Total synthesis of the mollu-series glycosyl ceramides alpha-D-Manp-(1----3)-beta-D-Manp-(1----4)-beta-D-Glcp-(1----1)-Cer and alpha-D-Manp-(1----3)-[beta-D-Xylp-(1----2)]-beta-D-Manp-(1----4)-beta- D-Glcp-(1----1)-Cer

Can Side-Chain Conformation and Glycosylation Selectivity of Hexopyranosyl Donors Be Controlled with a Dummy Ligand?

The use of a phenylthio group (SPh) as a dummy ligand at the 6-position to control the side-chain conformation of a series of hexopyranosyl donors is described. The SPh group limits side-chain conformation in a configuration-specific manner, which parallels that seen in the heptopyranosides, and so influences glycosylation selectivity. With both d- and l-glycero-d-galacto-configured donors, the equatorial products are highly favored as they are with an l-glycero-d-gluco donor. For the d-glycero-d-gluco donor, on the other hand, modest axial selectivity is observed. Selectivity patterns are discussed in terms of the side-chain conformation of the donors in combination with the electron-withdrawing effect of the thioacetal group. After glycosylation, removal of the thiophenyl moiety and hydrogenolytic deprotection is achieved in a single step with Raney nickel.

Preparation of Tea Aroma Precursor Glycosides: An Efficient and Sustainable Approach via Chemical Glycosidation

Tea aroma precursor glycosides are plant-derived natural products with great economic value. However, the preparation of these glycosides remains largely overlooked in the past decades. Herein, we report a mild, efficient, and sustainable chemocatalytic procedure for the production of tea aroma precursor glycosides. During the study of the glycosidation, the catalysts were found to be decisive in the product formation favoring different reaction pathways; in addition, the influence of molecular sieves was elucidated. With regard to these findings, the serious problem of the competing orthoester formation side reaction was successfully overcome with low catalyst loading (1 mol %) and the use of 5 Å molecular sieves, leading to the preparation of a variety of tea aroma precursor β-d-glucopyranosides and β-primeverosides on a gram scale in high yields in an economical way. Taken together, the current approach features catalytic glycosidation with non-toxic and low-cost catalysts, demonstrates highly favorable greenness and sustainability, and promises industrial production of tea aroma precursor glycosides.

Total Synthesis of (+)-Erogorgiaene and the Pseudopterosin A-F Aglycone via Enantioselective Cobalt-Catalyzed Hydrovinylation

Due to their pronounced bioactivity and limited availability from natural resources, metabolites of the soft coral Pseudopterogorgia elisabethae, such as erogorgiaene and the pseudopterosines, represent important target molecules for chemical synthesis. We have now developed a particularly short and efficient route towards these marine diterpenes exploiting an operationally convenient enantioselective cobalt-catalyzed hydrovinylation as the chirogenic step. Other noteworthy C-C bond forming transformations include diastereoselective Lewis acid-mediated cyclizations, a Suzuki coupling and a carbonyl ene reaction. Starting from 4-methyl-styrene the anti-tubercular agent (+)-erogorgiaene (>98 % ee) was prepared in only 7 steps with 46 % overall yield. In addition, the synthesis of the pseudopterosin A aglycone was achieved in 12 steps with 30 % overall yield and, surprisingly, was found to exhibit a similar anti-inflammatory activity (inhibition of LPS-induced NF-κB activation) as a natural mixture of pseudopterosins A-D or iso-pseudopterosin A, prepared by β-D-xylosylation of the synthetic aglycone.

Synthesis of l-glycero- and d-glycero-d-manno-Heptose Building Blocks for Stereoselective Assembly of the Lipopolysaccharide Core Trisaccharide of Vibrio parahemolyticus O2

Synthesis of bacterial cell surface l-glycero-d-manno-heptose (l,d-Hep)- and d-glycero-d-manno-heptose (d,d-Hep)-containing higher carbon sugars is a challenging task. Here, we report a convenient and efficient approach for the synthesis of the l,d-Hep and d,d-Hep building blocks. Using l-lyxose and d-ribose as starting materials, this approach features diastereoselective Mukaiyama-type aldol reactions as the key steps. On the basis of the synthetic l,d-Hep and d,d-Hep building blocks, we achieved the first stereoselective synthesis of the unique α-l,d-Hep-(1→3)-α-d,d-Hep-(1→5)-α-Kdo core trisaccharide of the lipopolysaccharide of Vibrio parahemolyticus O2.

