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.