Glycosyl Acceptors Research Articles

Regioselective and 1,2-cis-α-Stereoselective Glycosylation Utilizing Glycosyl-Acceptor-Derived Boronic Ester Catalyst.

Regioselective and 1,2-cis-α-stereoselective glycosylations using 1α,2α-anhydro glycosyl donors and diol glycosyl acceptors in the presence of a glycosyl-acceptor-derived boronic ester catalyst. The reactions proceed smoothly to give the corresponding 1,2-cis-α-glycosides with high stereo- and regioselectivities in high yields without any further additives under mild reaction conditions. In addition, the present glycosylation method was successfully applied to the synthesis of an isoflavone glycoside.

Organocatalytic glycosylation by using electron-deficient pyridinium salts.

A new organocatalytic glycosylation method based on electron-deficient pyridinium salts is reported. At ambient temperature and catalyst loadings as low as 1 mol %, 2-deoxyglycosides were formed from benzyl- and silyl-protected glycals and primary or secondary glycosyl acceptors, with excellent yields and anomeric selectivity. Mechanistic investigations point to alcohol-pyridinium conjugates (1,2-addition products) as key intermediates in the catalytic cycle.

Chemical Synthesis of a Glycopeptide Derived from Skp1 for Probing Protein Specific Glycosylation.

Skp1 is a cytoplasmic and nuclear protein, best known as an adaptor of the SCF family of E3-ubiquitin ligases that label proteins for their degradation. Skp1 in Dictyostelium is posttranslationally modified on a specific hydroxyproline (Hyp) residue by a pentasaccharide, which consists of a Fucα1,2-Galβ-1,3-GlcNAcα core, decorated with two α-linked Gal residues. A glycopeptide derived form Skp1 was prepared to characterize the α-galactosyltransferase (AgtA) that mediates the addition of the α-Gal moieties, and to develop antibodies suitable for tracking the trisaccharide isoform of Skp1 in cells. A strategy was developed for the synthesis of the core trisaccharide-Hyp based on the use of 2-naphthylmethyl (Nap) ethers as permanent protecting groups to allow late stage installation of the Hyp moiety. Tuning of glycosyl donor and acceptor reactivities was critical for achieving high yields and anomeric selectivities of glycosylations. The trisaccharide-Hyp moiety was employed for the preparation of the glycopeptide using microwave-assisted solid phase peptide synthesis. Enzyme kinetic studies revealed that trisaccharide-Hyp and trisaccharide-peptide are poorly recognized by AgtA, indicating the importance of context provided by the native Skp1 protein for engagement with the active site. The trisaccharide-peptide was a potent immunogen capable of generating a rabbit antiserum that was highly selective toward the trisaccharide isoform of full-length Skp1.

Chemical Fucosylation of a Polysaccharide: A Semisynthetic Access to Fucosylated Chondroitin Sulfate.

Chemical O-glycosylation of polysaccharides is an almost unexplored reaction. This is mainly due to the difficulties in derivatizing such complex biomacromolecules in a quantitative manner and with a fine control of the obtained structural parameters. In this work, chondroitin raw material from a microbial source was chemo- and regioselectively protected to give two polysaccharide intermediates, that acted in turn as glycosyl acceptors in fucosylation reactions. Further manipulations on the fucosylated polysaccharides, including multiple de-O-benzylation and sulfation, furnished for the first time nonanimal sourced fucosylated chondroitin sulfates (fCSs)-polysaccharides obtained so far exclusively from sea cucumbers (Echinoidea, Holothuroidea) and showing several very interesting biological activities. A semisynthetic fCS was characterized from a structural point of view by means of 2D-NMR techniques, and preliminarily assayed in an anticoagulant test.

