake isonitrile- or isothiocyanate-substituted terpenes.112,114 Numerous comparative structural studies on the chemical composition of sponge samples and their nudibranch predators reveal the uptake of sponge metabolites.Sponge metabolites can be selectively transferred or are subject to chemical modification by the nudibranch.99,106,112,113 Two experimental studies have elegantly demonstrated transfer of sponge metabolites to nudibranchs. The nudibranch Hyperselodoris webbi, which normally feeds on sponges containing longifolin was shown to acquire terpene metabolites such as the furodysin and the spiniferins from the sponges Dysidea fragilis and Pleraplysilla spinifera when supplied with these alternative sponge diets.115 Specimens of Phyllidiella pustulosa, when placed on samples of Acanthella cavernosa labelled with 14C cyanide or thiocyanate, acquired radioactive terpenes from their sponge hosts.In a separate 14C labelling study, the nudibranchs were shown to be incapable of de novo biosynthesis.114,116 In general, nudibranchs dependent on a sponge diet show variation in their chemical constituents from one collection site to another, whereas nudibranchs that synthesise compounds de novo contain the same metabolites irrespective of collection site.117,118 So far, it seems that temperate nudibranchs are more likely to carry out de novo biosynthesis than tropical specimens. 4.2 Polypropionate biosynthesis in marine molluscs Marine molluscs can be exclusively herbivorous (sacoglossans, pulmonates, anaspideans), feeding on algae or cyanobacteria, or carnivorous (some cephalaspideans, nudibranchs), feeding on other smaller molluscs or invertebrates such as sponges and bryozoans, as well as ascidians.Since metabolites containing polypropionate motifs have not yet been found in algae, bryozoans or ascidians, and their occurrence in sponges is restricted, it would thus appear that these characteristic molluscal metabolites are likely to be biosynthesised de novo; to date, experimental evidence is however limited. Chloroplasts are functional symbionts in herbivorous opisthobranchs of the order Sacoglossa. The chloroplasts carry out photosynthesis within the animal and organic carbon is fixed then transported to various parts of the animal, notably the pedal glands which are involved in mucus secretion.The fixed carbon appears in a wide variety of primary metabolites including lipids and sugars. Various members of the Elysioidea and Polybranchioidea superfamilies have been studied, including Tridachia (=Elysia65) crispata, Tridachiella diomedea, Placobranchus ianthobapsus (=ocellatus99), Elysia hedgpethi, and Placida dendritica.99,119–123 In a pioneering marine biosynthetic study in 1979, Ireland and Scheuer placed the sacoglossan P.ocellatus in seawater containing [14C] sodium hydrogen carbonate, and isolated [14C] labelled 9,10-deoxytridachione 92 and photodeoxytridachione 95.62 The data were consistent with an in vivo non-enzymic photoconversion of 92 to 95 triggered when animals were exposed to excess UV radiation. In biosynthetic experiments with Elysia viridis, sodium [1-14C]propionate was supplied by injection into the hepatopancreas or by addition to the aquarium water.Animals were incubated for 1–3 days after which time labelled elysione (105) was isolated and converted by NaBH4 reduction to a radioactive alcohol, confirming that the labelling was genuine.67 This experiment suggests the probable metabolic link between the photosynthetic activity detected in sacoglossans and their polypropionate metabolites.Photosynthetically-derived sugars can be converted to propionate via the TCA cycle and succinate. Attempts to repeat this propionate labelling experiment in Elysia timida, and to test the putative role of acetate in the biosynthesis of 15-norphotodeoxytridachione (100) were unsuccessful;67,68 in each experiment, the polypropionate fraction containing metabolites 92, 95, 100 and 107 was labelled, however attempts at purification then led to loss of radioactivity.No algal terpenoid metabolites have been found in the tissues of E. viridis and E. timida, unlike their congener E. translucens.67 Biosynthetic studies have been reported on other sacoglossans with mixed results. Slugs of the genus Cyerce are the only polybranchoid molluscs which do not ingest functional chloroplasts from their algal diet,99 yet two species have been reported to contain polypropionates.26–28 An interesting structural feature is the absence of methyl groups at some sites in the molecules, which could be explained by: (i) demethylation of a polypropionate intermediate;27 (ii) use of a standard acetate pathway with introduction of methyl groups at selected sites by use of the C1-tetrahydrofolate metabolism; (iii) a combined propionate-acetate pathway.These last two biosynthetic possibilities have not been adequately considered in the literature. Cyerce crystallina is a brightly-coloured sacoglossan that associates with the alga Chlorodesmis fastigiata and which contains the cyercenes 24–28 and 39–40; all these metabolites except cyercene A (39)27–28 contain some non-methylated centres.When the animal is attacked, it defends itself by detaching dorsal appendages (cerata) which continue writhing, and produce mucus. The mollusc then regenerates cerata over a 5–6 day period. The polypropionate compounds are variously localised in the mantle tissue, the cerata, and in the mucus extract produced by the animals when molested, but not in the digestive gland.27 This tissue distribution is consistent with de novo biosynthesis.It was first considered that the cyercenes were products of propionate metabolism, with demethylation at selected sites (C-3, C-5, C-7 or C-11) occuring during the late stages of biosynthesis.27 Specimens of C. crystallina which lacked cerata were placed in seawater containing [2-14C]propionate for 5 days, then the freshly-formed cerata removed and extracted.The seven cyercene metabolites were all found to be radioactive when isolated by reversedphase HPLC, with the highest incorporation into cyercene A 39 and the lowest into cyercene 1 24. The authors ascribed the high level of labelling in 39 to its role as a growth-inducing agent in the cerata. The results are consistent with the operation of a propionate-based pathway rather than an acetate plus C1 route.Chemical degradation of cyercenes 1 or 4 (24 or 28) by ozonolysis would test the unusual demethylation proposal. Additionally, an acetate labelling experiment could provide an alternative biosynthetic origin for the terminal unit in these compounds. The cyercenes appear to be examples of compounds formed from mixed propionate-acetate biosynthesis with occasional use of an isobutyrate unit. Cyercene A 39 can formally derive from five propionate units, while cyercene 5 (28) has four propionate units and an acetate chain terminating unit.Cyercenes 2 (25), 4 (27) and B (40) contain three propionates together with two acetates as either a chain starter unit or chain building unit, while cyercene 1 (24) and cyercene 3 (26) have two propionate units. Cyercene 1 has three acetate units and cyercene 3 has two acetates and an isobutyrate chain 486 Natural Product Reports, 1998starter unit. The relative incorporations of [2-14C]propionate into cyercene A and cyercene 1 is in accordance with the number of propionate units each metabolite contains. Given the absence of functional chloroplasts, the biosynthetic origin of the propionate used by the animals is of interest.It has not yet been determined whether the congeneric species C. nigricans, which contains the polypropionates 23 and 38,26 manufactures these metabolites de novo. The stiligerid sacoglossan Ercolania funerea contains cyercene B (40) together with its 12-demethyl, 7-methyl and 7-methyl-12- demethyl analogues, (41), (42) and (43), and 7-methylcyercene- 2 (29) and -1 (30).29 When an incubation experiment using [1-14C]propionate was carried out with cerata-free specimens of E.funerea for 8 days, radioactivity was associated with the pyrone fraction; individual compounds were not isolated because of the small amount of extract. The data suggest that E. funerea, like C. crystallina, is capable of de novo biosynthesis using a propionate pathway.Again the precursor status of acetate in this presumably mixed biosynthesis requires testing, since metabolites 29, 41 and 43 appear to derive from use of an acetate chain starter unit instead of propionate. The cyercenelike metabolites of Placida dendritica,35 a mollusc which contains functional chloroplasts which quickly lose their photosynthetic capacity,123 also warrant biosynthetic attention. Placidene-A 44 and isoplacidene-A 46 contain five propionate units, while placidene-B 45 and the isomeric isoplacidene-B 47 appear to use an acetate chain starter unit and four propionate-derived units.Consistent with a probable de novo origin, these compounds are not found in the algal diet (Bryopsis plumosa) of the mollusc. Gavagnin et al. have traced a hypothetic evolutionary path for the two sacoglossan families Oxynoidea and Polybranchioidea, in which the more ancestral species are chemically related to their algal diets, whereas more evolved species are capable of de novo biosynthesis.67 According to their scheme, the Polybranchioidea (e.g.Cyerce, Ercolania) and some species of Elysia (timida, viridis) would produce polypropionates whereas oxynoid molluscs and other species of Elysia (translucens) acquire dietary metabolites. Molluscs that can produce their own defensive allomones are clearly advantaged over those dependent on a dietary source of metabolites. The first studies on marine polypropionate biosynthesis investigated metabolites from siphonariid limpets and con- firmed the operation of a polypropionate pathway rather than an acetate-methionine route.Pulmonates of the genus Siphonaria are air-breathing, intertidal molluscs which may represent an evolutionary link between marine and terrestrial gastropods. Although siphonariid limpets feed by moving around rock platforms and pools, grazing on algae at low tides, there is no evidence that they assimilate algal metabolites or acquire chloroplasts. The denticulatins A and B, 108 and 109, were found to incorporate radioactive label after a six day incubation period when [1-14C]propionate was injected into specimens of S.denticulata; in contrast, incorporation of [1-14C]acetate into these animals did not provide significantly labelled denticulatins. These data were consistent with a propionate-derived biosynthesis rather than the alternative addition of methyl groups from C1-tetrahydrofolate metabolism onto a polyacetate backbone (Scheme 1).Kuhn–Roth degradation revealed that only the carbons anticipated to originate from C-1 of propionate were labelled, but these data did not demonstrate a uniform distribution of 14C label along the carbon chain. An alternative incorporation protocol, less injurious to the limpets, involved adding the precursor directly to the aquarium water from where it was absorbed directly through the skin tissue.124 Injection of [1-14C]propionate into the foot tissue of S.zelandica followed by a four day incubation gave radiolabelled siphonarins A and B 62 and 63 confirming their propionate origin.45,125 The lower homologue, siphonarin A 62 was shown to be assembled from an acetate chain starter unit with nine propionate-derived chainbuilding units by determining that C-19 was not labelled by [1-14C]propionate (Scheme 2). The selective isolation of C-19 plus an attached methyl carbon (as p-bromophenacyl acetate) was achieved by ozonolysis, hydrolysis of the ensuing anhydride, and derivatisation.45 Furthermore, acetate was utilised by S.zelandica for the synthesis of 62. The role of succinate in furnishing propionate units was consistent with the presence of a functioning methylmalonyl mutase in these molluscs.45,125 In summary, all marine molluscs studied to date use a C3-based de novo pathway rather than a C2 plus C1 route; in some molluscs, the C3 precursor may be ultimately derived from photosynthetically-fixed carbon.Precursors used in labelling experiments can be injected into the mantle tissue or hepatopancreas of the molluscs or taken up by absorption through the skin. Carnivorous molluscs which feed on other propionatecontaining molluscs can acquire metabolites by dietary transfer; unlike the nudibranchia, there is as yet no experimental evidence (other than the isolation of metabolites) to support the dietary origin of the polypropionate compounds 12–14, 17–19, and 36–37 in carnivorous molluscs such as Aglaja,19,20 Philinopsis,21 and Navanax.22 The recent isolation of two diVerent types of metabolites, the polypropionate 5 and an aminoalkylpyridine, from two collections of the herbivorous cephalaspidean Smaragdinella calyculata23 raises an intriguing question as to the origin of the polypropionate metabolite.The O O OH HO O H O ONa O OR 108/109 1 • 17 • • • • • • • • R = –CH2COC6H4- p-Br i = 14C • S. denticulata 6-7 days Scheme 1 The biosynthesis of the denticulatins.Reagents and conditions: (i) CrO3–H2SO4 then p-Br-C6H4COCH2Br, EtOH–H2O 9:1. O O O OH O O Me O OH O O O O O O Me HO2C O O O * * * CH3CO2R + 8 CH3CO2R CH3CO2R • • • • • • • • and • i ii S. zelandica 4 days 62 R = –CH2COC6H4- p-Br * CH3CO2Na CH3CH2CO2Na Scheme 2 The biosynthesis of siphonarin A. Reagents and conditions: (i) CrO3–H2SO4 then p-Br-C6H4COCH2Br, EtOH–H2O 9:1; (ii) O3, DCM, "78)C, then H2O, then p-Br-C6H4COCH2Br, EtOH–H2O 9:1.Davies-Coleman and Garson: Marine polypropionates 487origin of the polypropionate metabolites 88, 89 and 127 from the herbivorous anaspidean molluscs Dolabella auricularia58 and Dolabrifera dolabrifera,81 and 48–50 from the carnivorous notaspidean Pleurobranchus membranaceus35is as yet completely unexplained. 4.3 The biosynthetic status of marine polypropionate metabolites The formation of ‘metabolites’ such as muamvatin, the siphonarins and the denticulatins reflects thermodynamic factors which dictate the direction of ring closure.47,48 Energy calculations show that the preferred cyclisation mode for the siphonarin B 63 vs.caloundrin B 68 is dictated by the syn vs. anti relationship between adjacent methyls in the precursor 69 and its C8 epimer 149 (Scheme 3).48 The alternative cyclisation products 150 and 151 have not been isolated. Likewise, the denticulatins are likely to be the preferred cyclisation products of the acyclic precursor 152 (Scheme 4); three of the four total syntheses of the denticulatins in the literature utilise the thermodynamic cyclisation of deprotected open chain precursors such as 112, 116 or 117,70,73–76 while a fourth relies on spontaneous isomerisation of an alternative hemiacetal ring system 114 to a more stable ring system.71,72 Natural products containing a 4-pyrone ring are likely to derive spontaneously from an acyclic triketone which may easily undergo ring closure during isolation and chromatography on silica, hence the siphonarins, the baconipyrones, caloundrin, vallartanones, maurenones, cyercenes, illikonapyrone, the peroniatriols and other pyrone-based marine metabolites are unlikely to be genuine natural products.Once formed, the cyclic polypropionates also undergo some non-enzymatic reactions which are characteristic of ‚-dicarbonyl and related systems, and which thereby confuse the structural nature of the biosynthetic products.Hemiacetal-containing polypropionates dehydrate to dihydropyrones under acidic, basic or buVered conditions,16,70–72,74,80 hence the status of products such as the membrenones and maurenone is also suspect. Retrohemiketalisation via a ‚-diketone enolate occurs on brief treatment of polypropionates such as the denticulatins with mild base and provides epimeric mixtures.47,73,74,76 Retro-aldol or retro-Claisen rearrangements can also occur,15,16,36,42,44,81 yielding modified polypropionates such as 6, 48, 49, 58–61, 66, 67 or 127, whose natural product status must again be viewed with suspicion.Some of the non-enzymic transformations have been readily mimicked in the laboratory. 16,43,47,74 These facile rearrangements may explain the lack of biological activity, either phamacological or ecological, associated with many of the polypropionates isolated from marine molluscs (see below). Definition of the structures and stereochemistry of the intended polypropionate biosynthetic products remains a considerable challenge to the organic chemist.As explored below, biosynthetic considerations may facilitate structural definition of the true metabolites. 