Abstract Condensed selenophenes on the one hand, and isothiazoles or isoselenazoles on the other hand, show a ring opening by organolithium compounds. We studied here the competition of ring opening in two isomers of the bicyclic unsubstituted system, selenoloisothiazole. Synthesis of these unknown isomers, namely selenolo [3, 2-d] and [2, 3- d] isothiazoles, and of the corresponding selenolo [2, 3-d] isoselenazole, is described.
