7,9-dihydro-1H-purine-2,6,8(3H)-trione uric acid 2,6-dihydroxy-7,9-dihydro-8H-purin-8-one 9H-purine-2,6,8-triol 7H-purine-2,6,8-triol 1H-purine-2,6,8-triol 5,7-dihydro-1H-purine-2,6,8(9H)-trione