First preparative synthesis of a 3-acetamido-3,6-dideoxy-d-galactopyranose glycosyl donor via intramolecular cyclization of an epoxytrichloroacetimidate

The preparative synthesis of a 3-acetamido-3,6-dideoxy-d-galactopyranose N-phenyl-trifluoroacetimidate donor has been accomplished using as key step a silica gel mediated cyclization of an epoxytrichloroacetimidate, while other more conventional routes to aminosugars failed. Test glycosylations with the N-phenyl-trifluoroacetimidate donor are also reported.     

Convergent synthesis of the tetrasaccharide repeating unit related to the O-antigenic polysaccharide of Escherichia coli 78

A convergent synthesis of the tetrasaccharide repeating unit of the O-antigenic cell wall polysaccharide of Escherichia coli 78, as the corresponding methyl glycoside (I), is being reported. It involved stereoselective glycosidation of a β-linked mannodisaccharide acceptor with a β-linked glucosamine based disaccharide thioglycoside donor, which were prepared from the corresponding functionalised monosaccharide based glycosyl donors and acceptors. The resulting tetrasaccharide derivative was finally converted to (I) by selective deprotection and also by global protection and deprotection techniques.     

Synthesis of a tetrasaccharide related to the triterpenoid saponin Bellisoside isolated from Bellis perennis (compositae)

A concise synthesis of a tetrasaccharide related to the triterpenoid saponins Bellisoside has been accomplished from commercially available monosaccharides through rational protecting group manipulations and stereoselective glycosylations. For the glycosylation reactions, H₂SO₄–silica has been successfully used as an alternative to conventional Lewis acids such as TfOH or TMSOTf. The target tetrasaccharide has been synthesized in the form of its p-methoxyphenyl glycoside which leaves scope for further glyco-conjugate formation through the selective deprotection of p-methoxyphenyl glycoside followed by trichloroacetimidate chemistry.     

Anomeric reactivity-based one-pot oligosaccharide synthesis: a rapid route to oligosaccharide libraries

The assembly of an oligosaccharide library has been achieved in a practical and efficient manner employing a' one-pot sequential approach. With the help of the anomeric reactivity values of thioglycosides, using a thioglycoside (mono- or disaccharide) with one free hydroxyl group as acceptor and donor coupled with another fully protected thioglycoside, a di- or trisaccharide is selectively formed without self-condensation and subsequently reacted in situ with an anomerically inactive glycoside (mono- or disaccharide) to form a tri- or tetrasaccharide in high overall yield. The approach enables the rapid assembly of 33 linear or branched fully protected oligosaccharides using designed building blocks. These fully protected oligosaccharides have been partially or completely deprotected to create 29 more structures to further increase the diversity of the library.     

A new, lipophilic p-alkoxybenzyl ether protecting group and its use in the synthesis of a disaccharide

[formula: see text] In contrast to major advances in the chemical synthesis of oligosaccharides, the methods of purification of the intermediates are essentially the same as they were decades ago. Here, the synthesis of p-(dodecyloxy)benzyl chloride is described and it is demonstrated that the new p-(dodecyloxy)benzyl ether protecting group can render a protected disaccharide sufficiently lipophilic for selective adsorption on C18 silica, thus sidestepping the expensive silica gel chromatography traditionally used for the isolation of protected oligosaccharides.     

Propargyl mediated intramolecular aglycon delivery (IAD): applications to the synthesis of core N-glycan oligosaccharides

The use of propargyl mediated intramolecular aglycon delivery (IAD) for the synthesis of the key Manβ(1→4)GlcNAc linkage of N-glycan oligosaccharides, including the core N-glycan pentasaccharide, is investigated. Isomerisation of a 2-O-progargyl group of manno thioglycoside donors to an allene is followed by iodonium ion mediated mixed acetal formation with the 4-OH of protected GlcNAc acceptors, and subsequent intramolecular glycosylation occurs with complete control of anomeric stereochemistry to form the Manβ(1→4)GlcNAc linkage. A variety of linear and convergent approaches (1+2, 3+1, 3+2) to the core pentasaccharide are investigated as means of probing the generality and limitations of this type of intramolecular aglycon delivery for the formation of β-mannoside linkages in complex oligosaccharides.     

