Regioselective lithiation of the tris-O-silylated uridine (I) with sec-butyllithium in the presence of TMEDA, followed by addition to dibenzosuberenone (II) afforded the 5-(dibenzocycloheptenyl)uridine derivative (III). Subsequent reduction of the tertiary alcohol of (III) with triethylsilane in the presence of boron trifluoride, followed by re silylation with tert-butyldimethylsilyl chloride furnished (IV). The 4-thiouridine derivative (V) was then obtained by thionation of (IV) with phosphorus pentasulfide in refluxing pyridine. Removal of the O-silyl protecting groups of (V) was effected by treatment with HF-pyridine, yielding (VI). Phosphorylation of the primary hydroxyl of (VI) with phosphoryl chloride in the presence of 1,8-bis-(dimethylamino)naphthalene in trimethyl phosphate, followed by condensation with dichloromethylenebisphosphonic acid gave rise to the target triphosphate analogue, which was finally converted to the corresponding tetrasodium salt by passage through a cation-exchange resin.

Regioselective lithiation of the tris-O-silylated uridine (I) with sec-butyllithium in the presence of TMEDA, followed by addition to dibenzosuberenone (II) afforded the 5-(dibenzocycloheptenyl)uridine derivative (III). Subsequent reduction of the tertiary alcohol of (III) with triethylsilane in the presence of boron trifluoride, followed by re silylation with tert-butyldimethylsilyl chloride furnished (IV). The 4-thiouridine derivative (V) was then obtained by thionation of (IV) with phosphorus pentasulfide in refluxing pyridine. Removal of the O-silyl protecting groups of (V) was effected by treatment with HF-pyridine, yielding (VI). Phosphorylation of the primary hydroxyl of (VI) with phosphoryl chloride in the presence of 1,8-bis-(dimethylamino)naphthalene in trimethyl phosphate, followed by condensation with dichloromethylenebisphosphonic acid gave rise to the target triphosphate analogue, which was finally converted to the corresponding tetrasodium salt by passage through a cation-exchange resin.