O-(α-D-Glucopyranosyl)trichloroacetimidate as a Glucosyl Donor

One-pot expeditious synthesis of glycosylated esters through activation of carboxylic acids using trichloroacetonitrile

Acetimidates, a valuable intermediate has been well explored as versatile synthon in a number of organic transformations particularly as suitable donors in glycosylation reactions. Herein, we explored acetimidates to furnish high-to-excellent yield of diverse glycosylated esters under one-pot mild reaction condition. The commercially available trichloroacetonitrile is implemented for the activation of carboxylic acid via in situ generation of trichloroacetimidate, which was subsequently attacked by sugar alcohols to deliver high-to-excellent yields of desired glycosylated esters. The devised method has some notable features such as metal-free condition, one-pot mild reaction condition, easy-handling, high-to-excellent yields, and broad substrate scope.

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.

Self-Promoted Stereoselective Glycosylations - Past, Present, Future

Self-promoted glycosylations have generally not received much attention, despite having the advantages of being easy to perform and often highly stereoselective. This account covers the work done in this field and the mechanistic aspects of self-promoted glycosylations are discussed, with a main focus on the stereoselectivity of the reactions. Most self-promoted glycosylations utilize trichloroacetimidate donors, but examples of self-promoted reactions with other donors have been described and will be discussed. Self-promoted glycosylation strategies can provide a basis for new stereoselective glycosylation methodologies.

Quantitative analysis of auxin metabolites in lychee flowers

To investigate the modulation of endogenous indole-3-acetic acid (IAA) level by biosynthesis and inactivation during floral development, IAA and its metabolites were analyzed by LC-ESI/MS/MS in Lychee (Litchi chinensis Sonn.) flowers. In the bloomed flowers, the level of free IAA was higher in males than in females. In contrast, the total sum level of IAA metabolites was higher in females than in males, suggesting a higher biosynthetic activity of IAA in the females before the bloom. A detailed time-course analysis from the bud stage to the developing flower stage showed higher levels of IAA in females than males. The major metabolites were oxidized IAA in both sexes. The results suggest that IAA is involved in the maturation of female floral tissues in lychee, and oxidative metabolism plays an essential role in controlling the free IAA levels therein.

Multigram synthesis of an orthogonally-protected pentasaccharide for use as a glycan precursor in a Shigella flexneri 3a conjugate vaccine: application to a ready-for-conjugation decasaccharide

Fine-tuned catalytic processes facilitating regio- and stereoselective conversions for the large-scale synthesis of a pentasaccharide and its oligomerization into ready-for-conjugation haptens.

Glycosidic linkage, N-acetyl side-chain, and other structural properties of methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-β-D-mannopyranoside monohydrate and related compounds

The crystal structure of methyl 2-acetamido-2-deoxy-β-D-glycopyranosyl-(1→4)-β-D-mannopyranoside monohydrate, C₁₅H₂₇NO₁₁·H₂O, was determined and its structural properties compared to those in a set of mono- and disaccharides bearing N-acetyl side-chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C-N (amide) and C-O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N-H hydrogen. Relative to N-acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen-bond acceptor display elongated C-O and shortened C-N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C-N and C-O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis-trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter-residue hydrogen bonding and some bond angles in or proximal to β-(1→4) O-glycosidic linkages on linkage torsion angles φ and ψ. Hypersurfaces correlating φ and ψ with the linkage C-O-C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.

Structural properties of D-mannopyranosyl rings containing O-acetyl side-chains

The crystal structures of 1,2,3,4,6-penta-O-acetyl-α-D-mannopyranose, C₁₆H₂₂O₁₁, and 2,3,4,6-tetra-O-acetyl-α-D-mannopyranosyl-(1→2)-3,4,6-tri-O-acetyl-α-D-mannopyranosyl-(1→3)-1,2,4,6-tetra-O-acetyl-α-D-mannopyranose, C₄₀H₅₄O₂₇, were determined and compared to those of methyl 2,3,4,6-tetra-O-acetyl-α-D-mannopyranoside, methyl α-D-mannopyranoside and methyl α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside to evaluate the effects of O-acetylation on bond lengths, bond angles and torsion angles. In general, O-acetylation exerts little effect on the exo- and endocyclic C-C and endocyclic C-O bond lengths, but the exocyclic C-O bonds involved in O-acetylation are lengthened by ∼0.02 Å. The conformation of the O-acetyl side-chains is highly conserved, with the carbonyl O atom either eclipsing the H atom attached to a 2°-alcoholic C atom or bisecting the H-C-H bond angle of a 1°-alcoholic C atom. Of the two C-O bonds that determine O-acetyl side-chain conformation, that involving the alcoholic C atom exhibits greater rotational variability than that involving the carbonyl C atom. These findings are in good agreement with recent solution NMR studies of O-acetyl side-chain conformations in saccharides. Experimental evidence was also obtained to confirm density functional theory (DFT) predictions of C-O and O-H bond-length behavior in a C-O-H fragment involved in hydrogen bonding.

