C3 Epimerization of Glucose, via Regioselective Oxidation and Reduction

One-step synthesis of Ling's tetrol and its conversion into A,D-di-allo-α-cyclodextrin derivatives

2A-F,3B,C,E,F,6B,C,E,F-Tetradeca-O-benzyl-α-cyclodextrin or Ling's tetrol is a unique α-cyclodextrin derivative that is partially protected with specific access points on both rims of the cyclodextrin structure. Ling's tetrol is therefore potentially useful for the synthesis of more complex and sophisticated enzyme models and supramolecular structures. However, the original synthesis gave only 10% yield after a reaction time of 4 days, and a recent improvement that gave 52% yield required two steps and a reaction time in one step of 6 days. Here, a single-step synthesis is reported where Ling's tetrol is obtained in a yield of 59% with a reaction time of 40 hours. 2A-F,3B,C,E,F,6B,C,E,F-Tetradeca-O-benzyl-α-cyclodextrin was subsequently converted into 6A,D-dicarboxy-3A,D-diepi-α-cyclodextrin, 3A,D-dioxo-α-cyclodextrin and 3A,D-diamino-3A,D-dideoxy-3A,D-diepi-α-cyclodextrin. The binding of these compounds to CH4 and CO2 was determined.

Catalytic Approaches to Chemo- and Site-Selective Transformation of Carbohydrates

Carbohydrates are a fundamental unit playing pivotal roles in all the biological processes. It is thus essential to develop methods for synthesizing, functionalizing, and manipulating carbohydrates for further understanding of their functions and the creation of sugar-based functional materials. It is, however, not trivial to develop such methods, since carbohydrates are densely decorated with polar and similarly reactive hydroxy groups in a stereodefined manner. New approaches to chemo- and site-selective transformations of carbohydrates are, therefore, of great significance for revolutionizing sugar chemistry to enable easier access to sugars of interest. This review begins with a brief overview of the innate reactivity of hydroxy groups of carbohydrates. It is followed by discussions about catalytic approaches to enhance, override, or be orthogonal to the innate reactivity for the transformation of carbohydrates. This review avoids making a list of chemo- and site-selective reactions, but rather focuses on summarizing the concept behind each reported transformation. The literature references were sorted into sections based on the underlying ideas of the catalytic approaches, which we hope will help readers have a better sense of the current state of chemistry and develop innovative ideas for the field.

Site-Selective Electrochemical Oxidation of Glycosides

Quinuclidine-mediated electrochemical oxidation of glycopyranosides provides C3-ketosaccharides with high selectivity and good yields. The method is a versatile alternative to Pd-catalyzed or photochemical oxidation and is complementary to the 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated C6-selective oxidation. Contrary to the electrochemical oxidation of methylene and methine groups, the reaction proceeds without oxygen.

Site-Selective Modification of (Oligo)Saccharides

Oligosaccharides, either as such or as part of glycolipids, glycopeptides, or glycoproteins, are ubiquitous in nature and fulfill important roles in the living cell. Also in medicine and to some extent in materials, oligosaccharides play an important role. In order to study their function, modifying naturally occurring oligosaccharides, and building in reactive groups and reporter groups in oligosaccharides, are key strategies. The development of oligosaccharides as drugs, or vaccines, requires the introduction of subtle modifications in the structure of oligosaccharides to optimize efficacy and, in the case of antibiotics, circumvent bacterial resistance. Provided the natural oligosaccharide is available, site-selective modification is an attractive approach as total synthesis of the target is often very laborious. Researchers in catalysis areas, such as transition-metal catalysis, enzyme catalysis, organocatalysis, and photoredox catalysis, have made considerable progress in the development of site-selective and late-stage modification methods for mono- and oligosaccharides. It is foreseen that the fields of enzymatic modification of glycans and the chemical modification of (oligo)saccharides will approach and potentially meet each other, but there is a lot to learn and discover before this will be the case.

