An efficient route to per-O-acetylated hexofuranoses

Strategies toward protecting group-free glycosylation through selective activation of the anomeric center

Glycosylation is an immensely important biological process and one that is highly controlled and very efficient in nature. However, in a chemical laboratory the process is much more challenging and usually requires the extensive use of protecting groups to squelch reactivity at undesired reactive moieties. Nonetheless, by taking advantage of the differential reactivity of the anomeric center, a selective activation at this position is possible. As a result, protecting group-free strategies to effect glycosylations are available thanks to the tremendous efforts of many research groups. In this review, we showcase the methods available for the selective activation of the anomeric center on the glycosyl donor and the mechanisms by which the glycosylation reactions take place to illustrate the power these techniques.

Tetranuclear zinc cluster: a dual purpose catalyst for per-O-acetylation and de-O-acetylation of carbohydrates

A tetranuclear Zn cluster-catalyzed per-O-acetylation and de-O-acetylation of carbohydrates has been reported.

Galactofuranosyl glycosides: immunomodulatory effects on macrophages and in vivo enhancement of lethality on sepsis

Galactofuranoside derivatives were synthesised by the classic Fischer glycosydation method, and their immune modulation properties were studied in vitro and in vivo. NMR spectroscopic and ESI-MS analyses confirmed the purity and authenticity of all derivatives. Their phagocyte capacities were tested in resident macrophages. Methyl β-galactofuranoside (GFB-Me) and n-octyl β-galactofuranoside (GFB-O) had an immune stimulant effect at 25μmolml(-1) with an enhancement of 35.12%±0.06 SD and 17.49%±0.11 SD, respectively, but Methyl α-galactofuranoside (GFA-Me) and n-octyl α-galactofuranoside (GFA-O) gave a low immune response. Methyl α-galactofuranoside 5,6-O-isopropylidene (GFA-IP) and Methyl β-galactofuranoside 5,6-O-isopropylidene (GFB-IP) had negative values relative to the control group of minus 4.96%±0.10 SD and -40.72%±0.07 SD, respectively. Furthermore, GFB-Me and GFB-Me-IP were evaluated in vivo on the lethality induced by cecal ligation and puncture. Cytokine levels and iNOS expression were determined and correlated to mortality data. The results showed that the free HO-5 and HO-6 and the β-configuration are essential for the induction of phagocytic activity by the galactofuranosyl units. The methyl β-galactofuranosides also enhanced lethality during sepsis, increasing the levels of pro-inflammatory cytokines and iNOS expression.

Preparation and properties of the aldohexofuranose pentaacetates

The preparation of one isomer of each of the 16 enantiomeric pairs of the aldohexofuranose pentaacetates is described together with (1)H and (13)C NMR data. Eight of the isomers have been obtained crystalline. Equilibrium values for the anomeric pairs have been determined.

Nitrosation of sugar oximes: preparation of 2-glycosyl-1-hydroxydiazene-2-oxides

Oximes of glucose, xylose, lactose, fructose, and mannose have been prepared. Nitrosation of the oximes of glucose, xylose, and lactose with NaNO2/HCl afforded 2-(beta-glycopyranosyl)-1-hydroxydiazene-2-oxides, which were isolated as salts 13, 22, and 28. Nitrosation of fructose oxime 29 furnished fructose, whereas nitrosation of mannose oxime 30 with NaNO2/HCl afforded the 1-hydroxy-2-(beta-D-mannopyranosyl)diazene-2-oxide 32, from which the p-anisidinium salt 31 and the sodium salt 33 were prepared. However, nitrosation of 30 with isopentyl nitrite in aqueous solutions of CsOH or KOH resulted in the formation of the 2-(alpha-D-mannofuranosyl)-1-hydroxydiazene-2-oxide salts 34 and 35, respectively. Methylation of the ammonium 2-(beta-D-glucopyranosyl)-1-hydroxydiazene-2-oxide 13 yielded the 1-methoxy compound, which was benzoylated to afford the tetra-O-benzoate 14a, the structure of which was confirmed by X-ray diffraction analysis. From the glucose O-methyloximes 15 and 16 the N-methoxy-N-nitroso-2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosylamine 18 was prepared. The structure of this compound was confirmed by X-ray diffraction analysis. Treatment of acetobromoglucose with cupferron furnished the 1-(2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyloxy)-2-phenyldiazene-2-oxide 20.

First O-Glycosylation from Unprotected 1-Thioimidoyl Hexofuranosides Assisted by Divalent Cations

The preparation of O-hexofuranosides was accomplished from unprotected 1-thioimidoyl furanosides as donors. The present methodology was first used for the synthesis of octyl galactofuranoside and further extended to D-galactofuranose-containing disaccharides. Within this study, we emphasized the need for additional complexing cations to maintain the furanose ring in its initial size. After experimentation, calcium ion was first used concomitantly with trimethylsilyl trifluoromethanesulfonate, the latter being able to activate the thioimidate and the former being likely to inhibit ring expansion. Moreover, an improvement was performed by using copper(II) trifluoromethanesulfonate which could then meet the requirements as both promoter and complexing agent.

Thioimidoyl furanosides as first inhibitors of the α-l-arabinofuranosidase AbfD3

Two sets of five thioimidoyl alpha-L-arabino- and beta-D-galactofuranosides were designed, synthesized and subjected to docking studies to evaluate their ability to be recognized by the active site of the alpha-L-arabinofuranosidase AbfD3. Further in vitro assays showed that the targeted furanosides are the first potent inhibitors of this furanosyl hydrolase and that the most efficient one, the thiazolyl alpha-L-arabinofuranoside 1, is a competitive inhibitor having a KI of 1.4 microM.

