The R 1 -R 2 disubstitution pattern in 18-21 caused unexpected differences in affinities. The vicinal difluoro substitution in 19 may lead to an unfavorable electron withdrawing effect which could reduce the efficiency of T-stacking with Tyr98. Such edge-to-face interactions, where the hydrogen of one aromatic system points perpendicular to the center of an aromatic plane, are preferred in protein-ligand binding events. The two ligands with bicyclic arylidene cores, 22 and 23, had a similar moderate affinity to VHL. Their additional aryl ring is likely pointing out towards the solvent. Overall, in the majority of our first-series VHL ligands, modifications at the phenylene core did not improve affinity, indicating a rather narrow window for improvement. Among the introduced substituents, fluorine appeared to be the most promising, which we took forward into the design of the second series of VHL inhibitors. Two selected VHL ligands of the first series, 2 and 8, have also been investigated with respect to their ability to stabilize HIF-1α in a cellular context, the data is discussed below.