Expeditious oligosaccharide synthesis via selective, semi-orthogonal, and orthogonal activation

Traditional strategies for oligosaccharide synthesis often require extensive protecting and/or leaving group manipulations between each glycosylation step, thereby increasing the total number of synthetic steps while decreasing the efficiency of the synthesis. In contrast, expeditious strategies allow for the rapid chemical synthesis of complex carbohydrates by minimizing extraneous chemical manipulations. Oligosaccharide synthesis by selective activation of one leaving group over another is one such expeditious strategy. Herein, the significant improvements that have recently emerged in the area of the selective activation are discussed. The development of orthogonal strategy further expands the scope of the selective activation methodology. Surveyed in this article, are representative examples wherein these excellent innovations have been applied to the synthesis of various oligosaccharide sequences.

Programmable one-pot glycosylation

In oligosaccharide synthesis, protecting groups, possible participating groups, promoters/catalysts, reaction conditions, and donor leaving groups and acceptors must all be carefully designed in order to generate the correct regio- and stereochemistry for the new glycosidic bond. Programmable one-pot synthesis has been developed to address the above problems. This strategy is based on the sequential use of thioglycoside building blocks to form glycosidic bonds based on the reactivity difference of the building blocks. The activation of the anomeric leaving group can be attenuated through modification of the protecting group strategy and neighboring group participation. This reactivity-based strategy has been applied to one-pot glycosylation reactions where a series of building blocks with identical leaving groups react sequentially in one vessel without laborious intermediate purification steps. It provides rapid access to oligosaccharides with a wide-range of molecular diversity. In this chapter we outline the recent development of this strategy that can be applied to synthesize a wide variety of oligosaccharides and glycoconjugates that are associated with infectious diseases or carbohydrate-based cancer antigens.

Phenylpropanoid glycoside analogues: enzymatic synthesis, antioxidant activity and theoretical study of their free radical scavenger mechanism

Phenylpropanoid glycosides (PPGs) are natural compounds present in several medicinal plants that have high antioxidant power and diverse biological activities. Because of their low content in plants (less than 5% w/w), several chemical synthetic routes to produce PPGs have been developed, but their synthesis is a time consuming process and the achieved yields are often low. In this study, an alternative and efficient two-step biosynthetic route to obtain natural PPG analogues is reported for the first time. Two galactosides were initially synthesized from vanillyl alcohol and homovanillyl alcohol by a transgalactosylation reaction catalyzed by Kluyveromyces lactis β-galactosidase in saturated lactose solutions with a 30%-35% yield. To synthesize PPGs, the galactoconjugates were esterified with saturated and unsaturated hydroxycinnamic acid derivatives using Candida antarctica Lipase B (CaL-B) as a biocatalyst with 40%-60% yields. The scavenging ability of the phenolic raw materials, intermediates and PPGs was evaluated by the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) method. It was found that the biosynthesized PPGs had higher scavenging abilities when compared to ascorbic acid, the reference compound, while their antioxidant activities were found similar to that of natural PPGs. Moreover, density functional theory (DFT) calculations were used to determine that the PPGs antioxidant mechanism proceeds through a sequential proton loss single electron transfer (SPLET). The enzymatic process reported in this study is an efficient and versatile route to obtain PPGs from different phenylpropanoid acids, sugars and phenolic alcohols.

Enzymatic synthesis of capsaicin 4-O-β-xylooligosaccharides by β-xylosidase from Aspergillus sp

Capsaicin 4-O-β-xylooligosaccharides were synthesized by a biocatalytic xylosylation using Aspergillus sp. β-xylosidase. Capsaicin was converted into three new capsaicin glycosides, i.e. capsaicin 4-O-β-xyloside, capsaicin 4-O-β-xylobioside, and capsaicin 4-O-β-xylotrioside in 15, 12 and 10% yield, respectively. All products were isolated from the reaction mixtures by preparative HPLC. The structures of the products were determined by NMR spectroscopic method.

