Synthesis of a Tri- and Tetrasaccharide Fragment Specific for the Shigella flexneri Serotype 5a O-Antigen. A Reinvestigation

ABSTRACT

Stereocontrolled, stepwise synthesis of methyl α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside (A(E)B, 1) and methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→2)-α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside (DA(E)B, 2) is described; these constitute the methyl glycosides of fragments of the O-specific polysaccharide of Shigella flexneri serotype 5a. Two routes to trisaccharide 1 were considered. Route 1 involved the coupling of a precursor to residue A and a disaccharide EB, whereas route 2 was based on the condensation of a precursor to residue E and a disaccharide AB. Rather surprisingly, the latter afforded the β-anomer of 1, namely methyl α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside as the major product. Route 1 was preferred. Overall, several observations made during this study suggested that, for the construction of higher fragments, a suitable precursor to rhamnose A would require protecting groups of low bulkiness at position 3 and 4. Therefore, the 2-O-acetyl-3,4-di-O-allyl-α-L-rhamnopyranosyl trichloroacetimidate (35) was the precursor of choice to residue A in the synthesis of the tetrasaccharide 2. The condensation product of 35 and methyl 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl-4-O-benzyl-α-L-rhamnopyranoside was selectively deacylated and condensed to 2-trichloroacetamido-3,4,6-tri-O-acetyl-2-deoxy-α-D-glucopyranosyl trichloroacetimidate to afford the corresponding fully protected tetrasaccharide 45. Controlled stepwise deprotection of the latter proceeded smoothly to afford the target 2. It should be emphasised that the preparation of 45 was not straightforward, several donors and coupling conditions that were tested resulted only in the complete recovery of the acceptor. Distortion of several signals in the ¹³C NMR spectra of the fully or partially protected tetrasaccharide intermediates suggested that steric hindrance, added to the known low reactivity of HO-2 of rhamnosyl acceptors, probably played a major role in the outcome of the glycosidation attempts.     

Linear Synthesis of The Methyl Glycosides of Tri-, Tetra-, and Pentasaccharide Fragments of The Shigella Flexneri Serotype 5A O-Antigen

ABSTRACT

The stepwise synthesis of methyl α-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→3)-α-L-rhamnopyranoside (EBC-OMe, 1), methyl α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranosyl-(1→3)-α-L-rhamnopyranoside (A(E)BC-OMe, 2), and methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→2)-α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranosyl-(1→3)-α-L-rhamnopyranoside (DA(E)BC-OMe, 3) is described. Compounds 1, 2 and 3 constitute the methyl glycosides of fragments of the O-specific polysaccharide of Shigella flexneri serotype 5a. Methyl 2,4-di-O-benzoyl-α-L-rhamnopyranosyl-(1→3)-2,4-di-O-benzoyl-α-L-rhamnopyranoside was an appropriate BC precursor for the synthesis of 1. For the synthesis of the branched targets 2 and 3, a benzyl group was best suited at position 2 of rhamnose C. Thus, methyl 4-O-benzyl-α-L-rhamnopyranosyl-(1→3)-2,4-di-O-benzyl-α-L-rhamnopyranoside was the key intermediate to the BC portion. In all cases, 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl fluoride was a convenient E precursor, when used in combination with titanium tetrafluoride. All along, attention was paid to steric hindrance as a factor of major impact on the condensation steps outcome. Therefore, based on previous experience, 2-O-acetyl-3,4-di-O-allyl-α-L-rhamnopyranosyl trichloroacetimidate and 3,4,6-tri-O-acetyl-2-deoxy-2-trichloroacetamido-α-D-glucopyranosyl trichloroacetimidate were used as donors. Both suited all requirements when used as key precursors for residues A and D in the synthesis of 3, respectively.     

