2',5'SilySpiroT AIDS-028396 AIDS028396 [1-[2'-O-(tert-Butyldiphenylsilyl)-5'-O-(tert-butyldimethylsilyl)-.beta.-D-ribofuranosyl]-3-methylthymine]-3'-spiro-5-(4-amino-1,2-oxathiole-2,2-dioxide)