A stereoselective approach for the synthesis of alpha-sialosides

Total Synthesis of a PSGL-1 Glycopeptide Analogue for Targeted Inhibition of P-Selectin

The high affinity interaction between P-selectin glycoprotein ligand-1 (PSGL-1) and P-selectin is mediated by a multimotif glycosulfopeptide (GSP) recognition domain consisting of clustered tyrosine sulfates and a Core 2 O-glycan terminated with sialyl LewisX (C2-O-sLeX). These distinct GSP motifs are much more common than previously appreciated within a wide variety of functionally important domains involved in protein-protein interactions. However, despite the potential of GSPs to serve as tools for fundamental studies and prospects for drug discovery, their utility has been limited by the absence of chemical schemes for synthesis on scale. Herein, we report the total synthesis of GSnP-6, an analogue of the N-terminal domain of PSGL-1, and potent inhibitor of P-selectin. An efficient, scalable, hydrogenolysis-free synthesis of C2-O-sLeX-Thr-COOH was identified by both convergent and orthogonal one-pot assembly, which afforded this crucial building block, ready for direct use in solid phase peptide synthesis (SPPS). C2-O-sLeX-Thr-COOH was synthesized in 10 steps with an overall yield of 23% from the 4-O,5-N oxazolidinone thiosialoside donor. This synthesis represents an 80-fold improvement in reaction yield as compared to prior reports, achieving the first gram scale synthesis of SPPS ready C2-O-sLeX-Thr-COOH and enabling the scalable synthesis of GSnP-6 for preclinical evaluation. Significantly, we established that GSnP-6 displays dose-dependent inhibition of venous thrombosis in vivo and inhibits vaso-occlusive events in a human sickle cell disease equivalent microvasculature-on-a-chip system. The insights gained in formulating this design strategy can be broadly applied to the synthesis of a wide variety of biologically important oligosaccharides and O-glycan bearing glycopeptides.

Synthesis of sialyl halides with various acyl protective groups

Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.

Diacetyl strategy for synthesis of NHAc containing glycans: enhancing glycosylation reactivity via diacetyl imide protection

The presence of NHAc groups in the substrates (both glycosyl donors and acceptors) significantly reduced the reactivity of glycosylation. This decrease was attributed to the NHAc groups forming intermolecular hydrogen bonds by the NHAc groups, thereby reducing molecular mobility. Hence, a diacetyl strategy involving the temporary conversion of NHAc to diacetyl imide (NAc2) was developed for the synthesis of NHAc-containing glycans. This strategy has two significant advantages for oligosaccharide synthesis. The NAc2 protection of NHAc substantially enhances the rate of glycosylation reactions, resulting in improved yields. Moreover, NAc2 can be readily reverted to NHAc by the simple removal of one acetyl group under mild basic conditions, obviating the necessity for treating the polar amino group. We have achieved the efficient synthesis of oligosaccharides containing GlcNHAc and N-glycans containing sialic acid using the diacetyl strategy.

Stereoselective Synthesis of Sialyl Lewisa Antigen and the Effective Anticancer Activity of Its Bacteriophage Qβ Conjugate as an Anticancer Vaccine

Sialyl Lewisa (sLea ), also known as cancer antigen 19-9 (CA19-9), is a tumor-associated carbohydrate antigen. The overexpression of sLea on the surface of a variety of cancer cells makes it an attractive target for anticancer immunotherapy. However, sLea -based anticancer vaccines have been under-explored. To develop a new vaccine, efficient stereoselective synthesis of sLea with an amine-bearing linker was achieved, which was subsequently conjugated with a powerful carrier bacteriophage, Qβ. Mouse immunization with the Qβ-sLea conjugate generated strong and long-lasting anti-sLea IgG antibody responses, which were superior to those induced by the corresponding conjugate of sLea with the benchmark carrier keyhole limpet hemocyanin. Antibodies elicited by Qβ-sLea were highly selective toward the sLea structure, could bind strongly with sLea -expressing cancer cells and human pancreatic cancer tissues, and kill tumor cells through complement-mediated cytotoxicity. Furthermore, vaccination with Qβ-sLea significantly reduced tumor development in a metastatic cancer model in mice, demonstrating tumor protection for the first time by a sLea -based vaccine, thus highlighting the significant potential of sLea as a promising cancer antigen.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Improved synthesis of CD22-binding sialosides and its application for further development of potent CD22 inhibitors