Synthesis of the Methyl Glycosides of a Di- and Two Trisaccharide Fragments Specific for the Shigella flexneri Serotype 2a O-Antigen

ABSTRACT

The stereocontrolled synthesis of methyl α-D-glucopyranosyl-(1→4)-α-L-rhamnopyranoside (EC, 1), methyl α-L-rhamnopyranosyl-(1→3)-[α-D-glucopyranosyl-(1→4)]-α-L-rhamnopyranoside (B(E)C, 3) and methyl α-D-glucopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→3)-2-acetamido-2-deoxy-β-D-glucopyranoside (ECD, 4) is described; these constitute the methyl glycosides of branched and linear fragments of the O-specific polysaccharide of Shigella flexneri serotype 2a. Emphasis was put on the construction of the 1,2-cis EC glycosidic linkage resulting in the selection of 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl fluoride (8) as the donor. Condensation of methyl 2,3-O-isopropylidene-4-O-trimethylsilyl-α-L-rhamnopyranoside (11) and 8 afforded the fully protected αE-disaccharide 20, as a common intermediate in the synthesis of 1 and 3, together with the corresponding βE-anomer 21. Deacetalation and regioselective benzoylation of 20, followed by glycosylation with 2,3,4-tri-O-benzoyl-α-L-rhamnopyranosyl trichloroacetimidate (15) afforded the branched trisaccharide 25. Full deprotection of 20 and 25 afforded the targets 1 and 3, respectively. The corresponding βE-disaccharide, namely, methyl β-D-glucopyranosyl-(1→4)-α-L-rhamnopyranoside (βEC, 2) was prepared analogously from 21. Two routes to trisaccharide 4 were considered. Route 1 involved the coupling of a precursor to residue E and a disaccharide CD. Route 2 was based on the condensation of an appropriate EC donor and a precursor to residue D. The former route afforded a 1:2 mixture of the αE and βE condensation products which could not be separated, neither at this stage, nor after deacetalation. In route 2, the required αE-anomer was isolated at the disaccharide stage and transformed into 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl-(1→4)-2,3-di-O-benzoyl-α-L-rhamnopyranosyl trichloroacetimidate (48) as the EC donor. Methyl 2-acetamido-2-deoxy-4,6-O-isopropylidene-β-D-glucopyran-oside (19) was preferred to its benzylidene analogue as the precursor to residue D. Condensation of 19 and 48 and stepwise deprotection of the glycosylation product afforded the target 4.     

A novel dimeric flavonol glycoside from Cynanchum acutum subsp. sibiricum

A novel dimeric flavonol glycoside, Cynanflavoside A (1), together with six analogues, kaempferol-3-O-α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside (2), quercetin-3-O-α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside (3), kaempferol-3-O-α-L-rhamnopyranosyl-(1→2)-β-D-xylopyranoside (4), quercetin-3-O-α-L-rhamnopyranosyl-(1→2)-β-D-xylopyranoside (5), kaempferol-3-O-β-D-glucopyranosyl-7-O-α-L-rhamnopyranoside (6), and quercetin-3-O-galactoside (7) were isolated from the n-butyl alcohol extract of Cynanchum acutum subsp. sibiricum. Their structures were determined spectroscopically and compared with previously reported spectral data. All compounds were evaluated for their anti-complementary activity in vitro, and only compound 5 exhibited anti-complement effects with CH₅₀ value of 0.33 mM.     

Synthesis of a Single Repeat Unit of Type VIII Group B Streptococcus Capsular Polysaccharide 1

