Total synthesis of sialylated and sulfated oligosaccharide chains from respiratory mucins

The total syntheses of several complex oligosaccharide moieties that occur in the core structure of sulfated mucins are reported. A trisaccharide acceptor was obtained through regio- and stereoselective sialylation of methyl (6-O-pivaloyl-beta-D-galactopyanosyl)(1-->3)-4,6-O-benzylidene-2-a cetamido-2-deoxy-alpha-D-galactopyranoside with a novel sialyl donor. A tetrasaccharide, pentasaccharide, and hexasaccharide were constructed in predictable and controlled manner with high regio- and stereoselectivity after the successful preparation and employment of a disaccharide donor, trisaccharide donor, disaccharide acceptor, and trisaccharide acceptor building blocks. Finally, a mild oxidative cleaving method was adopted for the selective removal of 2-naphthylmethyl (NAP) in the presence of benzyl groups.     

Modular synthesis of heparin oligosaccharides

A general, modular strategy for the first completely stereoselective synthesis of defined heparin oligosaccharides is described. Six monosaccharide building blocks (four differentially protected glucosamines, one glucuronic and one iduronic acid) were utilized to prepare di- and trisaccharide modules in a fully selective fashion. Installation of the alpha-glucosamine linkage was controlled by placing a conformational constraint on the uronic acid glycosyl acceptors thereby establishing a new concept for stereochemical control. Combination of disaccharide modules to form trans-uronic acid linkages was completely selective by virtue of C2 participating groups. Coupling reactions between disaccharide modules exhibited sequence dependence. While the union of many glucosamine uronic acid disaccharide modules did not meet any problems, certain sequences proved not accessible. Elaboration of glucosamine uronic acid disaccharide building blocks to trisaccharide modules by addition of either one additional glucosamine or uronic acid allowed for stereoselective access to oligosaccharides as demonstrated on the example of a hexasaccharide resembling the ATIII-binding sequence. Final deprotection and sulfation yielded the fully synthetic heparin oligosaccharides.     

Study of carbohydrate-carbohydrate interactions: total synthesis of 6d-deoxy Lewis pentaosyl glycosphingolipid

This article describes total synthesis of 6d-deoxy Lewis^x pentaosyl glycosphingolipid, a useful tool for study of the Lewis^x-Lewis^x interaction. A 6-deoxy galactose donor was condensed with a diol of glucosamine to provide regioselectively a β 1→4 linked disaccharide, which was further fucosylated to a protected deoxy Lewis^x trisaccharide. Glycosylation of a lactoside diol with the 6d-deoxy Lewis^x trisaccharide gave regio- and stereoselectively a pentasaccharide in excellent yield. After chemical modification, this pentasaccharide reacted with the 3-O-benzoylated azidosphingosine to form a glycosphingolipid, which, after azide reduction followed by condensation with stearic acid and deprotection, afforded the target compound.     

Total synthesis of the aminopropyl functionalized ganglioside GM1

GM1 is a common ganglioside pentasaccharide present on mammalian cell surface.It has been shown to play important roles in cellular communications and initiation of β-amyloid aggregation.In order to synthesize GM1,an efficient synthetic route was developed via a [3+2] strategy.The GM3 trisaccharide acceptor bearing an azido propyl group at the reducing end was prepared using the traditional acetamide protected sialyl thioglycosyl donor,which gave better stereoselectivity than sialyl donors protected with trichloroacetamide or oxazolidinone.The glycosylation of the axial 4-hydroxyl group of GM3 by the disaccharide donor was found to be highly dependent on donor protective groups.Donor bearing the more rigid benzylidene group gave low glycosylation yield.Replacing the benzylidene with acetates led to productive coupling and formation of the fully protected GM1 pentasaccharide.Deprotection of the pentasaccharide produced GM1 functionalized with the aminopropyl side chain,which will be a valuable probe for biological studies.     

