Radical and Palladium-Catalyzed Cyclizations to Cyclobutenes: An Entry to the BCD Ring System of Penitrem D

A novel approach toward the synthesis of the BCD ring system of penitrem D is described. The
strategy capitalizes on the fast cyclization rates of aryl radicals into cyclobutenes and allows access
to a variety of fused tricyclic structures. Radical/polar crossover reactions of precursors 24−29
promoted by samarium diiodide in the presence of HMPA and acetone allow access to the fully
functionalized BCD ring system of penitrem D. The stereochemical implications of these processes
are evaluated, and a Pd-mediated cyclization approach toward the penitrems is also introduced.