Synthesis of β- and γ-Carbolines by the Palladium/Copper-Catalyzed Coupling and Cyclization of Terminal Acetylenes

A variety of 3-substituted β- and γ-carbolines have been synthesized from N-substituted 3-iodoindole-2-carboxaldehydes and 2-bromoindole-3-carboxaldehydes, respectively. The coupling of these aldehydes with various terminal acetylenes with PdCl₂(PPh₃)₂/CuI as the catalyst readily affords the corresponding alkynylindole carboxaldehydes, which have subsequently been converted to the corresponding tert-butylimines and cyclized to β- and γ-carbolines by either copper-catalyzed or thermal processes.