Molecular modeling, synthesis, and structures of N-methylated 3,5-linked pyrrolin-4-ones toward the creation of a privileged nonpeptide scaffold.

The molecular modeling, synthesis, and elucidations of the solid state and solution structures of N-methylated 3,5-linked bispyrrolin-4-ones are described. Prior investigations established that the 3,5-linked pyrrolin-4-one based scaffold can be incorporated into mimics of beta-sheet/beta-strands and into potent, orally bioavailable inhibitors of the HIV-1 protease. To extend the utility of this scaffold beyond that of the initially designed mimics of beta-sheet/beta-strands, we have now explored the structure of N-methylated pyrrolinones. Molecular modeling indicated that N-methylated bispyrrolinones could adopt three low-energy backbone conformations (ca. 165 degrees, 289 degrees, and 320 degrees). Upon their successful synthesis, structural elucidation both in the solid state and in solution revealed the existence of two of the three predicted backbone conformers (ca. 165 degrees and 289 degrees). Two structures were particularly noteworthy and completely unexpected. Mono-N-methyl bispyrrolinone (+)-1 self assembled in the solid state to form a novel helix, while the acetylene-linked dimer of (+)-1, designed to potentiate the observed helical array, instead associated via an intermolecular hydrogen bond in parallel columns. These serendipitous observations led us to speculate that the pyrrolinone moiety may in fact represent a privileged nonpeptide scaffold, able to mimic not only the extended beta-sheet/beta-strand conformation as initially targeted, but also diverse conformations including those analogous to beta-turns and helices. These seemingly unlimited conformations greatly expand the scope of this scaffold for the development of low-molecular weight ligands for biologically important macromolecules.

Design and synthesis of sulfur-free cyclic hexapeptides which contain the RGD sequence and bind to the fibrinogen GP IIb/IIIa receptor. A conformation-based correlation between propensity for imide formation and receptor affinity.

The Arg-Gly-Asp (RGD) sequence is the key recognition site in many adhesive interactions. To probe the structural and conformational requirements for potential antithrombotic agents, we have designed and synthesized three cyclic hexapeptides (1, 5 and 6) containing the RGD sequence. In the ELISA GP IIb/IIIa-fibrinogen receptor assay, 1, 5 and 6 bound with IC50 values of 1, 0.1 and 0.016 microM, respectively. All three peptides completely displaced fibrinogen from the receptor. No potent, sulfur-free cyclic hexapeptide had heretofore been described as a fibrinogen receptor antagonist. The enhanced binding affinity of 6, distinguished by the presence of two D-amino acids, is likely to reflect an increased conformational resemblance to the natural peptide ligands. Cyclization of H-Asp(OFm)-DSer-Phe-DPhe-Arg-Gly-OH with DPPA and NaHCO3 in DMF to afford 6 was attended by subsequent aspartimide formation with generation of 9-fluorenylmethanol. Interestingly, imide formation was not observed with any of the three linear hexapeptides (3, 8 and 9), with the all-L-cyclic peptide 1, nor with 5, which contains only Ser-1 in the D-configuration. The observed imide formation led us to use catalytic transfer hydrogenation rather than piperidine to remove the 9-fluorenylmethyl ester protecting group at the beta-carbonyl of aspartic acid. Further investigation revealed that imide formation was minimized by careful exclusion of water, reducing dissolution of NaHCO3. Thus the distinguishing conformational features of 6 express themselves both in receptor affinity and chemical propensity toward imide formation.