A mixture of hydroxylammonium chloride (0.38 g), sodium hydrogencarbonate (0.58 g) and dimethyl sulfoxide (4 mL) was stirred at 50° C. for 30 min, 4′-{[3-(2-{4-[2-(dimethylamino)ethoxy]phenyl}-2-oxoethyl)-6-ethyl-2,4-dioxo-3,4-dihydrothieno[2,3-d]pyrimidin-1(2H)-yl]methyl}biphenyl-2-carbonitrile (0.41 g) was added, and the mixture was stirred at 90° C. for 20 hr. After allowing to cool to room temperature, ethyl acetate and water were added to the reaction mixture, and the mixture was extracted with ethyl acetate. The organic layer was washed successively with water and saturated brine, and dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography and dissolved in tetrahydrofuran (4 mL). N,N′-Carbonyldiimidazole (0.13 g) and then 1,8-diazabicyclo[5.4.0]undec-7-ene (0.12 mL) were added, and the mixture was stirred at room temperature for 2 hr. The reaction mixture was diluted with ethyl acetate, washed successively with 1N hydrochloric acid and saturated brine, and dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography to give the title compound as a pale-yellow amorphous solid (0.26 g, 57%).
