摘要:

AbstractThe advantages and disadvantages of using enantiomeric excess, enantiomeric composition or enantiomer ratio to describe a mixture of enantiomers, are discussed with respect to various stereochemical situations.

ISSN:0165-0513    

DOI:10.1002/recl.19951140412

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractEnantiomerically pure 2‐[2‐(diphenylphosphino)aryl]oxazolines are readily prepared from 2‐bromobenzonitrile by transmetalation with BuLi, subsequent reaction with chloro‐diphenylphosphine and conversion of the resulting phosphinoaryl nitrile to the oxazoline by treatment with a chiral amino alcohol in the presence of ZnCl2. An alternative synthesis is based on the orthometalation of 2‐aryloxazolines followed by reaction with chlorodiphenyl

ISSN:0165-0513    

DOI:10.1002/recl.19951140413

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe preparative enantiomer separation of the inhalation anesthetics enflurane (1), isoflurane (2) and, for the first time, desflurane (3) in high chemical and enantiomeric purity by preparative gas chromatography on octakis(3‐O‐butanoyl‐2,6‐di‐O‐pentyl)‐γ‐cyclodextrin (4) dissolved in SE‐54 and coated on Chromosorb P (AW‐DMCS) is described. Thermodynamic data for the observed enantioselectivities are reported. A discrepancy between the chiroptical and gas‐chromatographic assignment of the relative configuration of2and3is discussed. The1H‐ and19F‐NMR spectra of racemic1in the presence of4show good separation of the resonances of the enantiomers of1. Thermodynamic complexation constants derived from the1H‐NMR spectra of the complexes formed by4and the enantiomers of1were in good agreeme

ISSN:0165-0513    

DOI:10.1002/recl.19951140414

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA method is presented to determine the enantiomeric excess(ee) of phosphoric acids1, which are extensively used as resolving agents, by means of diastereomeric salt formation with chiral amines. The unexpected dynamic behaviour of these systems leads us to postulate a model, which we refer to asnon‐reciprocal behaviou

ISSN:0165-0513    

DOI:10.1002/recl.19951140415

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe internal cavities of a dendritic box, constructed from poly(propylene imine) dendrimers and enantiomerically pureN‐tBOC‐L‐phenylalanine hydroxysuccinimide esters, possess distinct chiral properties as was demonstrated with the observation of induced optical activity of a variety of guests encapsulated in the box. This induced optical activity of guests is measured with CD (circular dichroism) spectroscopy and is remarkable as the optical activity of the amino acid outer shell itself is vanishingly small. Although small, the effects observed are significant in many cases, whilst in other cases the guests encapsulated do not show an induced CD. Indications for an enforced close proximity and some kind of ordering of guests is found by the observation of an exciton‐coupled CD spectrum in the case of four Bengal Rose molecules encapsulated in a single dendritic box. It is shown that the effects are dependent on the solvent used and the history of the sample. Therefore, the observation of induced optical activity can be used to study the mode of encapsulation of the guests into

ISSN:0165-0513    

DOI:10.1002/recl.19951140416

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractL‐α‐Methylphenylalanine, obtained by enzymatic resolution of the corresponding racemic amide with amidase fromOchrobactrum anthropiNCIMB 40321, was used in the synthesis of dipeptides containing α‐substituted amino acids. The 2‐hydroxynaphthalene‐1‐carboxaldehyde Schiff bases of the dipeptides (1and2) were tested as Ti(IV) and Al(III) complexes in asymmetric Lewis‐acid‐catalyzed reactions. Only the Al(III) complex of2showed moderate enantioselectivity in the cyanation reaction of benzald

ISSN:0165-0513    

DOI:10.1002/recl.19951140417

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe hydroxylation of Baylis‐Hillman adducts1is shown to proceed with high levels of diastereocontrol. A mechanistic rationale to explain this selectivity is discusse

ISSN:0165-0513    

DOI:10.1002/recl.19951140418

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA method for the preparation of diphosphineruthenium complexes from a range of diphosphine precursors is described. The stable products, of general formula P2Ru(allyl)acac (P = phosphine ligand), do not catalyse the hydrogenation of alkenes but can easily be converted into catalytically active species by reaction with trimethylsilyl trifluoromethanesulfonate. The chemistry involved in this transformation is discussed. Catalysis of the hydrogenation of a range of alkenes, and direct comparison with related rhodium reductions, is described. Some mechanistic insights are gained through additions of D2to alkenes and parallel hydrogenations on deuterated solvent, supporting the intervention of conventional dihydride and alkyl hydride intermediates which undergo exchange with solvent at both stages.

ISSN:0165-0513    

DOI:10.1002/recl.19951140419

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractReadily available structural modules were combined to a new chiral receptor for carboxylic acids which shows triple hydrogen‐bonding to various substrates. Large chemical shift anisochronies were observed with chiral and prochiral carboxylic acid

ISSN:0165-0513    

DOI:10.1002/recl.19951140420

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe enantioselectivity of serine proteases in the transesterification of N‐acetyl‐(D,L)‐ phenylalanine esters with propan‐1‐ol in cyclohexane is strongly influenced by the leaving ability of the alcoholate group. Moreover the enantioselectivity is greatly influenced by the addition of small amounts (0.33 M) of organic additives. Addenda with a small molecular volume likee.g.ethanol and acetonitrile increase the rate for the L‐enantiomer whereas alcohols with bulky alkyl groups like e.g.tert‐butanol and 2‐methylbutan‐2‐ol enhance the activity of the enzyme towards the D‐enantiomer. This enables a rational tuning of the enantioselectivity as was demonstrated for four different proteases (α‐chymotrypsin, subtilisin Carlsberg,Aspergillus oryza

ISSN:0165-0513    

DOI:10.1002/recl.19951140421

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY