Methanol → 4-N-acetyl-5'-O-(4,4'-dimethoxytrityl)-2'-O-(2-cyanoethoxymethyl)cytidine

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2′-O-(2-Cyanoethoxymethyl)cytidine (9.9 g, 26.8 mmol) was subjected to azeotropic distillation with pyridine, and then was dried with a vacuum pump for 30 minutes. The residue was dissolved in 190 mL of THF, and 43 g of pyridine (538 mmol) and 20 g of molecular sieve 4 A were added under an argon atmosphere, and the mixture was stirred for 10 minutes. To the reaction solution was added 11.8 g of 4,4′-dimethoxytrityl chloride 11.8 g (34.9 mmol) by 3 portions every 1 hour, and the mixture was further stirred for 1 hour. After 2 mL of methanol was added and the reaction solution was stirred for 2 minutes, the reaction solution was filtrated with a celite, and was washed with ethylacetate. After concentrating the filtrate with evaporation, the residue was dissolved in ethyl acetate, and was separated with a saturated aqueous sodium bicarbonate solution. After the organic layer was washed with saturated brine and dried over anhydrous magnesium sulfate, the solvent was distilled off. The obtained residue was purified by silica gel chromatography to obtain the objective compound (15 g, yield 83%).

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