CD22, one of the sialic acid-binding immunoglobulin-like lectins (Siglecs), regulates B lymphocyte signaling via its interaction with glycan ligands bearing the sequence Neu5Ac/Gcα(2→6)Gal. We have developed the synthetic sialoside GSC-718 as a ligand mimic for CD22 and identified it as a potent CD22 inhibitor. Although the synthesis of CD22-binding sialosides including GSC-718 has been reported by our group, the synthetic route was unfortunately not suitable for large-scale synthesis. In this study, we developed an improved scalable synthetic procedure for sialosides which utilized 1,5-lactam formation as a key step. The improved procedure yielded sialosides incorporating a series of aglycones at the C2 position. Several derivatives with substituted benzyl residues as aglycones were found to bind to mouse CD22 with affinity comparable to that of GSC-718. The new procedure developed in this study affords sialosides in sufficient quantities for cell-based assays, and will facilitate the search for promising CD22 inhibitors that have therapeutic potential.

Pathways in the synthesis of functionalized glycolipids for liposomal preparations

We describe in this paper the synthesis of two glycolipids containing a mannosyl residue functionalized with malonic acid and azide groups at the C6 position. Two synthetic routes have been successfully implemented: the first one involves Schmidt's glycosylation procedure using functionalized carbohydrates, whereas the second one involves nucleophilic substitutions in the C6 position of an iodinated intermediate obtained using Koenigs-Knorr reaction. A comparative discussion of reactions and yields is realized. The two glycolipids served as material for the preparation of liposomes.

Advanced Chemical Methods for Stereoselective Sialylation and Their Applications in Sialoglycan Syntheses

Sialic acid is an important component of cell surface glycans, which are responsible for many vital body functions and should therefore be thoroughly studied to understand their biological roles and association with disorders. The difficulty of isolating large quantities of homogenous-state sialoglycans from natural sources has inspired the development of the corresponding chemical synthesis methods affording acceptable purities, yields, and amounts. However, the related syntheses are challenging because of the difficulties in α-glycosylation of sialic acid, which arises from its certain structural features such as the absence of a stereodirecting group at the C3 position and presence of carboxyl group at the anomeric position. Moreover, the structural complexities of sialoglycans with diverse numbers and locations of sialic acid on the glycan chains pose additional barriers. Thus, efficient α-stereoselective routes to sialosides remain highly sought after, although various types of sialyl donors/acceptors have been developed for the straightforward synthesis of α-sialosides. Herein, we review the latest progress in the α-stereoselective synthesis of sialosides and their applications in the preparation of gangliosides and other sialoglycans.

Synthesis of an Aminooxy Derivative of the GM3 Antigen and Its Application in Oxime Ligation

The anomeric aminooxy GM3 trisaccharide cancer antigen (Neu5Acα2,3Galβ1,4Glcβ-ONH2) has been chemically synthesized using a linear glycosylation approach. The key step involves a highly α(2,3)-stereoselective sialylation to a galactose acceptor. The Neu5Acα2,3Gal intermediate was functionalized as a donor for a [2 + 1] glycosylation, including a glucose acceptor that featured an O-succinimidyl group on the reducing end as an aminooxy precursor. The fully deprotected anomeric aminooxy GM3 trisaccharide was then conjugated to the immunologically relevant zwitterionic polysaccharide PS A1 via an oxime link.

