6-amino-9-((1R,3R,4R,5R)-4-hydroxy-2,6-dioxabicyclo[3.2.0]heptan-3-yl)-3,9-dihydro-2H-purin-2-one,6-amino-9-((1R,3R,4R,5R)-4-hydroxy-2,6-dioxabicyclo[3.2.0]heptan-3-yl)-3,9-dihydro-2H-purin-2-one,Impurity in commercial preparations of Valganciclovir,Guanine Therapeutic