Synthesis of beta- and gamma-carbolines by the palladium/copper-catalyzed coupling and cyclization of terminal acetylenes.
A variety of 3-substituted beta- and gamma-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 PdCl(2)(PPh(3))(2)/CuI as the catalyst readily affords the corresponding alkynylindole carboxaldehydes, which have subsequently been converted to the corresponding tert-butylimines and cyclized to beta- and gamma-carbolines by either copper-catalyzed or thermal processes.