Enantioselective palladium-catalyzed ring-opening reaction of azabenzonorbornadienes with methyl 2-iodobenzoate: An efficient access to cis-dihydrobenzo[c]phenanthridinones

Abstract

The palladium-catalyzed asymmetric ringopening (ARO) reaction of azabenzonorbornadienes with iodobenzoate derivatives was realized by using chiral spirophosphine ligands with zinc powder as a reducing reagent. Various enantiomerically enriched cis-dihydrobenzo[c]phenanthridinones, the core structure of numerous chiral natural products, were prepared by the ARO reaction and tandem amidation from easily obtained starting materials with favorable enantioselectivity (up to 86% ee) under mild reaction conditions. The present palladium-catalyzed ARO reaction avoids the direct use of organometallic reagents, enables tandem cyclization, and thus improves synthetic efficiency. The formal total synthesis of a chiral natural product, (+)-chelidonine, was accomplished with the ARO reaction as a key step, which demonstrates the potential synthetic applications of the present methodology. © 2013 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.