Total Synthesis of Natural Disaccharide Sambubiose

A practical and robust synthetic method to obtain the natural disaccharide sambubiose (2-O-β-D-xylopyranosyl-D-glucopyranose) is reported, exploring the key step in the synthesis, i.e., stereoselective O-glycosylation. Specifically, the best combinations of glycoside donors and acceptors were identified, stereospecific control of the reaction was achieved by screening several catalysts and protection/deprotection steps were evaluated and improved. The best result was obtained by coupling allyl 3,5,6-tri-O-benzyl-β-D-glucofuranoside with 2,3,4-tri-O-acetyl-D-xylopiranosyl-α-trichloro acetimidate in the presence of trimethylsilyl triflate as a catalyst giving the corresponding protected target compound as a correct single isomer. The latter was transformed accordingly into the desired final product by deprotection steps (deallylation, deacetylation, and debenzylation). Sambubiose was synthesized into a satisfactory and higher overall yield than previously reported and was also characterized.

Multivalent Carbohydrate-Functionalized Polymer Nanocrystals

Nanoparticles with a covalently bound shell of carbohydrate or sulfate groups, respectively, and a polyethylene core were generated by Ni(II)-catalyzed aqueous copolymerization of ethylene with comonomers undec-10-en-1-yl sulfate, undec-10-en-1-yl β-d-glucoside or undec-10-en-1-yl α-d-mannoside, respectively. Via remote substituents of the catalyst, the degree of branching and consequently degree of crystallinity of the polyethylene core of the glyconanoparticles could be controlled. This in turn impacts particle shapes, from spherical to anisotropic platelets, as observed by cryo-transmission electron microscopy. Enzyme-linked lectin assays revealed the mannose-decorated nanocrystals to be efficient multivalent ligands for concavalin A.

Chemistry of xylopyranosides

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

A hydrophobic disordered peptide spontaneously anchors a covalently bound RNA hairpin to giant lipidic vesicles

The attraction of nucleic acids to lipidic compartments is the first step for carriers of potentially inheritable information to self-organise in functionalised synthetic cells. Confocal fluorescence imaging shows that a synthetic amphiphilic peptidyl RNA molecule spontaneously accumulates at the outer bilayer membranes of phospho- and glycolipidic giant vesicles. Cooperatively attractive interactions of -3.4 to -4.0 kcal mol(-1) between a random coil hydrophobic peptide and lipid membranes can thus pilot lipophobic RNA to its compartmentation. The separation of mixed lipid phases in the membranes further enhances the local concentration of anchored RNA.

Synthesis and microarray-assisted binding studies of core xylose and fucose containing N-glycans

The synthesis of a collection of 33 xylosylated and core-fucosylated N-glycans found only in nonmammalian organisms such as plants and parasitic helminths has been achieved by employing a highly convergent chemo-enzymatic approach. The influence of these core modifications on the interaction with plant lectins, with the human lectin DC-SIGN (Dendritic Cell-Specific Intercellular adhesion molecule-3-Grabbing Nonintegrin), and with serum antibodies from schistosome-infected individuals was studied. Core xylosylation markedly reduced or completely abolished binding to several mannose-binding plant lectins and to DC-SIGN, a C-type lectin receptor present on antigen presenting cells. Employing the synthetic collection of core-fucosylated and core-xylosylated N-glycans in the context of a larger glycan array including structures lacking these core modifications, we were able to dissect core xylose and core fucose specific antiglycan antibody responses in S. mansoni infection sera, and we observed clear and immunologically relevant differences between children and adult groups infected with this parasite. The work presented here suggests that, quite similar to bisecting N-acetylglucosamine, core xylose distorts the conformation of the unsubstituted glycan, with important implications for the immunogenicity and protein binding properties of complex N-glycans.

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.

Mining for hemicellulases in the fungus-growing termite Pseudacanthotermes militaris using functional metagenomics

BACKGROUND

The metagenomic analysis of gut microbiomes has emerged as a powerful strategy for the identification of biomass-degrading enzymes, which will be no doubt useful for the development of advanced biorefining processes. In the present study, we have performed a functional metagenomic analysis on comb and gut microbiomes associated with the fungus-growing termite, Pseudacanthotermes militaris.

