A facile, large-scale preparation of the methyl 2-thioglycoside of N-acetylneuraminic acid, and its usefulness for the alpha-stereoselective synthesis of sialoglycosides

An efficient and concise regioselective synthesis of alpha-(1 --> 5)-linked L-arabinofuranosyl oligosaccharides

A series of alpha-(1 --> 5)-linked L-arabinofuranosyl di-, tetra-, hexa- and octameric derivatives were synthesized efficiently. The process was carried out in a regio- and stereoselective manner using perbenzoylated arabinofuranosyl trichloroacetimidates as glycosyl donors and unprotected or partially protected arabinofuranosides as glycosyl acceptors in the presence of a catalytic amount of trimethylsilyl trifluoromethanesulfonate (TMSOTf).

Continuous production of sialyllactose from colominic acid using a membrane reactor

Continuous production of sialyllactose, as a typical sialylsaccharide, was examined using a membrane reactor. The synthesis of sialyllactose through the transfer reaction catalyzed by neuraminidase, has been reported to be a suitable process for industrial production, but it still has drawbacks such as a low yield, high enzyme cost, and hydrolysis by the enzyme of sialyllactose formed. We attempted to solve these problems by utilizing an appropriate membrane reactor system. We first investigated the effects of various reaction conditions on sialyllactose productivity, and found that the productivity was independent of the enzyme concentration and reaction temperature but dependent on the substrate and buffer concentrations and the hydraulic retention time (HRT). We then selected a suitable membrane that allowed sialyllactose to permeate but rejected the substrate and enzyme. Finally, we constructed a membrane reactor system with a cut-off molecular weight of 3000 and applied it to continuous sialyllactose production from colominic acid at an HRT of 80 min. Using substrate concentration of 25 g/l the system performed with a high level of productivity and gave a good yield, while maintaining high transfer ratio of 4-5% over a 160-h test period.

Total synthesis of a cholinergic neuron-specific ganglioside GT1a alpha: a high affinity ligand for myelin-associated glycoprotein (MAG)

An efficient total synthesis of a cholinergic neuron-specific ganglioside GT1a alpha (IV3NeuAcIII6NeuAcII3NeuAc-GgOse4Cer) is described. The suitably protected sialyl-alpha(2-->6)-gangliotriose (III6NeuAc-GgOse3) derivative was glycosylated with the phenyl 2-thioglycoside of sialic acid in the presence of N-iodosuccinimide (NIS) and trimethylsilyl trifluoromethane-sulfonate (TMSOTf) in acetonitrile medium, giving the disialogangliotriose (III6NeuAcII3NeuAc-GgOse3) derivative which contains both sialyl-alpha(2-->6)-GalNAc and sialyl-alpha(2-->3)-Gal structures (Route I). This pentasaccharide was efficiently synthesized also by the coupling of (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D-galacto -2-nonulopyranosylonate)-(2-->6)-2-deoxy-3,4-O-isopropylidene-2-ph thalimido-D-galactopyranosyl trichloroacetimidate with 2-(trimethylsilyl)ethyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D-galacto -2-nonulopyranosylonate)-(2-->3)-(2,6-di-O-benzyl-beta-D-galactopy ranosyl)-(1-->4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside, followed by conversion of the phthalimido group to the acetamido group (Route II). O-Deisopropylidenation and further glycosylation with methyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D-gala ct-2-nonulopyranosylonate)-(2-->3)-2,4,6-tri-O-benzoyl-1-thio-b eta-D-galactopyranoside, promoted by dimethyl(methylthio)sulfonium triflate (DMTST), gave the desired trisialogangliotetraose (IV3NeuAcIII6NeuAcII3NeuAc-GgOse4) derivative, which was converted stepwise into the title ganglioside GT1a alpha by the introduction of the ceramide part and then complete deprotection. The ganglioside obtained was shown to be identical with the native GT1a alpha on TLC-immunostaining.

