GENERAL STEPS: (2R,3R,4R,5R)-2-(Acetoxymethyl)-5-(6-oxo-1H-purin-9(6H)-yl)tetrahydrofuran-3,4-diyl diacetate (5.00 g, 12.68 mmol, 1.00 eq.), benzyltriethylammonium chloride (5.77 g, 25.36 mmol, 2.00 eq.) and N,N- dimethylaniline (1.8 mL, 13.94 mmol, 1.10 equiv) were dissolved in 50 mL of anhydrous acetonitrile. The reaction mixture was placed in a preheated oil bath (70°C) and trichlorophosphorus (5.9 mL, 63.40 mmol, 5.00 eq.) was slowly added dropwise and stirred at the same temperature for 2 hours. After completion of the reaction, the solvent and the remaining phosphorous trichloride were removed under reduced pressure (high vacuum, 70°C). The residue was poured into a chloroform/ice mixture and stirred for 20 minutes. The organic phase was separated and the aqueous phase was extracted with chloroform three times. The organic phases were combined and washed with 5% sodium bicarbonate solution until the aqueous layer was weakly basic. Subsequently, the organic phases were separated, dried with anhydrous magnesium sulfate and concentrated under reduced pressure to remove the solvent. Purification by silica gel column chromatography (eluent: ethyl acetate) afforded the target product (2R,3R,4R,5R)-2-(acetyloxymethyl)-5-(6-chloro-9H-purin-9-yl)tetrahydrofuran-3,4-diacetic acid diester (Compound 17) as a yellow oil (5.23 g, quantitative yield).