Influence of a δ-lactam-type cyclic protecting group on the reactivity of a 4-hydroxy glycosyl acceptor derivative of d-glucosamine

A new 4-hydroxy glycosyl acceptor derivative of d-glucosamine containing a lactam-type N-protecting group was synthesized. The new glycosyl acceptor containing a hydroxy group at the C(4) atom was shown to be highly reactive in the glycosylation despite the presence of the amide (lactam) NH group. The high efficiency of the synthesized acceptor derivative is apparently attributed to the transformation of the (Z)-amide function that is present in amino sugar derivatives containing an acyclic acyl N-protecting group into the (E)-amide group in derivatives containing a lactam-type cyclic N-protecting group. This transformation leads to a radical change in the hydrogen bonding conditions and the character of molecular association. The observed effect can be employed to design new N-protecting groups providing high reactivity of glycosyl acceptor derivatives of D-glucosamine.

A Convergent Synthesis of 6-O-Branched β-Glucan Oligosaccharides.

β-Glucans are important carbohydrate antigens on the surface of fungal cells useful for antifungal vaccine development. This paper has described a highly convergent and efficient strategy for the synthesis of structurally defined branched β-glucan oligosaccharides that can be used for detailed studies of β-glucans and for the design of β-glucan-based vaccines. The strategy was highlighted by assembling the title compounds via preactivation-based glycosylation with thioglycosides as glycosyl donors. It was used to successfully prepare β-glucan oligosaccharides that had a β-1,3-linked nonaglucan backbone with β-1,6-glucotetraose, β-1,3-glucodiose and β-1,3-glucotetraose branches at the 6-O-position of the nonaglucan central sugar unit. The structure and size of the glycosyl donors and acceptors used in the syntheses did not significantly affect the glycosylation efficiency, suggesting that the strategy can be generally useful for the synthesis of more complex structures.

Synthesis of the oligosaccharides related to branching sites of fucosylated chondroitin sulfates from sea cucumbers.

Natural anionic polysaccharides fucosylated chondroitin sulfates (FCS) from sea cucumbers attract great attention nowadays due to their ability to influence various biological processes, such as blood coagulation, thrombosis, angiogenesis, inflammation, bacterial and viral adhesion. To determine pharmacophore fragments in FCS we have started systematic synthesis of oligosaccharides with well-defined structure related to various fragments of these polysaccharides. In this communication, the synthesis of non-sulfated and selectively O-sulfated di- and trisaccharides structurally related to branching sites of FCS is described. The target compounds are built up of propyl β-d-glucuronic acid residue bearing at O-3 α-l-fucosyl or α-l-fucosyl-(1→3)-α-l-fucosyl substituents. O-Sulfation pattern in the fucose units of the synthetic targets was selected according to the known to date holothurian FCS structures. Stereospecific α-glycoside bond formation was achieved using 2-O-benzyl-3,4-di-O-chloroacetyl-α-l-fucosyl trichloroacetimidate as a donor. Stereochemical outcome of the glycosylation was explained by the remote participation of the chloroacetyl groups with the formation of the stabilized glycosyl cations, which could be attacked by the glycosyl acceptor only from the α-side. The experimental results were in good agreement with the SCF/MP2 calculated energies of such participation. The synthesized oligosaccharides are regarded as model compounds for the determination of a structure-activity relationship in FCS.

ChemInform Abstract: N‐Iodosuccinimide and Acid‐Washed Molecular Sieves (NIS/AW‐300 MS) as Promoters in Thioglycoside Activation.

AbstractThe methodology is applied to the glycosylation of various thioglycosides with glycosyl acceptors as well as a reactivity‐based one‐pot sequential glycosylation to a branched trisaccharide.

A synthetic strategy to xylose-containing thioglycoside tri- and tetrasaccharide building blocks corresponding to Cryptococcus neoformans capsular polysaccharide structures.

