Synthesis of Membrane-Permeable Macrocyclic Peptides via Imidazopyridinium Grafting

Macrocyclic peptides (MPs) are a class of compounds that have been shown to be particularly well suited for engaging difficult protein targets. However, their utility is limited by their generally poor cell permeability and bioavailability. Here, we report an efficient solid-phase synthesis of novel MPs by trapping a reversible intramolecular imine linkage with a 2-formyl- or 2-keto-pyridine to create an imidazopyridinium (IP⁺)-linked ring. This chemistry is useful for the creation of macrocycles of different sizes and geometries, including head-to-side and side-to-side chain configurations. Many of the IP⁺-linked MPs exhibit far better passive membrane permeability than expected for "beyond Rule of 5" molecules, in some cases exceeding that of much lower molecular weight, traditional drug molecules. We demonstrate that this chemistry is suitable for the creation of libraries of IP⁺-linked MPs and show that these libraries can be mined for protein ligands.