The oxidation of the hydroxymethyl group of 3,5-di-O-benzyl-4-C-(hydroxymethyl)-1,2-O-isopropylidene-alpha-D-ribo-pentofuranose (I) with oxalyl chloride and TEA in DMSO gives the aldehyde (II), which is treated with CBr4 and PPH3 in dichloromethane to yield the dibromovinyl compound (III). The reaction of (III) with BuLi in THF affords the ethynyl derivative (IV), which is silylated with Tes-Cl and BuLi in THF to provide the triethylsilylethynyl compound (V). Elimination of the acetonide group of (V) with HOAc and TFA gives the dihydroxycompound (VI), which is treated with Ac2O and pyridine, yielding the diacetate (VII). The condensation of the sugar(VII) with 5-fluorouracil (VIII) by means of Tms-OTf and N,O-bis(trimethylsilyl)acetamide in dichloroethane affords the adduct (IX), which is deacetylated with TEA in methanol to provide the hydroxy derivative (X). The debenzylation of (X) by means of BCl3 in dichloromethane furnishes the trihydroxy compound (XI), which is treated with acetyl bromide in acetonitrile to give the bromo diacetoxy derivative (XII). The debromination of (XII) is performed with Bu3SnH and AIBN in hot toluene, yielding the diacetate (XIII), which is condensed with 1,2,4-triazole (XIV) by means of 4-chlorophenyl dichlorophosphate in pyridine to afford the triazolopyrimidinone (XV). Finally, (XV) is converted into the target compound by cleavage of the triazole ring with NH4OH in dioxane and deacetylation with NaOH in methanol/water.