(2r,3r,4s,5s)-2-{6-(2,2-Diphenyl-ethylamino)-2-[2-(1-methyl-1h-imidazol-4-yl)-ethylamino]-purin-9-yl}-5-(3-propyl-[1,2,4]oxadiazol-5-yl)-tetrahydro-furan-3,4-diol formate