Synthesis of Benzobicycloheptanones via the Trap of Photogenerated Ketene Methide Intermediate with Olefins

Irradiation of ortho-formyl dienes with UV light led to benzobicycloheptanones in high yields and chemoselectivities via a photogenerated ketene methide/Diels–Alder cascade reaction. The reaction mechanism was proposed to be a [1,5]-H shift process rather than a radical pathway based on control experiments. DFT calculations indicate that the energy of transition states is responsible for the high chemoselectivity observed in this protocol.