Asymmetric synthesis of chiral fluorinated compounds using N-acyloxazolidinones

A variety of optically active fluorinated molecules have been prepared by diastereoselective reactions of chiral N-acyloxazolidinones. The trifluoromethyl radical generated from iodotrifluoromethane with a radical initiator, triethylborane, reacts on the si-face of the lithium enolates of the oxazolidinones with good diastereomeric excess. The perfluoroalkylation and ethoxycarbonyldifluoromethylation of the lithium enolates mediated by triethylborane also proceeds diastereoselectively via a radical mechanism. The bromodifluoromethylation of the lithium enolates proceeds not via a radical mechanisim involving bromodifluoromethyl 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 selectivity. The boron enolate derived from chiral N-acyloxazolidinones reacts with trifluoroacetaldehyde to give anti and ''non-Evans'' syn aldols with stereoselectivity in the range of 7:3 similar to 17:3. This unexpected and ususal reversal of pi-face selectivity was reproduced by semiempirical molecular orbital calculations.