Synthesis of a model of an inner chain of cell-wall proteoheteroglycan isolated from Piricularia oryzae: Branched d-mannopentaosides

Efficient syntheses are described of the branched d-mannopentaosides methyl 2,6-di-O-(2-O-α-d-mannopyranosyl-α-d-mannopyranosyl)α-d-mannopyranoside and methyl 2,4-di-O-(2-O-α-d-mannopyranosyl-α-d-mannopyranosyl)-α-d-mannopyranoside, starting from the glycosyl acceptors methyl 3,4-di-O-benzyl-α-d-mannopyranoside and methyl 3,6-di-O-benzyl-α-d-mannopyranoside, and employing the protected d-mannotriosides methyl 3,4-di-O-benzyl-2,6-di-O-(3,4,6-tri-O-benzyl-α-d-mannopyranosyl)-α-d-mannopyranoside, and methyl 3,6-di-O-benzyl-2,4-di-O-(3,4,6-tri-O-benzyl-α-d-mannopyranosyl)-α-d-mannopyranoside as key intermediates.     

Synthesis of a branched d-mannopentaoside and a branched d-mannohexaoside: Models of the outer chain of the glycan of soybean agglutinin

Synthetic routes are described to the d-mannopentaoside methyl 3-O-(3,6-di-O-α-d-mannopyranosyl-α-d-mannopyranosyl)-6-O-α-d-mannopyranosyl-α-d-mannopyranoside, and the d-mannohexaoside methyl 3-O-(3,6-di-O-α-d-mannopyranosyl-α-d-mannopyranosyl)-6-O-(2-O-α-d-mannopyranosyl-α-d-mannopyranosyl)-α- d-mannopyranoside, formed in a regio- and stereo-controlled way by employing the properly protected d-mannobioside methyl 2,4-di-O-benzyl-3-O-(2,4-di-O-benzyl-α-d-mannopyranosyl)-α-d-mannopyranoside and d-mannotrioside methyl 2,4-di-O-benzyl-3-O-(2,4-di-O-benzyl-α-d-mannopyranosyl)-6-O-(3,4,6-tri-O-benzyl-α-d- mannopyranosyl)-α-d-mannopyranoside as key intermediates.     

Studies on the structure of a polysaccharide from Epidermophyton floccosum and approach to a synthesis of the basic trisaccharide repeating units

An alkali-soluble polysaccharide, designated as S-Iawe, has been isolated from the maycelia of Epidermophyton floccosum. Methylation, periodate oxidation, and acetolysis studies suggested that S-lawe is composed of (1→6)-Oα-d-mannopyranosyl-(1→6)-O-[α-d-mannopyranosyl-(1→2)]-O-α-d-mannopyranosyl repeating units. Condensation of 2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl bromide with methyl 3-O-benzyl-4,6-O-benzylidene-α-d-mannopyranoside in the presence of mercuric cyanide gave in 70% yield methyl 3-O-benzyl-4,6-O-benzylidene-2-O-(2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl)-α-d-mannopyranoside. Condensation of the debenzylidenated disaccharide with 2,3,4,6-tetra-O-acctyl-α-d-mannopyranosyl bromide afforded the corresponding trisaccharide repeating unit.     

Synthesis of three oligosaccharides that form part of the complex type of carbohydrate moiety of glycoproteins

Silver trifluoromethanesulfonate-promoted condensation of 3,4,6-tri-O-acetyl-2-deoxy-phthalimido-β-d-glucopyranosyl bromide with benzyl 3,6-di-O-benzyl-α-d-mannopyranoside and benzyl 3,4-di-O-benzyl-α-d-mannopyranoside gave the protected 2,4- and 2,6-linked trisaccharides in yields of 54 and 32%, respectively. After exchanging the 2-deoxy-2-phthalimido groups for 2-acetamido-2-deoxy groups and de-blocking, the trisaccharides 2,4-di-O-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-d-mannose and 2,6-di-O-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-d-mannose were obtained. Similar condensation of 3,6-di-O-acetyl-2-deoxy-2-phthalimido-4-O-(2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-β-d-glucopyranosyl bromide with benzyl 3,4-di-O-benzyl-α-d-mannopyranoside gave a pentasaccharide derivative in 52% yield. After transformations analogous to those applied to the trisaccharides, 2,6-di-O-[β-d-galactopyranosyl-(1→4)-O-(2-acetamido-2-deoxy-β-d-glucopyranosyl)]-d-mannose was obtained.     

