Synthesis of Cyclopenta[c]quinolines by Palladium-Catalyzed Cyclization of 3-Bromoindoles with Internal Alkynes via Spirocyclic Cyclopentadiene Intermediates

A novel method for the construction of a cyclopenta[c]quinoline ring via cyclization of 3-bromoindoles withinternal alkynesin the presence of palladium is described. The formation of the cyclopenta[c]quinoline ring is proposed from a double [1,5] carbonsigmatropic rearrangement of the spirocyclic cyclopentadiene intermediate,which is generated in situ from the cyclization of 3-bromoindoleswith internal alkynes involving a sequential double alkyne insertioninto the carbon-palladium bond and dearomatization of indole.The present studies have developed a novel ring-expansion reactionof the pyrrole ring to pyridine via one carbon insertion into theC2-C3 bond of indoles and provided a simple and distinct routefor the construction of tricyclic fused-quinoline derivatives thatare not easy to access with conventional methods.