摘要:

Abstractα‐SubstitutedN‐acylbornane‐10,2‐sultams6, 9, and10can be converted into enantiomerically pure ketones5. 13, and14, respectively,viaa two‐step procedure involving a known mercaptolysis reaction followed by an [Fe(acac)3]‐mediated coupling of the resultingS‐benzyl thioesters withGrignardreagents. Furthermore, enantiomerically pure aldehydes23can be obtained from α‐substitutedN‐acylbornane‐10,2‐sultams6viaa one‐step reduction with (i‐Bu)2AIH. No epimerization at the α‐chiral center is observed during the cleavage reaction whereby the chiral auxiliary, bornane‐10,2‐sultam1orent‐l, was recovered. By using this methodology, several natural products o

ISSN:0018-019X    

DOI:10.1002/hlca.19970800502

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe conformational analysis of naturally occurring cytostatic cyclic heptapeptides axinastatin 2, 3, and 4 was carried out by two‐dimensional NMR spectroscopy in combination with distance‐geometry (DG) and molecular‐dynamics (MD) calculations in explicit solvents. The synthesized secondary metabolites were examined in (D6)DMSO. Axinastatin 2 was also investigated in CD3OH. In all structures, Pro2is in thei+ 1 position of a βI turn and Pro6occupies thei+ 2 position of a βVIa turn about thecisamide bond between residue 5 and Pro6.In all peptides, a bifurcated H‐bond occurs between residue 4 CO and the amide protons of residue 1 and 7. For axinastatin 2 and 3, an Asn Igturn was found about Asn1and Pro2.We compared these structures with conformations of cyclic heptapeptides obtained by X‐ray and NMR studies. A β‐bulge motif with two β turns and one bifurcated H‐bond is found as the dominating backbone conformation of cyclic all‐L‐heptapeptides. Axinastatin 2, 3, and 4 can be characterized by sixtransand onecisamide bond resulting in a β/βVI(a)‐turn motif, a conformation found for many cyclic heptapeptides. Detailed biological tests of the synthetic compounds in different human cancer cell lines indicates these axinastatins to be in

ISSN:0018-019X    

DOI:10.1002/hlca.19970800503

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe partial synthesis of 10,22‐dihydro‐4,5‐dioxo‐4,5‐secopheophorbide a (1) from pheophorbide a methyl ester (2) is described. A regioselective, photooxygenolytic reaction of (pheophorbidato a methyl ester)cadmium(II)(3) provides the entry to the crucial 4,5‐secoporphinoid structure in form of the (10,22‐dihydro‐4,5‐dioxo‐4,5‐seco‐pheophorbidato a methyl ester)cadmium(II) (4). The hydride reduction of this 4,5‐dioxo‐4,5‐secophytoporphyrin ester occurs selectively at the ‘eastern’meso‐position to lead (after demetallation) to 10,22‐dihydro‐4,5‐dioxo‐4,5‐secopheophorbide a methyl ester (5). This oxobilin‐carbaldehyde has the structure assigned earlier to an ester of an isolation form of the red pigment(s) fromChlorella protothecoides.Hydrolysis of the propanoate ester function of 5, selectively catalyzed by pig liver esterase, then yields the title compound1. The red tetrapyrrole1may represent an intermedi

ISSN:0018-019X    

DOI:10.1002/hlca.19970800504

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractAbsolute rate constants for the addition of the highly nucleophilic 2‐hydroxy‐2‐propyl radical to eight fast‐reacting 1‐ and 1,1‐disubstituted alkenes in MeOH at room temperature have been determined by laser flash photolysis. Also the absorption spectra of the 2‐hydroxy‐2‐propyl and the benzylic and alkyl‐type adduct radicals are presented. The rate constants were obtained using various methods for the analysis of the kinetic traces and suppo

ISSN:0018-019X    

DOI:10.1002/hlca.19970800505

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractBenzo[a]heptalene has been synthesized by two different approaches. The first one follows a pathway to hexahydrobenzo[a]heptalenone19athat has been already described byWenkertandKim(Scheme). Indeed,19awas obtained in a mixture with its double‐bond‐shifted isomer19b. Reduction of this mixture to the corresponding secondary alcohols26a/26band elimination ofH2Olead to a mixture of the tetrahydrobenzo[a]heptalenes23a‐d(Scheme7and8). Reaction of23a‐dwith 2 equiv. of triphenylmethylium tetrafluoroborate in boiling CHCl3, followed by treatment with Me3N in CH2Cl2, generated directly2, unfortunately in a mixture with Ph3CH that could not be separated from2(Scheme 10and11). The second approachviadimethyl benzo[a]heptalene‐6,7‐dicarboxylate (30) (Scheme 12) that was gradually transformed into the corresponding carbaldehydes37and43(Scheme 14) both of which, on treatment with theWilkinsoncatalyst [RhCl(PPh3)3] at 130° in toluene, smoothly decarbonylated, finally gave pure2as an unstable orange, viscous oil. UV/VIS, NMR, and mass spectra of2are discussed in detail (

