摘要:

AbstractThe β‐hexapeptide (H‐β‐HVal‐β‐HAla‐β‐HLeu)2‐OH (2) was prepared from the componentL‐β‐amino acids by conventional peptide synthesis, including fragment coupling. A cyclo‐β‐tri‐ and a cyclo‐β‐hexapeptide were also prepared. The β‐amino acids were obtained from α‐amino acids byArndt‐Eisterthomologation. All reactions leading to the β‐peptides occur smoothly and in high yields. The β‐peptides were characterized by their CD and NMR spectra (COSY, ROESY, TOCSY, and NOE‐restricted modelling), and by an X‐ray crystal‐structure analysis. β‐Sheet‐type structures (in the solid state) and a compact, left‐handed or (M)31helix of 5‐Å pitch (in solution) were discovered. Comparison with the analogous secondary structures of α‐peptides shows fundamental differences, the most surprising one at this point being the greater stability of β‐peptide helices. There are structural relationships of β‐peptides with oligomers of β‐hydroxyalkanoic acids, and dissimilarities between the two classes of compounds are a demonstration of the power of H‐bonding. The β‐hexapeptide2is stable to cleavage by pepsin at pH 2 in H2O for at least 60 h at 37°, while the corresponding α‐peptide H‐(Val‐Ala‐Leu)2‐OH is cleaved

ISSN:0018-019X    

DOI:10.1002/hlca.19960790402

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe novel H2O‐soluble cyclophanes1and2incorporating different anion‐recognition sites were prepared in short synthetic routes (Schemes 1and2) as first‐generation mimics of the natural,D‐Ala‐D‐Ala binding antibiotic vancomycin. The X‐ray crystal structure of1, a tris(hydrochloride)salt, revealed an open, preorganized cavity of sufficient size for the incorporation of small aliphatic residues (Fig. 3). In the crystal, molecules of1are arranged in parallel stacks, generating two types of channels, an ‘intra‐stack’ channel passing through the cyclophane cavities and an ‘inter‐stack’ channel located between cyclophane stacks (Fig. 4). The strongest intermolecular interactions between macrocycles in the crystal are CO…︁HN H‐bonds between the carboxamide residues of adjacent cyclophanes in neighboring stacks (Fig. 5). The ‘intra‐’ and ‘inter‐stack’ channels incorporate the three ordered Cl−counterions and several, partially ordered solvent molecules (4 MeOH, 1 H2O) (Fig. 6). Counterion Cl(2) is located within the ‘intra‐stack’ channel and interacts with a protonated piperazinium N‐atom and both ‘intra‐stack’ MeOH molecules. The two other counterions, Cl(1) and Cl(3), are located within the ‘inter‐stack’ channel. They are connected to two MeOH and one H2O molecules and also interact both with the NH 2+group of the protonated spiropiperidinium ring in1, forming an infinite, chain‐like H‐bonding network …︁Cl(1)…︁HOH…︁MeOH…︁Cl(3)…︁HNH…︁Cl(1′)…︁. Both ‘intra‐’ and ‘inter‐stack’ MeOH molecules undergo weak CH…︁π interactions with neighboring aromatic rings. Cyclophane1complexed aromatic sulfonates in 0.5MKCl/DCl buffer in D2O, whereas the tetrakis(quaternary ammonium) receptor2bound the sodium salts of aliphatic and aromatic carboxylates and sulfonates, ofN‐acylated α‐amino acids as well as ofN‐acetyl‐D‐alanyl‐D‐alanine (Ac‐D‐Ala‐D‐Ala), a substrate of vancomycin, in pure H2O. In all of these complexes, ion pairing between the cationic recognition site in the periphery of the cyclophane receptor and the anionic substrates represents the major driving force for host‐guest association. The1H‐NMR analysis of complexation‐induced changes in chemical shift clearly demonstrated that, in solution, this ion pairing exclusively takes place outside the cavity. Nevertheless, the macrocyclic bridges are essential for the efficiency of the anion‐recognition sites in the two cyclophane receptors1and2. Control compounds3and4possess nearly the same anion‐recognition sites than1and2, but lack their macrocyclic preorgani

ISSN:0018-019X    

DOI:10.1002/hlca.19960790403

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

Abstract‘Semicorrin’ cobalt complexes, preparedin situfrom cobalt(II) chloride and the corresponding ligands, are efficient catalysts for the enantioselective reduction of electrophilic CC bonds with NaBH4. The best selectivities (>90% ee) are achieved with α,β‐unsaturated carboxamides and carboxylates. Analogous α,β‐unsaturated nitriles, sulfones, and phosphonates afford enantiomeric excesses of 50–70%. Interestingly, in the reduction of α,β‐unsaturated sulfones, the highest enantioselectivities were obtained with unsymmetrical ‘semicorrins’, whereas in all other casesC2‐symmetric ‘semicor

