Photoredox-catalyzed protecting-group-free C-glycosylation with glycosyl sulfinate via the Giese reaction

One step aqueous synthesis of unprotected glycosyl C-sulfonates

A one-step reaction for the production of unprotected glycosyl C-sulfonates directly from reducing sugars in aqueous solution has been developed, avoiding the use of any protecting groups. The reaction is equally applicable to disaccharides. The structure of the β-gluco C-sulfonate was confirmed by X-ray crystallography. Investigations showed that it did not inhibit almond β-glucosidase at a concentration of 100 μM.

One-Step Aqueous Synthesis of Glycosyl Pyridinium Salts, Electrochemical Study, and Assessment of Utility as Precursors of Glycosyl Radicals Using Photoredox Catalysis

A single step method for the production of unprotected glycosyl pyridinium salts has been developed involving treatment of the unprotected sugar with a pyridine, triethylamine, and either 2-chloro-1,3-dimethylimidazolinium chloride (DMC) or 2-chloro-1,3-dimethyl-1 H-benzimidazol-3-ium chloride (CDMBI) as an activator, in aqueous solution. Reaction efficiency is sensitive to steric effects, and in particular, ortho-substitution of the pyridine ring significantly decreased conversion to product; para-substitution of the pyridine ring is well tolerated. Cyclic voltammetry reveals that glycosyl pyridinium salts possess reduction potentials in the range of -0.9 to -1.4 V versus the standard calomel electrode, which are modulated by the electron effects of ring substituents. However, glycosyl pyridiniums are not found to be useful precursors for the production of glycosyl radicals under photoredox conditions.

Convenient Synthesis of β-C-Acyl Glycosides and its Application in the Synthesis of Scleropentaside A, Scleropentaside B and the Derivatives of Dapagliflozin

C-Glycosides are a common feature in numerous bioactive natural compounds and play a crucial role as mimics of O/N-glycosides. Our process for synthesizing β-C-acyl glycosides involves a reductive cross-coupling of protected glycosyl bromides with the corresponding carboxylic acid, followed by base-assisted deprotection and isomerization. This method is compatible with diverse glycosyl donors, including disaccharides. Consequently, we achieved the total synthesis of the natural products scleropentaside A and scleropentaside B with exceptional efficiency. These β-C-acyl glycosides can be readily transformed into novel forms of C-glycosides capable of disrupting signaling pathways linked to various pathological conditions, such as diabetes.

Recent Advances in Organic Photocatalyst-Promoted Carbohydrate Synthesis and Modification under Light Irradiation

Photoredox catalysis has been developed as a sustainable and eco-friendly catalytic strategy, which might provide innovative solutions to solve the current synthetic challenges and barriers in carbohydrate chemistry. During the last few decades, the study of organic photocatalyst-promoted carbohydrate synthesis and modification has received significant attention, which provides an excellent and inexpensive metal-free alternative to photoredox catalysis as well as introduces a new fastest-growing era to access complex carbohydrates simply. In this review, we aim to provide an overview of organic photocatalyst-promoted carbohydrate synthesis and modification under light irradiation, which is expected to provide new directions for further investigation.

Radical Strategy Towards N-glycosides: Current Advances and Future Prospects

N-glycosides exhibit diverse biological and pharmacological activities, making their efficient synthesis crucial for both biological research and drug development. Traditional acid-promoted N-glycosylation methods, which rely on the formation of oxocarbenium intermediates, often face significant challenges. These methods are water-sensitive and typically require neighboring group participation to achieve high selectivity. Furthermore, they depend on acid activation, rendering them incompatible with alkyl amine. Additionally, low-nucleophilicity amides often need to be converted into their TMS-derivatives to enhance reactivity, limiting the direct use of such substrates. In contrast, radical-based strategies have emerged as a promising alternative, addressing many of these limitations and leading to notable advances in N-glycosylation. This review explores the unique properties of N-glycosides, the inherent challenges of traditional N-glycosylation techniques, and the transformative advantages offered by radical-based approaches. Specifically, it highlights recent advancements in radical-mediated N-glycosylation, including photoredox radical strategies, radical/ionic hybrid approaches, and metallaphotoredox catalysis, accompanied by a detailed discussion of the underlying mechanisms. Finally, the ongoing challenges and potential future directions of N-glycoside synthesis using radical strategies are presented.