Electron-impact mass spectrometry of methyl O-methylglucopyranosiduranamides

6-O-Acetyl-2,4-diazido-3-O-benzyl-2,4-dideoxy-β-D-glucopyranosyl chloride and 2,6-diazido-3,4-di-O-benzyl-2,6-dideoxy-β-D-glucopyranosyl chloride are two valuable building units suitable for the synthesis of α-linked disaccharides containing 2,4-diamino-2,4-dideoxy- or 2,6-diamino-2,6-dideoxy-D-glucose as nonreducing moieties. The glycoside synthesis is accomplished stereoselectively under mild conditions in the presence of silver perchlorate. The α-(1→3)-linked disaccharides 2,4-diacetamido-2,4-dideoxy-3-O-(2,4-diacetamido-2,4-dideoxy-α-D-glucopyranosyl)-D-glucopyranose and 2-acetamido-2-deoxy-3-O-(2,6-diacetamido-2,6-dideoxy-α-D-glucopyranosyl)-D-glucopyranose have been prepared.     

The stepwise synthesis of a methyl β-xylotetraoside related to branched xylans

3,4-Di-O-acetyl-2-O-benzyl-α-d-xylopyranosyl bromide (1) reacts with methyl 2,3-anhydro-α-d-ribopyranoside (2) to afford, in high yield, methyl 2,3-anhydro-4-O- (3,4-di-O-acetyl-2-O-benzyl-β-d-xylopyranosyl)-β-d-ribopyranoside (3). Deacetylation of 3 gave 4, which reacted with 2,3,4-tri-O-acetyl-α-d-xylopyranosyl bromide to give the branched tetrasaccharide derivative 5, which, in turn, was converted by a series or conventional reactions into methyl 4-O-[3,4-di-O-(β-d-xylopyranosyl)-β-d- xylopyranosyl]-β-d-xylopyranoside. The reaction of 1 with its hydrolysis product gave 3,4-di-O-acetyl-2-O-benzyl-α-d-xylopyranosyl 3,4-di-O-acetyl-2-O-benzyl-β-d-xylopyranoside, which was also isolated after the reaction of 1 with 2.     

Syntheses of model oligosaccharides of biological significance.Synthesis of methyl 3,6-DI-O-(α-d-mannopyranosyl)-α-d-mannopyranoside and the corresponding mannobiosides

Methyl 2-O-allyl-4,6-O-benzylidene-3-O-(2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl)-α-d-mannopyranoside(12) was prepared in 90 % yield by Helferich glycosylation of methyl 2-O-allyl-4,6-O-benzylidene-α-d-mannopyranoside (9) with tetra-O-acetyl-α-d-mannopyranosyl bromide (11). Removal of the benzylidene group and second Helferich glycosylation with 11 led to methyl 2-O-allyl-3,6-di-O-(2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl)-α-d-mannopyranoside (14) which, after deallylation and Zemplén deacetylation, gave the title compound 5. The disaccharides methyl 3-O-(α-d-mannopyranosyl)-α-d-mannopyranoside (7) and methyl 6-O-(α-d-mannopyranosyl)-α-d-mannopyranoside (6) have also been synthesized. Complete assignments of the ¹H-n.m.r. spectra of the compounds 5, 6, and 7 are given.     

A synthesis of 3-O-(α-d-mannopyranosyl)-d-mannose and its protein conjugate

Methods for the synthesis of 3-O-(α-d-mannopyranosyl)-d-mannose and 2-(4-aminophenyl)ethyl 3-O-(α-d-mannopyranosyl)-α-d-mannopyranoside have been investigated by a number of sequences. Glycosidations with 2,3-di-O-acetyl-4,6-di-O-benzyl-d-mannopyranosyl and 2-O-benzoyl-3,4,6-tri-O-benzyl-d-mannopyranosyl p-toluenesulfonates were found to give better yields than the Helferich modification, the use of a peracylated d-mannopyranosyl halide, or the use of triflyl leaving group. Only the α anomer was obtained. Factors influencing glycosidation reactions are discussed. A mercury(II) complex was used for selective 2-O-acylation of 4,6-di-O-benzyl-α-d-mannopyranosides. A disaccharide—protein conjugate was prepared by the isothiocyanate method.     

Synthesis of the tetrasaccharide repeating-unit of the O-specific polysaccharide from Salmonella senftenberg

The oligosaccharide β-d-Man-(1 → 4)-α-l-Rha (1 → 3)-d-Gal-(6 ← 1)-α-d-Glc, which is the repeating unit of the O-specific polysaccharide chain of the lipopolysaccharide from Salmonella senftenberg, was obtained by glycosylation of benzyl 2,4-di-O-benzyl-6-O-(2,3,4-tri-O-benzyl-6-O-p-nitrobenzoyl-α-d-glucopyranosyl)-β-d-galactopyranoside or benzyl 2-O-acetyl-6-O-(2,3,4-tri-O-benzyl-6-O-p-nitrobenzoyl-α-d-glucopyranosyl)-β-d-galactopyranoside with 3-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-β-d-mannopyranosyl)-β-l-rhamnopyranose 1,2-(methyl orthoacetate) followed by removal of protecting groups.     

