Vinyl carbonate (2.496 g, 28.3 mmol) and pinacol ester of 4-pyrazoleboronic acid (5 g, 25.8 mmol) were added with sodium hydroxide (0.103 g, 2.58 mmol) and N,N-dimethylformamide (DMF, 18 mL). The reaction mixture was stirred at 140°C for 16 h. Subsequently, it was cooled to room temperature and activated carbon (200 mg) was added. The mixture was continued to be stirred at room temperature for 1 h, followed by filtration. The insoluble material was washed with ethyl acetate (EtOAc, 50 mL) and ethanol (EtOH, 50 mL). The filtrate and washings were combined and concentrated under reduced pressure. The residue was initially purified by silica gel column chromatography (silica gel 50 g, elution gradient: 0 to 40% methanol/dichloromethane). Further purification by SP4 system using silica gel column (100 g, elution gradient: 0 to 20% methanol in dichloromethane solution) afforded the target product 1-(hydroxyethyl)pyrazole-4-boronic acid pinacol ester (5.58 g, 23.44 mmol, 91% yield) as a colorless oil, which was used directly in the subsequent reaction.LCMS (Method A): retention time 0.68 min, [ M + H]+ = 239.13.
