Glycopeptides by Linch‐Pin C−H Activations for Peptide‐Carbohydrate Conjugation by Manganese(I)‐Catalysis

Fluorescent coumarin-alkynes for labeling of amino acids and peptides via manganese(I)-catalyzed C-H alkenylation

The late-stage fluorescent labeling of structurally complex peptides bears immense potential for molecular imaging. Herein, we report on a manganese(I)-catalyzed peptide C-H alkenylation under exceedingly mild conditions with natural fluorophores as coumarin- and chromone-derivatives. The robustness and efficiency of the manganese(I) catalysis regime was reflected by a broad functional group tolerance and low catalyst loading in a resource- and atom-economical fashion.

Recent progress in the synthesis of natural product inspired bioactive glycohybrids

Carbohydrates are a basic structural component that are indispensable to all cellular processes. In addition to being employed as chiral starting materials in the synthesis of a variety of natural products, carbohydrates are recognized as naturally occurring molecules having an enormous variety of functional, stereochemical, and structural properties. The understanding and biological roles of carbohydrate derived molecules can be greatly improved by selectively synthesizing functional carbohydrates through incorporating them with privileged scaffolds. For a deeper understanding of their roles and the development of functional materials based on sugar, it is crucial to develop new techniques for efficiently synthesizing, functionalizing, and modifying carbohydrates. Glycohybrids have a wide range of structural and functional characteristics along with protein-carbohydrate interactions that are crucial to mammalian biology and a number of disease states. This review, consisting the literature from January 2017 to July 2023 and provide an overview of recent developments in the chemical synthesis of glycohybrids based on natural product scaffolds of coumarin, quinolone, naphthalene diimide, indole, isatin, naphthoquinone, imidazole and pyrimidine. The biological activity of active glycohybrids are discussed in this review.

Iso-Pentadienyl Carbonate as a Five Carbon Synthon in Manganese(I)-Catalyzed Selective Linear 1,3-Dienylation

Selective linear 1,3-dienylations are essential transformations, and numerous synthetic efforts have been documented. However, a general method enabling access to electron-rich, -poor, and biologically relevant dienyl molecules is in high demand. Hence, we report a straightforward method of manganese(I)-catalyzed C-H dienylation of arenes by using iso-pentadienyl carbonate as a five carbon synthon. This is a highly unprecedented report for selective linear 1,3-dienylation using manganese C-H activation catalysis. Our method facilitates the synthesis of varieties of dienes, including those suitable for normal or inverse electron demand Diels-Alder reactions, dienyl glycoconjugates, and unnatural amino acids. Extensive mechanistic studies, including isolation of C-H activated organo-manganese complex and isotopic analyses, have supported the proposed mechanism of this dienylation. The synthetic applicability of this method eased to deliver a 6/6/5-fused tricyclic nagilactone scaffold.

Robust Synthesis of Terpenoid Scaffolds under Mn(I)-Catalysis

The 6/6/5-fused tricyclic scaffold is a central feature of structurally complex terpenoid natural products. A step-economical cascade transformation that leads to a complex molecular skeleton is regarded as a sustainable methodology. Therefore, we report the first Mn(I)-catalyzed C(sp²)-H chemoselective in situ dienylation and diastereoselective intramolecular Diels-Alder reaction using iso-pentadienyl carbonate to access 6/6/5-fused tricyclic scaffolds. To the best of our knowledge, there is no such report thus far to utilize iso-pentadienyl carbonate as a substrate in C-H activation catalysis. Extensive mechanistic studies, such as the isolation of catalytically active organo-manganese(I) complexes, 1,3-dienyl-intermediates, and isotopic labeling experiments have supported the proposed mechanism of this cascade reaction.

Late-stage peptide labeling with near-infrared fluorogenic nitrobenzodiazoles by manganese-catalyzed C-H activation

Late-stage diversification of structurally complex amino acids and peptides provides tremendous potential for drug discovery and molecular imaging. Specifically, labeling peptides with fluorescent tags is one of the most important methods for visualizing their mode of operation. Despite major recent advances in the field, direct molecular peptide labeling by C-H activation is largely limited to dyes with relatively short emission wavelengths, leading to high background signals and poor signal-to-noise ratios. In sharp contrast, here we report on the fluorescent labeling of peptides catalyzed by non-toxic manganese(i) via C(sp²)-H alkenylation in chemo- and site-selective manners, providing modular access to novel near-infrared (NIR) nitrobenzodiazole-based peptide fluorogenic probes.

