Boron trifluoride diethyl ether compound (0.31 g, 2.21 mmol) was added as m-bromobenzoic acid (0.2 g, 1.64 mmol) and 2-(tert-butoxy)pyridine (0.33 g, 2.21 mmol) after dissolution in anhydrous toluene (2 mL). The reaction mixture was stirred at room temperature for 30 min, followed by quenching the reaction with anhydrous sodium bicarbonate. The reaction mixture was diluted with ethyl acetate (30 mL) and washed sequentially with water (20 mL) and brine (20 mL). The organic layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure. The resulting residue was purified by fast column chromatography on silica gel with dichloromethane/hexane (0:4 to 1:4) as eluent to afford the target product tert-butyl m-bromobenzoate (5a) as a colorless oil.