Improved α-Sialylation through the Synergy of 5-N,4-O-Oxazolidinone Protection and Exocyclic C-1 Neighboring Group Participation

Stereoselective construction of α-sialyl linkages is one of the most significant challenges in carbohydrate chemistry. In this research, we developed a novel strategy for stereoselective synthesis of α-linked sialosides by protecting the 5-N,4-O-positions of a sialyl donor with an oxazolidinone group and its C-1 carboxylic functionality with a cyanoethyl ester to promote α-glycosylation. We also adopted the more electrophilic N-bromosuccinimide as a promoter to readily activate p-tolyl thiosialoside at -78 °C. The sialylation using this sialyl donor gave excellent yields and α-selectivity. The new synthetic method was used to successfully construct naturally occurring α-sialosides having sialic acid linked to the 6-O- or 3-O-position of galactoside, or 8-O-position of another sialic acid, respectively, as well as other α-linked sialosides.

Synthesis of a dimer of the repeating unit of type Ia Group B Streptococcus extracellular capsular polysaccharide and immunological evaluations of related protein conjugates

Type Ia group B Streptococcus (GBS) is one of the major causes of fatal infections in neonates. Its extracellular capsular polysaccharide (CPS) is a useful target for the development of anti-type Ia GBS vaccines. To explore the structure-activity relationships of type Ia GBS CPS and design more effective vaccines, a dimer of the branched pentasaccharide repeating unit of this CPS was synthesized by a highly convergent strategy highlighted by constructing the key intermediate via one-pot iterative glycosylation and imposing two side chains in one step via dual glycosylation. This represented the first total synthesis of a dimer of the repeating unit of any GBS CPS reported so far and the strategy should be applicable to higher oligomers of this repeating unit. The synthetic dimer and its monomeric analog were coupled with CRM197 carrier protein to generate conjugates that were evaluated in mice. Immunological results revealed that both carbohydrate antigens could induce robust total and IgG antibody responses and the elicited antibodies were cross-reactive with both carbohydrate antigens. It was concluded that both the monomeric and the dimeric repeating units may be employed as haptens for anti-type Ia GBS vaccine development.

Constrained sialic acid donors enable selective synthesis of α-glycosides

Sialic acid is a sugar residue present in many biologically significant glycans of mammals, commonly as a terminal α-glycoside. The chemical structure of sialic acid, which features an anomeric center with carboxyl and methylene substituents, poses a challenge for synthesis of the α-glycoside, thus impeding biological and therapeutic studies on sialic acid–containing glycans. We present a robust method for the selective α-glycosidation of sialic acid using macrobicyclized sialic acid donors as synthetic equivalents of structurally constrained oxocarbenium ions to impart stereoselectivity. We demonstrate the power of our method by showcasing broad substrate scope and applicability in the preparation of diverse sialic acid–containing architectures.

Chemical Synthesis of Glycosides of N-Acetylneuraminic Acid

Investigations of methodologies aimed on improving the stereoselective synthesis of sialosides and the efficient assembly of sialic acid glycoconjugates has been the mission of dedicated research groups from the late 1960s. This review presents major accomplishments in the field, with the emphasis on significant breakthroughs and influential synthetic strategies of the last decade.

Synthesis and Evaluation of GM2-Monophosphoryl Lipid A Conjugate as a Fully Synthetic Self-Adjuvant Cancer Vaccine

An efficient method was developed for the synthesis of a GM2 derivative suitable for the conjugation with various biomolecules. This GM2 derivative was covalently linked to keyhole limpet hemocyanin (KLH) and monophosphoryl lipid A (MPLA) to form novel therapeutic cancer vaccines. Immunological evaluations of the resultant conjugates in mice revealed that they elicited robust GM2-specific overall and IgG antibody responses. Moreover, the GM2-MPLA conjugate was disclosed to elicit strong immune responses without the use of an adjuvant, proving its self-adjuvant property. The antisera of both conjugates showed strong binding and mediated similarly effective complement-dependent cytotoxicity to GM2-expressing cancer cell line MCF-7. Based on these results, it was concluded that both GM2-MPLA and GM2-KLH are promising candidates as therapeutic cancer vaccines, whereas fully synthetic GM2-MPLA, which has homogeneous and well-defined structure and self-adjuvant property, deserves more attention and studies.

