142102-78-1 2',5'diSilylSpiroC AIDS-004710 AIDS004710 TSAO analog TSAO-C [1-[2',5'-Bis-O-(tert-butyldimethylsilyl)-.beta.-D-ribofuranosyl]cytosine]-3'-spiro-5-(4-amino-1,2-oxathiole-2,2-dioxide)