Discovery of rare sulfated N-unsubstituted glucosamine based heparan sulfate analogs selectively activating chemokines

Achieving selective inhibition of chemokines with structurally well-defined heparan sulfate (HS) oligosaccharides can provide important insights into cancer cell migration and metastasis. However, HS is highly heterogeneous in chemical composition, which limits its therapeutic use. Here, we report the rational design and synthesis of N-unsubstituted (NU) and N-acetylated (NA) heparan sulfate tetrasaccharides that selectively inhibit structurally homologous chemokines. HS analogs were produced by divergent synthesis, where fully protected HS tetrasaccharide precursor was subjected to selective deprotection and regioselectively O-sulfated, and O-phosphorylated to obtain 13 novel HS tetrasaccharides. HS microarray and SPR analysis with a wide range of chemokines revealed the structural significance of sulfation patterns and NU domain in chemokine activities for the first time. Particularly, HT-3,6S-NH revealed selective recognition by CCL2 chemokine. Further systematic interrogation of the role of HT-3,6S-NH in cancer demonstrated an effective blockade of CCL2 and its receptor CCR2 interactions, thereby impairing cancer cell proliferation, migration and invasion, a step towards designing novel drug molecules.     

Efficient synthesis of core 2 class glycosyl amino acids by one-pot glycosylation approach

We describe the one-pot synthesis of core 2 class branched oligosaccharides initiated by chemo-selective glycosylation of silyl ether. Glycosylation of 6-O-silyl-4-benzyl-2-azido-thiogalactoside with glycosyl fluoride provided selectively 6-glycosylated thioglycoside without both O-glycosylation at the 3 position and S-glycosylation. Subsequent coupling of galactosyl fluoride and amino acids afforded the protected branched oligosaccharides in good yields.     

Synthesis of a Systematic 64-Membered Heparan Sulfate Tetrasaccharide Library

Heparan sulfate (HS) has multifaceted biological activities. To date, no libraries of HS oligosaccharides bearing systematically varied sulfation structures are available owing to the challenges in synthesizing a large number of HS oligosaccharides. To overcome the obstacles and expedite the synthesis, a divergent approach was designed, where 64 HS tetrasaccharides covering all possible structures of 2-O-, 6-O- and N-sulfation with the glucosamine-glucuronic acid-glucosamine-iduronic acid backbone were successfully produced from a single strategically protected tetrasaccharide intermediate. This extensive library helped identify the structural requirements for HS sequences to have strong fibroblast growth factor-2 binding but a weak affinity for platelet factor-4. Such a strategy to separate out these two interactions could lead to new HS-based potential therapeutics without the dangerous adverse effect of heparin-induced thrombocytopenia.     

Synthesis of an aryl 2-deoxy-β-glycosyl tetrasaccharide to probe retaining endo-glycosidase mechanism

The synthesis of the 1,3–1,4-β-glucanase substrate analogue 4-nitrophenyl O-β-d-glucopyranosyl-(1→4)-O-β-d-glucopyranosyl-(1→4)-O-β-d-glucopyranosyl-(1→3)-2-desoxi-β-d-glucopyranoside 2 is reported. Starting from the main tetrasaccharide obtained by enzymatic depolymerization of barley β-glucan, the synthetic scheme involves preparation of the corresponding 3-O-substituted glycal which was converted into a 2-deoxy-α-glycosyl iodide as a glycosyl donor. The key glycosylation step was successfully achieved by nucleophilic substitution of the iodide donor with 4-nitrophenolate with high β-selectivity.     

Reactions on a surface. 1. Condensation of 3-methyl-3-buten-1-ol with carbonyl compounds on SiO2 and Al2O3

3-Methyl-3-buten-1-ol reacts with various aldehydes and ketones on silica gel and Al₂O₃ surfaces in the absence of solvents and conventional catalysts for this reaction to form derivatives of di- and tetrahydropyrans.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1326–1328, October, 1989.     

