合成路线1

The selective tosylation of L-gulose with tosyl chloride in pyridine, followed by complete acetylation with acetic anhydride gives tetraacetyl-6-O-tosyl-L-gulose (II), which is treated with HBr in AcOH to yield the bromo derivative (III). The reaction of (III) with potassium O-ethylxanthate in refluxing acetone, followed by deacetylation with NH4OH in methanol affords 1,6-thioanhydro-L-gulopyranose (IV). The selective oxidative cleavage of (IV) by means of NaIO4, followed by reduction with NaBH4 and protection of the resulting diol with acetone and TsOH provides the acetonide (V), which is silylated at the primary OH group with TBDMS-Cl giving the silyl ether (VI). Elimination of the acetonide group with TsOH in methanol yields the diol (VII), which is cleaved with Pb(OAc)4 and oxidized with PDC in DMF to afford the carboxylic acid (VIII). The treatment of (VII) with Pb(OAc)4/pyridine in THF furnishes the diacetate (IX), which is condensed with N4-acetyl-5-fluoro-O-(trimethylsilyl)cytosine (X) by means of TBDMS-OTf in dichloromethane yielding a mixture of the desired (-)(2R,5S)-isomer (XI) along with its (+)-(2R,5R)-isomer that is separated by column chromatography. Finally, deacetylation of (XI) with ammonia in methanol, followed by desilylation with TBAF in THF afforded the target compound.