Synthesis and reactivity of new β-enamino acid derivatives:: A simple and general approach to β-enamino esters and thioesters

A new strategy has been developed for the synthesis of several beta-enamino acid derivatives. N,N'-Carbonyldiimidazole has been used as C-acylating agent of methyl ketimines, providing a direct and simple route to new beta-enamino carbonyl imidazole derivatives 2. These derivatives 2 were cleanly and efficiently transformed into beta-enamino esters 4 (X = O) and thioesters 4 (X = S) by reaction with a great variety of alcohols and thiols, including tertiary ones. Alternative and complementary routes to compounds 4 were also investigated. In addition, beta-keto esters 6 have been obtained by mild acid hydrolysis of beta-enamino esters 4.