RESULTS

Using whole termite abdomens and fungal-comb material respectively, two fosmid-based metagenomic libraries were created and screened for the presence of xylan-degrading enzymes. This revealed 101 positive clones, corresponding to an extremely high global hit rate of 0.49%. Many clones displayed either β-d-xylosidase (EC 3.2.1.37) or α-l-arabinofuranosidase (EC 3.2.1.55) activity, while others displayed the ability to degrade AZCL-xylan or AZCL-β-(1,3)-β-(1,4)-glucan. Using secondary screening it was possible to pinpoint clones of interest that were used to prepare fosmid DNA. Sequencing of fosmid DNA generated 1.46 Mbp of sequence data, and bioinformatics analysis revealed 63 sequences encoding putative carbohydrate-active enzymes, with many of these forming parts of sequence clusters, probably having carbohydrate degradation and metabolic functions. Taxonomic assignment of the different sequences revealed that Firmicutes and Bacteroidetes were predominant phyla in the gut sample, while microbial diversity in the comb sample resembled that of typical soil samples. Cloning and expression in E. coli of six enzyme candidates identified in the libraries provided access to individual enzyme activities, which all proved to be coherent with the primary and secondary functional screens.

CONCLUSIONS

This study shows that the gut microbiome of P. militaris possesses the potential to degrade biomass components, such as arabinoxylans and arabinans. Moreover, the data presented suggests that prokaryotic microorganisms present in the comb could also play a part in the degradation of biomass within the termite mound, although further investigation will be needed to clarify the complex synergies that might exist between the different microbiomes that constitute the termitosphere of fungus-growing termites. This study exemplifies the power of functional metagenomics for the discovery of biomass-active enzymes and has provided a collection of potentially interesting biocatalysts for further study.

Supported Membranes Meet Flat Fluidics: Monitoring Dynamic Cell Adhesion on Pump-Free Microfluidics Chips Functionalized with Supported Membranes Displaying Mannose Domains

In this paper we demonstrate the combination of supported membranes and so-called flat microfluidics, which enables one to manipulate liquids on flat chip surfaces via “inverse piezoelectric effect”. Here, an alternating external electric field applied to the inter-digital transducers excites a surface acoustic wave on a piezoelectric substrate. Employing lithographic patterning of self-assembled monolayers of alkoxysilanes, we successfully confine a free-standing, hemi-cylindrical channel with the volume of merely 7 µL . The experimentally determined maximum flow velocity scales linearly with the acoustic power, suggesting that our current setup can drive liquids at the speed of up to 7 cm/s (corresponding to a shear rate of 280 s⁻¹) without applying high pressures using a fluidic pump. After the establishment of the functionalization of fluidic chip surfaces with supported membranes, we deposited asymmetric supported membranes displaying well-defined mannose domains and monitored the dynamic adhesion of E. Coli HB101 expressing mannose-binding receptors. Despite of the further technical optimization required for the quantitative analysis, the obtained results demonstrate that the combination of supported membranes and flat fluidics opens a large potential to investigate dynamic adhesion of cells on biofunctional membrane surfaces with the minimum amount of samples, without any fluidic pump.

Glycosylation of Nα-lauryl-O-(β-D-xylopyranosyl)-L-serinamide as a saccharide primer in cells

N(α)-Lauryl-O-(β-D-xylopyranosyl)-L-serinamide (Xyl-Ser-C12) was synthesized as a saccharide primer to obtain oligosaccharides of glycosaminoglycan using the glycan biosynthetic potential of mouse osteosarcoma FBJ-S1 cells and Chinese hamster ovary (CHO) cells. The glycosylated products secreted into the culture medium were collected and analyzed by liquid chromatography-mass spectrometry and glycosidase digestion. The structure of the Xyl-Ser-C12 derivatives was investigated. Several glycosaminoglycan-type oligosaccharides, such as GalNAc-(GlcA-GlcNAc)(n)-GlcA-Gal-Gal-Xyl-Ser-C12, were detected, and identified as intermediates of the biosynthesis of heparan sulfate glycosaminoglycans. Xyl-Ser-C12 exhibited greater acceptor activity for the glycosylation of glycosaminoglycan-type oligosaccharides than p-nitrophenyl-β-D-xylopyranoside.

