title: Thiazolo[3,4-b]indazole-2,2-dioxides as masked extended dipoles: pericyclic reactions of benzodiazafulvenium methides.
abstract: 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 [8π + 2π] 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.
authors.affiliation: Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
authors.forename: Maria I L
authors.initials: MI
authors.lastname: Soares
authors.forename: Cláudio M
authors.initials: CM
authors.lastname: Nunes
authors.forename: Clara S B
authors.initials: CS
authors.lastname: Gomes
authors.forename: Teresa M V D
authors.initials: TM
authors.lastname: Pinho e Melo