摘要:

Abstract(S)‐Perilla alcohol (5) was transformed into (S)‐7‐(phenylthio)‐p‐menth‐1‐en‐8‐amine (11) in five steps. Condensation of this building block with 1‐(4‐methoxyphenylsulfonyl)‐1H‐indole‐3‐acetaldehyde (12) led to the expected imine15which cyclized in 54% yield to protected 20‐(phenylthio)hobartine16upon exposure to anh. HCOOH. Treatment of this intermediate with an alkylating reagent led to (+)‐aristofruticosine protected in the indole moietyviaan intramolecular, allylic nucleophilic displacement reaction. Subsequent reductive removal of the protecting group completed the first synthesis of theAristoteliaalkaloid (+)‐aristofruticosine ((+)‐4). This straightforward synthesis confirmed the tentative structure (+)‐4, proposed byBickandHai, and established the hitherto unknown ab

ISSN:0018-019X    

DOI:10.1002/hlca.19910740113

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractIngestion of yellow star thistle (Centaurea solstitialis L.) by horses produces parkinsonism due to nigro‐pallidal degeneration. The toxin responsible has not been identified so far. A CH2Cl2extract from the aerial parts ofC. solstitialisexhibited significant neurotoxicity against primary neuronal cultures of foetal rat brain. Activity‐guided fractionation yielded the known sesquiterpene lactones solstitialin A (1), 13‐0‐acetylsolstitialin A (3), cynaropicrin (4), and the hitherto unknown 3‐O‐acetylsolstitialin A (2). In the bioassay with rat foetal full cell culture,3and4were toxic in a concentration‐dependent manner and may be responsible for the ability of the plant to cause neurodegenerative changes in the br

ISSN:0018-019X    

DOI:10.1002/hlca.19910740114

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractAn enzyme is extracted from the red peel ofAmanita muscariawhich cleaves the C(2)–C(3) and the C(4)–C(5) bond of the aromatic ring ofL‐dopa (1) to form a mixture of 4,5‐secodopa (= salt of 6‐amino‐2‐hydroxy‐4‐(2′‐oxoethylidene)hept‐2‐enedioic acid;2) and 2,3‐secodopa ( = salt of 7‐amino‐5‐formyl‐2‐hydroxyocta‐2,4‐dienedioic acid;3), two hitherto hypothetical biosynthetic intermediates (seeScheme). Though isolation of these products has not been possible, structural evidence is inferred from reaction products, kinetics, and spectroscopical characteristics in comparison with known compounds. Secodopas2and3are characterized in dilute solution by HPLC and UV/VIS spectroscopy (anions; λmax424 and 414 nm, resp., ϵ420= 25500; on acidification, shift to 380 and 372 nm, resp.). They cyclize without enzyme catalysis, optimally at pH 4.5–5;3produces muscaflavin (5) and2betalamic acid (4). The products arc identified by direct comparison with authentic samples in HPLC, by1H‐NMR of5, and by condensation of4withL‐proline to form the well known betalain indicaxanthin (7). The enzymatic conversion ofL‐dopa (1)via2to betalamic acid (4; (S)) and its condensation withL‐proline leads to pure natural indicaxanthin (7; (2S,115)); correspondingly, the enzymatic conversion ofD‐dopa to (R)‐betalamic acid and its condensation withL‐proline produces isoindicaxanthin ((2S,11R)) which is unknown in nature. Particularly relevant is the fact that the same enzyme cleaves pyrocatechol to produce a solution of the enolate form of the known 2‐hydroxy‐6‐oxohexa‐2,4‐dienoate (secopyrocatechol;9; seeFig. 5). Dissociation constants of the corresponding enolic functions in the cleavage products are determi

ISSN:0018-019X    

DOI:10.1002/hlca.19910740115

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe (−)‐(2S)‐Diethyl 2‐Hydroxyhexanedioate, a New Chiral Building Block for Enantioselective Synthesis(−)‐(2S)‐Diethyl 2‐hydroxyhexanedioate ((2S)‐3) has been obtained by enantioselective reduction of diethyl 2‐oxohexanedioate (1) with baker's yeast. The key intermediate (−)‐(5S)‐ethyl 5,6‐dihydroxyhexanoate ((5S)‐5) is proved to be a useful synthon for the synthesis of chiral δ‐lactones and a precur

ISSN:0018-019X    

DOI:10.1002/hlca.19910740116

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe tin hydride promoted and the reductive vitamin B12catalysed radical cyclisation of mixed 2‐bromo‐acetaldehyde acetals and of (2‐bromomethyl)dimethylsilyl ethers of allylic terpenoid alcohols has been investigated: 3‐oxadeca‐5,9‐dien‐l‐yl radicals undergo 5‐‘exo’ cyclisation to oxolanes (Scheme 4), 3‐oxa‐2‐siladeca‐5,9‐dien‐1‐yl radicals sequential 6‐‘endo’→5‐‘exo’ tandem cyclisation tocis‐3‐oxa‐4‐silabicyclo[4.3.0]nonanes (Scheme 5), and 3‐oxa‐2‐silatetradeca‐5,9,13‐trien‐l‐yl radicals sequential 6‐‘endo’→6‐‘endo’→5‐‘exo’ triple

