Synthesis of a sialic acid dimer derivative, 2'alpha-O-benzyl Neu5Ac-alph-(2-->5)Neu5Gc

Synthesis of N-Glycolylneuraminic Acid (Neu5Gc) and Its Glycosides

Sialic acids constitute a family of negatively charged structurally diverse monosaccharides that are commonly presented on the termini of glycans in higher animals and some microorganisms. In addition to N-acetylneuraminic acid (Neu5Ac), N-glycolyl neuraminic acid (Neu5Gc) is among the most common sialic acid forms in nature. Nevertheless, unlike most animals, human cells loss the ability to synthesize Neu5Gc although Neu5Gc-containing glycoconjugates have been found on human cancer cells and in various human tissues due to dietary incorporation of Neu5Gc. Some pathogenic bacteria also produce Neu5Ac and the corresponding glycoconjugates but Neu5Gc-producing bacteria have yet to be found. In addition to Neu5Gc, more than 20 Neu5Gc derivatives have been found in non-human vertebrates. To explore the biological roles of Neu5Gc and its naturally occurring derivatives as well as the corresponding glycans and glycoconjugates, various chemical and enzymatic synthetic methods have been developed to obtain a vast array of glycosides containing Neu5Gc and/or its derivatives. Here we provide an overview on various synthetic methods that have been developed. Among these, the application of highly efficient one-pot multienzyme (OPME) sialylation systems in synthesizing compounds containing Neu5Gc and derivatives has been proven as a powerful strategy.

Stereoselective synthesis of a 9-O-sulfo Neu5Gc-capped O-linked oligosaccharide found on the sea urchin egg receptor

The first total synthesis of a serine bearing α2→5-O_glycolyl-linked oligoNeu5Gc found on sea urchin egg cell surfaces has been accomplished.

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.

Structure-Activity Relationship Study of the Neuritogenic Potential of the Glycan of Starfish Ganglioside LLG-3 (‡)

LLG-3 is a ganglioside isolated from the starfish Linchia laevigata. To clarify the structure-activity relationship of the glycan of LLG-3 toward rat pheochromocytoma PC12 cells in the presence of nerve growth factor, a series of mono- to tetrasaccharide glycan derivatives were chemically synthesized and evaluated in vitro. The methyl group at C8 of the terminal sialic acid residue was crucial for neuritogenic activity, and the terminal trisaccharide moiety was the minimum active motif. Furthermore, the trisaccharide also stimulated neuritogenesis in human neuroblastoma SH-SY5Y cells via mitogen-activated protein kinase (MAPK) signaling. Phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 was rapidly induced by adding 1 or 10 nM of the trisaccharide. The ratio of phosphorylated ERK to ERK reached a maximum 5 min after stimulation, and then decreased gradually. However, the trisaccharide did not induce significant Akt phosphorylation. These effects were abolished by pretreatment with the MAPK inhibitor U0126, which inhibits enzymes MEK1 and MEK2. In addition, U0126 inhibited the phosphorylation of ERK 1/2 in response to the trisaccharide dose-dependently. Therefore, we concluded that the trisaccharide promotes neurite extension in SH-SY5Y cells via MAPK/ERK signaling, not Akt signaling.

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.

First synthesis of a pentasaccharide moiety of ganglioside GAA-7 containing unusually modified sialic acids through the use of N-Troc-sialic acid derivative as a key unit

The pentasaccharide part of the potent neuritogenic ganglioside GAA-7 has been synthesized for the first time. The unique branched terminus constituting partially modified sialic acids and N-acetylgalactosamine was successfully established by stereoselective double-sialylation using 8-O-methyl-N-Troc-sialic acid as a donor. The final 4 + 1 coupling reaction provided a high yield of pentasaccharide, which was deprotected to deliver the target molecule.

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.

Advances in the biology and chemistry of sialic acids

Sialic acids are a subset of nonulosonic acids, which are nine-carbon alpha-keto aldonic acids. Natural existing sialic acid-containing structures are presented in different sialic acid forms, various sialyl linkages, and on diverse underlying glycans. They play important roles in biological, pathological, and immunological processes. Sialobiology has been a challenging and yet attractive research area. Recent advances in chemical and chemoenzymatic synthesis, as well as large-scale E. coli cell-based production, have provided a large library of sialoside standards and derivatives in amounts sufficient for structure-activity relationship studies. Sialoglycan microarrays provide an efficient platform for quick identification of preferred ligands for sialic acid-binding proteins. Future research on sialic acid will continue to be at the interface of chemistry and biology. Research efforts not only will lead to a better understanding of the biological and pathological importance of sialic acids and their diversity but also could lead to the development of therapeutics.

