Oxalyl chloride (5.68 mL, 64.9 mmol) was slowly added dropwise to a suspension of 1-ethyl-3-methyl-1H-pyrazole-5-carboxylic acid (5 g, 32.4 mmol) dissolved in dichloromethane (40 mL) at room temperature and two drops of N,N-dimethylformamide (DMF) were added as catalyst. The reaction mixture was stirred at room temperature for 2 h, subsequently concentrated by rotary evaporator and dried under vacuum. Anhydrous ethanol (50 mL, 856 mmol) was added to the resulting residue and the reaction mixture continued to be stirred at room temperature for 1 hour. Upon completion of the reaction, the reaction was again concentrated by rotary evaporator and dried under vacuum. The light yellow oil obtained was dissolved in ethyl acetate (100 mL), washed sequentially with saturated sodium bicarbonate solution and brine, and the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated. Finally, the residue was dried under vacuum to give ethyl 3-methyl-1-ethyl-1H-pyrazole-5-carboxylate (5.5 g, 30.2 mmol, 93% yield) as a light yellow oil. The product was characterized by 1H NMR (400 MHz, chloroform-d) and LCMS (LCMS method E): 1H NMR δ 6.63 (s, 1H), 4.56 (q, J = 7.11 Hz, 2H), 4.35 (q, J = 7.11 Hz, 2H), 2.30 (s, 3H), 1.44 (t, J = 7.28 Hz, 3H), 1.39 (t, J = 7.28 Hz, 3H); LCMS Rt = 0.81 min, [M + H]+ = 183.1.
