Metal-Free Domino Oligocyclization Reactions of Enynals and Enynones with Molecular Oxygen.

A novel metal-free direct addition of molecular oxygen to the C-C triple bond toward benzannulated oxygen-bridged seven-membered ring systems and aza[3.1.0]bicycle skeletons under 3O2 atmosphere has been described. The reaction proceeds through at least three intramolecular C-O and C-C bond forming steps via green, simple, and unprecedented domino radical processes with high selectivity and good yields.