(1R,5R,7R,8S)-7-[4-(Benzoylamino)-5-methyl-2-oxo-1(2H)-pyrimidinyl]-5-[[bis(4-methoxyphenyl)phenylmethoxy]methyl]-3-methyl-2,6-dioxa-3-azabicyclo[3.2.1]oct-8-yl 2-cyanoethyl N,N-bis(1-methylethyl)phosphoramidite