Radioprotective properties of Cumaside, a complex of triterpene glycosides from the sea cucumber Cucumaria japonica and cholesterol

The radioprotective activity has been studied of a new immunomodulatory lead material, Cumaside, which is a complex of monosulfated triterpene glycosides from the edible sea cucumber Cucumaria japonica and cholesterol. Female CD-1 strain mice administered with prophylactic doses of Cumaside were irradiated using a Gamma-therapeutic device with a 60Co source (exposure dose 6.5 Gy, dose rate 1.14 Gy/min) and the average life span of the mice was determined. The animals administrated with Cumaside and irradiated were killed by pervisceral dislocation on days 4 and 9. Peripheral blood cell composition indexes, blood forming function and cell number in blood-forming organs and the number of pluripotent blood-forming stem cells were determined using standard procedures and the results compared with those of non-treated irradiated mice. The survivability percentage and average life span of the irradiated mice that were not treated with Cumaside were decreased in comparison with the Cumaside-treated groups. Especially, the leukocyte and neutrophil numbers in the blood (bone marrow from hip), and the weight and cell number of lymphoid organs were higher in the Cumaside-treated groups compared with the non-treated irradiated mice. It was concluded that at low prophylactic doses Cumaside possesses moderate radioprotective properties.

Ethylene in organic synthesis. Repetitive hydrovinylation of alkenes for highly enantioselective syntheses of pseudopterosins

In this report we highlight the significant potential of ethylene as a reagent for the introduction of a vinyl group with excellent stereoselectivity at three different stages in the synthesis of a broad class of natural products, best exemplified by syntheses of pseudopterosins. The late-stage applications of the asymmetric hydrovinylation reaction further illustrate the compatibility of the catalyst with complex functional groups. We also show that, depending on the choice of the catalyst, it is possible to either enhance or even completely reverse the inherent diastereoselectivity in the reactions of advanced chiral intermediates. This should enable the synthesis of diastereomeric analogs of several classes of medicinally relevant compounds that are not readily accessible by the existing methods, which depend on 'substrate control' for the installation of many of the chiral centers, especially in molecules of this class.

Transformation of cellulose into biodegradable alkyl glycosides by following two different chemical routes

The transformation of cellulose into long-chain alkyl glycoside surfactants has been carried out following two different routes: (1) Direct transformation of cellulose to butyl-, hexyl-, octyl-, decyl- and dodecyl-α,β-glycosides in an ionic liquid media and Amberlyst-15Dry as catalysts, with mass yield of up to 82 %; and (2) two steps reaction with transformation of cellulose into methyl glucosides, with a procedure described by Zhang et al., followed by transacetalation with 1-octanol and 1-decanol in the presence of Amberlyst-15Dry. A kinetic study for the direct transformation of cellulose using 1-octanol has shown that depolymerisation of cellulose continues during the Fischer glycosidation. Increasing the chain length of the alcohol decreases the global reaction rate owing to an increase in the lipophilicity of the alcohol that decreases its contact with the carbohydrates. Finally, several acid catalysts were tested and the best results were obtained with Amberlyst-15Dry.

Stereoelectronic factors in the stereoselective epoxidation of glycals and 4-deoxypentenosides

Glycals and 4-deoxypentenosides (4-DPs), unsaturated pyranosides with similar structures and reactivity profiles, can exhibit a high degree of stereoselectivity upon epoxidation with dimethyldioxirane (DMDO). In most cases, the glycals and their corresponding 4-DP isosteres share the same facioselectivity, implying that the pyran substituents are largely responsible for the stereodirecting effect. Fully substituted dihydropyrans are subject to a "majority rule", in which the epoxidation is directed toward the face opposite to two of the three groups. Removing one of the substituents has a variable effect on the epoxidation outcome, depending on its position and also on the relative stereochemistry of the remaining two groups. Overall, we observe that the greatest loss in facioselectivity for glycals and 4-DPs is caused by removal of the C3 oxygen, followed by the C5/anomeric substituent, and least of all by the C4/C2 oxygen. DFT calculations based on polarized-π frontier molecular orbital (PPFMO) theory support a stereoelectronic role for the oxygen substituents in 4-DP facioselectivity, but less clearly so in the case of glycals. We conclude that the anomeric oxygen in 4-DPs contributes toward a stereoelectronic bias in facioselectivity whereas the C5 alkoxymethyl in glycals imparts a steric bias, which at times can compete with the stereodirecting effects from the other oxygen substituents.

Large-scale synthesis of all stereoisomers of a 2,3-unsaturated C-glycoside scaffold

All stereoisomers of a highly functionalized 2,3-unsaturated C-glycoside can be accessed in 10-100 g quantities from readily available starting materials and reagents in 3-7 steps. These chiral scaffolds contain three stereogenic centers along with orthogonally protected functional groups for downstream reactivity.