Direct Synthesis of Unprotected C-Glycosides via Photoredox Activation of Glycosyl Ester

Synthetic C-glycosides play a crucial role in molecular biology and medicine. With the surge of interest in C-glycosides and the demand to provide efforts with sufficient feedstock, it is highly significant to pursue novel methodologies to access C-glycosides in a concise and efficient manner. Here, we disclose an attractive strategy that diverges itself from conventional multistep reaction sequences involving the manipulations of protecting groups. Widely available native sugars first react with 1,4-dihydropyridine acids via a site-selective Mitsunobu reaction, converting them into bench-stable radical precursors. Under visible-light-enabled photoredox catalysis conditions, the resulting glycosyl radicals undergo C-C bond formation reactions, yielding a variety of C-glycosides with excellent stereoselectivity. Our method demonstrates good tolerance to a wide range of functional groups and has been successfully applied in the post-transformation of drug molecules and the preparation of C-glycosyl amino acids.

Site-Selective Construction of N-Linked Glycopeptides through Photoredox Catalysis

The glycosylation of peptides and proteins can significantly impact their intrinsic properties, such as conformation, stability, antigenicity, and immunogenicity. Current methods for preparing N-linked glycopeptides typically rely on amide bond formation, which can be limited by the presence of reactive functional groups like acids and amines. Late-stage functionalization of peptides offers a promising approach to obtaining N-linked glycopeptides. In this study, we demonstrate the preparation of N-linked glycopeptides through a photoredox-catalyzed site-selective Giese addition between N-glycosyl oxamic acid and peptides containing dehydroalanine (Dha) under visible light conditions. Unlike traditional methods that rely on the coupling of aspartic acid and glycosylamine, this approach utilizes the conjugation of N-glycosylated carbamoyl radicals with Dha, facilitating the straightforward modification of complex peptides.

Organometallic Photocatalyst-Promoted Synthesis and Modification of Carbohydrates under Photoirradiation

Carbohydrates are natural, renewable, chemical compounds that play crucial roles in biological systems. Thus, efficient and stereoselective glycosylation is an urgent task for the preparation of pure and structurally well-defined carbohydrates. Photoredox catalysis has emerged as a powerful tool in carbohydrate chemistry, providing an alternative for addressing some of the challenges of glycochemistry. Over the last few decades, Ir- and Ru-based organometallic photocatalysts have attracted significant interest because of their high stability, high-energy triplet state, strong visible-light absorption, long luminescence lifetime, and amenability to ligand modification. This review highlights the recent progress in the organometallic photocatalyst-promoted synthesis and modification of carbohydrates under photoirradiation, as well as the related benefits and drawbacks.

Photoredox Chemistry of Sugars without Protecting Groups: Two-Step Production of C-Glycosides via Intermediate Dihydropyridine Glycosyl Esters

Unprotected sugars are converted directly into their corresponding dihydropyridine esters, which can be activated under photoredox conditions to produce glycosyl radicals, which in turn can react with a range of electron deficient alkenes to provide C-glycosides. This method does not involve any protection of sugar hydroxyl groups and represents a simple two-step method for the conversion of reducing sugars into unprotected C-glycosides.