Synthesis of 2-O-benzyl- and 2,3- and 2,6-di-O-benzyl-D-galactose

The following ethers, of potential value for the synthesis of α-D-galactopyranosides, were prepared: 2-O-benzyl-D-galactose, 2,6-di-O-benzyl-D-galactose, and 2,3-di-O-benzyl-D-galactose. Isopropylidenation of methyl α-D-galactopyranoside in the presence of phosphorus pentaoxide gave its 3,4-, and 4,6-O-isopropylidene derivatives. Treatment of the 3,4-acetal with trityl chloride in pyridine produced the 6-trityl ether, which was benzylated with benzyl chloride and sodium hydride in N,N-dimethylformamide to yield the 2-benzyl ether. Acid hydrolysis of this product gave 2-O-benzyl-D-galactose. Benzylation of methyl 3,4-O-isopropylidene-α-D-galactopyranoside, followed by hydrolysis, gave 2,6-di-O-benzyl-D-galactose. Similarly, 2,3-di-O-benzyl-D-galactose was obtained by acid hydrolysis of methyl 2,3-di-O-benzyl-4,6-O-isopropylidene-α-D-galactopyranoside and of methyl 2,3-di-O-benzyl-4,6-O-benzylidene-β-D-galactopyranoside.     

Silylmethylene radical cyclization. A stereoselective approach to branched sugars

Ethyl 6-O-benzyl-2,3-dideoxy-α-d-erythro-hex-2-enopyranoside (2) was converted, in three steps and in 73% overall yield, into ethyl 6-O-benzyl-2,3-dideoxy-3-C-(hydroxymethyl)-α-d-ribo-hex-2-enopyranoside. This transformation involved silylation of 2 with (bromomethyl)chlorodimethylsilane and application of the Nishiyama-Stork radical cyclisation, followed by Tamao oxidation of the sila cycle. Ethyl 6-O-benzyl-2,3-dideoxy-α-d-threo-hex-2-enopyranoside and benzyl 2,6-di-O-benzyl-α-l-threo-hex-4-enopyranoside were similarly transformed into, respectively, ethyl 6-O-benzyl-2,3-dideoxy-3-C-(hydroxymethyl)-α-d-lyxo-hex-2-enopyranoside (50%), and benzyl 2,6-di-O-benzyl-4-deoxy-4-C-(hydroxymethyl)-β-d-galactopyranoside (71%).     

Synthesis of O-β-d-mannopyranosyl-(1→4)-O-α-l-rhamnopyranosyl-(1→3)-d-galactopyranose,the trisaccharide repeating-unit of the O-specific polysaccharide from Salmonella anatum

Glycosylation of 1,2:5,6-di-O-isopropylidene-α-d-galactofuranose with 2,3-di-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-β-d-mannopyranosyl)-α-l-rhamnopyranosyl bromide, followed by removal of the protecting groups, gave O-β-d-mannopyranosyl-(1→4)-O-α-l-rhamnopyranosyl-(1→3)-d-galactose, which is the trisaccharide repeating-unit of the O-specific polysaccharide chain of the lipopolysaccharide from Salmonella anatum. The formation of the β-d-mannopyranosyl linkage was achieved by a glucose-mannose conversion via stereoselective reduction of the corresponding oxo-disaccharide.     

Studies on the koenigs-knorr reaction : Part IV: The effect of participating groups on the stereochemistry of disaccharide formation

Partial benzylation of methyl 2-O-benzyl-α-L-fucopyranoside afforded a mixture of methyl 2,3-, and 2,4-di-O-benzyl-α-L-fucopyranoside which were separated by means of their monoacetates. Partial benzylation of methyl α-L-fucopyranoside gave the 2,4-, and 3,4-dibenzyl ethers in the ratio of 3:2, and no 2,3-isomer could be detected in the reaction mixture. The structures of the three dibenzyl ethers were established: (a) by analysis of the n.m.r. spectra of their acetates, and (b) by methylation, removal of benzyl groups by hydrogenolysis, and characterization of the methyl ethers of the methyl glycosides. Acid hydrolysis of these compounds gave the monomethyl ethers of L-fucose, two of which were identical with known compounds, whereas the third, 4-O-methyl-L-fucose, was a new compound. Selective p-nitrobenzoylation of 2,3-, 2,4-, and 3,4-di-O-benzyl-L-fucose, followed by acetylation and treatment with hydrogen bromide in dichloromethane, gave the three possible mono-O-acetyl-di-O-benzyl-α-L-fucopyranosyl bromides, which were condensed with benzyl 2-acetamido-3,4-di-O-acetyl-2-deoxy-α-D-glucopyranoside. The disaccharide derived from the 2-O-acetyl substituted bromide was enriched in β-L-fucopyranoside, whereas the other two bromides gave mainly the α-L-linked anomer. The α-directing influence of the 3- and 4-O-acetyl substituents is not less than the β-directing influence of the 2-O-acetyl group in similar bromides; participation of acyl groups and electronic-steric influences are discussed as possible explanations for the steric course of the reaction.     

