Site‐Selective Peptide and Protein Functionalization with Cyclopropenium Cations

Abstract In the realm of organic chemistry, carbocations play a pivotal role as highly reactive intermediates in the synthesis of complex molecules. While cyclase enzymes construct terpenoid natural products through carbocation intermediates, the use of these electrophilic reactive species for peptide and protein bioconjugation in aqueous media remains unexplored. Herein, we disclose the discovery and development of a new chemical modification of peptides and proteins with aromatic cyclopropenium cations, selective at cysteine residues. The bioconjugation is fast, operationally simple, and occurs at low concentration in aqueous media, allowing for the installation of a tetrasubstituted cyclopropene ring with excellent site selectivity. Moreover, the cyclopropenylation is preferential to internal cysteines, thus complementing current methodologies for selective terminal cysteine bioconjugation. These studies further showcased the bioconjugates' utility as radical traps in a thiol–ene process, enabling the formation of cyclopropane‐linked conjugates.