Sialyl Donor Research Articles

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.

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.

An Efficient Convergent Synthesis of GP1c Ganglioside Epitope

In this report, we describe an efficient convergent synthesis of the GP1c glycolipid epitope, which is one of the most complex c-series gangliosides. The alpha(2,3) and alpha(2,8) sialylations were accomplished by use of 5N,4O-carbonyl and 7,8-O-isopropyliden as well as 5N,4O-carbonyl- and 7,8-di-O-chloroacetyl-protected sialyl donors in good yields with excellent alpha-selectivity, respectively. The two sialyl donors enable synthesis of the di- and trisialylgalactosides by simple glycosylation and deprotection. We synthesized the protected GP1c glycolipid epitope, which is a compact, rigid, branched structure, via direct coupling of tetarasaccharide and pentasaccharide units.

Efficient Synthesis of an α(2,9) Trisialic Acid by One-Pot Glycosylation and Polymer-Assisted Deprotection

An efficient synthesis of alpha(2,9) trisialic acid has been achieved via one-pot glycosylation and polymer-assisted deprotection. The synthesis involves chemo- and regioselective alpha-sialylation of ethylthiosialoside with the S-benzoxazolyl (S-Box) sialyl donor. Use of a prelinker to link an activated ester and a vinyl ether via a carbon chain enables polymer-assisted deprotection of the protected trisialic acids.

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.

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.

Reversible Sialylation: Synthesis of Cytidine 5‘-Monophospho-N-acetylneuraminic Acid from Cytidine 5‘-Monophosphate with α2,3-Sialyl O-Glycan-, Glycolipid-, and Macromolecule-Based Donors Yields Diverse Sialylated Products†

Sialyltransferases transfer sialic acid from cytidine 5‘-monophospho-N-acetylneuraminic acid (CMP-NeuAc) to an acceptor molecule. Trans-sialidases of parasites transfer α2,3-linked sialic acid from one molecule to another without the involvement of CMP-NeuAc. Here we report another type of sialylation, termed reverse sialylation, catalyzed by mammalian sialyltransferase ST3Gal-II. This enzyme synthesizes CMP-NeuAc by transferring NeuAc from the NeuAcα2,3Galβ1,3GalNAcα unit of O-glycans, 3-sialyl globo unit of glycolipids, and sialylated macromolecules to 5‘-CMP. CMP-NeuAc produced in situ is utilized by the same enzyme to sialylate other O-glycans and by other sialyltransferases such as ST6Gal-I and ST6GalNAc-I, forming α2,6-sialylated compounds. ST3Gal-II also catalyzed the conversion of 5‘-uridine monophosphate (UMP) to UMP-NeuAc, which was found to be an inactive sialyl donor. Reverse sialylation proceeded without the need for free sialic acid, divalent metal ions, or energy. Direct sialylation with CMP-NeuAc as well as the formation of CMP-NeuAc from 5‘-CMP had a wide optimum range (pH 5.2−7.2 and 4.8−6.4, respectively), whereas the entire reaction comprising in situ production of CMP-NeuAc and sialylation of acceptor had a sharp optimum at pH 5.6 (activity level 50% at pH 5.2 and 6.8, 25% at pH 4.8 and 7.2). Several properties distinguish forward/conventional versus reverse sialylation: (i) sodium citrate inhibited forward sialylation but not reverse sialylation; (ii) 5‘-CDP, a potent forward sialyltransferase inhibitor, did not inhibit the conversion of 5‘-CMP to CMP-NeuAc; and (iii) the mucin core 2 compound 3-O-sulfoGalβ1,4GlcNAcβ1,6(Galβ1,3)GalNAcα-O-benzyl, an efficient acceptor for ST3Gal-II, inhibited the conversion of 5‘-CMP to CMP-NeuAc. A significant level of reverse sialylation activity is noted in human prostate cancer cell lines LNCaP and PC3. Overall, the study demonstrates that the sialyltransferase reaction is readily reversible in the case of ST3Gal-II and can be exploited for the enzymatic synthesis of diverse sialyl products.

Synthesis of N,N‐Ac,Boc Laurylthio Sialoside and Its Application to O‐Sialylation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Alpha-selective sialylations at -78 degrees C in nitrile solvents with a 1-adamantanyl thiosialoside.

Novel 1-adamantanylthio sialosides were synthesized and coupled to acceptors under NIS/TfOH promotion conditions. These donors showed higher reactivity than the phenylthio sialosides and could be activated by NIS/TfOH in nitrile solvents at -78 degrees C to afford improved alpha-sialylations. With the N-acetyl-5-N,4-O-oxazolidinone-protected 1-adamantanylthio sialyl donor high alpha-selectivities could be achieved in the sialylations of both primary and sterically hindered secondary acceptors, including the important galactose 3-OH acceptors.

