Photoaddition Reactions of Benzoxazole‐2(3H)‐thiones to Cycloalkenes and Heteroaromatics

Abstract

Photoaddition reactions of benzoxazole-2-thiones 1 with cycloalkenes 2 and heteroaromatics 3 were examined. Irradiation of N-acylbenzoxazole-2-thiones 1a and 1b, and 3-(methoxycarbonyl)-and 3-(phenoxycarbonyl)benzoxazole-2(3H)-thiones (1f and 1h, resp.) in the presence of cycloalkenes 2a ± 2c, cyclohexa-1,4-diene (2d), and indene (2e) yielded 2-substituted benzoxazoles 4 ± 13, 18 and 19, and iminothietanes 14 ± 17, by intramolecular trapping of the acyl or MeOCO and PhOCO groups by thiolate anion of the zwitterionic intermediate I and by the phenolate anion of the zwitterionic intermediate II, respectively, derived from the spirocyclic amino-thietanes AT formed by [2 2] cycloaddition of the CS bond of 1 and CC bond of 2. Irradiation of 1 in the presence of heteroaromatics 3 gave the 2-substituted benzoxazoles 20 ± 33 exclusively. . To investigate the scope and limitation of the photocycloaddition of benzoxazole-2(3H)-thiones 1 and alkenes, we examined the photoreactions of 1 and cycloalkenes 2a ± 2c, cyclohexa-1,4-diene (2d) and indene (2e), and heteroaromatics 3, and our results are described in this paper (Schemes 1 and 2).

Reactivity of Benzofuran Derivatives

“…The photolytic reaction of benzoxazole-2-thione 143 with unsubstituted benzo[b]furan 1a was utilized to make 2-benzofuryl-benzoxazole 144 [53] (Scheme 48).…”