Ester Formation via Symbiotic Activation Utilizing Trichloroacetimidate Electrophiles

Trichloroacetimidates are useful reagents for the synthesis of esters under mild conditions that do not require an exogenous promoter. These conditions avoid the undesired decomposition of substrates with sensitive functional groups that are often observed with the use of strong Lewis or Brønsted acids. With heating, these reactions have been extended to benzyl esters without electron-donating groups. These inexpensive and convenient methods should find application in the formation of esters in complex substrates.

Synthesis and O-Glycosidic Linkage Conformational Analysis of 13C-Labeled Oligosaccharide Fragments of an Antifreeze Glycolipid

NMR studies of two ¹³C-labeled disaccharides and a tetrasaccharide were undertaken that comprise the backbone of a novel thermal hysteresis glycolipid containing a linear glycan sequence of alternating [βXyl p-(1→4)-βMan p-(1→4)] _n dimers. Experimental trans-glycoside NMR J-couplings, parameterized equations obtained from density functional theory (DFT) calculations, and an in-house circular statistics package ( MA'AT) were used to derive conformational models of linkage torsion angles ϕ and ψ in solution, which were compared to those obtained from molecular dynamics simulations. Modeling using different probability distribution functions showed that MA'AT models of ϕ in βMan(1→4)βXyl and βXyl(1→4)βMan linkages are very similar in the disaccharide building blocks, whereas MA'AT models of ψ differ. This pattern is conserved in the tetrasaccharide, showing that linkage context does not influence linkage geometry in this linear system. Good agreement was observed between the MA'AT and MD models of ψ with respect to mean values and circular standard deviations. Significant differences were observed for ϕ, indicating that revision of the force-field employed by GLYCAM is probably needed. Incorporation of the experimental models of ϕ and ψ into the backbone of an octasaccharide fragment leads to a helical amphipathic topography that may affect the thermal hysteresis properties of the glycolipid.

Silver-catalyzed stereoselective formation of glycosides using glycosyl ynenoates as donors

A silver-catalyzed glycosylation reaction employing readily accessible and stable glycosyl ynenoates is developed. This reaction is mostly high yielding and exhibits varying levels of stereoinversion at the anomeric position. Compared to established and versatile Yu's gold catalysis, this chemistry features the use of substantially cheaper AgNTf₂.

Promoter free allylation of trichloroacetimidates with allyltributylstannanes under thermal conditions to access the common 1,1'-diarylbutyl pharmacophore

1,1'-Diarylbutyl groups are a common pharmacophore found in many biologically active small molecules. To access these systems under mild conditions, the reaction of diarylmethyl trichloroacetimidates with allyltributylstannanes was explored. Simply heating allyltributylstannane with the trichloroacetimidate resulted in substitution of the imidate with an allyl group. Unlike other methods used to access these systems, no strong base, transition metal catalyst, Brønsted acid or Lewis acid promoter was required to affect the transformation. Conversions are best with electron rich benzylic trichloroacetimidate systems, where excellent yields are achieved just by refluxing the reactants together in nitromethane.

Synthesis of 1,1'-Diarylethanes and Related Systems by Displacement of Trichloroacetimidates with Trimethylaluminum

Benzylic trichloroacetimidates are readily displaced by trimethylaluminum under Lewis acid promoted conditions to provide the corresponding methyl substitution product. This method is a convenient way to access 1,1'-diarylethanes and related systems, which play a significant role in medicinal chemistry, with a number of systems owing their biological activity to this functionality. Most benzylic substrates undergo ready displacement, with electron deficient systems being the exception. The use of an enantiopure imidate showed significant racemization, implicating the formation of a cationic intermediate.

Rearrangement of Benzylic Trichloroacetimidates to Benzylic Trichloroacetamides

The rearrangement of allylic trichloroacetimidates is a well-known transformation, but the corresponding rearrangement of benzylic trichloroacetimidates has not been explored as a method for the synthesis of benzylic amines. Conditions that provide the trichloroacetamide product from a benzylic trichloroacetimidate in high yield have been developed. Methods were also investigated to transform the trichloroacetamide product directly into the corresponding amine, carbamate, and urea. A cationic mechanism for the rearrangement is implicated by the available data.

