A 100-mL single-neck round-bottomed flask equipped with a magnetic stirrer and reflux condenser was charged with 28 (400 mg, 0.642 mmol), THF (5 mL) and ethanol (5 mL). A solution of lithium hydroxide (124 mg, 10.2 mmol) in water (5 mL) was added, and the mixture was heated at 60° C. for 48 h. After this time, the reaction was cooled to room temperature, and the mixture was slowly acidified to pH 4.0 with 10% aqueous citric acid. The resulting precipitate was filtered, and the filter cake was washed with water (20 mL), then triturated with a 1:10 mixture of methylene chloride/ethyl acetate (20 mL). The resulting residue was dried to a constant weight under vacuum at 45° C. to afford 29 in 55% yield (210 mg) as an off-white solid: mp 197-198° C.; 1H NMR (500 MHz, DMSO-d6) δ 9.91 (s, 1H), 9.29 (s, 1H), 7.99 (d, 2H, J=8.5 Hz), 7.95 (d, 2H, J=8.5 Hz), 7.55 (d, 2H, J=8.5 Hz), 7.39 (d, 2H, J=8.5 Hz), 7.35 (dd, 1H, J=7.0, 1.5 Hz), 7.32-7.28 (m, 2H), 7.24 (s, 1H), 4.23 (s, 1H), 3.56 (s, 3H), 3.30 (m, 1H), 2.30 (s, 3H), 2.23 (s, 3H), 1.33 (s, 9H), 1.13 (d, 3H, J=6.5 Hz), 1.10 (d, 3H, J=6.5 Hz); MS (ESI+) m/z 596 (M+H).
