Uridine 5′-monophosphate (Sigma, Milwaukee, 3.0 g, 9.26 mmol) was dissolved in dry DMF (10 mL) and tributylamine (Aldrich, 2 mL). The solution was evaporated in vacuo at 40° C. to an oil. The residue was dissolved in dry DMF (Aldrich, 8 mL) to form a solution. Carbonyldiimidazole (Aldrich, 1.65 g, 10.18 mmol) was added to this solution. The reaction was heated at 50° C. for one hour. Uridine 5′-triphosphate (Yamasa, 5.60 g, 10.18 mmol) prepared as the anhydrous tributylammonium salt in DMF (5 mL) and tributylamine (2 mL), as described in Example 3 below, was added to the reaction solution. The mixture was allowed to stir at 50° C. for three days when the solution was evaporated in vacuo to an oil, redissolved in water (5 mL) and purified by column (300×50 mm) chromatography (Sephadex DEAE-A25, 40-120μ, Aldrich, pre-swollen in 1.0 M NaHCO3 and washed with 2 column volumes of deionized H2O (H2O→0.3 M NH4HCO3 gradient). The pure fractions were concentrated in vacuo at 35° C., and H2O added and reevaporated 5 times to obtain diuridine tetraphosphate tetraammonium salt as a white solid (2.37 g, 30% yield): 92.11% pure by HPLC with the same retention time as the standard. In addition, the tetraammonium salt was analyzed by FABMS to give a mass of [C18H25N4O23P4 (M−H+)−: calculated 788.9860] 788.9857, confirming a parent formula of C18H26N4O23P4 for the free acid].