Alkylation of Sulfonamides with Trichloroacetimidates under Thermal Conditions

An intermolecular alkylation of sulfonamides with trichloroacetimidates is reported. This transformation does not require an exogenous acid, base, or transition metal catalyst; instead the addition occurs in refluxing toluene without additives. The sulfonamide alkylation partner appears to be only limited by sterics, with unsubstituted sulfonamides providing better yields than more encumbered N-alkyl sulfonamides. The trichloroacetimidate alkylating agent must be a stable cation precursor for the substitution reaction to proceed under these conditions.

Formation of DPM ethers using O-diphenylmethyl trichloroacetimidate under thermal conditions

Alcohols are effectively converted to their corresponding diphenylmethyl (DPM) ethers by reaction with O-diphenylmethyl trichloroacetimidate in refluxing toluene without the requirement of a catalyst or other additives. A number of acid and base sensitive substrates were protected in excellent yield using this new method without disturbing the pre-existing functionality present in these molecules. This reaction is the first example of the formation of an ether from stoichiometric amounts of a trichloroacetimidate and an alcohol without the addition of a Brønsted or Lewis acid catalyst.

Stereospecific, High-Yielding, and Green Synthesis of β-Glycosyl Esters

A new method of synthesizing β-glycosyl esters stereospecifically has been developed by treating O-benzyl-protected glycosyl chlorides with Cs2CO3, tetrabutylammomium bromide (TBAB), a carboxylic acid, water, and granular polytetrafluoroethylene (PTFE) at 80 °C under mechanical agitation. D-Glucosyl, D-xylosyl, and D-galactosyl chlorides and 20 carboxylic acids were used to demonstrate the scope of the reaction. Control experiments showed that the water and granular PTFE had indispensable roles. Water-soluble TBAB has been found to be as efficient as N-methyl-N,N,N-trioctyloctan-1-ammonium chloride (Aliquat 336) in the reactions. After scaling up to 5-12 g, all of the products were obtained quantitatively via simple filtration and no organic solvents or chromatography was needed for the entire process.

β-Xylopyranosides: synthesis and applications

In recent years, β-xylopyranosides have attracted interest due to the development of biomass-derived molecules. This review focuses on general routes for the preparation of β-xylopyranosides by chemical and enzymatic pathways and their main uses.

Modulating glycosylation with exogenous nucleophiles: an overview

The major challenge in carbohydrate synthesis is stereochemical control of glycosidic bond formation. Different glycosylation methods have been developed that are based on the modulation effect of external nucleophiles. This review highlights the development, synthetic application, challenges and outlook of the modulated glycosylation methods.

Investigations of scope and mechanism of nickel-catalyzed transformations of glycosyl trichloroacetimidates to glycosyl trichloroacetamides and subsequent, atom-economical, one-step conversion to α-urea-glycosides

The development and mechanistic investigation of a highly stereoselective methodology for preparing α-linked-urea neo-glycoconjugates and pseudo-oligosaccharides is described. This two-step procedure begins with the selective nickel-catalyzed conversion of glycosyl trichloroacetimidates to the corresponding α-trichloroacetamides. The α-selective nature of the conversion is controlled with a cationic nickel(II) catalyst, [Ni(dppe)(OTf)2 ] (dppe=1,2-bis(diphenylphosphino)ethane, OTf=triflate). Mechanistic studies have identified the coordination of the nickel catalyst with the equatorial C2 -ether functionality of the α-glycosyl trichloroacetimidate to be paramount for achieving an α-stereoselective transformation. A cross-over experiment has indicated that the reaction does not proceed in an exclusively intramolecular fashion. The second step in this sequence is the direct conversion of α-glycosyl trichloroacetamide products into the corresponding α-urea glycosides by reacting them with a wide variety of amine nucleophiles in presence of cesium carbonate. Only α-urea-product formation is observed, as the reaction proceeds with complete retention of stereochemical integrity at the anomeric CN bond.