4.4 A comparison of polypropionate biosynthesis in marine molluscs and in terrestrial bacteria Contemporary polyketide research is an impressive crossfertilisation of chemistry, genetics and molecular biology.2,3,5–7 The molecular basis of aromatic polyketide, polyether and macrolide biosynthesis is under active investigation; the systems which are most closely understood are those that produce the erythromycin4,8,126–128 and actinorhodin2,3,5,127 and tetracenomycin5,129 antibiotics, and the immunosuppressant rapamycin.8 In spite of the obvious structural dissimilarity, there are remarkable enzymic similarities between polyketide biosynthesis, which occurs primarily in bacteria and fungi, and the synthesis of fatty acids which is ubiquitous.1 So-called type I fatty acid synthases (FAS), present in fungi and vertebrates, O O O OH O O O OH O OH HO O O O O O HO O O O O OH O O O OH O O O OH O O OH O O O O HO O OH HO O O O O O O O O OH O O O OH 1 8 21 1 8 21 63 1 69 21 epimerise C-8 68 1 21 21 1 150 149 151 1 21 Scheme 3 Cyclisation preferences for acyclic siphonariid precursors O O OH HO O H HO O O OH OH O O 17 1 1 152 108 17 Scheme 4 Cyclisation mode of denticulatin A precursor 488 Natural Product Reports, 1998consist of a set of large multifunctional proteins with a separate catalytic activity for each discrete biochemical step, while type II fatty acid synthases repetitively use a set of discrete, dissociable enzymes, clustered together as a multienzyme complex, to make fatty acids in bacteria and plants;130 however both types of FAS may occur within the same organism.130,131 The aromatic polyketides, such as the actinorhodin and tetracenomycin antibiotics of Streptomycetes, are assembled using polyketide synthase (PKS) enzymes which resemble the bacterial FASs and are therefore referred to as type II PKSs.PKSs which resemble type I FASs have been found in fungi and, surprisingly, in some bacteria.5,7 Key bacterial polyketides, such as the commercially-important erythromycin, avermectin and rapamycin are assembled using type I PKSs, as are the fungal products 6-methylsalicylic acid, sterigmatocystin and mevinolin.7 The Hutchinson132 and Cane133 research groups simultaneously demonstrated the so-called processive nature of bacterial type I polyketide biosynthesis; it is now firmly established that the functionality of each newly-introduced propionate unit is adjusted before addition of the next propionate building block.134 The catalytic system for erythromycin biosynthesis consists of three multifunctional proteins (DEBS1, DEBS2 and DEBS3), each one of which consists of two modules which each have catalytic activity corresponding to one cycle of the condensation process (Fig. 1). In each of the six condensation cycles, catalytic steps include some or all of the following: (i) transacylation (catalysed by an acyltransferase enzyme; AT) in which a methylmalonyl unit is loaded onto a panthotheinylated-acyl carrier protein domain (ACP); (ii) ‚-keto thioester formation (ketosynthase; KS) in which a propionyl starter unit or intermediate polypropionate is acylated, with decarboxylation, by a methylmalonyl acyl carrier protein (ACP); (iii) keto reduction (ketoreductase; KR); (iv) dehydration (enoyl dehydratase; DH) and (v) enoyl reduction (enoyl reductase; ER), leading to either a keto, hydroxy, alkene, or methylene centre (Fig. 1). The PKS is programmed like an assembly line with the growing polypropionate chain passed from one catalytic site to the next.127,128 The chain-building process is terminated by a thioesterase (TE), giving 6-deoxyerythronolide B 153, which is subjected to enzymic modification by discrete oxidases or glycosidases to give the erythromycin antibiotics exemplified here by erythromycin A 154. Genes coding for erythromycin production have been cloned and expressed in heterologous hosts,135–136 while combinatorial engineering has produced non-natural analogues of commercially-important polyketide products in normal or heterologous hosts.137–149 The reader is referred to some excellent reviews for further details.4,6,137 The Celmer model for macrolide antibiotics such as erythromycin140 and the Cane–Celmer–Westley PAPA model for polyethers141 explore structural and stereochemical trends among these diverse antibiotics.When compared to each other, the polyether and macrolide models show significant regions of structural and stereochemical homology, consistent with a common evolutionary origin.1,142 Configurational models have also been developed for the siphonariid polypropionates. 48 The 2-pyrone metabolites of siphonariids are all characterised by (S)-stereochemistry at the contiguous methylsubstituted centres of the alkenyl chain; indeed this proposed homology guided a reinvestigation of the relative stereochemistry of pectinatone 33.48 The siphonarin metabolites 62 and 63 have been shown to derive from the linear combination of one acetate and nine propionates or ten propionates respectively.The acyclic precursor 155 which generates 62 and 63 was defined by precursor labelling studies using [1-14C]propionate, assuming that a processive mode of biosynthesis operates.45 The siphonarin configurational model was then extended to the denticulatins 108 and 109 and to muamvatin 121 whose chain building patterns were not known. Acyclic precursors 152 (leading to 108 and 109) and 156 (leading to 121) were suggested, guided by some biosynthetic considerations, namely (i) maximum number of (R)-stereocentres at secondary hydroxy sites as demonstrated in fatty acid biosynthesis;126– 128,130 (ii) structural variation at the beginning of the polyketide chain,1 and (iii) by an entirely reasonable mechanistic preference for decarboxylation at a ‚-keto carboxylic acid terminus.When the siphonariid precursors 152, 155 and 156 were compared with each other, a common tetraketide unit (shown by the dashed box in Fig. 2) was identified. Furthermore, a closely similar structural motif could be identified in the PAPA model 157.48 In another comparison (Fig. 3), the siphonariid model shows a good homology with the macrolide model 158 derived by Celmer140 which includes the erythromycins. If these biosynthetic speculations on siphonariids are correct, then a common genetic origin for the bacterial macrolides and the siphonariid hemiketal polypropionates would be predicted; the siphonariid enzymes would thus likely resemble type I fatty acid or polyketide synthases. A consequence of the suggested processive biosynthetic model for the siphonarins and the denticulatins is that diketides 159–161 and triketides 162–165 are predicted as biosynthetic intermediates.Since the first three propionate units incorporated into siphonarin B 63 retain their keto functionality, the processive nature of marine polypropionate biosynthesis should preferentially be tested in S. denticulata. The first three cycles of a plausible biosynthetic pathway to denticulatin A are shown in Scheme 5. Key questions to address include: (i) is the product of the KR step an (R) or an (S) hydroxy-substituted acid? (ii) does the DH step proceed with syn- or anti-stereoselectivity? Structural diversity is achieved in the polyether antibiotics by interspersing acetate and butyrate building blocks into the growing polyketide chain; typical examples include monensin 166 and narasin A 167.In contrast, macrolide antibiotics are composed almost exclusively of propionate building blocks,1 although acetate and butyrate primers have been converted by modified forms of DEBS1 into triketide products.143–145 S O OH OH O OH OH O OH S O S O OH O S O OH O S OH O S O S O O O OH OH OH Et O OR2 OR1 OH OH OH OH OH OH KR AT KR ACP KS ACP DEBS 1 KS KS TE DEBS 3 AT KS ER KR ACP AT DEBS 2 AT ACP KS KS ACP KR AT AT ACP AT KR AT = acyltransferase ACP = acyl carrier protein DH = dehydratase KS = b-ketoacyl-ACP synthase ER = enoylreductase KR = b-ketoreductase TE = thioesterase ACP DH 153 R1 = R2 = H 154 R1 = desosamine, R2 = mycarose Figure 1 The erythronolide B PkSs: relating proteins and biosynthetic steps Davies-Coleman and Garson: Marine polypropionates 489Marine polypropionates may use acetate or isobutyrate chain starter units or acetate chain-building units.The 2-pyrone metabolites fortunately retain the chain-terminating carboxy group and so the chain starter unit is easily identified. Thus the cyercenes 24, 27 and 29, norpectinatone 34, aglajne-3 36 and its 5,6-dehydro analogue 37, can all be seen to possess an acetate starter unit, as do the furanones 17–19.Among the 4-pyrones, the cyercenes 41, 45 and 47, the baconipyrones 59 and 61, the siphonarins 62 and 64, and the 15-norphotodeoxytridachione 100 all seem likely to have an acetate chain starter unit; as mentioned above, this has been demonstrated for siphonarin A 62. Depending on the direction of chain growth, the dihydropyrone 57 and the auripyrones 88 and 89 may have an acetate chain starter, while reasonable biosynthetic pathways to the acyclic metabolites 1, 5, 10, 11, 15 and 16 also suggest that polypropionate formation is initiated by acetate.Examples of metabolites which may have an isobutyrate starter unit are 13, 26, 51, 55 and 56, while 53 and 54 may perhaps have a pentanoate starter unit. Tridachiapyrone 104 appears to have a branched C5 starter unit which could possibly derive from acetate plus propionate with a methyl migration;64 an isoprene unit is an interesting alternative. A number of metabolites, chiefly the majority of the cyercenes, appear to contain acetate in chain building positions.Notably, in 27, 28 and 43, acetate must occupy an unprecedented chain-terminating position; biosynthetic proposals for metabolites such as 1–14, which have lost the terminal carboxy group must be interpreted with caution. A recent study demonstrated the use of acetate and butyrate in the biosynthesis of the diacylguanidine metabolite triophamine 168 produced by the dorid nudibranch Triopha catalinae.111 Branched chain amino acids have been implicated as sources of C4 and C3 units for polyketide biosynthesis,146,147 and their role in marine polypropionate biosynthesis is definitely worth exploring.As our understanding of terrestrial polypropionate biosynthesis and of combinatorial synthesis of ‘non-natural’ polyketide and polypropionate metabolites increases, the marine polypropionates represent additional targets; the experiments described in section 4.2 demonstrate well that meaningful precursor incorporation data can be obtained.A number of the most pharmacologically-potent marine metabolites (e.g. halichondrins, spongistatins, bryostatins) appear to have polyketide-derived skeletons, but are present in minute quantities in their marine invertebrate hosts. Research on the more-accessible marine polypropionates should provide ideal model studies, through which to tackle these other more challenging and advanced marine biosynthetic problems. 5 Ecology and biological activity of marine polypropionates The tissue distribution of the cyercene metabolites in C.cristallina, Placida dendritica and Ercolania funera has been determined by anatomical dissection.27,148 The cyercenes -1, -2 and -3 (24–26) were present in mantle tissue of C. crystallina, and the mucus contained all metabolites except cyercene A 39 which was only localised in the cerata; no metabolites were found in digestive tissue.27 In E.funerea, cyercenes 29, 30 and 42 were found mainly in the cerata, while the pyrone distribution in mantle, cerata and mucus of P. dendritica was not specific. The most active metabolites, when tested as potential growth factors in an assay with Hydra vulgaris, were 29 and 39. Slime extracts from C. cristallina and from P. dendritica were highly toxic to the mosquitofish Gambusia aYnis, while E. funerea extracts were less toxic.148 In feeding deterrency assays, individual cyercene metabolites were less eVective than crude extracts from the molluscs;25,26 the presence of the algal feeding deterrent chlorodesmin in the crude extracts did not account for the observed feeding inhibition.25 There are two possible explanations, namely pyrone metabolites may act synergistically to inhibit fish feeding, or that the true polypropionate-derived bioactive metabolites have yet to be isolated.An interesting comparison of the defensive strategies of C.crystallina, P. dendritica and E. funera has been made. The highly deterrent nature of Cyerce allows the mollusc to fully exploit diverse habitats and nutritional possibilities.148 O O O O OH O O HO O O R O O OH O O OH O O O O OH OH O O HO O OH O O HO O OH P A P A 157 Cane–Celmer–Westley prototype * 156 Siphonarin A R = H Siphonarin B R = Me Denticulatin A Muamvatin 155 152 Figure 2 Stereochemical models for selected polypropionates and homology with the Cane–Celmer–Westley PAPA model HO OH OH OH OH O O HO O O R O O OH O O OH O O Erythronolide B Siphonarin A R = H Siphonarin B R = Me 155 158 Figure 3 Stereological homologies between the siphonarins and Celmer macrolide model 490 Natural Product Reports, 1998The chemical ecology of the Elysioidea has also been studied.Metabolites 92, 95, 100, and 107 are present in the mucus secretion of Elysia timida, consistent with a defensive role; metabolites 95, 100 and 107 were toxic to Gambusia aYnis.68 In E.viridis the ichthyotoxic elysione 105 is found in mucous secretions.67 The membrenones are present in the skin of the notaspidean mollusc Pleurobranchus membranaceus where, together with an acid secretion, they may function as a chemical defence.36 Siphonariid polypropionate compounds appear to play a limited role in chemical defence. Although the denticulatins are ichthyotoxic69 and have been shown to be located in the foot tissue and mucus of S.denticulata, the molluscs are commonly eaten by predators,124 as are specimens of S. maura. which produce the vallartanones.37 Vallartanone B 51 showed some fish-feeding deterrency,37 while vallartanone A 52 induced larval settlement in the tubeworm Phragmatopoma californica.124,149 Molluscal polypropionates show cytotoxic,1,17,53,55,58,63 antibiotic and antifungal12,17,30,31,41,66 and antiviral53,55 properties. Compared to the pronounced ecological eVects described above, the pharmaceutical potential of marine molluscs is unlikely to be realised since so many of the metabolites isolated in the laboratory are likely to be artifacts, or are products of non-enzymic transformations as discussed in Section 4.3.Acknowledgements. 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ISSN:0265-0568    

DOI:10.1039/a815477y

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Inhibitors of enzymes of androgen biosynthesis: cytochrome P45017· and 5·-steroid reductase Michael Jarman,a H. John Smith,b Paul J. Nichollsb and Claire Simonsb aCancer Research Campaign Centre for Cancer Therapeutics at the Institute of Cancer Research, Cotswold Road, Sutton, Surrey, UK SM2 5NG bWelsh School of Pharmacy, CardiV University, Redwood Building, King Edward VII Avenue, Cathays Park, CardiV, UK CF1 3XF Reviewing the literature up to July 1997 1 Introduction 2 Inhibitors of cytochrome P45017· 2.1 Non-steroidal inhibitors: ketoconazole 2.2 Non-steroidal inhibitors: other imidazole derivatives 2.3 Non-steroidal inhibitors: benzimidazole and triazole derivatives 2.4 Non-steroidal inhibitors: pyridine derivatives 2.5 Non-steroidal inhibitors: other structural classes 2.6 Steroidal inhibitors: rationale and early studies 2.7 Steroidal inhibitors: mechanism-based inhibitors 2.8 Steroidal inhibitors: C-17-heteroaryl steroids 2.9 Steroidal inhibitors: substrate analogues 2.10 Steroidal inhibitors: combined inhibitors of cytochrome P45017· and 5·-steroid reductase 3 Inhibitors of 5·-steroid reductase 3.1 Enzyme characteristics 3.2 5·-SR inhibitors 3.3 Steroidal inhibitors 3.4 Non-steroidal inhibitors 3.5 5·-SR inhibitors in prostatic cancer chemotherapy 4 References 1 Introduction The male androgens testosterone and 5·-dihydrotestosterone, in addition to their normal physiological function, also control prostatic disease in elderly men.Inhibitors of enzymes of androgen biosynthesis are therefore potential weapons in the fight against prostatic carcinoma and benign prostatic hypertrophy. The pathways leading to these steroids are shown in Scheme 1. The two enzymes to be targeted for the treatment of prostatic disease are cytochrome P45017· and 5·-steroid reductase. Cytochrome P45017· is of interest in relation to the treatment of malignant disease, whereas the 5·-reductase, though potentially having some role as a target for the treatment of prostatic carcinoma, is primarily of interest in relation to benign prostatic disease.This review elaborates on the reasons for this distinction, and describes various inhibitors of these target enzymes, with an emphasis in each case on those aspects which are less well covered in other recent reviews. 2 Inhibitors of cytochrome P45017· About 80% of patients with prostatic cancer have androgendependent disease and respond to hormonal ablation.Whereas 5·-dihydrotestosterone is the principal active androgen in the prostate, testosterone is also an active stimulant of the growth of prostatic cancer tissue. Hence inhibition of the synthesis or action of testosterone is judged necessary for the treatment of advanced hormone-dependent prostatic cancer. Surgical castration or the medically equivalent use of gonadotrophin releasing hormone (GnRH) agonists remove testicular androgens. However, concern that the continued production of androgens of adrenal origin would maintain growth of the cancer led to the concept of maximal androgen ablation.This treatment combines castration or a GnRH agonist with an antiandrogen to counteract the action of residual circulating testosterone or dihydrotestosterone on the androgen receptor in the prostate cancer cell. A discussion of maximal ablation is outside the scope of this review, but for the present purposes, its relevance is that an inhibitor of cytochrome P45017· could, by inhibiting the production of testosterone precursors in both the testes and the adrenals, achieve the same result as the combined therapy.The enzymology of the reactions catalysed by cytochrome P45017· has been discussed in a recent review1 but the salient features are summarised here. The individual steps catalysed by cytochrome P45017· are shown in Scheme 1. The enzyme catalyses two successive steps in the pathways leading to cleavage of the 2-carbon side chain at C-17 and can utilise either pregnenolone (the ƒ5 pathway) or progesterone (the ƒ4 pathway) as the initial substrate.The preferred pathway is species dependent: in humans it is ƒ5, but in the rat, for example, the ƒ4 pathway is preferred. Moreover, other features of the pathway are species dependent. Thus with the human enzyme, dissociation of the 17-hydroxylated intermediate before the second step appears obligatory but with the rat enzyme it is not.Species diVerences can reflect both incomplete sequence homology between cytochrome P45017· from diVerent species, the rat and human enzymes showing 70% homology, and the cofactor environment. For all these reasons the use of the human enzyme to evaluate inhibitors is preferred, though not always practicable. 2.1 Non-steroidal inhibitors: ketoconazole Although inhibitors of cytochrome P45017· have been known for over 30 years, consideration as a potential target for prostatic cancer treatment is much more recent, and arose serendipitously.Ketoconazole 1, Scheme 2, is an imidazole antifungal agent that acts by inhibiting the cytochrome P450 mediated 14‚-demethylation of lanosterol, disrupting fungal membranes.2 Reports of drug related gynaecomastia in male patients led to the realisation of its antihormonal eVects, to elucidation of the principal mechanism as inhibition of cytochrome P45017·, and to clinical trials in patients with prostatic cancer. Ketoconazole as used therapeutically is the racemate of the cis isomer (Scheme 2).Hence four stereoisomers, two cis and two trans, are possible, and these have been synthesised (see Scheme 3 for an example) and evaluated against a variety of cytochrome P450 enzymes.3 Results relevant to the present applications (Table 1) are on five of the steroidal enzymes shown in Scheme 1. Although the 2S,4R-isomer was by far the most potent stereoisomer against pig testicular cytochrome P45017· (IC50 0.05 ÏM) it retained suYcient undesirable activity against steroidal 11‚-hydroxylase to discourage further development of individual stereoisomers as a more selective therapy for prostatic cancer.The lack of selectivity for cytochrome Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 495P45017· is a drawback in the use of ketoconazole to treat prostatic cancer. Nevertheless, it remains to date the only inhibitor having this target enzyme as its primary target to have been evaluated clinically and, as such, is a standard against which future inhibitors may be judged. 