Lipase-catalyzed acetylation of N-acetylneuraminic acid derivative

A facile preparation of triacetylated derivative of 2-phenylthioglycoside of N-acetylneuraminic acid (4) was achieved by treatment with lipase PS in vinyl acetate. The major product 4 has a free hydroxyl group at C-7. Results of time-course HPLC analysis indicate that the reactivity of the hydroxyl groups under this condition is in the following order; C-9 > C-4 > C-8 > C-7.

Design and synthesis of a water-soluble taxol analogue: taxol-sialyl conjugate

Glycosidation, using the methylthio derivative of N-acetylneuraminic acid 3, of linker alcohol 4 in DME with "long-range participation" produced the alpha-glycosyl linkage with high stereoselectivity. The alpha-linked sialic acid 2 was introduced in taxol without protection of the alcohol functionality in sialic acid.

First total synthesis of ganglioside GT1a alpha

A first total synthesis of ganglioside GT1a alpha (IV3III6II3)Neu5AcGg4Cer) is described. The suitably protected sialyl alpha-(2-->6) ganglioside was glycosylated with the phenylthioglycoside of sialic acid in the presence of N-iodosuccinimide (NIS)-trimethylsilyl trifluoromethanesulfonate (TMSOTf), followed by further glycosylation with the methyl thioglycoside promoted by dimethyl(methylthio)sulfonium triflate (DMTST), to give the heptasaccharide. The oligosaccharide obtained was converted into the title ganglioside by the introduction of ceramide and then complete deprotection.

Studies on alpha-sialylation using sialyl donors with an auxiliary 3-thiophenyl group

Reaction of the methyl ester of 2-chloro-3-S-phenyl-3-thiosialic acid (4) with sodium thiomethoxide in acetonitrile at 0 degrees C affords the methyl ester of 2-S-methyl-3-S-phenyl-2,3-di-thio-alpha-sialic acid (6a) in quantitative yield. Sialylation of tetrahydropyran-2-methanol (7) and 2-(trimethylsilyl)ethyl 2,2'3,6,6'-penta-O-benzyl-alpha-lactoside (8) with 6a in the presence of phenylsulfenyl triflate (PST) as promotor in CH3 CN at -40 degrees C gives alpha-sialosides 9 and 10 in good yield and excellent stereoselectivity. No beta-sialosides are formed in either case. Acetylation of product 10, and the subsequent reductive removal of the 3-thiophenyl group using Ph3SnH, affords 12--protected GM3 trisaccharide--in 82% yield after two steps. Sialylation of acceptor 8 with chloride 4 using silver triflate as promotor afforded 10 in 48% yield after two days at -15 degrees C in THF. A possible mechanism of sialylation with 6a that involves intermediate alpha- and beta-nitrilium ions is discussed.

Synthesis of the Sda determinant and two analogous tetrasaccharides

To contribute to the possibility of studying in greater detail the biological significance of Sda-containing glycans as occur in Tamm-Horsfall glycoprotein, the following three spacer-linked tetrasaccharides have been synthesized: the Sda determinant alpha-Neu p5Ac-(2-->3)-[beta- D-GalpNAc-(1-->4)]-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)(CH2)5NH 2 (1), the Gal-analogue alpha-Neup5Ac-(2-->3)-[beta-D-Galp-(1-->4)]-beta-D-Galp-(1-->4) -beta-D-GlcpNAc-(1-->O)(CH2)5NH2 (2), and the GlcNAc-analogue alpha-Neup5Ac-(2-->3)-[beta-D-GlcpNAc-(1-->4)]- beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)(CH2)5NH2 (3). The general trisaccharide acceptor 5-azidopentyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy- D-glycero-alpha-D-galactonon-2-ulopyranosylonate)-(2-->3)-(2 ,6-di-O-benzyl- beta-D-galactopyranosyl)-(1-->4)-3,6-di-O- benzyl-2-deoxy-2-phthalimido-beta-D-glucopyranoside was prepared, using methyl (phenyl 5- acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto- non- 2-ulopyranosid)- onate as the sialyl donor. For the syntheses of 1, 2, and 3 the glycosyl donors 3,4,6-tri-O- acetyl-2-deoxy-2-phthalimido-alpha-D-galactopyranosyl bromide, 2,3,4,6-tetra-O-acetyl-alpha-D- galactopyranosyl bromide, and 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido -beta-D-glucopyranosyl trichloroacetimidate, respectively, proved to be the most suitable.

