[Synthesis]
A mixed solution of acetophenone (100 g) with diethyl oxalate (123 g) was added slowly and dropwise to a mixed system of potassium tert-butoxide (132 g) and tetrahydrofuran (600 mL), and the reaction was stirred at room temperature for 15 hours. Subsequently, acetic acid (119 mL) and hydrazine monohydrate (45.8 g) were added to the reaction system and stirred at reflux for 2 hours. After completion of the reaction, the reaction solution was cooled to room temperature and concentrated under reduced pressure. The concentrated residue was dissolved in a solvent mixture of water and ethyl acetate for partition extraction. The ethyl acetate layer was separated, washed sequentially with saturated aqueous sodium carbonate and saturated saline, dried over anhydrous magnesium sulfate and concentrated. The resulting crude product was recrystallized by diisopropyl ether to give ethyl 3-phenyl-1H-pyrazole-5-carboxylate (145 g, yield: brown solid). The product was characterized by 1H-NMR (300 MHz, CDCl3): δ 1.36 (3H, t, J = 7.2 Hz), 4.36 (2H, q, J = 7.2 Hz), 7.10 (1H, s), 7.30-7.46 (3H, m), 7.71-7.80 (2H, m), 11.86 (1H, br s). |