2.2 Non-steroidal inhibitors: other imidazole derivatives The discovery of the inhibition of cytochrome P45017· by ketoconazole has led to the testing of other imidazole derivatives. Etomidate 2 inhibited the pig testicular lyase conversion Scheme 1 Pathway of steroidal biosynthesis Cl Cl O Me + HO OH OH Cl Cl O O OH Cl Cl O O OCOPh Br Cl Cl O O OH N Cl Cl O O O N N N Ac N N vii 1 Ketoconazole cis-isomer (2 S,4 R)[+(2 R,4 S)] i ii–iv v, vi Scheme 2 Synthesis of cis 1: Reagents and conditions: i, p-TsOH, BunOH, PhH; ii, Br2; iii, PhCOCl, pyridine; iv, recrystallize from EtOH; v, imidazole, DMA; vi, aq. NaOH; vii, NaH, DMSO Cl Cl O Me Cl Cl O O OTs Cl Cl O CH2Br CH2Br Cl Cl O O OTs CH2Br (2 S,4 S) O O OTs + (2 R,4 S) Cl Cl O O O N NAc Br (2 S,4 R) i iv (2 S,4 R)-1 , ii HO N N Ac , iii Scheme 3 Synthesis of diastereoisomers of 1: example of (2S,4R)-1.Reagents: i, CuBr2, EtOAc–CH2Cl2, reflux; ii, p-TsOH, BunOH, toluene, reflux; iii, NaH, DMSO, 80)C; iv, K2CO3, imidazole, DMF, reflux 496 Natural Product Reports, 1998of 17·-hydroxyprogesterone to androstenedione less potently than ketoconazole, with a Ki value of 16.0 ÏM vs 2.6 ÏM for ketoconazole, 4 but was a more potent inhibitor of 11‚-hydroxylase (Scheme 1, 11-deoxycorticosterone] corticosterone).5 Econazole 3 and miconazole 4 were submicromolar inhibitors of human foetal adrenal 17·-hydroxylase with IC50 values of 0.3 ÏM (ketoconazole 0.02 ÏM), but all were ÏM inhibitors of 21-hydroxylase, again showing lack of selectivity. 6 A systematic study of a variety of imidazole derivatives has rationalised cytochrome P45017· (and aromatase) inhibitory potency in terms of superimposition of their structures onto the steroid substrate, locating an imidazole nitrogen to coordinate with haem iron.7 The most active compounds (Table 2) were those in which a 3-carbon spacer group linked a functionalised phenyl residue to the imidazole in such a way as to allow overlay of the phenyl residue and its substituent X onto ring A of the steroid substrate and its 3-oxygen substituent.However, even the most potent of these compounds (11–13) were only about half as active as ketoconazole. Molecular modelling was also used8 to rationalise in part the ranking of inhibitory activity9 (Table 3) of a variety of imidazole drugs against rat testicular microsomal cytochrome P45017·.Again the existence of conformations allowing both coordination of imidazole nitrogen to the haem residue and interaction with steroidal ring A favoured activity, and the model accounted for the order of potency of the ketoconazole stereoisomers (Table 1) as well. A favourable log P value was also important, exemplified by the contribution to this of the biphenyl residue in bifonazole 15, the most potent compound in the series.Finally a recent Japanese patent10 describes a series of imidazolylcarbazoles which are potent (low nM IC50) inhibitors of 17,20-lyase, one of which, YM 116 18 is reported to be in clinical development. 2.3 Non-steroidal inhibitors: benzimidazole and triazole derivatives The benzimidazole derivative liarozole 19 is a hydroxylase/ lyase inhibitor of potency similar to ketoconazole which appeared more potent in studies in male volunteers.It suppressed circulating testosterone to castrate levels within 8 h at a dose of 300 mg, compared with 600 mg for ketoconazole11 Table 1 Inhibition of selected steroidogenic enzymes by stereoisomers of ketoconazole 1 Cytochrome P450 Stereoisomer IC50/ÏM 2S,4S 2R,4R 2S,4R 2R,4S Aromatase 3.98 38.3 110.4 39.6 C17,20-lyase 0.589 2.04 0.050 2.38 11‚-hydroxylase 0.247 0.135 0.152 0.608 P450scc 2.95 3.92 1.24 5.40 21-hydroxylase 4.46 6.57 9.01 11.2 Me N N O O Et 2 Etomidate Cl Cl O N N Cl 3 Econazole Cl Cl O N N Cl Cl 4 Miconazole N N N N X X 5–9 10–14 Table 2 Inhibition of rat testicular cytochrome P45017· by certain imidazole derivatives Compound X IC50/(ÏM 1 — 12.1 5 H 46 6 NO2 30.3 7 NH2 27.6 8 Cl 184 9 F 50.6 10 H 171 11 NO2 25 12 NH2 22 13 Cl 29 14 F 39 Table 3 Inhibition of rat testicular microsomal 17·-hydroxylase and 17,20-lyase by various imidazole drugs Compound Ki/nmol l"1 17·-Hydroxylase 17,20-Lyase 1 160 84 3 668 325 4 599 243 15 86 56.5 16 170 81.5 17 901 505 N N N N Cl Cl O N N S Cl 15 Bifonazole 16 Clotrimazole 17 Tioconazole 18 YM 116 Ph Ph Cl NH N HN Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 497and has shown eYcacy in clinical trials against prostate cancer.12 Potentially of greater therapeutic significance is its inhibition of another P450 enzyme, mediating retinoic acid 4-hydroxylation (ref. 10 and refs cited therein). Liarozole also inhibits other steroidal P450 enzymes, though its profile diVers from that of ketoconazole: for example, it is a powerful inhibitor of human placental aromatase13 unlike ketoconazole. Liarozole is chiral and syntheses of its enantiomers have been reported14,15 but not their activities against cytochrome P45017·.The triazole derivative R 76713 20 has been developed as a potent inhibitor of aromatase (IC50 2.6 nM) but possesses moderate inhibition of rat testicular 17,20-lyase. Its enantiomers R 83839 (") and R 83842 [(+), vorozole] have also been evaluated against both enzymes.Against human placental aromatase the (+)-enantiomer was 32 times more active than the (")-enantiomer but against 17,20-lyase, inhibition originates mainly from the (")-enantiomer (IC50 1.8 ÏM). 2.4 Non-steroidal inhibitors: pyridine derivatives The discovery of inhibitors of cytochrome P45017· predates by some two decades the therapeutic use of ketoconazole against prostate cancer. Examples of active pyridine derivatives are the indenylpyridine SU 8000 21 and the naphthylpyridine SU 10603 22, having Ki values of 0.04 and 0.3 ÏM respectively.16 In rats, SU 10603 ablated prostate and seminal vesicles but not the adrenals, indicating some selectivity for the target enzyme.17 More recently, a series of aromatase inhibitors structurally related to 21 and 22 was evaluated for inhibition of rat testicular cytochrome P45017·.18 The most active compounds (23–28) were better inhibitors than ketoconazole (Table 4). The structurally related 2-(4-pyridyl)methylene-1- tetralone and its 3-pyridyl analogue, themselves weak inhibitors of rat testicular microsomal cytochrome P45017· were converted (Scheme 4) into the cyclopropanaphthalenes 29 and 30.19 The 3-pyridyl analogue 30 (IC50 6.3 ÏM) proved much more potent than the 4-pyridyl derivative 29 (36% inhibition at 125 ÏM) and 10-fold more potent than ketoconazole (IC50 54 ÏM in this study).During investigations of aromatase inhibitory esters of 4-pyridylacetic acid, the serendipitous discovery was made that some of these derivatives were also inhibitors of rat testicular cytochrome P45017·.20 These esters were synthesised by transesterification between methyl 4-pyridylacetate and the appropriate alcohol using n-butyllithium.21 The more potent inhibitors (Table 5) contained a cyclohexyl or, particularly, a N N N HN 19 Liarozole N N N N N Me N Cl Cl 20 R 76713 Me N Cl N O Cl 21 SU 8000 22 SU 10603 (CH2) n X N 5 6 7 23–28 Table 5 Inhibition of rat testicular microsomal 17·-hydroxylase and 17,20-lyase by various esters of 4-pyridylacetic acid Compound R IC50/ÏM 17·-Hydroxylase 17,20-Lyase 31 cyclohexyl 15 20 32 trans-4-methylcyclohexyl 4.2 4.4 33 cis-4-methylcyclohexyl 25 36 34 4-ethylcyclohexyl 2.0 2.7 35 trans-4-tert-butylcyclohexyl 2.1 3.0 36 1-methylcyclohexyl 10 11 37 (")-menthyl 4.5 5.1 38 (+)-menthyl 12 15 39 (")-borneyl 1.5 2.2 40 (+)-borneyl 0.59 0.54 41 (1S,2S,3S,5R)-(+)-isopinocampheyl 0.26 0.28 42 (1R,2R,3R,5S)-(")-isopinocampheyl 1.7 1.8 43 1-adamantyl 0.56 0.61 Table 4 Inhibition of rat testicular cytochrome P45017· by 2-(4- pyridylalkyl)tetral-1-ones Compound X n IC50/ÏM 23 5-OMe 1 13 24 5-OMe 0 13 25 6-OMe 1 19 26 5-OH 1 25 27 7-OH 1 39 28 H 1 22 O Py Py 29 Py = 4-pyridyl 30 Py = 3-pyridyl i ii Scheme 4 Reagents and conditions: i, PyCHO, piperidine–AcOH, 5 h, 130)C; ii, N2H4 · H2O, KOH, HO(CH2)2OH, 5 h, 150)C 498 Natural Product Reports, 1998substituted cyclohexyl moiety.Substitution on the pyridylmethylene residue appeared beneficial, the IC50 values for the monomethyl derivative 44 being 2.0 and 2.2 ÏM for hydroxylase and lyase, respectively and for the dimethyl analogue (45) 3.6 and 4.5 ÏM. This study was extended to include further examples 46–55 of ·-alkyl substituted esters, 3- as well as 4-pyridyl substituents and the use of human testicular enzyme.22 These further studies revealed several inhibitors with greater potency than ketoconazole, having IC50 values in the low nM range (Table 6).Their activity was rationalised in terms of an overlay22 (Fig. 1) of the ester onto the substrate. Lability to esterases is an important consideration for the further development of these compounds.23 Several potent compounds e.g. 47, 50 and 55 with a tertiary carbon atom adjacent to the oxygen and to the carbonyl function were apparently stable in this respect.22 In a further series of compounds 56–63, the ester linkage was reversed.24 These pyridylalkyl esters of adamantane-3- carboxylic acid included examples (Table 7) even more potent than the best in the aforementioned ‘normal’ ester series.The influence of chirality was also studied in the enantiomers 57 and 58 of the monomethyl derivative. The inhibitory potency was markedly enantioselective, with the (S)-3-(4-pyridyl) ethyl ester 58 (IC50 for lyase=1.8 nM) being some 40-fold more potent than the (R)-enantiomer 57 (IC50=74 nM).Again, based on its resistance to hydrolysis by esterases, the slightly less potent (IC50=2.7 nM) dimethyl derivative 59 was the preferred candidate for further development as a potential therapeutic agent.23 In this reverse ester series, 3-pyridyl derivatives 60–63 were substantially less potent and there was little to diVerentiate the enantiomers 61, 62 of the corresponding monomethyl derivative. 2.5 Non-steroidal inhibitors: other structural classes Novel pyrrolidine-2,5-diones, structurally related to the aromatase inhibitor aminoglutethimide, and synthesised as potential aromatase inhibitors also inhibited cytochrome P45017·.25 The several compounds that had potencies comparable with that of ketoconazole are shown in Table 8. All the non-steroidal inhibitors so far discussed contain a basic functionality capable of coordinating the haem residue of cytochrome P45017·, but this is not a mandatory requirement for inhibition.Thus, bifluranol 69, known to act on androgen responsive organs in animals26 was found to inhibit cytochrome P45017· and was more potent than its non-fluorinated analogue 70 in this respect (Table 9). The most potent analogue 74, was about one-third as active as ketoconazole N RO O 31–43 N O O R2 R1 44 R1 = Me; R2 = H 45 R1, R2 = Me N O O R2 R1 41 R1, R2 = H 46 R1 = Me; R2 = H 47 R1 = R2 = Me N O O R2 R1 48 R1, R2 = H 49 R1 = Me; R2 = H 50 R1 = R2 = Me N O O R2 R1 N O O R2 R1 51 R1, R2 = H 52 R1 = Me; R2 = H 53 R1 = R2 = Me 54 R1, R2 = H 55 R1 = R2 = Me Table 6 Inhibition of human testicular microsomal 17·-hydroxylase and 17,20-lyase by various 3- and 4-pyridyl esters Compound IC50/ÏM 17·-Hydroxylase 17,20-Lyase 41 14 5 46 19 6 47 26 10 48 260 88 49 82 14 50 29 15 51 930 130 52 220 35 53 90 13 54 1900 320 55 75 13 1 (comp) 65 26 O HO O O R1 N B D 20 17 16 13 R2 Fig. 1 Overlay of cyclohexyl 4-pyridylacetate onto pregnenolone N O N O O O R1 R2 R1 R2 56 R1, R2 = H 57 R1 = Me; R2 = H 58 R1 = H; R2 = Me 59 R1 = R2 = Me 60 R1, R2 = H 61 R1 = Me; R2 = H 62 R1 = H; R2 = Me 63 R1 = R2 = Me Table 7 Inhibition of human testicular microsomal 17·-hydroxylase and 17,20-lyase by 3- and 4-pyridylalkyl esters of adamantane-3- carboxylic acid Compound IC50/ÏM 17·-Hydroxylase 17,20-Lyase 56 43 18 57 340 74 58 3.3 1.8 59 8.8 2.7 60 1500 460 61 580 150 62 840 230 63 390 92 Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 499(respective IC50 values for 17·-hydroxylase; 18.7 and 6.0 ÏM, for 17,20-lyase; 27.9 and 10.6 ÏM).Loosely related structurally to bifluranol were several hydroxylated compounds derived from decafluoroazobenzene27 (Scheme 5) (Table 10). Both 4- (75) and 2-hydroxynonafluoroazobenzene (76) were inhibitors of cytochrome P45017·. The 2-hydroxy-derivative was interesting in being more selective for the second (lyase) step. It appeared to be a non-competitive inhibitor with respect to the substrate, progesterone, and to act by spontaneous conversion, at physiological pH, into the oxadiazepine 77. 2.6 Steroidal inhibitors: rationale and early studies In contrast to the majority of non-steroidal inhibitors of cytochrome P45017·, most of the steroidal inhibitors that have been synthesised have an element of rational design in their conception.Invariably this has involved functionalisation of the steroid residue at the C-17 position in such a way as to mimic the natural substrates, their intermediary 17·-hydroxy derivatives, or transition states for the production of these intermediates or the end products dehydroepiandrosterone or androstenedione. In some cases the potential for irreversible binding to the enzyme is built into the structure.The earliest synthetic steroidal inhibitors appear to be the 17·-acylamido and ureido steroids described by Arth et al.28 The general approach to the synthesis of these compounds is given in Scheme 6. The Beckmann rearrangement of the oxime of pregnenolone 3-O-acetate gave the 17-acetamide intermediate.Where amino substituents other than acetyl were required, this was deacylated with strong alkali and the 17-amino derivative converted into other acylamino or ureido derivatives by standard reactions. An exception was the formylamino group, which was introduced by reaction of pregnenolone with formamide–formic acid.29 Stepwise oxidations, under various conditions, gave inter alia 3-oxo-4-ene and 3-oxo-1,4-diene structures.The best compounds e.g. 78–84 were comparable with the non-steroidal inhibitors SU 8000 (21) and SU 10603 (22) (Table 11). In rats fed with the ureido derivative 84 at 500 mg kg"1 in their diet for 6 weeks, testicular testosterone levels fell by 75–90% and adrenal weights were unaVected, indicating some selectivity for the target enzyme. 2.7 Steroidal inhibitors: mechanism-based inhibitors The mechanism-based inhibitor MDL 27302 85 (Scheme 7)30 was designed to be activated by enzymatic one-electron Table 8 Inhibition of rat testicular cytochrome P45017· by pyrrolidinediones Compound R1 R2 % Inhibition (100 ÏM) 64 — — 88 65 H NH2 95 66 Me NH2 81 67 Me(CH2)3 NH2 75 68 Me(CH2)4 N2 89 1 (comp) — — 91.5 (CH2)7Me N O O R1 NH2 NH2 64 65–68 N O O R2 HO X OH R3 R4 HO R1 R4 OH R2 R3 69 Bifluranol R1 = F; R2 = H; R3 = Et; R4 = Me 70–72 73,74 Table 9 Inhibition of rat testicular cytochrome P45017· by bifluranol 69 and analogues Compound R1 R2 R3 R4 % Inhibition (100 ÏM) 17·-Hydroxylase 17,20-Lyase 69 F H Et Me 77 61 70 H H Et Me 65 40 71 F F Et Me 64 40 72 F H Et Et 85 73 73 X=H 94 88 74 X=F 95 91 N N F F F F F F F F F F N N F F HO F F F F F F F N N HO F F F F F F F F F + O N N F F F F F F F F decafluoroazobenzene 75 76 77 i ii Scheme 5 Reagents and conditions: i, KOH, ButOH, reflux; ii, phosphate buVer pH 7.6, 18 h, rt Table 10 Inhibition of rat testicular cytochrome P45017· by perfluoroazobenzene derivatives Compound IC50/ÏM 17·-Hydroxylase 17,20-Lyase 75 30 33 76 63 16 77 50 11 1 (comp) 6 11 500 Natural Product Reports, 1998oxidation of the cyclopropylamino nitrogen followed by opening of the ring to form a ‚-iminium radical which could react covalently with the enzyme whilst the drug is bound to the active site.31 It was a potent (Ki,app=90 nM) competitive irreversible inhibitor of the cynomolgus monkey testicular cytochrome P45017· with evidence of selectivity since it did not inhibit steroid 21-hydroxylase or cholesterol side-chain cleavage enzyme.32 This group has also described mechanistically analogous cyclopropyloxy derivatives (Scheme 8) of which three (Table 12) proved potent inhibitors of human or cynomolgous monkey testicular cytochrome P45017·.33 Inhibition was stronger after preincubation of enzyme with inhibitor, supporting the hypothesis of mechanism-based activation.The final step in cholesterol side-chain cleavage to give pregnenolone involves fission of the C-20,22 bond of 20·,22Rdihydroxycholesterol to generate the carbonyl function of pregnenolone. Since this site of oxidation is close to those involved in the further oxidations of pregnenolone by cytochrome P45017· it is not surprising that the cholesterol side-chain cleavage inhibitor (Ki=40 nM) 22-amino-23,24- dinorchol-5-en-3‚-ol 89 was, in addition, a potent inhibitor (Ki 29 nM) of cytochrome P45017·.34 The route of synthesis35 is shown in Scheme 9.NOH AcO NHCOMe AcO NHR HO NHR O NHR O 79 R = HCO 80 R = NH2CO 81 R = MeCO 82 R = NH2CO 83 R = HCO 84 R = NH2CO 78 R = MeCO Scheme 6 General synthetic routes to 17·-acylamido and ureido steroids Table 11 Inhibition of rat testicular 17,20-lyase by acylamido and ureido steroids Compound % Inhibition of 17,20-lyase (conc. of inhibitor in Ïg ml"1) 0.5 0.25 0.125 78 80 70 60 79 100 90 85 80 80 70 60 81 85 80 60 82 90 85 75 83 95 — 70 84 85 80 70 21 (comp) 90 80 70 22 (comp) 90 80 65 O HO HO N HO NH 85 ii NH P-450 P-450 [FeO]2+ [FeO]3+ NH enzyme products +• + i • Scheme 7 Reagents and conditions: i, C3H5NH2, MeOH, 48 h, reflux; ii, NaBH4, EtOH, 3 h, rt TBDMSO OH TBDMSO O HO O O O O O O i iii 86 87 88 ii iv Scheme 8 Reagents and conditions: i, C2H3OEt, Hg(OAc)2; ii, CH2I2, Et2Zn; iii, Bun 4NF; iv, Cr2O3, H2SO4.Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 501Closely related structurally to the aziridine 85 and the amine 89 were a group of aziridinyl- and amino-steroids 90–93 designed as mechanism based inhibitors.36 The aziridinyl derivatives 90 and 91, and the amino derivative 93 were obtained as enantiomers.The inhibition proved markedly enantioselective, the (S)-enantiomers being much the more potent (Table 13), though even the (R)-enantiomers were ca. twice as potent as ketoconazole.In contrast, the amino steroids 92 and 93 were very weak inhibitors indicating the importance of the spacing of the amino group relative to C-17 for coordination with haem iron. There was evidence from the type II binding spectrum for coordination of the aziridine nitrogen of the inhibitors with haem iron. Also, the potent inhibitor (S)-90 (Ki=1.7 nM) was shown to be tightly bound: enzyme activity was only slowly restored after preincubation with the inhibitor. 2.8 Steroidal inhibitors: C-17-heteroaryl steroids Tight binding to the enzyme was also a feature23 of 17-(3- pyridyl) steroids designed by consideration of how a pyridyl substituent might be incorporated into the steroid skeleton, such that the pyridyl nitrogen lone pair would coordinate to the iron atom of the haem cofactor in the active site of the enzyme.Studies of the detailed mechanism of the two enzymatic steps have informed such approaches. These are not reviewed here, but the reader is referred to a recent commentary37 and to references cited therein.Consideration of putative transition-state geometry for intermediates in the hydroxylase (TS*2) and lyase (TS*3) steps (Scheme 10), led to the prediction that attachment of a 2-pyridyl or a 3-pyridyl substituent to the 17-position of a steroid carrying a ƒ16,17- bond might lead to optimal binding to the haem iron in the respective transition states.38 In fact, the 3-pyridyl steroid 95 (abiraterone), was substantially more potent (Table 14) than its 2-pyridyl counterpart 96 whereas the 4-pyridyl derivative 97 was as predicted the least potent.Structural variations in rings A–C of the steroid skeleton were widely tolerated e.g. compounds 98–102. However, reduction of the ƒ16,17-bond led to ca. 10-fold lower inhibition, cf. 95 and 103. The general synthetic route to these ƒ16,17 17-(3-pyridyl) steroids involved a palladium catalysed cross-coupling reaction between 3-pyridyldiethylborane and the 17-O-enol triflate of a suitably protected 17-oxo steroid, as illustrated for 95 [Scheme 11, route (a)].An alternative route had been described earlier by Wicha and co-workers who had prepared similar Table 12 Inhibition of human and testicular cynomolgus monkey testicular C17,20-lyase by steroidal cyclopropyl ethers Compound Concentration/ ÏM Preincubation/ min Inhibition (%) 86 0.8 0 11 30 64 87 0.1 0 26 40 55 88 0.1 0 17 40 51 AcO CO2H AcO CONH2 HO CH2NH2 i, ii iii, iv 89 Scheme 9 Reagents and conditions: i, C3H5NH2, MeOH, 48 h, reflux; ii, NaBH4, EtOH, 3 h, rt Table 13 Inhibition of rat testicular cytochrome P45017· by aziridinyl and amino steroids Compound IC50/ÏM 90 0.21 90 34 91 1.2 91 36 92 >125 93 >125 93 >125 1 (comp) 67 HO NH H HO HN H NH H HN H O O S-90 R-90 S-91 R-91 HO NH2 92 H O NH2 H O H2N S-93 R-93 502 Natural Product Reports, 1998compounds as potential cardiotonic agents by reacting 3-pyridyllithium with a 17-oxo steroid and dehydrating the resulting tertiary alcohol. 39 The discovery that 95, given as its acetate prodrug 94, profoundly suppressed circulating testosterone levels in mice and rats40 led to its selection for clinical trials and the need for a more economical route than the synthesis via the enol triflate. It was found that the cheaply prepared41 vinyl iodide [Scheme 11, route (b)] slowly coupled with 3-pyridyldiethylborane to aVord 95.42 Analogous compounds bearing other heteroaromatic substituents at C-17 have been prepared and evaluated.43,44 The first of these studies describes thiazole, furan and thiophene derivatives.Synthetic routes to the most active of the compounds tested are shown in Scheme 12. The aminothiazole 104 had an IC50 of 63 nM against cynomolgus monkey testicular C17,20-lyase and the furan 105 produced 53% inhibition at 0.1 ÏM. Again the ƒ16,17-double bond was important. The 16,17-reduced analogue of 104 had an IC50 of 10 ÏM.The second study describes imidazolyl, pyrazolyl and isoxazolyl derivatives. The most active compounds 106–114, having IC50¶100 nM against the human enzyme, are shown in Table 15. The imidazol-4*- and -2*-yl derivatives 106 and 111 were the most potent. Acetylation 106]108 did not reduce potency as much as in the corresponding 3-pyridyl series. It would appear that abiraterone 95 remains the most potent of these ƒ16,17 17-heteroaryl substituted steroidal derivatives, and its failure to alter adrenal weights in mice at doses profoundly suppressing testosterone40 attests to its potential selectivity and promise as a clinical candidate for the treatment of prostatic cancer. 2.9 Steroidal inhibitors: substrate analogues The azasteroid 115 was prepared45 as a potential analogue of the substrate pregnenolone (Scheme 13). The main point of interest in its synthesis was the use of the perfluorotolyl group46 to protect the 3-hydroxy function. Unlike the acetyl group previously used in a synthesis of 11547 the perfluorotolyl group was stable during all the steps of the synthesis.The product was a moderate inhibitor (IC50 4.9 ÏM) of the conversion of pregnenolone into dehydroepiandrosterone by the human testicular enzyme. The sulfoxides 117 and 118 (Scheme 14) may be regarded as substrate analogues although the intermediate thioether 116 also has the potential to coordinate to haem iron through the lone electron pairs on sulfur. The inhibitory potencies of these compounds have been compared (Table 16) with that of the mechanism-based inhibitor 85.48 Trifluorinated analogues having the potential for greater metabolic stability than the natural substrates have been prepared49 (Scheme 15).The progesterone analogue 120 was a better inhibitor than ketoconazole (IC50 0.6 vs. 1.9 ÏM) of the lyase step of the rat testicular enzyme using O H+ ET O• O O O Fe OH HO Fe H2O2 H2O O2 HOO Fe HO O O Fe O H TS*3 LYASE HO Fe+ O=Fe e• ET e• O=Fe+ H+ O TS*1 HYDROXYLASE ET = electron transport system ET H2O TS*2 Scheme 10 Postulated catalytic cycle for cytochrome P45017·.For simplicity only the steroid D-ring and haem iron are shown. Table 14 Inhibition of human testicular cytochrome P45017· by various C-17 pyridyl steroids Compound IC50/ÏM 17·-Hydroxylase C17,20-Lyase 94 18 17 95 4 2.9 96 270 76 97 4000 1000 98 2.8 2.1 99 2.6 1.8 100 4.3 2.5 101 4.7 3 102 13 2.9 103 47 23 1 (comp) 65 26 N N HO HO 96 97 O N 98 N HO 99 HO N H 100 O N H 101 O N O 102 N HO H 103 Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 50317·-hydroxypregnenolone as substrate, though interestingly its pregnenolone counterpart 119 was not inhibitory at 5 ÏM. 2.10 Steroidal inhibitors: combined inhibitors of cytochrome P45017· and 5·-steroid reductase Potent inhibitors of cytochrome P45017· might also, by eYcient blockade of the biosynthesis of 17·-hydroxyprogesterone, deplete cortisol synthesis (see Scheme 1).If a less than maximal inhibition of cytochrome P45017· could be combined with eYcient depletion of dihydrotestosterone, then suYcient ablation of active androgens might be maintained whilst allowing cortisol synthesis. There are pharmacological advantages of combining these activities within a single drug, and some progress has been made in the identification of such dual inhibitors. For the most part the discovery of dual inhibition appears to have been a serendipitous outcome of programmes aimed at inhibitiors of one or other target, although Brodie and co-workers in particular have focused specifically on the discovery of such dual inhibitors.44 6-Methyleneprogesterone 121 (LY 207320), long known as a 5·-reductase inhibitor50 is also a dual inhibitor with an IC50 of 60 nM against both target enzymes.51 4-Hydroxynonafluoroazobenzene 75 inhibited human testicular testosterone-5· reductase (Ki=10 ÏM).Among the 17-pyridylsteroids, 98 was also an inhibitor of the human 5·-reductase (IC50=10 ÏM)38 as was the corresponding ƒ16,17 17-imidazolyl derivative 110 (IC50=120 nM).52 Interestingly, these two compounds both possessed the 4-en-3-one structure of the natural substrate testosterone and their 5-en-3-ol counterparts, respectively, 95 and 108, were not 5·-reductase inhibitors. 3-Oxopregn-4-ene-20‚-carbaldehyde 122 inhibited the activities of rat testicular 17·-hydroxylase (Ki=8.48 ÏM), C17,20-lyase (Ki=0.41 ÏM) and human prostatic 5·-reductase (Ki=15.6 nM), the Ki values in all cases being below the corresponding Km values.53 This was reflected in potent depletion of circulating testosterone in the rat in vivo.The oxime 123 was even more potent (respective Ki values for the foregoing O AcO OTf AcO AcO O HO I HO N HO N NNH2 HO 95 95 abiraterone 94 ii i iii iii (a) (b) 94 iv i ii Scheme 11 Reagents: (a) i, Tf2O, 2,6-di-But-4-Me pyridine, 12 h, rt; ii, 3-PyBEt2, Pd(PPh3)2Cl2, THF, H2O, Na2CO3, 1 h, 80)C; iii, NaOH, H2O, MeOH; (b) i, N2H4 · H2O, N2H4 · H2SO4, EtOH, 5 d, rt; ii, I2–THF, (Me2N)2C=NH, 2 h, 0)C then 4 h, 80)C; iii, as (a) ii, 48 h; iv, PyH, Ac2O HO O HO N S H2N 104 (a) TBDMSO O TBDMSO OH O i HO O HO O OMe Br ii ii 105 (b) i Scheme 12 Reagents and conditions: (a) i, CuBr2, MeOH, 24 h, reflux; ii, thiourea, Et3N, EtOH, 90 min, reflux; (b) i, furan, BunLi, THF, 2 h, "78)C; ii, 4 M HCl in dioxan, N2, 20min, rt Table 15 Inhibition of human testicular cytochrome P45017· by various C-17 heteroaryl steroids Compound IC50/nM 106 24 107 75 108 66 109 50 110 58 111 21 112 42 113 59 114 39 1 (comp) 77 504 Natural Product Reports, 1998activities 74, 18 and 1.1 nM) and was reported to be more eVective in vivo.54 Finally, (20S)-4-amino-20-hydroxymethylpregn- 4-en-3-one 124 (MDL 19687) (Scheme 16) was a potent dual inhibitor of 17,20-lyase (Ki=14 nM) and 5·-reductase (Ki=27 nM).55 3 Inhibitors of 5·-steroid reductase (5·-SR) A strategy for the treatment of prostatic cancer in the early hormone-dependent stage involves inhibition of the enzyme 5·-steroid reductase (5·-SR; EC 1.3.99.5) responsible for the metabolism of testosterone to dihydrotestosterone (DHT) (Scheme 1).56 DHT is the androgen responsible for development and growth of the prostate57 and inhibition of 5·-SR would reduce plasma and prostate DHT levels with removal of the growth stimulus to the prostatic cancer and metastases.Many potent inhibitors of 5·-SR have been discovered over the past 15 years (reviewed58,59,60) and whereas some have been developed to the clinical stage for the treatment of the related disease, benign prostatic hypertrophy (BPH) (a nonmalignant growth of the prostate in elderly men leading to restriction in urinary flow), there has been little reported on their use for the treatment of prostatic cancer. The requirements for agents for the treatment of BPH diVer from those required for prostatic cancer in certain respects.For successful marketing for the treatment of BPH, which is not a life-threatening disease, the agent must be free from side eVects associated with, (1) testosterone biosynthesis or antagonism of its action on the androgen receptor so as to be devoid of the long term eVects associated with chronic administration such as loss of libido, impotence, gynaecomastia, hot flushes and risk of bone brittleness;61,62 (2) agonist activity at the androgen receptor leading to overexcessive masculinity, (3) interference with other NADPH-dependent oxidoreductases in the steroidogenesis pathway essential for hormone production and normal cellular development.For example 3‚-hydroxysteroid dehydrogenase (3‚-HSD, 3‚-hydroxy-5- ene-steroid dehydrogenase/3-keto-5-ene-steroid isomerase) converts pregnenolone to progesterone, an essential step in corticosterone and cortisol production, and 17‚-hydroxysteroid dehydrogenase (17‚-HSD) in the testis (type 3 isozyme) reduces the weak androgen androstenedione to the potent testosterone (Scheme 1).As previously mentioned, prostatic cancer chemotherapy is aimed at total supression of androgenic action through several strategies: castration, androgen receptor antagonism and blockade of androgen synthesis (cytochrome P45017· inhibitors or GnRH agonists), the associated loss of libido being tolerable due to the life-threatening nature of prostatic cancer.Consequently the selectivity of 5·-SR inhibitors in this context is less important than that required for treatment of BPH. This section of the review briefly describes the design and development of the main types of 5·-SR inhibitors as prospective agents for the treatment of BPH and other DHT-induced N HN HO N HN 106 R = H 107 R = MeCO 108 O 109 N HN O 110 RO N HN HO N HN HO NH N NH N O O N O 111 112 113 114 H H O HO NOH O C7F7O C7F7O HO i, ii CF3 F F F F C7F7O H N O HN N O iii vi vii N O 115 iv, v Scheme 13 Reagents and conditions: i, C7F8, (C4 H9)4N+HSO4 ", CH2Cl2, NaOH (aq.); ii, NH2OH · HCl, EtOH; iii, SOCl2, dioxane; iv, CF3SO3Me, CH2Cl2, v, Me2NCHO, NaBH4, EtOH; vi, Ac2O, C5H5N; vii, NaOMe, Me2NCHO Table 16 Inhibition of porcine testicular cytochrome P45017· by methylthio steroids and derived sulfoxides Compound IC50/ÏM Ki,app 116 5.4 0.70 117 1.9 0.38 118 1.9 0.38 85 (comp) 4.6 3.62 Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 505conditions (male pattern baldness,63 acne,64 hirsuitism65) and aspects of their use as agents for the treatment of prostatic cancer.The structure–activity relationships within the various chemical types as agents for BPH have been comprehensively reviewed by Kenny et al.,60 Abell and Henderson59 and Frye58 and only those under further investigation are mentioned here. 3.1 Enzyme characteristics Human 5·-SR is a membrane-bound enzyme requiring NADPH as coenzyme.There are two distinct human isozymes: type 1 (5·-SR1) which predominates in skin and type 2 (5·-SR2) which predominates in prostate and genital skin.66 The two forms exhibit diVerent pH-rate profiles (5·-SR1, pH 6–8 maximum activity; 5·-SR2, pH 5–6)66 and aYnities for testosterone (5·-SR1, Km=2.9 ÏM; 5·-SR2, Km=0.5 ÏM).66 The isoforms were isolated using expression cloning of human cDNAs. 5·-SR1 is comprised of 259 amino acids and encoded by the SRD5A1 gene (short arm of chromosome 5) whereas 5·-SR2 has 254 amino acids and is encoded by the SRD5A2 gene (short arm of chromosome 2).The isozymes exhibit 50% homology in their sequences (see Chen et al.67 for relevant references). 3.2 Steroidal inhibitors The first reported 5·-SR inhibitors with potential clinical application were the 4-aza steroids developed by Merck from the early 1980s onwards. 4-MA was described as a potent competitive reversible inhibitor of rat prostatic enzyme (human 5·-SR1 and 5·-SR2, Ki,app=5 and 0.23 nM respectively58) but had undesirable agonist activity at the rat androgen receptor and furthermore lacked specificity towards other oxidoreductases (3‚-HSD) (see Frye58).Clinical studies were not pursued when it was shown to possess hepatic toxicity in the dog.68 Modification of 4-MA with optimisation of the C-17 side chain to contain a mix of a hydrogen bonding acceptor and hydrophobic residues led to the ƒ1,2-steroid, finasteride (Proscar>, 125).Finasteride is a potent inhibitor of 5·-SR2 (IC50=9.4 nM) with much lower activity towards 5·-SR1 (410 nM).69 Although introduction of a ƒ1,2-bond decreases in vitro activity by comparison with the saturated analogue, in vivo activity is relatively increased due to irreversible inhibition of the enzyme in a time-dependent manner.70 Introduction of a 4-trifluoromethyl substituent into the reduced compound increases in vitro potency over two-fold.71 HO O HO HO SH HO HO S HO S S S O S O 116 117 118 + i ii iii iv Scheme 14 Reagents and conditions: i, HS(CH2)2SH, BF3 · Et2O; ii, BunLi; iii, BunLi, Et2O, HMPA, MeI, "78)C then rt; iv, Me2CO, 30% H2O2, 3 d, rt OAc AcO O H HO O F3C AcO F3C HO O F3C O OH O i, ii iii–v vi, vii viii 119 120 Scheme 15 Reagents and conditions: i, LiAIH4 ; ii, NaIO4; iii, Ac2O, pyridine, rt; iv, Me3SiCF3, Bun 4NF, THF, rt; v, 1 M HCl, 3 h, rt; vi, TPAP-NMO, N2, rt; vii, KOH, MeOH, H2O, N2, 1 h, rt; viii, Al(PriO)3, 1-methylpiperidin-4-one, toluene, 4 h, reflux O H X i X = CHO X = CH2OH O H ii NO2 OH O H NH2 OH 124 iii Scheme 16 506 Natural Product Reports, 1998The azasteroids 126 are considered to mimic the enolate intermediate for the enzymic reaction [Eqn.(1)]. Mechanism-based inactivation (irreversible inhibition) of both isozymes by finasteride is suggested to occur through initial hydride reduction by NADPH at C-1 in a Michael reaction [Eqn.