2'-O-{2-{{{1-[2'-deoxy-5'-O-pivaloyl-3'-O-(tetrahydro-2H-pyran-2-yl)-β-D-ribofuranosyl]-1,2,3,4-tetrahydro-2,4-dioxopyrimidin-5-yl}methyl}amino}-2-oxoethyl}-5'-O-(4,4'-dimethoxytrityl)uridine 3'-(S,S'-diphenyl phosphorodithioate)