Synthesis of 2H-[1,2,3]thiadiazolo[5,4-b]indoles from Isatin-Derived Hydrazines

Significance

A new method for the synthesis of [1,2,3]thiadiazolo[5,4-b]indoles 5 involving oxidative cyclization of 3-hydrazono-2,3-dihydroindol­thiones 4 is reported. Treatment of 3-hydrazono-1,3-dihydroindol-2-ones 1 and 3-[(thiazol-2-yl)-hydrazono]-1,3-dihydroindole-2-ones 3a-c with Lawesson’s reagent afforded modest to excellent yields of products 4 (X-ray crystal structure obtained for cis-4, R = Me). Of the three methods studied for the oxidative cyclization leading to products 5, NCS/NBS, Br₂, and I₂, the former two proved to be most effective.