2,6-Dichloro-5-(5-O-(tert-butyldimethylsilyl)-1-acetoxy-2,3-O-isopropylidene-.alpha.-D-ribofuranosyl)imidazo[1,2-a]pyridine AIDS-060759 AIDS060759 Imidazo[1,2-.alpha.]pyridine C-nucleoside analog