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.