Copper-Catalyzed Heteroannulation of Propargylic Pyridines to Indolizines

Significance

A variety of propargylic pyridines were converted into the corresponding functionalized indolizines or indolizinones under copper catalysis in good to excellent yields. The presence of a base significantly accelerates the rate of the reaction (2 h vs. 18 h with and without base, respectively). Propargylic secondary acetates lead to indolizines and propargylic tertiary alcohols to indolizinones, the latter reaction involving a cyclization-1,2-migration cascade. The substrate scope was well studied and two possible mechanisms were suggested, involving either copper-coordinated allenic or alkynyl intermediates. No evidence in favor of either of the proposed mechanisms was provided.