Sialylation Reactions Research Articles

Direct Experimental Characterization of a Sialyl Cation.

Sialic acids are monosaccharide residues involved in several biological processes. Controlling the stereoselectivity of sialylation reactions is challenging and mechanistic studies on the structure of its intermediate, the sialyl cation, are scarce. Here it is shown that a sialyl cation can be generated and isolated from an ionized sialic acid precursor. This short-lived species is structurally characterized for the first time using cryogenic infrared spectroscopy. In combination with quantum chemical calculations, the results reveal that the positive charge at the anomeric carbon of the sialyl cation is stabilized by remote participation of the C5-NHAc group leading to the formation of a bridged structure. In this structure, the β-side is shielded from nucleophilic attack, potentially explaining the α-selectivity of this building block in SN1-type sialylation reactions. Other modes of participation are energetically unfavored and cannot be observed experimentally.

Sialylations reactions: Expanding the effect of silicon protecting groups at C-4

As a part of a general research goal into evaluating the effect of O-protecting groups in sialylation reactions, we compared the reactivities of phenylthio α- and β-sialyl donors protected at C-4 with tert-butyldimethyl silyl (TBDMS) and tri-isopropylsilyl (TIPS) groups in the coupling with primary and secondary galactosyl acceptors. The effect of the solvent and the donor's anomeric configuration were investigated and compared to previously published data. It is demonstrated that silicon groups at C-4 have a significant influence on sialylation reactions, and in general their overall performance is optimized with the use of acetonitrile.

Cover Feature: Donor‐Reactivity‐Controlled Sialylation Reactions (Eur. J. Org. Chem. 32/2021)

The Cover Feature shows that the Relative Reactivity Value (RRV) is an effective stereoselectivity navigator for a sialylation reaction under the NIS/TfOH promotion system for the driver (chemist). The right turn and left turn signals are symbolizing the α- and β-sialosides, respectively, using donor RRV as a direction indicator. Thiosialoside donors having RRV <11.5 enable selective synthesis of α-sialosides (right turn signal). On the other hand, donors with RRV ≥11.5 perform β-selective sialylation reactions (left turn signal). The authors thank Ms. Chiu-Yun Chen in our Institute for her assistance in creating the artwork. More information can be found in the Full Paper by C.-C. Wang et al.

Donor‐Reactivity‐Controlled Sialylation Reactions

AbstractAlthough tremendous efforts have been made for the efficient preparation of sialosides, controlling the stereochemical outcome of sialylation reaction still remains one of the most challenging tasks due to the unique chemical structure of sialic acid. We developed a new strategy to statistically analyze the stereoselectivity of sialylation reactions on six types of p‐tolyl thiosialosides in NIS/TfOH system using Relative Reactivity Value (RRV) as the indicator. Analysis of the reaction mechanism showed the formation of the relatively stable glycosyl bromide and glycosyl chloride intermediates from halide‐ and triflate‐containing promotors in the absence of an acceptor. We found that the α/β‐stereoselectivity, yields, and intermediate changes were associated with their donor reactivity. These findings enable to tailor the most suitable building blocks for stereo‐controlled sialylation reactions.

Multivalent sialylation of β-thio-glycoclusters by Trypanosoma cruzi trans sialidase and analysis by high performance anion exchange chromatography.

The synthesis of multivalent sialylated glycoclusters is herein addressed by a chemoenzymatic approach using the trans-sialidase of Trypanosoma cruzi (TcTS). Multivalent β-thio-galactopyranosides and β-thio-lactosides were used as acceptor substrates and 3'-sialyllactose as the sialic acid donor. High performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) was shown to be an excellent technique for the analysis of the reaction products. Different eluting conditions were optimized to allow the simultaneous resolution of the sialylated species, as well as their neutral precursors. The TcTS efficiently transferred sialyl residues to di, tri, tetra and octa β-thiogalactosides. In the case of an octavalent thiolactoside, up to six polysialylated compounds could be resolved. Preparative sialylation reactions were performed using the tetravalent and octavalent acceptor substrates. The main sialylated derivatives could be unequivocally assigned by MALDI mass spectrometry. Inhibition of the transfer to the natural substrate, N-acetyllactosamine, was also studied. The octalactoside caused 82% inhibition of sialic acid transfer when we used equimolar concentrations of donor, acceptor and inhibitor.

