Synthetic Studies on Sialoglycoconjugates 22: Total Synthesis of Tumor-Associated Ganglioside, Sialyl Lewis X1

Chemical Synthesis of Gangliosides

Synthetic methodologies for gangliosides have evolved over the past three decades. The strategies for constructing ganglioside skeletons can generally be classified as late-stage ceramide coupling, the glucosyl ceramide cassette strategy, or late-stage sialylation. Using these synthetic strategies, numerous natural gangliosides and their structural analogs, including functional probes, have been synthesized. This chapter describes the synthetic strategies for gangliosides and provides examples of the total synthesis of several gangliosides using each strategy.

Synthesis of Type-I and Type-II LacNAc-Repeating Oligosaccharides as the Backbones of Tumor-Associated Lewis Antigens

Type-I and Type-II LacNAc are Gal-GlcNAc disaccharides bearing a β1,3- or β1,4-linkage respectively. They exist as the backbones of Lewis antigens that are highly expressed in several cancers. Owing to the promise of developing carbohydrate-based anti-cancer vaccines, glycan synthesis at a large scale is indeed an important task. Synthesis of Type-I and Type-II tandem repeat oligomers has been hampered by the presence of GlcNAc residues. Particularly, N-protecting group plays a determining role in affecting glycosyl donor’s reactivity and acceptor’s nucleophilicity. This review discusses several representative studies that assembled desirable glycans in an efficient manner, such as chemoselective one-pot synthesis and chemoenzymatic methods. Additionally, we also highlight solutions that have been offered to tackle long-lasting problems, e.g., prevention of the oxazoline formation and change of donor/acceptor reactivity. In retrospect of scientific achievements, we present the current restrictions and remaining challenges in this less explored frontier.

Chemical Synthesis of Gangliosides

Various methods for the chemical synthesis of gangliosides have been investigated to date and numerous natural gangliosides and their structural analogues have been synthesized during the past three decades. Key technologies in the synthesis of gangliosides include α-selective sialylation and introduction of the ceramide moiety into the oligosaccharide chain. This chapter introduces two major strategies for ganglioside synthesis-the most commonly used strategy and the recently developed glucosylceramide cassette approach. Synthetic procedures for selected reactions are also presented.

The Synthesis and Biological Characterization of Acetal-Free Mimics of the Tumor-Associated Carbohydrate Antigens

Carcinomas express unique carbohydrates, known as tumor-associated carbohydrate antigens (TACAs), on their surface. These are potential targets for anticancer vaccines; however, to date, no such vaccine has reached the clinic. One factor that may complicate the success of this effort is the lability of the glycosidic bond. Acetal-free carbohydrates are analogues that lack the glycosidic linkage by replacing either the endo or exo oxygen with a methylene. This chapter summarizes the seminal syntheses of the mucin TACAs, provides an overview of common techniques for the synthesis of carbasugars and C-glycosides, reviews the syntheses published to date of acetal-free TACA analogues, and provides an overview of their observed biological activity. We conclude by offering a summation of the challenges remaining to the field biologically and the potential that acetal-free TACAs have of answering several basic questions in carbohydrate immunology.

Role of binding in Mycoplasma mobile and Mycoplasma pneumoniae gliding analyzed through inhibition by synthesized sialylated compounds

Mycoplasmas, which have been shown to be the causative pathogens in recent human pneumonia epidemics, bind to solid surfaces and glide in the direction of the membrane protrusion at a pole. During gliding, the legs of the mycoplasma catch, pull, and release sialylated oligosaccharides fixed on a solid surface. Sialylated oligosaccharides are major structures on animal cell surfaces and are sometimes targeted by pathogens, such as influenza virus. In the present study, we analyzed the inhibitory effects of 16 chemically synthesized sialylated compounds on the gliding and binding of Mycoplasma mobile and Mycoplasma pneumoniae and concluded the following. (i) The recognition of sialylated oligosaccharide by mycoplasma legs proceeds in a "lock-and-key" fashion, with the binding affinity dependent on structural differences among the sialylated compounds examined. (ii) The binding of the leg and the sialylated oligosaccharide is cooperative, with Hill constants ranging from 2 to 3. (iii) Mycoplasma legs may generate a drag force after a stroke, because the gliding speed decreased and pivoting motion occurred more frequently when the number of working legs was reduced by the addition of free sialylated compounds.

