Stereoselective radical bromination of alpha-chloro hydrocinnamic acid derivatives

Reaction of (S)-2-chloro-3-phenylpropanoic acid derivatives with NBS gave t he corresponding 3-bromo-2-chloro derivatives with a preference for the for mation of the (2R,3S) isomers over the (2R,3R) isomers. The stereoselectivi ty was affected by the nature of the carboxylic acid derivative. Reaction o f ester derivatives was highly stereoselective while the reaction of amide derivatives showed varied stereoselectivity which depended on the nature of the amide. Theoretical studies at UHF/3-21G* level showed that the interme diate benzylic radical of the methyl ester, the methyl amide and the diisop ropyl amide derivatives had different energy profiles with respect to rotat ion of the C2-C3 bond. The different stereoselectivity observed from reacti on of the various acid derivatives could be attributed, at least in part, t o different distribution of conformers of the radical intermediate. (C) 200 0 Elsevier Science Ltd. All rights reserved.