[O:1]=[C:2]1[C:10]2([C:14]3=[CH:15][C:16]4[O:20][CH2:19][O:18][C:17]=4[CH:21]=[C:13]3[O:12][CH2:11]2)[C:9]2[C:4](=[CH:5][CH:6]=[CH:7][CH:8]=2)[N:3]1[CH2:22][C:23]([O:25]CC)=[O:24].O.[OH-].[Li+].Cl>C1COCC1.O>[O:1]=[C:2]1[C:10]2([C:14]3=[CH:15][C:16]4[O:20][CH2:19][O:18][C:17]=4[CH:21]=[C:13]3[O:12][CH2:11]2)[C:9]2[C:4](=[CH:5][CH:6]=[CH:7][CH:8]=2)[N:3]1[CH2:22][C:23]([OH:25])=[O:24]
To a suspension of ethyl (2′-oxospiro[furo[2,3-f][1,3]benzodioxole-7,3′-indol]-1′(2′H)-yl)acetate (10.5 g, 24.5 mmol) in THF (200 mL) and water (100 mL) was added lithium hydroxide monohydrate (3.98 g, 95.0 mmol) at 0° C. The reaction mixture was stirred at 0° C. for 30 min and at ambient temperature for 17 h. The mixture was neutralized with of 4 M HCl (15.0 mL). The residue obtained upon removing the solvent was acidified by the addition of 4 M HCl (6.2 mL) to pH 3. The solid was filtered, washed with water and hexane, and dried under the reduced pressure to give the title compound (8.48 g, 87%) as a white solid: 1H NMR (300 MHz, CDCl3) δ 7.32-7.06 (m, 4H), 6.79 (d, 1H), 6.49 (s, 1H), 6.23 (s, 1H), 5.84 (m, 2H), 4.92 (m, 1H), 4.69-4.63 (m, 2H), 4.45 (m, 1H); 13C NMR (75 MHz, CDCl3) δ 177.8, 171.8, 155.8, 149.0, 142.4, 141.2, 132.0, 129.0, 124.1, 124.0, 119.2, 108.3, 103.4, 101.5, 93.5, 80.2, 58.2, 41.1; MS (ES−) m/z 338.2 (M−1).
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