A first total synthesis of ganglioside HLG-2

A neuritegenic ganglioside from sea cucumber, HLG-2 (see figure), has been synthesized for the first time. The unique tandem of sialic acids, Neu5Gc-alpha(2,4)-NeuAc, was established by the combination of a reactive N-Troc sialyl donor and a 1,5-lactamized sialyl acceptor. The ceramide counterpart was assembled in a stereoselective manner. Direct connection of the trisaccharide and the ceramide successfully afforded a precursor of HLG-2, which was converted to ganglioside HLG-2 in pure form.A first synthesis of the neuritegenic ganglioside HLG-2, which was identified in extracts of the sea cucumber Holothuria leucospilota, is described. The characteristic sequence of the trisaccharide part, alpha-N-glycolylsialyl-(2,4)-alpha-N-acetylsialyl-(2,6)-glucoside, was efficiently assembled by coupling of a highly active N-2,2,2-trichloroethoxycarbonyl (Troc)-protected sialyl donor and a 1,5-lactamized sialyl acceptor with high stereoselectivity. The corresponding trisaccharyl imidate donor was directly glycosidated with the primary hydroxyl group of the ceramide part, producing protected HLG-2 in relatively high yield, global deprotection of which furnished ganglioside HLG-2 in highly pure form.

Synthesis of the starfish ganglioside LLG-3 tetrasaccharide

The first synthesis of the ganglioside LLG-3 tetrasaccharide, which has attractive biological activities as well as a unique structure, is described. A C8-methoxy decorated sialic acid building block was initially prepared and a glycolic acid moiety was then introduced by sialylation. Amide condensation between the sialyl glycolic acid and an amino group at C5 on the sialyllactoside unit afforded the fully protected LLG-3 tetrasaccharide. Finally, the desired tetrasaccharide part of LLG-3 was obtained after careful global deprotection. [structure: see text].

Synthesis of sialic acid derivatives as ligands for the myelin-associated glycoprotein (MAG)

The trisaccharide substructure 13 of the ganglioside GQ1balpha shows a remarkable affinity for the myelin-associated glycoprotein (MAG). In the search for structurally simplified and pharmacokinetically improved mimics of 13, sialosides with modifications at the reducing and non-reducing end were synthesized. The biological evaluation of mimics 12a-o was performed in a competitive target-based assay. It was found that the relative inhibitory potency (rIP) of antagonist 12h was enhanced by more than 1000-fold in comparison to the reference trisaccharide 13, despite the former having a much simpler structure. In addition, the sialic acid derivatives, for example, 12h, have clearly improved pharmacokinetic properties due to the presence of aromatic moieties, a lower molecular weight, and a reduced number of polar hydroxy functions compared to the reference compound 13.

One-Pot Synthesis of Sialo-Containing Glycosyl Amino Acids by Use of anN-Trichloroethoxycarbonyl-?-thiophenyl Sialoside

We describe an efficient synthesis of 2,6- and 2,3-sialyl T antigens linked to serine in a one-pot glycosylation. We first investigated the glycosidation of thiosialosides by varying the N-protecting group. Modification of the C-5 amino group of beta-thiosialosides into the N-9-fluorenylmethoxycarbonyl, N-2,2,2-trichloroethoxycarbonyl (N-Troc), and N-trichloroacetyl derivatives enhanced the reactivity of these compounds towards glycosidation. Addition of a minimum amount of 3 A molecular sieves was also effective in improving the yield of alpha-linked sialosides. Next, we conducted one-pot syntheses of the glycosyl amino acids by using the N-Troc sialyl donor. The N-Troc derivative can be converted into the N-acetyl derivative without racemization of the amino acids. Branched-type one-pot glycosylation, initiated by regioselective glycosylation of the 3,6-dihydroxy galactoside with the N-Troc-beta-thiophenyl sialoside, provided the protected 2,6-sialyl T antigen in good yield. Linear-type one-pot glycosylation, initiated by chemoselective glycosylation of galactosyl fluoride with the N-Troc-beta-thiophenyl sialoside, afforded the protected 2,3-sialyl T antigen in excellent yield. Both protected glycosyl amino acids were converted into the fully deprotected 2,6- and 2,3-sialyl T antigens linked to serine in good yields.