ISSN:0018-019X    

DOI:10.1002/hlca.19970800506

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractTreatment of celery leaves (Apium graveolens, cv. secalinum) with Jasmonic acid (JA,1) or analogues of amino‐acid conjugates of Jasmonic acid such as the leucine conjugate with 1‐oxoindan‐4‐carboxylic acid (IN‐ILE,2) stimulated the biosynthesis of the furanocoumarins psoralen (6), xanthotoxin (8), bergapten (7), and isopimpinellin (9). Besides the increase of the compounds within the leaf, a significant amount (ca.20%) of the total furanocoumarins was deposited on the surface of the leaf. The two monomethoxy furanocoumarins,7and8, began to increase steadily and simultaneously within the leaf and on the leaf surfaceca.40 h after the onset of the jasmonic‐acid stimulus. Within the leaf, the ratio7/8wasca.1.3:1.0, while among the surface lipids8dominated (7/80.8:1.0), indicating that the export of the compounds to the surface is not a simple diffusive translocation along the oil‐ducts. Females of the carrot fly (Diptera, Psilidae) responded with an increased oviposition to the altered leaf surface chemistry of the JA‐treated celery plants. The effect shown for total leaves was corroborated by surrogate leaves treated with leaf‐surface extracts of JA‐induced leaves. Based on the known stimulation of oviposition by furanocoumarins, we conclude that the enhanced amount of furanocoumarins on the surface can explain the insects' preference for the JA‐stimulated plants. This is the first report of a JA‐induced change of the surface chemistry of a plant and an increase of the acceptability of treated leaves for a specialist inse

ISSN:0018-019X    

DOI:10.1002/hlca.19970800507

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

Chemistry of α‐Aminonitriles. Regioselective Synthesis and Crystal Structure of Uroporphyrinogen (Type I) OctanitrileA regioselective synthesis of uroporphyrinogen‐octanitrile (type I) based on the strategy of multiple use of (dimethylmethylidene)ammonium iodide for stepwise regioselective functionalization of the pyrrole nucleus is described. This uroporphyrinogen derivative is remarkably stable and beautifully crystallizes in space groupP1with one molecule per unit cell. The crystal structure of the compound shows interesting conformational characteristics which are interpreted to be caused by subtle stereoelectronic effects. TheEnglish FootnotestoSchemes 1‐3andFigs. 1‐12provide an extension of this

ISSN:0018-019X    

DOI:10.1002/hlca.19970800508

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractIn the presence of activating agents, theN‐acylglycine8reacts with electrophilic alkynesviathemünchnone9to the pyrrolopyridines (= indolizines)10, 18, and19(Scheme 1), Depending on the nature of the activating agent and the reaction temperature, the formation of the pyrroles was accompanied by partial epimerization to themanna‐configurated epimers16and17. Thegluco‐configurated pyrrolopyridine10was deprotected to12. Silylation of12, followed by reduction and desilylation, gave the hexol15. Cycloaddition of9to 4‐toluenesulfonyl cyanide yielded 53% of the imidazole23, while cycloaddition to phenyl cyanate gave the phenoxyimidazole28in low yields only (Scheme 2). As expected, the deprotected pyrroles12, 15, 20, and21are weak inhibitors of retaining β‐glucosidases, while the deprotected imidazole24derived from23proved a good inhibitor of sweet‐almond β‐glucosidases and a powerful inhibitor ofCaldocellum saccharolyti

ISSN:0018-019X    

DOI:10.1002/hlca.19970800509

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe stereochemistry of 1,3‐dipolar cycloaddition of azomethine ylides derived from aromatic aldehydes and i‐proline alkyl esters with several nitroolefins was investigated. Cyclic and acyclic nitroolefins add to theantiform of the ylide in a highly diastereoselective but poorly regioselective manner to give pyrrolizidine derivatives. In a few cases, the stereochemical results strongly support a stepwise mechan

ISSN:0018-019X    

DOI:10.1002/hlca.19970800510

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY

摘要:

AbstractThe synthesis of glycopeptides carrying tumour‐associated antigens is of interest for cancer diagnosis and treatment. Here, a very efficient route lo disaccharide threonine building block8is presented which allows the introduction of the sialyl‐Tn antigen into a peptide. The syntheses of the undecapeptide and the sialyl‐Tn‐containing glycoundecapeptide, which are a part of the repeating unit of MUC1, were performed by solid‐phase synthesis with an allylic anchor cleavable under neutral conditions. After detachment from the resin, the peptide and the glycopeptide arc completely deprotected giving the target compounds13and15, res

ISSN:0018-019X    

DOI:10.1002/hlca.19970800511

出版商:WILEY‐VCH Verlag GmbH    

年代:1997

数据来源: WILEY