Synthesis of the sialyl Lewis X epitope attached to glycolipids with different core structures and their selectin-binding characteristics in a dynamic test system

Sialyl Lewis X (sLeX)/selectin-mediated leukocyte rolling along endothelial cells has recently gained wide interest. In this paper the influence of the spacer length of laterally clustered neoglycolipids 1a-d on cell rolling in a dynamic test system is investigated. The required di-O-hexadecyl glycerols with none, and with three, six, or nine ethylene glycol units as spacer groups (compounds 4a-d) could be readily obtained. The synthesis of 1-O-thexyldimethylsilyl-protected sLeX 24 was based on sialylation of 2,3,4-O-unprotected galactose derivative 11 with sialyl phosphite 8 as donor; this afforded the desired disaccharide 12, which was transformed into trichloroacetimidate 14 as disaccharide donor. Reaction of 3-O-unprotected glucosamine derivative 18 with fucosyl donor 20 afforded disaccharide 21, which was transformed into the 4-O-unprotected derivative 23. Reaction of 14 with 23 furnished the desired tetrasaccharide 24 in good yield. Transformation of 24 into the trichloroacetimidate 26 as donor, followed by the reaction with 4a-d as acceptor gave, after deprotection, the target molecules 1a-d. For comparison, 4d was also connected with a sialyl residue (-->31) and with an N-acetylglucosamine residue (-->34). Compounds 1c and 1d with a hexaethylene glycol and a nonaethylene glycol spacer, respectively, were much more efficient in mediating selectin-dependent cell rolling in the dynamic test system than compounds 1a and 1b, which had no spacer (1a), or only a triethylene glycol spacer (1b).     

Divergent Chemoenzymatic Synthesis of Asymmetrical‐Core‐Fucosylated and Core‐Unmodified N‐Glycans

A divergent chemoenzymaytic approach for the preparation of core‐fucosylated and core‐unmodified asymmetrical N‐glycans from a common advances precursor is described. An undecasaccharide was synthesized by sequential chemical glycosylations of an orthogonally protected core fucosylated hexasaccharide that is common to all mammalian core fucosylated N‐glycans. Antennae‐selective enzymatic extension of the undecasaccharide using a panel of glycosyl transferases afforded core fucosylated asymmetrical triantennary N‐glycan isomers, which are potential biomarkers for breast cancer. A unique aspect of our approach is that a fucosidase (FucA1) has been identified that selectively can cleave a core‐fucoside without affecting the fucoside of a sialyl Lewis^X epitope to give easy access to core‐unmodified compounds.     

Studies on the synthesis of landomycin A: synthesis and glycosidation reactions of L-rhodinosyl acetate derivatives

An efficient, eight-step synthesis of L-rhodinosyl acetate derivative 3 is described. The synthesis originates from methyl (S)-lactate and involves a highly stereoselective, chelate-controlled addition of allyltributylstannane to the lactaldehyde derivative 7. The beta-anomeric configuration of 3 was established with high selectivity by acetylation of the pyranose precursor with Ac(2)O and Et(3)N in CH(2)Cl(2). Preliminary studies of glycosidation reactions of 3 and L-rhodinosyl acetate 10 containing a 3-O-TES ether revealed that these compounds are highly reactive glycosidating agents and that trialkylsilyl triflates are effective glycosylation promoters. The best conditions for reactions with 15 as the acceptor involved use of diethyl ether as the reaction solvent and 0.2 equiv of TES-OTf at -78 degrees C. However, the TES ether protecting group of 10 proved to be too labile under these reaction conditions, and mixtures of 16a, 17, and 18a are obtained in reactions of 10 and 15. Disaccharide 17 arises via in situ cleavage of the TES ether of disaccharide 16a, while trisaccharide 18a results from a glycosidation of in situ generated 17 (or of 16a itself) with a second equivalent of 10. These problems were largely suppressed by using 3 with a 3-O-TBS ether protecting group as the glycosyl donor and 0.2 equiv of TES-OTf as the reaction promoter. Attempts to selectively glycosylate the C(3)-OH of diol acceptors 20 or 28 gave a 70:30 mixture of 21 and 22 in the reaction of 20 and a 43:27:30 mixture of regioisomeric trisaccharides 29 and 30 and tetrasaccharide 31 from the glycosidation reaction of 28. However, excellent results were obtained in the glycosidation of differentially protected disaccharide 34 using 1.5 equiv of 3 and 0.05 equiv of TBS-OTf in CH(2)Cl(2) at -78 degrees C. The latter step is an important transformation in the recently reported synthesis of the landomycin A hexasaccharide unit.     

