Synthesis of the methyl thioglycosides of deoxy-N-acetyl-neuraminic acids for use as glycosyl donors

Chemoenzymatic synthesis and enzymatic analysis of 8-modified cytidine monophosphate-sialic acid and sialyl lactose derivatives

The sialic acids found on eukaryotic glycans have remarkably diverse core structures, with a range of modifications at C5, C7, C8 and C9. These carbohydrates have been found to play key roles in cell-cell interactions within eukaryotes and often serve as the initial site of attachment for viruses and bacteria. Consequently simple changes to the structures of the sialic acids can result in profoundly different and often opposing biological effects. Of particular importance are modifications at the 8-position. These include O-acetylation, carried out by an acetyl transferase, and particularly polysialylation, catalyzed by a polysialyltransferase. As part of a structural and mechanistic study of sialyltransferases and polysialyltransferases, access was needed to sialic acid-containing oligosaccharides that are modified at the 8-position of the sialic acid to render this center non-nucleophilic. The free 8-modified sialic acid analogues were synthesized using a concise, divergent chemical synthetic approach, and each was converted to its cytidine monophosphate (CMP) sugar donor form using a bacterial CMP-sialic acid synthetase. The transfer of each of the modified donors to lactose by each of two sialyltransferases was investigated, and kinetic parameters were determined. These yielded insights into the roles of interactions occurring at that position during enzymatic sialyl transfer. A transferase from Campylobacter jejuni was identified as the most suitable for the enzymatic coupling and utilized to synthesize the 8''-modified sialyl lactose trisaccharides in multimilligram amounts.

Chemoenzymatic synthesis of CMP-N-acetyl-7-fluoro-7-deoxy-neuraminic acid

7-Fluoro sialic acid was prepared and activated as cytidine monophosphate (CMP) ester. The synthesis started with d-glucose, which was efficiently converted into N-acetyl-4-fluoro-4-deoxy-d-mannosamine. Aldolase catalyzed transformation yielded the corresponding fluorinated sialic acid which was activated as CMP ester using three different synthetases in the presence as well as in the absence of pyrophosphatase which possesses inhibitory properties. Finally, conditions were optimized to perform a one-pot reaction starting from fluorinated mannosamine, which yielded the 7-fluoro-7-deoxy-CMP-sialic acid by incubation with three enzymes.

Ganglioside GD1 alpha analogues as high-affinity ligands for myelin-associated glycoprotein (MAG)

The first systematic synthesis of ganglioside GD1 alpha analogues carrying N-acetyldeoxyneuraminic acids linked to C-6 of the GalNAc residue was accomplished. The suitably protected GM1b pentasaccharide derivative was regioselectively glycosylated with the phenyl 2-thioglycosides of 7-deoxy, 8-deoxy, and 9-deoxy-N-acetylneuraminic acid promoted by N-iodosuccinimide (NIS)-trifluoromethanesulfonic acid (TfOH) in acetonitrile, and the resulting hexasaccharides were converted to the target GD1 alpha analogues. All of the analogues retained excellent efficiency in supporting the adhesion to myelin-associated glycoprotein (MAG), raising the possibility that the internal sialic acid linked to the GalNAc residue may be replaced by other anionic substituents, in contrast to the terminal sialic acid, which is essential for MAG binding.

An efficient short-step total synthesis of ganglioside GM3: effective usage of the neighbouring group participation strategy

We have developed an efficient methodology for highly stereoselective sialylation using 3-position substituted sialic acids and have prepared 2a having a 3 beta-phenylthio group as a sialic donor. Glycosylation of suitably protected lactoside 3 with 2a gave only the alpha-sialyl trisaccharide 16 in good yield. Condensation of the azidosphingosine 4 with the acetate 17 using promotors, DMTST or NIS-TfOH, afforded the glycolipid 18, which was directly transformed to 20 by reduction with Bu3P and subsequent acylation with octadecanoic acid in the presence of WSC. Removal of the protecting groups generated ganglioside GM3 (1).

Synthesis of phosphites and phosphates of neuraminic acid and their glycosyl donor properties--convenient synthesis of GM3

The importance and requirements for catalytic activation of sialyl donors are discussed, leading to the acid sensitive phosphite and phosphate moiety, respectively, as leaving group and nitriles as solvent. Therefore, from readily available N-acetylneuraminic acid, derivative 1 with phosphochloridites 2a-f and Huenigs' base sialyl phosphites 3a-f were prepared and isolated in high yields. Oxidation of 3a,c with tert-butyl-hydroperoxide afforded the corresponding phosphates 4a,c. As expected, phosphites 3 could be activated in acetonitrile by catalytic amounts of TMSOTf; thus, from 3a-e as donors and lactose derivatives 8A,B as acceptors the ganglioside building blocks 9A and 9B, respectively, were obtained in good yields. The best results were obtained with diethyl phosphite derivative 3a as sialyl donor, which exceeded by far the results obtained with the corresponding phosphate derivative 4a. Trisaccharide 9B was transformed into known 9A and into the fully O-acetylated GM3-trisaccharide 10.

Synthesis of a series of ganglioside GM3 analogs containing a deoxy-N-acetylneuraminic acid residue

Ganglioside GM3 analogs containing 4-, 7-, 8-, and 9-deoxy-N-acetylneuraminic acids in the place of N-acetylneuraminic acid (Neu5Ac) have been synthesized. Glycosylation of 2-(trimethylsilyl)ethyl O-(6-O-benzoyl-beta-D-galactopyranosyl)-(1----4)-2,6-di- O-benzoyl-beta-D-glucopyranoside with the methyl 2-thioglycoside derivatives of the respective deoxy-N-acetylneuraminic acids, using dimethyl(methylthio)sulfonium triflate as a promoter, gave the four required 2-(trimethylsilyl)ethyl alpha-sialosyl-(2----3b)-beta-lactosides. These were converted via O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, and subsequent imidate formation, into the corresponding alpha-sialosyl-(2----3b)-alpha-lactose trichloroacetimidates 15, 17, 19, and 21. Glycosylation of (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol with 15, 17, 19, and 21 in the presence of boron trifluoride etherate afforded the expected beta glycosides, which were transformed in good yields, via selective reduction of the azido group, coupling with octadecanoic acid, O-deacylation, and de-esterification, into the target compounds.