Photoaddition of Alkenes to Conjugated α-Diketones:  Tandem Cyclizations Leading to Tetrasubstituted Furans^1a

Photocycloaddition of 9a−c with 2 leads cleanly to tetrasubstituted furans 12a−c, respectively, in yields of ∼85%. The reactive triplet is efficiently sensitized by 2-benzoylnaphthalene and quenched by anthracene, indicating that E_T is in the range 43−58 kcal/mol. A mechanism is proposed involving an alkyl propargyl biradical (as 10) that closes first to a vinyl carbene (as 11) and then to product. Reaction of 9c with 20 furnishes only 22, and this result rules out an alternative mechanism in which the order of steps leading to the carbene is reversed.