To a 100 mL round-bottomed flask equipped with a magnetic stirrer and a reflux condenser tube, 1,2-dihydro-3H-pyrazol-3-one (4.97 g, 59.11 mmol) and pyridine (25 mL, 309.1 mmol) were added sequentially. The reaction mixture was heated to 95 °C with continuous stirring. A solution consisting of acetic anhydride (5.6 mL, 59.35 mmol) dissolved in pyridine (10 mL, 123.6 mmol) was slowly added dropwise over 3 minutes. After the dropwise addition was completed, the reaction mixture continued to be stirred at 95 °C for 3 hours. Upon completion of the reaction, the solvent was removed by distillation under reduced pressure. The resulting solid residue was ground with 40 mL of ether, subsequently filtered and washed with ether and finally dried to afford 1-acetyl-1H-pyrazol-3(2H)-one (6.96 g, 93% yield). The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 10.96 (s, 1H), 8.13 (d, J = 3.0 Hz, 1H), 6.01 (d, J = 3.0 Hz, 1H), 2.48 (s, 3H).
