Thiazolo[3,4-b]indazole-2,2-dioxides as Masked Extended Dipoles: Pericyclic Reactions of Benzodiazafulvenium Methides

Herein we report the first examples of 1,3-dipolar cycloadditions of thiazolidine-derived sydnones with benzyne leading to 1,3-dihydrothiazolo[3,4-b]indazoles. These heterocycles were converted into the corresponding sulfones which were used as precursors of novel benzo-2,3-diazafulvenium methides. These reactive intermediates reacted with N-substituted maleimides affording new 1H-indazoles characterized by an intense yellow color, a property that gives them a potential application as dyes. The synthesis of these heterocycles was rationalized considering the initial 1,3-cycloaddition of benzodiazafulvenium methides to maleimides. This chemical behavior is in contrast with the previously observed reactivity for 4,5-(methoxycarbonyl)diazafulvenium methides, which participate exclusively in [8 pi + 2 pi] cycloadditions to give 1,7-cycloadducts. Quantum chemical calculations, carried out at the DFT level of theory, were in agreement with the rationalization of the observed reactivity. Under flash vacuum pyrolysis or under microwave irradiation, 1-methyl- and 7,7-dimethylbenzo-2,3-diazafulvenium methides undergo sigmatropic [1,8]H shifts allowing the efficient synthesis of N-vinyl- and C-vinyl-2H-indazoles.