142102-77-0 2',5'diSilyl-5MeSpiroC AIDS-003932 AIDS003932 TSAO analog TSAO-m5C [1-[2',5'-Bis-O-(tert-butyldimethylsilyl)-.beta.-D-ribofuranosyl]-5-methylcytosine]-3'-spiro-5-(4-amino-1,2-oxathiole-2,2-dioxide)