Synthesis and anti-fungal activity of seven oleanolic acid glycosides

In order to develop potential anti-fungal agents, seven glycoconjugates composed of α-L-rhamnose, 6-deoxy-α-L-talose, β-D-galactose, α-D-mannose, β-D-xylose-(1→4)-6-deoxy-α-L-talose, β-D-galactose-(1→4)-α-L-rhamnose, β-D-galactose-(1→3)-β-D-xylose-(1→4)-6-deoxy-α-L-talose as the glycone and oleanolic acid as the aglycone were synthesized in an efficient and practical way using glycosyl trichloroacetimidates as donors. The structures of the new compounds were confirmed by MS, ¹H-NMR and ¹³C-NMR.Preliminary studies based on means of mycelium growth rate, indicated that all the compounds possess certain fungicidal activity against Sclerotinia sclerotiorum (Lib.) de Bary, Rhizoctonia solani Kuhn, Botrytis cinerea Pers and Phytophthora parasitica Dast.

Stereoelectronic effects determine oxacarbenium vs β-sulfonium ion mediated glycosylations

Activation of a glycosyl donor protected with a 2-O-(S)-(phenylthiomethyl)benzyl ether chiral auxiliary results in the formation of an anomeric β-sulfonium ion, which can be displaced with sugar alcohols to give corresponding α-glycosides. Sufficient deactivation of such glycosyl donors by electron-withdrawing protecting groups is, however, critical to avoid glycosylation of an oxacarbenium ion intermediate. The latter type of glycosylation pathway can also be suppressed by installing additional substituents in the chiral auxiliary.

Synthesis and characterisation of α-Glycosyloxyamides derived from cyanogenic glycosides

INTRODUCTION

After exposure to oxidative stress, the leaves of some cyanogenic plants contain primary α-glycosyloxyamides with structures corresponding to their original cyanogenic glycosides.

OBJECTIVES

The aim of this study was to prepare such amides from their nitrile precursors and to characterise the new substances in order to facilitate their early identification in forthcoming studies. Methods - A simple but highly specific method is described for the in-vitro synthesis of the amides from their nitrile glycoside precursors using the Radziszewski reaction with hydrogen peroxide and a single-step purification of the reaction product. A TLC method is presented for the preliminary and fast identification of the α-glycosyloxyamides.

RESULTS

Following this procedure, seven representative α-glycosyloxyamides, five of them new, were obtained and analytically characterised by means of (1) H, (13) C NMR and ATR-IR spectroscopy, highlighting the differences from their respective nitrile glycoside precursors.

CONCLUSION

Thus, α-glycosyloxyamides can be obtained in sufficient amounts and purity to serve as references for further studies on the catabolism of cyanogenic glycosides and the detoxification of cyanogenic foodplants using the new aspect of nitrile hydrolysis with (endogenous) hydrogen peroxide.

The antibacterial properties of 6-tuliposide B. Synthesis of 6-tuliposide B analogues and structure-activity relationship

6-Tuliposide B is a secondary metabolite occurring specifically in tulip anthers. Recently, a potent antibacterial activity of 6-tuliposide B has been reported. However, its molecular target has not yet been established, nor its action mechanism. To shed light on such issues, 6-tuliposide B and tulipalin B analogues were synthesized and a structure-activity relationship (SAR) was examined using a broad panel of bacterial strains. As the results of SAR among a total of 25 compounds, only tulipalin B and the compounds having 3',4'-dihydroxy-2'-methylenebutanoate (DHMB) moieties showed any significant antibacterial activity. Moreover, the 3'R analogues of these compounds displayed essentially the same activities as 6-tuliposide B and the structure of the 3'R-DMBA moiety was the same as that of the proposed active moiety of cnicin. These results suggest that 6-tuliposide B has the same action mechanism as proposed for cnicin and bacterial MurA is one of the major molecular targets of 6-tuliposide B.

Parallel chemoenzymatic synthesis of sialosides containing a C5-diversified sialic acid

A convenient chemoenzymatic strategy for synthesizing sialosides containing a C5-diversified sialic acid was developed. The alpha2,3- and alpha2,6-linked sialosides containing a 5-azido neuraminic acid synthesized by a highly efficient one-pot three-enzyme approach were converted to C5''-amino sialosides, which were used as common intermediates for chemical parallel synthesis to quickly generate a series of sialosides containing various sialic acid forms.

A sequential indium-mediated aldehyde allylation/palladium-catalyzed cross-coupling reaction in the synthesis of 2-deoxy-beta-C-aryl glycosides

Indium-mediated allylation of aldehydes with 2-chloro-3-iodopropene, followed by a palladium-catalyzed cross-coupling reaction with triarylindium reagents or arylboronic acids, leads to aryl-substituted homoallylic alcohols in good to excellent yields and diastereoselectivities. The products obtained from reactions conducted with d-glyceraldehyde acetonide can be transformed into 2-deoxy-beta-C-aryl ribofuranosides in high overall yields. Similarly, 2-deoxy-beta-C-aryl allopyranosides may be prepared efficiently from 2,4-O-benzylidene erythrose.

