To a mixture of 4′-{[6-ethyl-3-[2-(4-hydroxyphenyl)-2-oxoethyl]-2,4-dioxo-3,4-dihydrothieno[2,3-d]pyrimidin-1(2H)-yl]methyl}biphenyl-2-carbonitrile (0.49 g), (2-bromoethoxy)(tert-butyl)dimethylsilane (0.40 mL) and N,N-dimethylformamide (5 mL) was added cesium carbonate (0.6 g), and the mixture was stirred at 60° C. for 5 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 obtained residue was purified by silica gel column chromatography, and added to a mixture of hydroxylammonium chloride (0.52 g), sodium hydrogencarbonate (0.78 g) and dimethyl sulfoxide (5 mL) stirred at 50° C. for 30 min, 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 (3 mL). N,N′-Carbonyldiimidazole (0.11 g) and then 1,8-diazabicyclo[5.4.0]undec-7-ene (0.1 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 dissolved in tetrahydrofuran (3 mL), tetrabutylammonium fluoride (0.2 g) was added, and the mixture was stirred at room temperature for 3 hr. To the reaction mixture were added ethyl acetate and water, 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 obtained residue was purified by silica gel column chromatography to give the title compound as a colorless solid (0.18 g, 31%).