The 2-naphthylmethyl (NAP) group in carbohydrate synthesis: first total synthesis of the GlyCAM-1 oligosaccharide structures

Total syntheses of the GlyCAM-1 (glycosylation-dependent cell adhesion molecule-1) oligosaccharide structures: [alpha-NeuAc-(2 --> 3)-beta-Gal-(1 --> 4)-[alpha-Fuc-(1 --> 3)]-beta-(6-O-SO3Na)-GlcNAc-(1 --> 6)]-[alpha-NeuAc-(2 --> 3)-beta-Gal-(1 --> 3)]-alpha-GalNAc-OMe (1) and [alpha-NeuAc-(2 --> 3)-beta-Gal-(1 --> 4)-[alpha-Fuc-(1 --> 3)]-beta-GlcNAc-(1 --> 6)]-[alpha-NeuAc-(2 3)-beta-Gal-(1 --> 3)]-alpha-GalNAc-OMe (2) through a novel sialyl LewisX tetrasaccharide donor are described. Employing sequential glycosylation strategy, the starting trisaccharide was regio- and stereoselectively constructed through coupling of a disaccharide imidate with the monosaccharide acceptor phenyl-6-O-naphthylmethyl-2-deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside with TMSOTf as a catalyst without affecting the SPh group. The novel sialyl Lewisx tetrasaccharide donor 3 was then obtained by alpha-L-fucosylation of trisaccharide acceptor with the 2,3,4-tri-O-benzyl-1-thio-beta-L-fucoside donor. The structure of the novel sialyl Lewisx tetrasaccharide was established by a combination of 2D DQF-COSY and 2D ROESY experiments. Target oligosaccharides 1 and 2 were eventually constructed through heptasaccharide which was obtained by regioselective assembly of advanced sialyl Lewisx tetrasaccharide donor 3 and a sialylated trisaccharide acceptor in a predictable and controlled manner. Finally, target heptasaccharides 1 and 2 were fully characterized by 2D DQF-COSY, 2D ROESY, HSQC, HMBC experiments and FAB mass spectroscopy.     

Total synthesis of woodrosin I--part 1: preparation of the building blocks and evaluation of the glycosylation strategy

The preparation of three building blocks required for the total synthesis of woodrosin I (1) is outlined, a complex resin glycoside bearing a macrolide ring which spans four of the five sugars of its oligosaccharide backbone. Key steps involve the enantioselective, titanium-catalyzed addition of dipentylzinc to 5-hexenal, the glycosylation of the resulting alcohol 18 with the glucose-derived trichloroacetimidate 7, and further elaboration of the resulting product 19 into disaccharide 22 on treatment with the orthogonally protected glycosyl donor 15. The trichloroacetimidate method is also used for the formation of the second synthon represented by disaccharide 38. A model study shows that the assembly of the pentasaccharidic perimeter of 1 depends critically on the phasing of the glycosylation events between fragments 22, 38 and the rhamnosyl donor 27 due to the severe steric hindrance in the product. A particularly noteworthy finding is the fact that diol 22 can be regioselectively glycosylated at the 3'-OH group in high yield without protection of the neighboring 2'-OH function.     

First synthesis of 3′-deoxy Lewisx pentasaccharide

The total synthesis of 3′-deoxy Lewis^x pentasaccharide is reported. 4-O-Acetyl-2,6-di-O-benzoyl-3-deoxy-β-d-xylo-hexopyranosyl trichloroacetimidate was condensed with a diol of glucosamine to give a disaccharide, which was further fucosylated to a Lewis^x trisaccharide analogue. Glycosylation of a lactoside diol with this trisaccharide provided a pentasaccharide, which after deprotection, afforded the target pentasaccharide.     

Multi-component one-pot synthesis of the tumor-associated carbohydrate antigen Globo-H based on preactivation of thioglycosyl donors

