摘要:

AbstractRacemic diaminosugars were easily obtained by a regio‐ and stereoselective three‐step synthetis. In the first step, regiospecific hetero‐Diels‐Aldercycloadditions between 1,2‐dihydropyridines1aand10and nitrosobenzene led to the bicyclic compounds2aand11, respectively,cis‐Glycol formation starting from these latter adducts, followed by hydrogenolysis of the NO bond and by exhaustive acetylation of the OH‐groups, led to the diaminolyxose5, the diaminomannose14and the diaminoallose15. When starting from benzyl 1,2‐dihydropyridine‐1‐carboxylate (1b) and using the same reaction sequence, the racemic piperidine de

ISSN:0018-019X    

DOI:10.1002/hlca.19850680112

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

AbstractIn the AlBr3‐catalyzed adamantane rearrangement in CS2of 1,2‐exo‐trimethylenenorbornane (1) to 2‐endo,6‐endo‐trimethylenenorbornane (3), hydride‐ion abstraction occurs at C(6) from theexo‐side. ThekH/kDvalue for competition between1and5(Dexo‐C(6)) was 1.58 ± 0.05, whereas no kinetic isotope effect was operative for competition between unlabeled1and4(Dendo‐C(5)) and betw

ISSN:0018-019X    

DOI:10.1002/hlca.19850680113

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

AbstractCyclic octa‐ and decapeptides have been prepared, and their ion selectivity in solvent polymeric membranes was studied. Cyclo(‐LPro‐DLeu‐)5behaves as an ionophore which selects Mg2+over Ca2+by a factor of 100. Cyclo(LPro‐LLeu)5induces selectivity in membranes for Mg2+over Li+, Na+and K+by a factor of 400, 200 and 10, respectively, while selectivity of Mg2+over Ca2+is poor. Some evidence is presented which indicates that certain ionophores exhibit high selectivity for monovalent magnesium

ISSN:0018-019X    

DOI:10.1002/hlca.19850680114

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

AbstractAn empirical force‐field for carbenium ions has been incorporated inAllinger'sMM2 programme. Structural parameters of secondary carbenium ions calculated by this method are compared with those obtained withSchleyer'sBIGSTRN calculations. The strain changes occurring upon solvolysis of secondaryp‐toluenesulfonates are evaluated by means of this force‐field and correlated with the rate constants for solvolysis. The equation for correlation of acetolysis, relative to cyclohexylp‐toluenesulfonate, of 28kcsubstrates is ΔG rel+= 0.67 ΔEst‐ 0.2

ISSN:0018-019X    

DOI:10.1002/hlca.19850680115

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

AbstractTheFriedel‐Craftsmonoacylation oftrans‐η‐[(1RS,2RS,4SR,5SR,6RS,7SR,8SR)‐C,5,6,C‐η:C,7,8,C‐η‐(5,6,7,8‐tetramethylidene‐2‐bicyclo[2.2.2]octyl acetate)]‐bis(tricarbonyliron) ((±)‐5) is highly stereoselective and yieldstrans‐η‐[(1RS,2RS,4RS,5SR,6RS,7RS,8SR)‐C,6‐η,oxo‐σ:C,7,8,C‐η‐(6,7,8‐trimethylidene‐5‐((Z)‐2‐oxopropylidene)‐2‐bicyclo[2.2.2]octyl acetate)]‐bis(tricarbonyliron) ((±)‐8) which equilibrates with thetrans‐η‐[(1RS,2RS,4RS,5SR,6RS,7RS,8SR)‐C,5,6,C‐η:C,7,8,C‐η‐(6,7,8‐trimethylidene‐5‐((Z)‐2‐oxopropylidene)‐2‐bicyclo[2.2.2]octyl acetate)]‐bis(tricarbonyliron) ((±)‐9) on heating. Optically pure (–)‐9has been prepared from the corresponding optically pure alcohol (+)

ISSN:0018-019X    

DOI:10.1002/hlca.19850680116

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

Preparation of the Enantiomerically Purecis‐ andtrans‐Configurated 2‐(tert‐Butyl)‐3‐methylimidazolidin‐4‐ones from the Amino Acids (S)‐Alanine, (S)‐Phenylalanine, (R)‐Phenylglycine, (S)‐Methionine, and (S)‐ValineIn contrast to α‐hydroxy and α‐mercapto carboxylic acids, simple α‐amino acids do not form acetal‐type derivatives (2, X = NH) with pivalaldehyde. For the generation of amino‐acid‐derived chiral, nonracemic enolates (cf.3), and hence, for the α‐alkylation of amino acids without racemization and without an external chiral auxiliary, the imidazolidinones12–14were prepared diastereoselectively. To this end, the methyl or ethyl esters of amino‐acid hydrochlorides were first converted toN‐methylamides of amino acids which in turn were condensed with pivalaldehyde to give (neopentylidenamino)amides (11). TheseSchiffbases could be cyclized either totrans‐or tocis‐imidazolidinones (12, 14and13, respectively), which were obtained in enantiomerically pure form after recrystallization. The enantiomeric purities were confirmed by HPLC with chiral stationary phases or by1H‐NMR spectroscopy in the presence of chiral shift reagents. The configurations (cis,trans) were assigned by

