3-[5-O-[Bis(4-methoxyphenyl)phenylmethyl]-3-O-[[bis(1-methylethyl)amino](2-cyanoethoxy)phosphino]-2-deoxy-β-D-erythro-pentofuranosyl]-5,11-dihydro-5-(2-methyl-1-oxopropyl)-11-oxo-3H-benzimidazo[1,2-a]purin-7-yl 2-methylpropanoate