Asymmetric reactions based on 1,3-oxathianes3̄: Secondary α-hydroxyacids, RCHOHCO 2H and glycols RCHOHCH 2OH

Abstract

Reduction of the previously prepared 1 chiral 2-acyl-l,3-oxathianes derived from (+)-pulegone with various metal hydride combinations proceeds stereoselectively, with diastereomer excess (d.e.) of as much as 97% in the case of reduction of phenyl ketones with lithium tri-sec.butylborohydride. Lesser selectivity (maximum 82% d.e.) is achieved with primary or tertiary alkyl ketones: the predominant diastereomer is readily purified by chromatography. The major product in these cases is that predicted by Cram's chelate rule. The product ratio is reversed with diiasobutylaluminum hydride and also in the reduction of secondary alkyl ketones with lithium sec. butylborohydride, where stereoselectivity is low. The 2-hydroxyalkyl-l,3-oxathianes are cleaved to α-hydroxyaldehydes with N-chlorosuccinimide-silver nitrate and the aldehydes reduced to glycols, RCHOHCH 2OH with sodium borohydride with little or no racemization. Esters, RCHOHCO 2CH 3, are obtained in high enantiomeric purity by O-benzylating the 2-hydroxyalkyl-l,3-oxathianes prior to cleavage, oxidizing with sodium chlorite following cleavage, esterifying and debenzylating. A method for measuring the enantiomeric purity of glycols RCHOHCH 2OH by conversion to 2-phenyl-l,3-dioxolanes with benzaldehyde, followed by proton NMR analysis of the resulting 2-phenyl-4-alkyl-l,3-dioxolane diastereomer pair in the presence of a chiral europium shift reagent is described.