Photoaddition of benzoxazole-2-thiones with alkenes

The photochemical reactions of the benzoxazole-2-thiones 6 have been examined. Irradiation of 3-unsubstituted benzoxazole-2-thione 6a in the presence of alkenes 2 gave 2-alkylated benzoxazoles 7,9-15 and 8 (in the case of 2a). Irradiation of 3-substituted benzoxazole-2-thiones 6b-e and acrylonitrile 2f yielded the 2-alkylidenebenzoxazoles 17, 22 and 24-25. Irradiation of 6b-e in the presence of 1,1-di- and tetra-substituted alkenes 2a,c and e gave the amide derivatives 18-20, 23 and 26. The 3-(alk-omega-enyl)-benzoxazole-2-thiones 6g-i gave the lactam derivatives 28g-i upon irradiation. The formation of these photoproducts can be explained in terms of the intermediacy of amino spiro-thietanes, which are derived by [2 + 2] photocycloaddition of the C=S bond of benzoxazole-2-thiones and the C=C bond of alkenes.