The general procedure for the synthesis of ethyl 5-keto-4,5-dihydro-1H-pyrazole-3-carboxylate from the sodium salt of diethyl oxaloacetate was as follows: acetic acid (150 mL) was slowly added dropwise to a solution of 1,4-diethoxy-1,4-dioxobut-2-en-2-ol (30.0 g, 0.143 mol) in toluene (150 mL). The reaction mixture was stirred at room temperature for 30 minutes before 85% hydrazine hydrochloride (17 g, 0.29 mol) was added. Stirring was continued at room temperature for 30 minutes, after which the reaction system was heated to 100 °C and kept overnight. Upon completion of the reaction, the mixture was concentrated under reduced pressure and extracted with ethyl acetate (500 mL). The organic phase was washed sequentially with saturated aqueous sodium bicarbonate (200 mL) and saturated aqueous sodium chloride (200 mL), dried over anhydrous sodium sulfate and filtered, and concentrated under reduced pressure to give a yellow solid product. The yield was 17 g (0.11 mol) with 77% yield. The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 12.75 (br s, 1H), 5.91 (br s, 1H), 4.24 (q, J = 7 Hz, 2H), 1.27 (t, J = 7 Hz, 3H).
