Differentiation between [1,2,4]triazolo[1,5-a] pyrimidine and [1,2,4]triazolo[4,3-a]-pyrimidine regioisomers by 1H-15N HMBC experiments.

The condensation of malonoaldehyde derivatives with either a 3-amino-[1,2,4]-triazole or a 3,5-diamino-[1,2,4]-triazole precursor was studied. In agreement with previous reports, two different bicycles, namely, bearing the regioisomeric [1,2,4]triazolo[1,5-a]pyrimidine (1) or [1,2,4]triazolo[4,3-a]pyrimidine (2) structural surrogates, could be obtained. We found that, depending on the triazole precursor, only one regioisomer resulted, either of the 1 or 2 series. We also observed that these two structural surrogates could be unambiguously differentiated by indirectly measuring their (15)N chemical shifts by (1)H-(15)N HMBC experiments. The occasional conversion of [1,2,4]triazolo[4,3-a]pyrimidines to the [1,2,4]triazolo[1,5-a]pyrimidine counterparts could be unequivocally determined by (15)N NMR data.