[Cl:1][C:2]1[C:10]2[CH:9]([CH2:11][C:12]([OH:14])=[O:13])[O:8][B:7]([OH:15])[C:6]=2[CH:5]=[C:4]([OH:16])[CH:3]=1.[OH-].[Na+].Cl[C:20]([O:22][CH2:23][CH3:24])=[O:21].Cl>O>[Cl:1][C:2]1[C:10]2[CH:9]([CH2:11][C:12]([OH:14])=[O:13])[O:8][B:7]([OH:15])[C:6]=2[CH:5]=[C:4]([O:16][C:20]([O:22][CH2:23][CH3:24])=[O:21])[CH:3]=1
To the suspension of the crude 2-(4-chloro-1,6-dihydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-3-yl)acetic acid (180 mg, 0.74 mmol) and NaOH (30 mg, 0.74 mmol) in 8 mL H2O was added ethyl chloroformate (96 mg, 0.89 mol) dropwise at 0° C. The reaction mixture was stirred at room temperature for 30 min and acidified to pH=5.0 using 1N HCl. The resulting mixture was extracted with EtOAc (3×10 mL) and the combined organic layers were dried over anhydrous Na2SO4 and concentrated in vacuo. The residue was purified by prep HPLC to give the title compound as an off-white powder (50 mg, yield: 21.5%, two steps). 1H NMR (400 MHz, DMSO) δ12.43 (s, 1H), 9.50 (s, 1H), 7.55 (d, J=2.4 Hz, 1H), 7.50 (d, J=2.4 Hz, 1H), 5.52 (m, 1H), 4.28 (m, 2H), 3.19 (m, 1H), 2.36 (m, 1H), 1.31 (t, J=7.0 Hz, 1H).
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