Mechanistic insights into the 1,3-xylanases: useful enzymes for manipulation of algal biomass

Xylanases capable of degrading the crystalline microfibrils of 1,3-xylan that reinforce the cell walls of some red and siphonous green algae have not been well studied, yet they could prove to be of great utility in algaculture for the production of food and renewable chemical feedstocks. To gain a better mechanistic understanding of these enzymes, a suite of reagents was synthesized and evaluated as substrates and inhibitors of an endo-1,3-xylanase. With these reagents, a retaining mechanism was confirmed for the xylanase, its catalytic nucleophile identified, and the existence of -3 to +2 substrate-binding subsites demonstrated. Protein crystal X-ray diffraction methods provided a high resolution structure of a trapped covalent glycosyl-enzyme intermediate, indicating that the 1,3-xylanases likely utilize the (1)S(3) → (4)H(3) → (4)C(1) conformational itinerary to effect catalysis.

A highly efficient deprotection of the 2,2,2-trichloroethyl group at the anomeric oxygen of carbohydrates

Commercially available zinc dust in the presence of ammonium chloride in acetonitrile at reflux removes the 2,2,2-trichloroethyl (TCE) group at anomeric centers with excellent yields (>95%) in short reaction times. This present method is easily implemented on substrates containing acyl and benzyl groups and large-scale reactions also proceed in high yield.

Regulation of adhesion behavior of murine macrophage using supported lipid membranes displaying tunable mannose domains

Highly uniform, strongly correlated domains of synthetically designed lipids can be incorporated into supported lipid membranes. The systematic characterization of membranes displaying a variety of domains revealed that the equilibrium size of domains significantly depends on the length of fluorocarbon chains, which can be quantitatively interpreted within the framework of an equivalent dipole model. A mono-dispersive, narrow size distribution of the domains enables us to treat the inter-domain correlations as two-dimensional colloidal crystallization and calculate the potentials of mean force. The obtained results demonstrated that both size and inter-domain correlation can precisely be controlled by the molecular structures. By coupling α-D-mannose to lipid head groups, we studied the adhesion behavior of the murine macrophage (J774A.1) on supported membranes. Specific adhesion and spreading of macrophages showed a clear dependence on the density of functional lipids. The obtained results suggest that such synthetic lipid domains can be used as a defined platform to study how cells sense the size and distribution of functional molecules during adhesion and spreading.

Synthetic arabinomannan glycolipids and their effects on growth and motility of the Mycobacterium smegmatis

Arabinomannan-containing glycolipids, relevant to the mycobacterial cell-wall component lipoarabinomannan, were synthesized by chemical methods. The glycolipids were presented with tri- and tetrasaccharide arabinomannans as the sugar portion and a double alkyl chain as the lyophilic portion. Following synthesis, systematic biological and biophysical studies were undertaken in order to identify the effects of the glycolipids during mycobacterium growth. The studies included mycobacterial growth, biofilm formation and motility assays. From the studies, it was observed that the synthetic glycolipid with higher arabinan residues inhibited the mycobacterial growth, lessened the biofilm formation and impaired the motility of mycobacteria. A surface plasmon resonance study involving the immobilized glycan surface and the mycobacterial crude lysates as analytes showed specificities of the interactions. Further, it was found that cell lysates from motile bacteria bound oligosaccharide with higher affinity than non-motile bacteria.

Chemoenzymatic elaboration of monosaccharides using engineered cytochrome P450BM3 demethylases

Polysaccharides comprise an extremely important class of biopolymers that play critical roles in a wide range of biological processes, but the synthesis of these compounds is challenging because of their complex structures. We have developed a chemoenzymatic method for regioselective deprotection of monosaccharide substrates using engineered Bacillus megaterium cytochrome P450 (P450(BM3)) demethylases that provides a highly efficient means to access valuable intermediates, which can be converted to a wide range of substituted monosaccharides and polysaccharides. Demethylases displaying high levels of regioselectivity toward a number of protected monosaccharides were identified using a combination of protein and substrate engineering, suggesting that this approach ultimately could be used in the synthesis of a wide range of substituted mono- and polysaccharides for studies in chemistry, biology, and medicine.

Synthesis and characterization of mannosylated oligoribonucleotides

Oligoribonucleotide (RNA)-carbohydrate conjugates bearing mono- and divalent mannosides were readily obtained using 3,4-diethoxy-3-cyclobutene-1,2-dione as the linking agent in the presence of trace amount of triethylamine. The glycoconjugates were purified by HPLC and characterized by electrospray mass spectroscopy.

