Partially hydrogenated 2-amino1,2,4triazolo1,5-apyrimidines as synthons for the preparation of polycondensed heterocycles: reaction with alpha-bromoketones

摘要

2-Aminosubstituted 4,5,6,7-tetrahydro- and 4,7-dihydro1,2,4triazolo1,5-apyrimidines heated with alpha-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 imidazo2',1':3,41,2,4triazolo1,5-apyrimidines by heating with alkali in ethanol solution. 6-Ethoxycarbonyl-4,7-dihydro1,2,4triazolo1,5-apyrimidines, which are resistant to oxidative aromatization, in analogous conditions with a-bromoketones produce bromides of 5,8-dihydro-1H-imidazo2',1':3,41,2,4triazolo1,5-apyrimidines in one step. The unusual reaction of acid catalyzed hydrolytic cleavage of the imidazole ring of imidazo2',1':3,41,2,4triazolo1,5-apyrimidines in the presence of hydrobromic acid on heating is revealed. Tautomerism of partially hydrogenated imidazo 2',1':3,411,2,4triazolo1,5-apyrimidines was studied by experimental and computational methods. Some of these polycyclic compounds represent new partially hydrogenated mesoionic heterocycles. (C) 2015 Elsevier Ltd. All rights reserved.