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

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(3 H )‐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.