Naturally occurring monosaccharides: properties and synthesis

Synthesis of the Three Most Expensive l-Hexose Thioglycosides from d-Glucose

The biologically important l-hexoses, which are less widespread than d-hexoses, cannot be obtained from natural sources or can only be extracted very costly. Due to the complexity of their synthesis, their commercially available derivatives (which are sold mostly in free form) are also very expensive, which is further exacerbated by the current rapid rise in prices. In the present work, starting from the cheapest d-hexose, d-glucose, using inexpensive and readily available chemicals, a reaction pathway was developed in which the three most expensive l-hexoses (l-idose, l-altrose, and l-talose) were successfully prepared in orthogonally protected thioglycoside form, ready for glycosylation. The l-ido and l-talo derivatives were synthesized by C-5 epimerization of the corresponding 5,6-unsaturated thioglycosides. From the l-ido derivatives, the orthogonally protected thioglycosides of l-altrose were then prepared by C-4 epimerization. Different approaches to the preparation of the key intermediates, 5,6-unsaturated thioglycoside derivatives, were systematically investigated in the presence of various protecting groups (ether and ester) and using commercially available reagents.

Light-empowered contra-thermodynamic stereochemical editing

Creating, conserving and modifying the stereochemistry of organic compounds has been the subject of significant research efforts in synthetic chemistry. Most synthetic routes are designed according to the stereoselectivity-determining step. Stereochemical editing is an alternative strategy, wherein the chiral-defining or geometry-defining steps are independent of the construction of the major scaffold or complexity. It enables late-stage alterations of stereochemistry and can generate isomers from a single compound. However, in many instances, stereochemical editing processes are contra-thermodynamic, meaning the transformation is unfavourable. To overcome this barrier, photocatalysis uses photogenerated radical species and introduces thermochemical biases. A range of synthetically valuable contra-thermodynamic stereochemical editing processes have been invented, including deracemization of chiral molecules, positional alkene isomerization and dynamic epimerization of sugars and diols. In this Review, we highlight the fundamental mechanisms of visible-light photocatalysis and the general reactivity modes of the photogenerated radical intermediates towards contra-thermodynamic stereochemical editing processes.

Direct Synthesis of 2,6-Dideoxy-β-glycosides and β-Rhamnosides with a Stereodirecting 2-(Diphenylphosphinoyl)acetyl Group

Anomeric stereocontrol is usually one of the major issues in the synthesis of complex carbohydrates, particularly those involving β-configured 2,6-dideoxyglycoside and d/l-rhamnoside moieties. Herein, we report that 2-(diphenylphosphinoyl)acetyl is highly effective as a remote stereodirecting group in the direct synthesis of these challenging β-glycosides under mild conditions. A deoxy-trisaccharide as a mimic of the sugar chain of landomycin E was prepared stereospecifically in high yield. The synthetic potential was also highlighted in the synthesis of Citrobacter freundii O-antigens composed of a [→4)-α-d-Manp-(1→3)-β-d-Rhap(1→4)-β-d-Rhap-(1→] repeating unit, wherein the convergent assembly up to a nonasaccharide was realized with a strongly β-directing trisaccharide donor. Variable-temperature NMR studies indicate the presence of intermolecular H-bonding between the donor and the bulky acceptor as direct spectral evidence in support of the concept of hydrogen-bond-mediated aglycone delivery.

Synthesis of Four Orthogonally Protected Rare l-Hexose Thioglycosides from d-Mannose by C-5 and C-4 Epimerization

l-Hexoses are important components of biologically relevant compounds and precursors of some therapeuticals. However, they typically cannot be obtained from natural sources and due to the complexity of their synthesis, their commercially available derivatives are also very expensive. Starting from one of the cheapest d-hexoses, d-mannose, using inexpensive and readily available chemicals, we developed a reaction pathway to obtain two orthogonally protected l-hexose thioglycoside derivatives, l-gulose and l-galactose, through the corresponding 5,6-unsaturated thioglycosides by C-5 epimerization. From these derivatives, the orthogonally protected thioglycosides of further two l-hexoses (l-allose and l-glucose) were synthesized by C-4 epimerization. The preparation of the key intermediates, the 5,6-unsaturated derivatives, was systematically studied using various protecting groups. By the method developed, we are able to produce highly functionalized l-gulose derivatives in 9 steps (total yields: 21–23%) and l-galactose derivatives in 12 steps (total yields: 6–8%) starting from d-mannose.

