Organokatalytische Glycosylierung durch elektronenarme Pyridiniumsalze

Solvent-Free Strategy for Direct Access to Versatile Quaternary Ammonium Salts with Complete Atom Economy

A solvent-free method for the synthesis of quaternary ammonium salts (QAS) by iodoquaternization of alkenes with N-heteroarenes was reported. Its advantages lie in energy-saving and clean production by using iodine as the oxidant and manual grinding the starting materials, together with the complete atom economy and low process mass intensity (PMI) value. Demonstrated by 50 examples, the generated QAS was proved to be able to produce valuable chemicals, such as biological protease inhibitors, anti-cancer agents, and organic fluorescent materials.

Iterative Synthesis of 2-Deoxyoligosaccharides Enabled by Stereoselective Visible-Light-Promoted Glycosylation

The photoinitiated intramolecular hydroetherification of alkenols has been used to form C-O bonds, but the intermolecular hydroetherification of alkenes with alcohols remains an unsolved challenge. We herein report the visible-light-promoted 2-deoxyglycosylation of alcohols with glycals. The glycosylation reaction was completed within 2 min in a high quantum yield (ϕ=28.6). This method was suitable for a wide array of substrates and displayed good reaction yields and excellent stereoselectivity. The value of this protocol was further demonstrated by the iterative synthesis of 2-deoxyglycans with α-2-deoxyglycosidic linkages up to a 20-mer in length and digoxin with β-2-deoxyglycosidic linkages. Mechanistic studies indicated that this reaction involved a glycosyl radical cation intermediate and a photoinitiated chain process.

Deoxy sugars. General methods for carbohydrate deoxygenation and glycosidation

Deoxy sugars represent an important class of carbohydrates, present in a large number of biomolecules involved in multiple biological processes. In various antibiotics, antimicrobials, and therapeutic agents the presence of deoxygenated units has been recognized as responsible for biological roles, such as adhesion or great affinity to receptors, or improved efficacy. The characterization of glycosidases and glycosyltranferases requires substrates, inhibitors and analogous compounds. Deoxygenated sugars are useful for carrying out specific studies for these enzymes. Deoxy sugars, analogs of natural substrates, may behave as substrates or inhibitors, or may not interact with the enzyme. They are also important for glycodiversification studies of bioactive natural products and glycobiological processes, which could contribute to discovering new therapeutic agents with greater efficacy by modification or replacement of sugar units. Deoxygenation of carbohydrates is, thus, of great interest and numerous efforts have been dedicated to the development of methods for the reduction of sugar hydroxyl groups. Given that carbohydrates are the most important renewable chemicals and are more oxidized than fossil raw materials, it is also important to have methods to selectively remove oxygen from certain atoms of these renewable raw materials. The different methods for removal of OH groups of carbohydrates and representative or recent applications of them are presented in this chapter. Glycosidic bonds in general, and 2-deoxy glycosidic linkages, are included. It is not the scope of this survey to cover all reports for each specific technique.

Stereoselective O-Glycosylations by Pyrylium Salt Organocatalysis

Despite many years of invention, the field of carbohydrate chemistry remains rather inaccessible to non-specialists, which limits the scientific impact and reach of the discoveries made in the field. Aiming to increase the availability of stereoselective glycosylation chemistry for non-specialists, we have discovered that several commercially available pyrylium salts catalyze stereoselective O-glycosylations of a wide range of phenols and alkyl alcohols. This catalytic reaction utilizes trichloroacetimidates, an easily accessible and synthetically proven electrophile, takes place under air and only initiates when all three reagents are mixed, which should provide better reproducibility by non-specialists. The reaction exhibits varying degrees of stereospecificity, resulting in β-selective glycosylations from α-trichloroacetimidates, whilst an α-selective glycosylation proceeds from β-trichloroacetimidates. A mechanistic study revealed that the reaction likely proceeds via an S_N 2-like substitution on the protonated electrophile.

Stereoselective Electro-2-deoxyglycosylation from Glycals

We report a novel and highly stereoselective electro-2-deoxyglycosylation from glycals. This method features excellent stereoselectivity, scope, and functional-group tolerance. This process can also be applied to the modification of a wide range of natural products and drugs. Furthermore, a scalable synthesis of glycosylated podophyllotoxin and a one-pot trisaccharide synthesis through iterative electroglycosylations were achieved.

Direct Addition of Amides to Glycals Enabled by Solvation-Insusceptible 2-Haloazolium Salt Catalysis

The direct 2-deoxyglycosylation of nucleophiles with glycals leads to biologically and pharmacologically important 2-deoxysugar compounds. Although the direct addition of hydroxyl and sulfonamide groups have been well developed, the direct 2-deoxyglycosylation of amide groups has not been reported to date. Herein, we show the first direct 2-deoxyglycosylation of amide groups using a newly designed Brønsted acid catalyst under mild conditions. Through mechanistic investigations, we discovered that the amide group can inhibit acid catalysts, and the inhibition has made the 2-deoxyglycosylation reaction difficult. Diffusion-ordered two-dimensional NMR spectroscopy analysis implied that the 2-chloroazolium salt catalyst was less likely to form aggregates with amides in comparison to other acid catalysts. The chlorine atom and the extended π-scaffold of the catalyst played a crucial role for this phenomenon. This relative insusceptibility to inhibition by amides is more responsible for the catalytic activity than the strength of the acidity.

Mechanism-Based Condition Screening for Sustainable Catalysis in Single-Electron Steps by Cyclic Voltammetry

A cyclic-voltammetry-based screening method for Cp₂ TiX-catalyzed reactions is introduced. Our mechanism-based approach enables the study of the influence of various additives on the electrochemically generated active catalyst Cp₂ TiX, which is in equilibrium with catalytically inactive [Cp₂ TiX₂ ]^- . Thioureas and ureas are most efficient in the generation of Cp₂ TiX in THF. Knowing the precise position of the equilibrium between Cp₂ TiX and [Cp₂ TiX₂ ]^- allowed us to identify reaction conditions for the bulk electrolysis of Cp₂ TiX₂ complexes and for Cp₂ TiX-catayzed radical arylations without having to carry out the reactions. Our time- and resource-efficient approach is of general interest for the design of catalytic reactions that proceed in single-electron steps.

Palladium-Catalyzed Direct Stereoselective Synthesis of Deoxyglycosides from Glycals

Palladium(II) in combination with a monodentate phosphine ligand enables the unprecedented direct and α-stereoselective catalytic synthesis of deoxyglycosides from glycals. Initial mechanistic studies suggest that in the presence of N-phenyl-2-(di-tert-butylphosphino)pyrrole as the ligand, the reaction proceeds via an alkoxy palladium intermediate that increases the proton acidity and oxygen nucleophilicity of the alcohol. The method is demonstrated with a wide range of glycal donors and acceptors, including substrates bearing alkene functionalities.

Stereoselective Koenigs-Knorr Glycosylation Catalyzed by Urea

A stereoselective Koenigs-Knorr glycosylation reaction under the catalysis of urea is described. This method is characterized by urea-mediated hydrogen-bond activation and subsequent glycosylation with glycosyl chlorides or bromides. Excellent yields and high anomeric selectivity can be achieved in most cases. Moreover, the low α-stereoselectivity of glycosylations observed when using perbenzylated glucosyl donors can be greatly improved by the addition of tri-(2,4,6-trimethoxyphenyl)phosphine (TTMPP).