4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole (300 mg, 1.55 mmol) was dissolved in N,N-dimethylformamide (DMF, 5 mL) at 0 °C and under argon protection. Sodium hydride (60% oil solution, 40.8 mg, 1.70 mmol) was added in batches, followed by stirring the reaction mixture for 20 min at room temperature. A DMF (1 mL) solution of bromocyclobutane (209 mg, 1.55 mmol) was added and stirring was continued for 2 hours at room temperature. Upon completion of the reaction, the reaction was quenched with saturated aqueous ammonium chloride solution and then extracted three times with ethyl acetate (EtOAc). The organic phases were combined, washed with brine, dried over anhydrous sodium sulfate, filtered and concentrated. Purification by silica gel column chromatography afforded the target product N-cyclobutyl-pyrazole-4-boronic acid pinacol ester (20A, 40 mg, 5.21% yield). Mass spectrum (ESI): m/z 249.0 ([M+H]+).
