Heterocycles by cycloaddition. Part 10. Cycloaddition–extrusion–ring expansion of mesoionic compounds with benzocyclobutene. Approaches to seven- and eight-membered fully conjugated systems

Abstract

Several mesoionic compounds undergo dipolar cycloaddition with benzocyclobutene. The sydnone 12 directly gave the 2, 3-benzodiazepine 14 by cycloaddition–extrusion–ring expansion, while primary cycloadducts 10, 11 and 16 could be isolated from the thiazolium-4-olate 7, 1, 3-dithiolium4-olate 8, and thiazolium-5-olate 15. The 2-Methylthiothiazolium-5-olate 20 gave secondary products 21, 22 and 23. Pyrolysis of the thiazolium-5-olate adduct 16 gave the 3-benzazepine 17 by extrusion of SCO. The benzazocinone 35 was formed by oxidation of the sulfoxide 32 or the sulfenate 36, which were derived from the thiazolium-4-olate adduct 10.