Rhodium-Catalyzed Regioselective Synthesis of Isocoumarins through Benzothiadiazine-Fused Frameworks

An unprecedented two-step, one-pot synthesis of benzimidazothiadiazine 5,5-dioxides is presented. Reaction condition based regioselectivity has been achieved where fused benzimidazo­[1,2-b]­[1,2,4]­thiadiazines are exclusively formed under thermal conditions, whereas benzimidazo­[2,1-c]­[1,2,4]­thiadiazines were created only under microwave irradiation. The salient features of this protocol include a regioselective sulfonylation of 2-aminobenzimidazole with o-halo sulfonyl chlorides followed by N–C bond formation. The acid forms of these fused regioisomers have been used to introduce novel guanidine-containing isocoumarin frameworks.