Abstract

We have synthesized a single repeat unit of type VIII Group B Streptococcus capsular polysaccharide, the structure of which is {L-Rhap(β1→4)-D-Glcp(β1→4)[Neu5Ac(α2→3)]-D-Galp(β→4)}_n. The synthesis presented three significant synthetic challenges namely: the L-Rhap(β→4)-D-Glcp bond, the Neu5Ac(α2→3)-D-Galp bond and 3,4-D-Galp branching. The L-Rhap bond was constructed in 60% yield (α:β 1:1.2) using 4-O-acetyl-2,3-di-O-benzoyl-α-L-rhamnopyranosyl bromide 6 as donor, silver silicate as promotor and 6-O-benzyl-2,3-di-O-benzoyl-1-thio-β-D-glucopyranoside as acceptor to yield disaccharide 18. The Neu5Ac(α2→3) linkage was synthesized in 66% yield using methyl [phenyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto-nonulopyranosid]onate as donor and triol 2-(trimethylsilyl) ethyl 6-O-benzyl-β-D-galactopyranoside as acceptor to give disaccharide 21. The 3,4-D-Galp branching was achieved by regioselective glycosylation of disaccharide diol 21 by disaccharide 18 in 28% yield to give protected tetrasaccharide 22. Tetrasaccharide 22 was deprotected to give as its 2-(trimethylsilyl)ethyl glycoside the title compound 1a. In addition the 2-(trimethylsilyl)ethyl group was cleaved and the tetrasaccharide coupled by glycosylation (via tetrasaccharide trichloroacetimidate) to a linker suitable for conjugation.

     

Synthetic α,β(1→4)-Glucan Oligosaccharides as Models for Heparan Sulfate

Abstract

α,β-(1→4)-Glucans were devised as models for heparan sulfate with the simplifying assumptions that carboxyl-reduction and sulfation of heparan sulfate does not decrease the SMC antiproliferative activity and that N-sulfates in glucosamines can be replaced by O-sulfates. The target oligo-saccharides were synthesized using maltosyl building blocks. Glycosylation of methyl 2,3,6,2′,3′,6′-hexa-O-benzyl-β-maltoside (1) with hepta-O-acetyl-α-maltosyl bromide (2) furnished tetrasaccharide 3 which was deprotected to α-D-Glc-(1→4)-β-D-Glc-(1→4)-α-D-Glc-(1→4)-β-D-Glc-(1→OCH₃) (5) or, alternatively, converted to the tetrasaccharide glycosyl acceptor (8) with one free hydroxyl function (4?′-OH). Further glycosylation with glucosyl or maltosyl bromide followed by deblocking gave the pentasaccharide [β-D-Glc-(1→4)-α-D-Glc-(1→4)]₂-β-D-Glc-(1→OCH₃) (11) and hexasaccharide [α-D-Glc-(1→4)-β-D-Glc-(1→4)₂-α-D-Glc-(1→4)-β-D-Glc-(1→OCH₃) (14). The protected tetrasaccharide 3 and hexasaccharide 12 were fully characterized by ¹H and ¹³C NMR spectroscopy. Assignments were possible using 1D TOCSY, T-ROESY, ¹H,¹H 2D COSY supplemented by ¹H-detected one-bond and multiple-bond ¹H,¹³C 2D COSY experiments.     

Antibacterial activity of a triterpenoid saponin from the stems of Caesalpinia pulcherrima Linn.

A new compound 1 was isolated from the methanolic extract of the stems of the Caesalpinia pulcherrima Linn. along with a reported compound (2) 3-O-β-D-glucopyranosyl-(1→4)-β-D-xylopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl hederagenin 28-O-β-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester. The new compound 1 has m.p. 272–274°C, m.f. C₄₆H₇₄O₁₇, [M]⁺ m/z 898. It was characterised as 3-O-β-D-glucopyranosyl-(1→4)-α-L-arabinopyranosyl hederagenin 28-O-β-D- xylopyranosyl ester by various colour reactions, chemical degradations and spectral analyses. Antibacterial activity of compound 1 was screened against various Gram-positive and Gram-negative bacteria and showed significant results.     