Sialylation reactions with N,N-dibenzylthiosialoside donor

A new sialyl donor, N,N-dibenzyl-protected p-toluenethiosialoside, was synthesized and its sialylation reaction was investigated. This reaction proceeded in a preactivation manner and preferred to provide β-anomeric coupling products. The β-selectivity was rationalized based on the assumption that N,N-dibenzylamino group could serve as an oriented group to activate the hydroxyl group of acceptor from β-face.

Silylene/Oxazolidinone Double‐Locked Sialic Acid Building Blocks for Efficient Sialylation Reactions in Dichloromethane

AbstractWe describe efficient sialylation reactions in CH2Cl2 with the use of silylene/oxazolidinone double‐locked sialic acid building blocks. The building blocks were synthesized from 4,5‐oxazolidinone‐protected phenylthiosialoside. In sialylation reactions towards primary and relatively reactive secondary hydroxy groups on the galactosides, the double‐locked building blocks provided desired coupling products in good yields with excellent α‐selectivities. In the sialylation reaction with the C3‐OH of the galactoside, the double‐locked building blocks expressed significantly better α‐selectivity in comparison with the results obtained by using the oxazolidinone‐locked building block. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2009)

Sialylation reactions with N, N-acetyl, benzoyl- O-perbenzoyl-protected sialyl donor

A new sialyl donor, N, N-acetyl, benzoyl- O-perbenzoyl-protected p-toluenethiosialoside, was synthesized and its sialylation reaction was investigated. This reaction proceeded in dichloromethane and β anomeric selectivity was achieved when NIS–TfOH was used as promoter. This method may be employed to construct the unnatural β-linked sialosides.

Solvent Effect in the Synthesis of Sialosyl α(2–6) Galactosides: Is Acetonitrile the only Choice?

AbstractIn spite of notable achievements for the synthesis of α‐sialosides that have been made in the past decades, sialylation reactions often require low temperatures (–40 to –78 °C) and the use of acetonitrile as a solvent. Herein we report that a C‐5 oxazolidinone sialosyl donor gives high yields andstereoselectivities in the presence of dichloromethane and/or tetrahydrofuran at 0 °C. Surprisingly, high stereoselectivities and yields are obtained even at ambient temperature in tetrahydrofuran.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Thioester-assisted α-sialylation reaction

α-Selective sialylation reactions were carried out using novel sialic acid building blocks that possess a thioester auxiliary. In contrast to other arylthio- and benzylthioester derivatives, sialyl phosphite 1a (with the phenylthioester moiety) was employed as the α-selective building block, and was reacted with various primary alcohols, including the C6–OH group of galactose and glucose, with moderate to excellent α-selectivities. For C3–OH of the galactose, 4,6-di- O-benzylgalactal afforded desired α-linkage with excellent selectivity.

Sialylation reactions with 5- N,7- O-carbonyl-protected sialyl donors: unusual stereoselectivity with nitrile solvent assistance

A 5- N,7- O-carbonyl-protected sialyl donor was synthesized, and, unexpectedly, this donor showed β-selectivity (α/β = 1/2.4–1/20) on coupling with sugar acceptors in acetonitrile upon treatment with various promoter systems. For the coupling reaction in dichloromethane, a modified Ellervik’s method (IBr and AgClO 4·H 2O) was highly effective in activating the 5- N,7- O-carbonyl donor, providing moderate α-selectivity (α/β = ∼1.8/1).

Efficient Synthesis of MUC4 Sialylglycopeptide through the New Sialylation Using 5-Acetamido-Neuraminamide Donors

Sialylation reactions using a new sialyl donor, diethyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-O-beta-D-glycero-D-galacto-2-nonulopyranosylonamide phosphite (Neu5Ac-1-amide-2-phosphite) derivatives, and the synthesis of the sialyl-T N-MUC4 glycopeptide are described. The sialylation was performed in CH2Cl2 solvent toward the 6-hydroxyl group of several monosugar acceptors and generated alpha-sialoside in good yield under low temperature and TMSOTf activation system. Amide derivatives of sialoside were easily converted into naturally occurring sialoside after hydrolysis of the amide group. Sialyl-alpha(2,6)-GalN3 was also prepared by this new sialylation protocol, and then this sialoside was further converted into a Fmoc-protected sialyl-TN serine derivative for solid-phase glycopeptides synthesis. The solid-phase glycopeptide synthesis using this sialyl-TN serine derivative in which the sugar hydroxyl group was free afforded the target sialyl-TN-MUC4 glycopeptide.

