Highly efficient and regiospecific photocyclization of 2,2'-diacyl bixanthenylidenes.

In contrast to the reversible photochemistry of the 2,2'-substituted bixanthenylidenes (1a-f), the photocyclization of 2,2'-diacyl bixanthenylidenes (1g-j) reveals an irreversible process where the initial cyclic intermediate C(E) can undergo a rapid [1,11] hydrogen shift to form stable isomer C'(E) in a degassed solution, which cannot revert to the starting compound, so giving a highly efficient and regiospecific photocyclization.