A protecting group-free approach for synthesizing C-glycosides through glycosyl dithiocarbamates

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.

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.

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.

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.

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

C-Glycoside analogs of naturally occurring glycoconjugates are useful tools for chemical biology studies, but their synthesis usually requires protection of the hydroxyl groups of the glycosyl donors. Here we report protecting-group-free and photoredox-catalyzed C-glycosylation with glycosyl sulfinates and Michael acceptors via the Giese radical addition.

Selective Anomeric Acetylation of Unprotected Sugars with Acetic Anhydride in Water

Unprotected sugars are selectively acetylated simply by stirring in aqueous solution in the presence of acetic anhydride and a weak base such as sodium carbonate. The reaction is selective for acetylation of the anomeric hydroxyl group of mannose, 2-acetamido, and 2-deoxy sugars and can be performed on a large scale. Competitive intramolecular migration of the 1-O-acetate to the 2-hydroxyl group when these two substituents are cis causes over-reaction and the formation of product mixtures.

Chemical synthesis of oligosaccharides and their application in new drug research

Oligosaccharides are the ubiquitous molecules of life. In order to translate human bioglycosylation into clinical applications, homogeneous samples of oligosaccharides and glycoconjugates can be obtained by chemical, enzymatic or other biological methods for systematic studies. However, the structural complexity and diversity of glycans and their conjugates present a major challenge for the synthesis of such molecules. This review summarizes the chemical synthesis methods of oligosaccharides, the application of oligosaccharides in the field of medicinal chemistry according to their related biological activities, and shows the great prospect of oligosaccharides in the field of pharmaceutical chemistry.

Copper-Catalyzed Stereoselective Borylation and Palladium-Catalyzed Stereospecific Cross-Coupling to Give Aryl C-Glycosides

Metabolically stable C-glycosides are an essential family of compounds in bioactive natural products, therapeutic agents, and biological probes. For their application, development of synthetic methods by connecting glycosides and aglycons with strict stereocontrol at the anomeric carbon, as well as with high functional-group compatibility and environmental compatibility is a pivotal issue. Although Suzuki-Miyaura-type C(sp³ )-C(sp² ) cross-coupling using glycosyl boronates is a potential candidate for the construction of C-glycosides, neither the cross-coupling itself nor the facile synthesis of the coupling precursor, glycosyl boronates, have been achieved to date. Herein, it was succeeded to develop a copper-catalyzed stereoselective one-step borylation of glycosyl bromides to glycosyl boronates and palladium-catalyzed stereospecific cross-coupling of β-glycosyl borates with aryl bromides to give aryl β-C-glycosides, in which the β-configuration of the anomeric carbon of the glycosyl trifluoroborates is stereoretentively transferred to that of the resulting aryl C-glycosides.

Synthesis of functionalized S-benzyl dithiocarbamates from diazo-compounds via multi-component reactions with carbon disulfide and secondary amines

Triflic acid promoted multi-component synthesis of functionalized S-benzyl dithiocarbamates from diazo compounds, carbon disulfide and secondary amines is reported. The reactions proceeded at room temperature and gave the desired dithiocarbamates in good yields. Wide-substrate scope and easy operation are the important features of this methodology.

Multicomponent Synthesis of S-Benzyl Dithiocarbamates from para-Quinone Methides and Their Biological Evaluation for the Treatment of Alzheimer's Disease

Multicomponent synthesis of biologically relevant S-benzyl dithiocarbamates from para-quinone methides, amines, and carbon disulfide are described under catalyst and additive-free conditions. The reactions proceeded at room temperature in a short span of time with excellent yields. One of the synthesized compounds, 3e showed considerable acetylcholinesterase (AChE) inhibitory (51.70 + 5.63% at 20 μm) and antioxidant (63.52 ± 1.15 at 20 μm) activities.

Protecting group free glycosylation: one-pot stereocontrolled access to 1,2-trans glycosides and (1→6)-linked disaccharides of 2-acetamido sugars

Unprotected 2-acetamido sugars may be directly converted into their oxazolines using 2-chloro-1,3-dimethylimidazolinium chloride (DMC), and a suitable base, in aqueous solution. Freeze drying and acid catalysed reaction with an alcohol as solvent produces the corresponding 1,2-trans-glycosides in good yield. Alternatively, dissolution in an aprotic solvent system and acidic activation in the presence of an excess of an unprotected glycoside as a glycosyl acceptor, results in the stereoselective formation of the corresponding 1,2-trans linked disaccharides without any protecting group manipulations. Reactions using aryl glycosides as acceptors are completely regioselective, producing only the (1→6)-linked disaccharides.