Facile Preparation of Deuterium-Labeled Standards of Indole-3-Acetic Acid (IAA) and Its Metabolites to Quantitatively Analyze the Disposition of Exogenous IAA inArabidopsis thaliana

[2',2'-(2)H(2)]-indole-3-acetic acid ([2',2'-(2)H(2)]IAA) was prepared in an easy and efficient manner involving base-catalyzed hydrogen/deuterium exchange. 1-O-([2',2'-(2)H(2)]-indole-3-acetyl)-beta-D-glucopyranose, [2',2'-(2)H(2)]-2-oxoindole-3-acetic acid, and 1-O-([2',2'-(2)H(2)]-2-oxoindole-3-acetyl)-beta-D-glucopyranose were also successfully synthesized from deuterated IAA, and effectively utilized as internal standards in the quantitative analysis of IAA and its metabolites in Arabidopsis thaliana by using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS). The use of this technique shows that these metabolites were accumulated in the roots of Arabidopsis seedlings. Dynamic changes in the metabolites of IAA were observed in response to exogenous IAA, revealing that each metabolic action was regulated differently to contribute to the IAA homeostasis in Arabidopsis.

Stereoselective α-glycosylation of C(6)-hydroxyl myo-inositols via nickel catalysis-application to the synthesis of GPI anchor pseudo-oligosaccharides

Glycosylphosphatidyl inositol (GPI) anchors play a key role in many eukaryotic biological pathways. Stereoselective synthesis of GPI anchor analogues have proven to be critical for probing the biosynthesis, structure, and biological properties of these compounds. Challenges that have emerged from these efforts include the preparation of the selectively protected myo-inositol building blocks and the stereoselective construction of glucosamine α-linked myo-inositol containing pseudodisaccharide units. Herein, we describe the effectiveness of the cationic nickel(II) catalyst, Ni(4-F-PhCN)4(OTf)2, at promoting selective formation of 1,2-cis-2-amino glycosidic bonds between the C(2)-N-substituted benzylideneamino trihaloacetimidate donors and C(6)-hydroxyl myo-inositol acceptors. This catalytic coupling process allows rapid access to pseudosaccharides of GPI anchors in good yields and with excellent levels of α-selectivity (α:β=10:1-20:1). In stark contrast, activation of trichloroacetimidate donors containing the C(2)-N-substituted benzylidene group with TMSOTf and BF3(.)OEt2 provided the desired pseudodisaccharides as a 1:1 mixture of α- and β-isomers.

Methyl 4-O-β-D-xylopyranosyl β-D-mannopyranoside, a core disaccharide of an antifreeze glycolipid

Methyl β-D-xylopyranosyl-(1→4)-β-D-mannopyranoside, C12<!?tlsb=-0.02pt>H22O10, crystallized as colorless block-like needles from methanol–water solvent. Comparisons to the internal linkage conformations in the two crystallographic forms of the structurally related disaccharide methyl β-D-mannopyranosyl-(1→4)-β-D-xylopyranoside are discussed. Intramolecular inter-residue hydrogen bonding is observed between one mannopyranosyl hydroxy O atom and the ring O atom of the xylopyranosyl residue. Intermolecular hydrogen bonding yields a bilayered two-dimensional sheet of molecules that are located parallel to thebcplane.

Recent Advances in Transition Metal-Catalyzed Glycosylation

Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of α- and β-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation.

Disorder and conformational analysis of methyl β-D-galactopyranosyl-(1→4)-β-D-xylopyranoside

Methyl β-D-galactopyranosyl-(1→4)-β-D-xylopyranoside, C12H22O10, (II), crystallizes as colorless needles from water with positional disorder in the xylopyranosyl (Xyl) ring and no water molecules in the unit cell. The internal glycosidic linkage conformation in (II) is characterized by a ϕ′ torsion angle (C2′Gal—C1′Gal—O1′Gal—C4Xyl) of 156.4 (5)° and a ψ′ torsion angle (C1′Gal—O1′Gal—C4Xyl—C3Xyl) of 94.0 (11)°, where the ring atom numbering conforms to the convention in which C1 denotes the anomeric C atom, and C5 and C6 denote the hydroxymethyl (–CH2OH) C atoms in the β-Xyl and β-Gal residues, respectively. By comparison, the internal linkage conformation in the crystal structure of the structurally related disaccharide, methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside], (III) [Stenutz, Shang & Serianni (1999).Acta Cryst.C55, 1719–1721], is characterized by ϕ′ = 153.8 (2)° and ψ′ = 78.4 (2)°. A comparison of β-(1→4)-linked disaccharides shows considerable variability in both ϕ′ and ψ′, with the range in the latter (∼38°) greater than that in the former (∼28°). Inter-residue hydrogen bonding is observed between atoms O3Xyland O5′Galin the crystal structure of (II), analogous to the inter-residue hydrogen bond detected between atoms O3Glcand O5′Galin (III). The exocyclic hydroxymethyl conformations in the Gal residues of (II) and (III) are identical (gauche–transconformer).