Total synthesis of adicardin

The first synthesis of adicardin, a compound with anti-chronic renal failure activity isolated from Hydrangea macrophylla, has been described. The structures of the target compound and intermediates have been validated by MS, NMR, and identical with the natural product.

Synthesis of the Mixed Acetal Segment of S-Glyceroplasmalopsychosine

In this report the concept of converting carbohydrate to non-carbohydrate asymmetric molecules has been successfully exploited. The mixed acetal segment of glyceroplasmalopsychosine, a novel glycolipid has been synthesized in a stereo-specific manner using two simple sugar units. The glycosidation reaction between these two monosaccharides ensured the correct acetal stereocenter of the target molecule. Either olefin metathesis or heterogeneous Wittig reactions were used for constructing the long aliphatic chain of glyceroplasmalopsychosine.

Non-reducing end alpha-mannosylated glycolipids as potent activators for invariant Valpha19 TCR-bearing natural killer T cells

A novel invariant Valpha19-Jalpha33 T cell receptor alpha chain, first found in mammalian blood cells, was primarily expressed by natural killer T cell repertoire (Valpha19 NKT cell). Attempts have been made to find specific antigens for Valpha19 NKT cells. A series of alpha- and beta-glycosyl ceramides were synthesized and tested whether they had potential to stimulate the cells isolated from invariant Valpha19-Jalpha33 TCR transgenic mice (where the development of Valpha19 NKT cells is facilitated). Comprehensive examinations revealed substantial antigenic activity in alpha-ManCer that was presented by MR1, one of the MHC class Ib molecules. Next, naturally occurring and synthetic alpha-mannosyl glycolipids were further analyzed to determine structural requirements for natural ligands for Valpha19 NKT cells. As a result, alpha-mannosyl phosphatidyl inositols (PI) such as (alpha-Man)(2)-PI and alpha-Man-alpha-GlcNH(2)-PI (a partial structure of mycobacterial lipoarabinomannan and GPI-anchors) as well as alpha-ManCer derivatives were found to activate Valpha19 NKT cells in vivo and in vitro. Thus, Valpha19 NKT cells are possibly responsive to certain alpha-mannosyl glycolipids and may have roles in the innate and adaptative immune systems to protect against various antigens expressing alpha-mannosyl glycolipids and to regulate the adaptive immune system responding to the intracellular ligands.

Highly β-Selective and Direct Formation of 2-O-Glycosylated Glucosides by Ring Restriction into Twist-Boat

Three disaccharide donors, ethyl 2-O-(2,3,4-tris-O-tert-butyldimethylsilyl-beta-D-xylopyranosyl)-3,4,6-tris-O-tert-butyldimethylsilyl-1-thio-beta-D-glucopyranoside, ethyl 2-O-(2,3,4-tris-O-tert-butyldimethylsilyl-alpha-L-rhamnopyranosyl)-3,4,6-tris-O-tert-butyldimethylsilyl-1-thio-beta-D-glucopyranoside, and ethyl 2-O-(2,3,4,6-tetrakis-O-tert-butyldimethylsilyl-beta-D-glucopyranosyl)-3,4,6-tris-O-tert-butyldimethylsilyl-1-thio-beta-D-glucopyranoside, produced a highly beta-selective glycosidation up to alpha/beta = 2/98 using MeOTf as the activator and 2,6-lutidine as an additive. The ring conformations of the glucose part in these disaccharide donors were all restricted to 3S1, and the conformation would lead to the stereoselectivity.

Glycolipids with nonreducing end alpha-mannosyl residues that have the potential to activate invariant Valpha19 NKT cells

