The general procedure for the synthesis of 1-THP-4-pyrazoleboronic acid pinacol ester from 3,4-dihydro-2H-pyran and 4-pyrazoleboronic acid pinacol ester was as follows: 3,4-dihydro-2H-pyran (5.6 g, 67 mmol) and trifluoroacetic acid (1.17 g, 10.3 mmol) were added to a 4-(4,4,5,5-tetramethyl-1,3,2-dioxoborono pentan-2-yl)-1H-pyrazole (10.0 g, 51.5 mmol) in a solution of toluene (200 mL). The reaction mixture was heated to 90 °C and kept for 2 hours. After completion of the reaction, it was cooled to room temperature and the reaction mixture was partitioned between ethyl acetate (200 mL) and saturated aqueous sodium bicarbonate solution (100 mL). The aqueous layer was further extracted with ethyl acetate (100 mL). The organic layers were combined, washed with saturated aqueous sodium chloride (100 mL), dried over anhydrous sodium sulfate, filtered, and concentrated in vacuo. Purification by silica gel column chromatography (elution gradient: 10% to 50% petroleum ether solution of ethyl acetate) afforded the target product 1-THP-4-pyrazoleboronic acid pinacol ester as a white solid. Yield: 13.4 g, 48.2 mmol, 94% yield.1H NMR (400 MHz, CDCl3) δ 7.94 (s, 1H), 7.83 (s, 1H), 5.41 (dd, J = 9.5, 2.5 Hz, 1H), 4.01-4.08 (m, 1H), 3.65-3.74 (m, 1H), 1.98- 2.18 (m, 3H), 1.60-1.76 (m, 3H), 1.32 (s, 12H).
