Cytidine, N-acetyl-5′-O-[bis(4-methoxyphenyl)phenylmethyl]-5-methyl-2′-O,4′-C-methylene-3′-O-[(1R,3R,3aS)-tetrahydro-3-phenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaphosphol-1-yl]-