7-Oxa-6-thia-2,5-diazaoctanoic acid, 8-[(3aR,4R,6R,6aR)-6-(6-amino-9H-purin-9-yl)hexahydrofuro[3,4-d]oxazol-4-yl]-3-(2-methylpropyl)-4-oxo-, 1,1-dimethylethyl ester, 6,6-dioxide, (3S)-