Stereoselective sialylation with O-trifluoroacetylated thiosialosides: hydrogen bonding involved?

A series of novel sialyl donors containing O-trifluoroacetyl (TFA) groups at various positions was synthesized. The choice of protecting groups in sialyl donors was based on hypothesis that variations in ability of different acyl groups to act as hydrogen bond acceptors would influence the supramolecular structure of reaction mixture (solution structure), hence the outcome of sialylation. These glycosyl donors were examined in the model glycosylation of the primary hydroxyl group of 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose in comparison with sialyl donors without O-TFA groups. The presence of O-TFA groups in a sialyl donor strongly affected the outcome of sialylation. Several sialyl donors studied showed promising results: yields of disaccharides can be as high as 86% as can be the stereoselectivities (α/β up to 15:1). The results obtained suggest that varying acyl O-protecting groups in sialyl donor may result in dramatic changes in the outcome of sialylation although further studies are required to dissect the influence of intermolecular hydrogen bonding and intramolecular substituent effects related to variations of electron-withdrawing properties of different acyl groups.

Chemical Synthesis of a Complex-Type N-Glycan Containing a Core Fucose

A chemical synthesis of a core fucose containing N-glycan was achieved. Asparagine was introduced at an early stage of the synthesis, and the sugar chain was convergently elongated. As for the fragment synthesis, we reinvestigated α-sialylation, β-mannosylation, and N-glycosylation to reveal that precise temperature control was essential for these glycosylations. Intermolecular hydrogen bonds involving acetamide groups were found to reduce the reactivity in glycosylations: the protection of NHAc as NAc₂ dramatically improved the reactivity. The dodecasaccharide-asparagine framework was constructed via the (4 + 4) glycosylation and the (4 + 8) glycosylation using the tetrasaccharide donor and the tetrasaccharide-asparagine acceptor. An ether-type solvent enhanced the yields of these key glycosylations between large substrates. After the whole deprotection of the dodecasaccharide, the target N-glycan was obtained.

Linear synthesis and immunological properties of a fully synthetic vaccine candidate containing a sialylated MUC1 glycopeptide

A strategy for the linear synthesis of a sialylated glycolipopeptide cancer vaccine candidate has been developed using a strategically designed sialyl-Tn building block and microwave-assisted solid-phase peptide synthesis. The glycolipopeptide elicited potent humoral and cellular immune responses. T-cells primed by such a vaccine candidate could be restimulated by tumor-associated MUC1.

Chemical synthesis of the repeating unit of type Ia group B Streptococcus capsular polysaccharide

The structure of the capsular polysaccharide (CPS) of serotype Ia group B Streptococcus (GBS) has been characterized for years, but its repeating unit, which is a challenging pentasaccharide with a branch and a difficult α-sialic acid linkage, has not been synthesized yet. In this report, an effective synthesis was developed for the serotype Ia GBS CPS repeating unit, which had a reactive functionality linked to its main-chain reducing end to enable further elaboration, such as coupling with carrier proteins. The target molecule was accomplished by a convergent [2 + 3] glycosylation strategy employing a sialo-disaccharide as donor and a branched trisaccharide as acceptor. The strategy was designed to suit the synthesis of oligomers of the repeating unit.

Improvement of the immune efficacy of carbohydrate vaccines by chemical modification on the GM3 antigen

N-modified GM3 glycoconjugates improved the efficiency of the vaccination without the combination of metabolic oligosaccharide engineering technology.

