Development of a chiral Lewis acid-promoted asymmetric aldol reaction using oxazaborolidinone

The reaction of an N-sulfonylamino acid with 1 equiv. of BH3 . THF easily gives the oxazaborolidinone which promotes the aldol reaction of a variety of aldehydes with silyl ketene acetals with very high enantioselectivity. For example, isobutyrate aldols are obtained with >90% ee and acetate aldols are obtained with >98% ee using a silyl nucleophile derived from ethyl 1, 3-dithiolane-2-carboxylate. The level of selectivity is acceptable for practical syntheses although a stoichiometric amount of the promoter is used. The scope and limitation of the oxazaborolidinone-promoted aldol reaction is discussed by surveying the stereochemical outcome of reactions inducing high enantioselectivity.