A novel, reductive ring-opening of carbohydrate benzylidene acetals

Purification of subunit B of Shiga toxin using a synthetic trisaccharide-based affinity matrix

The blood group P1 antigenic trisaccharide (3), which is the receptor-binding ligand of Shiga-like toxins, is synthesized in a spacer-equipped form (32) from 2-(trimethylsilyl)ethyl glucoside 5 and the 1-thiogalactoside building blocks 10 and 22 in a stereocontrolled, stepwise fashion. Covalent attachment of 32 to hydrazine group-containing agarose gel by reductive amination provided the P1 trisaccharide-containing affinity sorbent which was used for preparative scale isolation of subunit B of Shiga toxin.

Syntheses of deoxy analogues of methyl 3,6-di-O-alpha-D-mannopyranosyl-alpha-D-mannopyranoside for studies of the binding site of concanavalin A

All of the monodeoxy analogues of methyl 3,6-di-O-alpha-D-mannopyranosyl-alpha-D-mannopyranoside have been synthesized together with two dideoxy (2,3'- and 4,3'-) analogues. The 2'- and 2"-deoxy functions were introduced by NIS-promoted couplings with 2,3,4-tri-O-acetyl-D-glucal as donor, followed by reduction of the resulting 2'- and 2"-iodo derivatives, whereas the 3'-, 3"-, 4'-, 4"-, 6'-, and 6"-deoxy functions were introduced by using known deoxyglycosyl chlorides as donors in coupling reactions promoted by silver trifluoromethanesulfonate. The 2- and 4-deoxy functions on the central mannose residue were introduced by displacement, using triiodoimidazole and triphenylphosphine, of a hydroxyl group by iodine on suitably protected derivatives followed by reduction of the resulting iodo analogues.

Synthesis of the tetrasaccharide repeating unit of the antigen from Klebsiella type 2

The disaccharide ethyl 2,4,6-tri-O-acetyl-3-O-(2,3,4,6-tetra-O-benzyl-alpha- D-glucopyranosyl)-1-thio-beta-D-glucopyranoside (6) and methyl 2,6-di-O-benzyl-3-O-(methyl 2,3,4-tri-O-benzyl-alpha-D- glucopyranosyluronate)-beta-D-mannopyranoside (21) have been synthesized and condensed in the presence of methyl triflate to afford a tetrasaccharide derivative. Removal of protecting groups gave methyl 3-O-(methyl alpha-D-glucopyranosyluronate)-4-O-(3-O-alpha-D- glucopyranosyl-beta-D-glucopyranosyl)-beta-D-mannopyranoside (23), the repeating unit of the antigen from Klebsiella type 2, in the form of its methyl ester methyl glycoside.

Analogues of disaccharides and glycosides containing a cyclic guanidinium structure show varying inhibitory effects on glycoside hydrolases

