To a solution of diethyl cyanomethylphosphonate (0.173 mL) in dimethylsulfoxide (6 mL) was added potassium tert-butoxide (127 mg) at room temperature, and the mixture was stirred for 30 minutes. Then a solution of tert-butyl 3-(6-formylpyridin-2-ylmethyl)-6-methoxy-2-phenyl-1H-indole-1-carboxylate (385 mg) in dimethylsulfoxide (3 mL) was added dropwise at room temperature, and the mixture was stirred overnight. To the reaction mixture were added water and saturated brine, followed by extraction with ethyl acetate. The aqueous layer was extracted with ethyl acetate. The combined organic layers were washed with water and saturated brine successively, dried over anhydrous magnesium sulfate, and then concentrated under reduced pressure. The residue was purified by silica gel column chromatography (eluting solvent: ethyl acetate-hexane) to obtain the title compound (192 mg). 1H-NMR (CDCl3) δ ppm: 1.23 (9H, s), 3.89 (3H, s), 4.04 (2H, s), 6.55 (1H, d, J=15.8 Hz), 6.82-6.90 (1H, m), 6.96-7.03 (1H, m), 7.04-7.12 (1H, m), 7.23-7.46 (7H, m), 7.48-7.57 (1H, m), 7.82-7.90 (1H, m).
