The aza–ene or the Michael addition? Examination of an unusual substituent effect on the reaction of heterocyclic ketene aminals with ethyl propiolate

Heterocyclic ketene aminals having at least one secondary amino group underwent the aza–ene reaction with ethyl propiolate under various conditions to yield the corresponding adducts, which were readily transformed into the δ-lactam fused heterocyclic products with or without the aid of sodium ethoxide in refluxing ethanol, while the ester- and cyano-substituted tertiary enediamines acted as the strong Michael donor to add to ethyl propiolate in a polar or a protic solvent. The unusual substituent effect on the reactivity and mechanism was discussed.