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A solution of (3S)-(1,6-dihydroxy-4-methyl-1,3-dihydro-benzo[c][1,2]oxaborol-3-yl)-acetic acid ethyl ester (1.04 g, 4.16 mmol) in THF (5 mL) was treated with lithium hydroxide (0.478 g, 2.08 mmol) in water (5 mL) at 0° C. The solution was stirred at 0° C. for 1 hour then quenched with 2N HCl to pH 2 and concentrated to approximately half of the entire volume. The precipitated solid was collected by vacuum filtration, then rinsed with cold water and dried to give (1,6-dihydroxy-4-methyl-1,3-dihydro-benzo[c][1,2]oxaborol-3-yl)-acetic acid. The filtrate was neutralized with NaHCO3 and lyophilized. The residue was suspended in water (5-10 mL) and stirred for 10 minutes. The solid was collected by vacuum filtration to give a second portion of (1,6-dihydroxy-4-methyl-1,3-dihydro-benzo[c][1,2]oxaborol-3-yl)-acetic acid as a yellow solid (total product; 0.49 g, 53%). 1H NMR (400 MHz, DMSO-d6) δ 12.31 (s, 1H), 9.30 (s, 1H), 9.08 (s, 1H), 6.88 (s, 1H), 6.66 (s, 1H), 5.38 (d, J=9.38 Hz, 1H), 3.00 (d, J=15.24 Hz, 1H), 2.20 (s, 3H), 2.00 (dd, J=15.24, 10.16 Hz, 1H). MS (ESI) m/z: 221 [M−1].

1. customthen quenched with 2N HCl to pH 2
2. concentrationconcentrated to approximately half of the entire volume
3. filtrationThe precipitated solid was collected by vacuum filtration
4. washrinsed with cold water
5. customdried