A novel route for the synthesis of benzo[b]thiepins

3,4 - Di(methoxycarbonyl) - 5 - pyrrolidinobenzob]thiepin 5 is synthesized by a 2+2]cycloaddition of 3 - pyrrolidinobenzob]thiephene 3 to dimethyl acetylenedicarboxylate, with subsequent ring opening of the cyclobutene moiety in the intermediate 6,7-di(methoxycarbonyl)-5-pyrrolidino - 2 - thiabenzob]bicyclo3.2.0]hepta - 3,6 - diene 4 (detectable by PMR spectroscopy at ?30°C). Hydrolysis of the pyrrolidino group in 5 affords 3,4 - di(methoxycarbonyl) - 5 - hydroxybenzob]thiepin 8. Peracid oxidation of 5 yields the corresponding 1,1-dioxide. The benzob]thiepins are thermally unstable and (depending on the substituents) either extrude sulphur or rearrange to a 4-mercapto-1-naphthol, probably via the thianorcaradiene as the intermediate. The kinetics of the latter reaction are discussed. On being irradiated, the benzob]thiepins isomerize to 2-thiabenzob]bicyclo3.2.0]hepta-3,6-dienes.