Non-classical C-saccharide linkage of dehydroalanine: synthesis of C-glycoamino acids and C-glycopeptides

Herein, a non-classical C-saccharide linkage is reported via a C5 radical of pentose or C6 radical of hexose addition to Michael acceptors. C(sp³)-S cleaved glycosyl thianthrenium salts are developed as the glycosyl radical agents. The reaction provides an efficient toolkit to synthesize β-glycosyl substituted unnatural amino acids as well as for the late-stage C-saccharide modification of peptides.

Glycosyl Radical-Based Synthesis of C-Glycoamino Acids and C-Glycopeptides

Radical-based reactions usually exhibit excellent functional-group compatibilities due to their mild initiation conditions. Glycosyl radical involved C-glycosylation modifications are important strategies to achieve highly regio- and chemoselective constructions of C-glycosidic bonds or C-glycoside linkages of peptides and proteins. In this Concept, we cover recent developments in glycosyl radical-based synthesis of unnatural amino acids and late-stage modification of peptides and proteins, and provide a preliminary outlook on the possible development of this direction in the future.

Visible-Light-Induced Copper-Catalyzed Asymmetric C(sp3)-C(sp3)-H Glycosylation: Access to C-Glycopeptides

Herein, a practical and highly efficient method for visible-light-induced copper-catalyzed N-aminoquinoline-directed asymmetric C(sp³)-C(sp³)-H glycosylation was reported. At the same time, C(sp³)-C(sp³)-H glycosylation of nondeoxysugars with amino acids to construct C-glycopeptides was achieved. This approach promoted the synthesis of various C-glycopeptides and provided a new model for the synthesis of C-glycoamino acids.

Selective Labeling of Peptides with o-Carboranes via Manganese(I)-Catalyzed C-H Activation

A robust method for the selective labeling of peptides via manganese(I) catalysis was devised to achieve the C-2 alkenylation of tryptophan containing peptides with 1-ethynyl-o-carboranes. The manganese-catalyzed C-H activation was accomplished with high catalytic efficiency, and featured low toxicity, high functional group tolerance and excellent E-stereoselectivity. This approach unravels a promising tool for the assembly of o-carborane with structurally complex peptides of relevance to applications in boron neutron capture therapy.

Visible-Light-Promoted Stereoselective C(sp3 )-H Glycosylation for the Synthesis of C-Glycoamino Acids and C-Glycopeptides

The glycosylative modification of peptides could improve the pharmacological properties of peptide drugs and deliver them efficiently to the target sites. Compared with O-/N-glycosides, C-glycosides exhibit more metabolic stability. We here disclose the first example of visible-light-promoted and Cu-catalyzed stereoselective C-glycosylation. The mild reaction conditions are compatible with various carbohydrate substrates, as demonstrated with a series of monosaccharides and a disaccharide, and are amenable to the synthesis of a wide variety of C-glycoamino acids and C-glycopeptidomimetics with good yields and excellent stereoselectivities. The dual-functional photocatalyst formed in situ via coordination of the glycine derivative and the chiral phosphine Cu complex could not only catalyze the photoredox process but also control the stereoselectivity of the glycosylation reaction.

Stereoselective Synthesis of C-Vinyl Glycosides via Palladium-Catalyzed C-H Glycosylation of Alkenes

C-vinyl glycosides are an important class of carbohydrates and pose a unique synthetic challenge. A new strategy has been developed for stereoselective synthesis of C-vinyl glycosides via Pd-catalyzed directed C-H glycosylation of alkenes with glycosyl chloride donors using an easily removable bidentate auxiliary. Both the γ C-H bond of allylamines and the δ C-H bond of homoallyl amine substrates can be glycosylated in high efficiency and with excellent regio- and stereoselectivity. The resulting C-vinyl glycosides can be further converted to a variety of C-alkyl glycosides with high stereospecificity. These reactions offer a broadly applicable method to streamline the synthesis of complex C-vinyl glycosides from easily accessible starting materials.

Sustainable manganese catalysis for late-stage C-H functionalization of bioactive structural motifs

The late-stage C–H functionalization of bioactive structural motifs is a powerful synthetic strategy for accessing advanced agrochemicals, bioimaging materials, and drug candidates, among other complex molecules. While traditional late-stage diversification relies on the use of precious transition metals, the utilization of 3d transition metals is an emerging approach in organic synthesis. Among the 3d metals, manganese catalysts have gained increasing attention for late-stage diversification due to the sustainability, cost-effectiveness, ease of operation, and reduced toxicity. Herein, we summarize recent manganese-catalyzed late-stage C–H functionalization reactions of biologically active small molecules and complex peptides.

