ASYMMETRIC-SYNTHESIS OF CHIRAL FLUORINATE D COMPOUNDS USING N-ACYLOXAZOLIDINONES

A variety of optically active fluorinated molecules have been prepared by diastereoselective reactions of chiral N-acyloxazolidinones. The t rifluoromethyl radical generated from iodotrifluoromethane with a radi cal initiator, triethylborane, reacts on the si-face of the lithium en olates of the oxazolidinones with good diastereomeric excess. The perf luoroalkylation and ethoxycarbonyldifluoromethylation of the lithium e nolates mediated by triethylborane also proceeds diastereoselectively via a radical mechanism. The bromodifluoromethylation of the lithium e nolates proceeds not via a radical mechanisim involving bromodifluorom ethyl radical but via an ionic chain mechanism involving the insertion of difluorocarbene. The Evans aldol reaction of hexafluoroacetone and trifluoroacetaldehyde causes complete reversal of diastereofacial sel ectivity. The boron enolate derived from chiral N-acyloxazolidinones r eacts with trifluoroacetaldehyde to give anti and ''non-Evans'' syn al dols with stereoselectivity in the range of 7:3 similar to 17:3. This unexpected and ususal reversal of pi-face selectivity was reproduced b y semiempirical molecular orbital calculations.