Carbohydrate microarrays reveal sulphation as a modulator of siglec binding

Siglecs are receptors on cells of the immune, haemopoietic, and nervous systems that recognize sialyl-glycans with differing preferences for sialic acid linkage and oligosaccharide backbone sequence. We investigate here siglec binding using microarrays of Lewis(x) (Le(x))- and 3'-sialyl-Le(x)-related probes with different sulphation patterns. These include sulphation at position 3 of the terminal galactose of Le(x), position 6 of the galactose of Le(x) and sialyl-Le(x), position 6 of N-acetylglucosamine of Le(x) and sialyl-Le(x), or both positions of sialyl-Le(x). Recombinant soluble forms of five siglecs have been investigated: human Siglec-7, -8, -9, and murine Siglec-F and CD22 (Siglec-2). Each siglec has a different binding pattern. Unlike two C-type lectins of leukocytes, L-selectin and Langerin, which also bind to sulphated analogues of sialyl-Le(x), the siglecs do not give detectable binding signals with sulphated analogues that are lacking sialic acid. The sulphate groups modulate, however, positively or negatively the siglec binding intensities to the sialyl-Le(x) sequence.

Studies on the endogenous L-selectin ligands: systematic and highly efficient total synthetic routes to lactamized-sialyl 6-O-sulfo Lewis X and other novel gangliosides containing lactamized neuraminic acid

Systematic syntheses of lactamized neuraminic acid-containing gangliosides GM4, sulfated sialylparagloboside, and sulfated/nonsulfated sialyl Lewis X are described. The highly efficient, one-step lactamization of neuraminic acid was accomplished by treatment of the N-deacetylated sialic acid (neuraminic acid)-containing gangliosides with HBTU and HOBt in DMF at 65 degrees C. Both the lactamized neuraminic acid residue and the sulfate group at O-6 of the GlcNAc residue were found to be involved in the antigenic determinant defined by G159 monoclonal antibody, while the fucose residue may not be critical for the recognition by G159 mAb.

Synthesis and selectin-binding activity of N-deacetylsialyl Lewis X ganglioside

A novel analogue of sialyl Lewis X ganglioside, N-deacetylsialyl Lewis X ganglioside, was synthesized. Methyl 4,7,8,9-tetra-O-acetyl-3,5-dideoxy-5-trifluoroacetamido-D-glycero-alpha-D-galacto-2-nonulopyranosylonate-(2 --> 3)-2,4,6-tri-O-benzoyl-D-galactopyranosyl trichloroacetimidate was coupled with 2-(trimethylsilyl)ethyl [2-acetamido-6-O-benzyl-2-deoxy-3-O-(4-methoxybenzyl)-beta-D-glucopyranosyl]-(1 --> 3)-[2,4,6-tri-O-benzyl-beta-D-galactopyranosyl]-(1 --> 4)-2,3,6-tri-O-benzyl-beta-D-galactopyranoside to give the desired pentasaccharide in high yield. The glycosylation of the pentasaccharide acceptor, which was derived from its precursor by removal of the 3-methoxybenzyl group, with the phenyl 1-thioglycoside derivative of L-fucose using N-iodosuccinimide-trifluoromethanesulfonic acid as promoter, produced the hexasaccharide. Proper manipulation of the protecting groups of the hexasaccharide afforded the corresponding glycosyl imidate, which was coupled with (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol. Selective reduction of the azido group, N-acylation with octadecanoic acid, and the complete removal of the protecting groups gave the desired N-deacetylsialyl Lewis X ganglioside. L-Selectin bound more strongly to N-deacetylsialyl Lewis X ganglioside than to the sialyl Lewis X ganglioside, whereas E- and P-selectins bound equally well to the two gangliosides.

