The photochemical reactions of alkyl and aryl 2-thioxo-3H-benzoxazole-3-carboxylates 1 have been examined. Irradiation of 1 in the presence of tetra-and trisubstituted alkenes 2a and 2b, 2-methylprop-2-ene nitrile 2e, and dienes 2f and 2g gave [2 2] cycloadducts of the CS bond of 2-thioxobenzoxazoles and the CC bond of alkenes, spiro [benzoxazole-thietanes] 3, 4, 8 ± 13, 15, 18, 20, 23 ± 26 in moderate-to-good yields. The photoaddition reactions proceed in a regiospecific manner. The spirocyclic compounds obtained are indefinitely stable at room temperature. Irradiation of 1a in the presence of 1,1-and 1,2-disubstituted alkenes 2c and 2d yielded the products 5 ± 7 of oxazole-ring cleavage. Compound 1d also underwent photoaddition with alkenes to yield spiro[benzoxazole-thietanes] and/or 2-substituted benzoxazoles and/or iminothietanes, depending on the nature of the substituents present in the alkenes. On intramolecular [2 2] photoadduct, tetracyclic 27, was obtained, when ethenyl 2-thioxobenzoxazole-3-carboxylate 1e was irradiated.1. Introduction. ± The photochemistry of thiocarbonyl compounds, including thioimides and thioamides, has been of considerable synthetic and mechanistic interest [1 ± 4], and these compounds behave like carbonyl compounds in many respects. In the course of our studies on the photochemistry of cyclic conjugated nitrogen-thiocarbonyl systems [1i ± k] [2] [3], we have reported the photoaddition reactions of the benzoxazole-2-thiones [2] and benzothiazole-2-thiones [3] with alkenes. In these reactions, [2 2] cycloadducts, aminothietanes are believed to be primary photoproducts. However, they are usually unstable, probably because the N-atom lone-pairelectron-assisted cleavage of either the CÀS bond of the thietane ring or the CÀO bond of the oxazole ring facilitates the formation of zwitterions. In contrast, thietanes have been isolated from photochemical cycloaddition of alkenes and thioimides, which have a cross-conjugated CO system [1a] [1h] [1j] [4]. This has been explained in terms of difference between the p-electron-donating effects of N-atoms of thioamides and those of thioimides. We are interested in exploring the effect of the substituents at the N-atom of thioamides upon photocycloaddition of thioamides and alkenes. Here 1 ), we describe the results of photochemical reactions of alkyl and aryl 2-thioxobenzoxazole-3-carboxylates 1, substituted with an electron-withdrawing group at the N-atom, with alkenes 2, yielding the stable isolable spirocyclic aminothietanes.
