To the solution of 2-(5-amino-2-(N-(1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-6-yl)sulfamoyl)phenyl)acetic acid (2.2 g, 6 mmol) in dry THF (20 ml) under nitrogen, added the solution of tert-butyl 2,2,2-trichloroacetate (4.3 ml, 24 mmol) in dry DCM (10 ml), followed by BF3Et2O (0.1 ml, 0.72 mmol). The reaction was stirred at rt overnight. Quenched the reaction with sodium bicarbonate (sat. in water), diluted with EtOAc, washed with 1N HCl, water and brine, dried over anhydrous sodium sulfate. Concentrated, the result was purified by flash chromatography with EtOAc and hexane. Got 650 mg light yellow solid as product. MS calcd for (C19H23BN2O6S): 418.14. MS found (ESI negative): (M−H)−=417.0. 1H NMR (400 MHz, DMSO-d6) δ (ppm): 9.83 (s, 1H), 7.46 (d, J=8.8 Hz, 1H), 7.40 (d, J=1.6 Hz, 1H), 7.19 (d, J=8 Hz, 1H), 7.09 (dd, J=8 Hz, 1H), 6.39 (d, J=6.8 Hz, 1H), 6.37 (s, 1H), 4.84 (s, 2H), 3.77 (s, 2H), 1.37 (s, 9H).