Synthesis of L-Rhamnose and N -Acetyl-D-Glucosamine Derivatives Entering in the Composition of Bacterial Polysaccharides by Use of Glucansucrases

Transglucosylation reactions using sucrose as glucosyl donor and either N-acetyl-D-glucosamine, L-rhamnose, or methyl α -L-rhamnopyranoside as acceptors were carried out with recombinant glucansucrases from families 70 and 13 of glycoside-hydrolases. Depending on the enzyme specificity, various carbohydrate structures were synthesized and characterized including α -D-glucopyranosyl-(1 → 6)-N-acetyl-D-glucosamine, α -D-glucopyranosyl-(1 → 4)-N-acetyl-D-glucosamine, α -D-glucopyranosyl-(1 → 1)-β -L-rhamnopyranoside, α -D-glucopyranosyl-(1 → 4)-α -D-glucopyranosyl-(1 → 1)-β -L-rhamnopyranoside, methyl α -D-glucopyranosyl-(1 → 4)-α -L-rhamnopyranoside, and methyl α -D-glucopyranosyl-(1 → 3)-α -L-rhamnopyranoside. Disaccharides were obtained with yields going up to 64%. The structural diversity generated as well as the obtained yields appear to be related to enzyme active site architecture, which can be modulated and improved by enzyme engineering. Several of the obtained disaccharides enter in the composition of surface polysaccharides of pathogenic bacteria, among which is Shigella flexneri. Our results outline the potential of glucansucrases in the chemo-enzymatic synthesis of complex carbohydrates of biological interest whose chemical synthesis may be seen as a limitation.     

Preparation of trans Diequatorial 2,3-Pyruvate Acetals in a Di- and a Trisaccharide Related to the K-Antigen from E. Coli 0101:K103:H−

Abstract

Using methyl 2,2-bis(ethylthio)propionate as acetalating agent and triflic acid-sulfuryl chloride as catalyst, synthesis of 2,3-trans diequatorial pyruvate ketal was achieved. Starting from D-galactose and L-rhamnose derivatives, methyl 2,3,4,6-tetra-O-benzyl-α-D-galactopyranosyl-(1→4)-6-O-benzyl-2,3-O-(1-methoxycarbonyl)ethylidene- α-D-galactopyranosyl-(1→3)-2,4-di-O-benzyl-α-L-rhamnopyranoside and methyl 4,6-di-O-benzyl-2,3-O-(1-methoxy-carbonyl)ethylidene-α-D-galactopyranosyl-(1→3)-2,4-di-O-benzyl-α-L-rhamnopyranoside were synthesized. Removal of the protecting groups from the former, afforded the trisaccharide repeating unit of the K-antigen from E.coli O101:K103:H^? in the form of its methyl glycoside methyl ester.     

Synthetic Studies on Sialoglycoconjugates 88: Synthesis of Ganglioside GM3 and GM4 Analogs Containing 2- OR 3-Branched Fatty-Alkyl Residues in Place of Ceramide

ABSTRACT

Each of four ganglioside GM₄ and GM₃ analogues containing 2- or 3-branched fatty alkyl residues in place of ceramide have been synthesized. Coupling of O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-α-D-galacto-2-nonulopyranosylonate)-(2→3)-2,4,6-tri-O-benzoyl-α-D-galactopyranosyl trichloroacetimidate (13) or O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-α-D-glacto-2-nonulopyranosylonate)-(2→3)-O-(2,4-di-O-acetyl-6-O-benzoyl-β-D-galactopyranosyl)-(1→4)-3-O-acetyl-2,4-di-O-benzoyl-α-D-glucopyranosyl trichloroacetimidate (14) with 2- or 3-branched fatty-alkyl-1-ols (9-12), prepared from the corresponding branched fatty acids by methyl esterification and reduction, using BF₃Ot₂ gave the corresponding ganglioside analogues (15, 17, 19, 21, 23, 25, 27, 29) in good yields, which were coverted, via O-deacylation and de-esterification, into the title compounds.     

