141845-83-2 2',5'diSilySpiroT AIDS-003924 AIDS003924 TSAO-U [1-[2',5'-Bis-O-(tert-butyldimethylsilyl)-.beta.-D-ribofuranosyl]uracil]-(R)(ribo)-3'-spiro-5-(4-amino-1,2-oxathiole-2,2-dioxide)