摘要:

AbstractStructural prerequisites for the stability of the31helix of β‐peptides can be defined from inspection of models (Figs. 1and2): lateral non‐H‐substituents in 2‐ and 3‐position on the 3‐amino‐acid residues of the helix are allowed, axial ones are forbidden. To be able to test this prediction, we synthesized a series of heptapeptide derivatives Boc‐(β‐HVal‐β‐HAla‐β‐HLeu‐Xaa‐β‐HVal‐β‐HAla‐β‐HLeu)‐OMe13–22(Xaa = α‐ or β‐amino‐acid residue) and a β‐depsipeptide25with a central (S)‐3‐hydroxybutanoic‐acid residue (Xaa = –OCH(Me)CH2C(O)–) (Schemes 1 3). Detailed NMR analysis (DQF‐COSY, HSQC, HMBC, ROESY, and TOCSY experiments) in methanol solution of the β‐hexapeptide H(‐β‐HVal‐β‐HAla‐β‐HLeu)2‐OH (1) and of the β‐heptapeptide H‐β‐HVal‐β‐HAla‐β‐HLeu‐(S,S)‐β‐HAla(αMe)‐β‐HVal‐β‐HAla‐ β‐HLeu‐OH (22), with a central (2S,3S)‐3‐amino‐2‐methylbutanoic‐acid residue, confirm the helical structure of such β‐peptides (previously discovered in pyridine solution) (Fig.3andTables 1–5). The CD spectra of helical β‐peptides, the residues of which were prepared by (retentive)Arndt‐Eisterthomologation of the (S)‐ orL‐α‐amino acids, show a trough at 215 nm. Thus, this characteristic pattern of the CD spectra was taken as an indicator for the presence of a helix in methanol solutions of compounds13–22and25(including partially and fully deprotected forms) (Figs.4–6). The results fully confirm predicted structural effects: incorporation of a single ‘wrong’ residue ((R)‐β‐HAla, β‐HAib, (R,S)‐β‐HAla(α Me), orN‐Me‐β‐HAla) in the central position of the β‐heptapeptide derivativesA(see17, 18, 20, or21, resp.) causes the CD minimum to disappear. Also, the β‐heptadepsipetide25(missing H‐bond) and the β‐heptapeptide analogs with a single α‐amino‐acid moiety in the middle (13and14) are not helical, according to this analysis. An interesting case is the hepta

ISSN:0018-019X    

DOI:10.1002/hlca.19960790802

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractAt 0° in MeCN, 2,2‐disubstituted 3‐amino‐2H‐azirines1and 4,4‐disubstituted 1,2‐thiazetidin‐3‐one 1,1‐dioxides7react smoothly to give 1,2,5‐thiadiazepin‐6‐one 1,1‐dioxides of type8(Scheme 2). The reaction mechanism of this regiospecific ring enlargement to seven‐membered heterocycles follows previously described pathways. The structures of7aand8bwere established by X‐ray cr

ISSN:0018-019X    

DOI:10.1002/hlca.19960790803

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractNortopsentin D (5), a bis(indole) alkaloid unique for bearing a 2‐amino‐methylimidazole appendage at the central 1H‐imidazol‐5(4H)‐one nucleus, was isolated in abundance, besides the putative biogenetic precursor6of its appendage, from the deep‐water axinellid spongeDragmacidonsp. Structural elucidation of5by NMR and MS methods heavily relied on itsN‐methyl derivatives8–11. Unusually for topsentin‐type structures, natural5and semisynthetic methyl derivatives8and10proved inactive on KB tumoural cells, while introduction of the last three methyl groups, amazingly led to h

ISSN:0018-019X    

DOI:10.1002/hlca.19960790804

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractRecrystallization of Cs3P7from liquid NH3yields the triammoniate Cs3P7·3 NH3, which loses the weakly bound NH3of crystallization below 253 K. A low‐temperature crystal‐structure analysis shows that Cs3P7· NH3consists of a framework of heptaphosphanortricyclane anions P 73−and Cs+cations with NH3molecules completing the coordination of the cations. The framework is built from Cs3P7layers connected by only few Cs…︁P interactions, the interlayer gap being filled by a two‐dimensional network of NH3. The Cs7P7part of the structure completes a family of alkali‐metal‐polyphosphide substructures which range from ∞1[RbP7]2−or ∞1[CsP11]2−chains over ∞2[Cs2Pn]−layers (

