Synthesis, molecular structure, and reactions of 1H-1,2,4-triazolo[4,3-a] pyrimidinium betaines

Novel 1-substituted 1,2,4-triazolo[4,3-a]pyrimidinium betaines have been prepared and characterised. Treatment of 1-alkyl-1-(4,6-dimethylpyrimidin-2-yl) hydrazines (2f-j) with phosgene gave a series of 1,2,4-triazolo[4,3-a] pyrimidinium chlorides (3a-e) which were subsequently converted by ammonia into 1,2,4-triazolo[4,3-a]pyrimidinium-3-olates (5a-e). Analogous 3-thiolates (5g-k) were best synthesized by treating the appropriate hydrazines (2f-j) with carbon disulphide. The 1-(3-hydroxypropyl) derivatives (5f) and (5l) were obtained by deprotection of benzyl derivatives (5e) and (5k) using BCl₃ in dichloromethane at -10°C. Methylation of the 3-thiolate derivative (5g) with iodomethane gave the salt (9) which was subsequently transformed by methylamine into the triazole derivative (10). 1,5,7-Trimethyl-1H-1,2,4-triazolo[4,3-a] pyrimidinium-3-thiolate gave 5-amino-1-methyl-1H-1,2,4-triazole-3-thiol (11) on treatment with hydrazine hydrate. The molecular structure of 1-benzyl-5,7-dimethyl-1H-1,2,4-triazolo[4,3-a]pyrimidinium-3-olate (5b) has been determined by X-ray crystallography.