Free-Radical-Mediated [2+2+1] Cycloaddition of Acetylenes, Amidines, and CO Leading to Five-Membered α,β-Unsaturated Lactams

A free-radical-mediated [2 + 2 + 1] cycloaddition reaction comprising acetylenes, amidines, and CO was achieved by radical chain reaction to give five-membered alpha,beta-unsaturated lactams in good yields. Both acyclic and cyclic amidines reacted with a variety of terminal acetylenes to afford monocyclic, bicyclic, and tricyclic lactams. We propose that vinyl radical carbonylation and nucleophilic addition of the amidine onto the resulting a-ketenyl radical give stable intermediates that are ready to undergo five-membered ring closure with elimination of tin radical.