A vinylogous Norrish reaction as a strategy for light-mediated ring expansion

The reactions of bicyclic divinyl ketones display wavelength-dependent changes in product formation. UV irradiation results in the formation of competitive [6,3,5] and [7,3,5] tricyclic unsaturated ketones that subsequently undergo ring expansion and reaction with a range of nucleophiles. DFT calculations and transient absorption experiments were completed that are consistent with a vinylogous Type II Norrish pathway. Photo-mediated ring expansion using UV-A light. 22 examples, 75-96% yield. The mechanism was investigated by DFT and Transient Absorption Spectroscopy.