Reaction of 1-substituted 3,5-diamino-1,2,4-triazoles with β-keto esters: synthesis and new rearrangement of mesoionic 3-amino-2H-[1,2,4]triazolo-[4,3-a]pyrimidin-5-ones

Reaction of 3,5-diamino-1-R-1,2,4-triazoles (R=Ph, Bn) with β-keto esters results in the reversible formation of N-(5-amino-1-R-1,2,4-triazol-3-yl)-substituted enaminoesters (8). Subsequent transformations depended on the reaction conditions. Compounds 8 undergo intermolecular reactions of condensation and amidation in the absence of solvent. However, in the presence of acetic acid they form 3-amino-5-oxo-2-R-2,5-dihydro-[1,2,4]triazolo[4,3-a]pyrimidin-4-ium-8-ides (10) followed by rearrangement to 3-amino-1-R-[1,2,4]triazolo[4,3-a]pyrimidin-5-ones (11). The transformation of 10 into 11 represents a new type of rearrangement in the azolopyrimidine series. Heating of enaminoesters 8 in ethanol with sodium ethoxide present, proved to be a suitable method for the preparation of the mesoionic compounds 10.