Base catalyzed rearrangement of π-conjugated sulfur and selenium bridged propargylic systems

A series of novel π-conjugated bis-propargylic sulfides, selenides, sulfoxides and sulfones have been prepared. In the presence of amine bases these compounds underwent facile isomerization to the corresponding allenes, followed by a tandem cyclization and aromatization of the latter via a diradical intermediate. While the reactions of sulfones and sulfoxides were almost spontaneous and proceeded in practically quantitative yield, the sulfides and selenides exhibited modest reactivity and required more polar solvents to afford the corresponding thiophenes and selenophenes. Some mechanistic studies and DNA cleaving properties are also presented.