Modeling stages of domino reaction of thiopyrano[4,3-b]indole-3(5H)-thiones and dimethyl acetylenedicarboxylate: a new synthetic route to γ-carbolines with thione group

Modeling stages of the cascade reaction of thiopyrano[4,3-b]indole-3(5H)-thiones and dimethyl acetylenedicarboxylate were studied. Alkylation of thiones with alkyl halides or dimethyl sulfate affording to thiopyrano[4,3-b]indole-2-ium salts models the first stage of the process. The attack at the 3-position of the obtained salts by secondary amines can serve as a model for the second stage of the cascade transformation; it gives 3-benzoylindoles with thioamide-group. Reactions of thiopyrano[4,3-b]indole-2-ium salts with primary amines lead to a series of γ-carbolines with thione group. The action of ammonia on these salts causes dealkylation and gives thiopyrano[4,3-b]indole-3(5H)-thiones.