SYNTHESIS OF A XYLOSYLATED RHAMNOSE PENTASACCHARIDE—THE REPEATING UNIT OF THE O-SPECIFIC SIDE CHAIN OF LIPOPOLYSACCHARIDES FROM THE REFERENCE STRAINS FOR Stenotrophomonas maltophilia SEROGROUP O18

A xylosylated rhamnose pentasaccharide, α- l-Rha p-(1→3)-[β- l-Xyl p-(1→2)-] [β- l-Xyl p-(1→4)-]α- l-Rha p-(1→3)- l-Rha p, the repeating unit of the O-specific side chain of the lipopolysaccharides from the reference strains for Stenotrophomonas maltophilia serogroup O18, was synthesized by a highly regio- and stereoselective procedure. Thus coupling of methyl rhamnopyranoside (9) with 2,3,4-tri- O-acetyl-α- l-rhamnopyranosyl trichloroacetimidate (8) gave the (1→3)-linked disaccharide (10), and subsequent benzoylation and deacetylation afforded the disaccharide acceptor 12. Condensation of 12 with 8 yielded methyl 2,3,4-tri- O-acetyl-α- l-rhamnopyranosyl-(1→3)-α- l-rhamnopyranosyl-(1→3)-2,4-di- O-benzoyl-α- l-rhamnopyranoside (13). Coupling of 13 with 2,3,4-tri- O-benzoyl-α- l-xylopyranosyl trichloroacetimidate (4) followed by deprotection gave the target pentasaccharide (15).     

Synthesis of Two Tetrasaccharides Related to the O-Antigen from Azospirillum brasilense S17 and Azospirillum lipoferum SR65

Synthesis of two isomeric tetrasaccharides, β-D-Glup-(1→2)-α-L-Rhap-(1→3)-α-L- Rhap-(1→2)-α-L-Rhap (I) and β-D-Glup-(1→3)-α-L-Rhap-(1→3)-α-L-Rhap-(1→3)-α-L-Rhap (II), the repeating units from the lipopolysaccharides of the nitrogen-fixing bacterium Azospirillum brasilense S17 and Azospirillum lipoferum SR65, was achieved via assembly of the building blocks 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl trichloroacetimidate (2), p-methoxyphenyl 3,4-di-O-benzoyl-α-L-rhamnopyranoside (3), 3-O-allyloxycarbonyl-2,4-di-O-benzoyl-α-L-rhamnopyranosyl trichloroacetimidate (6), 2,3,4,6-tetra-O-benzoyl-β-D-glucopyranosyl trichloroacetimidate (8), and p-methoxy phenyl 2,4-di-O-benzoyl-α-L-rhamnopyranoside (14). Condensation of 3 with 6 or 8 provided the disaccharides 9 or 11, respectively. Deallyloxycarbonylation of 11 gave the disaccharide aceptor 12, while removal of the p-methoxyphenyl group in 9 followed by trichloroacetimidation of the anomeric hydroxyl group afforded the disaccharide donor 10. Meanwhile, disaccharide donor 16 and acceptor 18 were prepared from 6, 8, and 14 similarly. Finally, condensation of 10 with 12 or 16 with 18, followed by deprotection, gave the target tetrasaccharides I or II, respectively.     

Synthesis of Methyl 2-O-, 3-O-, 4-O-, 6-O-, 2,3-Di-O- and 4,6-Di-O-β-D-Galactopyranosyl-β-D-glucopyranoside

Abstract

Synthesis of methyl O-β-D-galactopyranosyl-(1→2)-β-D-glucopyranoside 1, methyl O-β-D-galactopyranosyl-(1→3)-β-D-glucopyranoside 2, methyl O-β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside 3, methyl O-β-D-galactopyranosyl-(1→6)-β-D-glucopyranoside 4, methyl O-β-D-galactopyranosyl-(1→4)-[O-β-D-galactopyranosyl-(1→6)]-β-D-glucopyranoside 5, and methyl O-β-D-galactopyranosyl-(1→2)-[O-β-D-galactopyranosyl-(1→3)]-β-D-glucopyranoside 6, using 2,3,4,6 tetra-O-acetyl-α-D-galactopyranosyl trichloroacetimidate or 2,3,4,6 tetra-O-acetyl-α-D-galactopyranosyl bromide as a glycosyl donor and selectively protected derivatives of methyl O-β-D-glucopyranoside as glycosyl acceptors are described.     

