1-(2,3-Dideoxy-2-fluoro-.beta.-D-glycero-pent-2-eno-furanosyl)cytosine 122757-53-3 2'-Fd4C AIDS-001231 AIDS001231 Cytidine, 2',3'-didehydro-2',3'-dideoxy-2'-fluoro- D-2'-Fd4CL .BETA.-D-Fd4C