Facile access to C-glycosyl amino acids and peptides via Ni-catalyzed reductive hydroglycosylation of alkynes

C-Glycosyl peptides/proteins are metabolically stable mimics of the native glycopeptides/proteins bearing O/N-glycosidic linkages, and are thus of great therapeutical potential. Herein, we disclose a protocol for the syntheses of vinyl C-glycosyl amino acids and peptides, employing a nickel-catalyzed reductive hydroglycosylation reaction of alkyne derivatives of amino acids and peptides with common glycosyl bromides. It accommodates a wide scope of the coupling partners, including complex oligosaccharide and peptide substrates. The resultant vinyl C-glycosyl amino acids and peptides, which bear common O/N-protecting groups, are amenable to further transformations, including elongation of the peptide and saccharide chains.

C-Glycosyl peptides/proteins are metabolically stable mimics of the native glycopeptides/proteins of great therapeutic potential, but their chemical synthesis is challenging. Here, the authors report a protocol for the synthesis of vinyl C-glycosyl amino acids and peptides, via a Ni-catalyzed reductive hydroglycosylation reaction of alkyne derivatives of amino acids and peptides with glycosyl bromides.

Visible-Light-Driven Redox Neutral Direct C-H Amination of Glycine Derivatives and Peptides with N-Acyloxyphthalimides

A room temperature, visible-light-promoted and redox neutral direct C-H amination of glycine and peptides has been firstly accomplished by using N-acyloxyphthalimide or -succinimide as nitrogen-radical precursor. The present strategy provides ways to introduce functionalities such as N-acyloxyphthalimide or -succinimide specifically to terminal glycine segment of peptides. Herein, mild conditions and high functional-group tolerance allow the preparation of non-natural α-amino acids and modification of corresponding peptides in this way.

Recent advances and perspectives in manganese-catalyzed C–H activation

Manganese-catalyzed C–H activation has become an emerging area in organic chemistry. These efficient and eco-friendly manganese catalysed reactions provides new opportunities in the field of synthetic organic chemistry.

Peptide late-stage C(sp3)-H arylation by native asparagine assistance without exogenous directing groups

There is a strong demand for novel native peptide motifs for post-synthetic modifications of peptides without pre-installation and subsequent removal of directing groups. Herein, we report an efficient method for peptide late-stage C(sp3)-H arylations assisted by the unmodified side chain of asparagine (Asn) without any exogenous directing group. Thereby, site-selective arylations of C(sp3)-H bonds at the N-terminus of di-, tri-, and tetrapeptides have been achieved. Likewise, we have constructed a key building block for accessing agouti-related protein (AGRP) active loop analogues in a concise manner.

Late-stage C(sp2)-H and C(sp3)-H glycosylation of C-aryl/alkyl glycopeptides: mechanistic insights and fluorescence labeling

C(sp³)–H and C(sp²)–H glycosylations of structurally complex amino acids and peptides were accomplished through the assistance of triazole peptide-isosteres. The palladium-catalyzed peptide–saccharide conjugation provided modular access to structurally complex C-alkyl glycoamino acids, glycopeptides and C-aryl glycosides, while enabling the assembly of fluorescent-labeled glycoamino acids. The C–H activation approach represents an expedient and efficient strategy for peptide late-stage diversification in a programmable as well as chemo-, regio-, and diastereo-selective fashion.

C–H glycosylations of complex amino acids and peptides were accomplished through the assistance of triazole peptide-isosteres. The palladium-catalyzed glycosylation provided access to complex C-glycosides and fluorescent-labeled glycoamino acids.

Visible-Light-Promoted C(sp3 )-H Alkylation by Intermolecular Charge Transfer: Preparation of Unnatural α-Amino Acids and Late-Stage Modification of Peptides

Disclosed herein is the visible-light-promoted deaminative C(sp³ )-H alkylation of glycine and peptides using Katritzky salts as electrophiles. Simple reaction conditions and excellent functional-group tolerance provide a general strategy for the efficient preparation of unnatural α-amino acids and precise modification of peptides with unnatural α-amino-acid residues. Mechanistic studies suggest that visible-light-promoted intermolecular charge transfer within a glycine-Katritzky salt electron donor-acceptor (EDA) complex induces a single-electron transfer process without the assistance of photocatalyst.

Synthesis of C-terminal glycopeptidesviaoxime resin aminolysis

We developed a general solid-phase approach to complex C-terminal glycopeptides.