ISSN:0018-019X    

DOI:10.1002/hlca.19960790805

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

Abstract(all‐Z)‐(9,10,12,13,15,16‐2H6)Octadeca‐9,12,15‐trienoic acid ( = α‐linolenic acid; D6‐4) was synthesized to investigate the biochemical formation of linolenic‐acid‐derived aroma compounds in cultures of the yeastSporobolomyces odorus, using an established gas chromatographic/mass spectrometric (GC/MS) method. Three compounds were identified as labeled: (Z)‐dec‐7‐eno‐5‐lactone (δ‐jasmin lactone), (Z,Z)‐dodeca‐6,9‐dieno‐4‐lactone, and (2E,4Z)‐hepta‐2,4‐dienoic acid. Both lactones were biosynthesized mostly under conservation of the initial configuration from their corresponding oxygenated linolenic‐acid intermediates. The application of (13S,9Z,11E,15Z)‐13‐hydroxy(9,10,12,13,15, 16‐2H6)octadeca‐9,11,15‐trienoic acid (D6‐7) as a OH‐functionalized precursor of δ‐jasmin lactone allowed to gain insight into the stereochemical course of the biosynthesis to both enantiomers of this lactone. In this experiment, 88.3% of the metabolized labeled precursor was transformed under retention of the original configuration of the (R)‐enantiomer. This investigation is also a contribution to a better understanding of the CC bond isomerization step

ISSN:0018-019X    

DOI:10.1002/hlca.19960790806

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe interaction between a vitamin B12derivative containing a peripheral C18alkyl chain (see1a) and a (methyl)thiomalonate substrate bearing alkyl chains of various length at the thioester group (see5) was investigated. A catalytic cycle was established for the methylmalonyl→succinyl rearrangement by using electrochemistry and photolysis (seeScheme 3). Increased yields of the succinate relative to the reduction product were obtained (2:3 ratio), when the reaction was run in MeOH/H2O, and when both the substrate and the catalyst had an octadecyl substituent capable of hydrophobic interaction

ISSN:0018-019X    

DOI:10.1002/hlca.19960790807

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractIntensive studies on the diazomethane methylation of the common ribonucleosides uridine, cytidine, adenosine, and guanosine and its derivatives were performed to obtain preferentially the 2′‐O‐methyl isomers. Methylation of 5′‐O‐(monomethoxytrityl)‐N2‐(4‐nitrophenyl)ethoxycarbonyl‐O6‐[2‐(4‐nitrophenyl)ethyl]‐guanosine (1) with diazomethane resulted in an almost quantitative yield of the 2′‐ and 3′‐O‐methyl isomers which could be separated by simple silica‐gel flash chromatography (Scheme 1). Adenosine, cytidine, and uridine were methylated with diazomethane with and without protection of the 5′ ‐O‐position by a mono‐ or dimethoxytrityl group and the aglycone moiety of adenosine and cytidine by the 2‐(4‐nitrophenyl)ethoxycarbonyl (npeoc) group (Schemes 2–4). Attempts to increase the formation of the 2′‐O‐methyl isomer as much as possible were based upon various solvents, temperatures, catalysts, and con

ISSN:0018-019X    

DOI:10.1002/hlca.19960790808

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe three mono substitutedN‐[(E)‐3‐(4‐hydroxyphenyl)prop‐2‐enoyl]spermidines1–3have been studied by positive‐ion electrospray‐ionization tandem mass spectrometry (ESI‐MS/MS). Because of the neighboring‐group participation, the MS/MS of [1+ H]+and [2+ H]+are essentially similar, while compound3can be easily distinguished from1and2because of the characteristic ions atm/z218. However, with the source collision‐induced dissociation (source‐CID) MS/MS technique, the compounds1and2can be unambiguously distinguished by the signal of the pyrrolidinium ion (m/z72) from their daughter ion (m/z275). The source‐CID MS/MS of the labeled compoundN‐(4‐aminobutyl)‐N‐(3‐aminopropyl)‐N‐[3‐(4‐ hydroxyphenyl)prop‐2‐en[15N]amide]([15N(4)]‐2) provide more information on the decomposition mechanisms and proved the occurrence of a parti

ISSN:0018-019X    

DOI:10.1002/hlca.19960790809

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe electrospray tandem mass spectra (ESI‐MS/MS) of the threeN,N′‐bis[(E)‐3‐(4‐hydroxyphenyl)prop‐2‐enoyl]spermidines1–3displayed the same fragment‐ion signals. These isomers could not be differentiated by ESI‐MS/MS, since their fragmentation patterns are similar. (E,E)‐N‐(3‐[15N]Aminopropyl)‐3,3′‐bis(4‐ hydroxyphenyl)‐N,N′‐(butane‐1,4‐diyl)bis[prop‐2‐enamide] ([15N(1)])‐(1) was synthesized in order to get further information about the fragmentation mechanisms. The comparison of the ESI‐MS/MS of1and [15N(1)]‐1revealed a transamidation, theZipreaction, under mass‐spectral conditions of the [1+ H]+ions. Because of this

ISSN:0018-019X    

DOI:10.1002/hlca.19960790810

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe cyclododecadiynes3b–d, 8b–d, and10b–cwith functionalities in two propargylic positions, as well as the monofunctionalized diyne13bhave been prepared from simple open‐chain building blocks. In the DMPU ( =N,N'‐dimethylpropyleneurea)‐assisted ring‐closing alkylation of 1,7‐diynes, the twelve‐membered ring compounds have been prepared in yields of 16–55%. The preparation of the diene‐diyne15and the cyclododeca‐5,11‐diyne

ISSN:0018-019X    

DOI:10.1002/hlca.19960790811

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY