Visible Light-Induced [2+2+1] Cycloaddition of 1,6-Enones with α-Bromocarbonyls

A visible light-induced dehydrobrominative [2+2+1] cycloaddition of 1,6-enones with α-carbonyl alkyl bromides as the one-carbon synthon is accomplished, providing facile and direct access to <i>cis</i>-hexahydro-2<i>H</i>-cyclopenta[<i>b</i>]furans in promising yields with excellent diastereoselectivity. Mechanistic studies indicate a radical cascade featuring the radical addition to C═O bonds, polarity-mismatched 1,5-hydrogen atom transfer of alkoxy radicals, bromination, and an uncommon S_N2 cyclization at the quaternary carbon center, thus significantly extending the boundary of the existing [2+2+1] cycloaddition methodology.