[(2R,3R,4R,5R)-5-(4-amino-2-oxo-pyrimidin-1-yl)-2-[[[(2R,3R,4R,5R)-5-( 6-aminopurin-9-yl)-4-hydroxy-2-(hydroxymethyl)oxolan-3-yl]oxy-hydroxy- phosphoryl]oxymethyl]-4-hydroxy-oxolan-3-yl]oxyphosphonic acid