To a cooled (0° C.) solution of 2,3-dihydro-6H-[1,4]dioxino[2,3-f]indole-7,8-dione (Lackey and Sternbach, Synthesis 1993:993-997) (1.73 g, 8.42 mmol) in anhydrous N,N-dimethylformamide (28 mL) was added sodium hydride (60% in mineral oil, 0.37 g, 9.26 mmol). The mixture was stirred at ambient temperature for 1 h and bromodiphenylmethane (2.39 g, 9.69 mmol) was added. After stirring at ambient temperature for 16 h, further bromodiphenylmethane (0.42 g, 1.68 mmol) was added. The mixture was stirred at ambient temperature for 8 h and then diluted with water (200 mL) and ethyl acetate (200 mL). The layers were separated and the aqueous layer was extracted with ethyl acetate (2×100 mL). The combined organic layers were washed with brine (150 mL), dried over anhydrous magnesium sulfate, filtered, concentrated in vacuo. The residue was purified by column chromatography, eluted with a 35% to 50% gradient of ethyl acetate in hexanes to afford 6-(diphenylmethyl)-2,3-dihydro-6H-[1,4]dioxino[2,3-f]indole-7,8-dione (0.7 g, 22%) as a red solid: 1H NMR (300 MHz, CDCl3) δ 7.41-7.28 (m, 10H), 7.16 (s, 1H), 6.94-6.91 (m, 1H), 5.96 (s, 1H), 4.27-4.14 (m, 4H); MS (ES+) m/z 371.9 (M+1).
