To a solution of methyl 3-[(2′-oxo-5,6-dihydrospiro[benzo[1,2-b:5,4-b′]difuran-3,3′-indol]-1′(2′H)-yl)methyl]benzoate (2.80 g, 6.5 mmol) in tetrahydrofuran (30 mL) and water (10 mL) was added lithium hydroxide monohydrate (0.82 g, 19 mmol). The reaction mixture was stirred at ambient temperature for 16 h. Most of the tetrahydrofuran was removed in vacuo and the resultant solution was washed with diethyl ether (2×50 mL). The aqueous phase was rendered acidic to litmus by the addition of 1 M hydrochloric acid and extracted with ethyl acetate (3×50 mL). The combined organic solution was dried over anhydrous sodium sulfate, filtered and concentrated in vacuo. The residue was triturated with diethyl ether (50 mL) and the solid was collected by vacuum filtration, washed with diethyl ether (20 mL), air-dried and dried under high vacuum to afford 3-[(2′-oxo-5,6-dihydrospiro[benzo[1,2-b:5,4-b′]difuran-3,3′-indol]-1′(2′H)-yl)methyl]benzoic acid (2.20 g, 81%) as a colorless solid: mp>250° C. (water); 1H NMR (300 MHz, DMSO-d6) 13.06 (br s, 1H), 7.89-7.82 (m, 2H), 7.66 (d, J=7.6 Hz, 1H), 7.55-7.47 (m, 1H), 7.29-7.16 (m, 2H), 7.07-6.96 (m, 2H), 6.49 (s, 1H), 6.42 (s, 1H), 5.17-4.72 (m, 4H), 4.55-4.45 (m, 2H), 3.06-2.88 (m, 2H); 13C NMR (75 MHz, DMSO-d6) δ177.2, 167.1, 161.2, 160.7, 142.0, 136.9, 132.2, 131.8, 131.2, 129.1, 128.8, 128.4, 127.3, 123.8, 123.2, 120.5, 120.0, 118.9, 109.3, 92.5, 79.7, 72.1, 57.0, 42.5; MS (ES+) m/z 414.0 (M+1).
