[(2R,3S,4R,5R)-3-[[(2R,3S,4R,5R)-5-(2-amino-6-oxo-3H-purin-9-yl)-3,4-dihydroxyoxolan-2-yl]methoxy-hydroxyphosphoryl]oxy-5-(2,4-dioxopyrimidin-1-yl)-4-hydroxyoxolan-2-yl]methyl [(2R,3S,4R,5R)-5-(6-aminopurin-9-yl)-4-hydroxy-2-(hydroxymethyl)oxolan-3-yl] hy