Preorganization of the Hydroxyethylene Dipeptide Isostere: The Preferred Conformation in Solution Resembles the Conformation Bound to BACE

Design of Pseudodiproline Dimers as Mimetics of Pro-Pro Units: Stereocontrolled Synthesis, Configurational Relevance, and Structural Properties

Stereocontrolled methods are described for the synthesis of hitherto unreported pseudodiproline dimers in which a cyclopentane carboxylic acid is linked to a pyrrolidine residue by a stereochemically defined hydroxymethylene tether. These proline-cyclopentane (Pro-Cyp) dimers have interesting structural characteristics as seen in their X-ray crystal structures as well as their nuclear magnetic resonance (NMR) spectra in CDCl₃. They can be considered to be novel Pro-Pro mimetics, which can be used to replace natural diproline sequences with potential applications in medicinal chemistry. They also represent a new concept in the peptidomimetic design of chimeric proline-based amino acids as carbocyclic hydroxyethylene isosteres of inhibitor molecules, in which the stereodefined bridging hydroxyl group can simulate a tetrahedral intermediate in an enzyme complex.

Optimization of Hydroxyethylamine Transition State Isosteres as Aspartic Protease Inhibitors by Exploiting Conformational Preferences

NMR conformational analysis of a hydroxyethylamine peptide isostere developed as an aspartic protease inhibitor shows that it is a flexible architecture. Cyclization to form pyrrolidines, piperidines, or morpholines results in a preorganization of the whole system in solution. The resulting conformation is similar to the conformation of the inhibitor in the active site of BACE-1. This entropic gain results in increased affinity for the enzyme when compared with the acyclic system. For morpholines 27 and 29, the combination of steric and electronic factors is exploited to orient substituents toward S1, S1', and S2' pockets both in the solution and in the bound states. These highly preorganized molecules proved to be the most potent compounds of the series. Additionally, the morpholines, unlike the pyrrolidine and piperidine analogues, have been found to be brain penetrant BACE-1 inhibitors.