Demethylative Alkylation of Methionine Residue by Employing the Sulfonium as the Key Intermediate

Methionine (Met) offers a valuable handle to achieve peptide chemical modification owing to its unique thioether functional group. In contrast with cysteine, the site-selective functionalization of the hydrophobic and redox-sensitive thioether motif on peptides is still challenging, and strategies for diversification on the Met residue are rarely disclosed. Herein we report a transition-metal-free and redox-neutral approach for Met diversification with substrate diversity, which could be applied to synthesize cyclic peptides.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

Late-Stage C-H Acylation of Tyrosine-Containing Oligopeptides with Alcohols

The selective tagging of amino acids within a peptide framework while using atom-economical C-H counterparts poses an unmet challenge within peptide chemistry. Herein, we report a novel Pd-catalyzed late-stage C-H acylation of a collection of Tyr-containing peptides with alcohols. This water-compatible labeling technique is distinguished by its reliable scalability and features the use of ethanol as a renewable feedstock for the assembly of a variety of peptidomimetics.

An Electrochemical Approach to Designer Peptide α-Amides Inspired by α-Amidating Monooxygenase Enzymes

Designer C-terminal peptide amides are accessed in an efficient and epimerization-free approach by pairing an electrochemical oxidative decarboxylation with a tandem hydrolysis/reduction pathway. Resembling Nature's dual enzymatic approach to bioactive primary α-amides, this method delivers secondary and tertiary amides bearing high-value functional motifs, including isotope labels and handles for bioconjugation. The protocol leverages the inherent reactivity of C-terminal carboxylates, is compatible with the vast majority of proteinogenic functional groups, and proceeds in the absence of epimerization, thus addressing major limitations associated with conventional coupling-based approaches. The utility of the method is exemplified through the synthesis of natural product acidiphilamide A via a key diastereoselective reduction, as well as bioactive peptides and associated analogues, including an anti-HIV lead peptide and blockbuster cancer therapeutic leuprolide.

Biocompatible Photoinduced Alkylation of Dehydroalanine for the Synthesis of Unnatural α-Amino Acids

A site-selective alkylation of dehydroalanine to access protected unnatural amino acids is described. The protocol is characterized by the wide nature of alkyl radicals employed, mild conditions, and functional group compatibility. This protocol is further extended to access peptides, late-stage functionalization of pharmaceuticals, and enantioenriched amino acids.

Synthesis of γ-Oxo-α-amino Acids via Radical Acylation with Carboxylic Acids

Herein we present a highly efficient, light-mediated, deoxygenative protocol to access γ-oxo-α-amino acid derivatives. This radical methodology employs photoredox catalysis, in combination with triphenylphosphine, to generate acyl radicals from readily available (hetero)aromatic and vinylic carboxylic acids. This approach allows for the straightforward synthesis of γ-oxo-α-amino acids bearing a wide range of functional groups (e.g., Cl, CN, furan, thiophene, Bpin) in synthetically useful yields (∼60% average yield). To further highlight the utility of the methodology, several deprotection and derivatization reactions were carried out.

Post-synthetic functionalization of tryptophan protected peptide sequences through indole (C-2) photocatalytic alkylation

We report a selective, mild, and efficient C-H functionalization of tryptophan and tryptophan-containing peptides with activated α-bromo-carbonyl compounds under visible-light irradiation. The protocol efficiency is outlined by the wide substrate scope and excellent tolerance of sensitive functional groups present in the amino acid side chains. The method can be successfully extended to access pharmaco-peptide conjugate scaffolds.

A general asymmetric synthesis of artificial aliphatic and perfluoroalkylated α-amino acids by Luche's cross-electrophile coupling reaction

Aliphatic artificial α-amino acids (α-AAs) have attracted great interest in biochemistry and pharmacy. In this context, we developed a promising practical protocol for the asymmetric synthesis of these α-AAs through the selective and efficient intermolecular cross-electrophile coupling of Belokon's chiral dehydroalanine Ni(ii) complex with different alkyl and perfluoroalkyl iodides mediated by a dual Zn/Cu system. The reaction afforded diastereomeric complexes with dr up to 21.3 : 1 in 24-95% yields (19 examples). Exemplarily, three enantiomerically pure aliphatic α-AAs were obtained through acidic decomposition of (S,S)-diastereomers of Ni(ii) complexes. Importantly, the chiral auxiliary ligand (S)-BPB ((S)-2-(N-benzylprolyl)aminobenzophenone) was easily recycled by simple filtration after acidic complex decomposition and reused for the synthesis of the initial dehydroalanine Ni(ii) complex.

Targeting Tryptophan for Tagging Through Photo-induced Electron Transfer

The chemical modification of tryptophan (Trp) has been the subject of interest for nearly 100 years, yet the development modification conditions that exploit Trp's inherent photolability have remained elusive. In this perspective, we discuss our recently reported method for Tryptophan (Trp) photobioconjugation that uses N -carbamoyl pyridinium salts to engage Trp in photo-induced electron transfer. We detail our inspiration and rationale as well as place our report in the context of select prior art in the field.

Stereoselective synthesis of unnatural α-amino acid derivatives through photoredox catalysis

A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids. The developed protocol allows the use of ubiquitous carboxylic acids as radical precursors without prior derivatization. The protocol utilizes near-stoichiometric amounts of the imine and the acid radical precursor in combination with a catalytic amount of an organic acridinium-based photocatalyst. Alternative mechanisms for the developed transformation are discussed and corroborated by experimental and computational studies.

Construction of Peptide Macrocycles via Radical-Mediated Intramolecular C-H Alkylations

Enzyme-catalyzed radical-mediated C-H functionalization reactions allow nature to create natural products of unusual three-dimensional structures from simple linear peptide precursors. In comparison, chemist's ability to harness radical C-H functionalization reactions for synthesis of complex peptides remains limited. In this work, new methods have been developed to construct peptide macrocycles via radical-mediated intramolecular C-H alkylation reactions under photoredox catalysis. Linear peptide precursors equipped with a C-terminal N-(acyloxy)phthalimide ester can cyclize with the α C-H bond of N-terminal glycine or aryl C-H bond of N-heteroarene capping units in high yield and selectivity under mild conditions. The strategy uses the C-H cyclization step to incorporate lysine, homolysine, and various heteroarene-derived amino acid linchpins into peptide macrocycles, enabling convergent and flexible synthesis of complex peptide macrocycles from simple building blocks.

Small peptide diversification through photoredox-catalyzed oxidative C-terminal modification

A photoredox-catalyzed oxidative decarboxylative coupling of small peptides is reported, giving access to a variety of N,O-acetals. They were used as intermediates for the addition of phenols and indoles, leading to novel peptide scaffolds and bioconjugates. Amino acids with nucleophilic side chains, such as serine, threonine, tyrosine and tryptophan, could also be used as partners to access tri- and tetrapeptide derivatives with non-natural cross-linking.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.