A mixture of 4-(benzyloxy)benzoic acid (1.04 g) and phosphorus oxychloride (12.2 mL) was stirred at room temperature for 5 min, and 2,4-dichloropyrimidin-5-amine (500 mg) was added thereto, and the obtained mixture was stirred at 100° C. for 2 hr. The reaction mixture was added to 1M aqueous sodium hydroxide solution at 0° C., and the precipitate was collected by filtration, and dissolved in THF and ethyl acetate. The obtained solution was dried over anhydrous sodium sulfate, and the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/ethyl acetate) to give 2-(4-(benzyloxy)phenyl)-5-chloro[1,3]oxazolo[5,4-d]pyrimidine (3.07 g) as a mixture with impurity. A mixture of 2-(4-(benzyloxy)phenyl)-5-chloro[1,3]oxazolo[5,4-d]pyrimidine (100 mg, a mixture with impurity), tert-butyl (2S)-but-3-yn-2-ylcarbamate (50.1 mg), dichlorobis(triphenylphosphine)palladium(II) (10.4 mg), copper(I) iodide (5.64 mg), triethylamine (0.041 mL) and DMF (2 mL) was stirred at 100° C. for 6 hr under an argon atmosphere. In another reaction container, A mixture of 2-(4-(benzyloxy)phenyl)-5-chloro[1,3]oxazolo[5,4-d]pyrimidine (100 mg, a mixture with impurity), tert-butyl (2S)-but-3-yn-2-ylcarbamate (50.1 mg), dichlorobis(triphenylphosphine)palladium(II) (10.4 mg), copper(I) iodide (5.64 mg), triethylamine (0.041 mL) and toluene (2 mL) was stirred at 100° C. for 6 hr under an argon atmosphere. The two reaction mixtures were combined, water was added thereto, and the mixture was extracted with ethyl acetate. The combined organic layer were washed with saturated brine, and dried over anhydrous magnesium sulfate, and the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (hexane/ethyl acetate) to give the title compound (44.4 mg).
