摘要:

AbstractZ‐Protected diphenyl (decarboxy‐dipeptidyl)phosphonates5a‐cwith a (decarboxysarcosinyl)phosphonate moiety are prepared from Z‐L‐alanine (1a). Z‐L‐valine (1b), and Z‐L‐phenylalanine (1c) by the following series of steps: coupling with methyl sarcosinate (→2a–c), saponification (→3a–c),Hofer‐Moestoxidative decarboxyiation by electrolysis in MeOH (→4a–c), andArbuzovreaction with P(OPh)3/TiCl4(Scheme 3). Double deprotonation and alkylation lead to non‐stereoselective incorporation of side chains next to the phosphonate group (products of type6–8, nine examples, seeScheme 4). In the cases of6a–cand8c, the diastereoisomers could be separated and the configuration of the newly formed stereogenic center deduced. We assign the L,D‐configuration to the diastereoisomers for which t

ISSN:0018-019X    

DOI:10.1002/hlca.19910740702

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

Regioselective 1,3‐Dipolar Cycloadditions of Thiocarbonyl Ylides with 1,3‐Thiazole‐5(4H)‐thionesThe thiocarbonyl ylides13and 1,3‐thiazol‐5(4H)‐thiones1undergo a smooth reaction to yield spirocyclic 1,3‐dithiolanes14–16(Schemes 4–6). The 1,3‐dipolar cycloadditions occur in a regioselective manner, but the orientation of the thiobcnzophenone‐S‐methylide (13b) differs from that of the cycloalkane thione‐S‐methylides13aand13c. Whereas the 1,3‐cycloadduct with13bis formed in accordance with frontier‐orbital considerations, the inverse orientation in the reactions with13aand13cmost likely is the result of steric hindrance in the transition state. The thiocarbonyl ylides have been preparedin situfrom the corresponding 2,5‐dihydro‐1,3,4‐thiadiazoles12. The more stable aliphatic precursors12aand12cundergo decom

ISSN:0018-019X    

DOI:10.1002/hlca.19910740703

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractO‐Methylation of the optically active 3′‐hydroxycoclaurines3aand3band of theN‐methylated analogs5a,bwithS‐adenosyl‐L‐[methyl‐14C]methionine in presence of mammalianCOMTwas investigatedin vitro.TheN‐unsubstituted (1S)‐ and (1R)‐isomers3aand3b, respectively, afforded almost equal amounts of the correspondingN‐norreticuline4andN‐nororientaline19, besides two unknown by‐products (seeFig.andTable 1). TheN‐methylated (1S)‐isoquinoline5a, on the other hand, afforded largely (S)‐orientaline ((S)‐19), while an almost equal mixture of (R)‐reticuline (6b) and (R)‐orientaline ((R)‐19) was obtained from the (1R)‐enantiomer5b. The isoquinolines3a,band5a,bwere prepared by aBischler‐Napieralskicyclization yieldingO‐benzyl‐protected isoquinoline10(Scheme 1). The optical resolution of10was accomplished with 2′‐bromotartranilic acid. TheN‐methylated isoquinolines were prepared byN‐formylation of10a,band reduction of the formamides13a,bwith diborane (→14a,b). Deblocking of the benzyl‐ether moieties of10a,band14a,bwas accomplished by catalytic hydrogenation in presence

ISSN:0018-019X    

DOI:10.1002/hlca.19910740704

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractA series of didehydro‐3‐O‐methyl‐estrones having a styrenic framework, with the ring‐A‐conjugated double bond in all three possible positions (8,9‐didehydro‐ (6),9,11‐didehydro‐ (1b), 6,7‐didehydro‐ (9), and the 12,18‐di‐nor‐8,9‐didehydroestrone analog11), were compared for their reactivity towards singlet oxygen. Under dye‐sensitized photo‐oxygenation conditions, both, products derived from ene‐type reactions with the isolation of a stable hydroperoxide and a fragmentation product, were obtained from6(seeScheme 3), while only fragmentation took place for1b(Scheme 1), Kinetic studies indicated that6is more reactive towards1O2than1b(β = 9.2·10−3mol·1−1vs 3.3·10−2mol·1−1, resp.). The observed reactivity, apparently, does not match with ene‐type reaction and [2 + 2]cycloaddition being in competition, since the most activated substrate6preferentially yields ene‐type products and their derivatives. Conformational analysis on the structure of6and1b, both calculated by molecular‐mechanic techniques (MMPMI) and determined by X‐ray diffraction, show that the allylic H‐atoms satisfy the orthogonality rule for ene‐type reactions. The product distribution is best rationalized by applyingFukui's rule which takes into account a combination of electronic and geometric factors. Substrates9and11yielded photo‐products arising from ene‐type reaction with no stable primary products isolated (Scheme 4). Geometric considerations based on the calculated structur