Light-Driven Site-Selective Glycosylation of Native Carbohydrates

Carbohydrates constitute the largest source of biomass on Earth, but their synthetic modification is challenging due to their high content in oxygen functionalities. The site- and stereoselective modification of native sugars is a definite goal of glycochemistry research. Recent efforts to bypass the need for protecting groups, leveraging selective activation through photochemical mechanisms for site-selective C-C bond formation from native sugars, are likely to largely impact all glycochemistry-related areas. Davis, Koh, and co-workers have recently presented their use of photocatalysis to develop a "cap and glycosylate" approach for the site- and stereoselective C-glycosylation of native sugars. A modernized direct radical functionalization of in situ formed thioglycoside using photocatalysis was used in the synthetic manipulation of unprotected carbohydrates. This allowed reaching complex saccharides, and post-translational modification of proteins.

C-OH Bond Activation for Stereoselective Radical C-Glycosylation of Native Saccharides

Radical C-glycosylation presents a flexible and efficient method for synthesizing C-glycosides. Existing methods always require multistep processes for generating anomeric radicals. In this study, we introduce a streamlined approach to produce anomeric radicals through direct C-OH bond homolysis of unmodified saccharides, eliminating the need for protection, deprotection, or activation steps. These anomeric radicals selectively couple with activated alkenes, yielding C-glycosylation products with high stereoselectivity (>20:1). This method is applicable to a variety of native monosaccharides, such as l-arabinose, d-arabinose, d-xylose, l-xylose, d-galactose, β-d-glucose, α-d-glucose, and l-ribose, as well as oligosaccharides including α-lactose, d-(+)-melibiose, and acarbose. We also extend this approach to C-glycosylation of amino acid and peptide derivatives, and demonstrate a streamlined synthesis of an anti-inflammatory agent.

Visible-light-promoted direct desulfurization of glycosyl thiols to access C-glycosides

C-Glycosides are essential for the study of biological processes and the development of carbohydrate-based drugs. Despite the tremendous hurdles, glycochemists have often fantasized about the efficient, highly stereoselective synthesis of C-glycosides with the shortest steps under mild conditions. Herein, we report a desulfurative radical protocol to synthesize C-alkyl glycosides and coumarin C-glycosides under visible-light induced conditions without the need of an extra photocatalyst, in which stable and readily available glycosyl thiols that could be readily obtained from native sugars are activated in situ by pentafluoropyridine. The benefits of this procedure include high stereoselectivity, broad substrate scope, and easy handling. Mechanistic studies indicate that the in situ produced tetrafluoropyridyl S-glycosides form key electron donor-acceptor (EDA) complexes with Hantzsch ester (for C-alkyl glycosides) or Et3N (for coumarin C-glycosides), which, upon irradiation with visible light, trigger a cascade of glycosyl radical processes to access C-glycosides smoothly.

Exploration of Glycosyl Dithioimidocarbonates in Photoinduced Desulfurative Cross-Coupling Reactions

Readily synthesized bench-stable glycosyl dithioimidocarbonates are useful C-glycoside precursors. Under mild photochemical conditions, these glycosides undergo desulfurative glycosyl radical generation in the presence of weak acid, 4CzIPN, and Hantzsch ester. These radicals perform well in Geise-like reactions to yield C-glycosides with high stereoselectivity.

Radical C-Glycosylation Using Photoexcitable Unprotected Glycosyl Borate

We have developed radical C-glycosylation using photoexcitable unprotected glycosyl borate. The direct excitation of glycosyl borate under visible light irradiation enabled the generation of anomeric radical without any photoredox catalysts. The in situ generated anomeric radical was applicable to the radical addition such as Giese-type addition and Minisci-type reaction to introduce alkyl and heteroaryl groups at the anomeric position. In addition, the radical-radical coupling between the glycosyl borate and acyl imidazolide provided unprotected acyl C-glycosides.

