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

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To a suspension of (5) (62.013 g, 280 mmol) in 1.4 L of dry MeCN under argon, was added 70 mL (280 mmol) of BSA. The reaction mixture was stirred at room temperature for 15 min to give a clear solution. To this solution were added 1,2,3,5-Tetra-O-acetyl-β-D-ribofuranose (89.096 g, 280 mmol) and then 60 mL (310 mmol) of TMSOTf was added dropwise over 20 min. After the addition had been completed, stirring was continued at room temperature for an additional 30 min. The reaction mixture was diluted with 4 L of EtOAc. The EtOAc solution was washed with 1:1 saturated aqueous NaHCO3 /saturated aquenous NaCl (3×2 L), dried (with Na2SO4, 100 g), decolorized (activated charcoal, 8 g) and filtered through Celite. The filtrate was evaporated and the residue was recrystallized from MeOH to give 116.88 g (3 crops, 87%) of (42) as white needles. The recrystallization was accomplished by heating the crude product in MeOH at reflux on a steam bath for a few minutes and then decanting the supernatant while hot into a clean beaker. Immediate crystallization resulted. A second portion of fresh MeOH was added to the remaining solid and the whole process was repeated until all of the solid was dissolved. MP: 145°-146° C. This product was pure by 1H NMR.

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