(2S,3R,4R,5R)-2-(2,2-bis(4-methoxyphenyl)-2-phenylethoxy)-4-fluoro-5-(2-isobutyramido-6-oxo-3,6-dihydro-9H-purin-9-yl)tetrahydrofuran-3-yl (2-cyanoethyl) diisopropylphosphoramidite