Diazafulvenium Methides in Synthesis of Functionalized Pyrazoles

Significance

New chemistry of diazafulvenium methide species, generated by pyrolytic or solution-phase SO₂-extrusion from 2,2-dioxo-1H,3H-pyrazolo[1,5-c][1,3]thiazoles, is reported. Two types of reactions, one involving an [8+2] cyclo­addition with different dipolarophiles and the other, a sigmatropic [1,8]-H shift lead to a range of pyrazolo-annulated heterocycles and simple N-vinyl-1H-pyrazoles and C-vinyl-1H-pyrazoles, respectively. The starting pyrazolothiazoles are read­ily prepared from thiazolidine precursors, albeit in low overall yields. The yields of the cycloaddition reactions range from moderate to excellent, while the pyrazole products resulting from hydrogen shifts are obtained in good to excellent yields.