some chiral diamines and amino alcohols leads, via a dynamic resolution process, to single atropisomers of tertiary amides bearing chiralimidazolidines or oxazolidines. Hydrolysis of the new heterocycle competes a dynamic thermodynamic resolution of the starting aldehyde, and rapid reduction allows the isolation of atropisomeric amides bearing 2-hydroxymethyl substituents in enantiomerically enriched
Using amide conformation to ‘project’ the stereochemistry of an (−)-ephedrine-derived oxazolidine: a pair of pseudoenantiomeric chiral amido-phosphine ligands
Protection of a tertiary 2-formylbenzamide as an (-)-ephedrine-derived oxazolidine both Fords the amide's stereogenic Ar-CO axis to adopt one of two possible diastereoisomeric conformations and protects the formyl group from attack during amide ortho-lithiation. By functionalising the amide in the 6-position, reactive sites (such as -CHO, -SR, -PR2 groups) may be introduced which fall under the stereochemical influence, relayed by the amide: of the (-)-ephedrine-derived oxazolidine. This 'projection' of stereochemistry is exemplified by a pair of amido-phosphines. members of the first ever class of non-biaryl atropisomeric ligands, which are made functionally pseudoenantiomeric by the intervention of either one or two amide groups between the (-)-ephedrine-derived oxazolidine and the PPh2 group. The pseudoenantiomeric amido-phosphines promote the palladium-catalysed asymmetric allylation of dimethyl malonate in 82 and -53% e.e., respectively. (C) 2001 Published by Elsevier Science Ltd.
(−)-Ephedrine as an auxiliary for the asymmetric synthesis of atropisomeric amides by dynamic resolution under thermodynamic control
作者:Jonathan Clayden、Lai Wah Lai
DOI:10.1016/s0040-4039(01)00416-6
日期:2001.4
N,N-Dialkyl-2-formylbenzamides and N,N-dialkyl-2-formyl-1-naphthamides condense with (-)-ephedrine in refluxing toluene to give oxazolidines both as single diastereoisomers with respect to the new stereogenic centre and as single conformers with respect to the slowly-rotating Ar-CONR2 bond. In the naphthamide series, removal of the ephedrine auxiliary by hydrolysis returns the starting aldehyde (isolated by reduction to the more stable alcohol) in enantiomerically enriched form (up to 94% ee). Overall, the two-step sequence amounts to a dynamic resolution under thermodynamic control. (C) 2001 Elsevier Science Ltd. All rights reserved.
Conformational preference in aromatic amides bearing chiral ortho substituents: its origin and application to relayed stereocontrol
作者:Mark S. Betson、Jonathan Clayden、Madeleine Helliwell、Paul Johnson、Lai Wah Lai、Jennifer H. Pink、Christopher C. Stimson、Neoclis Vassiliou、Neil Westlund、Samreen A. Yasin、Latifa H. Youssef
DOI:10.1039/b514557k
日期:——
Tertiary aromatic amides bearing stereogenic centres ortho to the amide group may adopt two diastereoisomeric conformations which interconvert slowly on the NMR timescale at ambient temperature, and are therefore detectable by NMR. Certain classes of stereogenic centre—particularly sulfoxides, ephedrine-derived oxazolidines, and proline-derived imidazolidines—strongly bias the population of the two conformers. We propose a model, supported by molecular mechanics calculations, which rationalises the sense and magnitude of the conformational selectivity attained in terms of the steric and electronic properties of the controlling centre. The control over conformation may be exploited either by trapping the favoured conformer as an atropisomer, or by using it to relay information about the stereochemistry of the controlling centre.