Table . Data for nucleotides 5a-e, and 5h-i: % yield, 31 P NMR chemical shift, and HPLC Rt a General procedure for the synthesis of 5a-i: To a solution of uridine-2′,3′-methoxymethylidene-5′-O-(2-thio-1,3,2dithiaphospholane) (0.34 g, 5 mmol) in acetonitrile (10 mL) the appropriate nucleophile (10 mmol) was added followed by DBU (0.76 g , 5 mmol). When the reaction was completed (as indicated by 31 P NMR), the solvent was evaporated and the residue was treated with 10% HCl, pH 2.3, for 3 h, and then with 24% NH4OH, pH 9.8, for 45 min. The aqueous solution was freeze dried and separated by liquid chromatography on Sephadex® DEAE A-25 column, using a linear gradient of ammonium bicarbonate (0.02-0.4 M). The products were obtained as a white solid at high yields as mentioned in Table b n.r. -no reaction c 31 P NMR spectrum in D2O, at 242.9 MHz and 300K.