Synthetic studies on glycosphingolipids from protostomia phyla: synthesis of amphoteric glycolipid analogues from the porcine nematode, Ascaris suum

A novel amphoteric glycosphingolipid, cholinephosphoryl-(-->6)-beta-D-GlcpNAc-(1-->3)-beta-D-Manp-(1-->4)-beta-D-Glcp-(1-->)-Cer, isolated from the porcine parasitic nematode, Ascaris suum, may be expected to be involved in host-parasite interactions. This glycosphingolipid analogue containing octyl residue in place of ceramide was synthesized as follows: The key reaction of this synthetic procedure is the formation of a intramolecular aglycon delivery (IAD) approach for beta-selective mannosylation. Then, a coupling of phosphocholine group at the position C-6'' of 16 was attempted using 2-chloro-2-oxo-1,3,2-dioxaphospholane, followed by reaction of the resulting cyclic phosphate intermediate with anhydrous trimethylamine to give 17. Subsequent debenzylation and debenzylidenation afforded target compound (2).

Linear synthesis of a protected H-type II pentasaccharide using glycosyl phosphate building blocks

A linear synthesis of a fully protected H-type II blood group determinant pentasaccharide utilizing glycosyl phosphate and glycosyl trichloroacetimidate building blocks is reported. Envisioning an automated solid-phase synthesis of blood group determinants, the utility of glycosyl phosphates in the stepwise construction of complex oligosaccharides, such as the H-type II antigen, is demonstrated. Installation of the central glucosamine building block required the screening of a variety of nitrogen protecting groups to ensure good glucosamine donor reactivity and protecting group compatibility. The challenge to differentiate C2 of the terminal galactose in the presence of other hydroxyl and amine protecting groups prompted us to introduce the 2-(azidomethyl)benzoyl group as a novel mode of protection for carbohydrate synthesis. The compatibility of this group with traditionally employed protecting groups was examined, as well as its use as a C2 stereodirecting group in glycosylations. The application of the 2-(azidomethyl)benzoyl group along with a systematic evaluation of glycosyl donors allowed for the completion of the pentasaccharide and provides a synthetic strategy that is expected to be generally amenable to the solid support synthesis of blood group determinants.

Enhanced effect of sulfonylurea (SU) in copolymer comprising a sugar moiety and SU derivative as double ligands on insulin secretion from MIN6 cells

Copolymers composed of sulfonylurea (SU) as an antagonist of ATP-sensitive K+ channel and sugar moieties as double ligands were synthesized. Insulin secretion from MIN6 cells (insulinoma cell line) in contact with the copolymers was evaluated. MIN6 cells attached to the poly(N-p-vinylbenzyl-D-maltonamide-co-SU) [P(VMA-co-SU)]-coated dishes were in the more aggregated form as compared to other polymer-coated surfaces. By introducing SU into the sugar bearing homopolymer, an enhanced effect on insulin secretion from MIN6 cells was observed due to the specific interaction between SU ligands and SU receptors on the beta-cell membrane. P(VMA-co-SU) composed of SU and non-reducing glucose moieties demonstrated enhanced insulin secretion from MIN6 cells and faster proliferation of MIN6 cells as compared to poly(N-p-vinylbenzyl-D-lactonamide-co-SU) [P(VLA-co-SU)] probably owing to the glucose transporters presence on the MIN6 cell membrane. Insulin secretion from MIN6 cells pretreated with diazoxide as an agonist of ATP-sensitive K+ channel was suppressed.

6-O-sulfo de-N-acetylsialyl Lewis X as a novel high-affinity ligand for human L-selectin: total synthesis and structural characterization

Total synthesis and structural characterization of a novel 6-O-sulfo de-N-acetylsialyl Lewis X, which was originally discovered as a minor by-product of the parent 6-O-sulfo N-acetylsialyl Lewis X, a high-affinity endogenous ligand for human L-selectin, are described. The total synthesis has been achieved by a highly efficient, regio- and alpha-stereoselective glycosylation of N-trifluoroacetylneuraminic acid, selective protections of the 3- and 6-hydroxyl groups of N-acetylglucosamine that undergo fucosylation and sulfation, and construction of the glycolipid structure containing a ceramide. The structure of 6-O-sulfo de-N-acetylsialyl Lewis X ganglioside was characterized by fast atom bombardment mass spectrometry (FAB-MS).