By condensation of 1,3-diamino-2,4-(R)-O-benzylidene-1,3-dideoxy-D-erythritol (3) and 1,3-diamino-2,4-di-O-benzyl-1,3-dideoxy-D-threitol (4) with methyl 2,3,6-tri-O-benzyl-4-deoxy-4-iso-thiocyanato-beta-D-glucopyranosid e (9) the (1-->4)-linked disaccharide analogues 4-deoxy-4-[(4R,5S)-5-hydroxy-4-(hydroxymethyl)-1,4,5,6-tetrahydropyri midin-2- yl[amino-alpha,beta-D-glucopyranose hydrochloride (15) and 4-deoxy-4-[(4R,5R)-5-hydroxy-4-(hydroxymethyl)-1,4,5,6-tetrahydropyri midin- 2-yl]amino-alpha,beta-D-glucopyranose hydrochloride (18) were synthesized. By the same reaction sequence, using 3 and methyl isothiocyanate, the glycoside analogue (4R,5S)-5-hydroxy-4-(hydroxymethyl)-2-methylamino-1,4,5,6- tetrahydropyrimidine hydrochloride (20) was obtained. All compounds possess in their 'glyconic' moiety the flat guanidinium group, mimicking a glucopyranosyl cation. Together with the previously synthesized (1-->6)-linked disaccharide analogues 6-deoxy-6-[(4R,5S)-5-hydroxy-4-(hydroxymethyl)-1,4,5,6- tetrahydropyrimidin-2-yl]amino-alpha,beta-D-glucopyranose hydrochloride (1) and 6-deoxy-6-[(4R,5R)-5-hydroxy-4-(hydroxy-methyl)-1,4,5,6- tetrahydropyrimidin-2-yl]amino-alpha,beta-D-glucopyranose hydrochloride (2), a possible inhibitory effect on the action of alpha-D-glucosidase, beta-D-glucosidase, alpha-D-galactosidase, and beta-D-galactosidase was investigated. All compounds, except 20 with alpha-D-glucosidase where no inhibition could be detected, showed either competitive or mixed competitive inhibition with all enzymes. The effects of the disaccharide analogues were generally weaker as compared to the effect of the previously synthesized configurationally related nitrophenyl glycoside analogues (4R,5S)-5-hydroxy-4-(hydroxymethyl)-2-(p-nitrophenyl)amino-1,4,5,6- tetrahydropyrimidine hydrochloride (21) and (4R,5R)-5-hydroxy-4-(hydroxymethyl)-2-(p-nitrophenyl)amino-1,4,5,6- tetrahydropyrimidine hydrochloride (22). On the basis of experimental results, different binding hydrochloride (22). On the basis of experimental results, different binding modes of competitive inhibitors to the active site of corresponding enzymes are discussed.

Synthesis of sialyl Lewis X ganglioside analogues containing modified L-fucose residues

Sialyl Le(x) ganglioside analogues containing 2-epi-, 2,3-di-epi-, 4-epi-, and 2-O-methyl-L-fucose in place of the L-fucose residue have been synthesized. Glycosylation of 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-beta-D-glucopyranosyl)-(1-->3)- 2,4,6- tri-O-benzyl-beta-D-galactopyranoside with the methyl 1-thioglycoside derivatives of the respective fucose analogues, using dimethyl(methylthio)sulfonium triflate (DMTST) as a promoter, gave the corresponding protected 2-(trimethylsilyl)ethyl deoxy-alpha-L-hexopyranosyl-(1-->3)-O-(2- acetamido-2-deoxy-beta-D-glucopyranosyl)-(1-->3)-beta-D-galactopyrano sid es. These were transformed by reductive ring-opening of their benzylidene acetal groups into the glycosyl acceptors. Dimethyl (methylthio)sulfonium triflate-promoted glycosylation of these compounds with methyl O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D-galacto -2- nonulopyranosylonate)-(2-->3)-2,4,6-tri-O-benzoyl-1-thio-beta-D- galactopyranoside afforded the desired pentasaccharides, which were converted via reductive removal of their benzyl groups, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, and reaction with trichloroacetonitrile, into the corresponding alpha-tri-chloroacetimidates. Glycosylation of (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol these 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 deesterification, into the target gangliosides. The 2-(trimethylsilyl)ethyl glycosides of sialyl Le(x) oligosaccharides containing modified fucose were also prepared from the intermediates of the ganglioside synthesis.

Chemical and enzymatic synthesis of glycopeptides

Progress recently made in the synthesis of biologically relevant N- and O-glycopeptides is illustrated by examples. In this context, developments in the preparation of complex saccharide side chains and in the subsequent coupling to peptide portions is described. Special emphasis is given to the synthesis of Lewis antigen-type structures. Furthermore, modern methods in solid phase peptide syntheses utilizing glycosylated building blocks are presented. Recent advances in glycopeptide syntheses employing enzymatic methods in deprotection steps as well as in peptide/saccharide chain elongation are reported.

