AIDS-072978 AIDS072978 Spironucleoside TSAO derivative [1-[2',5'-Bis-O-(tert-butyldimethylsilyl)-.beta.-D-xylofuranosyl]-3-N-methylthymine]-3'-spiro-5''-(4''-amino-1'',2'',3''-oxathiazole-2'',2''-dioxide)