Elsevier

Tetrahedron

Volume 52, Issue 20, 13 May 1996, Pages 7025-7036
Tetrahedron

Neighbouring group effects promote substitution reactions over elimination and provide a stereocontrolled route to chloramphenicol

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Abstract

In reactions of β-brominated valine and p-nitrophenylalanine derivatives to give β-hydroxy amino acid derivatives the carboxyl group, when protected as an amide, exerts a neighbouring group effect to facilitate the substitution process, and reduce competing elimination reactions. As a consequence of the effect, the (2R,3R)- and (2R,3S)-stereoisomers of 3-bromo-N-tert-butyl-Nα-phthaloyl-p-nitrophenylalaninamide both react to give (2S,3R)-3-hydroxy-N-tert-butyl-Nα-phthaloyl-p-nitrophenylalaninamide, providing a stereoconvergent route to chloramphenicol.

The diastereoselectivity and course of the reaction shown reflects a neighbouring group effect by the amide substituent and provides a stereocontrolled route to Chloramphenicol.

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