The synthesis of a series of unprecedented tricyclic heterocycles, 1,2,4-triazolo[3,2-d][1,5]benzothiazepine 6-oxides (5a-i), was accomplished via cycloaddition of the allene-like cations 3, derived from thiochroman-4-one arylhydrazones, to the triple bond of nitriles and ensuing ring expansion annulation. The structure of one product (5h) has been unambiguously determined by X-ray diffraction analysis.