Further investigation on the reaction of heterocyclic ketene aminals with ethyl propiolate: ene-type reaction of enamines and an unusual substituent effect

Abstract

The reaction between heterocyclic ketene aminals and ethyl propiolate 1 has been investigated and an unusual substituent effect observed. Although 2-(benzoylmethylene)-1, 3-dimethylimidazolidines 2 failed to add to 1, 2-(benzoylmethylene)imidazolidine 3 reacted with it to give the fused heterocycle 5via the intermediate 4. With one methylated nitrogen, the heterocyclic ketene aminals 6 or 7 reacted with 1 to afford the heterocyclic compounds 9 or 10 and ethoxycarbonylvinyl substituted heterocycles 11 or 12. By controlling the reaction conditions, 9 or 10 and 11 or 12 can be synthesized selectively. The results are interpreted in terms of an ene-addition mechanism with steric interaction in the intermediates. A correlation between reactivity and intramolecular hydrogen bonding is also discussed.