Synthetic studies toward pyruvate acetal-containing saccharides: en route to the efficient synthesis of Rhizobium-related exopolysaccharide fragments

The disaccharide building block benzyl O-(2,3-di-O-benzoyl-4,6-O-[(R)-1-(methoxycarbonyl) ethylidene]-beta-D-galactopyranosyl)-(1-->3)-2-O-benzoyl-4,6-O-[(S)-1- (methoxycarbonyl)ethylidene]-alpha-D-glucopyranoside (13), related to a Rhizobium exopolysaccharide, was prepared by coupling various 4,6-O-[(R)-1-(methoxycarbonyl)ethylidene]-D-galactosyl donors (benzoyl-protected chloride 1, pivaloyl-protected chloride 2, and benzoyl-protected fluorides 3 and 4, and trichloroacetimidate 5) with benzyl 2-O-benzoyl-4,6-O-[(S)-1- (methoxycarbonyl)ethylidene]-alpha-D-glucopyranoside (10) and the corresponding 2,3-O-tetraisopropyldisiloxane-protected glucoside 12. The best results, with respect to beta-selectivity and yield of the coupling, were obtained with 5 and 10 in dichloromethane. The beta-linked (13) and alpha-linked (14) disaccharides were efficiently converted via the 1-OH derivatives 17 and 21 into the corresponding trichloroacetimidates 18 and 22. The latter were used for the synthesis of the disaccharide ligands 4,6-(R)-pyruvate-beta-D-Galp-(1-->3)-4,6-(S)-pyruvate-beta-D-Glcp-O(CH2) 5NH2 (20), and 4,6-(R)-pyruvate-alpha-D-Galp-(1-->3)-4,6-(S)-pyruvate-beta-D-Glcp-O (CH2)5NH2 (24). The corresponding tri- and tetra-saccharide derivatives 4,6-(R)-pyruvate-beta-D-Galp-(1-->3)-4,6-(S)-pyruvate-beta-D-Glcp-(1-->4 )-beta- D-Glcp-O(CH2)5NH2 (28) and 4,6-(R)-pyruvate-beta-D-Galp-(1-->3)-4,6-(S)-pyruvate-beta-D-Glcp-(1-->4 )-beta- D-Glcp-(1-->4)-beta-D-Glcp-O(CH2)25NH2 (36) were obtained similarly.

Synthesis of glycopeptides and neoglycoproteins containing the fucosylated linkage region of N-glycoproteins

N-Glycoproteins fucosylated in the core region occur in tumor membranes and virus envelopes. Partial structures of such N-glycoproteins containing fucosylated chitobiosyl asparagine conjugates were synthesized using the allyloxycarbonyl (Aloc) and the tert-butyl ester protecting groups in the peptide portion. As the alpha-fucosidic bond of the conjugates revealed to be very sensitive to acids when carrying ether-type protecting groups, a method for exchanging the protecting groups of the fucose portion of saccharides was developed. Conjugates containing O-acetyl protected fucose proved to be stable against acids used in glycopeptide syntheses. These methods were applied in the synthesis of a fucosyl chitobiose hexapeptide with the partial sequence of a leukemia virus envelope glycoprotein. The glycopeptide was coupled to bovine serum albumin yielding a neoglycoprotein which contains a glycoconjugate of exactly specified structure.

First synthesis of the 3'-sulfated Lewis(a) pentasaccharide, the most potent human E-selectin ligand so far

Tri- and pentasaccharides of Lewis(a)-type, sulfated at position 3 of the outer galactose, have been prepared using the new 4-methoxybenzyl glycoside of N-acetylglucosamine 5 as starting material. The synthesis of the pentasaccharide 2 was achieved through a beta-stereoselective coupling of an alpha-trichloroacetimidate activated form of the N-acetamido protected trisaccharide 18 on to a 3',4'-unprotected lactose derivative.

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.

