Fluorescent Isoindole Crosslink (FlICk) Chemistry: A Rapid, User‐friendly Stapling Reaction

The stabilization of peptide secondary structure via stapling is a ubiquitous goal for creating new probes, imaging agents, and drugs. Inspired by indole-derived crosslinks found in natural peptide toxins, we employed ortho-phthalaldehydes to create isoindole staples, thus transforming inactive linear and monocyclic precursors into bioactive monocyclic and bicyclic products. Mild, metal-free conditions give an array of macrocyclic α-melanocyte-stimulating hormone (α-MSH) derivatives, of which several isoindole-stapled α-MSH analogues (K_i ≈1 nm) are found to be as potent as α-MSH. Analogously, late-stage intra-annular isoindole stapling furnished a bicyclic peptide mimic of α-amanitin that is cytotoxic to CHO cells (IC₅₀ =70 μm). Given its user-friendliness, we have termed this approach FlICk (fluorescent isoindole crosslink) chemistry.