Together, this provided the incentive for us to design and synthesize the tripyrimidonamide 5a, which mimics the hot spots I688, I692, and L696. In compound 5a, the side chain of V was used instead of I to avoid diasteromeres. 6, which lacks the isopropyl side chain, was also designed to probe the influence of the absence of the third side chain in a tripyrimidonamide. Next, we aimed to design compounds that can also form polar interactions, as these should confer specificity of binding. In addition, the binding to a well-defined cleft or groove in a PPI region has been described to yield a particularly effective PPI inhibitor. The Y689 side chain of 7a should be accommodated in an indentation in the binding epitope of helix H4' (Figure ); we also designed the homolog 7b with a prolonged (4-hydroxy-phenyl)-ethyl side chain. Although both compounds mimic the three hot spots Y689, I692, L696, with the longer side chain in 7b, we intended to accommodate for the apparent mismatch between the preferred side chain orientations in tripyrimidonamides and the side chain pattern of the hot spots (i, i+3, i+4). The side chain patterns of 5b and 7a are almost identical to that of the α-aminoxy-peptide AX, which was shown to bind to the CTD. Further analysis of the physicochemical properties of the CTD dimerization interface revealed a particular hydrophobic patch there (Figure ). Interestingly, the 4-methoxy-benzyl side chain of 5b should act as a (weak) hydrogen bond acceptor for S673' and T669' on helix H4', but at the same time decrease the side chain's hydrophilicity for a more favorable burial in the overall hydrophobic interface.