Head-to-tail peptide macrocyclisations are significantly improved, as measured by isolated yields, reaction rates and product distribution, by substitution of one of the backbone amide CO bonds with an oxetane ring.

Passive membrane permeability is a fundamental challenge in the development of bioactive macrocycles. To achieve this objective, chemists have resorted to various strategies, the most common of which is deployment of N‐methylated amino acids and/or D‐amino acids. Here we investigate the effect of heterocyclic grafts on the passive membrane permeability of macrocycles and report the structural consequences of iterative amino acid replacement by azole rings. Through stepwise substitution of amino acid residues for heterocycles, we show that lipophilicity and PAMPA permeability of a macrocycle can be vastly improved. Overall, changes in permeability do not scale linearly as more heterocycles are incorporated, underscoring the subtleties of conformation‐property relationships in this class of molecule. NMR analysis and molecular dynamics simulations provide insights into the structural consequences of the added heterocycles and these frameworks can now be applied as macrocyclic scaffolds for drug discovery.

Identification of turn motifs that are stabilized by intramolecular hydrogen bonds can be useful in describing the conformation of peptide systems. However, this approach is somewhat insufficient for cyclic peptides...

Site-selective transformations of densely functionalized scaffolds have been a topic of intense interest in chemical synthesis. Herein we have repurposed the rarely used Cornforth rearrangement as a tool to effect a single-atom ring contraction in cyclic peptide backbones. Investigations into the kinetics of the rearrangement were carried out to understand the impact of electronic factors, ring size, and linker type on the reaction efficiency. Conformational analysis was undertaken and showed how subtle differences in the peptide backbone result in substrate-dependent reaction profiles. This methodology can now be used to perform conformation-activity studies. The chemistry also offers an opportunity to install building blocks that are not compatible with traditional C-to-N iterative synthesis of macrocycle precursors.