Tetrasubstituted furans from novel [3 + 2] photocycloaddition of conjugated acetylenic α-diketones with alkenes

Photocycloaddition of ketones 1a–c with tetramethylethylene 2 furnishes tetrasubstituted furans 3a–c in isolated yields of ≈85%; a suggested mechanism for formation of 3a involves [3 + 2] cycloaddition by way of an initial biradical 5 to form an intermediate carbene 6, followed by cyclization to the furan.