A new, improved synthesis of the trisaccharide repeating unit of the O-antigen from Xanthomonas campestris pv. campestris 8004

Recent studies have revealed that lipid-A and core fragments of the lipopolysaccharide from Xanthomonas campestris pv. campestris 8004 (Xcc), a phytopathogenic Gram-negative bacterium, are able to elicit plant immunity with two independent mechanisms. To date, nothing is known about the effect of the O-antigen portion. Since its separation from the core region by selective chemical degradation is very difficult, the chemical synthesis of related oligosaccharides is strictly necessary. In this paper a new, improved synthesis of the O-antigen repeating unit is presented. The main improvements in the synthesis are: (1) a shorter, high-yielding preparation of an efficient glycosyl donor of the rare sugar 3-acetamido-3,6-dideoxy-d-galactopyranose (3-acetamido-d-fucose, d-Fucp3NAc); (2) a new protecting group pattern, which is demonstrated to open a path to the future synthesis of higher oligomers.     

Novel reaction systems for the synthesis of O-glucosides by enzymatic reverse hydrolysis

Our studies are presented to replace alcohols as solvents in reverse hydrolytic reactions catalyzed by immobilized β-glucosidase to synthesize O-substituted β-d-glucopyranosides in preparative-scale. We found that 1,2-diacetoxyethane is a suitable solvent and O-alkyl or aryl β-d-glucosides were synthesized in moderate yields (after isolation 12-19%). In these reactions proportion of glucose and glucosyl acceptor hydroxy compounds was 1:20. We suggest that 1,2-diacetoxyethane can be useful not only for alcohols but for other glucosyl donor compounds unsuitable for the role of solvent (e.g., phenols) in the synthesis of O-β-d-glucosides by reverse hydrolysis.     

Stereoselective Synthesis of α‐3‐Deoxy‐D‐manno‐oct‐2‐ulosonic Acid (α‐Kdo) Glycosides Using 5,7‐O‐Di‐tert‐butylsilylene‐Protected Kdo Ethyl Thioglycoside Donors

An efficient methodology for the synthesis of α‐Kdo glycosidic bonds has been developed with 5,7‐O‐di‐tert‐butylsilylene (DTBS) protected Kdo ethyl thioglycosides as glycosyl donors. The approach permits a wide scope of acceptors to be used, thus affording biologically significant Kdo glycosides in good to excellent chemical yields with complete α‐selectivity. The synthetic utility of an orthogonally protected Kdo donor has been demonstrated by concise preparation of two α‐Kdo‐containing oligosaccharides.     

Synthesis of a Single Repeat Unit of Type VIII Group B Streptococcus Capsular Polysaccharide 1

Abstract

We have synthesized a single repeat unit of type VIII Group B Streptococcus capsular polysaccharide, the structure of which is {L-Rhap(β1→4)-D-Glcp(β1→4)[Neu5Ac(α2→3)]-D-Galp(β→4)}_n. The synthesis presented three significant synthetic challenges namely: the L-Rhap(β→4)-D-Glcp bond, the Neu5Ac(α2→3)-D-Galp bond and 3,4-D-Galp branching. The L-Rhap bond was constructed in 60% yield (α:β 1:1.2) using 4-O-acetyl-2,3-di-O-benzoyl-α-L-rhamnopyranosyl bromide 6 as donor, silver silicate as promotor and 6-O-benzyl-2,3-di-O-benzoyl-1-thio-β-D-glucopyranoside as acceptor to yield disaccharide 18. The Neu5Ac(α2→3) linkage was synthesized in 66% yield using methyl [phenyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto-nonulopyranosid]onate as donor and triol 2-(trimethylsilyl) ethyl 6-O-benzyl-β-D-galactopyranoside as acceptor to give disaccharide 21. The 3,4-D-Galp branching was achieved by regioselective glycosylation of disaccharide diol 21 by disaccharide 18 in 28% yield to give protected tetrasaccharide 22. Tetrasaccharide 22 was deprotected to give as its 2-(trimethylsilyl)ethyl glycoside the title compound 1a. In addition the 2-(trimethylsilyl)ethyl group was cleaved and the tetrasaccharide coupled by glycosylation (via tetrasaccharide trichloroacetimidate) to a linker suitable for conjugation.