Synthesis of Tetrasaccharide Repeating Unit of the K-Antigen from Klebsiella Type-16

Abstract

Starting from L-fucose, D-glucose and lactose, methyl O-[2,3-di-O-benzoyl-4, 6-O-(4-methoxybenzylidene)-β-D-glucopyranosyl]-(1→4)-2,3-di-O-benzoyl-α-L-fucopyranoside and methyl O-(2,3,4,6-tetra-O-benzyl-β-D-galactopyranosyl)-(1→4)-O-(2,3,6-tri-O-benzyl-α-D-glucopyranosyl)-(1→4)-O-(methyl 2,3-di-O-benzoyl-β-D-glucopyranosyluronate)-(1→4)-2,3-di-O-benzoyl-α-L-fucopyranoside were synthesized. Removal of protecting groups gave the tetrasaccharide repeating unit of the antigen from Klebsiella type-16 in the form of its methyl ester methyl glycoside.     

The Synthesis of Four Dideoxygenated Analogues of β-Maltosyl-(1→4)-Trehalose

Abstract

Four derivatives of β-maltosyl-(1→4)-trehalose were prepared, each with two deoxy functions in one of the constitutive disaccharide building blocks. 2,3-Di-O-acetyl-4,6-dideoxy-4,6-diiodo-α-D-galactopyranosyl- (1→4) ?1,2,3,6-tetra-O-acetyl-D-glucopyranose (3) was employed as a precursor for the 4?,6?-dideoxygenated tetrasaccharide 9: coupling of 3 with 2,3,6-tri-O-benzyl-α-D-glucopyranosyl 2,3,6-tri-O-benzylidene-α-D-glucopyranoside (4) furnished the tetrasaccharide 5 which was deiodinated and deprotected to yield the target tetrasaccharide 9. Secondly, the dideoxygenated maltose derivative 3-deoxy-4,6-O-isopropylidene-2-O-pivaloyl-β-D-glucopyranosyl- (1→4) ?1,6-anhydro-3-deoxy-2-O-pivaloyl-β-D-glucopyranose (10) was ring-opened to the anomeric acetate 11. A [2+2] block synthesis with 4 in TMS triflate mediated glycosylation gave a tetrasaccharide which was deprotected to the 3″,3?-dideoxygenated analogue of β-maltosyl-(1→4)-trehalose. For the third tetrasaccharide, 2,3,2″,3′-tetra-O-benzyl-α,α-trehalose was iodinated at the primary positions and deiodinated in the presence of palladium-on-carbon, then this acceptor was selectively glycosylated with hepta-O-acetyl-maltosyl bromide (20). Removal of protective groups furnished the maltosyl trehalose tetrasaccharide deoxygenated at positions C-6 and C-6′. to prepare a 3,3′-dideoxygenated trehalose, the free hydroxyl groups of 2-O-benzyl-4,6-O-(R)-benzylidene-α-D-glucopyranosyl 2-O-benzyl-4,6-O-(R)-benzylidene-α-D-glucopyranoside (25) were reduced by Barton-McCombie deoxygenation. One of the benzylidene groups was opened reductively with sodium cyanoborohydride. The resulting free hydroxyl group at the 4′-position was glycosylated in a Koenigs-Knorr reaction with 20 to yield the 3,3′-dideoxygenated tetrasaccharide 32, the fourth target oligosaccharide, after deprotection.     

