To a stirred solution of (2′-oxo-5,6-dihydrospiro[benzo[1,2-b:5,4-b′]difuran-3,3′-indol]-1′(2′H)-yl)acetic acid (0.25 g, 0.74 mmol), isobutylchloroformate (0.11 mL, 0.81 mmol), N-methylmorpholine (0.09 mL, 0.81 mmol) in dichloromethane (10 mL) was added 3,3-dimethylbutylamine (0.11 g, 1.1 mmol). The solution was stirred at ambient temperature for 16 h, diluted with dichloromethane (10 mL) and washed with 2 M sodium carbonate (2×25 mL). The organic layer was dried over magnesium sulfate, filtered and concentrated in vacuo to dryness. The residue was purified by flash chromatography with ethyl acetate in hexanes (30% to 50% gradient) to afford N-(3,3-dimethylbutyl)-2-(2′-oxo-5,6-dihydrospiro[benzo[1,2-b:5,4-b′]difuran-3,3′-indol]-1′(2′H)-yl)acetamide (0.13 g, 41%) as a colorless solid: mp 161-163° C.; 1H NMR (300 MHz, DMSO-d6) δ8.14 (dd, J=5.4, 5.4 Hz, 1H), 7.25 (dd, J=7.6, 7.6 Hz, 1H), 7.11 (d, J=6.9 Hz, 1H), 7.03-6.95 (m, 1H), 6.90 (d, J=7.8 Hz, 1H), 6.53 (s, 1H), 6.37 (s, 1H), 4.71 (ABq, 2H), 4.51-4.40 (m, 2H), 4.38-4.20 (m, 2H), 3.12-3.01 (m, 2H), 2.93 (t, J=8.6 Hz, 2H), 1.37-1.27 (m, 2H), 0.85 (s, 9H); 13C NMR (75 MHz, DMSO-d6) δ177.6, 166.3, 161.5, 160.9, 143.1, 132.7, 129.0, 123.8, 123.3, 121.3, 120.2, 119.8, 109.4, 92.8, 80.2, 72.5, 57.3, 43.2, 43.0, 35.9, 30.0, 29.7, 28.8; MS (ES+) m/z 421.2 (M+1).