Short synthetic sequence for 2-sulfation of alpha-L-iduronate glycosides

Hunter syndrome (mucopolysaccharidosis-II) is caused by deficiency of the lysosomal enzyme iduronate-2-sulfatase. The assay of this sulfatase requires the use of alpha-L-iduronate glycosides containing a sulfate at the 2-position. We report a simple, three-step procedure for the introduction of sulfate at the 2-position starting with the methyl ester of alpha-L-iduronate glycosides. The procedure involves protection of the 2- and 4-hydroxyl groups of the iduronate moiety as the dibutyl stannylene acetal, selective sulfation with sulfur trioxide-trimethylamine, and deprotection of the methyl ester to afford the desired 2-sulfate in 61% overall yield.

Colon cancer releases alpha-tocopherol from its O-glycosides better than normal colon tissue

BACKGROUND/AIMS

Free radicals, in a colon, may damage DNA, make difficult DNA repair and change course of post-translational modifications of regulatory proteins, which promote tumor initiation and progression. Therefore risk of colon cancer is closely related to diet and other lifestyle factors. Dietary antioxidants, such as vitamin E, should reduce the levels of harmful oxidation products. However vitamin E is not soluble in water, which decreases its bioavailability. As O-glycosides of alpha-tocopherol are better soluble in water and penetrate to tissues easier than free alpha-tocopherol, the aim of our work was to investigate the rate of release the free tocopherol from its O-glycosides in colon cancer, in comparison to human healthy colon tissue.

METHODOLOGY

The activities of enzymes catalysing hydrolysis of alpha-tocopheryl glucoside (1a) and mannoside (1b) as well as p-nitrophenyl beta-glucoside (2a) and mannoside (2b) in cancer and healthy human colon tissues, were determined according to the modified method described by Zwierz et al.

RESULTS

The alpha-tocopherol and p-nitrophenol were significantly better released from the respective glucosides and mannosides in cancer tissue than in "healthy" human colon tissues, with p = 0.000947 for la, p = 0.033024 for 1b; p = 0.0028 for 2a, and p = 0.0033 for 2b, respectively.

CONCLUSION

Alpha-tocopherol and p-nitrophenol are released from the O-glycosides of glucose and mannose in significantly higher amount in colon cancer than in healthy tissues. The alpha-tocopherol O-glycosides can be considered as prodrugs in prevention and treatment of the colon cancer.

Dimethylthexylsilyl 2-acetamido-3-O-allyl-2-deoxy-6-O-(4-methoxybenzyl)-beta-D-glucopyranoside, dimethylthexylsilyl 3,4,6-tri-O-benzyl-beta-D-mannopyranosyl-(1-->4)-2-acetamido-3-O-allyl-2-deoxy-6-O-(4-methoxybenzyl)-beta-D-glucopyranoside, and dimethylthexylsilyl 2-O-(benzylsulfonyl)-3,4,6-tri-O-benzyl-beta-D-mannopyranosyl-(1-->4)-2-acetamido-3-O-allyl-2-deoxy-6-O-(4-methoxybenzyl)-beta-D-glucopyranoside: synthesis of authentic samples

Dimethylthexylsilyl 2-acetamido-3-O-allyl-2-deoxy-6-O-(4-methoxybenzyl)-beta-D-glucopyranoside was prepared by reduction of the corresponding 4,6-O-(4-methoxybenzylidene) acetal with sodium cyanoborohydride and trifluoroacetic acid. This alcohol was coupled to 2-O-benzoyl-3,4,6-tri-O-benzyl-alpha-D-glucopyranosyl trichloroacetimidate to give a beta-glucoside that was converted to dimethylthexylsilyl 3,4,6-tri-O-benzyl-beta-D-mannopyranosyl-(1-->4)-2-acetamido-3-O-allyl-2-deoxy-6-O-(4-methoxybenzyl)-beta-D-glucopyranoside by saponification, Dess-Martin oxidation, and sodium borohydride reduction. Sulfonylation then gave dimethylthexylsilyl 2-O-(benzylsulfonyl)-3,4,6-tri-O-benzyl-beta-D-mannopyranosyl-(1-->4)-2-acetamido-3-O-allyl-2-deoxy-6-O-(4-methoxybenzyl)-beta-D-glucopyranoside.

