Synthesis of tricyclic azetidinones by intramolecular free radical cyclization

The synthesis of strained tricyclic azetidinones was achieved by free radical cyclization of N-substituted azetidinones involving a tributyltin hydride/AIBN mediated chain reaction. The 5-exo cyclization led to benzo carbapenems 10, 12, and 13, which were relatively unstable in solution. The 6-exo mode, however, afforded stable benzo carbacephems 20, 22, 24, and 25.