Partially hydrogenated 2-amino[1,2,4]triazolo[1,5-a]pyrimidines as synthons for the preparation of polycondensed heterocycles: reaction with α-bromoketones

2-Aminosubstituted 4,5,6,7-tetrahydro- and 4,7-dihydro[1,2,4]triazolo[1,5-a]pyrimidines heated with α-bromoketones in acetonitrile undergo selective quaternization at the N-3 atom of the triazolopyrimidine core with simultaneous oxidative aromatization of the dihydropyrimidine ring. The quaternized products can be cyclized into imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines by heating with alkali in ethanol solution. 6-Ethoxycarbonyl-4,7-dihydro[1,2,4]triazolo[1,5-a]pyrimidines, which are resistant to oxidative aromatization, in analogous conditions with α-bromoketones produce bromides of 5,8-dihydro-1H-imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines in one step. The unusual reaction of acid catalyzed hydrolytic cleavage of the imidazole ring of imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines in the presence of hydrobromic acid on heating is revealed. Tautomerism of partially hydrogenated imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines was studied by experimental and computational methods. Some of these polycyclic compounds represent new partially hydrogenated mesoionic heterocycles.

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