Reaction Steps
Lithium hydroxide monohydrate (1.48 g, 35.2 mmol) was added to a solution of methyl 2-[(2′-oxo-5,6-dihydrospiro[benzo[1,2-b:5,4-b′]difuran-3,3′-indol]-1′(2′H)-yl)methyl]benzoate (6.00 g, 14.2 mmol) in a mixed solvent (tetrahydrofuran/water=2/1 v/v, 180 mL), and stirred at ambient temperature for 16 h. Most of tetrahydrofuran was removed under vacuum, and 150 mL water was added. The solution was extracted with 50 mL of mixed solvent (ethyl acetate/hexanes:1/3 v/v). The water layer was acidified with 1 N HCl solution until pH 2. After filtration and air dry, 2-[(2′-oxo-5,6-dihydrospiro[benzo[1,2-b:5,4-b′]difuran-3,3′-indol]-1′(2′H)-yl)methyl]benzoic acid was obtained (5.60 g, 96%) as a colorless solid: 1H NMR (300 MHz, DMSO-d6) δ13.2 (br s, 1H), 7.95 (dd, J=7.7, 1.2 Hz, 1H), 7.51 (ddd, J=7.5, 7.5, 1.2 Hz, 1H), 7.37 (dd, J=7.5, 7.5 Hz, 1H), 7.24-7.14 (m, 2H), 7.12-6.96 (m, 2H), 6.81 (d, J=7.7 Hz, 1H), 6.58 (s, 1H), 6.40 (s, 1H), 5.39-5.18 (m, 2H), 4.86 (d, J=8.9 Hz, 1H), 4.74 (d, J=8.9 Hz, 1H), 4.47 (t, J=8.7 Hz, 2H), 3.95 (s, 3H), 2.95 (t, J=8.7 Hz, 2H); 13C NMR (75 MHz, DMSO-d6) δ 177.8, 168.7, 161.6, 161.2, 143.0, 137.6, 133.0, 132.6, 131.5, 129.8, 129.2, 127.7, 126.4, 124.2, 123.6, 120.9, 120.4, 119.7, 109.7, 92.9, 80.5, 72.6, 57.5, 42.5, 28.8; MS (ES+) m/z 414.0 (M+1).