Synthetic approach toward complexity of sialic acid-containing glycans

The biological relevance of sialic acid-containing glycans (sialo-glycans) to various interactions at the cell surface has increased the demand for the supply of structurally defined sialo-glycans. Due to their unique structures, the stereoselective synthesis of glycosides of sialic acid is inherently difficult, which makes the synthetic approach to synthesize diverse and complex sialo-glycans far from successful. However, the gap between the chemical and natural synthesis of complex sialo-glycans is narrowing through the emergence of synthetic methods. This review highlights recent progress made in the synthesis of complex sialo-glycans via cutting-edge synthetic methods.

Chemical approach for the syntheses of GM4 isomers with sialic acid to non-natural linkage positions on galactose

Cell-surface glycans containing sialic acid are involved in various biological phenomena. However, the syntheses of GM4 derivatives with (2 → 2) and (2 → 4) linkages have not been investigated to date. In this study, sialylation of all of the hydroxyl groups on galactose were investigated for the syntheses of GM4 isomers. Regioselective sialylation was achieved via protection of galactosyl acceptors using electron-rich benzyl groups. These synthetic sialylated glycans will prove to be useful tools for studying unidentified carbohydrate-mediated biological roles.

Comparative studies on the O-sialylation with four different α/β-oriented (N-acetyl)-5-N,4-O-carbonyl-protected p-toluenethiosialosides as donors

Four types of 5-N,4-O-carbonyl-protected p-toluenethiosialosides were synthesized and their couplings with different acceptors were thoroughly investigated. The results indicate that the sialyl donor structure, the amount of glycosyl acceptor, and the detailed promotion conditions have great influence on the sialylation stereoselectivties and product yields. Under the (p-Tol)2SO/Tf2O activation conditions, the glycosylations with simple alcohols provided declined α-selectivities and higher yields with increasing the amounts of acceptors from 1.1 equiv to 2.0equiv. However, the outcome of same sialylation was independent of the relative amounts of sugar alcohol acceptors. With NIS/TfOH as promoter, the α-selectivities of the sialylations were significantly improved compared with the cases activated by (p-Tol)2SO/Tf2O. In general, the difference in configuration of N-acetylated sialyl donors (D2 and D4) has little effect on the sialylation yield and stereoselectivity. In contrast, the N-deacetylated α/β sialyl donors (D1 and D3) show complex sialylation profiles with different acceptors.

Automated synthesis of sialylated oligosaccharides

Sialic acid-containing glycans play a major role in cell-surface interactions with external partners such as cells and viruses. Straightforward access to sialosides is required in order to study their biological functions on a molecular level. Here, automated oligosaccharide synthesis was used to facilitate the preparation of this class of biomolecules. Our strategy relies on novel sialyl α-(2→3) and α-(2→6) galactosyl imidates, which, used in combination with the automated platform, provided rapid access to a small library of conjugation-ready sialosides of biological relevance.

General consideration on sialic acid chemistry

Sialic acids, also known as neuraminic acids, are a family of negatively charged α-keto acids with a nine-carbon backbone. These unique sugars have been found at the termini of many glycan chains of vertebrate cell surface, which play pivotal roles in mediating or modulating a variety of physiological and pathological processes. This brief review covers general approaches for synthesizing sialic acid containing structures. Recently developed synthetic methods along with structural diversities and biological functions of sialic acid are discussed.

Toward automated oligosaccharide synthesis

Carbohydrates have been shown to play important roles in biological processes. The pace of development in carbohydrate research is, however, relatively slow due to the problems associated with the complexity of carbohydrate structures and the lack of general synthetic methods and tools available for the study of this class of biomolecules. Recent advances in synthesis have demonstrated that many of these problems can be circumvented. In this Review, we describe the methods developed to tackle the problems of carbohydrate-mediated biological processes, with particular focus on the issue related to the development of the automated synthesis of oligosaccharides. Further applications of carbohydrate microarrays and vaccines to human diseases are also highlighted.