General Experimental Procedures. All solvents and reagents were purchased from the suppliers and used without further purification. IR spectra were recorded on a JASCO FT/IR-460 Plus spectrophotometer. Optical rotations were measured with a JASCO P-2300. MS spectra were obtained using the Waters UPLC-MS system (Aquity UPLC XevoQTof). NMR spectra were recorded with JEOL JNM-ECS 400 and JNM-ECA600 spectrometers with tetramethylsilane as an internal standard. Silica gel column chromatography (CC) was performed on silica gel N-60 (40-50 μm).Thin-layer chromatography (TLC) spots on plates pre-coated with silica gel 60 F 254 were detected with a UV lamp (254 nm). Fractionations for all CCs were based on TLC analyses. Synthesis of 5′-(4,4′-dimethoxytrityl)-2′-trimethylsilylethyluridine (4):After stirring of 2-(trimethylsilyl)ethyl diselenide (1, 1.14 mmol), NaBH 4 (1.14 mmol), and EtOH (2.85 mmol) in DMF (5 ml) at 0 o C for 30 min, 3 (0.95 mmol) was added to the solution, and the reaction was continued at 60 o C for an additional 1 h. The resultant solution was quenched by 5% NH 3 Cl aq., and then was poured into distilled water,