Selective Axial-to-Equatorial Epimerization of Carbohydrates

Radical-mediated transformations have emerged as powerful methods for the synthesis of rare and unnatural branched, deoxygenated, and isomeric sugars. Here, we describe a radical-mediated axial-to-equatorial alcohol epimerization method to transform abundant glycans into rare isomers. The method delivers highly predictable and selective reaction outcomes that are complementary to other sugar isomerization methods. The synthetic utility of isomer interconversion is showcased through expedient glycan synthesis, including one-step glycodiversification. Mechanistic studies reveal that both site- and diastereoselectivities are achieved by highly selective H atom abstraction of equatorially disposed α-hydroxy C-H bonds.

General Strategy for the Synthesis of Rare Sugars via Ru(II)-Catalyzed and Boron-Mediated Selective Epimerization of 1,2-trans-Diols to 1,2-cis-Diols

Human glycans are primarily composed of nine common sugar building blocks. On the other hand, several hundred monosaccharides have been discovered in bacteria and most of them are not readily available. The ability to access these rare sugars and the corresponding glycoconjugates can facilitate the studies of various fundamentally important biological processes in bacteria, including interactions between microbiota and the human host. Many rare sugars also exist in a variety of natural products and pharmaceutical reagents with significant biological activities. Although several methods have been developed for the synthesis of rare monosaccharides, most of them involve lengthy steps. Herein, we report an efficient and general strategy that can provide access to rare sugars from commercially available common monosaccharides via a one-step Ru(II)-catalyzed and boron-mediated selective epimerization of 1,2-trans-diols to 1,2-cis-diols. The formation of boronate esters drives the equilibrium toward 1,2-cis-diol products, which can be immediately used for further selective functionalization and glycosylation. The utility of this strategy was demonstrated by the efficient construction of glycoside skeletons in natural products or bioactive compounds.

Photocatalytic, site-selective oxidations of carbohydrates

Site-selective oxidations of carbohydrates, employing acridinium photocatalysis and quinuclidine hydrogen atom transfer catalysis, are presented. Protocols have been developed for oxidations of all-equatorial carbohydrates as well as those containing cis-1,2-diols. Site-selectivity reflects the relative rates of hydrogen atom transfer from the carbohydrate C-H bonds, and can be enhanced using a phosphate hydrogen-bonding or boronic acid catalyst.

Selective Transformations of Carbohydrates Inspired by Radical-Based Enzymatic Mechanisms

Enzymes are a longstanding source of inspiration for synthetic reaction development. However, enzymatic reactivity and selectivity are frequently untenable in a synthetic context, as the principles that govern control in an enzymatic setting often do not translate to small molecule catalysis. Recent synthetic methods have revealed the viability of using small molecule catalysts to promote highly selective radical-mediated transformations of minimally protected sugar substrates. These transformations share conceptual similarities with radical SAM enzymes found in microbial carbohydrate biosynthesis and present opportunities for synthetic chemists to access microbial and unnatural carbohydrate building blocks without the need for protecting groups or lengthy synthetic sequences. Here, we highlight strategies through which radical reaction pathways can enable the site-, regio-, and diastereoselective transformation of minimally protected carbohydrates in both synthetic and enzymatic systems.

A Unified Strategy to Access 2- and 4-Deoxygenated Sugars Enabled by Manganese-Promoted 1,2-Radical Migration

The selective manipulation of carbohydrate scaffolds is challenging due to the presence of multiple, nearly chemically indistinguishable O-H and C-H bonds. As a result, protecting-group-based synthetic strategies are typically necessary for carbohydrate modification. Here we report a concise semisynthetic strategy to access diverse 2- and 4-deoxygenated carbohydrates without relying on the exhaustive use of protecting groups to achieve site-selective reaction outcomes. Our approach leverages a Mn²⁺-promoted redox isomerization step, which proceeds via sugar radical intermediates accessed by neutral hydrogen atom abstraction under visible light-mediated photoredox conditions. The resulting deoxyketopyranosides feature chemically distinguishable functional groups and are readily transformed into diverse carbohydrate structures. To showcase the versatility of this method, we report expedient syntheses of the rare sugars l-ristosamine, l-olivose, l-mycarose, and l-digitoxose from commercial l-rhamnose. The findings presented here validate the potential for radical intermediates to facilitate the selective transformation of carbohydrates and showcase the step and efficiency advantages attendant to synthetic strategies that minimize a reliance upon protecting groups.

