Trimethylsilyl triflate → [(2R,3R,4R,5R)-3,4-diacetoxy-5-(2-bromo-5,6-dichloro-benzimidazol-1-yl)tetrahydrofuran-2-yl]methyl acetate

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To a suspension of (7) (47.867 g, 180 mmol) in 540 mL of dry. MeCN, was added 45 mL (180 mmol) of BSA. The reaction mixture was stirred at room temperature for 15 min to give a clear solution. A small amount of solid impurity remained unsilylated, believed to be 5,6-dichlorobenzimidazol-2-one from the starting material(7). 1,2,3,5-Tetra-O-acetyl-β-D-ribofuranose (60.14 g, 189 mmole) was added to this solution and then TMSOTf (38.267 mL 198 mmol) was added dropwise over 20 min. After the addition had been completed, stirring was continued at room temperature for 30 min. The reaction mixture was diluted with 2 L of EtOAc. The EtOAc solution was washed with 1:1 saturated aquenous NaHCO3 /saturated aquenous NaCl (1.5 L×2), dried (Na2SO4, 100 g), decolorized (activated charcoal, 6 g), and filtered through Celite. The filtrate was evaporated and the residue was recrystallized from MeOH to give 82.21 g (4 crops, 87%) of 52a as white needles. MP: 140°-144° C. This product was pure by 1H NMR.

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