ROESY and 13C NMR to distinguish between D- and L-rhamnose in the α-D-Manp-(1 → 4)-β-Rhap-(1 → 3) repeating motif

Many children suffering from autism spectrum disorder (ASD) experience gastrointestinal (GI) conditions. Enterocloster bolteae has been regularly detected in the stool of individuals suffering from GI symptoms and autism. Literature has suggested that E. bolteae strains WAL 16351 and WAL 14578 produce an immunogenic capsular polysaccharide (CPS) comprised of disaccharide repeating units: α-D-Man-(1 → 4)-β-Rha-(1 → 3) that could be used for the development of an immunotherapeutic vaccine. Ambiguity in the configuration of rhamnose led to the synthesis of tri- and disaccharide analogues containing D-rhamnose and L-rhamnose, respectively. ROESY-NMR spectra showed that CH₃-6 of rhamnose and H-2 of mannose in the L-Rha containing disaccharide gave correlation. No such correlation was seen between the CH₃-6 of rhamnose and the H-2 of mannose in the D-Rha containing trisaccharide. Molecular dynamics studies on hexasaccharide containing L-Rha or D-Rha confirmed that these structures adopt conformations resulting in different distances between the C6-rhamnose and the H-2 mannose of the preceding residue. We also demonstrate that assignment of the absolute configuration of the rhamnosyl residue in the β-Rhap-(1 → 3)-D-Man linkage can be determined using the ¹³C chemical shift of C-2 in of D-Mannose. While β-D-Rha will lead to an upfield shift of C-2 due to γ-gauche interaction between H-1 Rha and H-2 Man, β-L-Rha will not. Our results provide insights to distinguish between D- and L-rhamnose in the α-D-Manp-(1 → 4)-β-Rhap-(1 → 3) repeating motif.

Synthesis of Reverse Glycosyl Fluorides and Rare Glycosyl Fluorides Enabled by Radical Decarboxylative Fluorination of Uronic Acids

An efficient protocol for synthesizing reverse glycosyl fluorides is described, relying on silver-promoted decarboxylative fluorination of structurally diverse pentofuran- and hexopyranuronic acids under the mild reaction conditions. The potential applications of the reaction are further demonstrated by converting readily available d-uronic acid derivatives into uncommon d-/l-glycosyl fluorides through a C1-to-C5 switch strategy. The reaction mechanism is corroborated by 5-exo-trig radical cyclization of allyl α-d-C-glucopyranuronic acid triggered by decarboxylative fluorination.

From d- to l-Monosaccharide Derivatives via Photodecarboxylation-Alkylation

Photodecarboxylation-alkylation of conformationally locked monosaccharides leads to inversion of stereochemistry at C5. This allows the synthesis of l-sugars from their readily available d-counterparts. Via this strategy, methyl l-guloside was synthesized from methyl d-mannoside in 21% yield over six steps.

Raman Optical Activity and Raman spectroscopy of carbohydrates in solution

This comprehensive study on selected 14 carbohydrates in water solution is an extension of previously published one focused only on solid state analysis. Here, Raman spectroscopy was used as a dedicated method for analysis of carbohydrates in solution, both using a normal effect (RS) and its chiral analogue: Raman Optical Activity spectroscopy (ROA). The compounds were selected as biologically important and representative of all groups: monosaccharides, disaccharides, trisaccharides, cyclodextrines and polysaccharides. RS and ROA spectra are presented together with an expanded discussion on various structures and conformations of studied carbohydrates in the solution taking into account particular regions, i.e. (1) low wavenumber region (250-600 cm^-1), (2) anomeric region (600-950 cm^-1), (3) fingerprint region (950-1200 cm^-1) and (4) CH₂and COH deformations region (1200-1500 cm^-1). So, the following information can be obtained about: (1) the absolute configuration of the anomeric centre; (2) the configuration of the anomeric centre and the orientation of the anomeric hydroxyl group; (3) the ring structures and the relative orientation of substituents and (4) the conformation of the exocyclic CH₂OH (4), respectively. Raman spectroscopy and Raman Optical Activity were shown as unique tools to study complex structures of carbohydrates.

Synthesis of l-Pyranosides by Hydroboration of Hex-5-enopyranosides Revisited

Extensive study of the diastereoselective synthesis of l-pyranosides utilizing hydroboration of substituted exo-glucals (5-enopyranosides) obtained from d-sugars is presented. On the basis of this study we present the empirical rules describing the reaction stereoselectivity and the correlation between the yield of the l-ido product and the size of protecting groups used. Application of these guidelines revealed that the hydroboration of methyl 2,3-O-methyl-6-deoxy-α-d-xylo-hex-5-enopyranoside resulted in exclusive formation of l-ido product with high yield. This method can be successfully applied to the synthesis of l-iduronic acid being an essential component of anticoagulant drugs with diastereoselectivity superior to previously published protocols.

Characterization of NADP+-specific L-rhamnose dehydrogenase from the thermoacidophilic Archaeon Thermoplasma acidophilum

Thermoplasma acidophilum utilizes L-rhamnose as a sole carbon source. To determine the metabolic pathway of L-rhamnose in Archaea, we identified and characterized L-rhamnose dehydrogenase (RhaD) in T. acidophilum. Ta0747P gene, which encodes the putative T. acidophilum RhaD (Ta_RhaD) enzyme belonging to the short-chain dehydrogenase/reductase family, was expressed in E. coli as an active enzyme catalyzing the oxidation of L-rhamnose to L-rhamnono-1,4-lactone. Analysis of catalytic properties revealed that Ta_RhaD oxidized L-rhamnose, L-lyxose, and L-mannose using only NADP(+) as a cofactor, which is different from NAD(+)/NADP(+)-specific bacterial RhaDs and NAD(+)-specific eukaryal RhaDs. Ta_RhaD showed the highest activity toward L-rhamnose at 60 °C and pH 7. The K (m) and k (cat) values were 0.46 mM, 1,341.3 min(-1) for L-rhamnose and 0.1 mM, 1,027.2 min(-1) for NADP(+), respectively. Phylogenetic analysis indicated that branched lineages of archaeal RhaD are quite distinct from those of Bacteria and Eukarya. This is the first report on the identification and characterization of NADP(+)-specific RhaD.

Structure-guided engineering of human thymidine kinase 2 as a positron emission tomography reporter gene for enhanced phosphorylation of non-natural thymidine analog reporter probe

Positron emission tomography (PET) reporter gene imaging can be used to non-invasively monitor cell-based therapies. Therapeutic cells engineered to express a PET reporter gene (PRG) specifically accumulate a PET reporter probe (PRP) and can be detected by PET imaging. Expanding the utility of this technology requires the development of new non-immunogenic PRGs. Here we describe a new PRG-PRP system that employs, as the PRG, a mutated form of human thymidine kinase 2 (TK2) and 2'-deoxy-2'-18F-5-methyl-1-β-L-arabinofuranosyluracil (L-18F-FMAU) as the PRP. We identified L-18F-FMAU as a candidate PRP and determined its biodistribution in mice and humans. Using structure-guided enzyme engineering, we generated a TK2 double mutant (TK2-N93D/L109F) that efficiently phosphorylates L-18F-FMAU. The N93D/L109F TK2 mutant has lower activity for the endogenous nucleosides thymidine and deoxycytidine than wild type TK2, and its ectopic expression in therapeutic cells is not expected to alter nucleotide metabolism. Imaging studies in mice indicate that the sensitivity of the new human TK2-N93D/L109F PRG is comparable with that of a widely used PRG based on the herpes simplex virus 1 thymidine kinase. These findings suggest that the TK2-N93D/L109F/L-18F-FMAU PRG-PRP system warrants further evaluation in preclinical and clinical applications of cell-based therapies.