SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Anomer

28

The origin of the anomeric effect has remained an open question. After Mo demonstrated that hyperconjugation is not responsible for the anomeric effect [Y. Mo, Nature Chem., 2010, 2, 666.], electrostatic interactions and Pauli repulsions have been at the center of this debate. In this work, the total energies of the most stable rotamers of the equatorial and axial anomers of fluoro, hydroxyl, cyano and amino groups in cyclohexane and 2-substituted tetrahydropyran rings are decomposed into their fundamental kinetic, electrostatic and exchange components. In this partitioning scheme, the differences in the total energies among the most stable rotamers of each anomer correlate very well with the differences in the exchange components, revealing that the anomeric effect has no electrostatic origin. Indeed, the anomeric effect is dominated by the exchange energy. This proposal for the origin of the anomeric effect brings new insights that, once incorporated, may improve qualitative chemical models. Implications of this new proposal for the origin of the anomeric effect on geometric parameters and solvation are also discussed.

Concepts: Energy, Physics, Chemistry, Kinetic energy, Pauli exclusion principle, Anomer, Anomeric effect, Mutarotation

0

B3LYP and M06-2X optimization and MP2 single point calculations are reported for the4H5and5H4conformations of 3,4,6-tri-O-acetyl-D-allal, 3,4,6-tri-O-acetyl-D-galactal, 3,4,6-tri-O-acetyl-D-glucal, and 3,4,6-tri-O-acetyl-D-gulal. Significant discrepancies in predictions of relative energies and conformers' population for B3LYP and M06-2X optimized geometries are observed. Generally, B3LYP overestimates the conformers' energies with respect to MP2, whereas M06-2X slightly underestimates the conformers' energies. B3LYP failed to estimate the4H5⇄5H4conformational equilibrium for 3,4,6-tri-O-acetyl-D-galactal and 3,4,6-tri-O-acetyl-D-glucal. The M06-2X functional showed good agreement with experimental results for all glycals studied. The4H5⇄5H4conformational equilibrium for 3,4,6-tri-O-acetyl-D-allal and 3,4,6-tri-O-acetyl-D-gulal is governed by the vinylogous anomeric effect (VAE), whereas competition between the VAE and quasi 1,3-diaxial interactions influence this equilibrium for 3,4,6-tri-O-acetyl-D-galactal and 3,4,6-tri-O-acetyl-D-glucal. The orientation of the 4-OAc group influences the strength of the quasi 1,3-diaxial interactions between the 3-OAc and 5-CH2OAc groups. AIM analysis shows weak bonding interaction between the 3-OAc and 5-CH2OAc groups.

Concepts: Statistics, Mathematics, Interaction, Cultural studies, Social influence, Anomer, Anomeric effect, Single

0

A family of seven mannuronan C5-epimerases (AlgE1-AlgE7) produced by Azotobacter vinelandii is able to convert β-d-mannuronate (M) to its epimer α-l-guluronate (G) in alginates. Even sharing high sequence homology at the amino acid level, they produce distinctive epimerization patterns. The introduction of new G-blocks into the polymer by in vitro epimerization is a strategy to improve the mechanical properties of alginates as biomaterial. However, epimerization is hampered when the substrate is modified or in the gelled state. Here it is presented how native and engineered epimerases of varying size perform on steric hindered alginate substrates (modified or as hydrogels). Reducing the size of the epimerases enables the epimerization of otherwise inaccessible regions in the alginate polymer. Even though the reduction of the size affects the productive binding of epimerases to the substrate, and hence their activity, the smaller epimerases could more freely diffuse into calcium-alginate hydrogel and epimerize it.

Concepts: Protein, Amino acid, Enzyme, Stereochemistry, Nitrogen, Cell wall, Anomer, Epimer

