Discovery of an Acid-Promoted [3+2] Cyclodimerization of 3-Vinylindoles and the Development of a General Lewis Acid Catalyzed Process

A novel proton-catalyzed [3+2]-type cyclodimerization of 3-vinylindoles is reported leading to fused cyclopenta[b]indoles, contrasting to the normal [4+2]-cycloaddition pathway. The development of a general, high yielding version of the reaction catalyzed by zinc bromide in toluene is described.