Synthesis
3.0 g (8.85 mmol) of potassium phosphate heptahydrate, 1.50 g (5.90 mmol) of bis(boronic acid) pinacol ester B2(pin)2, 12 mg (0.015 mmol) of Xphos-Pd-G2, and 4 mg (0.008 mmol) of Xphos were added sequentially to a reaction vial with 6 mL of ethanol stirred to mix well, and then 0.36 mL (2.95 mmol) 5-methyl-1H-indazole-4-chloride was added and the reaction was carried out at room temperature for 1 hour. The reaction solution was diluted by adding 5mL of ethyl acetate, filtered through diatomaceous earth, washed with ethyl acetate, the filtrates were combined and concentrated under reduced pressure to obtain the crude product, which was separated by silica gel column chromatography, eluted with petroleum ether-ethyl acetate to obtain 5-methyl-1H-indazole-4-boronic acid pinacol ester. 5-Methyl-1H-indazole-4-boronic acid pinacol ester was added to the reaction flask and hydrolyzed dropwise with dilute HCl. The solution first produced a precipitate, with the precipitate gradually disappeared, adjust the system pH to 1. To the solution, 25% NaOH solution was added dropwise to pH 13, stirring for 1 h. The solution was partitioned, and the organic phase was extracted with 15 mL of 10% NaOH, the aqueous phases were combined, and the bases were extracted with 15 mL of THF for two times, respectively. The pH of the obtained base solution was adjusted with dilute HCl, turbidity was generated at the beginning, flocculent appeared slowly, and the pH was adjusted to 5.0. The aqueous phase was extracted with 70 mL of THF, and the organic phase was spin-dried and purified to obtain 5-methyl-1H-indazole-4-boronic acid.