As part of an ongoing project aimed at developing vaccine candidates against Cryptococcus neoformans the preparation of tri- and tetrasaccharide thioglycoside building blocks, to be used in construction of structurally defined part structures of C. neoformans GXM capsular polysaccharide, was investigated. Using a naphthalenylmethyl (NAP) ether as a temporary protecting group and trichloroacetimidate donors in optimized glycosylations the target building blocks, ethyl 6-O-acetyl-2,4-di-O-benzyl-3-O-(2-naphthalenylmethyl)-α-D-mannopyranosyl-(1→3)-[2,3,4-tri-O-benzyl-β-D-xylopyranosyl-(1→2)]-4,6-di-O-benzyl-1-thio-α-D-mannopyranoside (16) and ethyl 2,3,4-tri-O-benzyl-β-D-xylopyranosyl-(1→2)-4,6-di-O-benzyl-3-O-(2-naphthalenylmethyl)-α-D-mannopyranosyl-(1→3)-[2,3,4-tri-O-benzyl-β-D-xylopyra-nosyl-(1→2)]-6-O-acetyl-4-O-benzyl-1-thio-α-D-mannopyranoside (21), were efficiently prepared. These synthesized thiosaccharide building blocks were then used as donors in high-yielding (~90%) DMTST promoted glycosylations to a spacer-containing acceptor to, after deprotection, afford GXM polysaccharide part structures ready for protein conjugation to give vaccine candidates. Also, the NAP groups in the building blocks were removed to obtain tri- and tetrasaccharide acceptors suitable for further elongation towards larger thiosaccharide building blocks.

Synthesis of uniformly deuterated n-dodecyl-β-D-maltoside (d39-DDM) for solubilization of membrane proteins in TROSY NMR experiments.

This work reports the first synthesis of uniformly deuterated n-dodecyl-β-D-maltoside (d39-DDM). DDM is a mild non-ionic detergent often used in the extraction and purification of membrane proteins and for solubilizing them in experimental studies of their structure, dynamics and binding of ligands. We required d39-DDM for solubilizing large α-helical membrane proteins in samples for [(15)N-(1)H]TROSY (transverse relaxation-optimized spectroscopy) NMR experiments to achieve the highest sensitivity and best resolved spectra possible. Our synthesis of d39-DDM used d7-D-glucose and d25-n-dodecanol to introduce deuterium labelling into both the maltoside and dodecyl moieties, respectively. Two glucose molecules, one converted to a glycosyl acceptor with a free C4 hydroxyl group and one converted to a glycosyl donor substituted at C1 with a bromine in the α-configuration, were coupled together with an α(1 → 4) glycosidic bond to give maltose, which was then coupled with n-dodecanol by its substitution of a C1 bromine in the α-configuration to give DDM. (1)H NMR spectra were used to confirm a high level of deuteration in the synthesized d39-DDM and to demonstrate its use in eliminating interfering signals from TROSY NMR spectra of a 52-kDa sugar transport protein solubilized in DDM.

Acceptor-induced modification of regioselectivity in CGTase-catalyzed glycosylations of p-nitrophenyl-glucopyranosides

Cyclodextrin glycosyltransferases (CGTase) are reported to selectively catalyze α(1→4)-glycosyl transfer reactions besides showing low hydrolytic activity. Here, the effect of the anomeric configuration of the glycosyl acceptor on the regioselectivity of CGTase catalyzed glycosylations was investigated. For this purpose, the α and β anomers of p-nitrophenyl-d-glucopyranoside were used as glycosyl acceptors, Bacillus macerans and Thermoanaerobacter sp. CGTases were used as biocatalysts and β-cyclodextrin as the glycosyl donor. As expected, p-nitrophenyl-α-d-glucopyranosyl-(1→4)-O-α-d-glucopyranoside was produced when p-nitrophenyl-α-d-glucopyranoside was used as acceptor with B. macerans CGTase. Surprisingly, when p-nitrophenyl-β-d-glucopyranoside was used as glycosyl acceptor, besides the expected α(1→4)-glycosylation products both α(1→3)- and α(1→6)-transfer products were also obtained. This unexpected change in B. macerans CGTase regioselectivity leading to α(1→4)-, α(1→3)- and α(1→6)-glycosylation products was also observed for Thermoanaerobacter sp. CGTase with the β anomer. It is shown, applying time course analyses, that all isomers can be synthesized efficiently by adequate selection of enzyme and reaction conditions. In fact, when using Thermoanaerobacter sp. CGTase the yield of p-nitrophenyl-β-d-isomaltoside (the α(1→6)-transfer product) was the highest at long reaction time (19% yield). The previously unknown capacity of α(1→6)-glycosidic linkages formation by CGTases demonstrates an unexpected broader regioselectivity of CGTases in glycosyl-transfer reactions as well as an acceptor dependent transfer selectivity.