Marine Toxins with Spiroimine Rings: Total Synthesis of Pinnatoxin A

This microreview provides a compilation of synthetic approaches and total syntheses of pinnatoxin A in a survey of the literature up to early 2010. Pinnatoxin A is the first discovered and representative member of a fascinating group of potent marine toxins that share a spiroimine subunit as a unifying structural element.

Perturbation of indole-3-butyric acid homeostasis by the UDP-glucosyltransferase UGT74E2 modulates Arabidopsis architecture and water stress tolerance

Reactive oxygen species and redox signaling undergo synergistic and antagonistic interactions with phytohormones to regulate protective responses of plants against biotic and abiotic stresses. However, molecular insight into the nature of this crosstalk remains scarce. We demonstrate that the hydrogen peroxide-responsive UDP-glucosyltransferase UGT74E2 of Arabidopsis thaliana is involved in the modulation of plant architecture and water stress response through its activity toward the auxin indole-3-butyric acid (IBA). Biochemical characterization of recombinant UGT74E2 demonstrated that it strongly favors IBA as a substrate. Assessment of indole-3-acetic acid (IAA), IBA, and their conjugates in transgenic plants ectopically expressing UGT74E2 indicated that the catalytic specificity was maintained in planta. In these transgenic plants, not only were IBA-Glc concentrations increased, but also free IBA levels were elevated and the conjugated IAA pattern was modified. This perturbed IBA and IAA homeostasis was associated with architectural changes, including increased shoot branching and altered rosette shape, and resulted in significantly improved survival during drought and salt stress treatments. Hence, our results reveal that IBA and IBA-Glc are important regulators of morphological and physiological stress adaptation mechanisms and provide molecular evidence for the interplay between hydrogen peroxide and auxin homeostasis through the action of an IBA UGT.

Palladium-controlled beta-selective glycosylation in the absence of the C(2)-ester participatory group

The development of a new glycosylation method for the stereoselective synthesis of beta-glycosides in the absence of the traditional C(2)-ester neighboring group effect is described. This process relies on the ability of the cationic palladium catalyst, Pd(PhCN)(2)(OTf)(2) generated in situ from Pd(PhCN)(2)Cl(2) and AgOTf, to direct beta-selectivity. The new glycosylation reaction is highly beta-selective and proceeds under mild conditions with 1-2 mol % of catalyst loading. This beta-glycosylation method has been applied to a number of glucose donors with benzyl, allyl, and p-methoxybenzyl groups incorporated at the C(2)-position as well as tribenzylated xylose and quinovose donors to prepare various disaccharides and trisaccharides with good to excellent beta-selectivity. Mechanistic studies suggest that the major operative pathway is likely to proceed via a seven-membered ring intermediate, wherein the cationic palladium complex coordinates to both the C(1)-imidate nitrogen and C(2)-oxygen of the trichloroacetimidate donor. Formation of this seven-membered ring intermediate directs the selectivity, leading to the formation of beta-glycosides.

Synthesis of the beta-1,3-glucan, laminarahexaose: NMR and conformational studies

The synthesis of laminarahexaose is described. NMR studies of several of the intermediates leading to the beta-1,3-glucan show anomalously small coupling constants for some of the C-1 hydrogens. An X-ray structure for the protected hexasaccharide shows that the small coupling constants are due to some of the glucopyranose rings adopting a twist-boat conformation. The X-ray studies also explain other unexpected NMR observations.

Catalytic stereoselective glycosidation with glycosyl diphenyl phosphates: rapid construction of 1,2-cis-alpha-glycosidic linkages

A commercially available 0.1 M solution of HClO(4) in dioxane has been shown to catalyze the glycosidation of glycosyl diphenyl phosphates. The per-O-benzyl-protected glucosyl and galactosyl donors and the 3,4,6-tri-O-acetyl-2-azido-2-deoxygalactosyl donor each react with a range of acceptor alcohols in the presence of 0.05-0.2 equiv of HClO(4) in dioxane/Et(2)O (1:1) to afford glycosides in good yields with good to excellent alpha selectivities. The synthetic utility of this glycosidation method was demonstrated by a stereoselective synthesis of the alpha-galactosylceramide KRN7000, an activator of natural killer (NK) T cells through CD1d molecules.