     

Fluoridolysis of 5,6-epoxy carbohydrates: application to the synthesis of 5-fluoro lactosamine and isolactosamine glycosides

The synthesis of 6-selenophenyl derivatives of β-1,3 and β-1,4 disaccharides has been explored for the purpose of extending our epoxide fluoridolysis methodology to the synthesis of 5-fluoro analogues of N-acetyl isolactosamine (isoLacNAc, lacto-N-biose) and N-acetyl lactosamine (LacNAc) glycosides. Successful synthesis of the C-6 selenium-containing disaccharides was achieved via Lewis acid-mediated donor and acceptor substrates, the latter containing a selectively protected C-6 hydroxyl group for ultimate conversion to the desired 6-selenophenyl disaccharides. In contrast, the use of selenium-containing acceptor substrates under a variety of conditions failed to yield the desired selenium-containing disaccharides. Oxidation of the 6-selenophenyl derivatives to the corresponding selenoxides followed by thermal elimination yielded the exocyclic olefins, which were converted to the 5,6-epoxides. Epoxide fluoridolysis yielded the desired target compounds, 5-fluoro β-octyl glycoside analogues of type 1 and type 2 glycans. The newly synthesized fluorine-containing disaccharides have potential application as fucosyltransferase substrates, both for mechanistic studies and in the chemoenzymatic synthesis of fluorine-containing oligosaccharides.     

Pre-activation-based one-pot synthesis of an alpha-(2,3)-sialylated core-fucosylated complex type bi-antennary N-glycan dodecasaccharide

Synthesis of N-glycans is of high current interests due to their important biological properties. A highly efficient convergent strategy based on the pre-activation method for assembly of the complex type core fucosylated bi-antennary N-glycan dodecasaccharide has been developed. Retrosynthetically, this extremely challenging target is broken down to three modules: a sialyl disaccharide, a glucosamine building block and a hexasaccharide diol acceptor. The sialyl disaccharide was easily obtained by selective activation of a new 5-N-trichloroacetyl protected sialyl donor in the presence of a thiogalactoside acceptor. The hexasaccharide diol module was produced by double mannosylation of a fucosylated tetrasaccharide acceptor, which in turn was generated by glycosylation of a alpha-fucosylated disaccharide with a beta-mannose containing disaccharide donor. The union of the three modules was performed in one-pot giving the fully protected dodecasaccharide in high yield. This synthesis is characterized by minimum protective group and aglycon adjustment on oligosaccharide intermediates, thus greatly enhancing the overall synthetic efficiency. The modular feature of this strategy suggests that this method can be readily adapted to the synthesis of a wide variety of N-glycan structures.     

Chemoenzymatic synthesis of heparan sulfate tetrasaccharide from a N-acetyl-α-d-glucosamine-O-methylglycoside acceptor

Heparan sulfate tetrasaccharide 2 was efficiently prepared in seven steps through chemoenzymatic synthesis. A monosaccharide 5, N-acetyl-α-d-glucosamine- O-methylglycoside (GlcNAc-OMe), was successfully used as an acceptor in a heparosan synthase (pmHS2)-catalyzed glycosylation reaction. This avoided the multi-step synthesis of a more complex disaccharide acceptor 3, greatly simplifying the route to tetrasaccharide target 2. This approach provides a critical tetrasaccharide intermediate for subsequent chemoenzymatic transformation to fondaparinux and its analogues.     

Expedient synthesis of a tetrasaccharide and a pentasaccharide corresponding to the cell wall O-antigen of Escherichia coli O77 and Escherichia coli O17

A concise chemical synthetic strategy has been developed for the synthesis of a tetrasaccharide and a pentasaccharide corresponding to the O-antigen of Escherichia coli O77 and E. coli O17 strains, respectively using [2+2] and [3+2] block glycosylation approaches from suitably functionalized common monosaccharide intermediates. All of the intermediate steps are high yielding while the glycosylation steps are highly stereoselective. A number of recently developed methodologies have been used in the synthesis.