Synthesis of galactofuranose-containing disaccharides using thioimidoyl-type donors

Four galactofuranose-containing disaccharides have been prepared utilising various thioimidates [Galf-SC(NR)XR'] and suitably protected acceptors as key precursors. We observed that the efficiency of the coupling reactions was particularly dependent on the aglycon present on the furanosyl donor when copper(II) ions were used as the promoter, and that activation could be correlated with the nature of the third heteroatom, X.

Facile synthesis of benzyl β-d-galactofuranoside. A convenient intermediate for the synthesis of d-galactofuranose-containing molecules

Benzyl beta-D-galactofuranoside was efficiently obtained from 1,2,3,5,6-penta-O-benzoyl-alpha,beta-D-galactofuranose, via benzyl 2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranoside. Conditions for the O-debenzylation were investigated in order to evaluate the synthetic application of the benzyl group as an anomeric protector of a galactofuranose moiety in synthetic strategies involving galactofuranose.

Total Synthesis of Agelagalastatin

The total synthesis of agelagalastatin, an antineoplastic glycosphingolipid, has been achieved. The synthesis involved an alpha-selective glycosylation of the ceramide moiety with the trisaccharide fluoride. The trisaccharide component was constructed employing the CB glycoside method which permitted a completely alpha-stereoselective galactofuranosylation.

General one-step synthesis of free hexofuranosyl 1-phosphates using unprotected 1-thioimidoyl hexofuranosides

A general one-step strategy is developed for the synthesis of hexofuranosyl 1-phosphates starting from new unprotected glycofuranosyl donors. It required first the preparation of new 1-thiohexofuranosides bearing a thioimidoyl heterocycle as a leaving group. The presence of sulfur and/or nitrogen atom(s) on the aglycon allowed remote activation of these thioglycofuranosides by anhydrous phosphoric acid and led to the target phosphates 9, 27, 29, and 30 in good to excellent selectivities and, more importantly, with very limited or no ring expansion. Moreover, this one-step phosphorylation reaction could be significantly improved by avoiding any tedious protecting group manipulations on negatively charged compounds and by focusing on a simple but general procedure of purification. This approach was applied to the diastereocontrolled synthesis of d-galacto- and d-glucofuranosyl 1-phosphates and also to the preparation of rare epimer and/or deoxy counterparts, that is, d-manno- and d-fucofuranosyl derivatives.

An original chemoenzymatic route for the synthesis of β-d-galactofuranosides using an α-l-arabinofuranosidase

DGalactofuranose is a widespread component of cell wall polysaccharides in bacteria, protozoa and fungi, but is totally absent in mammals. Importantly, galactofuranose is a key constituent of major cell envelope polysaccharides in pathogenic mycobacteria. In this respect, galactofuranose-based glycoconjugates are interesting target molecules for drug design. O-Glycosidases and notably beta-D-galactofuranosidases could be useful tools for the chemoenzymatic synthesis of galactofuranosides, but to date no studies of this type have been reported. Here we report the use of a GH 51 alpha-l-arabinofuranosidase for the synthesis of beta-D-galactofuranosides. We have demonstrated that this enzyme can catalyse both the autocondensation of p-nitrophenyl-beta-D-galactofuranoside and the transgalactofuranosylation of benzyl alpha-D-xylopyranoside, forming p-nitrophenyl beta-D-galactofuranosyl-(1-->2)-beta-D-galactofuranoside and benzyl beta-D-galactofuranosyl-(1-->2)-alpha-D-xylopyranoside, respectively. Both reactions were very regiospecific and the reaction involving benzyl alpha-D-xylopyranoside afforded very high yields (74.8%) of the major product. To our knowledge, this demonstration of chemoenzymatic synthesis of galactofuranosides constitutes the very first use of an O-glycosidase for the synthesis of galactofuranosides.

Facile Cu(OTf)2-Catalyzed Preparation of Per-O-acetylated Hexopyranoses with Stoichiometric Acetic Anhydride and Sequential One-Pot Anomeric Substitution to Thioglycosides under Solvent-Free Conditions

Solvent-free per-O-acetylation of hexoses with a stoichiometric amount of acetic anhydride employing 0.03 mol % Cu(OTf)2 proceeded in high yields (90-99%) at room temperature to give exclusively pyranosyl products as an anomeric mixture, the alpha/beta ratio of which was dependent on the temperature and amount of catalyst used. Sequential anomeric substitution with p-thiocresol in the presence of BF3.etherate gave the thioglycosides, isolated exclusively or predominantly as one anomer in 66-75% yields.

Convenient approach to higher carbon sugars. First synthesis of the free C12-sugar: D-erythro-L-manno-D-manno-dodecose

The model synthesis of a C12-aldose was initiated from the easily available dimethyl(benzyl 2,3,4-tri-O-benzyl-alpha-D-manno-heptopyranos-6-ulos-7-yl)phosphonate and 2,3:4,5-di-O-isopropylidene-D-arabinose.

A novel synthesis of D-galactofuranosyl, D-glucofuranosyl and D-mannofuranosyl 1-phosphates based on remote activation of new and free hexofuranosyl donors

The selective synthesis of 1,2-cis-hexofuranosyl 1-phosphates was readily accomplished according to a procedure based on the 'Remote Activation Concept'. This approach required (i) the preparation of suitable 1,2-trans-hexofuranosyl donors, so that new heterocyclic thiofuranosides were designed and synthesized, (ii) the stereocontrolled phosphorylation of the corresponding unprotected donors and (iii) the simple and fast purification of the resulting anomeric phosphates. This approach showed to be equally efficient in the galactose, glucose and mannose series.