Visible-Light-Promoted 6π Photocyclization of ortho-Biaryl-Appended ß-Ketoesters.

A photocatalyst- and additive-free visible-light-induced 6π-photocyclization of ortho-biaryl-appended ß-ketoesters has been developed. Upon irradiation with visible light, substrates undergo 6-endo-trig cyclization/1,5-H shift to 9,10-dihydrophenanthren-9-ols with high efficiency and selectivity. The reaction proceeds via conrotatory ring closure followed by a suprafacial 1,5-hydrogen shift leading to the observed single trans-fused products. Preliminary mechanistic studies reveal the feasibility of both 1,5-H shift and intersystem crossing of the diradical intermediate.