Photoaddition of Alkenes to Conjugated α-Diketones:  Tandem Cyclizations Leading to Tetrasubstituted Furans<sup>1a</sup>

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