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
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