Synthesis of two purple-membrane glycolipids and the glycolipid sulfate O-(β-d-glucopyranosyl 3-sulfate)-(1→6)-O-α-d-mannopyranosyl-(1→2)-O-α-d-glucopyranosyl-(1→1)-2, 3-di-O-phytanyl-sn-glycerol

The two purple-membrane glycolipids O-β-d-glucopyranosyl- and O-β-d-galactopyranosyl-(1→6)-O-α-d-mannopyranosyl-(1→2)-O-α-d-glucopyranosyl-(1→1)-2, 3-di-O-phytanyl-sn-glycerol were prepared by coupling O-(2,3,4-tri-O-acetyl-α-d-mannopyranosyl)-(1→2)-O-(3,4,6-tri-O-acetyl-α-d-glucopyranosyl)-(1→1)-2, 3-di-O-phytanyl-sn-glycerol (9) with 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl bromide or 2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl bromide, respectively, followed by deacetylation. The glycolipid sulfate O-(β-d-glucopyranosyl 3-sulfate)-(1→6)-O-α-d-mannopyranosyl-(1→2)-O-α-d-glucopyranosyl-(1→1)-2,3-di-O-phytanyl-sn-glycerol was prepared by coupling of 9 with 2,4,6-tri-O-acetyl-3-O-trichloroethyloxycarbonyl-α-d-glucopyranosyl bromide in the presence of Hg(CN)₂/HgBr₂ followed by selective removal of the 3?-trichloroethyloxycarbonyl group, sulfation of HO-3?, and deacetylation. The suitably protected key-intermediate 9 could be prepared by two distinct approaches.     

Unambiguous syntheses of the 3,4-di- and 3,4,6-tri-methyl ethers of methyl α-d-mannopyranoside

The unambiguous syntheses of methyl 3,4,6-tri-O-methyl-α-d-mannopyranoside (6) and methyl 3,4-di-O-methyl-α-d-mannopyranoside (10) were performed by routes involving methyl 3-O-benzoyl-4,6-O-benzylidene-α-d-mannopyranoside (1) to form methyl 2-O-p-tolylsulfonyl-d-mannopyranoside (4). Compound 4 directly led to 6, and, via a 6-trityl derivative, to 10.     

Selective benzylation of some D-galactopyranosides. Unusual relative reactivity of the hydroxyl group at C-4

Partial benzylation of methyl 2,3-di-O-benzyl-α-D-galactopyranoside gave methyl 2,3,6-tri-O-benzyl-α-D-galactopyranoside as the major product, whereas the isomeric 2,6-di-O-benzyl ether gave a mixture of products in which the ratio of methyl 2,4,6- to methyl 2,3,6-tri-O-benzyl-α-D-galactopyranoside was ≈4:1. The proportion of unreacted starting-material was low in both cases, whereas after a similar reaction of methyl 2,6-di-O-benzyl-β-D-galactopyranoside more than 50% of the dibenzyl ether was recovered unchanged. In this case also, considerably higher reactivity was exhibited by the hydroxyl group at C-4 than that at C-3. Acid hydrolysis of the methyl glycosides of the tribenzyl ethers afforded crystalline 2,4,6-tri-O-benzyl-α-D-galactose and syrupy 2,3,6-tri-O-benzyl-D-galactose. Structures of intermediates were established by acetylation, examination of their n.m.r. spectra, and conversion into the known 3-O and 4-O-methyl-D-galactose.     

