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 15N chemical shifts by 1H15N 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 15N NMR data. Copyright © 2010 John Wiley & Sons, Ltd. Regioisomeric [1,2,4]triazolo[1,5‐a] pyrimidine (1) and [1,2,4]triazolo[4,3‐a]pyrimidine derivatives (2), respec‐ tively obtained by condensation of malonoaldehydes with 3‐amino‐ [1,2,4]‐triazole or 3,5‐diamino‐[1,2,4]‐ triazole precursors, were unambiguously characterized by 1H15N HMBC experiments.