Linkage-Editing of Melibiosamine: Synthesis and Biological Evaluation of CH2- and CHF-Linked Analogs

Melibiosamine (Gal-α(1,6)-GlcNH2), consisting of galactose and glucosamine linked by an α(1,6)-glycosidic bond, is an artificial disaccharide derivative that selectively inhibits the proliferation of K562 tumor cells relative to HUC-F2 normal cells. In this study, we employed a linkage-editing strategy to synthesize CH2- and CHF-linked melibiosamine analogs through chemo- and stereoselective hydrogenation of fluorovinyl-C-glycoside. (R)-CHF-Melibiosamine exhibited more potent antiproliferative activity than O-linked melibiosamine, while (S)-CHF-melibiosamine was less potent.

A Radical Activation Strategy for Versatile and Stereoselective N-Glycosylation

Previous N-glycosylation approaches have predominately involved acidic conditions, facing challenges of low stereoselectivity and limited scope. Herein, we introduce a radical activation strategy that enables versatile and stereoselective N-glycosylation using readily accessible glycosyl sulfinate donors under basic conditions and exhibits exceptional tolerance towards various N-aglycones containing alkyl, aryl, heteroaryl and nucleobase functionalities. Preliminary mechanistic studies indicate a pivotal role of iodide, which orchestrates the formation of a glycosyl radical from the glycosyl sulfinate and subsequent generation of the key intermediate, a configurationally well-defined glycosyl iodide, which is subsequently attacked by an N-aglycone in a stereospecific SN2 manner to give the desired N-glycosides. An alternative route involving the coupling of a glycosyl radical and a nitrogen-centered radical is also proposed, affording the exclusive 1,2-trans product. This novel approach promises to broaden the synthetic landscape of N-glycosides, offering a powerful tool for the construction of complex glycosidic structures under mild conditions.

Synthesis of C-Alkyl Glycosides from Alkyl Bromides and Glycosyl Carboxylic Acids via Ni/Photoredox Dual Catalysis

C-Alkyl glycosides, an important class of C-glycosides, are widely found in various drugs and natural products. The synthesis of C-alkyl glycosides has attracted considerable attention. Herein, we developed a Ni/photoredox catalyzed decarboxylative C(sp3)-C(sp3) coupling reaction of stable glycosylcarboxylic acids with simple aliphatic bromides to generate C-alkyl glycosides. The method successfully linked several functional molecular fragments (natural products or drugs) to a sugar moiety, showing the extensive application prospects of this transformation. Controlled experiments and DFT calculations demonstrated that the reaction pathway contains a free radical process, and a possible mechanism is proposed.

Development of a flow photochemical process for a π-Lewis acidic metal-catalyzed cyclization/radical addition sequence: in situ-generated 2-benzopyrylium as photoredox catalyst and reactive intermediate

A flow photochemical reaction system for a π-Lewis acidic metal-catalyzed cyclization/radical addition sequence was developed, which utilizes in situ-generated 2-benzopyrylium intermediates as the photoredox catalyst and electrophilic substrates. The key 2-benzopyrylium intermediates were generated in the flow reaction system through the intramolecular cyclization of ortho-carbonyl alkynylbenzene derivatives by the π-Lewis acidic metal catalyst AgNTf2 and the subsequent proto-demetalation with trifluoroacetic acid. The 2-benzopyrylium intermediates underwent further photoreactions with benzyltrimethylsilane derivatives as the donor molecule in the flow photoreactor to provide 1H-isochromene derivatives in higher yields in most cases than the batch reaction system.

Transformations of carbohydrate derivatives enabled by photocatalysis and visible light photochemistry

This review article highlights the diverse ways in which recent developments in the areas of photocatalysis and visible light photochemistry are impacting synthetic carbohydrate chemistry. The major topics covered are photocatalytic glycosylations, generation of radicals at the anomeric position, transformations involving radical formation at non-anomeric positions, additions to glycals, processes initiated by photocatalytic hydrogen atom transfer from sugars, and functional group interconversions at OH and SH groups. Factors influencing stereo- and site-selectivity in these processes, along with mechanistic aspects, are discussed.