‘Glyco-Epitope’ Assignments for the Selectins: Advances Enabled By the Neoglycolipid (Ngl) Technology in Conjunction with Synthetic Carbohydrate Chemistry’

The neoglycolipid (NGL) technology involving the preparation of lipid-linked oligosaccharide probes for binding experiments with carbohydrate-recognizing proteins, and their analysis by mass spectrometry, is a unique and powerful means of discovering oligosaccharide ligands for carbohydrate-binding proteins, and assigning details of their specificities. The key feature is that it enables the pinpointing and sequence determination of bioactive oligosaccharides within highly heterogeneous mixtures derived from natural glycoconjugates. A new generation of NGLs incorporating a fluorescent label now establishes the principles for a streamlined technology whereby oligosaccharide populations are carried through ligand detection and isolation steps, and sequence determination. Advances in selectin research made through applications of the NGL technology include (i) demonstration of the importance of density of selectin expression, and of oligosaccharide ligands, in the magnitude and the specificity of the binding signals; (ii) demonstration of the efficacy of lipid-linked oligosaccharides in supporting selectin-mediated cell interactions; (iii) the discovery of 3-sulphated Le(a)/Le(x) as selectin ligands; (iv) the isolation and sequencing of carbohydrate ligands for E-selectin on murine myeloid cells and kidney; (v) the finding that sulphation at position 6 of the penultimate N-acetylglucosamine confers superior L-selectin binding signals not only to 3-sialyl-Le(x) but also to 3'-sulpho-Le(x); and (vi) the finding that sialic acid de-N-acetylation, or further modification with formation of an intra-molecular amide bond in the carboxyl group, enhances or virtually abolishes, respectively, the potency of the 6'-sulfo-sialyl-Le(X) ligand. Working with biotinylated forms of the oligosaccharide ligands, we have observed that their presentation on a streptavidin matrix influences differentially the efficacy of interactions of the L- and P-selectins (but not E-selectin) with the sialylated and sulphated ligands.

Synthetic studies on glycosphingolipids from the parasite Echinococcus multilocularis

Novel neutral glycosphingolipids isolated from the metacestodes of Echinococcus multilocularis by Persat, may be expected to be involved in host-parasite interactions. We have synthesized these glycosphingolipid analogues containing 2-branched fatty alkyl residues in place of ceramide. The glycosylation of galactosyl donors 4 and 5 with each of the acceptors 2 and 11 in the presence of N-iodosuccinimide (NIS)/TfOH, and the glycosylation of fucosyl donor 13 with acceptors 12 and 20 in the presence of dimethyl(methylthio)sulfonium triflate (DMTST) gave the desired oligosaccharide derivatives at good yield. The fully per-O-acylated 2-(trimethylsilyl)ethyl glycosides 6, 15, 21, and 26 were converted to glycosylimidates 7, 16, 22, and 27, which were condensed with 2-(tetradecyl)hexadecanol and subsequently deacylated give four target glycosphingolipid analogues.

Synthesis of sialyl LeX ganglioside analogues modified at C-6 of the galactose residue to elucidate the mechanism of selection recognition

Sialyl Lewis X ganglioside analogues modified at C-6 of the galactose residue, having 6-O-methoxy, 6-acetamido and 6-amino functional groups, have been synthesized. Treatment of 2-(trimethylsilyl)ethyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5- dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylonate)-(2-->3)- 2-O-benzoyl -beta-D-galactopyranoside with trimethyloxonium tetrafluoroborate or Tf2O, followed by NaN3, gave the 6-O-methyl and 6-azido derivatives, respectively, which were converted into their respective methyl 1-thioglycoside derivatives. The glycosyl donors obtained were coupled with 2-(trimethylsilyl)ethyl 2- acetamido-6-O-benzyl-2-deoxy-3-O-(4-methoxybenzyl)-beta-D-glucopyranosyl - (1-->)-2,4,6-tri-O-benzyl-beta-D-galactopyranosyl-(1-->4)-2,3,6-tri-O-be nzyl- beta-D-galactopyranoside in the presence of dimethyl(methylthio)sulfonium triflate (DMTST) to give their respective pentasaccharides. The glycosylation of the pentasaccharide acceptors derived from their precursors by removal of the 4-methoxybenzyl group, with phenyl 1-thioglycoside derivative of L-fucose using N-iodosuccinimide-TfOH afforded the corresponding hexasaccharides, which were transformed in good yield, via reductive removal of the benzyl group, the simultaneous transformation of azide to amine and acetamide, Omicron-deacylation and saponification of the methyl ester into the target compounds without the ceramide groups. On the other hand, the proper manipulation of the protecting group of the hexasaccharides, including a transformation of azide into acetamide and trifluoroacetamide, gave the hexasaccharide imidates, which were coupled with (2S,3R,4E)-2-azido-o-O-tert-butyldipenhylsilyl-4-o ctadecene-1,3-diol. Selective reduction of the azido group, N-acylation with octadecanoic acid, and the complete removal of the protecting groups gave the desired sialyl LeX ganglioside analogues.