Efficient Allitol Bioproduction from D-Fructose Catalyzed by Recombinant E. coli Whole Cells, and the Condition Optimization, Product Purification

Allitol is a kind of rare sugar alcohol with potential application value. An engineered strain, which simultaneously expressed D-psicose-3-epimerase (DPE), ribitol dehydrogenase (RDH), and formate dehydrogenase (FDH) three enzymes, was constructed by cloning above three genes into one plasmid and transformed into the host E. coli strain, and used as the whole-cell catalysts for biotransformation of allitol from the low-cost substrate of D-fructose. The whole cell allitol biotransformation conditions were optimized. The medium, recombinant gene induction conditions, and the substrate feeding rate for cultivation of the catalytic cells were optimized. Then, the fed-batch culture was made and scaled up to 10 L fermentor. Finally, 63.44 g/L allitol was obtained from 100 g/L D-fructose after 3 h of biotransformation, and the allitol crystals of 99.9% purity were obtained by using cooling recrystallization. The allitol production method developed in this research has high product purity, and is highly efficient, easily scaled up, and suitable for large-scale production of highly purified allitol.

Selective Modification of Streptozotocin at the C3 Position to Improve Its Bioactivity as Antibiotic and Reduce Its Cytotoxicity towards Insulin-Producing β Cells

With the increasing resistance of bacteria to current antibiotics, novel compounds are urgently needed to treat bacterial infections. Streptozotocin (STZ) is a natural product that has broad-spectrum antibiotic activity, albeit with limited use because of its toxicity to pancreatic β cells. In an attempt to derivatize STZ through structural modification at the C3 position, we performed the synthesis of three novel STZ analogues by making use of our recently developed regioselective oxidation protocol. Keto-STZ (2) shows the highest inhibition of bacterial growth (minimum inhibitory concentration (MIC) and viability assays), but is also the most cytotoxic compound. Pre-sensitizing the bacteria with GlcNAc increased the antimicrobial effect, but did not result in complete killing. Interestingly, allo-STZ (3) revealed moderate concentration-dependent antimicrobial activity and no cytotoxicity towards β cells, and deoxy-STZ (4) showed no activity at all.

Regioselective Manipulation of GlcNAc Provides Allosamine, Lividosamine, and Related Compounds

Palladium-catalyzed oxidation of isopropyl N-acetyl-α-d-glucosamine (GlcNAc) is used to prepare the rare sugars allosamine, lividosamine, and related compounds with unprecedented selectivity. The Passerini reaction applied on 3-keto-GlcNAc provides an entry into branching of the carbon skeleton in this compound.

Regioselective Carbohydrate Oxidations: A Nuclear Magnetic Resonance (NMR) Study on Selectivity, Rate, and Side-Product Formation

Palladium/neocuproine catalyzed oxidation of glucosides shows an excellent selectivity for the C3-OH, but in mannosides and galactosides, unselective oxidation was initially observed. For further application in more-complex (oligo)saccharides, a better understanding of the reaction, in terms of selectivity and reactivity, is required. Therefore, a panel of different glycosides was synthesized, subjected to palladium/neocuproine catalyzed oxidation and subsequently analyzed by qNMR. Surprisingly, all studied glucosides, mannosides, galactosides, and xylosides show selective oxidation of the C3-OH. However, subsequent reaction of the resulting ketone moiety is the main culprit for side product formation. Measures are reported to suppress these side reactions. The observed differences in reaction rate, glucosides being the most rapidly oxidized, may be exploited for the selective oxidation of complex oligosaccharides.

Site-selective carbon–carbon bond formation in unprotected monosaccharides using photoredox catalysis

One-step site-selective, protection group-free synthesis of branched monosaccharides.