STEREOSELECTIVE BROMINATION OF DEHYDROAMINO ACIDS WITH CONTROLLABLE RETENTION OR INVERSION OF OLEFIN CONFIGURATION

Dehydroamino acid esters react with brominating reagents to produce syn-a-bromo imines as the major products, which undergo tautomerization to mixtures of the diastereomeric (E)- and (Z)-beta-bromo-alpha,beta-dehydroamino acids upon treatment with base. Herein, we examine the characteristics of dehydroamino acid substrates 1 that affect both the diastereofacial selectivity of the bromination and the configuration of the resulting olefin product(s) 3. In these studies, we demonstrate E- and Z-diastereoselectivity for this reaction using kinetic and thermodynamic reaction conditions. Z-Vinyl bromides were shown to be the thermodynamically more stable products and were formed from the kinetic E-isomers by DABCO-induced isomerization. High levels of kinetic E-selectivity were obtained by treatment of the intermediate alpha-bromo imines with hindered amine bases such as 2,2,6,6-tetramethylpiperidine or sodium hexamethyldisilazide. The reaction conditions detailed herein allow for the stereodivergent preparation of both E- and Z-vinyl bromide products from either diastereomeric E- or Z-olefin starting material with useful levels of diastereoselectivity.