Synthesis and biological activities of three sulfated sialyl Le(x) ganglioside analogues for clarifying the real carbohydrate ligand structure of L-selectin

Sulfated sialyl Le(x) ganglioside analogues at C-6 of D-galactose, N-acetyl-D-glucosamine, and of both D-galactose and N-acetyl-D-glucosamine residues have been synthesized, in order to clarify the structure of the real carbohydrate ligand of L-selectin. Coupling of the suitably protected N-acetyl-D-glucosaminyl-beta (1-->3)-lactose derivatives 13 and 16 with the sialyl alpha(2-->3)-D-galactopyranosyl trichloroacetimidates 10 and 12 (glycosyl donors), via glycosylation of 2-(trimethylsilyl)ethyl 4,6-O-benzylidene-beta-D-galactopyranoside (1) with the phenyl 2-thioglycoside derivative (2) of N-acetylneuraminic acid (Neu5Ac) using N-iodosuccinimide/TfOH, O-benzoylation, removal of the benzylidene group affording 5, selective 6-O-levulinoylation, O-benzoylation, removal of the 2-(trimethylsilyl)ethyl group, and imidate formation, or via O-acetylation of 5, removal of the 2-trimethylsilyl)ethyl group, then imidate formation, gave the pentasaccharides 18-20. The glycosylation of the pentasaccharide acceptors (21-23) derived from 18-20 by removal of the 4-methoxybenzyl group, with phenyl 1-thioglycoside derivative 27 of L-fucose using dimethyl(methylthio)sulfonium triflate (DMTST) afforded the corresponding hexasaccharides 28-30, which were transformed in good yields, via reductive removal of their benzyl groups, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, imidate formation, coupling with (2S,3R,4E)-2-azido-O-benzoyl-4-octadecene-1,3-diol (35) in the presence of boron trifluoride etherate, selective reduction of the azido group, coupling with octadecanoic acid, selective removal of the levulinoyl groups, treatment with sulfur trioxide-pyridine complex, then removal of the protecting groups, into the desired sulfated sialyl Le(x) ganglioside analogues 50-52.

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 deoxygalactose-containing sialyl Le(X) ganglioside analogues to elucidate the structure necessary for selectin recognition

Sialyl Lewis X ganglioside analogues containing 4-deoxy-, 6-deoxy-, and 4,6-dideoxy-D-galactopyranose in place of D-galactopyranose have been synthesized. Glycosylations of 2-(trimethylsilyl)ethyl 2,6-di-O-benzyl-beta-D-galactopyranoside and 2-(trimethylsilyl)ethyl beta-D-fucopyranoside with the phenyl 2-thioglycoside derivative of sialic acid, using N-iodosuccinimide (NIS)-trifluoromethanesulfonic acid (TfOH) as the promoter in acetonitrile, gave the desired 2-(trimethylsilyl)ethyl sialyl-alpha-2-->3)-beta-D-galactopyranoside and -beta-D-fucopyranoside, respectively. The sialylgalactose derivative obtained was then modified to 4-deoxy and 4,6-dideoxy derivatives. These were converted, by O-benzoylation, transformation of the 2-(trimethylsilyl)ethyl group to trichloroacetimidates, and introduction of the methylthio group with methylthiomethylsilane, into the corresponding glycosyl donors, which were then coupled with 2-(trimethylsilyl)ethyl O-(2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl)-(1->3)-O-(2-acetamido-6-O- benzyl-2-deoxy-beta-D-glucopyranosyl)-(1->3)-2,4,6-tri-O-benzyl-beta-D- galactopyranoside in the presence of dimethyl(methylthio)sulfonium triflate (DMTST). The resulting pentasaccharides were each converted to the corresponding alpha-trichloroacetimidates, which, on coupling with (2S, 3R, 4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol, gave the desired sphingosine derivatives. Selective reduction of the azide group, N-acylation with octadecanoic acid, O-deacylation, and saponification of the methyl ester afforded the target compounds.