Syntheses of R-beta-rutinosides by rutin-degrading reaction

Rutinose and five R-beta-rutinosides were obtained by means of rutin-degrading reaction in water or aqueous alcohol (ROH, R = methyl, ethyl, propyl, isopropyl, and benzyl) with rutin-degrading enzyme as catalyst and rutin as starting material in 84-94% yields, of which methyl-beta-rutinoside, propyl-beta-rutinoside, isopropyl-beta-rutinoside, and benzyl-beta-rutinoside are firstly reported in this paper. Based on spectral analysis, the structures of all products were elucidated.

Synthetic analogs of natural glycosides in drug discovery and development

Secondary metabolites, which have vital environmental and allelopathic functions for a host, and long tradition of ethnopharmacological applications preceding modern medicinal use, often occur in their native state as glycosides. The role of sugar moiety looks completely different from plant physiology point of view and from drug discovery and development perspective. Based on a short survey of cases, in which structural modification of natural glycone (saccharide part of a low molecular weight secondary metabolite) resulted in advantageous pharmacological changes, we postulate that glycosides of natural origin can be quite promising as drug leads, based on general rules of drug design. In particular, polyfunctional sugar moieties offer ample opportunities for almost continuous changes in shape, electron density and polarity. By the same token, glycosylation of other biologically active natural products, which are not natively glycosylated, can be viewed as a tool for tune up of their activity in direction of higher efficacy and better selectivity. Despite of considerable advances towards turning enzymatic glycosylations into biotechnological processes, chemical transformations still remain more practical, particularly for synthesis of modified glycosides, both: in research laboratory and in industry.

[Synthesis of heteroaromatic N-beta-glycosides of N-acetylglucosamine under phase transfer conditions: II]

Regioselective N-beta-glucosamination of various unsubstituted or C4-, C5-, or C6-monosubstituted indolin-2-ones under phase transfer conditions was studied. The regioselectivity was unambiguously proved by (1)H NMR spectroscopy and X-ray analysis. The presence of a substituent at C7 of the aromatic ring leads to the formation of either a mixture of isomeric N-beta- and O-beta-D-glucosaminides or only oxazoline and/or 2-acetamidoglycal irrespective of the reaction conditions.

Chemo-enzymatic synthesis of poly-N-acetyllactosamine (poly-LacNAc) structures and their characterization for CGL2-galectin-mediated binding of ECM glycoproteins to biomaterial surfaces

Poly-N-acetyllactosamine (poly-LacNAc) structures have been identified as important ligands for galectin-mediated cell adhesion to extra-cellular matrix (ECM) proteins. We here present the biofunctionalization of surfaces with poly-LacNAc structures and subsequent binding of ECM glycoproteins. First, we synthesized beta-GlcNAc glycosides carrying a linker for controlled coupling onto chemically functionalized surfaces. Then we produced poly-LacNAc structures with defined lengths using human beta1,4-galactosyltransferase-1 and beta1,3-N-acetylglucosaminyltransferase from Helicobacter pylori. These compounds were also used for kinetic characterization of glycosyltransferases and lectin binding assays. A mixture of poly-LacNAc-structures covalently coupled to functionalized microtiter plates were identified for best binding to our model galectin His(6)CGL2. We further demonstrate for the first time that these poly-LacNAc surfaces are suitable for further galectin-mediated binding of the ECM glycoproteins laminin and fibronectin. This new technology should facilitate cell adhesion to biofunctionalized surfaces by imitating the natural ECM microenvironment.

Facile synthesis of oleanolic acid monoglycosides and diglycosides

Oleanolic acid and its glycosides are important natural products, possessing various attractive biological activities such as antitumor, antivirus and anti-inflammatory properties. In the present work, fifteen oleanolic acid saponins bearing various saccharide moieties, including 3-monoglycoside, 28-monoglycoside and 3,28-diglycoside, were easily synthesized in high yields. Benzyl was chosen as the protective group for the COOH(28) group, instead of commonly used methyl and allyl, to avoid difficulties in the final deprotection. Alkali-promoted condensation of the carboxylic acid with bromo-glycosides was found to be more efficient in the synthesis of 28-glycosides. Two approaches were investigated and proved practicable in the preparation of 3,28-diglycosides. This method is suitable for preparing oleanolic acid glycosides with structural diversity for extensive biological evaluation and structure-activity relationship study, and it also apply new idea for the corresponding synthetic methods to the glycoside derivatives of other triterpenoid.