To assess the binding affinities of compounds 1-23, we employed a competitive fluorescence polarization (FP) assay. Binding of a ligand to the HIF binding site of VHL displaces a fluorescein-labeled, 19-mer HIF-1α oligopeptide, leading to a change in polarization of emitted light upon excitation of the competing fluorescent probe. According to our FP measurements (Table ), ligand 8 showed the highest binding affinity to VHL (Kd = 97 nM), being in the same range than that of the established VHL inhibitor VH298 (1) without a fluorine atom at the R 2 position.