General procedure for the synthesis of 5-chloro-4-ethoxycarbonyl-1-methylpyrazole from ethyl 5-amino-1-methylpyrazole-4-carboxylate: cuprous chloride (CuCl, 1.76 g, 1.77 mmol) was added to a solution of tert-butyl nitrite (tBuONO, 2.95 mL, 2.46 mmol) in acetonitrile (ACN), followed by the addition of 5-amino-1 -methylpyrazole-4-carboxylic acid ethyl ester (2.5 g, 1.48 mmol). The reaction mixture was stirred at room temperature for 2 h and then heated at 60 °C for 1 h. The reaction was carried out by thin layer chromatography (TLC). The reaction process was monitored by thin layer chromatography (TLC). After completion of the reaction, the mixture was cooled to room temperature, diluted with 2N aqueous hydrochloric acid and extracted with hydrochloric acid (3×). The combined organic phases were washed with brine, dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated, and finally purified by fast column chromatography to afford the target product 5-chloro-4-ethoxycarbonyl-1-methylpyrazole (2.08 g, 75% yield). The structure of the product was confirmed by 1H-NMR (300 MHz, CDCl3) with chemical shifts δ: 7.90 (1H, s), 4.30 (2H, q, J = 5.4 Hz), 3.86 (3H, s), 1.34 (3H, t, J = 5.4 Hz).