0

Stereoselective manipulations at the C1 anomeric position of saccharides are one of the central goals of preparative carbohydrate chemistry. Historically, the majority of reactions forming a bond with anomeric carbon has focused on reactions of nucleophiles with saccharide donors equipped with a leaving group. Here, we describe a novel approach to stereoselective synthesis of C-aryl glycosides capitalizing on the highly stereospecific reaction of anomeric nucleophiles. First, methods for the preparation of anomeric stannanes have been developed and optimized to afford both anomers of common saccharides in high anomeric selectivities. We established that oligosaccharide stannanes could be prepared from monosaccharide stannanes via O-glycosylation with Schmidttype donors, glycal epoxides, or under dehydrative conditions with C1 alcohols. Second, we identified a general set of catalytic conditions with Pd2(dba)3 (2.5 mol%) and a bulky ligand (JackiePhos, 10 mol%) controlling the b-elimination pathway. We demonstrated the glycosyl cross-coupling results in consistently high anomeric selectivities for both anomers with mono- and oligosaccharides, deoxysugars, saccharides with free hydroxyl groups, pyranose and furanose substrates. The versatility of the glycosyl crosscoupling reaction was probed in the total synthesis of salmochelins (siderophores) and commercial anti-diabetic drugs (gliflozins). Combined experimental and computational studies revealed that the b-elimination pathway is suppressed for biphenyl-type ligands due to the shielding of Pd(II) by sterically demanding JackiePhos whereas smaller ligands, which allow for the formation of a Pd-F complex, predominantly result in a glycal product. Similar steric effects account for the diminished rates of cross-couplings of 1,2cis C1-stannanes with aryl halides. DFT calculations also revealed that the transmetalation occurs via a cyclic transition state with retention of configuration at the anomeric position. Taken together, facile access to both anomers of various glycoside nucleophiles, a broad reaction scope, and uniformly high transfer of anomeric configuration make the glycosyl cross-coupling reaction a practical tool for the synthesis of bioactive natural products, drug candidates, allowing for late-stage glycodiversification studies with small molecules and biologics.

Concepts: Glucose, Stereochemistry, Carbohydrate, Carbohydrate chemistry, Carbohydrates, Monosaccharide, Anomer, Pyranose

0

The regioselective protection of both methyl galactopyranoside anomers at the 2 and 3-positions as the butane diacetal (BDA) is well known. Here we describe the formation of an unexpected byproduct, which mainly occurs when α-methyl galactopyranoside is reacted with 2,3-butanedione under BF3•OEt2 catalysis. The structure of the byproduct, which did not arise from anomerisation to the β-anomer or from BDA formation at the galactopyranoside 3,4-positions, was elucidated by NMR and X-ray crystallographic analysis, and proved to be the expected BDA protected galactopyranoside, but in which the stereochemistry of both its BDA acetal centres are inverted. Interestingly, the conformation of the resulting six-membered BDA ring was distorted to a skew boat conformation in order to maintain anomeric stabilisation.

Concepts: Crystallography, Stereochemistry, X-ray crystallography, Protection, Conformational isomerism, Anomer, Cyclohexane conformation, Butane

0

Two cyclic diastereoisomeric structures, known as α- and β-anomers of d-glucose with different configurations around C1 with OH groups in axial or equitroial positions, undergo the mutarotation conversion to each other in water. Two-dimensional correlation and codistribution spectroscopy (2DCOS and 2DCDS) analyses were applied to the time-dependent ATR IR spectra of aqueous solutions of α- and β-d-glucose undergoing such mutarotation conversion. 2DCOS analysis reveals that the increase and decrease in the IR intensities after the dissolution of α- or β-d-glucose are not fully synchronized, suggesting the mutarotation of d-glucose in water is not a simple binary conversion process but a multi-step reaction involving an intermediate species with a finite and observable concentration level and lifetime. 2DCDS analysis of the time-dependent ATR IR spectra clearly demonstrated the presence of intermediate species contributing to the band positions overlapped close to bands for α- and β-d-glucose. The fact that band positions identified for the intermediate species for α- to β-d-glucose conversion are the same for the reverse reaction suggests that they arise from the same species, most likely the open-ring structure produced by the hydrolysis.

Concepts: Concentration, Chemistry, Aqueous solution, Analysis, Infrared, The Band, Anomer, Band

0

The stereoselective peracetylation of α-d-xylose (1) and α-l-arabinose (4) using a combination of triethylamine and acetic anhydride in the presence or absence of a catalytic amount of dimethylaminopyridine (DMAP) is described. The peracetylated d-xylose and l-arabinose alpha pyranose anomers 2α and 5α are obtained in 97% and 56% yields respectively. The peracetylated d-xylose beta pyranose anomer 2β is obtained in 71% yield through simple modification of the reaction conditions. Details regarding synthesis and isolation optimization studies under different conditions are presented below. The stereoselective peracetylation reactions disclosed here have been used to separate mixtures of d-xylose and l-arabinose as their peracetylated derivatives 2β and 5α in 47% and 42% yields and can provide pure pentoses after deacetylation.