Synthesis of a pentasaccharide fragment of Polysaccharide II of Mycobacterium tuberculosis

Stereocontrolled, stepwise synthesis of decyl glycosides of alpha-(1-->2)-linked di- to pentaglucosides (1-5) is described; these constitute fragments of Polysaccharide II of Mycobacterium tuberculosis. Phenyl 3,4,6-tri-O-acetyl-2-O-benzyl-1-thio-alpha-D- glucopyranoside (7) was used as the single key intermediate, obtained from 1,3,4,6-tetra-O-acetyl-2-O-benzyl-beta-D-glucopyranose (6) and PhSSiMe3. Halogenolysis of 7 afforded the isolated beta bromide (10) and beta chloride (13). Solvolysis of 10 with decanol without heavy metal salts gave decyl 3,4,6-tri-O-acetyl-2-O-benzyl-alpha-D-glucopyranoside (14) in a highly stereoselective reaction, in high yield. Subsequent, iterative hydrogenolytic removal of the O-benzyl group and glycosylation with the beta-chloride 13 under catalysis by silver salts afforded the protected di- to penta-saccharide glycosides 16, 19, 21, and 23, which were conventionally deblocked.

Synthetic and structural studies of alpha-sialyl-(2-->6) and alpha-sialyl-(2-->3) 1-deoxynojirimycin derivatives potentially useful for biomedical applications

Suitably protected derivatives of 1-deoxynojirimycin (1,5-dideoxy-1,5-imino-D-glucitol, DNJ) and its D-galacto analog were coupled with 2-thioglycosides of N-acetylneuraminic acid. The resulting disaccharides were converted into a variety of alpha-sialyl-(2-->6)-and alpha-sialyl-(2-->3)-DNJ derivatives, including the cyclic lactams 6-O-(5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyrano sylono- 1',5-lactam)-1,5-dideoxy-1,5-imino-D-glucitol and -D-galactitol. The structural features of the synthetic compounds were investigated by ion-spray mass and 1H NMR spectrometry. The 1C4 conformation of N-tert-butoxycarbonyl-DNJ, a synthetic intermediate having the gluco configuration, was confirmed by X-ray crystallography.

Systematic synthesis of N-methyl-1-deoxynojirimycin-containing, Le(x), Le(a), sialyl-Le(x) and sialyl-Le(a) epitopes recognized by selectins

A systematic synthesis of the N-methyl-1-deoxynojirimycin-containing oligosaccharides related to the Lewis x, Lewis a, sialyl-Lewis x and sialyl-Lewis a antigens has been achieved. The couplings of the suitably protected 1-deoxynojirimycin derivative 10 with methyl-1-thioglycosides (glycosyl donors) of L-fucose (11), D-galactose (15) and alpha-sialyl-(2-->3)-D-galactose (27) were carried out by using dimethyl(methylthio)sulfonium triflate (DMTST) or N-iodosuccinimide/trifluoromethanesulfonic acid (NIS/TfOH) as the glycosyl promoter. The resulting di- and tri-saccharides were each converted, by further cross glycosylations with 11, 15 or 27, to the desired tri- and tetra-saccharides 3-6 that inhibit the recognition between sialyl-Lewis x and selectins, a family of leukocyte cell adhesion molecules.

