Development of Chiral Lewis Acid, Oxazaborolidinone, Promoted Asymmetric Aldol Reaction

Abstract

Reaction of N-sulfonyl amino acid with 1 equiv. of BH₃ THF easily gave the oxazaborolidinone which promoted the aldol reaction of a variety of aldehydes with silyl ketene acetals with very high enantioselectivity. For example, isobutyrate aldols were obtained with >96% ee and acetate aldols were obtained with >98% ee by using a silyl nucleophile derived from ethyl 1, 3-dithiolane-2-carboxylate. The level of the selectivity is surely acceptable for practical syntheses although the promoter is used as stoichiometric amounts. The scope and limitation of the oxazaborolidinone-promoted aldol reaction will be discussed by surveying the stereochemical outcome of the reaction inducing the high enantioselectivity.