ISSN:0018-019X    

DOI:10.1002/hlca.19850680117

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

AbstractEnantiomerically purecis‐ andtrans‐5‐alkyl‐1‐benzoyl‐2‐(tert‐butyl)‐3‐methylimidazolidin‐4‐ones (1, 2, 11, 15, 16) andtrans‐2‐(tert‐butyl)‐3‐methyl‐5‐phenylimidazolidin‐4‐one (20), readily available from (S)‐alanine, (S)‐valine, (S)‐methionine, and (R)‐phenylglycine are deprotonated to chiral enolates (cf.3, 4, 12, 21). Diastereoselective alkylation of these enolates to 5,5‐dialkyl‐ or 5‐alkyl‐5‐arylimidazolidinones (5, 6, 9, 10, 13a‐d, 17, 18, 22) and hydrolysis give α‐alkyl‐α‐amino acids such as (R)‐ and (S)‐α‐methyldopa (7and8a, resp.), (S)‐α‐methylvaline (14), and (R)‐α‐methyl‐methionine (19). The configuration of the products is proved by chemical correlation and by NOE1H‐NMR measurements (see23, 24). In the overall process, a simple, enantiomerically pure α‐amino acid can be α‐alkylated with retention or with inversion of configuration through pivaladehyde acetal derivatives. Since no chiral auxiliary is required, the process is coined ‘self‐reproduction of a center of chirality’. The method is compared with other α‐alkylations of amino acids occurring without racemization. The importance of enantiomerically pure, α‐branched α

ISSN:0018-019X    

DOI:10.1002/hlca.19850680118

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

Alkylation in the 2‐Position of (2S, 4R)‐4‐Hydroxyproline with Retention of ConfigurationO‐Acetyl‐4‐hydroxyproline (1b) is condensed with pivalaldehyde to give a single stereoisomer of the 2‐(tert‐butyl)‐4‐oxo‐3‐oxa‐1‐azabicyclo[3.3.0]oct‐7‐yl acetate (3). This is converted to the enolates4or5, reactions of which with alkyl halides, aldehydes, and acetone (→6,9,10,11) are diastereoselective (lk‐1,3‐induction). Cleavage of the corresponding products furnishes the enantiomerically pure 2‐deuterio‐, 2‐methyl‐, 2‐allyl‐

ISSN:0018-019X    

DOI:10.1002/hlca.19850680119

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

On the Steric Course of the Reaction of Enamines Derived from Open‐Chain Aldehydes and Ketones with Nitro‐olefins Yielding 2,3‐Disubstituted 4‐NitroketonesThe relative configuration of the nitro‐ketone12obtained by the title reaction has been proved by aBaeyer‐Villigerdegradation and subsequent reduction to the amino‐alcohol (±)‐(u)‐20, the relative configuration of which is identical with a sample prepared from 2‐methyl‐3‐hydroxybutyrate of known configuration (Scheme 1). The stereochemical course of the title reaction is thus as predicted by ourtopological rule. It is assumed that the other nitro‐ketones13–18described here are also formed by (lk)‐combination of the trigonal centers of the educts (diastereoselectivity>90%, seeFormulaeandFig. 1). It is not possible to reverse this relative topicity by employing (Z)‐ instead of (E)‐components of the starting materials, due to the reversibility of the primary step of theseMichaelad

ISSN:0018-019X    

DOI:10.1002/hlca.19850680120

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY

摘要:

Abstracttert‐Butyl α,β‐dioxobutyrate (hydrate;1d) undergoes, at medium or high pH, the benzilic‐acid rearrangement with exclusive 1,2‐shift of the COO(t‐Bu) group; the same is true for the corresponding isopropyl ester1cand ethyl ester1bat high pH, whereas at lower pH, the overall picture of these reactions is complicated by concurrent hydrolysis of the ester, followed by a 1,2‐shift of theCOO−group. Consequently, the shift of these electron‐attracting groups cannot be considered to be systematically disfavoured (compared,e.g., with alkyl‐group shifts). Kinetic measurements of the rearrangement show for both esters (as well as for the analogous ethyl ester1b, and also for ethyl 3‐cyclopropyl‐α,β‐dioxopropionate (4)) a characteristic rate profile: at relatively low pH,kis proportional to [HO−], approaching saturation with increasing [HO−] (interpreted as complete transformation of the substrate into the hydrate monoanion), which is followed at higher pH by another rate increase withkproportional to [HO−] (probably due to the reaction of the hydrate dianion). The similarity ofkvalues for1b‐dshows that in the shift of COOR steric

ISSN:0018-019X    

DOI:10.1002/hlca.19850680121

出版商:WILEY‐VCH Verlag GmbH    

年代:1985

数据来源: WILEY