The appropriate nucleophile (10 mmol) was added to a solution of uridine-2′,3′methoxymethylidene-5′-O-(2-thio-1,3,2-dithiaphospholane) (0.34 g, 5 mmol) in acetonitrile (10 mL). Next, DBU (5 mmol) was added with stirring. The progress of the reaction was monitored by 31 P-NMR. When the reaction was complete, the solvent was evaporated and the residue was treated first with 10% HCl, pH 2.3, for 3 h, and then with 24% NH4OH, pH 9.8, for 45 min. The aqueous phase was freeze dried and the residue was separated by liquid chromatography on Sephadex® DEAE A-25 column with a linear gradient of 0-0.5 M NH4HCO3 buffer (pH 7.5). After repeated freeze-drying of the relevant fraction, the product was obtained at 90-98% overall yield as a white solid.