A protocol for metal triflate catalyzed direct glycosylations with GalNAc 1-OPiv donors.

Herein we report on the development of novel glycosylation methodology for the concise synthesis of naturally occurring glycoconjugate motifs containing N-acetylgalactosamine (GalNAc) from the cheaper and commercially available N-acetylglucosamine (GlcNAc). The stereoselective glycosylations proceed with catalytic amounts of a promoter and without the need for N-protection other than the biologically relevant N-acetyl group. Among the catalysts explored, both Bi(OTf)3 and Fe(OTf)3 were found to be highly active Lewis acids for this reaction. It was also found that other less reactive metal triflates such as those of Cu(II) and Yb(III) can be beneficial in glycosylation reactions on more demanding glycosyl acceptors. We have furthermore demonstrated that it is possible to control the anomeric stereoselectivity in the glycosylation via postglycosylation in situ anomerization to obtain good yields of α-galactosides. The present protocol was used to prepare important naturally occurring carbohydrate motifs, including a trisaccharide fragment of the naturally occurring marine sponge clarhamnoside.

Synthesis of chlamydia lipopolysaccharide haptens through the use of α-specific 3-iodo-Kdo fluoride glycosyl donors.

A scalable approach towards high-yielding and (stereo)selective glycosyl donors of the 2-ulosonic acid Kdo (3-deoxy-D-manno-oct-2-ulosonic acid) is a fundamental requirement for the development of vaccines against Gram-negative bacteria. Herein, we disclose a short synthetic route to 3-iodo Kdo fluoride donors from Kdo glycal esters that enable efficient α-specific glycosylations and significantly suppress the elimination side reaction. The potency of these donors is demonstrated in a straightforward, six-step synthesis of a branched Chlamydia-related Kdo-trisaccharide ligand without the need for protecting groups at the Kdo glycosyl acceptor. The approach was further extended to include sequential iteration of the basic concept to produce the linear Chlamydia-specific α-Kdo-(2→8)-α-Kdo-(2→4)-α-Kdo trisaccharide in a good overall yield.

The intriguing dual-directing effect of 2-cyanobenzyl ether for a highly stereospecific glycosylation reaction.

The diverse presence as well as their very specific bio-responses of glycoconjugates found in all living species requires scientists to synthesize the precise structure of these complex oligosaccharides for various studies on glycoscience. Very few approaches were able to offer the sole α- or β-glycosylated products, even at the cost of complicating the preparative route or usage of exotic chiral auxiliaries to drive the stereoselectivity. In this report, the unification of solvent assistance and neighbouring group participation concepts have led us to the use of 2-cyanobenzyl ether as the dual-directing auxiliary for stereospecific construction of α- and β-glycosidic bonds from a single starting material, and both isomers can be obtained in exclusive stereoselectivity. This work demonstrates the difference in reactivities of glycosyl acceptors can be employed to completely drive the stereoselectivity, drawing the parallel comparison with the arming/disarming concept, which has been exclusively confined to glycosyl donors.

