[3-[[3-[[5-(2-amino-6-oxo-3H-purin-9-yl)-3-hydroxy-oxolan-2-yl]oxy-hyd roxy-phosphoryl]oxy-5-(6-aminopurin-9-yl)oxolan-2-yl]methoxy-hydroxy-p hosphoryl]oxy-5-(5-methyl-2,4-dioxo-pyrimidin-1-yl)oxolan-2-yl]methoxy -[5-(6-aminopurin-9-yl)-2-(hydroxymethyl)o