Metal‐Catalyzed C(sp 2 )−H Functionalization Processes of Phenylalanine‐ and Tyrosine‐Containing Peptides

o-Halogenation and -Alkoxylation of Phenylglycine Derivatives by Pd-Mediated C-H Functionalization: Scope and Limitations

Orthopalladated derivatives from substituted phenylglycines [Pd(μ-Cl)(C6H3R1C(R2)(R3)N(R4)2]2 (1) react with halogenating reagents (PhICl2, Br2, I2) (2) to give the corresponding o-halogenated amino acids C6H3(X)R1C(R2)(R3)N(R4)2 (3). The reaction is general and tolerates a variety of functional groups (R1 to R4) at the aryl ring, the Cα, and the N atom. On the other hand, the reaction of [Pd(μ-Cl)(C6H3R1C(R2)(R3)N(R4)2]2 (1) with PhI(OAc)2 in the presence of a variety of alcohols R5OH (4) gives the o-alkoxylated phenylglycines C6H3(OR5)R1C(R2)(R3)N(R4)2 (5), also as a general process. A partial loss of the enantiomeric excess is observed when the starting phenylglycine is enantiomerically pure, this arising from the formation of bridging azavinylidene (6) and imine intermediate species (7), which were characterized by X-ray diffraction methods.

Late-Stage Radical C-H Alkylamination of Tyrosine Compounds and Phenol-Containing Drugs

A modular site-selective iron-catalyzed radical amination of a number of phenol-containing biomolecules such as tyrosine-containing peptides, estrogens, and other phenol-based pharmaceuticals has been developed. The method features the use of the cost-efficient combination of FeBr3 as catalyst along with triflic acid as Brønsted acid, thereby enabling the predictable appendance of morpholine and related heterocycles at the ortho C-H bond of phenols in a late-stage fashion.

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.

Iridium-catalyzed diacylmethylation of tyrosine and its peptides with sulfoxonium ylides

Pyridyloxy-directed Ir(III)-catalyzed diacylmethylation of protected tyrosines was achieved with alkyl and (hetero)aryl sulfoxonium ylides, furnishing tyrosine-based unnatural amino acids in good yields. Furthermore, the late stage exemplification of the strategy was successfully accomplished in tyrosine-containing dipeptides, tripeptides and tetrapeptides in moderate yields. This methodology is distinguished by its site-selectivity, tolerance of sensitive functional groups, scalability, and retention of the chiral configuration for tyrosine motifs.

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

Organocatalytic asymmetric synthesis has evolved over the years and continues to attract the interest of many researchers worldwide. Enantiopure noncanonical amino acids (ncAAs) are valuable building blocks in organic synthesis, medicinal chemistry, and chemical biology. They are employed in the elaboration of peptides and proteins with enhanced activities and/or improved properties compared to their natural counterparts, as chiral catalysts, in chiral ligand design, and as chiral building blocks for asymmetric syntheses of complex molecules, including natural products. The linkage of ncAA synthesis and enantioselective organocatalysis, the subject of this perspective, tries to imitate the natural biosynthetic process. Herein, we present contemporary and earlier developments in the field of organocatalytic activation of simple feedstock materials, providing potential ncAAs with diverse side chains, unique three-dimensional structures, and a high degree of functionality. These asymmetric organocatalytic strategies, useful for forging a wide range of C-C, C-H, and C-N bonds and/or combinations thereof, vary from classical name reactions, such as Ugi, Strecker, and Mannich reactions, to the most advanced concepts such as deracemisation, transamination, and carbene N-H insertion. Concurrently, we present some interesting mechanistic studies/models, providing information on the chirality transfer process. Finally, this perspective highlights, through the diversity of the amino acids (AAs) not selected by nature for protein incorporation, the most generic modes of activation, induction, and reactivity commonly used, such as chiral enamine, hydrogen bonding, Brønsted acids/bases, and phase-transfer organocatalysis, reflecting their increasingly important role in organic and applied chemistry.

Split cross-coupling via Rh-catalysed activation of unstrained aryl-aryl bonds

Constructive functionalization of unstrained aryl-aryl bonds has been a fundamental challenge in organic synthesis due to the inertness of these bonds. Here we report a split cross-coupling strategy that allows two-fold arylation with diverse aryl iodides through cleaving unstrained aryl-aryl bonds of common 2,2'-biphenols. The reaction is catalyzed by a rhodium complex and promoted by a removable phosphinite directing group and an organic reductant. The combined experimental and computational mechanistic studies reveal a turnover-limiting reductive elimination step that can be accelerated by a Lewis acid co-catalyst. The utility of this coupling method has been illustrated in the modular and simplified syntheses of unsymmetrical 2,6-diarylated phenols and skeletal insertion of phenyl units.

Backbone-enabled modification of peptides with benzoquinone via palladium-catalyzed δ-C(sp2)-H functionalization

Backbone-enabled site-selective modification of peptides with benzoquinone via Pd-catalyzed δ-C(sp2)-H functionalization has been achieved. The amide groups of peptides serve as internal directional groups, facilitating C-H functionalization through a kinetically less favored six-membered palladacycle. This methodology presents novel opportunities for the late-stage site-selective diversification of peptides.