The recognition of three different epitopes for the H-type 2 human blood group determinant by lectins of Ulex europaeus, Galactia tenuiflora and Psophocarpus tetragonolobus (winged bean)

The chemical mapping of the regions of H-type 2 human blood group-related trisaccharide (Fuc alpha (1-2)Gal beta (1-4)GlcNAc beta Me) that are recognized by three different lectins, the so-called epitopes, are reviewed together with an account of how and why oligosaccharides form specific complexes with proteins as presently viewed in this laboratory. The occasion is used to report the synthesis of the various mono-O-methyl derivatives of the above trisaccharide that were used in these investigations. Also, Fuc alpha (1-2)Gal beta (1-4)Xyl beta Me was synthesized in order to examine whether or not the hydroxymethyl group of the GlcNAc residue participates in the binding reaction.

Synthesis of the 5-aminopentyl glycoside of beta-D-Gal p-(1-->4)-beta-D-Glc p NAc-(1-->3)-L-Fuc p and fragments thereof related to glycopeptides of human Christmas factor and the marine sponge Microciona prolifera

The marine sponge Microciona prolifera and human coagulation factor IX (Christmas factor)-related mono- to tri-saccharide 5-aminopentyl glycosides beta-D-Gal p-R (5), beta-D-Glc pNAc-R (16), beta-D-Gal p-(1-->4)-beta-D-Glc p NAc-R (26), beta-D-Glc p NAc-(1-->3)-beta-L-Fuc p-R (39), beta-D-Glc pNAc-(1-->3)-alpha-L-Fuc p-R (43), beta-D-Gal p-(1-->4)-beta-D- Glc pNAc-(1-->3)-beta-L-Fuc p-R (45), and beta-D-Gal p-(1-->4)-beta-D-Glc p NAc-(1-->3)-alpha-L-Fuc p-R (47), where R is a 5-aminopentyloxy spacer moiety, which allowed the construction of glycoconjugates, were prepared. Thus, 3,4,6-tri-O-acetyl-2-deoxy-2-(2,2,2- trichloroethoxycarbonyl-amino)-alpha-D-glucopyranosyl trichloroacetimidate (10) and 1,3,4,6-tetra-O-acetyl-2-chloro-acetamido-2- deoxy-beta-D-glucopyranose (13) were condensed with N-Z-protected 5-amino-pentanol (2) followed by conversion of the coupling products into the corresponding N-acetylglucosamine derivatives, to give compound 16 after deblocking. Similarly, the donors 10 and 13 were coupled to position 3 of suitably protected aminopentyl beta- (32) and alpha- (37) -L-fucopyranosides, to give the disaccharides 39 and 43, respectively. Starting from lactose, O-(2,3,4,6-tetra-O-benzoyl-beta-D-galactopyranosyl)-(1-->4)-3,6-di-O- benzoyl-2-deoxy-2-(2,2,2-trichloroethoxycarbonylamino)-alpha-D-glu copyranosyl trichloroacetimidate (23) was prepared and used as an efficient disaccharide donor for the construction of ligand 26 from 2 and of the trisaccharide ligands 45 and 47 from fucosides 32 and 37, respectively.

Synthesis of double-chain bis-sulfone neoglycolipids of the 2''-, 3''-, 4''-, and 6''-deoxyglobotrioses

2-(Trimethylsilyl)ethyl (Me3SiCH2CH2) 2,3,6-tri-O-benzoyl-4-O-(2,3-di-O-benzoyl-6-O-p-methoxybenzyl-beta-D- galactopyranosyl-beta-D-glucopyranoside was glycosylated with different 2-, 3-, 4-, or 6-"deoxy-D-galactose" derivatives to give the corresponding deoxytrisaccharides. Removal of the protecting groups gave the Me3SiCH2CH2 2''-, 3''-, 4''-, and 6''-deoxyglobotriosides. Transformation of the protected Me3SiCH2CH2 globotriosides into the corresponding trichloroacetimidates proceeded, via the hemiacetals, in 91-96% over-all yield. Glycosylation of 3-(hexadecylsulfonyl-2-[hexadecylsulfonyl)methyl]propanol with the trichloroacetimidates, followed by removal of protecting groups, gave the title neoglycolipids.

