A solution of 1′-(diphenylmethyl)-6,7-dihydro-5H-spiro[furo[3,2-g]chromene-3,3′-indol]-2′(1′H)-one (0.61 g, 1.32 mmol) and triethylsilane (0.67 mL, 4.2 mmol) in trifluoroacetic acid (4.5 mL) was stirred at reflux under nitrogen for 2.5 h. Once cooled, the reaction was diluted with water (40 mL) and extracted with ethyl acetate (3×25 mL). The combined organic solution was washed with brine (50 mL), dried (sodium sulfate), filtered and concentrated under reduced pressure. The residue was dissolved into 10% diethyl ether in dichloromethane (5 mL) and, after standing at ambient temperature, yielded a colorless precipitate. This material was collected by filtration and washed with hexanes. Drying in air provided 6,7-dihydro-5H-spiro[furo[3,2-g]chromene-3,3′-indol]-2′(1′H)-one (0.25 g, 63%) as a colorless solid. An additional portion (0.14 g, 37%) of product was also recovered by concentrating the filtrate and triturating the residual solid with hexanes: mp 216-220° C. (hexanes); 1H NMR (300 MHz, DMSO-d6) δ10.58 (s, 1H), 7.23 (ddd, J=7.8, 7.5, 0.9 Hz, 1H), 7.08 (d, J=7.2 Hz, 1H), 6.98-6.91 (m, 2H), 6.36 (s, 1H), 6.31 (s, 1H), 4.74 (d, J=9.0 Hz, 1H), 4.61 (d, J=9.0 Hz, 1H), 4.05 (dd, J=5.1, 4.8 Hz, 2H), 2.61-2.45 (m, 2H), 1.84-1.77 (m, 2H); 13C NMR (75 MHz, DMSO-d6) δ178.6, 159.5, 155.4, 141.8, 132.9, 128.6, 123.8, 123.6, 122.3, 121.4, 114.8, 109.7, 97.9, 79.5, 66.0, 57.3, 23.9, 21.7; MS (ES+) m/z 294.1 (M+1).