Sialyl-Lewis(x) sequence 6-O-sulfated at N-acetylglucosamine rather than at galactose is the preferred ligand for L-selectin and de-N-acetylation of the sialic acid enhances the binding strength

Oligosaccharide sequences based on sialyl-Lewis(x) with 6-O-sulfation at galactose (6'-sulfo) or at N-acetylglucosamine (6-sulfo) and expressed on high endothelial venules are considered likely endogenous ligands for the leukocyte adhesion molecule, L-selectin. In the course of high performance TLC of three hexaglycosylceramides 6'-sulfo sialyl Lewis(x), 6-sulfo sialyl Lewis(x), and 6',6-bis-sulfo sialyl Lewis(x), synthesized chemically for selectin recognition studies, two minor byproducts were detected and isolated from each parent compound. By liquid secondary ion mass spectrometry these were identified as isomers containing a de-N-acetylated sialic acid or having a modified carboxyl group. Binding experiments with the parent compounds and the non-sulfated sialyl Lewis(x) glycolipid show that 6-sulfation potentiates, whereas 6'-sulfation virtually abolishes L-selectin binding. Thus the hierarchy of binding strengths were 6-sulfo sialyl > sialyl = 6',6-bis-sulfo sialyl >> 6'-sulfo sialyl Lewis(x). Whereas modification of the sialic acid carboxyl group markedly impaired L-selectin binding, de-N-acetylation resulted in enhanced binding. The natural occurrence on high endothelial venules of this 'super-active' de-N-acetylated form of 6-sulfo sialyl Lewis(x), and related structures, now deserves investigation.

Glycosphingolipid antigens and cancer therapy

Specific types of glycosphingolipid (GSL), which are chemically detectable in normal cells, are more highly expressed in tumors. The high level of expression on the surfaces of tumor cells causes an antibody response to these GSLs, which can therefore be described as tumor-associated antigens. Some of these GSLs have been shown to be adhesion molecules involved in tumor cell metastasis, and to be modulators of signal transduction controlling tumor cell growth and motility. Tumor-associated GSL antigens have been used in the development of antitumor vaccines. GSLs and sphingolipids involved in adhesion and signaling are therefore targets for cancer therapy.

Synthesis and biological activities of three sulfated sialyl Le(x) ganglioside analogues for clarifying the real carbohydrate ligand structure of L-selectin

Sulfated sialyl Le(x) ganglioside analogues at C-6 of D-galactose, N-acetyl-D-glucosamine, and of both D-galactose and N-acetyl-D-glucosamine residues have been synthesized, in order to clarify the structure of the real carbohydrate ligand of L-selectin. Coupling of the suitably protected N-acetyl-D-glucosaminyl-beta (1-->3)-lactose derivatives 13 and 16 with the sialyl alpha(2-->3)-D-galactopyranosyl trichloroacetimidates 10 and 12 (glycosyl donors), via glycosylation of 2-(trimethylsilyl)ethyl 4,6-O-benzylidene-beta-D-galactopyranoside (1) with the phenyl 2-thioglycoside derivative (2) of N-acetylneuraminic acid (Neu5Ac) using N-iodosuccinimide/TfOH, O-benzoylation, removal of the benzylidene group affording 5, selective 6-O-levulinoylation, O-benzoylation, removal of the 2-(trimethylsilyl)ethyl group, and imidate formation, or via O-acetylation of 5, removal of the 2-trimethylsilyl)ethyl group, then imidate formation, gave the pentasaccharides 18-20. The glycosylation of the pentasaccharide acceptors (21-23) derived from 18-20 by removal of the 4-methoxybenzyl group, with phenyl 1-thioglycoside derivative 27 of L-fucose using dimethyl(methylthio)sulfonium triflate (DMTST) afforded the corresponding hexasaccharides 28-30, which were transformed in good yields, via reductive removal of their benzyl groups, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, imidate formation, coupling with (2S,3R,4E)-2-azido-O-benzoyl-4-octadecene-1,3-diol (35) in the presence of boron trifluoride etherate, selective reduction of the azido group, coupling with octadecanoic acid, selective removal of the levulinoyl groups, treatment with sulfur trioxide-pyridine complex, then removal of the protecting groups, into the desired sulfated sialyl Le(x) ganglioside analogues 50-52.

