Post-Assembly Modification of Peptides by Ligand-Enabled β-C(sp3)-H Arylation of Alanine at the C-Terminus: Overcoming the Inhibition Effect of Peptide Bonds

Methionine-enabled peptide modification through late-stage Pd-catalyzed β-C(sp3)-H olefination/cyclization

We present a method for site-selective diversification of peptides via Pd-catalyzed β-C(sp3)-H olefination/cyclization. In this protocol, the native methionine residue acts as a directing group, enabling site-specific olefination/cyclization of peptides. This chemistry demonstrates broad substrate scope, offering a versatile tool for peptide ligation.

Postassembly Modification of Peptides by Histidine-Directed β-C(sp3)-H Arylation of Alanine at the Internal Positions: Overcoming the Inhibitory Effect of Peptide Bonds

Peptide modification by C(sp3)-H functionalization of residues at the internal positions remains underdeveloped due to the inhibitory effect of backbone amides. In this study, using histidine (His) as an endogenous directing group, we developed a novel method for the β-C(sp3)-H functionalization of alanine (Ala) at diverse positions of peptides. Through this approach, a wide range of linear peptides were modified on the side-chain of Ala adjacent to His to afford the functionalized peptides in moderate to good yield and excellent position selectivity. Furthermore, conjugation of peptides with functional molecules such as glucuronide, oleanolic acid, dipeptide, and fluorophore derivatives was achieved.

Site-Selective Modification of Peptides via Late-Stage Pd-Catalyzed Tandem Reaction of Phenylalanine with Benzoquinone

An efficient and straightforward approach for site-selective functionalization of phenylalanine and phenylalanine-containing peptide via a Pd-catalyzed tandem reaction has been developed. The robust method underwent dual C-H activation, including C-C coupling with benzoquinone and intramolecular C-N cyclization, providing a feasible and rapid synthetic route to incorporate 4-benzoquinone-indoline fragments into peptides.

Decatungstate-Catalyzed C(sp3)-H Alkylation of a Val Residue Proximal to the N-Terminus Controlled by an Electrostatic Interaction

The decatungstate photocatalyst [W₁₀O₃₂]^4- efficiently promoted the C(sp³)-H alkylation of the trifluoroacetic acid salt of valine methyl ester (H-Val-OMe·TFA) with electron-deficient alkenes under UV irradiation. The electrostatic interaction between the cationic ammonium group (⁺NH₃) of the main chain and anionic [W₁₀O₃₂]^4- played an important role in this reaction. The influence of various protected amino acids in the C(sp³)-H alkylation was investigated as the model reaction for the alkylation of Val-containing peptides. The introduction of an alkyne moiety into Val through this alkylation was successful, and successive copper-catalyzed azide-alkyne cycloaddition (CuAAC) was demonstrated. The C(sp³)-H bond of a Val residue located at the second from the N-terminus was also successfully converted. C(sp³)-H alkylation of oligopeptides containing two Val residues selectively proceeded proximally to the N-terminus.

Post-Assembly Modification of Head-to-Tail Cyclic Peptides by Methionine-Directed β-C(sp3)-H Arylation

Peptide modification by C(sp³)-H functionalization of internal residues remains a major challenge due to the inhibitory effect of peptide bonds. In this work, we developed a methionine-directed β-C(sp³)-H arylation method for internal alanine functionalization. By tuning the σ_C-C bond rotation of internal Ala through head-to-tail cyclization, we overcame the inhibitory effect and functionalized a wide range of head-to-tail cyclic peptides with aryl iodides with excellent position selectivity.

Macrocyclization of bioactive peptides with internal thiazole motifs via palladium-catalyzed C-H olefination

Peptides containing thiazole fragments represent a large group of bioactive compounds with potential medicinal applications. However, methods for efficient synthesis of these compounds with structural diversity are limited. Herein, we report a method for modification and macrocyclization of thiazole-containing peptides through palladium-catalyzed δ-C(sp²)-H olefination. In this protocol, the thiazole and neighboring amide bonds act as directing groups, which allows site-specific olefination of phenylalanine, tryptophan and tyrosine residues. This chemistry exhibits broad substrate scope and provides facile access to peptide-peptide conjugates and peptide macrocycles. Our results highlight the potency and applicability of thiazole motifs in promoting Pd-catalyzed functionalization of peptides.