To a 350 mL of dry seal pressure flask (Chemglass) were added (2R,3R,4R,5R)-5-(2-amino-6-chloro-9H-purin-9-yl)-2-(benzoyloxymethyl)-4-fluoro-4-methyltetrahydrofuran-3-yl benzoate (6a, 3.6 g, 6.85 mmol) and 150 mL of absolute ethanol. Azetidine hydrochloride (2.56 g, 27.4 mmol) was added and then followed by triethylamine (4.16 g, 41.1 mmol). The supension was stirred and heated to 70° C. while sealed for 5 hours. All the starting material was consumed but the benzoyl groups remained as shown by TLC. Sodium methoxide (7.8 mL, 34.3 mmol, 25% solution in methanol) was added to the mixture and heated at 50° C. The reaction was complete after 3.5 h. The reaction mixture was allowed to cool to room temperature and neutralized by addition of glacial acetic acid (0.41 g, 6.85 mmol). The mixture was concentrated under reduced pressure and then the residue was triturated with ethyl acetate. The resulting solid was removed by filtration and the solid was washed with EtOAc (2×15 mL). The filtrate was concentrated under reduced pressure and the residue was purified via column chromatography (Analogix, 120 g cartridge, gradient of 0 to 15% MeOH in DCM). The pure product containing fractions were combined, concentrated under reduced pressure and dried (50° C., 0.2 mmHg, 17 h) to a light pink colored foam solid (2.15 g, 6.35 mmol, 93%).
