A solution of structural ambiguity: s‐Triazolo[1,5‐a] – and s‐triazolo[4.3‐a] pyrimidines

The condensation of 3‐amino‐5‐benzylthio‐s‐triazole (2) with acetylacetone in refluxing acetic acid has been reported to have given 3‐benzylthio‐5,7‐dimethyl‐s‐triazolo[4,3‐a]pyrimidine (3). However, it has now been established, with the aid of 13C spectra and a modification of the original synthetic work, that only 2‐benzylthio‐5,7‐dimethyl‐s‐triazolo[1,5‐a]pyrimidine (4) can be obtained by this method of condensation. The erroneously reported, but previously unknown 6 was synthesized and its structure and that of 4 was firmly established by ir, uv, pmr,13 C nmr, tlc and mixed melting point data. The correct structures of 3‐mercapto‐5,7‐dimethyl‐s‐triazolo‐[4,3‐a]pyrimidine (5) and 2‐mercapto‐5,7‐dimethyl‐s‐triazolo[1,5‐a]pyrimidine (6) were also established and the facile rearrangement of 5 to 6 was demonstrated.