2',5'SilySpiroT AIDS-028397 AIDS028397 TSAO-T [1-[2'-O-(tert-Hexyldimethylsilyl)-5'-O-(tert-butyldimethylsilyl)-.beta.-D-ribofuranosyl]-3-methylthymine]-3'-spiro-5-(4-amino-1,2-oxathiole-2,2-dioxide)