ISSN:0018-019X    

DOI:10.1002/hlca.19910740705

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe isomerization of 1,2‐epcxycyclopentane (1) to enantiomerically enriched (R)‐cyclopent‐2‐enol (2) in protic solvents is catalyzed by cob(I)alamin. The enantiomeric excess (e.e.) of (R)‐2is usuallyca.60%; it is only slightly dependent on the temperature, but increases with decreasing dielectric constant ε of the solvent. Standard kinetic methods show the reaction to be first order in vitamin B12and zero order in1. The rate constant increases exponentially with increasing ε of the solvent. AnArrheniusplot at ε = 40 gives activation parameters ΔH≠= 78 ± 4 kJ·mol−1and ΔS≠= −49 ± 1 J·mol−1·K−1. The isomerization1→2proceeds in two steps (Schemes 2 and 7): (i) The epoxide ring is first opened by the proton‐assisted fast and irreversible nucleophilic attack of the chiral CoIcatalyst to form diastereoisomeric (1R,2R)‐ and (1S,2S)‐(2‐hydroxycyclo‐pentyl)cob(III)alamins6in a ratio ofca.4:1 which are the dominant species in the steady state; (ii) The intermediates6then decompose in the rate‐limiting step to form2and recycled catalyst. Experiments with specifically2H‐labeled1showed the hydro‐cobalt elimination6→2to be non‐stereoselective. It proceedsviareversible CoC bond homolysis to a free 2‐hydroxycyclopentyl radical from which stereoe

ISSN:0018-019X    

DOI:10.1002/hlca.19910740706

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractAcetone extraction of cultures of the marine ascomyceteLeptosphaeria oraemaris(LINDER) on cornmeal disk gave the novel polyketide derivative leptosphaerolide ( = (+)‐7‐[(1E)‐l,3‐dimethylpent‐1‐enyl]‐10‐hydroxy‐3‐methoxybenzo[1,2‐b:5,4‐c′]dipyran‐2(9H)‐one; (4+)‐8) besides theo‐dihydroquinone 3‐[(1E)‐1,3‐dimethylpent‐1‐euyl]‐8,10‐dihydroxy‐7‐methoxy‐8‐(2‐oxopropyl)‐1H‐naphtho[2,3‐c]pyran‐9(8H)‐one (1) as a 10:9 mixture of epimers.retro‐Aldol reaction of1gave leptosphaerodione ( = (−)‐3‐[(1E)‐1,3‐dimethylpent‐1‐enyl]‐10‐hydroxy‐7‐methoxy‐1H‐naphtho[2,3‐c]pyran‐8,9(8H)‐dione; (−)‐6) which was also present in small amounts in the extracts and which gave1on reaction with acetone. It is thus likely that 1 is an artefact of the ex

ISSN:0018-019X    

DOI:10.1002/hlca.19910740707

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractTreatment of 3,3′‐alkylidene‐4,4′‐dihydroxybis[coumarins]1with NaBH3CN in refluxing MeOH affords 3‐alkyl‐4‐hydroxycoumarins2and 4‐hydroxycoumarin (3;Scheme 1). The reaction might take placeviahydride trapping of alkylidenechromandionesCformed from1in aretro‐Michaelreaction. Such aretro‐Michaelreaction of1might be biologically relevant. The presence ofCduring the reductive fragmentation1→2is suggested byDiels‐Alderand nucleophilic trapping of the alkylidenechromandionesCas well as from cross‐over experiments with coumarins other than3(see Scheme 2). The reductive fragmentation of1allows the chemo‐ and regioselective synthesis of a variety of 3‐alky

ISSN:0018-019X    

DOI:10.1002/hlca.19910740708

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractUV irradiation of the unsaturated (E)‐5,10‐secosteroidal ketones1and6results, in addition to (E/Z)‐isomeri‐zation, in an intramolecularPateino‐Büchireaction and, in the case of1, in transannular cyclization (along with AcOH elimination). The stereochemistry of the observed intramolecular photoprocesses can be explained in terms of ground‐state conformations of the (E)‐seco‐ketones

ISSN:0018-019X    

DOI:10.1002/hlca.19910740709

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe fluxional behaviour of [Ir4(CO)11(t‐BuNC)] (1) and [Ir4(CO)11(ArNC)] (Ar = 4‐methoxyphenyl, 4‐tolyl, phenyl, 4‐chlorophenyl, 4‐nitrophenyl;2–5) has been investigated by 2D and variable‐temperature13C‐NMR. These techniques give new evidence of the processes responsible for CO exchange, namely two successive merry‐go‐rounds causingCotton‐like scrambling and a CO rotation a

ISSN:0018-019X    

DOI:10.1002/hlca.19910740710

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe fused pentacyclic alkaloid 2‐bromoleptoclinidinone (1) isolated from an ascidian possesses a bidentate chelating site. Complexation of1with [cis‐RuCl2{(D8)bpy}2] · 2H2O forms an octahedral complex which was isolated both as hexafluorophosphate2and chloride3salts. Intercalation of3in calf‐thymusDNAwas shown to occur as well as photoactivated cleavage of double‐strand supercoiledDNApBR322 under visible‐light i

ISSN:0018-019X    

DOI:10.1002/hlca.19910740711

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY