Role of ion pairs in model glycosylation reactions of permethylated glucosyl and xylosyl triflates

Elucidating the molecular mechanisms of chemical O-glycosylation remains a significant challenge in glycochemistry. This study examines the mechanism of the nucleophilic substitution reaction between glycosyl triflates, which are extensively used in studies of glycosylation mechanisms, and several acceptor alcohols. The investigation was conducted through a comparative analysis of permethylated glucosyl triflate GTf and its xylosyl counterpart XTf. The glycosylation reactions, conducted in dichloromethane using GTf and XTf with EtOH, tBuOH, and CF3CH2OH, exhibited diverse α/β selectivities depending on the types of donor and acceptor molecules used. Identifying a unified mechanism to explain this range of selectivities proved challenging. Notably, we observed a distinct trend wherein the addition of excess triflate salt (Bu4NOTf) had a more pronounced effect on the α/β selectivity in glycosylation reactions utilizing XTf compared to those using GTf. Quantum chemical calculations performed at the SCS-MP2//DFT(M06-2X) level, with explicit inclusion of five solvent molecules, showed that contact ion pairs arising from XTf were significantly more stable than those from GTf. These experimental and computational results strongly suggest that ion pairs play a more crucial role in the glycosylation process involving XTf than GTf. Additionally, our quantum chemical analyses clarified that the enhanced stability of the ion pairs from XTf was attributed not to the strength of the C-1-OTf bond within XTf but to the flexibility in the conformational changes of XTf's pyranosyl ring.

Description of the Wild Strain Rhizobium rosettiformans DSM26376, Reclassified under Peteryoungia rosettiformans comb.nov., for Producing Glucuronan

Glucuronan is a polysaccharide composed of β-(1,4)-linked d-glucuronic acids having intrinsic properties and biological activities recoverable in many fields of application. Currently, the description of Sinorhyzobium meliloti M5N1CS mutant bacterial strain as the sole source of glucuronan makes it relevant to the exploration of new microorganisms producing glucuronan. In this study, the Peteryoungia rosettifformans strain (Rhizobia), was identified as a wild producer of an exopolysaccharide (RhrBR46) related to glucuronan. Structural and biochemical features, using colorimetric assays, Fourier infrared spectroscopy, nuclear magnetic resonance, high pressure size exclusion chromatography coupled to multi-angle light laser scattering, and enzymatic assays allowed the characterization of a polyglucuronic acid, having a molecular mass (Mw¯) of 1.85 × 10⁵ Da, and being partially O-acetylated at C-2 and/or C-3 positions. The concentration of Mg²⁺ ions in the cultivation medium has been shown to impact the structure of RhrBR46, by reducing drastically its Mw¯ (73%) and increasing its DA (10%). Comparative structural analyses between RhrBR46 and the glucuronan from Sinorhyzobium meliloti M5N1CS strain revealed differences in terms of molecular weight, degree of acetylation (DA), and the distribution of acetylation pattern. These structural divergences of RhrBR46 might contribute to singular properties or biological activities of RhrBR46, offering new perspectives of application.

O-Methylation in Carbohydrates: An NMR and MD Simulation Study with Application to Methylcellulose

Methylated carbohydrates are important from both biological and technical perspectives. Specifically, methylcellulose is an interesting cellulose derivative that has applications in foods, materials, cosmetics, and many other fields. While the molecular dynamics simulation technique has the potential for both advancing the fundamental understanding of this polymer and aiding in the development of specific applications, a general drawback is the lack of experimentally validated interaction potentials for the methylated moieties. In the present study, simulations using the GROMOS 56 carbohydrate force field are compared to NMR spin–spin coupling constants related to the conformation of the exocyclic torsion angle ω in d-glucopyranose and derivatives containing a 6-O-methyl substituent and a ¹³C-isotopologue thereof. A ³J_CC Karplus-type relationship is proposed for the C5–C6–O6–C_Me torsion angle. Moreover, solvation free energies are compared to experimental data for small model compounds. Alkylation in the form of 6-O-methylation affects exocyclic torsion only marginally. Computed solvation free energies between nonmethylated and methylated molecules were internally consistent, which validates the application of these interaction potentials for more specialized purposes.

The influence of neighboring group participation on the hydrolysis of 2-O-substituted methyl glucopyranosides

Does neighboring group participation actually enhance the reactivity of the anomeric center when the participating group is inherently disarming? To investigate the influence of the neighboring group effect from a 2-O protective group on acidic glycoside hydrolysis, 10 methyl glucosides having different protective groups on O2 have been synthesized and a clear trend between anomeric configuration, participation of the protective group, and the rate of hydrolysis could be observed.

Acid hydrolysis of cellulose as the entry point into biorefinery schemes

Cellulose is a major source of glucose because it is readily available, renewable, and does not compete with the food supply. Hydrolysis of cellulose is experiencing a new research and development cycle in which this reaction is carried out over solid catalysts and coupled to other reactions for increased efficiency. Cellulose is typically not soluble in conventional solvents and very resistant to chemical and biological transformations. This Review focuses on aspects related to the hydrolysis of cellulose as this process is a significant entry point into the biorefinery scheme based on carbohydrates for the production of biofuels and biochemicals. Structural features of cellulose, conventional acid-catalyzed reactions, and the use of solid acid catalysts for hydrolysis are discussed. The longterm success of the biorefinery concept depends on the development of energetically efficient processes to convert cellulose directly or indirectly into biofuels and chemicals.

Glycosidation-anomerisation reactions of 6,1-anhydroglucopyranuronic acid and anomerisation of beta-D-glucopyranosiduronic acids promoted by SnCl(4)

The reaction of silylated nucleophiles with 6,1-anhydroglucopyranuronic acid (glucuronic acid 6,1-lactones) catalysed by tin(IV) chloride provides 1,2-trans or 1,2-cis (deoxy)glycosides in a manner dependent on the donor structure. The alpha-glycoside was obtained for reactions of the donor with the 2-acyl group and 2-deoxydonors, whereas the 2-deoxy-2-iodo donor gave the beta-glycoside. Experimental evidence shows that when 1,2-cis-glycoside formation occurs, the anomerisation of initially formed 1,2-trans-glycosides catalysed by SnCl(4) is possible. The anomerisation of beta-D-glucopyranosiduronic acids was found to be faster, in some cases, than anomerisation of related beta-D-glucopyranosiduronic acid esters and beta-D-glucopyranoside derivatives and the rates are dependent on the structure of the aglycon. Moreover, the rates of anomerisation of beta-D-glucopyranuronic acid derivatives can be qualitatively correlated with rates of hydrolysis of beta-D-glucopyranosiduronic acids. Mechanistic possibilities for the reactions are considered.

The synthesis of cyclic imidates from amides of glucuronic acid and investigation of glycosidation reactions

The synthesis of novel cyclic glycosyl imidates and an investigation of their potential as donors in glycosidation reactions is described. The results show that 1,2-cis glycosides obtained from the reactions of glycosyl acetates or cyclic imidates, each derived from amides of glucuronic acid, result from the anomerisation of initially formed 1,2-trans glycosides.

Quantum mechanical and NMR spectroscopy studies on the conformations of the hydroxymethyl and methoxymethyl groups in aldohexosides

The potential energy surfaces of the hydroxymethyl and methoxymethyl groups in methyl hexopyranosides have been extensively studied, employing quantum mechanical calculations and high resolution NMR data. The structure and energy of the C-5-C-6 rotamers were calculated at the B3LYP level of the density functional theory (DFT). For all, geometry optimizations were carried out for 264 conformers of 16 methyl D-gluco- and methyl D-galactopyranoside derivatives 1-16 at the B3LYP/6-31G** level. For all calculated minima, single-point calculations were performed at the B3LYP/6-311++G** level. Solvent effects were considered using a self-consistent reaction field method. Values of the vicinal coupling constants 3J(H-5-H-6R), 3J(H-5-H-6S), 3J(C-4-H-6R), and 3J(C-4-H-6S) for methyl D-glucopyranosides, methyl D-galactopyranosides and their 6-O-methyl derivatives 9-16 were measured in two solvents, methanol and water. The calculated gg, gt, and tg rotamer populations of the hydroxymethyl and methoxymethyl groups in 9-16 agreed well with experimental data. The results clearly showed that the population of gg, gt, and tg rotamers is sensitive to solvent effects. It was concluded that the preference of rotamers in 1-16 is due to the hydrogen bonding and solvent effects.

Conformational flexibility in highly sulfated beta-D-glucopyranoside derivatives

Triggered by findings on heparin-like disaccharides, the conformation of sulfated glucopyranosides was investigated. Sodium (methyl 2,3,4-tri-O-sulfonato-beta-D-glucopyranosid)uronate tetrasodium salt is in a conformational equilibrium, to which a non-chair conformation contributes. The same is true for methyl (methyl 2,3,4-tri-O-sulfonato-beta-D-glucopyranosid)uronate trisodium salt, methyl 2,3,4,6-tetra-O-sulfonato-beta-D-glucopyranoside tetrasodium salt, and octa-O-sulfonato-beta, beta-trehalose octasodium salt, with less obvious non-chair contributions. The effect is charge related. The conformational effect, which does not occur in analogous alpha-D-glucopyranoside derivatives, is discussed in terms of the anomeric effect.

Comparative stability of 1-alkoxyalkyl alpha-D-glucosides in the presence of acid or alpha-D-glucosidase from yeast

The aminated 1-alkoxyalkyl glycosides [(S)-2-amino-1-methoxyethyl] 6-amino-6-deoxy-alpha-D-glucopyranoside (3) and [(R,S)-1-ethoxyethyl] 6-amino-6-deoxy-alpha-D-glucopyranoside (4) have been synthesised and characterised. These compounds as well as [(R)-2-amino-1-methoxyethyl] alpha-D-glucopyranoside (1) prepared earlier are resistant against alpha-D-glucosidase (maltase, alpha-D-glucoside glucohydrolase, E.C. 3.2.1.20) from yeast, yet undergo hydrolysis under relatively mild acidic conditions. The kinetic parameters of the interaction with alpha-D-glucosidase and with acid were determined. The relative rates of acid hydrolysis of aminated 1-alkoxyalkyl glycosides compared with aminated ordinary glycosides suggest essential differences in the mechanism of acid-catalysed hydrolysis.

Substitution of acyclic sugar acetals. Is the acid-catalyzed substitution of erythro and threo l,2,3,4,5-penta-O-acetyl-l-arabinose S-ethyl monothioacetals anchimerically assisted?

The rate constants for the inversion and for the 14C-acetoxy exchange of the diastereomeric 1,2,3,4,5-penta-O-acetyl S-ethyl monothioacetals show that the substitution is not anchimerically assisted by the C2-acetoxy group. The substitution of acyclic poly-O-acetyl sugar monoacetals is best explained by the formation of the acyclic intermediate

[Formula: see text] (X = S or O) in the rate-determining step.

The acid-catalyzed hydrolysis of methyl 2-chloro-2-deoxy-β-D-glucopyranoside

The kinetics of the hydrolysis of methyl 2-chloro-2-deoxy-β-D-glucopyranoside have been determined in hydrochloric acid solutions over a range of acid concentrations and temperatures. Chloro substitution reduces the rate by a factor of 35 compared with the hydroxy analogue. Application of the Hammett criterion indicates a unimolecular (A-1) mechanism of hydrolysis, as does application of the Bunnett criterion. The entropy of activation, however, is considerably smaller than that observed for the hydrolysis of methyl β-d-glucopyranoside. This is interpreted as being indicative of partial A-2 character.

FRACTIONATION OF PECTINS FROM SUNFLOWERS, SUGAR BEETS, APPLES, AND CITRUS FRUITS

Pectins from sunflower heads, sugar beet, apple, and citrus fruits were separated into acid-insoluble pectic acids and acid-soluble polysaccharides. The acid-soluble polysaccharides were shown by moving-boundary electrophoresis to be complex mixtures of four or five polysaccharides. The pectic acids were fractionated by precipitation with sodium acetate from aqueous solution. The fractions were examined for galacturonic acid content, specific rotation, limiting viscosity number, behavior on moving-boundary electrophoresis, and presence of neutral sugars. The results showed that each of the pectic acids contained two acidic components, one a galacturonan free of neutral sugars, the other a galacturonan to which neutral sugars were attached, probably as side chains.