(2)].70,72 Kinetic isotopic studies73 and detection70 of 127 in inhibited 5·-SR1 support this mechanism. Irreversible inhibition of 5·-SR accounts for the long duration of the inhibitory eVect (t1/2=7 days) despite the short biological half life (t1/2=6 h) in humans. Several other aza- and oxo-steroids have a similar irreversible action. Finasteride at the clinical dose used for the treatment of BPH lowers DHT in the plasma to 20–35% of its baseline level.74–76 Incomplete removal of DHT for such a potent irreversible agent is unlikely to be due to incomplete inhibition of 5·-SR2 and suggests that residual DHT arises from the action of 5·-SR1 either in the prostate or peripherally which would account for the modest response to finasteride for the treatment of BPH. Another agent in clinical trials for BPH is the related acylurea derivative turosteride (128, FCE 26073) which inhibits 5·-SR2 (IC50=55 nM) with little eVect on 3‚-HSD (IC50=2.5 ÏM).77,78 Unlike finasteride, 128 decreases DHT levels without an associated rise in testosterone (8-fold increase for finasteride) which could prove clinically beneficial.77 Although testosterone is a weaker agonist than DHT at the androgen receptor it is usually at low levels compared to DHT in the prostate due to rapid metabolism; enhanced levels could partially nullify the eVect of reducing DHT.Turosteride reduced prostate size in a rat model of BPH when administered orally and reduced growth (45%) in the Dunning R 3327 rat prostatic carcinoma model79 at a dose of 200 mg kg"1 day"1.At a lower dose (50 mg kg"1 day"1) it had no eVect on tumour growth whereas prostate weight was decreased79 by 53%. This would suggest a diVerent sensitivity of normal prostate and prostate tumour to therapy with 5·-SR inhibitors.79 In this model, finasteride (25 mg kg"1 day"1) was inactive on tumour growth but prostate weight was reduced80 by 64%. Incomplete reduction of DHT levels by finasteride directed research towards the development of dual inhibitors of the 5·-SRs. Optimisation of the C17-carboxamido substituents with the inclusion of more hydrophobic residues, especially in the anilides, provided inhibitory activity against 5·-SR1.Equipotency towards both enzymes with IC50 or Ki values in the low nanomolar range has been achieved. A few of the compounds which were further examined in vivo are described here (see Frye58 for a more comprehensive survey).The anilide GG 745 (129) is a potent irreversible inhibitor of both forms81,82 and is 60-fold more active than finasteride towards 5·-SR1. After a single oral dose it reduces DHT levels in man to >90% of baseline levels.82 FCE 28260 (130), another anilide, has IC50 values of 36 and 3.3 nM for 5·-SR1 and 5·-SR2 respectively.83 It reduced prostatic growth in testosterone–implanted castrated rats five times more eVectively than finasteride when given orally for 7 days and, in adult male rats, also reduced DHT levels to a three fold greater extent after 24 hours.83 It showed no anti-androgenic eVect which is required in an agent for BPH.Replacement of the C17-carboxamido side chain of the 4-aza steroid structure with a C17-cholesterol side chain together with introduction of a C7‚-methyl group led to MK 386 (131) which is a potent, selective recombinant 5·-SR1 inhibitor (IC50=0.9 nM) with decreased 5·-SR2 activity (IC50=150 nM).84 MK 386 is a slow binding inhibitor and is in combination in clinical trials with finasteride to provide a dual inhibitory eVect in BPH therapy. Another potential use would be for treatment of skin disorders84 and male pattern baldness.The conformation of the C17-carboxamido group is important in 4-aza steroids in determining potency. Modelling suggests that in finasteride and other unsymmetrically –O H H 5a-SR NADPH H+ Testosterone enolate intermediate DHT OH O O (1) N CONEt2 CH3 H NH CONHBu t H O O 4-MA 125 Finasteride NH H NH N N NH NH N H CONH2 NH H H H H O 126 NADPH 127 finasteride O –O CONH2 CONH2 O : + + –O (2) N CON CH3 H Pri CONHPri 128 O Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 507substituted C17-amides the favoured conformations are 132 and 133 for (O—C20—N—CBut) with the torsion angles (C13—H17—C20—O) in the ranges "20 to 25) and +110 to 115.4) respectively.85 In the anilides (CONHPh) preference for the (E)-conformation decreases on disubstitution (CONRPh) with CH3, C2H5 or Ph to give the favoured (Z)-form, this change being associated with a decrease in potency towards both enzymes.81 The 6-aza steroids (e.g. 134) are extended mimics of the intermediate enolate ion (see Fig. 2), and furnish a group of inhibitors with dual activity on optimisation of the C17- carboxamido substituent. Within the series, generally, sensitivity towards 3‚-HSD is increased by the introduction of small lipophilic groups whereas 5·-SR1 activity is increased by large lipophilic groups.60 Selectivity towards 5·-SR1 compared with 3‚-HSD has been predicted by modelling of a series of conformationally restricted C17-substituents.86 Compound 134 has Ki,app values of 8.8 and 1600 nM towards 5·-SR1 and 3-HSD respectively and an IC50=<0.1 nM towards 5·- SR2.86 Compound 134 has demonstrated good (67%) oral availability in the dog and equivalent activity to finasteride in the rat model for DHT-dependent prostate growth.86 The 6-aza steroids are slow oVset inhibitors of the enzyme leading in practical terms to an almost irreversible action [Eqn.(3)]. Inhibitors carrying a negative charge have been designed as mimics of the negatively charged enolate ion intermediate postulated in the reaction pathway [Eqn. (1)]. Several structural types with C17-diisopropyl or pivalyl carboxamides have been examined, 135, 136 and 137 as well as their phosphinic (—PO3H2), phosphonic (—PHO2H) and sulfonic acid (—SO3H) counterparts (see review59).This group of compounds are potent inhibitors of 5·-SR2 with little eVect on 5·-SR1. Unlike the competitive aza steroids they are uncompetitive inhibitors of the enzyme with respect to testosterone and NADPH. Kinetics are in agreement with formation of a dead end complex between the inhibitor and the enzyme–NADP+ complex remaining after release of DHT (Eqn. 387), the tendency for this to occur presumably being due to their negative charge.Synergism exists between binding of the inhibitor and NADP+ in support of this view.87 Epristeride 136 (R1=H, R2=But) is a selective 5·-SR2 inhibitor (Ki=0.7–2 nM) with little eVect on 5·-SR1 (400– 450 nM) and seven related steroidogenic oxido-reductases.87 A theoretical advantage of an uncompetitive inhibitor is that the associated rise in the testosterone level on inhibition does not reverse the extent of inhibition as may be the case for NH CONH CF3 CF3 H NH CONH H C CF3 CF3 129 GG 745 130 FCE 28260 O O N CH3 H CH3 131 MK 386 132 133 O C O N R2 R1 C N O R2 R1 C O CH C NH C –O CHC NH + Fig. 2 NH CONH Bu t CF3 134 O E NADPH E. NADPH T E. NADPH.T. E.NADPH.I Ki Competitive I DHT E.NADP+.DHT NADP+ E.NADP+ E uncompetitive E.NADP.I Ki I + (3) CONR1R2 H CONR1R2 CONR1R2 135 136 137 HO2C HO2C HO2C 508 Natural Product Reports, 1998competitive inhibition. However since epristeride is less eVective in reducing DHT levels in vivo than finasteride this advantage is not apparent.Progesterone is about 24-fold better than testosterone as a substrate of 5·-SR1,80 and the low 5·-SR1 potency of the acrylate type of inhibitors e.g. 136 and 137 has been improved using a C20-oxo function in place of the C17-carboxamido substituent.88 The uncompetitive inhibitors 138 and 139 had Ki=ca. 3 nM for both isozymes88 and 138 caused significant and prolonged supression of plasma DHT levels in the cynomolgus monkey after a single oral dose. 3.3 Non-steroidal inhibitors In the chemotherapy of hormone dependent cancers, the general tendency has been to move away from steroidal agents in view of their potential hormonal agonist activities (or those of their metabolites) to cheaper, readily available non-steroidal agents. Many non-steroidal inhibitors of 5·-SR are now known with potency equivalent to that of the steroidal inhibitors described previously. These have either been designed as partial structures of the aza steroids and steroidal acrylate type inhibitors or as analogues of a lead structure ONO-3805 or, more recently, discovered by high throughput screening of commercial libraries of aryl carboxylic acids as mimics of the benzoic acid analogues of the steroidal acrylate inhibitors.The benzoquinolinones 140, based on the 4-aza steroids, are potent inhibitors of human 5·-SR1, activity being enhanced by the presence of an electron withdrawing / donating substituent at C8 (i.e.Cl>Me>F>H). LY 191704 (R=H) is a selective competitive inhibitor of 5·-SR1 (Ki=4.6 nM)89 with little eVect on 5·-SR2 (1750 nM) and has progressed to clinical trials for the treatment of BPH. The corresponding acrylate mimic 141 based on epristeride is a weak inhibitor of 5·-SR2 (Ki=260 nM). ONO-3805 (142) is a dual inhibitor with Ki=27 and 31 nM for 5·-SR1 and 5·-SR2 respectively.89 It has an uncompetitive action89 and binds to the enzyme–NADP+ complex as suggested by kinetic studies which is in accord with that for the acrylate inhibitors.Modification of the o-alkoxyaniline function of ONO-3805 to an indole with changes elsewhere in the structure gave (S)-143 with improved potency (IC50=40 and 4 nM for 5·-SR1 and 5·-SR2 respectively).69,90 This compound had good oral availability and a long half life in the dog and the rat. In the rat in vivo prostate model of shrinkage, after 10 days of an oral dose of 1 mg kg"1 the prostate weight was reduced by 35%.69 The benzodioxolane 144 had IC50=ca. 23 nM for both isozymes.69,90 FK 143 (145), a further development, was a non-competitive inhibitor towards both isozymes (Kie and Kies for 5·-SR1, 27.0 and 19.6 nM; 5·-SR2, 19.9 and 14.5 nM).91 FK 143 in vivo suppressed DHT plasma levels in the dog and rat and reduced growth in an androgen-dependent rat prostate model.92 In male ACI/Seg rats which spontaneously develop prostate cancer, a 20 ppm FK 143-containing diet reduced the incidence of cancer to 45.7% (control=62.9%).93 At a higher dose (200 ppm) the incidence (67.6%) was similar to that of the control although intraprostate DHT levels were significantly lower.At the higher dose the testosterone concentration is increased two-fold which could have a stimulatory eVect on cancer growth by overriding the reduced DHT levels as discussed earlier.93 In this series of inhibitors the essential requirements for high potency are the carboxylic acid and carbonyl functions.Modelling studies suggest that the carbonyl function determines the two conformations 146 and 147 responsible for 5·-SR2 and 5·-SR1 inhibitory acitivity respectively. The 2-methylindole 148 adopts conformation 147 and is mainly an inhibitor of 5·-SR1.60 The indolizine, FR 146687 (149) is a non-competitive inhibitor of both isozymes94 (5·-SR1, Kie=13.9 nM, Kies=10 nM; 5·-SR2, Kie=49 nM, Kies=43.6 nM) with no eVect on other steroid oxidoreductases.In mature rats and castrated rats COCH2CH2 F COCH2CH2 OCH3 138 139 HO2C HO2C N CH3 R H 8 140 R = H or CH3 141 Cl O Cl HO2C N (CH2)3 CO2H O O CH3 N C O (CH2)3 CO2H CH2CH(CH3)2 CH2CH(CH3)2 144 145 FK 143 C O CH2CH(CH3)2 NH C O (CH2)3 CO2H CH3 CH3 O CH CH3 N C O (CH2)3 O CH CO2H CH3 142 ONO-3805 143 O CH2CH(CH3)2 CH2CH(CH3)2 HN CH Jarman, Smith, Nicholls and Simons: Inhibitors of enzymes of androgen biosynthesis 509treated with testosterone propionate, ventral prostate and seminal vesicle weights were reduced to a greater extent than with finasteride.DHT concentrations in the prostate were significantly reduced in a dose-dependent manner and testosterone levels increased inversely.94 High throughput screening led to the discovery of the benzophenone and indole carboxylic acid series of inhibitors. The benzophenones 150 are potent, uncompetitive inhibitors of 5·-SR2 (150, X=O, Ki=5 nM).95 The indole carboxylic acids 151 are also potent inhibitors of 5·-SR2 (151, Ki=40 nM) but introduction of a 5-OCH3 group with switching of the benzyloxy group to the 6-position enhances potency towards 5·-SR1 (Ki=460 nM) as well as 5·-SR2 (Ki=20–30 nM).95 3.4 5·-SR Inhibitors in prostatic cancer chemotherapy Finasteride has been used clinically for the treatment of BPH74,96,97,98 with some success but finasteride monotherapy for advanced prostate cancer gave only a minor eVect in a small trial99 whereas finasteride administration after radical prostatectomy suggested, but did not prove, that a delay in progression is possible.98,100 However a large Prostate Cancer Prevention Trial, under the auspices of the National Cancer Institute, is underway with 18 000 men aged 55 years and over taking randomised 5 mg drug or placebo over a period of 7 years.101 The results of this trial are not yet available.The paucity of information regarding the usefulness of 5·-SR inhibitors in prostatic cancer since their introduction into the area of hormone-dependent diseases in the early 1980s does not bode well for a successful outcome in this life-threatening disease.Perhaps the pathophysiology of the disease renders the current target, 5·-SR2, redundant. Glimpses of such a phenomenon have appeared in the literature. Recent studies with cultures of hormone-dependent LNCaP human prostate adenocarcinoma cells as well as hormonedependent DU 145 human prostatic adenocarcinoma cells suggest that 5·-SR1 inhibitors may have a role in therapy.Studies with LNCaP cultures showed that a potent 5·-SR1 inhibitor (cellular IC50=5.8 nM) supressed DHT formation and reduced the stimulatory eVects of testosterone on proliferation. 102 With DU 145 cultures, the presence of 5·-SR1 but not 5·-SR2 was shown using the relevant cDNAs and selective inhibitors of the two isozymes.103 The relative lack of clinical eVectiveness of finasteride in treating metastatic adenocarcinoma may be attributable to the absence of the target 5·-SR2.103 Studies with turosteride in the Dunning R 3327 rat prostatic carcinoma model79 showed a diVerent sensitivity of normal prostate and tumour prostate to the action of the inhibitor, the latter being the least sensitive.Taken together these three studies may suggest changes in the pathophysiology of prostatic tumour tissue which require further investigation. 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ISSN:0265-0568    

DOI:10.1039/a815495y

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Enzymatic cleavage of aromatic rings: mechanistic aspects of the catechol dioxygenases and later enzymes of bacterial oxidative cleavage pathways Timothy D. H. Bugg* and Christopher J. Winfield Department of Chemistry, University of Southampton, Highfield, Southampton, UK, SO17 1BJ 11.1 1.2 1.3 22.1 2.2 2.3 2.4 33.1 3.2 41 Significance of oxidative aromatic ring cleavage The subject of this review is the oxidative cleavage of aromatic rings: a remarkable reaction in biological chemistry which has little precedent in organic chemistry, but which has a major role in the maintenance of the global carbon cycle.The first section will explain the environmental significance of this reaction, the various strategies used by micro-organisms for the breakdown of aromatic compounds and the relevance of this reaction to the biosynthesis of some natural products. 1.1 Occurrence of aromatic compounds in the biosphere For millions of years the biosynthesis and breakdown of aromatic rings has been an important part of the global carbon cycle.1 The woody tissues of plants and trees are composed primarily of cellulose and lignin, the latter being a heterogeneous polymeric material containing phenolic aromatic rings attached to three-carbon aliphatic linkages (see diagram).2 The biosynthesis of lignin occurs by the radical polymerisation of p-coumaryl alcohol 1, coniferyl alcohol 2, and sinapyl alcohol 3.2 The breakdown of lignin is initiated by fungi such as Phanerocheate chrysosporium using extracellular lignin peroxidase enzymes,3 but the smaller aromatic fragments generated by this breakdown are then degraded by both fungi and bacteria.It has been estimated that 1.5#1010 tons of carbon dioxide is converted annually into wood, of which 18–35% by dry weight is lignin.1 Since the breakdown of cellulose and HO Significance of oxidative aromatic ring cleavage Occurrence of aromatic compounds in the biosphere Strategies for bacterial degradation of aromatic compounds Oxidative cleavage in natural product biosynthesis Mechanistic aspects of the catechol dioxygenases Intradiol and extradiol catechol cleavage reactions Model chemistry for catechol oxidative cleavage Intradiol catechol dioxygenases Extradiol catechol dioxygenases Subsequent enzymatic steps in oxidative cleavage pathways Enzymatic steps following intradiol cleavage Enzymatic steps following extradiol cleavage References OMe RO OH OH OMe O OMe O OH Bugg and Winfield: Enzymatic cleavage of aromatic rings MeO ORO MeO OMe Diagrammatic representation of lignin OR OH OMe CH2OH CH2OH CH2OH OH OH OH 1 2 lignin is unique to micro-organisms, the global significance of aromatic biodegradation is clear.Since the dawn of the Industrial era, a rapidly increasing number of man-made aromatic compounds have found their way into the environment, for example as pesticides, detergents, oils, solvents, paints or explosives. Many of these man-made compounds can be fairly readily degraded by micro-organisms using the same enzymes used for the degradation of naturally occurring compounds.1 For example, the carbamate insecticide carbaryl (4) can be degraded via the bacterial naphthalene degradative pathway.4 However, a significant number of these unnatural, or xenobiotic, chemicals are degraded very slowly or not at all by micro-organisms, and hence persist in the environment.The persistence of toxic man-made chemicals in the environment which can contaminate freshwater and food is an issue of considerable public concern, which has led to the tightening of legislation concerning the application of these materials, and their disposal. Many of the more persistent man-made aromatic hydrocarbons contain chlorine substituents.5 The eVects of chlorination are to reduce both the solubility and the chemical reactivity of the ring.The presence of a chlorine substituent can either block the normal degradation step, or give rise to a reactive metabolic intermediate which is no longer processed. Thus, the biodegradation of chlorinated aromatics, which has been reviewed,5 is generally slower and more complex than that of the parent hydrocarbons. Chlorinated phenols are produced industrially on a large scale, and are found in freshwater supplies.Chlorinated phenols can be degraded by several strains of bacteria by an adaptation of the normal phenol degradation pathway.6 This OH O OMe OMe MeOOMe O OH OMe OMeO OMe 3OH O OMe OH MeO 513CO2H O O Cl NHMe O H Cl C Cl Cl CCl3 6 5 4 Cl OH Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl H Cl Cl Cl 9 10 7 8 OO Cl n Cl n Cl n Cl n 11 514 Natural Product Reports, 1998 12 pathway is also used for the degradation of the herbicide 2,4-D (5), which is degraded via 2,4-dichlorophenol.7 However, there are several classes of polychlorinated hydrocarbons which show high persistence in the environment.5 One well-known example is the insecticide dichlorodiphenyltrichloroethane (DDT, 6), whose widespread use in the 1950s led, via bioaccumulation in the food chain, to the severe poisoning of higher animals, as described in Rachel Carson’s ‘Silent Spring’.8 Most of the ‘organochlorine insectides’ are no longer used in the Western world, but residues of DDT and hexachlorocyclohexanes (HCH, 7) are still widely found in the fatty tissues of sea animals.9 The fungicide pentachlorophenol (PCP, 8) is still in use, and is degraded slowly via a novel pathway.10 Trichloroethylene (TCE, 9) and tetrachloroethylene (PCE, 10) are common contaminants of freshwater arising from the manufacture of PVC plastics, and are degraded slowly.11 Two important classes of polychlorinated aromatic pollutants are the polychlorinated biphenyls (PCBs, 11) and polychlorinated benzodioxins (PCDDs, 12).PCBs are found in a range of industrial applications, notably