Synthetic Studies on Sialoglycoconjugates 104: Synthesis of Kdn-Lewis X Ganglioside Analogs Containing Modified Reducing Terminal and L-Rhamnose in Place of L-Fucose 1 2

Abstract

KDN-Le^x ganglioside analogs (10, 13, 16 and 19) containing the modified reducing terminal and L-rhamnose in place of L-fucose have been synthesized. Glycosidation of methyl 2,3,4-tri-O-benzyl-1-thio-α-L-rhamnopyranoside (1) with 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-β-D-glucopyranosyl)-(1→3)-O-(2,4,6-tri-O-benzyl-α-D-galacopyranoside (2), followed by reductive ring opening of the benzylidene acetal, gave 2-(trimethylsilyl)ethyl O-(2,3,4-tri-O-benzyl-α-L-rhamnopyranosyl)-(1→3)-O-(2-acet-amido-6-O-benzyl-2-deoxy-β-D-glucopyranosyl)-(1→3)-O-(2,4,6-tri-O-benzyl-β-D-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-D-glucopyranoside (4). The tetrasaccharide 4 was coupled with methyl O-(methyl 4,5,7,8,9-penta-O-acetyl-3-deoxy-D-glycero-α-D-galacto-2-nonulopyranosylonate)-(2→3)-2,4,6-tri-O-benzoyl-1-thio-β-D-galactopyranoside(5), using dimethyl(methylthio)sulfonium triflate (DMTST), to give the hexasaccharide 6, which was converted into compound 11 in the usual manner. Compounds 8 and 11 were transformed, via bromination of the reducing terminal, radical reduction, O-deacylation and saponification of the methyl ester, into the desired KDN-Le^x hexasaccharides (10, 13). On the other hand, glycosylation of 2-(tetradecyl)hexadecanol with α-trichloroacetimidates 14 and 17, afforded the target ganglioside analogs 16 and 19.

     

Chemical-Enzymatic Synthesis of A Branched Hexasaccharide Fragment of Type Ia Group B Streptococcus Capsular Polysaccharide

ABSTRACT

A branched hexasaccharide fragment of type Ia group B streptococcal polysaccharide, α-NeuAc(2→3)-β-D-Gal(1→4)-β-D-GlcNAc(1→3)-[β-D-Glc(1→4)]-β-D-Gal(1→4)-β-D-Glc-OMe (13), has been synthesized by chemical-enzymatic procedures. Chemical synthesis of a pentasaccharide, β-D-Gal(1→4)-β-D-GlcNAc(1→3)-[β-D-Glc(1→4)]-β-D-Gal(1→4)-β-D-Glc-OMe (12), was achieved from glycosyl donor, 4-O-(2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-3,6-di-O-acetyl-2-deoxy-2-phthalimido-β-D-glucopyranosyl trichloroacetimidate (9), and acceptor, methyl O-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)-(1→4)-O-(2,6-di-O-benzyl-β-D-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-D-glucopyranoside (6), by block condensation in 41% yield. Following enzymatic sialylation of 12 at the 3-O-position of its terminal galactopyranosyl residue using recombinant α-(2→3)-sialyltransferase and CMP-NeuAc afforded 13 in 59% yield.     

SYNTHESIS OF A MANNOTETRAOSE—THE REPEATING UNIT OF THE CELL-WALL MANNANS OF MICROSPORUM GYPSEUM AND RELATED SPECIES OF TRYCHOPHYTON

A tetrasaccharide, α-D-mannopyranosyl-(1→2)-α-D-mannopyranosyl-(1→6)-α-D-mannopyranosyl-(1→6)-D-mannopyranose (1), the repeating unit of the cell-wall mannans of Microsporum gypseum and related species of Trychophyton, was synthesized using 6-O-acetyl-2,3,4-tri-O-benzoyl-α-D-mannopyranosyl trichloroacetimidate (5) and 2-O-acetyl-3,4,6-tri-O-benzoyl-α-D-mannopyranosyl trichloroacetimidate (13) as the glycosyl donors in “the inverse Schmidt” procedure.     