Concepts: Chemical reaction, Acetylation, Monosaccharide, Acetyl, Acetic anhydride, Anomer, Pentose, Arabinose

0

Cellobiohydrolases (CBHs) make up an important group of enzymes for both natural carbon cycling and industrial deconstruction of lignocellulosic biomass. The consecutive hydrolysis of one cellulose strand relies on an intricate pattern of enzyme-substrate interactions in the long, tunnel-shaped binding site of the CBH. In this work, we have investigated the initial complexation mode with cellulose of the most thoroughly studied CBH, Cel7A from Hypocrea jecorina (HjCel7A). We found that HjCel7A predominantly produces glucose when it initiates a processive run on insoluble microcrystalline cellulose, confirming the validity of an even and odd product ratio as an estimate of processivity. Moreover, the glucose released from cellulose was predominantly α-glucose. A link between the initial binding mode of the enzyme and the reducing end configuration was investigated by inhibition studies with the two anomers of cellobiose. A clear preference for β-cellobiose in product binding site +2 was observed for HjCel7A, but not the homologous endoglucanase, HjCe7B. Possible relationships between this anomeric preference in the product site and the prevalence of odd-numbered initial-cut products are discussed, and a correlation between processivity and anomer selectivity is proposed.

Concepts: Enzyme, Glucose, Carbon, Starch, Cellulose, Hydrolysis, Cellulase, Anomer

0

Cucurbit[7]uril (CB[7]) is known to bind strongly to hydrophilic amino saccharide guests with exceptional α-anomer selectivities under aqueous conditions. Single-crystal X-ray crystallography and computational methods were used to elucidate the reason behind this interesting phenomenon. The crystal structures of protonated galactosamine (GalN) and glucosamine (GluN) complexes confirm the inclusion of α anomers inside CB[7] and disclose the details of the host-guest binding. Whereas computed gas-phase structures agree with these crystal structures, gas-phase binding free energies show preferences for the β-anomer complexes over their α counterparts, in striking contrast to the experimental results under aqueous conditions. However, when the solvation effect is considered, the binding structures drastically change and the preference for the α anomers is recovered. The α anomers also tend to bind more tightly and leave less space in the CB[7] cavity toward inclusion of only one water molecule, whereas loosely bound β anomers leave more space toward accommodating two water molecules, with markedly different hydrogen-bonding natures. Surprisingly, entropy seems to contribute significantly to both anomeric discrimination and binding. This suggests that of all the driving factors for the strong complexation of the hydrophilic amino saccharide guests, water mediation plays a crucial role in the anomer discrimination.

Concepts: DNA, Crystallography, Water, Chemistry, Atom, Chemical bond, Solid, Anomer

0

The mechanism of a bismuth(V)-mediated thioglycoside activation was examined using reaction kinetics and quantum chemical reaction models. NMR experiments show an unusual non-linear growth/decay curve for the glycosylation reaction. Further studies suggest an anomeric inversion of the beta-glycoside donor to the alpha-donor during its activation, even in the presence of a neighboring 2-position acetate. Interestingly, in situ anomerization was not observed in the activation of an alpha-glycoside donor and this anomer also showed faster reaction times and higher product diastereoselectivites. Density Functional Theory calculations identify the structure of the promoter Ph3Bi(OTf)2 (1) in solution and map out the energetics of its interactions with the two thioglycoside anomers. These calculations suggest that 1 must bind the thiopropyl arm to induce triflate loss. The computational analyses also show that, unlike most O-glycosides, the beta- and alpha-donor S-glycosides are similar in energy. One energetically reasonable anomerization pathway of the donors is an SN1-like mechanism promoted by forming a bismuth-sulfonium adduct with the Lewis acidic Bi(V) for formation of an oxacarbenium intermediate. Further 2D NMR correlation studies support the existence of these Bi-sulfonium intermediates during the glycosylation reaction. Finally, the computed energy compensations needed to form these alpha vs. beta Bi-adducts is a possible explanation for the differential reactivity of these donors.

Concepts: Chemical reaction, Chemistry, Computational chemistry, Density functional theory, Quantum chemistry, Computational physics, Anomer, Mutarotation