ISSN:0018-019X    

DOI:10.1002/hlca.19910740117

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

1‐Methoxycarbonyl‐Substituted 2,3‐Dihydropyridin‐4(1H)‐one(= Methyl 1,2,3,4‐Tetrahydro‐4‐oxopyridine‐1‐carboxylate) as Chromophore for Photochemical [2 + 2]‐CycloadditionsWith olefins having an electron‐acceptor as well as with olefins having an electron‐donor substituent, 1‐methoxycarbonyl‐substituted dihydropyridinone12undergoes [2 + 2] cycloaddition in good preparative yields. The photochemical cycloaddition is highly regioselective. For preparative purposes, the ring junction can be equilibrated to the thermodynamically more stablecis‐junction. Only the ‘endo’/‘exo’ selectivity at the C‐atom bearing the olefin substituent cannot be controlled. The photodimerization of12is the only side reaction. Using a slight excess of the olefin, the photodimerization can be suppressed. The protecting group at the N‐atom of the dihydropyridinone can be varied in order to introduce an internal sensitizer, as shown with 1‐acyl‐substituted compound29, which underwe

ISSN:0018-019X    

DOI:10.1002/hlca.19910740118

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe structure and synthesis of novel irregular C12terpenoids isolated from quince fruit (Cydonia oblongaMILL.) are described: quince oxepine (= (E)‐2,3,6,7‐tetrahydro‐4‐methyl‐2‐(3‐methylbuta‐1,3 dienyl)oxepine;3) and the quince oxepanes as a 1:1 mixture ofcis‐ andtrans‐isomers (=cis‐ andtrans‐(E)‐4‐methyl‐2‐(3‐methylbuta‐1,3‐dienyl)oxepane;4and5, resp.). The absolute configurations of the natural compounds have not been determined due to the minute amounts available, but both relative and absolute configurations of synthetic4and5were established by

ISSN:0018-019X    

DOI:10.1002/hlca.19910740119

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractCarbonyl(cycloheptatrienyl)iodo(phosphorus donor)tungstens ([WI(C7H7)(CO)L]; L = P(OMe)3,1a; L = P[O(i‐Pr)]3,1b; L = PPh3,1c) were prepared from dicarbonyl(cycloheptatrienyl)iodotungsten ([WI(C7H7)(CO)2)]viaa carbonyl‐substitution process. Similarly, bromocarbonyl(phosphorus donor)(1,2,4,6‐tetramethylcycloheptatrienyl)tungstens ([WBr(Me4C7H3)(CO)L]; L = P(OMe)3,6a; L = P[O(i‐Pr)]3,6b; L = PPh3,6c) were obtained from the reaction of bromodicarbonyl(1,2,4,6)‐tetramethylcycloheptatrienyl)tungsten ([WBr(Me4C7H3)(CO)2];4) with L. The reduction of1a‐c,4, and6a,bwith sodiumdihydridobis(2‐methoxyethoxy)aluminium in toluene led to stable hydrido complexes [WH(R4C7H3)(CO)L] (R = H, L = P(OMe)3,2a; R = H, L = P[O(i‐Pr)]3,2b; R = H, L = PPh3,2c; R = Me, L = P(OMe)3,7a; R = Me, L = P[O(i‐Pr)]3,7b; R = Me, L = CO,7d). Complexes2aand7bwere characterized by X‐ray

ISSN:0018-019X    

DOI:10.1002/hlca.19910740120

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe tripeptide and hexapeptide derivatives Boc‐Gly‐Sar‐MeLeu‐OH (5b), Boc‐Ala‐Sar‐Sar‐OH (6b), Boc‐Ala‐Sar‐MeLeu‐OH (7b), and Boc‐Abu‐Sar‐MeLeu‐Val‐MeLeu‐Ala‐OH (12b) can be poly‐deprotonated (tri‐ and pentalithio derivativesKandP, respectively), and thusC‐alkylated on sarcosine (Sar) moieties with MeI and allyl or PhCH2Br. The polylithiated species are solubilized in THF, and their reactivity modified by excess base (lithium diisopropylamide (LDA)), by added LiCl, and/or the cosolventN,N′‐dimethylpropyleneurea (DMPU). Optimization of the reaction conditions for methylation in the cases of7b(Table 3) and12b(Scheme 8) gave products in which the Sar residue of the educt has been transformed into a Me‐D‐Ala unit in yields of 80 (9c/8c) and 67% (14c/13c), respectively, and with a diastereoselectivity ofca.4:1. Less selective methylations and benzylations were observed with the tripeptides5band6bcontaining only one stereogenic center; also, excess base and alkyl halide may lead to double alkylations in those latter two cases (Tables 1and2). No epimerization of stereogenic centers was detected under the strong‐base conditions. The analysis of the products was accomplished by a combination of NMR and FAB‐MS spectroscopy, as well as by hydrolysis to the parent amino acids, subsequent formation of derivatives with isopropyl isocya

ISSN:0018-019X    

DOI:10.1002/hlca.19910740121

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractPhotochemical behaviors of the pyrazinone derivatives 5,6,7,8‐tetrahydroquinoxalin‐2(1H)‐ones1a–cand 1,5,6,7,8,9‐hexahydro‐2H‐cyclohepta[b]pyrazin‐2‐one1dwere investigated. Dye‐sensitized photo‐oxygenation of1a‐cgave the 1:1 adducts5a–cof the corresponding 3,8a‐epidioxy‐3,5,6,7,8,8a‐hexahydroquinoxalin‐2(1H)‐one4and H2O, whereas1dgave 3,9a‐epidioxy‐1,3,5,6,7,8,9,9a‐octahydro‐2H‐cyclohepta[b]pyrazin‐2‐one4d(Scheme 2). The different kind of products was interpreted as being the result of the ring strain and steric hindrance of endoperoxides produced from1a–dwith singlet oxygen. Irradiation of1a–bin the presence of alkenes gave tricyclic azetidine derivatives9by [

ISSN:0018-019X    

DOI:10.1002/hlca.19910740122

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY