General procedure for the synthesis of 1-(methoxymethyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole from 4-pyrazole boronic acid pinacol ester and iodomethyl methyl ether: 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole (10.00 g, 51.5 mmol) was dissolved in acetonitrile (40 mL) and stirred under nitrogen protection at 35 °C for 5 min, followed by cooling to 0 °C. At room temperature, iodomethyl methyl ether (21.83 mL, 258 mmol) and potassium carbonate (35.6 g, 258 mmol) were added sequentially, and the reaction mixture was stirred at 35 °C for 3 h and then cooled to room temperature. The insoluble material was removed by filtration and the filtrate was concentrated under vacuum. The residue was dissolved in ethyl acetate, the organic phase was washed with water, dried over anhydrous magnesium sulfate and concentrated under vacuum. The residue was purified by silica gel column chromatography (100 g silica gel, elution gradient: 0 to 50% hexane solution of ethyl acetate) to afford 1-(methoxymethyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole (6.45 g, 50% purity by LCMS, 26% yield) as a yellow liquid, which was used directly in the next step of the reaction.LCMS (Method G): retention time 0.85 min, [M+H]+=238.8.