8-O-Sialylation of neuraminic acid acceptor reactivity and anomeric stereocontrol

2-O-Unprotected and 2-deoxy-2,3-dehydro-neuraminic acid derivatives 2, 4, and 10 exhibit enhanced acceptor properties at their 8-hydroxy group in sialylation reactions with phosphite 5 as donor; yet, mainly (10) or exclusively (2, 4) β-glycoside bond formation was observed. The 3-phenylthionocarbonate group as stereodirecting auxiliary group in the sialyl donor 11 led with 10 as acceptor to exclusive formation of α(2–8)-linked disaccharide.

A mathematical model of sialylation of N-linked oligosaccharides in the trans-Golgi network.

A mathematical model is developed of the compartmentalized sialylation of N-linked oligosaccharides in order to understand and predict the outcome of sialylation reactions. A set of assumptions are presented, including Michaelis-Menten-type dependency of reaction rate on the concentration of the glycoprotein substrate. The resulting model predicts the heterogeneous outcome of a posttranslational oligosaccharide biosynthesis step, a critical aspect that is not accounted for in the modeling of the cotranslational attachment of oligosaccharides to glycosylation sites (Shelikoff et al., Biotech. Bioeng., 50, 73-90, 1996) or general models of the secretion process (Noe and Delenick, J. Cell Sci., 92, 449-459, 1989). In the steady-state for the likely case where the concentration of substrate is much less than the Km of the sialyltransferase, the model predicts that the extent of sialylation, x, will depend upon the enzyme concentration, enzyme kinetic parameters and substrate residence time in the reaction compartment. The value of x predicted by the model using available literature data is consistent with the values of x that have been recently determined for the glycoproteins CD4 (Spellman et al., Biochemistry, 30, 2395-2406, 1991) and t-PA (Spellman et al., J. Biol. Chem., 264, 14100-14111, 1989) secreted by Chinese hamster ovary cells. For the unsaturated case, the model also predicts that x is independent of the concentration of secreted glycoprotein in the Golgi. The general modeling approach outlined in this article may be applicable to other glycosylation reactions and posttranslational modifications.

A Novel α-2,6-Sialyltransferase: Transfer of Sialic Acid to Fucosyl and Sialyl Trisaccharides

The substrate specificity and enzymatic sialylation ability of the bacterium α-2,6-sialyltransferase were examined. The enzyme assay displayed a remarkable ability to catalyze sialyl transfer to type-II oligosaccharides possessing fucoside or sialoside at the 2 or 3 position of the terminal galactoside. Enzymatic syntheses were performed in order to confirm the structure of unusual assay products found when using Neu5Ac β2,3Galβ1,4Glc and Fuc α1,2Galβ1,4Glc as the sialyl acceptors. Both sialylation reactions (10 μmol scales) were run using 83 munits of enzyme, were complete in 2 h, and afforded the sialoside analogues Neu5Ac α2,6(Fuc α1,2) Galβ1,4Glc (88%) and Neu5Ac α2,6(Neu5Ac β2,3) Galβ1,4Glc (92%).

Cytidine 5'-monophospho-N-acetylneuraminic acid or a related compound is the low Mr factor from human red blood cells which induces gonococcal resistance to killing by human serum.

A low-Mr factor which induces gonococcal resistance to complement-mediated serum killing has been partially purified from lysates of mixed red and buffy coat cells from human blood. The lysates were dialysed against Tris buffer for 24 h at 25 degrees C with the diffusate being continuously recycled through a column of QAE-Sephadex A25. After elution in an NaCl gradient, the active fractions were both desalted and further purified on Sephadex G10. A second fractionation on QAE-Sephadex A25 and desalting with Sephadex G10 preceded further purification by repeated high-pressure liquid chromatography (HPLC) using a DEAE anion exchange column and desalting with Sephadex G10. Less than 500 micrograms of material showing one peak in HPLC was obtained from 1 litre of blood. After NMR had indicated the possible presence of pyrimidine nucleotide, carbohydrate and N-acetyl groups, nanogram quantities of a commercial preparation of cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-NANA) were shown to induce gonococci to serum resistance. The synthetic CMP-NANA also co-eluted with the preparation from blood cells in HPLC, and the two materials were indistinguishable in their patterns of acid and heat lability. Furthermore, the resistance-inducing activity of both materials was inhibited by cytidine monophosphate, which is known to inhibit sialylation reactions by CMP-NANA. It appears therefore that the resistance-inducing factor is CMP-NANA or a closely related compound. If the factor is CMP-NANA, biological activities indicated that the cell lysate from 1 litre of blood contained about 40 micrograms, and the most purified preparation contained only about 1%. With this minute amount in a mixture, the presence of CMP-NANA or a closely related analogue could not be established unequivocally by NMR.