Mimics of the Structural Elements of Type III Group B Streptococcus Capsular Polysaccharide. Part I: Synthesis of a Carboxylate-Containing Pentasaccharide

Abstract

A carboxylate-containing pentasaccharide, methyl O-(β-d-galactopyranosyl)-(1→4)-O-(β-d-glucopyranosyl)-(1→6)-O-{3-O-[(S)-1-carboxyethyl]-β-d-galactopyranosyl-(1→4)-O}-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-(1→3)-β-d-galactopyranoside (27) was synthesized by block condensation of suitably protected donors and acceptors. Phenyl 3-O-benzyl-4,6-di-O-chloroacetyl-2-deoxy-2-phthalimido-1-thio-β-d-glucopyranoside (17) was condensed with methyl 2,4,6-tri-O-benzyl-β-d-galactopyranoside (4) to afford a disaccharide, methyl O-(3-O-benzyl-4,6-di-O-chloroacetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-(1→3)-2,4,6-tri-O-benzyl-β-d-galactopyranoside (18). Removal of chloroacetyl groups gave 4,6-diol, methyl 0-(3-O-benzyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-(1→3)-2,4,6-tri-O-benzyl-β-d-galactopyranoside (19), in which the primary hydroxy group (6-OH) was then selectively chloroacetylated to give methyl O-(3-O-benzyl-6-O-chloroacetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-(1→3)-2,4,6-tri-O-benzyl-β-d-galactopyranoside (20). This acceptor was then coupled with 2,4,6-tri-O-acetyl-3-O-[(S)-1-(methoxycarbonyl)ethyl]-α-d-galactopyranosyl trichloroacetimidate (14) to afford a trisaccharide, methyl O-{2,4,6-tri-O-acetyl-3-O-[(S)-l-(methoxycarbonyl)ethyl]-β-d-galactopyranosyl}-(1→4)-O-(3-O-benzyl-6-O-chloroacetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl)-(1→3)-2,4,6-tri-O-benzyl-β-d-galactopyranoside (21). Removal of the 6-O-chloroacetyl group in 21 gave 22, which was coupled with 4-O-(2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-2,3,6-tri-O-acetyl-α-d-glucopyranosyl trichloroacetimidate (23) to yield protected pentasaccharide 24. Standard procedures were used to remove acetyl groups and the phthalimido group, followed by N-acetylation, and debenzylation to yield pentasaccharide 27 and a hydrazide by-product (28) in a 5:1 ratio, respectively. Compound 27 contains a complete repeating unit of the capsular polysaccharide of type III group B Streptococcus in which terminal sialic acid is replaced by an (S)-1-carboxyethyl group.     

SYNTHESIS OF β-d-Glcp-(1→2)-[β-d-Ribf-(1→3)-]α-l-Rhap-(1→3)-α-l-Rhap-(1→2)-α-l-Rhap,THE REPEATING UNIT OF THE LIPOPOLYSACCHARIDE OF Acetobacter diazotrophicus PAL 5

A pentasaccharide, the major repeating unit of the lipopolysaccharide (LPS) of the nitrogen fixing bacterium Acetobacter diazotrophicus PAL 5 was efficiently synthesized as its allyl glycoside using a regio- and stereo-selective strategy. The key acceptor, allyl 3-O-acetyl-4-O-benzoyl-α-l-rhamnopyranoside (3), was prepared by selective 3-O-acetylation of allyl 4-O-benzoyl-α-l-rhamnopyranoside. Condensation of 3 with 2,3,4,6-tetra-O-benzoyl-α-d-glucopyranosyl trichloroacetimidate furnished the disaccharide 5. Deallylation and subsequent trichloroacetimidation of 5 afforded 2,3,4,6-tetra-O-benzoyl-β-d-glucopyranosyl-(1→2)-3-O-acetyl-4-O-benzoyl-α-l-rhamnopyranosyl trichloroacetimidate (10). Selective 3-O-glycosylation of allyl α-l-rhamnopyranoside (1) with 10 followed by benzoylation gave trisaccharide (12), which could be conveniently converted to a donor (14). Condensation of 14 with allyl 3,4-di-O-benzoyl-α-l-rhamnopyranoside (15) gave tetrasaccharide 16. Selective deacetylation of 16 gave the acceptor 17 which was ribosylated to furnish the protected pentasaccharide, and finally deprotection led to the title compound.