Two efficient routes for the rapid assembly of the tumor-associated carbohydrate antigen Globo-H hexasaccharide 2 by a preactivation based iterative one-pot strategy are reported. The first method involves the sequential coupling of four glycosyl building blocks, leading to the desired hexasaccharide in 47% overall yield in one-pot synthesis. Although model studies on constructing the challenging Gal-alpha-(1-4)-Gal linkage in Gb3 trisaccharide yielded the desired alpha linkage almost exclusively, a similar approach to assemble the hexasaccharide led to the formation of a significant amount of beta anomer. As an alternative, the second synthesis utilized three components in one pot with the Gal-alpha-(1-4)-Gal linkage preformed, producing the desired hexasaccharide in a similar overall yield as the four component approach. Both methods demonstrate that oligosaccharides containing alpha and beta linkages within the same molecule can be constructed in one pot via a preactivation based approach with higher glyco-assembly efficiencies than the automated solid-phase synthesis strategy. Furthermore, because glycosylations can be carried out independent of anomeric reactivities of donors, it is not necessary to differentiate anomeric reactivities of building blocks through extensive protective group adjustment for chemoselective glycosylation. This confers great flexibilities in the building block design, allowing matching of the donor with the acceptor, leading to improved overall yield.     

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.

     

Convergent synthesis of a common pentasaccharide-repeating unit corresponding to the O-specific polysaccharide of Escherichia coli O4:K3, O4:K6, and O4:K12

A convergent chemical synthesis of a pentasaccharide found in the O-specific polysaccharide of Escherichia coli O4:K3, O4:K6, and O4:K12 has been achieved in excellent yield. A [3+2] block synthetic strategy has been adopted to couple a disaccharide donor 11 with a trisaccharide acceptor 10 for the construction of the pentasaccharide derivative 12 which on deprotection furnished target pentasaccharide 1 as its 4-methoxyphenyl glycoside. Disaccharide thioglycoside donor 11 and trisaccharide acceptor 10 were prepared from suitably protected monosaccharide intermediates. Yields were excellent in all steps.     

Chemical and chemoenzymatic synthesis of glycosyl-amino acids and glycopeptides related to Trypanosoma cruzi mucins

This study describes the synthesis of the alpha- and beta-linked N-acetyllactosamine (Galp-beta-1,4-GlcNAc; LacNAc) glycosides of threonine (LacNAc-Thr). LacNAc-a-Thr was prepared by direct chemical coupling of a 2-azido-2-deoxy-lactose disaccharide donor to a suitable partially protected threonine unit. In contrast, stepwise chemical generation of beta-linked N-acetylglucosamine followed by enzymatic galactosylation to give LacNAc-beta-Thr proved effective, whereas use of a 2-azido-2-deoxy-lactose donor in acetonitrile failed to give the desired beta-linked disaccharyl glycoside. This study illustrates that it is possible to overcome the inherent stereoselection for 1,2-trans chemical glycosylation with a GlcNAc donor, and that the well-established preference of bovine beta-1,4-galactosyltransferase for beta-linked acceptor substrates can also be overcome. Using this knowledge, short glycopeptide fragments based on T. cruzi mucin sequences, Thr-Thr-[LacNAcThr]-Thr-Thr-Gly, were synthesised. All LacNAc-based compounds outlined were shown to serve as acceptor substrates for sialylation by T. cruzi trans-sialidase.     

Synthesis of a Fucosylated Trisaccharide Via Transglycosylation by α-<Emphasis Type="SmallCaps">l</Emphasis>-Fucosidase from <Emphasis Type="Italic">Thermotoga maritima</Emphasis>

Fucosylated oligosaccharides, such as 2′-fucosyllactose in human milk, have important biological functions such as prebiotics and preventing infection. In this work, the effect of an acceptor substrate (lactose) and the donor substrate 4-nitrophenyl-α-l-fucopyranoside (pNP-Fuc) on the synthesis of a fucosylated trisaccharide was studied in a transglycosylation reaction using α-l-fucosidase from Thermotoga maritima. Conducting a matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), it was demonstrated that synthesized oligosaccharide corresponded to a fucosylated trisaccharide, and high-performance liquid chromatography (HPLC) of the hydrolyzed compound confirmed it was fucosyllactose. As the concentration of the acceptor substrate increased, the concentration and synthesis rate of the fucosylated trisaccharide also increased, and the highest concentration obtained was 0.883 mM (25.2% yield) when using the higher initial lactose concentration (584 mM). Furthermore, the lower donor/acceptor ratio had the highest synthesis, so at the molar ratio of 0.001, a concentration of 0.286 mM was obtained (32.5% yield).     

A stereochemical surprise at the late stage of the synthesis of fully N-differentiated heparin oligosaccharides containing amino, acetamido, and N-sulfonate groups

The glucosamine residues in heparin-like glycosaminoglycans have been found to exist as amines, acetamides, and N-sulfonates. To develop a completely general, modular synthesis of heparin, three degrees of orthogonal nitrogen protection are required. Reported herein is a strategy for the synthesis of fully N-differentiated heparin oligosaccharides in the context of target octasaccharide 1, which contains an N-acetate, N-sulfonates, and a free amine. The protecting group scheme used in the synthesis blocked the N-acetate as a N-diacetate, the N-sulfonates as azido groups, and the amine as a N-CBz; free hydroxyls were masked as benzyl ethers and O-sulfonates as acetate esters. Disaccharide and tetrasaccharide modules were synthesized using this strategy; however, the union of tetrasaccharide trichloroacetimidate 4 with disaccharide acceptor 5 unexpectedly formed the undesired beta-linked glycoside in addition to the alpha-linkage anticipated for iduronic acid nucleophiles, resulting in an inseparable 6:1 alpha/beta mixture of products. Detailed studies into the basis for this unexpected result were conducted and are also reported.     

Fucosyltransferases as synthetic tools: glycan array based substrate selection and core fucosylation of synthetic N-glycans

Two recombinant fucosyltransferases were employed as synthetic tools in the chemoenzymatic synthesis of core fucosylated N-glycan structures. Enzyme substrates were rapidly identified by incubating a microarray of synthetic N-glycans with the transferases and detecting the presence of core fucose with four lectins and one antibody. Selected substrates were then enzymatically fucosylated in solution on a preparative scale and characterized by NMR and MS. With this approach the chemoenzymatic synthesis of a series of α1,3-, α1,6-, and difucosylated structures was accomplished in very short time and with high yields, which otherwise would have required extensive additional synthetic effort and a complete redesign of existing synthetic routes. In addition, valuable information was gathered regarding the specificities of the lectins employed in this study.     

Stereoselective Synthesis of the Hexasaccharide Repeating Unit of the Cell Wall Polysaccharide from Kineosporia aurantiaca VKM Ac-720T Employing the Direct Glycosylation with Anomeric Hydroxy Sugars Involving Glycosyl Phthalate Intermediates

Synthesis of a suitably protected form of the hexasaccharide repeating unit of the cell wall polymer from Kineosporia aurantiaca VKM Ac-720 ^T has been achieved by the stereoselective direct glycosylation of a trisaccharide acceptor with a trisaccharide donor having an anomeric hydroxy group involving a glycosyl phthalate intermediate. Both the trisaccharide acceptor and the trisaccharide donor were obtained from a common trisaccharide, of which two β-mannopyranosyl linkages were constructed stereoselectively by employing the direct glycosylation method with the anomeric hydroxy sugar involving a glycosyl phthalate intermediate and the 2′-carboxybenzyl glycoside method, respectively.     

Synthesis of Monomeric and Dimeric Repeating Units of the Zwitterionic Type 1 Capsular Polysaccharide from Streptococcus pneumoniae

Zwitterionic polysaccharides (ZPSs) from Bacteroides fragilis and Streptococcus pneumoniae display unique T‐cell activities. The first synthesis of a hexasaccharide representing two repeating units of the zwitterionic capsular polysaccharide from S. pneumoniae type 1 (Sp1) is reported. Key elements of the approach are stereoselective construction of 1,4‐cis‐α‐galactose linkages based on a reactive trichloroacetimidate donor that incorporates a 6‐O‐acetyl group, which may contribute to the high α selectivity in glycosylation. After assembly of the fully protected hexasaccharide from five monosaccharide synthons 2 – 4 , 24 and 25 , selective deprotection of the primary hydroxyl groups of the four galactose residues followed by oxidation to the corresponding uronic acids provides hexasaccharide 19 . The trisaccharide counterpart 1 was synthesized in similar fashion from three synthons, 2 – 4 . This approach employed both conventional and dehydrative glycosylation methodologies and avoids the use of poorly reactive uronic acid derived glycosyl donors and acceptors.     

Synthesis of alpha-D-idoseptanosyl glycosides using an S-phenyl septanoside donor

[reaction: see text] An S-phenyl alpha-D-idoseptanoside donor was used in the selective preparation of a series of alpha-D-idoseptanosyl glycosides. Glycosylation of a methyl beta-D-glycero-D-guloseptanoside acceptor with the new donor constituted the first synthesis of a septanose disaccharide.