2',5'-Bis(t-butyldimethylsilyl)-3'-spiro-5-(4-amino-1,2-oxathiole-2,2-dioxide)ribofuranosylhypoxanthine AIDS-003938 AIDS003938 SpiroOxathioldiTBSRiboHx TSAO-H TSAO-Hypoxanthine TSAO-Inosine