Step 3: To a pre-oven-dried vial at room temperature was added a lithium isopropylmagnesium chloride solution (1.0 M, dissolved in THF, 6.32 mL, 8.22 mmol). Subsequently, 4-bromo-1-(1-ethoxyethyl)-1H-pyrazole (1.00 g, 4.56 mmol) was slowly added dropwise to this solution. The reaction mixture was stirred continuously at room temperature for 16 hours. Upon completion of the reaction, the mixture was cooled to -20 °C and 2-methoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.73 g, 10.95 mmol) was added via syringe. Subsequently, the reaction mixture was slowly warmed up to room temperature. After continued stirring at room temperature for 2 h, the reaction was quenched by the addition of saturated aqueous ammonium chloride solution (15 mL), at which time the formation of a white precipitate was observed. The mixture was diluted with additional water (20 mL) and extracted with hexane (140 mL each time, 2 times). The organic phases were combined and washed sequentially with saturated aqueous sodium bicarbonate and brine, then dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated to give 1.20 g (99% yield) of the target product 1-(1-ethoxyethyl)-4-pyrazoleboronic acid pinacol ester as a colorless oil.
