Photoaddition of benzoxazole-2-thiones with alkenes

Abstract

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-ω-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 CS bond of benzoxazole-2-thiones and the CC bond of alkenes.