transformer oils, heat transfer fluids, dielectric fluids, and plasticisers.12 Although production of PCBs has eVectively ceased, approximately 5#109 lb are still in use or are stockpiled for disposal.PCB degradation is very slow, due to their very low solubility and reactivity, but is also complicated by the fact that these materials are produced as a complex mixture of isomers. Their industrial synthesis involves reaction of biphenyl with chlorine gas under high temperature and pressure, resulting in a possible 208 isomers. Industrial samples of PCB are therefore named according to the level of chlorine content, for example Arochlor 1248 contains 48% chlorine by weight.Since enzymes are inherently selective in their action, the presence of such a range of isomers further reduces the rate of degradation.12 Small quantities of PCDDs arise from the manufacture of (chlorophenoxy)acetic acid herbicides and other chlorinated phenolic chemicals.13 They are especially hazardous environmental pollutants, since they have carcinogenic and teratogenic properties. Their release in 1976 in an industrial explosion at Seveso, Italy was implicated in the widespread poisoning of livestock and chemical contamination of the local population.14 One final class of man-made aromatics worthy of note are the nitro-aromatics, used widely as explosives by the armed forces.The presence of a nitro group also reduces the inherent reactivity of the aromatic ring, hindering degradation, and leading to the persistence of nitroaromatic explosives in contaminated soil.15 We shall discuss in Section 1.2 the strategies used by micro-organisms for their degradation.1.2 Strategies for bacterial degradation of aromatic compounds The resonance stabilisation of the benzene ring, relative to a cyclic triene, is approximately 150 kJ mol"1. In order to degrade the benzene ring, this resonance stabilisation must somehow be overcome. There are two strategies used by micro-organisms for aromatic ring breakdown: (1) by aerobic bacteria: oxidation of the ring to a dihydroxy-aromatic compound (a catechol or a hydroquinone), followed by oxidative cleavage of the ring; (2) by anaerobic bacteria: reductive hydrogenation of the ring, followed by fragmentation of the cyclohexane ring skeleton.This review is primarily concerned with the former strategy, however references will be given to the latter strategy. Many simple aromatic compounds such as benzene, toluene, xylenes, benzoic acid, phenylacetic acid and phenylpropionic acid are degraded by aerobic soil bacteria such as Pseudomonas, Acinetobacter (Gram negative) and Rhodococcus (Gram positive).1 The latter two compounds are also degraded by Escherichia coli (Gram negative), an enteric bacterium.The catabolic pathways used by these bacteria are oxidative, proceeding via hydroxylation of the ring to give a catechol intermediate, in the following ways. Unsubstituted aromatics such as benzene, naphthalene, and biphenyl are converted into the corresponding 1,2- dihydroxycatechols via conversion to the cis-1,2-dihydrodiol, followed by oxidation (see Scheme 1a).The dihydroxylation R R OH dioxygenase O2, Fe2+ a FADNAD+ NADH NAD+ OH dehydrogenase NADH R R OH OH mono oxygenase FAD, O2 b oxidative ring cleavage OH NADH CO2 R R CO2H NAD+ dioxygenase O2, Fe2+ CO2– OH c FAD OH O NADH NAD+ Scheme 1 Strategies for aromatic degradation reaction is carried out by a family of three-component dioxygenase enzymes comprising: (1) an NADH-dependent flavin reductase; (2) a ferredoxin electron transfer protein containing two Rieske [2Fe/2S] clusters; and (3) a terminal dioxygenase subunit, containing a mononuclear iron(II) cofactor and two further [2Fe/2S] clusters.16 The subsequent oxidation of the cis-dihydrodiol is carried out by a family of NAD+-dependent dehydrogenases.Catechols are also commonly produced by orthohydroxylation of phenols, carried out by FAD-dependent monooxygenases (see Scheme 1b), whose mechanism of action has been reviewed.17 Other oxidative transformations can also give rise to catechols, including the oxidative decarboxylation of salicyclic acid (see Scheme 2c), which occurs via a dihydroxylation reaction similar to that in Scheme 1a.16Oxidative ring cleavage of the catechol intermediate can occur in one of two ways: intradiol (or ortho cleavage to give a muconic acid 13; or extradiol (or meta) cleavage to give a hydroxymuconaldehydic acid derivative 14, as shown in Scheme 2.The non-haem iron-dependent dioxygenase enzymes OH Intradiol cleavage: OH OH OH CO2– HO 15 Scheme 2 Dioxygenase catalysed aromatic ring cleavage which catalyse these reactions will be discussed in detail in Section 2. Oxidative cleavage is also observed in some cases using a p-hydroquinone substrate, such as gentisic acid 15.Thus there is a requirement for a 1,2- or 1,4-dihydroxyaromatic moiety for oxidative ring cleavage, which is consistent with the single electron transfer chemistry involved in the cleavage mechanisms of these enzymes. The ortho cleavage pathway for benzene in Pseudomonas putida is completed as shown in Scheme 3. The cis,cis-muconic Extradiol cleavage: –O2C CO OH 2– 13 Scheme 3 Catechol ortho-cleavage pathway O2C acid ring cleavage product 13 is cyclised enzymatically to form a five-membered lactone, muconolactone 16.An isomerase enzyme then transforms 16 into the unsaturated lactone 17, which is hydrolytically cleaved to give 3-oxoadipic acid (‚-ketoadipate, 18). 3-Oxoadipic acid is subsequently cleaved to give acetic acid and succinic acid, which can be utilised for growth via the tricarboxylic acid (TCA) cycle. Mechanistic studies on enzymes on this pathway will be discussed in Section 3.1.This ortho-cleavage pathway can be used for the degradation of chlorinated phenols by Pseudomonas putida, as shown in Scheme 4.5 2-Chlorophenol is hydroxylated to give 2-chloro- 6-hydroxyphenol, which is processed to give chloromuconolactone 19. The isomerase enzyme which would normally isomerise this muconolactone catalyses instead the elimination of HCl. Hydrolysis of the lactone gives an unsaturated intermediate, which can be reduced to 2-oxoadipic acid by an NADH-dependent reductase. 4-Chlorophenol is hydroxylated to give 4-chloro-2-hydroxyphenol, which is processed to give Bugg and Winfield: Enzymatic cleavage of aromatic rings 18 catechol 1,2-dioxygenase –O2C O2, Fe3+ CO2– 13 catechol 2,3-dioxygenase CO2– O2, Fe2+ OH 14 OH O CO2– O2, Fe2+ OHC – CO gentisate dioxygenase 2– CO2– O O H O O 16 17 CO2– H O 3C CO2 – + CO2– COO2C isomerase 2– cycloisomerase– CO2– Cl CO2– OH O H O isomerase O O OH Cl– 19 CO2– CO2– OH Cl O Cl O O CO2– OH Cl– OH OH Cl 2– 20 Scheme 4 Degradation of chlorophenols via ortho-cleavage chloromuconolactone 20.Hydrolysis of this lactone liberates chloride, and generates the same unsaturated intermediate. There are two branches for completion of the meta-cleavage pathway for catechol in Pseudomonas putida mt-2, as shown in Scheme 5.1 The ring fission product 2-hydroxymuconaldehyde dehydrogenase CO2– 21 NAD+ NADH –O2C tautomerase 14 hydrolase O OH decarboxylase –O2C CO 22 CO2 OHC hydratase Mn2+ O OH O O aldolase + CO HCO2– 2–2– CO Scheme 5 Catechol meta-cleavage pathway CO2– 515 CO2– acid can be oxidised to give 2-hydroxymuconic acid 21 which is decarboxylated through the combined action of a tautomerase enzyme (to be discussed in Section 3.2), and a decarboxylase enzyme to give 22. Alternatively, 2-hydroxymuconaldehydic acid can be cleaved hydrolytically to give formic acid and 2-hydroxypentadienoic acid 22.The latter pathway is used exclusively for the degradation of 2-methylphenol (o-cresol), in which case hydrolytic cleavage yields acetic acid rather than formic acid. 2-Hydroxypentadienoic acid 22 is further broken down by hydration to 4-hydroxy-2-oxopentanoic acid, followed by aldolase-catalysed cleavage to give acetaldehyde and pyruvic acid.Degradation of 4-methylphenol (p-cresol) also proceeds via meta-cleavage, and results eventually in the production of propionaldehyde rather than acetaldehyde. Longer aliphatic side chains attached to aromatic rings are typically broken down via ‚-oxidation into short side chains, followed by the above pathways. As mentioned above, simple chlorinated aromatics can be degraded by ortho-cleavage.Nitro aromatics are degraded by two strategies: (1) elimination of nitrite from a nitrophenol intermediate by a FADdependent monooxygenase (see Scheme 6); (2) reduction to an amino substituent, followed by oxidative cleavage.15 Aromatic compounds can also be degraded anaerobically by a reductive strategy, as shown in Scheme 7. In a denitrifying Pseudomonas strain, benzoic acid is converted into benzoyl CoA.18 The aromatic ring is then reduced to give cyclohexenyl CoA derivative 23 which is then a substrate for hydration and ‚-cleavage, analogous to ‚-oxidation. This remarkable series of reactions has recently been reviewed.18NO2 OH O2, FAD + NO2– 2 NADH 2 NAD+ OH COSCoA COSCoA CO2– ligase OH Scheme 6 Enzymatic conversion of p-nitrophenol into hydroquinone reductase ATP, CoASH NADH 23 COSCoA CO2– O OH OH CO2H OMe OH OMe CO2– Scheme 7 Anaerobic degradation of benzoic acid 1.3 Oxidative cleavage in natural product biosynthesis Biosynthetic incorporation studies have revealed that a small number of plant and fungal natural products are formed by oxidative cleavage of an aromatic precursor.The majority of these natural products are of polyketide origin, in which case the aromatic precursor was formed by an earlier cyclisation of a polyketide chain. We shall discuss the evidence for oxidative cleavage, and insights into possible mechanisms of aromatic ring cleavage.The first such natural product to be studied was penicillic acid 24, an antibiotic and potent carcinogen produced by several fungi. Isotope labelling studies showed that penicillic acid is formed by oxidative cleavage of the aromatic precursor orsellinic acid 25.19 A biosynthetic pathway is shown in Scheme 8. Similarly, the biosynthesis of tetronic acid 26 dioxygenase O O OH OH OH 25 O OH OMe OMe O O O OH 24 Scheme 8 Biosynthesis of penicillic acid.The labelling pattern from [13C2]acetate is shown. from Penicillium multicolor was shown by 13C2-acetate labelling studies to be consistent with oxidative cleavage of precursor 27, which could in turn be generated from oxidative decarboxylation of a benzoic acid 28 (R=H) precursor (see Scheme 9). This hypothesis was confirmed by feeding of the labelled ethyl ester 28 (R=Et), which was incorporated into 26.Incorporation of [1-13C, 1-18O]acetate revealed the expected pattern of 18O labelling, consistent with a dioxygenasecatalysed oxidative cleavage reaction.20,21 Reductiomycin (29) is a further example of a polyketide natural product arising 516 Natural Product Reports, 1998 OH CO2H HO HO OH 28 27 OH dioxygenase cleavage OH HO O HO OH O HO O O O 26 HO Scheme 9 Biosynthetic pathway for tetronic acid 26. The labelling pattern from [13C2]acetate is indicated.OH O NHO O O O 29 from oxidative cleavage of an aromatic precursor, which has been verified by incorporation of [13C]-carboxy 4-hydroxybenzoic acid into 29.22 The betalains are a small family of unusual heterocyclic amino acids found as yellow and violet pigments in plants of the Centrospermae order and mushrooms of the Amanita and Hygrocybe genera. Incorporation of 14C-tyrosine con- firmed that betalamic acid 30 and its derivative indicaxanthin 31 are derived from oxidative cleavage of 3,4- dihydroxyphenylalanine (DOPA, 32), as shown in Scheme 10.23 An extradiol dioxygenase enzyme has been purified from Amanita muscaria which catalyses the conversion of 32 into 30 (cleavage b).24 Similarly, extradiol cleavage of the C-2—C-3 bond (cleavage a), followed by intramolecular cyclisation, gives stizolobinic acid 33 and muscaflavine 34.Two other mechanisms have been proposed for oxidative ring cleavage in natural product biosynthesis. Firstly, enzymatic Baeyer–Villiger oxidation of cyclic ketones to the corresponding lactones is precedented by the enzyme cyclohexanone monooxygenase.25 Ring expansions of this type have been proposed in a number of biosynthetic pathways, for example in the conversion of anthraquinone precursors into purpactin A (35),26 as shown in Scheme 11, and in the course of aflatoxin biosynthesis.27 Secondly, evidence has been obtained for the formation of quinone epoxide intermediates in the biosynthesis of patulin 36 in Penicillium patulum.28 A cell-free extract of P.patulum has been shown to convert gentisyl alcohol into the quinone epoxide phyllostine 37, which was subsequently converted into patulin, presumably through the ring cleavage mechanism shown in Scheme 12.29 This fragmentation would result in the same ring fission product as that resulting from dioxygenasecatalysed cleavage, although one atom of oxygen would be derived from water, rather than both from dioxygen.Finally, we note that several polyketide natural products biosynthesised via aromatic ring cleavage contain a methyl ketone (-COCH3) at the site of ring cleavage. This functional group is not reminiscent of dioxygenase-catalysed ring cleavage products, but might instead be the product of a retro-aldol cleavage reaction. Examples are botryodiplodin 38,30 verrucarin E 39,31 and spicifernin 41.32 Botryodiplodin has been shown to be derived from the common polyketide naturalHO2C NH CO2H CO2H NH2 dioxygenase cleavage a O CHO NH2 a 34 CO2H b CO2H OH OH 32 NH CO2H 2 OH dioxygenase cleavage b CO2H CO2H O O CO2H 33 NH2 NH2 O OH O O CO2H HO2C CO2H NH HO CO2H CO2H NH N + CO2H CO2– O 31 30 Scheme 10 Biosynthesis of the betalein pigments OH O OH OH OH O HO mono oxygenase (Baeyer-Villiger oxidation) O O O OH OH O O HO O oxidative phenolic coupling O OH OH OH OMe O OMe O O O AcO AcO O OHO OH 35 Scheme 11 Biosynthesis of purpactins A and B product orsellinic acid 25,30 which has also been proposed as a biosynthetic precursor for verrucarin E.31 In these examples the biosynthetic precursor is a 1,3- dihydroxy aromatic compound, which could exist in a minor tautomeric form as a cyclohexene 1,3-diketone.This tautomer could be cleaved via a retro-aldol reaction, as shown in Scheme 13(a).In this case the product of retro-aldol cleavage could readily decarboxylate, and could be converted biosynthetically into either botryodiplodin 38 or verrucarin E 39. This type of retro-aldol ring cleavage has been implicated in the catabolic pathway for degradation of phloroglucinol 40 in Bugg and Winfield: Enzymatic cleavage of aromatic rings CH2OH CH2OH CH2OH O HO O HO OH O OH O O 37 O CH2OH CH2OH OH O O O OH CO2H OH O O 36 O Scheme 12 Biosynthesis of patulin CO CO2H O O OH (a) O 2H CO2 H – HO OH OH O 25 O O COR O HO O O38 NH39 OH OH OH (b) O HO O OH NADPH NADP+ HO 40 Scheme 13 (a) Biosynthesis of botryodiplodin 38 and verrucarin E 39: a possible hydrolytic cleavage.(b) The pathway for degradation of phloroglucinol 40 in E. oxidoreducens. Eubacterium oxidoreducens, as shown in Scheme 13(b).33 A similar retro-aldol cleavage pathway can be drawn for spicifernin 41 (Scheme 14).32 O OH O HO2C• • O OH O MeO2C Scheme 14 41 2 Mechanistic aspects of the catechol dioxygenases 2.1 Intradiol and extradiol catechol cleavage reactions The intradiol and extradiol oxidative cleavage reactions, shown in Scheme 15, are catalysed by non-haem irondependent dioxygenase enzymes, which incorporate both atoms of molecular oxygen into the ring fission product. The intradiol and extradiol dioxygenases were discovered through the work of Hayaishi in the 1950s, who first proposed a mechanistic framework for their reactions.34 The intradiol dioxygenases utilise a mononuclear non-haem iron(III) cofactor for substrate binding and catalysis.These enzymes are a CO2H 517R a intradiol dioxygenase –O2C R O2, Fe3+ b CO2– OHa OH O CO2– b extradiol dioxygenase OH R O2, Fe2+ Scheme 15 Reactions catalysed by intradiol and extradiol dioxygenases characteristic red–brown colour when purified, due to a tyrosinate–iron(III) charge transfer interaction. The extradiol dioxygenases utilise a mononuclear iron(II) cofactor for substrate binding and catalysis.The fact that these two families of enzyme catalyse similar yet distinct oxidative cleavage reactions, and have similar but distinct cofactor requirements, poses several interesting questions. Does the oxidation state of the metal cofactor control the regiospecificity of the reaction, and if so how? What is the coordination geometry of the metal cofactor in each family, and what are the ligands for the metal cofactor? Does the coordination geometry control the regiospecificity of the reaction? Or are there stereoelectronic factors which control the intradiol vs.extradiol reaction mechanism? This section will describe first of all a range of model chemistry which is relevant to the intradiol/extradiol cleavage reactions.The enzymology of each family will then be discussed, in the light of recent X-ray crystal structures and mechanistic studies on members of both families of enzyme. 