Photocycloaddition of 2-oxopyran-3-carbonitriles to 2,3-dimethylbut-2-ene

On acetone-sensitized irradiation in the presence of 2,3-dimethylbut-2-ene, alpha-cyano-alpha,beta-unsaturated delta-lactone 5 is converted both to cy clopentapyrans resulting from stepwise addition of the alkene to the olefin ic C(beta)- and the nitrile C-atom of triplet excited 5, and cyclobutapyran es, i.e., [2 + 2] cycloadducts. Similarly, direct irradiation of 1-benzopyr an-3-carbonitrile 6 in the presence of the same alkene affords cyclobutaben zopyran 13 and cyclopentabenzopyran 14, the latter resulting from an upper excited triplet state of 6.