Gold-Catalyzed Oxidative Ring Expansion of 2-Alkynyl-1,2-Dihydropyridines or -quinolines: Highly Efficient Synthesis of Functionalized Azepine or Benzazepine Scaffolds

A gold-catalyzed highly regio- and chemoselective oxidative ring expansion of 2-alkynyl-1,2-dihydropyridines and its analogues using pyridine-N-oxide as the oxidant has been developed. Ring expansion proceeds through exclusive 1,2-migration of a vinyl or phenyl group, whereas no 1,2-H and 1,2-N migration take place. The reaction provides an efficient and attractive route to various types of medium-sized azepine derivatives in generally high to excellent yields with a broad functional group tolerance. DFT studies indicate that the reaction proceeds through the formation of a cyclopropyl gold intermediate, and no gold carbene species is involved.