A highly practical synthesis of the sialyl Lewis X pentasaccharide and an investigation of binding to E-, P-, and L-selectins

A practical synthesis of the sialyl Lewis X (sLex) pentasaccharide, NeuAc alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-3Gal beta OEt (1), as a potential blocker for E-selectin has been described. The glycosylation of a trisaccharide acceptor, Fuc alpha (1-3)GlcNAc beta (1-3)Gal beta OEt, with a disaccharide donor, NeuAc alpha (2-3)Gal beta SMe, did not yield the desired sLex pentasaccharide 1 at all. However, the glycosylation of a disaccharide acceptor, GlcNAc beta (1-3)Gal beta OEt, with a disaccharide donor, NeuAc alpha (2-3)Gal beta SMe, quantitatively yielded the tetrasaccharide NeuAc alpha (2-3)Gal beta (1-4)GlcNAc beta (1-3)Gal beta OEt. This tetrasaccharide is readily converted to the title compound in a high yield by fucosylation, followed by deprotection. The inhibitory activities of compound 1 toward the binding of the natural ligand (sLex) with the E-, P-, and L-selectins were stronger than those of the sLex tetrasaccharide.

Total synthesis of VIM-2 ganglioside isolated from human chronic myelogenous leukemia cells

A total synthesis of the tumor-associated glycolipid antigen, VIM-2, is described [2]. Phenyl 2,3,4-tri-O-benzoyl-6-O-benzyl-beta-D-galactopyranosyl-(1-->4)-6-O-benzy l-2- deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside (7), a key intermediate prepared by condensation of phenyl 6-O-benzyl-2-deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside (6) and 2,3,4-tri-O-benzoyl-6-O-benzyl-alpha-D-galactopyranosyl bromide (5), was glycosylated with methyl 2,3,4-tri-O-benzyl-1-thio-beta-L-fucopyranoside (8) to give the trisaccharide donor 9, which, on coupling with 2-(trimethylsilyl)ethyl 2,4,6-tri-O-benzyl-beta-D-galactopyranosyl-(1-->4)-2,3,6-tri-O-benzyl-be ta-D- glucopyranoside (10), afforded the pentasaccharide 11. The regioselective glycosylation of 12 (derived by O-debenzoylation of 11) with 7 gave the heptasaccharide 13, which was converted by treatment with hydrazine monohydrate and subsequent N-acetylation into the hexasaccharide acceptor 14. The stereo- and regio-selective glycosylation of 14 with methyl (phenyl 5-acetamido-4,7,8,9-O-benzoyl-3,5-dideoxy-2-thio-D-glycero-beta-D-galact o-2- nonulopyranosid)onate (16) gave the desired octasaccharide 18. Hydrogenolytic removal of the benzyl groups in 18 and successive O-acetylation, removal of the 2-(trimethylsilyl)ethyl group, and treatment with trichloroacetonitrile gave the alpha-trichloro-acetimidate 21, which was then coupled with (2S,3R,4E)-2-azido-3-O-(tert-butyldiphenylsilyl)-4-octade cene-1,3-diol (22) to give 23. Compound 23 was transformed, via selective reduction of the azido group, N-introduction of octadecanoic acid, O-desilylation, O-deacylation, and saponification of the methyl ester group, into the title VIM-2 ganglioside 26.