2.2 Model chemistry for catechol oxidative cleavage Since the discovery of the catechol dioxygenases, many attempts have been made to mimic these reactions nonenzymatically. The intriguing metal cofactor specificity shown by the catechol dioxygenases suggested that the metal cofactor had a major role in the active site chemistry, hence most attempts have involved metal ion complexes of catechol substrates.The most reactive model systems developed to date are based on iron(III) complexes. The first example was an FeIIInitrilotriacetate complex 42, which was reported to convert 3,5-di-tert-butylcatechol catalytically over a period of 4 days in the presence of oxygen to give the furanone derivative 43b in 80% yield.35,36 An X-ray crystal structure of this complex showed the catechol substrate bound in bidentate fashion, with the geometry around the central FeIII close to octahedral.36 Labelling studies with 18O2 on this system revealed the incorporation of one atom of 18O into furanone 43b, consistent with the existence of an anhydride intermediate 43a as shown in Scheme 16.36 Subsequent studies using a range of metal complexes showed a correlation between the reactivity of the FeIII-ligand system and the Lewis acidity of the metal centre, which could be quantitatively assessed by measuring the redox potential for the catechol-to-semiquinone oxidation.37 Of the complexes studies, the FeIII-nitrilotriacetate complex 42 showed the highest reactivity, and the highest redox potential of +59 mV (and hence the highest aYnity of the catechol ligand for the FeIII centre).37 Further studies by Que and co-workers led to the discovery of more reactive FeIII complexes,38,39 the most active of which was FeIII-tris(2-pyridylmethyl)amine 44.This complex was found to react with dioxygen within minutes to form furanone 43b in 98% yield, at a rate of 15 M"1 s"1, approximately 1000-fold faster than complex 42. Analysis of complex 44 by X-ray crystallography and 1H NMR spectroscopy revealed a very strong iron–catecholate interaction, and increased semiquinone character in the bound substrate.It was therefore 518 Natural Product Reports, 1998 But But But But O– –O FeIII –O O O– O– – O N O O O OO– L4Fe 42 But But But ButCO2R O O O O O43b 43a Scheme 16 Intradiol cleavage catalysed by FeIII complex 42 But But But But But But O– –O • FeIII N O N O –O O N O O– N O– O L4FeIII L4FeII 44 Scheme 17 Semiquinone activation proposed for FeIII complex 44 proposed that formation of a transient FeII-semiquinone intermediate preceded reaction with dioxygen, as shown in Scheme 17.Several other model systems based on iron(III) complexes have also been developed. A bipyridine(pyridine)–iron complex in THF has been reported to produce intradiol cleavage products,40 and a 5–10% yield of extradiol cleavage products.41 Labelling studies with 18O2 on this model system revealed a stepwise insertion of 18O into the reaction products, consistent with the mechanism shown in Scheme 16.41 More recently it has been reported that use of FeCl2 and FeCl3 in THF–water and THF–pyridine gives up to 40% yield of the extradiol lactone product 45, and up to 15% yield of the But But But But OHC O O OH HO2C O 46 45 Cl O– FeIII NH HN O– FeIII N O NH R 48 47 extradiol cleavage product 46, as well as the intradiol products 43a and 43b.42 Interestingly, use of FeCl2 gave rise to more extradiol cleavage, and use of FeCl3 to more intradiol cleavage, as found in the enzymatic systems.42 Further FeIII complexes 4743 and 4844 have been reported to give up to 80% yields of ring cleavage products in 1 h.Complexes of other transition metals have also been found to show catalytic activity for oxidative ring cleavage.Copper(I) chloride in pyridine–methanol converts catechol into cis,cismuconic acid monomethyl ester, probably via an anhydride intermediate, in an 80% yield on a 5 g scale.45 A range of copper(I) and copper(II) catecholate and semiquinone complexes have been studied by EPR spectroscopy and X-ray diVraction, and a correlation observed in this series between reactivity and the formation of a semiquinone intermediate.46 Similarly, studies of CoIII–tetramine complexes by NMR and EPR spectroscopy concluded that formation of an activated intermediate CoII–tetramine–semiquinone complex was required for reaction with dioxygen.47 Finally, two interesting model systems have been characterised by Bianchini and co-workers.A RhIII–triphos–catecholate complex† was found to react with dioxygen to form a stable RhIII–semiquinone–superoxide complex 49, which was characterised by X-band EPR spectroscopy.48 The corresponding But But O –• O O –O PPh2 PPh2 RhIII OIrIII •O O– O– PPh2 PPh2 Ph2P Ph2P 49 50 NHAc O But ButCO2R O CHO O NHOH 52 ". 2 2 CO2H 2 in tetrahydro- 51 iridium(III) complex was found to react with dioxygen to form a cyclic peroxide adduct 50, whose structure was elucidated by X-ray crystallography,49 and which slowly reacted to give intradiol cleavage products.50 The structural elucidation of 49 and 50 provides experimental support for later mechanistic proposals for the catechol dioxygenases.The observation that metal–semiquinone complexes appear to show higher reactivity towards dioxygen, together with the characterisation of a metal–semiquinone–superoxide complex 49, suggests that the initial steps of the metal catalysed reactions involve single electron transfer reactions mediated by a redox-active metal ion.Both catechol and dioxygen readily undergo single electron redox chemistry, as shown in Scheme 18: catechol can be oxidised to give a stable semiquinone, and thence to give an o-benzoquinone; and the unreactive triplet ground state of dioxygen can be reduced to give the reactive superoxide anion (O ),51 and further reduced to peroxide (O 2").Indeed, several metal(II) complexes are known to form stable metal(III)–superoxide complexes.51 The formation of a metal(III)–semiquinone– superoxide complex could therefore proceed via single electron transfer from catechol to metal, to give a metal(II)- semiquinone intermediate, followed by single electron transfer from metal to dioxygen. The cyclic peroxide 50 could be formed either by recombination of semiquinone with superoxide, or by reaction of the semiquinone ligand directly with dioxygen, followed by electron transfer from the metal ion.It is therefore of interest to note that oxidative cleavage of catechols is observed upon treatment with potassium superoxide in organic solvents, in the absence of metal ions. Treatment of 3,5-di-tert-butylcatechol with KO †Triphos=tris(diphenylphosphinomethyl)ethane.Bugg and Winfield: Enzymatic cleavage of aromatic rings catecholate semiquinone O– O• O – 1e– – 1e– O– O– O O • o-benzoquinone 519 O– Scheme 18 Single electron oxidation of catechol furan gives a 10–40% yield of the furanone intradiol cleavage product 43b, and a 55–90% yield of the hydroxylated furanone 51.52 The latter product could be envisaged to arise from extradiol cleavage, however subsequent studies imply that 51 arises from epoxidation of an o-benzoquinone intermediate.53 Treatment of catechol with KO2 in dimethyl-sulfoxide leads to the rapid formation of the extradiol cleavage product 46, although in only 5–10% yield.54 Treatment of N-acetyl-Ltryptophan with KO2 in DMSO also generates the oxidative cleavage product 52.55 These model studies support the intermediacy of superoxide in the oxidative cleavage mechanism. Finally, there is literature precedent to support possible reaction mechanisms for the later steps of the oxidative cleavage mechanism.The cyclic peroxide 50 provides experimental evidence for the existence of a hydroperoxide intermediate in the oxidative cleavage mechanism. How might such a hydroperoxide be processed to give the intradiol and/or extradiol cleavage products? The most likely mechanisms are illustrated in Scheme 19. Attack of the peroxide onto either the neighbouring C-1 carbonyl or the neighbouring C-3 alkene would generate dioxetane intermediates, which could fragment to give either the intradiol or extradiol cleavage products respectively.There is evidence to support this type of mechanism under alkaline conditions, in the absence of metal ions.56 Alternatively, a 1,2-rearrangement could occur, analogous to a Baeyer–Villiger oxidation or a Criegee rearrangement. Acyl migration of the C-1 carbonyl would generate a sevenmembered cyclic anhydride intermediate, as observed in model intradiol cleavage reactions.Alkenyl migration of C-3 would generate a seven-membered lactone intermediate, which could be hydrolysed to give the extradiol cleavage product. Baeyer– Villiger oxidation of 1,2-diketones is known to give anhydride products,57 in support of the acyl migration route. Furthermore, investigation of the mechanism of Baeyer–Villiger oxidation of o-benzoquinones using 18O labelling concluded that an acyl migration mechanism was operating.56 However, investigation of the mechanism of Baeyer–Villiger oxidation of benzil using 17O labelling, as shown in Scheme 20, has suggested that the mechanism in this case may involve formation of a 1,2-epoxide.58 Alkenyl migration of a cyclic hydroperoxide is also precedented, via the Criegee rearrangement shown in Scheme 21.59 However, there is an alternative mechanism for the formation of a lactone intermediate, as shown in Scheme 19, since the participation of the neighbouring alkene to give a 2,3-epoxide is possible.This type of alkene participation is precedented in the rearrangements shown in Scheme 22 and 23: the formation of cyclohexyl epoxides from a cyclohexyl hydroperoxide,60 and the formation of epoxide products from polyunsaturated fatty acid hydroperoxides by treatment with either trifluoroacetic anhydride,61 or with soybean lipoxygenase.62 In summary, there is good chemical precedent from model systems to support the metal ion-catalysed oxidative cleavage of catechols, but there are several plausible mechanistic possibilities for the later steps of the cleavage mechanism.CHO CO2– R OO RBUT OH EXTRADIOL CLEAVAGE 17 Ph O 17 Ph Scheme 20 Baeyer–Villiger oxidation of 1,2-diketones O OOH O (CF3CO)2O 2,6-dichloropyridine Scheme 21 Alkenyl migration of a cyclohexyl hydroperoxide 2.3 Intradiol catechol dioxygenases The intradiol dioxygenases catalyse the transformation of catechols into cis,cis-muconic acids by direct incorporation of both atoms of dioxygen into the substrate resulting in aromatic ring cleavage between the two hydroxy groups. The non-haem iron dependent dioxygenases have been a subject of great interest in recent years and their metallo-biochemistry has been reviewed.63,64 The best studied of this group of enzymes are the catechol-1,2-dioxygenases (1,2-CTD) and protocatechuate-3,4- dioxygenase (3,4-PCD), which have been extensively characterised as a result of the rich spectroscopic properties of the non-haem iron(III) centre.Detailed structural information is now available for 3,4-PCD from Pseudomonas aeruginosa (now reclassified as Pseudomonas putida), whose X-ray crystal structure has been solved with and without bound substrate.65–68 The catechol-1,2-dioxygenases have been divided into Type I and Type II enzymes by Dorn and Knackmuss.69 Type I are chromosomal enzymes characterized by their narrow substrate range and relatively high specificity for catechol.Type II are plasmid-mediated enzymes induced when the bacteria are grown on media containing halogenated aromatic compounds. They are relatively non-specific with a wider substrate range, catalysing faster breakdown of chloro and methyl substituted 520 Natural Product Reports, 1998 OH O O O OH O O– O– 1,2-dioxetane formation 2,3-dioxetane formation [attack at C-1] OH O 3 O OH alkenyl migration acyl migration CO O2 2– O O 1 [migration of C-3] CO [migration of C-1] 2– O O INTRADIOL CLEAVAGE 2,3-epoxide formation [p participation] O 1,2 epoxide formation OH O O OH O [attack at C-3] + O O– Scheme 19 Possible mechanistic courses for the later stages of oxidative ring cleavage O O O But Ar But But O But OOH R CH3COCl O O O R pyridine O O– O But But O R R O + O Ph O But acyl migration17 17 O MCPBA But But But O O O O O+ Ph AcO– But But Scheme 22 Epoxide formation in hydroperoxide rearrangements CF3 O CO2R O O O + O (CF3CO)2O 2,6-lutidine H OOH soybean lipoxygenase O CO2R O CF3 O CO2R O Scheme 23 OH catechols but with lower catalytic eYciency for catechol than type I.Type II enzymes share low sequence homology with type I enzymes; the conserved amino acids include structurally important residues and those shown to be involved in ferric ion coordination, indicating a possible common ancestor.65,70–73 1,2-CTD has a requirement for high spin ferric ion (Fe3+) for activity.Its quaternary structure is composed of two subunits with one metal ion per subunit, and exists usually as a homodimer (·2Fe2 although ·2Fe is known), but heterodimers (·‚Fe2) have been observed.74–80 Subunit molecular weights range between 34–40 and 27–30 kDa for type I andtype II respectively.Dissociation of the subunits at high ionic strengths usually produces inactive enzyme but recent gel filtration studies have shown that activity is retained on dissociation of a 78 kDa 1,2-PCD homodimer from Acinetobacter radioresistens at high salt concentrations.81 The protocatechuate-3,4-dioxygenase enzymes are chromosomally encoded proteins composed of equimolar quantities of two non-identical subunits, with a requirement for ferric ion.82 The most common Fe/·‚ stoichiometry is one iron per ·‚ heterodimer (·‚Fe) although ·2‚2Fe has also been observed.83 The quaternary structures are of the general composition (·‚Fe)n where n=4, 5, 8, 10 and 12.70,72,83–85 The molecular weights range between 22–24 and 25–40 kDa for ·- and ‚-subunits respectively.Sequence alignments between ·- and ‚-subunits show conservation of the iron binding residues indicating an evolutionary relationship, and also show substantial overall sequence identity with 1,2-CTD I and II indicating common ancestry.74,86,87 3,4-PCD displays a narrow substrate specificity but will metabolise halogenated catechols albeit with extremely reduced catalytic eYciency.88 Inhibitor studies have shown that the carboxylate moiety of the substrate is not essential for inhibition to occur but the presence of a para-hydroxy group greatly improves inhibitory properties.89 Bull et al.have observed two intermediates in the reaction of protocatechuic acid, O2 and 3,4-PCD from Pseudomonas putida using stopped flow kinetic analysis.90 They are denoted ESO2 and ESO2* in Scheme 24 and are characterised by their on 1 ES E + S kkoff Ok 2 ESO2 k2 k3 E + P ESO2* Scheme 24 Identification of reaction intermediates diVerent UV spectra.90 The formation of ESO2, as a function of O2 concentration, shifts the Tyr-FeIII charge transfer band at 460 nm of the E · S complex to 2400 nm, and reduces the catecholate-FeIII charge transfer band at 2700 nm.Using the slow substrate 3,4-dihydroxyphenyl propionate (3,4-DHP) a stabilised ESO2* intermediate with a distinct peak at 540 nm was observed. This is characteristic of an E ·P or FeIIIdicarboxylate complex.91,92 A visible absorbance remains in both ESO2 and ESO2* indicating that the Tyr-FeIII charge transfer interaction remains throughout the reaction cycle.93 Que and Mayer have also observed two distinct reaction intermediates, termed I and II, by stopped flow analysis of the reaction of 1,2-CTD from Pseudomonas arvilla with pyrogallol and O2.94 The spectrum of II corresponded closely to that of ESO2 observed by Bull et al.and not to the second intermediate ESO2*. However, Walsh et al. found with catechol as substrate that intermediate I was spectrally more like ESO2* than ESO2 indicating diVerent processing of catechol and pyrogallol by the enzyme.95 The crystal structure of 3,4-PCD from P.aeruginosa has been determined by X-ray analysis at 2.15 resolution.65,67 The enzyme is a highly symmetric (·‚Fe)12 aggregate, the protomer ·- and ‚-subunits showing similar tertiary structures, suggesting that the ancestral enzyme was a homodimer with two active sites. A non-functional active site remains in the ·-subunit which does not bind iron.The coordination geometry of the ‚-subunit active site is best described as trigonal bipyramidal in which Tyr447 and His462 occupy the two axial sites while Tyr408 and His460 bind equatorially (Fig. 1). This confirmed previous resonance Raman data which uncovered two distinct tyrosinate vibrations at 1254 and 1266 cm"1.93 The tyrosinate vibrations were subsequently Bugg and Winfield: Enzymatic cleavage of aromatic rings Scheme 25 18O2 labelling studies on the reaction of pyrogallol with catechol 1,2-dioxygenase from Pseudomonas arvilla 17 2 assigned on the basis of their excitation profiles.96 Histidine ligation had previously been identified by 54/56Fe isotope shifts in the resonance Raman spectra of native and labelled 3,4-PCD, and by prominent peaks in the Fourier transformed EXAFS data at 3.3 Å.97 Evidence for water ligation was first detected by line broadening of the EPR spectrum of lyophilised 3,4-PCD which had been rehydrated in H O.98 Subsequent EXAFS studies on the spectroscopically similar 3,4-PCD from Brevibacterium fuscum revealed a short Fe–O bond for the solvent ligand consistent with hydroxide rather than water.99 In the previous section the possible courses of the reaction mechanism, either via a dioxetane, epoxide or anhydride, and their chemical precedents were discussed (see Scheme 19).Insights into the intradiol dioxygenase reaction mechanism have been obtained from 18O2 labelling studies of pyrogallol cleavage by 1,2-CTD from Pseudomonas arvilla as summarised in Scheme 25.100 Mass spectrometric analysis of isolated and diazomethane derivatised products revealed the formation of two distinct metabolites: 2-pyrone-6-carboxylic acid 53, showing 99% incorporation of a single atom of 18O; and 2-hydroxymuconic anhydride, leading to acids with 74% incorporation of two atoms of 18O (54), and 24% incorporation of one atom of 18O 55.If the reaction proceeds via a dioxetane intermediate then complete incorporation of two atoms of 18O is expected. The results clearly show a significant amount of singly labelled product, ruling out a dioxetane intermediate. Single oxygen atom incorporation is only possible via a relatively long lived anhydride intermediate allowing free exchange of an 18O2 derived, iron bound hydroxide ion with solvent.Interestingly, single oxygen atom incorporation was not observed using the natural substrate, catechol. It has been hypothesised that the muconic anhydride produced is too reactive towards the iron bound hydroxide and therefore too short lived for exchange of solvent and iron bound label to occur. The singly labelled 2-pyrone-6-carboxylic acid is thought to be derived from attack of an active site nucleophile on the anhydride intermediate, followed by displacement by the internal enolate oxygen to give a pyrone, labelled only at the carboxylate, in agreement with prior studies.101 It is possible that the turnover-dependent inactivation of these enzymes occurs via attack of an active site nucleophile on the anhydride intermediate to form an irreversibly bound acyl–enzyme species.521Tyr447 Tyr408 O HN N FeIII His460 OH Wat827 O N NH TyrTyr447 408 O– His462 O– 3,4-PCD O O HN N O FeIII His460 O N NH His462 Tyr447 Anaerobic 3,4-PCD·PCA Tyr408 O– O– N O O HN N O FeIII C His460 O N N NH His462 3,4-PCD·INO·CN Figure 1 Schematic coordination geometries of E, E · S and E · I · CN complexes of 3,4-PCD from Pseudomonas aeruginosa Crystallographic analysis of 3,4-PCD complexes with its natural substrate protocatechuic acid (PCA) and the substrate analogues 2-hydroxyisonicotinic acid N-oxide (INO) and 6-hydroxynicotinic acid