K. Mukherjee, Ashis Margaretha, Paul Agosta, William C. 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. 9 c;Cyclization;Diketone;E T;Photocycloaddition;Alkene;alternative mechanism;Tetrasubstituted Furans 1;reactive triplet;vinyl carbene;alkyl propargyl biradical;Photoaddition;quenched;anthracene;result rules;yield;kcal;tetrasubstituted furans 12;Conjugated;benzoylnaphthalene;Tandem 1996-05-17
    https://acs.figshare.com/articles/journal_contribution/Photoaddition_of_Alkenes_to_Conjugated_-Diketones_Tandem_Cyclizations_Leading_to_Tetrasubstituted_Furans_sup_1a_sup_/3701679
10.1021/jo952079+.s001