To another reaction vessel, 0.12 g of cytosine and 0.53 mL of N,O-bis(trimethylsilyl)acetamide were added in a nitrogen atmosphere, and the obtained mixture was then stirred at 80° C. for 2 hours. After cooling in air, a methylene chloride solution containing (3S,4S,5R)-2-bromo-4-((4-chlorobenzyl)oxy)-5-(((4-chlorobenzyl)oxy)methyl)-3-fluorothiolane was added to the reaction mixture, and the obtained mixture was then stirred 60° C. for 2 hours. Thereafter, methylene chloride was added to the reaction mixture, and the thus obtained mixture was washed with a saturated sodium hydrogen carbonate aqueous solution, and was then dried over anhydrous sodium sulfate. After that, the solvent was distilled away under reduced pressure. The obtained residue was purified by silica gel column chromatography (chloroform/methanol=100/0 to 80/20), so as to obtain 0.18 g of (3S,4S,5R)-2-(4-amino-2-oxo-1,2-dihydropyrimidin-1-yl)-4-((4-chlorobenzyl)oxy)-5-(((4-chlorobenzyl)oxy)methyl)-3-fluorothiolane in the form of a light yellow solid.
