Peptide modification chemistry has long exploited the reactive double bond of dehydroalanine, Dha, through Michael additions, cycloadditions, and metal-catalyzed cross-couplings. Yet the amide bond flanking that double bond had remained an untapped handle. The challenge is formidable: peptides carry multiple amide bonds and sensitive residue side chains, making truly selective amide activation extraordinarily difficult to achieve without damaging the rest of the scaffold. Prior approaches to direct amide-bond activation were restricted to dipeptides or analogues carrying only a single amide, leaving the broader peptide landscape out of reach.
Researchers in the Liu and Wang Groups at the Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, with additional affiliation at Lanzhou University, published in Organic Letters, identified a unique property of the Dha amide N–H: its combination of a withdrawing carbonyl and the conjugated double bond raises the acidity of the amide proton well above that of all 20 canonical amino acids, enabling selective deprotonation by DBU or Cs2CO3 under mild conditions. The resulting electron-rich enamine anion undergoes a single electron transfer, SET, process with a diaryliodonium salt, generating a phenyl radical that couples at the β-carbon of Dha and delivers the arylated product while preserving the double-bond character of the residue.
Optimization settled on DMSO as solvent at 25 °C, where N-Ac-Dha methyl ester delivered an 83% isolated yield with DBU at 1.5 equiv and diphenyliodonium triflate. A broad scope of symmetrical and para/meta-electron-withdrawing-group-substituted diaryliodonium salts performed well; electron-donating p-methyl and p-tert-butyl variants required 405 nm irradiation to reach acceptable yields of 61–63%. Crucially, when the reaction was run in the presence of aromatic, acidic, and basic amino acid residues, no cross-reactivity was detected. Three control experiments confirmed that deprotonation of the specific Dha amide is the obligatory first step. Replacing the carbonyl with hydrogen abolished reactivity, as did substitution with a phenyl group. Independently, blocking both amide N–H protons in a bi-N-protected Dha derivative shut down the reaction entirely.
The method translated directly to complex substrates. A series of Dha-containing peptides in solution were decorated with functionalized phenyl groups in good yields, with only a modest yield penalty when Dha was flanked by bulky residues. Solid-phase peptide modification was equally successful: after on-resin deprotection and elimination to install Dha, treatment with diaryliodonium salt and DBU in DMSO produced arylated peptides efficiently. Mechanistic evidence, including radical-trapping with TEMPO and 1,1-diphenylethylene, NMR titration showing electron flow from deprotonated Dha to the iodonium salt, and UV/vis charge-transfer absorption, support a radical chain initiated by SET from the enamine anion intermediate. Thermodynamic preference for the Z-alkene, established by prior computational work, accounts for the observed stereoselectivity of the product.
Beyond its immediate synthetic utility, this deprotonated amide activation mode offers a metal-free, chemically selective route into the Dha residue that complements existing metal-catalyzed Heck-type arylations and expands the toolkit available for late-stage peptide diversification. The authors note that the same activation logic may illuminate biosynthetic pathways in thiopeptide natural products, where Dha intermediates are central to heterocycle formation. As peptide-based drug discovery increasingly demands rapid access to structurally diverse analogue libraries, a base-mediated, room-temperature arylation that tolerates the full complement of proteinogenic residues represents a practical and conceptually new addition to the field.