N-oxide (NNO) have given new insight into the events involving substrate binding to the metal centre.68 Chelation of the substrate (i) displaces the equatorial solvent derived hydroxide releasing a water molecule to give a monodentate E · S complex, (ii) displaces the axial Tyr447 which then swings away to hydrogen bond with Tyr16 and Asp413 producing a bidentate E · S complex, (iii) changes the FeIII coordination environment to near octahedral with a sixth empty ligand site trans to His460 and (iv) creates a small pocket adjacent to the sixth site and the site of cleavage of the aromatic ring.Active site representations of the anaerobic 3,4-PCD · PCA complex and the aerobic 3,4-PCD · INO · X and 3,4-PCD · NNO · X inhibitor complexes (where X=H2O or "CN) are shown in Fig. 1. The 3,4-PCD · PCA complex is a five coordinate complex while the inhibitor complexes of INO and NNO clearly show the presence of a sixth ligand. The ligand is initially water and can be displaced on treatment with cyanide, which is then bound in an unfavourable, bent conformation.The longer C3—O—Fe versus C4—O—Fe bond length in the E · S complex is rationalised by the ligand Tyr408, trans to C3—O", being more electron donating than His462, trans to C4—O". This favours ketonisation of the bound substrate at C3, and reaction with dioxygen at C4. INO and NNO are known to bind as ketonised tautomers analogous to the proposed intermediate (56–59, Scheme 26).Previous analysis of these E · I complexes has shown markedly diVerent spectroscopic properties to the anaerobic E · S com- 522 Natural Product Reports, 1998 CO CO2– 2– INO O– 57 O– 56 CO CO +N N O– O 2– 2– NNO N+ N O– O– O O– 59 2 surrogate NO in the 58 Scheme 26 INO and NNO tautomers plex suggesting the electronic nature of the complexes are diVerent.It may be this factor which allows for binding of the sixth ligand.102 complexes have shown that monodentate binding to the metal centre occurs.104 The high spin ferric centre acts as an internal paramagnetic shift reagent to shift ligand proton resonances out of the diamagnetic region in a manner characteristic of the ligand. Resonances at 105 ppm were observed with both 4-methylcatechol and p-cresol whereas resonances at "30 and Addition of nitric oxide to the EPR silent, reduced 3,4-PCD (containing ferrous ion in the active site) gave an EPR active, S=3/2 species which blocked subsequent binding of all other ligands.103 Binding of NO to preformed 3,4- PCDred · CN" complex gave rise to an EPR active, S=1/2 species [3,4-PCDred · (CN)xNO] exhibiting superhyperfine splitting from 13CN", 14/15NO and a protein derived 14N.103 Further treatment of the 3,4-PCDred · (CN)x · NO complex with INO displaces the cyanide ligands to aVord a 3,4- PCDred · INO · NO complex analogous to the known oxidised 3,4-PCDox · INO · CN complex.In contrast, proton NMR studies on 1,2-CTD-substrate "50 ppm were observable in model complexes showing meta and bidentate binding respectively.105 An assignment of the endogenous ligands has also been made based on observations in model complexes.106 Considering all the evidence a catalytic mechanism has been proposed, as illustrated in Scheme 27.The current mechanistic hypotheses state the importance of (i) multiple ligand sites for catalysis, (ii) dynamic ligand exchange and availability of binding sites during the catalytic cycle and (iii) asymmetry in the active site arising from ligands trans to the available binding sites evident from the EPR spectra of the 3,4- PCDred · (CN)x · NO complex. The iron(III) centre is thought to chelate the substrate then activate it towards electrophilic attack by dioxygen in the ketonised form.No direct evidence for O2 binding to the ferric centre has been observed, however the E · S complex is unable to bind the O +3 oxidation state, requiring reduction to the +2 oxidation state before chelation can occur. Comparison of the EPR spectra of E · S · NO complexes of extradiol dioxygenases and reduced intradiol dioxygenases reveals several diVerences. In the extradiol complexes both NO and substrate bind in a non-competitive manner to the ferrous iron centre (bidentate coordination) whereas in the reduced intradiol · PCA complexes the substrate binds through the C-4 OH while the C-3 OH and NO compete for the fifth coordination site.107,108 It is still possible that a fleeting FeII · semiquinone complex may be able to bind O2.This might impart some radical character on the substrate for direct combination with triplet oxygen or superoxide species (derived from a single electron transfer from FeII to dioxygen to regain the FeIII ion).The process would have to be suYciently fast to be unobservable by spectroscopic techniques, but would be consistent with the FeII · semiquinone model complexes.38,39CO2– Tyr447 Tyr447 O O N O N + PCA His460 OH FeIII FeIII :OH – PCA OH2 O O Tyr408 N H Tyr447 CO2– O O –O2C O CO N O CON His462 2– 2– FeIII OH O N –OH Once O2 and substrate have reacted, a C-4 peroxy intermediate has been postulated which would readily coordinate to the iron centre.Calculated bond lengths and angles would appear to be consistent with energy minimised simulations.68 The model transition metal complexes mentioned in section 2.2 support the formation of the peroxy intermediate.49,50 Conversion of the peroxy intermediate to an anhydride would be consistent with an acyl migration Criegee rearrangement. Subsequent hydrolysis by the iron bound hydroxide gives the ring fission product.The proposed overall reaction mechanism is shown in Scheme 27. 2.4 Extradiol catechol dioxygenases The extradiol dioxygenases catalyse the transformation of catechols into 2-hydroxymuconaldehyde acids by direct incorporation of both atoms of dioxygen into the substrate. In contrast to the intradiol dioxygenases, ring fission occurs adjacent to the two hydroxy groups of the catechol. Enzymes cleaving substituted catechols can be subdivided into the proximal and distal extradiol dioxygenases to distinguish the position of ring cleavage in relation to the substituent group.Protocatechuate-2,3-dioxygenase (2,3-PCD) and protocatechuate-4,5-dioxygenase (4,5-PCD) provide examples of proximal and distal dioxygenases respectively (Scheme 28). O CO2H R OH aDistal Extradiol Cleavage a OH OH R b Proximal Extradiol Cleavage b OH CO2H O Scheme 27 The proposed mechanism of intradiol aromatic ring cleavage H their instability and dependence on ferrous ion, FeII and occasionally MnII, for which there are fewer spectroscopic probes.The crystal structures of 2,3-dihydroxybiphenyl-1,2- dioxygenase (BphC) from Pseudomonas cepacia LB400 and Pseudomonas sp. KKS102 have recently been solved by two groups.109,110 There is no apparent relationship to the intradiol family of dioxygenases and comparison of the two reveal completely diVerent structures. A phylogenetic analysis of 35 known extradiol dioxygenases has been reported.111 The alignment based study suggests the ancestral dioxygenases were single domain enzymes (Mr 21–24 kDa) similar to 2,3-dihydroxybiphenyl-1,2-dioxygenase from the napthalenesulfonate degrading bacteria strain BN6 (a 21.7 kDa homodimer).112 Later, two domain enzymes arose from a single genetic duplication event (Mr 31–35 kDa).Among the extradiol dioxygenases surveyed nine strictly conserved amino acid residues were found.These comprised the three metal ligands of BphC LB400: His146, His210 and Glu260 and three catalytic active site residues, His195, His241 and Tyr250. The three other conserved residues are Gly28, Leu165 and Pro254 are found at the domain interface, distant from the active site, suggesting a likely structural role. Spence et al. have proposed a classification system involving three families or classes of extradiol dioxygenases:113 class I comprises the single domain enzymes; class II comprises the major family of two domain enzymes such as BphC LB400 in which the iron(II) cofactor has been shown by X-ray crystallography to be bound in the C terminal domain; and class III consists of a minor family of two domain enzymes where the iron(II) may be bound within the N-terminal domain.113 The evolution of the diVerent classes could be explained by the duplication of a class I gene followed by mutation in either the C- or N-terminal domains to give type II or III respectively.The diVerent families are summarised in Table 1. The class I family of single domain enzymes display sequence similarities with the two domain (class II and III) enzymes, implicating a single genetic duplication event. They bind one iron(II) cofactor per subunit (Mr 21–24 kDa) and are found as either a homodimer or homohexamer.112,114 The class II enzymes are the major known family of extradiol dioxygenases and all contain a consensus sequence of conserved residues.111 This sequence defines the substrate binding pocket and is found between residues 239 and 260 in the C-terminal domain of BphC LB400.They are two domain enzymes which bind one iron(II) cofactor per subunit (Mr 31–35 kDa). The enzymes utilising monocyclic substrates exist as homotetramers.115–118 Enzymes utilising bicyclic substrates such as biphenyl, exist as homooctamers.119–122 The R Scheme 28 The two modes of cleavage by extradiol dioxygenases Extradiol cleavage of catechols is the more common mode of aromatic ring fission but the enzymes themselves are not as well described in the literature.This is primarily a result of Bugg and Winfield: Enzymatic cleavage of aromatic rings Tyr447 CO2– Tyr OH 447 OH – O O CO2–+ O2 N O N O FeIII FeIII OH O 2 O O N OH2 N CO2– CO2– Tyr447 Tyr O 447 OH OH • O O O N O N O FeII FeIII • O O O O O • OH2 N H O N 523Table 1 The three classes of extradiol dioxygenases Class Cofactor Domain structure Quaternary structure Subunit Mr/kDa 2+ I Fe II III Fe2+/Mn2+ Fe2+ 21–24 31–35 31–36 Single domain 112, 114 Two domains, FeII bound in C-terminal domain 115–124 Two domains, FeII bound in N-terminal domain? 113, 125–127 ·2, ·6 ·4, ·8 ·4 manganese(II) dependent enzyme MndD from Arthrobacter globiformis CM-2 has a homotetramer quaternary structure binding three atoms of MnII per holoenzyme (subunit Mr238 kDa).123,124 There are similarities between the N-terminal and C-terminal domains of the class II enzymes suggesting co-evolution.However, diversity among the menbers of this major family may result from recombination of genes encoding diVerent dioxygenases.109 The class III extradiol dioxygenases are also FeII dependent, two domain enzymes whose amino acid sequences do not align well with those of class II.They do not contain the concensus sequence in the C-terminal domain shown in class II. The class III family includes 2,3-dihydroxyphenylpropionate-1,2- dioxygenase MhpB from E. coli,113 catechol-2,3-dioxygenase (MpcI) from Alcaligenes eutrophus,125 protocatechuate-4,5- dioxygenase (LigAB) from Pseudomonas paucimobilis126 and 3,4-dihydroxyphenyl acetate-2,3-dioxygenase HpcB from Escherichia coli.127 The class III enzymes show strongest sequence conservation within the N-terminal domain, which in turn shows sequence similarity to the Class I single domain sequences.On this basis it was proposed that the iron binding site may exist within the N-terminal domain of these enzymes,113 although this is yet to be established experimentally. X-Ray crystal structure analysis at 1.9 Å resolution of the BphC LB400 enzyme by Han et al. identified the presence of two similar N-terminal and C-terminal domains within the subunit.109 Each domain contains two copies of a ‚·‚‚‚ secondary structure motif giving a total of four per subunit. This suggests further genetic duplication events during evolution.A funnel shaped cavity is defined by the eight stranded mixed ‚ sheet structure within each domain. Iron(II) is bound at the centre of the cavity in the C-terminal domain only. Three amino acid ligands are found to coordinate the iron; His146, His210 and Glu260.Together with two water molecules the iron takes on a square pyramidal coordination geometry with His146 as the axial ligand (Fig. 2). This 2-His-1-carboxylate facial triad is found in several other classes of metalloenzyme such as tyrosine hydroxylase, isopenicillin N synthase, iron superoxide dismutase and soybean lipoxygenase and has been recently reviewed.128 17 2 17 2 The active site structure is in accordance with prior spectroscopic evidence confirming water binding to the FeII centre.Arciero et al. observed hyperfine broadening of the EPR spectra of 4,5-PCD · NO and 2,3-CTD · NO complexes by bound H O.129 The broadening was removed on substrate binding indicating loss of the H O ligand(s) and providing evidence for three exogenous ligand sites and direct binding of NO (an O2 surrogate) to the E · S complex. NO binding was also found to be substrate dependent as binding of the catecholic substrate greatly enhanced the aYnity for NO.Hyperfine broadening was observed in the EPR spectrum of the 4,5-PCD E · S · NO complex when the C-3 or C-4 hydroxys of the substrate were 17O-labelled indicating that both catechol hydroxys are ligated to the FeII co-factor.107 The presence of histidine ligands in the iron coordination sphere has been implicated by EXAFS studies on catechol- 2,3-dioxygenase (2,3-CTD) from P. putida MT2 and from proton NMR experiments on 2,2*,3-trihydroxybiphenyl- 1,2-dioxygenase from Sphingomonas sp. strain RW1.130,131 524 Natural Product Reports, 1998 References Figure 2 Representations of the active site geometry of BphC from Pseudomonas cepacia LB400 and oxidised E · S complex of BphC from Pseudomonas sp. strain KKS102 Mabrouk et al. confirmed the iron cofactor to be in a square pyramidal geometry in both the free enzyme and the E · S complex of 2,3-CTD by combination of optical absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) experiments.132 Their results implied that the substrate displaced the water ligands to give a complex with one substrate hydroxy bound equatorially and one bound axially. X-Ray absorption spectroscopic studies by Shu et al. con- firmed that both free enzyme and the E · S complex showed five coordinate geometry and the substrate coordinated in a bidentate, mono-anionic fashion (and therefore asymmetric) by comparison with the model complex [FeII-(6TLA)(DBCH)] (ClO4).133 The crystal structure of BphC KKS102 E · S complex has also been solved.110 During crystallisation the metal centre was oxidised from FeII to FeIII, so the trigonal bipyramidal coordination observed may not be a true representation of the active E · S complex. However the positioning of the two substrate hydroxy ligands, one axial and one equatorial (Fig. 2), concurs with the CD and MCD studies of Mabrouk et al. mentioned previously.132 In spite of the range of spectroscopic studies carried out on the extradiol dioxygenases, studies of the later steps of the catalytic mechanism have until recently been limited to the early proposal by Hayaishi of a dioxetane intermediate using 18O labelling studies.134 More recently a series of mechanistic approaches have been carried out on 2,3-dihydroxyphenylpropionate 1,2-dioxygenase (MhpB) from E. coli,135–138 which will be described below. The mhpB gene encoding this enzyme has been identified at minute 8 of the E. coli chromosome, and the enzyme overproduced and purified to near homogeneity.135 The purified enzyme was found to be a 36 kDa tetrameric protein, requiring activation by FeII and ascorbate,135 and was subsequently identified as a class III extradiol dioxygenase.113 2.136 The reaction mechanism for this enzyme was first investigated by incorporation of 18O from 18O The mechanism could proceed either via a dioxetane intermediate (65, incorporation of two 18O atoms expected) or via a lactone intermediate (66, incomplete incorporation of 18O into the carboxylate) as illustrated in Scheme 29. Enzymatic processing of the 18O-labelled ring fission product by the ensuing hydrolase enzyme MhpC allowed the accurate estimate of 18O

ISSN:0265-0568    

DOI:10.1039/a815513y

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Book ReviewHeterocycles in life and society. An introduction toheterocyclic chemistry and biochemistry and the roleof heterocycles in sciences, technology, medicine andagricultureA. F. Pozharskii, A. T. Soldatenkov and A. R. Katritzky,John Wiley and Sons ISBN0 471 96034 9Heterocyclic chemistry has a reputation amongst undergraduatesas being one of the more tedious courses requiringsignificant amounts of learning aimed at the �compare andcontrast the chemistry and pyrroles and furans� type of examquestion. In part this may reflect the available specialist texts inheterocyclic chemistry which almost singularly fail to bring thesubject to life. In contrast this book does so brilliantly. It is anupdated translation of an original text by the two Russianauthors and to paraphrase the title of the first chapter isstudded with jewels of information concerning heterocycles. Itis the authors� contention that heterocycles should be viewedas a discrete class of compounds worthy of study not only byspecialist organic chemists but by anyone interested in lifeprocesses. The first two chapters outline the basics of thestructure and properties of heterocycles, assuming a knowledgeof the fundamentals of organic chemistry. The subsequentfour chapters deal with the role of heterocycles in genetics,enzymes, biogenetics and photosynthesis. Organic chemistswithout prior knowledge of biology need not be afraid of thisbook since the relevant biochemistry is explained clearly andplaced in context. The chapters concerning the use of heterocyclesin health, agriculture and industry use well knownexamples from the chemistry of antibiotics, dyes and agrochemicals,but also include much that was new to this reviewer.Finally the book addresses the prebiotic origins of heterocyclesand is particularly good at placing their chemistry within thecontext of the biosphere.This is not the book of choice for those looking to teachheterocyclic chemistry in the �classic� manner (one would notexpect that in a text which prefaces each chapter with poetry),but it is one that may inspire students and researchers to takea lasting interest in the subject. It is diYcult to imagine achemist or biochemist who will not find something of interestin this excellent book.Christopher J.

ISSN:0265-0568    

DOI:10.1039/a815531y

出版商:RSC    

年代:1998

数据来源: RSC