To a solution of compound [(2S,4R,5R)-4-amino-5-(5-amino-2,7-dioxo-6H-thiazolo[4,5-d]pyrimidin-3-yl)tetrahydrofuran-2-yl]methyl benzoate (compound 33c, crude, 80 mg, 0.198 mmoL) in DCM (10 mL) and THF (2 mL) were added TEA (44 mg, 0.436 mmoL) and methanesulfonyl chloride (27 mg, 0.237 mmol) at 0° C. After being stirred at room temperature for 2 hours, the reaction mixture was diluted by DCM, washed with water, brine, dried over Na2SO4 and concentrated in vacuo to afford 80 mg crude product of [(2S,4R,5R)-5-(5-amino-2,7-dioxo-6H-thiazolo[4,5-d]pyrimidin-3-yl)-4-(methanesulfonamido)tetrahydrofuran-2-yl]methyl benzoate (compound 33d), which was used in next step directly. MS obsd. (ESI−) [(M−H)+]: 480.
