2,2-Dimethyl-propionic acid 2-{[5-(6-amino-purin-9-yl)-2,5-dihydro-furan-2-ylmethoxy]-[2-(2,2-dimethyl-propanoyloxy)-ethylsulfanyl]-phosphorylsulfanyl}-ethyl ester AIDS-072925 AIDS072925 tBu(iso)SATE ddA