3-((Bis(diisopropylamine)phosphanyl)oxy)propanenitrile (6.42 g, 21.3 mmol, 6.77 mL, 1.8 eq) was added to a solution of N-(9-((2R,3R,4R,5R)-5-((bis(4- methoxyphenyl)(phenyl)methoxy)methyl)-3-fluoro-4-hydroxytetrahydrofuran-2-yl)-9H- purine-6-yl)benzamide (8.0 g, 11.8 mmol, 1.0 eq) in DCM (42 mL) at 0 °C followed by DCI (2.24 g, 18.9 mmol, 1.6 eq). The mixture was warmed to 25 °C and stirred for 3 hours. Two parallel reactions were carried out. The combined remedies were then diluted with DCM (150 mL), the organic layer was washed with sat NaHCO3 solution (200 mL) and brine (100 mL), dried over Na2SO4, filtered and concentrated, the crude was purified by column chromatography to give Dmt-2'fluoro-da(bz) amidite (18.0 g, 18.5 mmol, 78.1% yield, 90% purity) as a white solid.
