Reaktionsschritte
A mixture of methyl 5-[(2′-oxo-2,3-dihydrospiro[furo[2,3-g][1,4]benzodioxine-8,3′-indol]-1′(2′H)-yl)methyl]furan-2-carboxylate (1.38 g, 3.17 mmol) and lithium hydroxide (0.76 g, 31.7 mmol) in tetrahydrofuran (10 mL) and water (10 mL) was stirred at 60° C. for 2 h. The reaction mixture was acidified with 1 M hydrochloric acid (50 mL) and extracted with dichloromethane (3×30 mL). The combined organic solution was washed with brine (50 mL), dried over sodium sulfate, filtered and concentrated under reduced pressure to afford 5-[(2′-oxo-2,3-dihydrospiro[furo[2,3-g][1,4]benzodioxine-8,3′-indol]-1′(2′H)-yl)methyl]furan-2-carboxylic acid (1.36 g, quantitative) as a colorless solid: mp 192-195° C. (diethyl ether); 1H NMR (300 MHz, CDCl3) δ8.40 (br s, 1H), 7.27 (dd, J=7.8, 7.8 Hz, 1H), 7.25 (d, J=3.3 Hz, 1H), 7.18 (d, J=7.5 Hz, 1H), 7.06 (dd, J=7.5, 7.5 Hz, 1H), 6.94 (d, J=7.8 Hz, 1H), 6.50 (s, 1H), 6.45 (d, J=3.3 Hz, 1H), 6.31 (s, 1H), 5.19 (d, J=16.5 Hz, 1H), 4.92 (d, J=9.3 Hz, 1H), 4.87 (d, J=16.5 Hz, 1H), 4.66 (d, J=9.3 Hz, 1H), 4.21-4.07 (m, 4H); 13C NMR (75 MHz, CDCl3) δ177.4, 162.8, 155.2, 154.4, 144.8, 143.7, 141.3, 138.6, 132.3, 129.1, 124.2, 124.0, 121.1, 120.9, 111.8, 110.7, 109.0, 99.5, 80.0, 64.7, 64.0, 58.2, 37.5; MS (ES+) m/z 419.9 (M+1).
