2′-O-(2-methoxyethyl)-5-methyluridine (MOE T, 15.8 g, 50 mmol) was dissolved in 2,6-lutidine (23.3 ml) and acetonitrile (150 ml). 4,4′-Dimethoxytrityl chloride (DMTCl, 18.6 g, 55 mmol) was added. The mixture was stirred at room temperature for 30 min. and methanol (MeOH, 0.2 ml) was added. After 10 min., triethylamine (20 ml) and succinic anhydride (15 g) were added and the mixture was stirred overnight. The mixture was concentrated under reduced pressure. The residue was dissolved in dichloromethane (200 ml) and washed with triethylammonium phosphate (0.5 M, pH 7, 3×100 ml). The organic layer was dried (Na2SO4) and evaporated to approximately 100 ml. Hexane (150 ml) was added. The mixture was shaken for 5 min. and the supernatant was decanted. The residue was dried to give a yellow solid (36.8 g, 90%).