Synthesis of KDN-lactotetraosylceramide, KDN-neolactotetraosylceramide, and KDN-Lewis X ganglioside

Analogues of sialyl-lactotetraosylceramide, sialyl-neolactotetraosylceramide, and sialyl Lewis X ganglioside, in which the N-acetylneuraminic acid residue is replaced by a 3-deoxy-D-glycero-D-galacto-2-nonulopyranosylonic acid (KDN) unit, have been synthesized. Methyl O-(methyl 4,5,7,8,9-penta-O-acetyl-3-deoxy-D-glycero-alpha-D-galacto-2- nonulopyranosylonate)-(2-->3)-2,4,6-tri-O-benzoyl-1-thio-beta-D- galactopyranoside (4) was prepared from 2-(trimethylsilyl)ethyl O-(methyl 4,5,7,8,9-penta-O-acetyl-3-deoxy-D-glycero-alpha-D-galacto-2- nonulopyranosylonate)-(2-->3)-6-O-benzoyl-beta-D-galactopyranoside , via O-benzoylation, replacement of the 2-(trimethylsilyl)ethyl group by acetyl, and introduction of the methylthio group with trimethyl(methylthio)silane. Glycosylation of 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-beta-D-glucopyranosyl)-(1-->3)- O- (2,4,6-tri-O-benzyl-beta-D-galactopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl- beta- D-glucopyranoside (5) or of 2-(trimethylsilyl)ethyl O-(2-acetamido-3,6-di-O-benzyl-2-deoxy-beta-D-glucopyranosyl)-(1-->3)-O- (2,4,6-tri-O-benzyl-beta-D-galactopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl- beta- D-glucopyranoside, prepared from 5 via O-benzylation and reductive opening of the benzylidene acetal ring, with 4 as a donor gave the corresponding pentasaccharides 9 and 13 in good yields. In the same way, 4 was reacted with 2-(trimethylsilyl)ethyl O-(2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl)-(1-->3)-O-(2-acetamido-6- O-benzyl-2-deoxy-beta-D-glucopyranosyl)-(1-->3)-O-(2,4,6-tri-O-benzyl-be ta- D-galactopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside to yield the hexasaccharide 17. These three oligosaccharides 9, 13, and 17 were converted via reductive removal of the benzyl groups and benzylidene group, O-acetylation, removal of the 2-(trimethylsilyl)ethyl group, and subsequent reaction with trichloroacetonitrile, into the corresponding trichloroacetimidates 12, 16, and 20, respectively. Glycosylation of (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol with 12, 16, and 20 in the presence of boron trifluoride etherate afforded the expected beta-glycosides, which were transformed via selective reduction of the azido group, coupling with octadecanoic acid, O-deacylation, and de-esterification, into the target gangliosides in high yields.

Synthesis of the methyl thioglycosides of 2-, 3-, and 4-deoxy-L-fucose

Methyl thioglycoside derivatives of 2-, 3-, and 4-deoxy-L-fucopyranose have been prepared as glycosyl donors for the synthesis of sialyl Le(x) ganglioside analogues containing modified alpha-L-fucopyranose residues. Reductive dethioacylation of 2-(trimethylsilyl)ethyl 3,4-di-O-benzoyl-2-O-(phenoxy)thiocarbonyl-beta-L-fucopyranoside, prepared from L-fucose in eight steps, gave the 2-deoxy compound, which was transformed via selective removal of the 2-(trimethylsilyl)ethyl group, subsequent acetylation, and displacement of the 1-acetoxy group by a methylthio group, into methyl 3,4-di-O-benzoyl-2,6-dideoxy-1-thio- alpha,beta-L-lyxo-hexopyranoside (11). 2-(Trimethylsilyl)ethyl 2,4-di-O-benzoyl-3-O-(phenoxy)thiocarbonyl-beta- L-fucopyranoside, prepared from the unsubstituted glycoside in four steps, and 2-(trimethylsilyl)ethyl 2,3-di-O-benzoyl-4-O-(phenoxy)thiocarbonyl-beta-L-fucopyranoside, similarly prepared in two steps, were transformed via reduction of the (phenoxy)thiocarbonyloxy group, selective removal of the 2-(trimethylsilyl)ethyl group, O-acetylation, displacement of the 1-acetoxy group by a methylthio group as described for 11, and finally replacement of the benzoyl groups by benzyl groups, into the analogous, protected methyl 3- and 4-deoxy-1-thio-beta-L-fucopyranosides.