Step 1: 5-Chloro-1H-pyrrolo[2,3-b]pyridine (3 g, 0.02 mol) was dissolved in dichloromethane (50 ml) and triethylamine (0.20 mol) and tert-butyldimethylchlorosilane (0.2 mol) were added sequentially under stirring. The reaction mixture was stirred overnight at room temperature and the reaction was monitored by thin layer chromatography (TLC) until completion. Upon completion of the reaction, it was quenched by adding 50 ml of water and 10 ml of 0.1 mol/L dilute hydrochloric acid. The organic phase was separated and washed sequentially with 50 ml of water, 50 ml of saturated sodium carbonate solution and 50 ml of saturated sodium chloride solution. After evaporating the solvent of dichloromethane phase, the residue was washed with acetone three times and dried to obtain a yellow solid.
Step 2: The yellow solid obtained in step 1 was dissolved in 50 ml of tetrahydrofuran (THF), keeping the temperature of the reaction system at -5°C. Sodium carbonate (0.53 g, 0.005 mol) was added, then trimethyl borate (4.6 ml, 0.04 mol) was added slowly dropwise. After addition, the reaction mixture was warmed up to room temperature and the solvent was evaporated under reduced pressure to obtain a yellow oily substance.
Step 3: To the yellow oily substance obtained in step 2, 80 ml of ethyl acetate and pinacol (0.02 mol) were added and the reaction was refluxed. The reaction was monitored by TLC until completion. After evaporation of the solvent, the residue was dissolved in 100 ml of acetone, stirred and filtered. The filtrate was concentrated to dryness to give a colorless oil.
Step 4: The colorless oily material obtained in step 3 was dissolved in 50 ml of ether, 30 ml of 0.1 mol/L dilute hydrochloric acid was added and stirred overnight. After evaporating the solvent, the residue was extracted with 30 ml of ethyl acetate. The organic phase was dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give the crude product. The crude product was further purified by 1:1 (v/v) chloroform-crystallization (20 ml) to give a white solid. The final product was 7-azaindole-5-boronic acid pinacol ester (2.78 g, HPLC purity 99.8%) in 56.8% overall yield.