Picolinamide-assisted ortho-C-H functionalization of pyrenylglycine derivatives using aryl iodides

Chemical transformations involving the pyrenylglycine motif (an unnatural amino acid) and practical methods toward it are seldom known. This work aimed at developing a method for synthesizing novel pyrenylglycine (pyrene-based glycine) unnatural amino acid derivatives. To realize this, initially, a new pyrenylglycine substrate possessing the picolinamide moiety was assembled via the Ugi multicomponent reaction. The picolinamide moiety linked to amine substrates is a well-known bidentate directing group for accomplishing the site-selective γ-C-H functionalization of amines. Subsequently, it was aimed at using a Pd(II)-catalyzed bidentate directing group-aided γ-C-H arylation strategy for generating a wide range of unprecedented examples of C(2)-H arylated pyrenylglycines. Accordingly, pyrenylglycine possessing the picolinamide moiety was subjected to Pd(II)-catalyzed C(2)-H arylation in the non-K-region to afford a library of C(2)-arylated pyrenylglycines (π-extended pyrenes). Additionally, pyrenylglycine-based small peptides were assembled using C(2)-arylated pyrenylglycines. The X-ray structure of a representative compound was obtained, which corroborated the structure of pyrenylglycine and the regioselectivity of C(2)-H arylation of the pyrene in the non-K-region.

Rhodium-Catalyzed Regioselective C3Ar Functionalization of Tyrosines with Maleimides and Its Late-Stage Peptide Exemplification

Pyridyloxy-directed Rh(III)-catalyzed regioselective C3Ar-H alkenylation of protected tyrosines was achieved with N-aryl and N-alkyl maleimides, furnishing a series of maleimide-appended tyrosine-based unnatural amino acids in good yields. Further, the late-stage exemplification of the strategy was successfully accomplished on tyrosine-containing dipeptides, tripeptides, and tetrapeptides in moderate reactivity. Also, the chemical applications of the strategy were successfully executed toward nailing tyrosine with other amino acids via a maleimide linker and intramolecular hydroarylation to produce tyrosine-centered stapled products and succinimide-glued macrocyclized products, respectively.

Recent developments in the cleavage, functionalization, and conjugation of proteins and peptides at tyrosine residues

Peptide and protein selective modification at tyrosine residues has become an exploding field of research as tyrosine constitutes a robust alternative to lysine and cysteine-targeted traditional peptide/protein modification protocols. This review offers a comprehensive summary of the latest advances in tyrosine-selective cleavage, functionalization, and conjugation of peptides and proteins from the past three years. This updated overview complements the extensive body of work on site-selective modification of peptides and proteins, which holds significant relevance across various disciplines, including chemical, biological, medical, and material sciences.

Site-Selective C-H Amination of Phenol-Containing Biomolecules

A C-N bond-forming cross-dehydrogenative coupling of a collection of Tyr-containing peptides and estrogens with heteroarenes is described. This oxidative coupling is distinguished by its scalability, operational simplicity, and air tolerance and enables the appendance of phenothiazines and phenoxazines in phenol-like compounds. When incorporated into a Tb(III) metallopeptide, the Tyr-phenothiazine moiety acts as a sensitizer for the Tb(III) ion, providing a new tool for the design of luminescent probes.

Palladium-Catalyzed C(sp2)-H Arylation of Peptides Directed by Aspartic Acid

Herein, we report a palladium-catalyzed C(sp²)-H di- or monoarylation of short peptides containing N-terminal benzamide groups using aspartic acid (Asp) as an endogenous directing group. This strategy has the following merits: a broad substrate scope, selective diarylation of peptides, and gram-scale synthesis. Furthermore, this strategy can be successfully utilized to synthesize peptide-peptide conjugates.

Rhodium-Catalyzed Annulation of Vinylated Tyrosines with Internal Alkynes to Access Oxepine-Mounted Unnatural Tyrosines and Its Peptide Late Stage Functionalization

A Rh(III)-catalyzed [5+2] annulation of vinyl tyrosines with symmetrical and unsymmetrical internal alkynes was achieved, furnishing a series of oxepine-mounted tyrosine-based unnatural amino acids. In addition, the chemical applicability of the developed strategy was exemplified by stapling amino acid/peptide-appended alkynes with vinyl tyrosines and late stage functionalization of tyrosine-containing dipeptides and tripeptide with internal alkynes.

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.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

Amine-Directed Palladium-Catalyzed C-H Halogenation of Phenylalanine Derivatives

An efficient primary-amine-directed, palladium-catalyzed C-H halogenation (X=I, Br, Cl) of phenylalanine derivatives is reported on a range of quaternary amino acid (AA) derivatives thanks to suitable conditions employing trifluoroacetic acid as additive. The extension of this original native functionality-directed ortho-selective halogenation was even demonstrated with the more challenging native phenylalanine as tertiary AA.

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.