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 deoxy-L-fucose-containing sialyl Lewis X ganglioside analogues

Sialyl Lex ganglioside analogs containing 2-, 3-, and 4-deoxyfucose in the place of L-fucose have been synthesized. Glycosylation of 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-beta-D-glucopyranosyl)-(1-->3)- 2,4,6-triO-benzyl-beta-D-galactopyranoside with the methyl 1-thioglycoside derivatives of the respective deoxyfucoses, using dimethyl(methylthio)sulfonium triflate (DMTST) as a promoter, gave the corresponding three protected 2-(trimethylsilyl)ethyl dideoxy-alpha-L-heoxpyranosyl-(1-->3)- O-2(acetamido-2-deoxy-beta-glucopyranosyl)-(1-->3)-beta-D-galactop yranosides. These were transformed by reductive ring-opening of their benzylidene acetal groups into the glycosyl acceptors 6, 8, and 10. Dimethyl(methylthio)sulfonium triflate promoted glycosylation of 6, 8, and 10 with methyl O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha- D-galacto-2-nonulopyranosylonate)-(2-->3)-2,4,6-tri-O-benzoyl-1-th io-beta-D-galactopyranoside afforded the desired pentasaccharides, which were converted via reductive removal of their benzyl groups, O-acetylation, selective removal of the 2-(trimethylsilyl)ethyl group, and reaction with trichloroacetonitrile, into the corresponding alpha-trichloroacetimidates 14, 18, and 22. Glycosylation of (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol with 14, 18, and 22 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 deesterification, into the target compounds.

Syntheses of all the possible monomethyl ethers and several deoxyhalo analogues of methyl beta-lactoside as ligands for the Ricinus communis lectins

The synthesis of all the possible monomethyl ethers of methyl beta-lactoside (1) has been performed from 1 in a straightforward way, making use of the different reactivity of the hydroxyl groups in alkylation and stannylation reactions. In addition, the deoxyfluoro derivatives of 1 at positions, 6,3',4',epi-4', and 6' have been prepared by reaction of the appropriate substrates with diethylaminosulfur trifluoride or tetrabutylammonium fluoride. Finally, the 6-deoxyiodo and 6'-bromodeoxy analogues of 1 have also been prepared.

A new method of anomeric protection and activation based on the conversion of glycosyl azides into glycosyl fluorides

Glycosyl azides provide reliable anomeric protection stable to conditions for hydrolytic removal of ester groups, for reductive opening or release of acetalic diol protection, for the introduction of ether-type protection, and for glycosylation processes. The utility of this anomeric protection is further enhanced as glycosyl azides may be converted into glycosyl fluorides, which can be activated for glycosylation reactions. To this end, glycosyl azides have been subjected to 1,3-dipolar cycloaddition with di-tert-butyl acetylenedicarboxylate. On treatment with hydrogen fluoride-pyridine complex the N-glycosyl triazole derivatives directly give glycosyl fluorides.

Synthesis of the monodeoxy derivatives of 2-(trimethylsilyl)ethyl beta-lactoside

Monodeoxy derivatives of 2-(trimethylsilyl)ethyl (Me3SiEt) beta-lactoside were synthesized, by deoxygenation at the disaccharide level, for the 2'-, 3'-, 4'-, and 6'-monodeoxylactosides. The 2-, 3-, and 6-deoxy derivatives were synthesized by beta-D-galactosylation of suitably protected monodeoxygenated Me3SiEt glucosides. Silver silicate was shown to be an efficient glycosylation promoter in the preparation of the 2- and 3-deoxylactosides.