ISSN:0018-019X    

DOI:10.1002/hlca.19960790404

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractFrom extracts ofOncinotis tenuilobaSTAPF, two novel polyamine alkaloids, oncinotin‐11‐one (5) and oncinotin‐12‐one (6), were isolated. Peracetylation of6provided theN‐acetyl derivative11as well asN,N′‐diacetylinandenin‐10‐en‐12‐one (12) due to a β‐elimination‐type side reaction resulting in ring enlargement of11(Scheme 1). Deuteration of12yielded13, showing the same retention time asN,N′‐diacetylinandenin‐12‐one (14), when co‐HPLC was performed together with different keto‐isomericN,N′‐diacetylinandeninones. Structure elucidation was extended bySchmidtdegradation of6andN,N′‐diacetyl(10,11‐2H2)inandenin‐12‐one (13); the degradation products were identified by GC and ESI‐MS. The structure of5was proposed on the basis of spectroscopic means. Comparison of the spectroscopic data of5with those obtained from synthetic material as well as co‐HPLC of theN‐acetyl derivative20together with the corresponding synthetic compound revealed the identity of the substances and confirmed the structure of5. Additionally, oncinotine (2) and neooncinotine (3) were isolated, separated, and identified with authentic samples

ISSN:0018-019X    

DOI:10.1002/hlca.19960790405

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractCholic‐acid‐type 3‐keto steroids1–8, all isolated fromDeltocyathus magnificus(2–8after CH2N2treatment), are the first secondary metabolites obtained from a deep‐water scleractinian. Steroids1–3, 5, and7are new, their main structural oddities being loss of C(24) (see5) or hydroxylation in the side chain (see3and7). Steroids4, 6, and8, from the same coral, were previously known from other mar

ISSN:0018-019X    

DOI:10.1002/hlca.19960790406

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractRearrangement of 5α‐ and 5β‐cholesta‐6,8(14)‐dienes (13aand13b, resp.) in the presence of anhydrous toluene‐4‐sulfonic acid in acetic acid leads to 5α‐ and 5β‐12(13 → 14)‐abeo‐cholesta‐8,13(17)‐dienes (15aand15b, resp.)via5α‐ and 5β‐cholesta‐8,14‐dienes (14aand14b, resp.), respectively. Epimerization at C(20) of the spirosteradienes15aand15boccurs with increasing reaction time. Molecular‐mechanics calculation of the relative stabilities of these compounds and of congeners thereof is in agr

ISSN:0018-019X    

DOI:10.1002/hlca.19960790407

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractA series of acetoxy derivatives of androstane was deacetylated in organic solvents by several lipases. The most satisfactory results were obtained with lipase fromCandida cylindracea(CCL) andCandida antarctica(CAL). In some derivatives, CCL and CAL showed an overwhelming regioselectivity towards the removal of the 3β‐ or the 17β‐acetyl group (seeTable 2). Three new steroid derivatives were obtained through this approach. A hypothetical rationale for the behaviour of these enzymes is

ISSN:0018-019X    

DOI:10.1002/hlca.19960790408

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe synthesis and spectroscopic properties of the (5′ → 5″)‐ester of adenosine 5′‐diphosphate with ribofuranose cyclic 1″, 2″‐phosphate3, a recently discovered metabolite produced during tRNA splic

ISSN:0018-019X    

DOI:10.1002/hlca.19960790409

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe 14‐membered macrocyclic Ni2+complexes of1and2, with a methylthio pendant chain, and those of3and4, with a methoxy pendant chain, have been synthesized and their chemistry has been studied. Solution spectra in H2O, MeCN, and DMF indicate no participation of the side‐chain donor group in metal coordination. This is also the case in the solid state as shown by the X‐ray structures of the Ni2+complexes with1and2, in which a tetrahedrally distorted square‐planar geometry around the Ni2+results by the coordination of the four N‐atoms of the macrocycle. Cyclic voltammetry of these complexes in MeCN reveals that Ni2+is reversibly reduced to Ni+between −0.7 and −0.8 Vvs.SCE. For the complexes with1and2, the thioether bond is cleaved at more negative potentials, whereby a thiol group is formed. This thiol group is then oxidized atca.+0.7 Vvs.SCE, when a glassy C electrode is used, or atca.0 Vvs.SCE at a dropping Hg electrode. No cleavage of the ether bond in the complexes with3and4is observed under similar conditions. Reduction of the Ni2+complexes of1and2with Na‐amalgam in DMF produces small amounts of methane only in the case of1, indicating the importance of the proximity between the NiIcentre an

ISSN:0018-019X    

DOI:10.1002/hlca.19960790410

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractSharplessepoxidation of (E)‐1‐(trimethylsilyl)[1‐2H1]oct‐1‐en‐3‐o1 (3a) yielded (1S,2S,3S)‐ and (1R,2R,3R)‐1‐(trimethylsilyl)‐1,2‐epoxy[1‐2H1]octan‐3‐ols (4aand4b, resp.) which were converted in three steps into (S)‐ and (R)‐fluoro[2H1]acetic acid (7aand7b, resp.) in good yields. Their high isotopic and optical purity was established by1H‐ and19F‐NMR, mass

ISSN:0018-019X    

DOI:10.1002/hlca.19960790411

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY