Synthesis of β- and γ-carbolines by the palladium/copper-catalyzed coupling and copper-catalyzed or thermal 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 using cat. PdCl2(PPh3)2/CuI readily affords the corresponding alkynylindole carboxaldehydes, which have subsequently been converted to the corresponding tert-butylimines, which have then been cyclized to β- and γ-carbolines by either copper-catalyzed or thermal processes.