A Rapid Synthesis of the 3-Methyl-5-(Methylsulfanyl)-1-Phenyl-1H-Pyrazolo[4,3-e]-[1,2,4]Triazine from Phenylhydrazone of 5-Acetyl-3-(Methylsulfanyl)-1,2,4-Triazine under Microwave Irradiation

The combination of microwave irradiation and solvent-free conditions leads to enhanced reaction rates, higher yields of pure products, easier work-up and several advantages of the eco-friendly approach in the framework of green-chemistry [1,2]. Here we report a new route for the preparation of 1H-pyrazolo[4,3-e][1,2,4]triazine derivative 2 starting from phenylhydrazone of 5-acetyl-3-(methylsulfanyl)-1,2,4-triazine (1) [3] and p-toluenesulfonic acid (p-TsOH) under microwave irradiation.

The mixture of the phenylhydrazone 1 (259 mg, 1mmol) and p-toluenesulfonic acid (344 mg, 2 mmol) was heated in quartz container under microwave irradiation [4]. After 2 min the product was eluted with CHCl₃. The crude product 2 was purified by column chromatography on silica gel (Merck, 230-400 mesh) using chloroform as eluent. Yield of pure 3-methyl-5-(methylsulfanyl)-1-phenyl-1H-pyrazolo[4,3-e][1,2,4]triazine (2) – 149 mg (0.58 mmol, 58%).

Melting Point: 105 ^oC.

¹H-NMR (200 MHz, CDCl₃): δ= 2.73 (s, 3H); 2.77 (s, 3H); 7.29-7.40 (m, 1H); 7.50-7.61 (m, 2H); 8.31-8.38 (m, 2H).

IR (KBr, cm^-1): 2920, 1590, 1500, 1390, 760.

MS (EI, 70eV; m/z, %): 257 (43) [M⁺]; 232 (3); 216 (22); 93 (41); 77 (100).

Elemental Analysis: Calculated for C₁₂H₁₁N₅S: C, 56.03%; H, 4.28%; N, 27.23%. Found: C, 55.67%; H, 4.13%; N, 27.05%.