Synthesis, structure-activity relationships, and drug resistance of β-D-3′-fluoro-2′,3′-unsaturated nucleosides as anti-HIV agents

Our recent studies demonstrated that D- and L-2'-fluoro-2',3'-unsaturated nucleosides (D- and L-2'-F-d4Ns) display moderate to potent antiviral activities against HIV-1 and HBV. As an extension of these findings, beta-D-3'-fluoro-2',3'-unsaturated nucleosides were synthesized as potential antiviral agents. The key intermediate (2S)-5-(1,3-dioxolan)-1-benzoyloxy-3,3-difluoropentan-2-ol 6 was prepared from 2,3-O-isopropylidene-D-glyceraldehyde 1, which was converted to 5-O-benzoxy-D-2-deoxy-3,3-difluoropentofuranosyl acetate 7 by the ring-closure reaction under acidic conditions. The acetate 7 was condensed with silylated purine and pyrimidine bases, which produced the alpha and beta isomers. The 3',3'-difluoro nucleosides were then treated with t-BuOK to give the desired X-fluoro-unsaturated nucleosides. We studied the structure-activity relationships of D-3'-fluoro-2',3'-unsaturated nucleosides against HIV-1 in human peripheral blood mononuclear cells, from which we found that the cytosine derivative 26 was the most potent among the synthesized compounds. To understand the mode of action and drug resistance profile, with particular regard to the role of fluorine, we performed the molecular modeling studies of the cytidine analogue D-3'F-d4C and found a good correlation between calculated relative binding energies and activity/resistance data. Our model also shows interactions of the X-fluorine and the 2',3' double bond, which can be correlated to the observed biological data. Differences between fluorine substitution at the 3' and 2' positions may account for the higher cross-resistance with lamivudine observed in the 2'-fluorinated series.