2',5'SilySpiroT AIDS-028391 AIDS028391 TSAO-T [1-[2'-O-(tert-Butyldimethylsilyl)-5'-O-(tert-butylmethoxyphenylsilyl)-.beta.-D-ribofuranosyl]-3-methylthymine]-3'-spiro-5-(4-amino-1,2-oxathiole-2,2-dioxide)