Cobalt-Catalyzed Dual Annulation of o-Halobenzaldimine with Alkyne: A Powerful Route toward Bioactive Indenoisoquinolinones.

A cobalt-catalyzed dual annulation reaction for the synthesis of variously substituted indenoisoquinolinones from 2-bromobenzaldehydes, amines, and methyl 2-(ethynyl)benzoates has been developed. This method could also be applied to the synthesis of an array of highly functionalized bioactive indenoisoquinolinones and their derivatives. A possible mechanism of the cobalt catalysis is proposed, involving imine formation from bromobenzaldehyde and the amine, followed by a series of oxidative addition, alkyne insertion, cyclization reactions, and carbon-carbon double-bond migration. The regioselective alkyne insertion plays an important role for the success of the second annulation.