Pyrolytic desulfurization ring contraction of condensed thiadiazines as a general route towards pyrazoloazines and pyrazoloazoles with a bridgehead (ring junction) nitrogen atom

Pyrolytic conversion of 1,2,4]triazino3,4-b]1,3,4]thiadiazin-4-ones, 1,3,4]thiadiazino2,3-b]quinazolin-10-ones and 1,2,4]triazolo3,4-b]1,3,4]thiadiazines into their corresponding pyrazolo5,1-c]1,2,4]triazin-4-ones, pyrazolo4,3-b]quinazolin-9-ones and pyrazolo5,1-b]1,2,4]triazoles via desulfurization ring contraction is described. The starting condensed 1,3,4-thiadiazines were prepared from the corresponding readily available 4-amino-3-thioxo-1,2,4-triazin-5(4H)-ones, 3-amino-2,3-dihydro-2-thioxoquinazolin-4(1H)-one and 4-amino-3(2H)-thioxo-1,2,4-triazoles upon reaction with the appropriate α-haloketones in two steps, or directly in one step in ethylpyridinium tetrafluoroborate (ionic liquid, IL).