Regioselective synthesis of a glycomimetic trisaccharide of Sialyl Lewis (sLe)

Sialyl Lewis (sLe^x) is the smallest naturally occurring carbohydrate ligand that binds to E-Selectin on the activated endothelium. We report here the total synthesis of acetic acid-sLe^x analog (12), for testing as a therapeutic agent. Methoxyethyl 4-O-(3,4-O-isopropylidene-β-d-galactopyranosyl)-β-d-glucopyranoside (3) was prepared starting from the methoxyethyl-β-d-lactoside (2), which was selectively benzoylated to give the methoxyethyl 2,6-di-O-benzoyl-4-O-(2,6-di-O-benzoyl-3,4-O-isopropylidene-β-d-galactopyranosyl)-β-d-glucopyranoside (4). Glycosylation of acceptor 4 with methyl 2,3,4-tri-O-benzyl-1-thio-β-l-fucopyranoside (5) in the presence of cupric bromide and tetrabutylammonium bromide afforded the corresponding methoxyethyl 2,6-di-O-benzyl-3-O-(2,3,4-tri-O-benzyl-α-l-fucopyranosyl)-4-O-(2,6-di-O-benzyl-3,4-O-isopropylidene-β-d-galactopyranosyl)-β-d-glucopyranoside (6). Selective removal of the 4″,6″-O-isopropylidene group from 6 gave the deprotected trisaccharide 7. The regioselective esterification of O-3″ of trisaccharide 8 (obtained from the dibutylstannylene derivative of 7) with benzyl-2-bromoacetate and tetrabutylammonium bromide afforded the 3″-O-carbobenzyloxymethyl trisaccharide derivative 9, which on saponification and hydrogenolysis with palladium-charcoal afforded the target trisaccharide 12 glycomimetic of Sialyl Lewis (sLe^x) trisaccharide omitting the sialic acid moiety.     

Purification and properties of an endo-(1 leads to 3)-beta-D-glucanase from malted barley

3,6-Anhydro-α-D-galactopyranose 1,2-(methyl orthoacetate) and its 4-acetate were synthesized from 2,3,4-tri-O-acetyl-6-O-tosyl-α-D-galactopyranosyl bromide. Condensation of the above-mentioned, acetylated ortho ester with 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose gave 6-O-(2,4-di-O-acetyl-3,6-anhydro-β-D-galactopyranosyl)-1,2:3,4-di-O-isopropylidene-α-D-galactopyranose. The same disaccharide derivative was synthesised from 6-O-β-D-galactopyranosyl-1,2:3,4-di-O-isopropylidene-α-D-galactopyranose by mono-O-tosylation followed by treatment with alkali and acetylation.     

The stepwise synthesis of an aldopentaouronic acid derivative related to branched (4-O-methylglucurono)xylans

Benzylation of methyl 2,3-anhydro-4-O-[2-O-benzyl-3,4-di-O-(β-D-xylop yranosyl]-β-D-xylopyranosyl]-β-D-ribopyranoside (1) afforded the crystalline. fully benzylated tetrasaccharide derivative 2. The octa-O-benzyl derivative 3, having only HO-2 unsubstituted, obtained by treatment of 2 with benzyl alcoholate anion in benzyl alcohol, was allowed to react in dichloromethane with methyl 2,3-di-O-benzyl- 1-chloro-1-deoxy-4-O-methy]-α,β-glucopyranuronate in the presence of silver perchlorate and triethylamine to give the branched, 4-O-methyl-α-D-glucuronic acid-containing pentasaccharide derivative 4a as the major product. Subsequent debenzylation afforded the aldopentaouronic acid derivative 5a, which contains all the basic structural features of branched, hardwood (4-O-methylglucurono)xylans. The structure of 5a was confirmed by analysis of its ¹³C-n.m.r. spectrum and the mass-spectral fragmentation pattern of the corresponding fully methylated derivative 6a.     

The synthesis of some deoxygenated analogues of early intermediates in the biosynthesis of glycosylphosphatidylinositol (GPI) membrane anchors

Syntheses are described of 2-azido-4,6-di-O-benzyl-2,3-dideoxy-d-ribo-hexopyranosyl fluoride, 6-O-acetyl-2-azido-3-O-benzyl-2,4-dideoxy-d-xylo-hexopyranosyl fluoride and 2-azido-3,4-di-O-benzyl-2,6-dideoxy-d-glucopyranosyl fluoride. These glycosyl donors were coupled with the acceptor 1d-2,3,4,5-tetra-O-benzyl-1-O-(4-methoxybenzyl)-myo-inositol and the α-coupled products were transformed into α-d-3dGlcpN-PI, α-d-4dGlcpN-PI and α-d-6dGlcpN-PI by way of the H-phosphonate route. Brief mention is made of the biological evaluation of these deoxy-sugar analogues and their N-acetylated forms as candidate substrate/inhibitors of the N-deacetylase and α-(1→4)-d-mannosyltransferase activities present in trypanosomal and HeLa (human) cell-free system.