We have previously demonstrated that alpha-mannosyl ceramide and its derivatives promote immune responses of NK1.1(+) invariant Valpha19-Jalpha33 T cell receptor (TCR) alpha(+) T cells (Valpha19 NKT cells). In this study, attempts were made to determine the structural requirements for natural ligands for Valpha19 NKT cells. Naturally occurring and synthetic glycolipids were analyzed for their ability to stimulate the cells prepared from invariant Valpha19-Jalpha33 TCR transgenic mice, in which development of Valpha19 NKT cells is facilitated. As a result, alpha-mannosyl phosphatidylinositols such as 2,6-di-alpha-mannosyl phosphatidylinositol and alpha-mannosyl-4alpha-glucosaminyl-6-phosphatidylinositol (alpha-Man-GlcNH(2)-PtdIns) as well as alpha-mannosyl ceramide derivatives were found to activate the cells from the transgenic mouse liver, gut lamina propria and spleen in vivo and in vitro. Thus, glycolipids with nonreducing end alpha-mannosyl residues are suggested to be potent antigens for Valpha19 NKT cells. Next, a series of invariant Valpha19-Jalpha33 TCR(+) hybridomas, each with variations in the sequence of the Valpha-Jalpha junction and the TCR beta chain, were tested for responsiveness toward the alpha-mannosyl glycolipids. A loose correlation between the primary structure of the TCR and the reactive glycolipids was observed. For instance, hybridomas expressing TCRs consisting of an alpha chain with a variation in the Valpha19-Jalpha33 junction and a Vbeta6(+)beta chain showed affinity towards alpha-mannosyl ceramide and alpha-Man-GlcNH(2)-PtdIns, whereas those expressing TCRs with an invariant Valpha19-Jalpha33 alpha chain and a Vbeta8(+)beta chain responded to 2,6-di-alpha-mannosyl phosphatidylinositol. Thus, it is suggested that Valpha19 NKT cells with microheterogeneity in the TCR structure have been generated for defense against various antigens expressing alpha-mannosyl glycolipids.

Synthesis and biology of bis-xylosylated dihydroxynaphthalenes

The 10 analogous bis-xylosylated dihydroxynaphthalenes have been synthesized and their chemical and biological properties investigated. The yield of the xylosylation reactions can be correlated to the electrostatic potential, and thus to the nucleophilicity, for the oxygen atoms of the dihydroxynaphthalenes. The bis-xylosylated compounds were more stable compared to the mono-xylosylated ones. They initiate priming of glycosaminoglycan chains to less extent but the priming proceeds in two directions. Contrary to the mono-xylosylated analogs, the tested compounds did not show any antiproliferative properties.

Total synthesis of natural product (R)-4-phenyl-2-O-[beta-D-xylopyranosyl(1-->6)-beta-D-glucopyranosyl]butane and its epimer

The new compound (R)-4-phenyl-2-O-[beta-d-xylopyranosyl(1 --> 6)-beta-D-glucopyranosyl]butane (1) and its epimer (2), together with (R)-4-phenyl-2-O-beta-D-glucopyranosyl butane (24) and (S)-4-phenyl-2-O-beta-D-glucopyranosyl butane (25) were firstly synthesized from 4-phenylbutan-2-one and glucose. The absolute configuration of C-2 for 1 was confirmed as R. Their anti-tumour activities were evaluated.

Synthesis of complex-type glycans derived from parasitic helminths

Chemical syntheses of complex-type glycans derived from the eggs of parasitic helminths, Schistosoma mansoni and Schistosoma japonicum were achieved. In addition, their analogs, which lack xylose and/or fucose residue(s), are described. These branched sugar chains were synthesized regio- and stereoselectively by using beta-mannosylation, desilylation under high-pressure and glycosylation in frozen solvent as key transformations.

Synthesis of 3-O-(β-d-xylopyranosyl-(1→2)-β-d-glucopyranosyl)-3′-O-(β-d-glucopyranosyl)tamarixetin, the putative structure of aescuflavoside A from the seeds of Aesculus chinensis

3-O-(beta-D-xylopyranosyl-(1-->2)-beta-D-glucopyranosyl)-3'-O-(beta-D-glucopyranosyl)tamarixetin, the putative flavonal glycoside named aescuflavoside A, isolated from the seeds of Aesculus chinensis, is synthesized via regioselective glycosylation of 7-O-benzyltamarixetin with glycosyl bromides under phase-transfer-catalyzed conditions.

Regioselective Reductive Openings of Acetals; Mechanistic Details and Synthesis of Fluorescently Labeled Compounds

Regioselective reductive openings of mixed phenolic-benzylic acetals, using BH3.NMe3-AlCl3, was investigated, and a mechanism where the outcome is directed by the electrostatic potential of the two oxygen atoms is presented. The regioselective acetal opening was used in the synthesis of a fluorescently labeled analogue to antiproliferative xylosides. The fluorescently labeled xyloside was tested for uptake, antiproliferative activity, and glycosaminoglycan priming in different cell lines. The xyloside was taken up by all cell lines but did not initiate glycosaminoglycan biosynthesis.

