Esterification of Maleamic Acids without Double Bond Isomerization

Abstract

Activation of the carboxylic acid group of a maleamic acid by treatment with an arenesulfonyl chloride followed by addition of an alcohol affords a fumaramate or a maleamate, depending on the reaction conditions. The E isomer is obtained when the acid is treated with nearly equimolar amounts of 2,4,6‐triisopropylbenzenesulfonyl chloride and an alcohol in pyridine. Replacement of pyridine by 2‐picoline and use of a larger excess of activating agent (mesitylenesulfonyl chloride) and alcohol affords the Z isomer. In both cases, high diastereomeric excesses and yields are achieved.