An efficient ionic liquid supported divergent assembly of 3,6-branched glucosamine-containing pentasaccharide

Abstract We utilized the glycosyl acceptor tagging method with ionic liquid support for synthesis of the core segment of Clostridium botulinum C2 toxin ligand through a divergent synthetic strategy without chromatographic purification. The total yield was 57.1% and the reaction was completed in 10 h. The efficient ionic liquid supported glycosylation and purification procedure was applied for the synthesis of branched glucosamine-containing oligosaccharides for the first time, which expanded the scope of ionic liquid supported synthesis of biologically important oligosaccharides.

Hydrogen bond mediated aglycone delivery: synthesis of linear and branched α-glucans.

A Hydrogen bond mediated aglycone delivery (HAD) method was applied to the synthesis of α-glucans, which are abundant in nature, but as targets represent a notable challenge to chemists. The synthesis of linear oligosaccharide sequences was accomplished in complete stereoselectivity in all glycosylations. The efficacy of HAD may diminish with the increased bulk of the glycosyl acceptor, and may be an important factor for the syntheses of oligomers beyond pentasaccharides. The synthesis of a branched structure proved more challenging, particularly with bulky trisaccharide acceptors.

Hydrogen Bond Mediated Aglycone Delivery: Synthesis of Linear and Branched α‐Glucans

AbstractA Hydrogen bond mediated aglycone delivery (HAD) method was applied to the synthesis of α‐glucans, which are abundant in nature, but as targets represent a notable challenge to chemists. The synthesis of linear oligosaccharide sequences was accomplished in complete stereoselectivity in all glycosylations. The efficacy of HAD may diminish with the increased bulk of the glycosyl acceptor, and may be an important factor for the syntheses of oligomers beyond pentasaccharides. The synthesis of a branched structure proved more challenging, particularly with bulky trisaccharide acceptors.

Characterization of novel galactosylated chitin-oligosaccharides and chitosan-oligosaccharides

This study determined whether chitin-oligosaccharides and chitosan-oligosaccharides (COS) are suitable acceptor carbohydrates for the LacLM-type β-galactosidase of Lactobacillus plantarum. Enzymatic reactions were carried out with lactose as galactosyl-donor and chitinbiose (N,N′-diacetyl chitobiose), chitintriose (N,N′N″-triacetyl chitotriose) or a highly deacetylated COS preparation with a degree of polymerization ranging from 2 to 4 as galactosyl acceptors. Transgalactosylated oligosaccharides were identified by liquid chromatography-mass spectrometry (LC-MS/MS). LC-MS/MS analysis demonstrated formation of mono- and digalactosylated chitinbiose, mono-galactosylated chitintriose, and mono-, di-, and trigalactosylated COS. β-(1→4)-Linkages were formed during the galactosylation of chitinbiose and chitintriose. In conclusion, the conversion of lactose with β-Gal from Lb. plantarum and chitin-oligosaccharides or COS as glycosyl acceptors allows the production of novel oligosaccharides.

N-Iodosuccinimide and acid-washed molecular sieves (NIS/AW-300 MS) as promoters in thioglycoside activation

The development of an alternative promoter system for thioglycoside activation comprising N-iodosuccinimide (NIS) and acid-washed molecular sieves (AW-300 MS) has been studied and found to mediate high-yielding coupling with a variety of glycosyl acceptors. The utility of this combination is further exemplified by its application to reactivity-based coupling of thioglycosides and sequential one-pot regioselective glycosylation.

Isoprenoid phosphonophosphates as glycosyltransferase acceptor substrates.

Glycosyltransferases that act on polyprenol pyrophosphate substrates are challenging to study because their lipid-linked substrates are difficult to isolate from natural sources and arduous to synthesize. To facilitate access to glycosyl acceptors, we assembled phosphonophosphate analogues and showed these are effective substrate surrogates for GlfT1, the essential product of mycobacterial gene Rv3782. Under chemically defined conditions, the galactofuranosyltransferase GlfT1 catalyzes the formation of a tetrasaccharide sequence en route to assembly of the mycobacterial galactan.