Synthesis of complex-type glycans derived from parasitic helminths

Chemical synthesis of complex-type glycans 1 and 2 derived from eggs of parasitic helminths, Schistosoma mansoni and Schistosoma japonicum, is described. These branched sugar chains were synthesized regio- and stereoselectively by using beta-mannosylation, desilylation under high pressure, and glycosylation in frozen solvent as key transformations.

Mannose–pepstatin conjugates as targeted inhibitors of antigen processing

The molecular details of antigen processing, including the identity of the enzymes involved, their intracellular location and their substrate specificity, are still incompletely understood. Selective inhibition of proteolytic antigen processing enzymes such as cathepsins D and E, using small molecular inhibitors such as pepstatin, has proven to be a valuable tool in investigating these pathways. However, pepstatin is poorly soluble in water and has limited access to the antigen processing compartment in antigen presenting cells. We have synthesised mannose-pepstatin conjugates, and neomannosylated BSA-pepstatin conjugates, as tools for the in vivo study of the antigen processing pathway. Conjugation to mannose and to neomannosylated BSA substantially improved the solubility of the conjugates relative to pepstatin. The mannose-pepstatin conjugates showed no reduction in inhibition of cathepsin E, whereas the neomannosylated BSA-pepstatin conjugates showed some loss of inhibition, probably due to steric factors. However, a neomannosylated BSA-pepstatin conjugate incorporating a cleavable disulfide linkage between the pepstatin and the BSA showed the best uptake to dendritic cells and the best inhibition of antigen processing.

Synthesis of two bidesmosidic ursolic acid saponins bearing a 2,3-branched trisaccharide residue

The focus of this work was on the synthesis of two bidesmosidic ursolic acid saponins bearing a 2,3-branched trisaccharide residue. Therefore, 3-O-{[beta-D-glucopyranosyl-(1-->2)]-[alpha-L-arabinopyranosyl-(1-->3)]-alpha-L-arabinopyranosyl}ursolic acid-28-O-[beta-D-glucopyranosyl] ester 1 was concisely synthesized by two strategies in 22% and 41% overall yield, respectively, and another congener 3-O-{[beta-D-xylopyranosyl-(1-->2)]-[beta-D-glucopyranosyl-(1-->3)]-alpha-L-arabinopyranosyl}ursolic acid-28-O-[beta-D-glucopyranosyl] ester 2 was also efficiently prepared in 81% overall yield. The (1)H NMR and (13)C NMR signals of saponin 2 are all consistent with those reported for the natural product.

The synthesis of a carbohydrate-like dihydrooxazine and tetrahydrooxazine as putative inhibitors of glycoside hydrolases: A direct synthesis of isofagomine

The treatment of benzyl 2,3-O-isopropylidene-β-L-xylopyranoside with N-hydroxyphthalimide under Mitsunobu conditions, followed by protecting-group interchange, gave benzyl 4-O-[(tert-butoxycarbonyl)amino]-2,3- O-isopropylidene-α-D-arabinoside. Mild acid hydrolysis and catalytic hydrogenolysis afforded 4-O-[(tert-butoxycarbonyl)amino]-D-arabinose that, upon heating in water, gave the dihydrooxazine [(4R,5S,6R)-5,6-dihydro-4,5-dihydroxy-6-hydroxymethyl-4H-1,2-oxazine] as a crystalline solid. A single-crystal structure determination of this solid showed it to exist in the 5H6 conformation. Reduction of the dihydrooxazine gave the tetrahydrooxazine [(4R,5S,6R)-4,5-dihydroxy-6-hydroxymethyl-3,4,5,6-tetrahydro-2H-1,2-oxazine]. The dihydrooxazine was an effective inhibitor of two β-glucosidases (Ki = 27 and 35 µM). Benzyl 2,3-O-isopropylidene-β-L-xylopyranoside, via the derived imidazylate, was converted into a nitrile that, upon reduction and protecting-group manipulations, gave benzyl 4-C-aminomethyl-4-deoxy-α-D-arabinoside. Treatment of this amine with hydrogen and palladium-on-carbon gave isofagomine.Key words: dihydrooxazine, tetrahydrooxazine, isofagomine, iminosugars, glycosidase inhibitors.