Synthesis of N, N-Ac,Boc laurylthio sialoside and its application to O-sialylation

The combination of the 5- N- tert-butoxycarbonyl (Boc) group of laurylthio sialoside and cyclopentyl methyl ether (CPME) as a solvent enhanced the reactivity and α-selectivity of the sialyl donor during sialylation. Selective deprotection of the N-Boc group of sialoside, including an acid-sensitive isopropylidene function, was successfully achieved by Yb(OTf) 3–SiO 2. Transformation of N, N-Ac,Boc into an N-acetylglycolyl group of sialoglycoside was easily performed via selective N-deacylation of the mixed Ac- N-Boc carbamate, subsequent Boc group removal, and acylation.

Anomeric Acetates of N‐Acetylneuraminic Acid are Useful C‐Sialyl Donors in Samarium‐Mediated Reformatsky Coupling Reactions

Teaching an old molecule new tricks: A new solution for an expeditious C-sialylation from N-acetylneuraminic acid involves the use of the peracetylated derivative 1, a very common starting material prepared 40 years ago in Heidelberg, through an anomeric umpolung by samarium diiodide without an additive, in the presence of a carbonyl compound. Excellent yields are obtained for the coupling with cyclic ketones (see scheme).

Novel Glycosylation Reactions Using Glycosyl Thioimidates of N‐Acetylneuraminic Acid as Sialyl Donors.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Ganglioside GM3 Derivatives with Truncated Ceramide Moiety: Facial Synthesis and Inhibitory Activity against KB Cell Growth

An expeditious sialylation reaction with phenylthioglycoside 4 as a sialyl donor and MeSOTf as a promoter was developed. These conditions are very useful for synthesizing ganglioside GM3 (1), its C8‐ceramide analog 2, and 3‐deoxy analog 3 of 2 in an efficient manner. The GM3 analog 2, whose hydrophilicity is increased by shortening the ceramide moiety, exhibits increased growth inhibiton of KB cells. The 3‐hydoxy group of ceramide does not influence its activity against KB cells.

Stereoselective Synthesis of Oligo-α-(2,8)-Sialic Acids

An efficient and elegant synthesis of alpha(2,8)-oligosialosides is described. The 5-N,4-O-carbonyl-protected sialyl donor undergoes alpha-sialylation in CH2Cl2 to give alpha(2,8)- and alpha(2,9)-disialosides in excellent yields. The 5-N,4-O-carbonyl protecting group was effective in improving the reactivity of the C8 hydroxyl groups toward glycosylation. Using the sialyl building block, the synthesis of tetra-alpha(2,8)-sialic acid was accomplished by using a simple glycosidation and deprotection protocol.

Novel glycosylation reactions using glycosyl thioimidates of N-acetylneuraminic acid as sialyl donors

Novel sialyl donors 4 bearing a thioimidolyl moiety as the leaving group were successfully prepared from the corresponding arylthio derivatives 3 and a peracetylated chloro derivative of Neu5Ac 2 in the presence of N,N-di-isopropylethylamine with moderate yields. The reaction of 4 with various alcohols 5 was effectively activated by AgOTf as the promoter to give the corresponding O-sialosides 6 with good yields. Selective activation of 4a over 4-pentenyl 2-glycoside of Neu5Ac 7 with AgOTf was also achieved.

Synthetic Study of a(2,8) Oligosialoside Using N-Troc Sialyl N-Phenyltrifluoroimidate

An effective approach for the synthesis of oligo-a(2,8) sialosides using N-Troc sialyl donors is described. Glycosylation of N-Troc sialoside with N-Troc sialyl N-phenyltrifluoroimidate and phosphites smoothly proceeded to provide a(2,8) disialosides in good yield and selectivity.

N-Trifluoroacetyl Sialyl Phosphite Donors for the Synthesis of α(2 → 9) Oligosialic Acids

[reaction: see text] A new method for the synthesis of alpha(2 --> 9) oligosialic acids is developed using phosphite sialyl donors that are protected with a C-5 N-trifluoroacetyl (NHTFA) substituent. Compared with conventional donors, these donors gave a higher degree of alpha-anomeric selectivity during glycosidic bond formation and better yields during iterative sialylation in the synthesis of oligosialic acids.

Thiomidoyl Approach to the Synthesis of α‐Sialosides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

One‐Pot Synthesis of Sialo‐Containing Glycosyl Amino Acids by Use of an N‐Trichloroethoxycarbonyl‐β‐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.