Synthesis of ld-Hepp and KDO containing di- and tetrasaccharide derivatives of Neisseria meningitidis inner-core region via iodonium ion promoted glycosidations

Stereocontrolled Synthesis of α-3-Deoxy-d-manno-oct-2-ulosonic Acid (α-Kdo) Glycosides Using C3-p-Tolylthio-Substituted Kdo Donors: Access to Highly Branched Kdo Oligosaccharides

3-Deoxy-d-manno-oct-2-ulosonic acid (Kdo) is an eight-carbon monosaccharide found widely in bacterial lipopolysaccharides (LPSs) and capsule polysaccharides (CPSs). We developed an indirect method for the stereoselective synthesis of α-Kdo glycosides with a C3-p-tolylthio-substituted Kdo phosphite donor. The presence of the p-tolylthio group enhanced the reactivity, suppressed the formation of elimination by-products (2,3-enes), and provided complete α-stereocontrol. A variety of Kdo α-glycosides were synthesized by our method in excellent yields (up to 98 %). After glycosylation, the p-tolylthio group can be efficiently removed by free-radical reduction. Subsequently, the orthogonality of the phosphite donor and thioglycoside donor was demonstrated by the one-pot synthesis of a trisaccharide in Helicobacter pylori and Neisseria meningitidis LPS. Moreover, an efficient total synthesis route to the challenging 4,5-branched Kdo trisaccharide in LPSs from several A. baumannii strains was highlighted. To demonstrate the high reactivity of our approach further, the highly crowded 4,5,7,8-branched Kdo pentasaccharide was synthesized as a model molecule for the first time. Additionally, the reaction mechanism was investigated by DFT calculations.

Chemoenzymatic Total Synthesis of GM3 Gangliosides Containing Different Sialic Acid Forms and Various Fatty Acyl Chains

Gangliosides are sialic acid-containing glycosphingolipids that have been found in the cell membranes of all vertebrates. Their important biological functions are contributed by both the glycan and the ceramide lipid components. GM3 is a major ganglioside and a precursor for many other more complex gangliosides. To obtain structurally diverse GM3 gangliosides containing various sialic acid forms and different fatty acyl chains in low cost, an improved process was developed to chemically synthesize lactosyl sphingosine from an inexpensive l-serine derivative. It was then used to obtain GM3 sphingosines from diverse modified sialic acid precursors by an efficient one-pot multienzyme sialylation system containing Pasteurella multocida sialyltransferase 3 (PmST3) with in situ generation of sugar nucleotides. A highly effective chemical acylation and facile C18-cartridge purification process was then used to install fatty acyl chains of varying lengths and different modifications. The chemoenzymatic method represents a powerful total synthetic strategy to access a library of structurally defined GM3 gangliosides to explore their functions.

Stereocontrolled Synthesis of the Equatorial Glycosides of 3-Deoxy-d-manno-oct-2-ulosonic Acid: Role of Side Chain Conformation

The pseudosymmetric relationship of the bacterial sialic acid, pseudaminic acid, and 3-deoxy-d-manno-oct-2-ulosonic acid (KDO) affords the hypothesis that suitably protected KDO donors will adopt the trans, gauche conformation of their side chain and consequently be highly equatorially selective in their coupling reactions conducted at low temperature. This hypothesis is borne out by the synthesis, conformational analysis, and excellent equatorial selectivity seen on coupling of per-O-acetyl or benzyl-protected KDO donors in dichloromethane at -78 °C. Mechanistic understanding of glycosylation reactions is advancing to a stage at which predictions of selectivity can be made. In this instance, predictions of selectivity provide the first highly selective entry into KDO equatorial glycosides such as are found in the capsular polysaccharides of numerous pathogenic bacteria.

Synthesis of the β-linked GalNAc-Kdo disaccharide antigen of the capsular polysaccharide of Kingella kingae KK01

The first construction of the challenging β-(1 → 5)-linked GalNAc-Kdo skeleton is described for the synthesis of the disaccharide antigen of the capsular polysaccharide of Kingella kingae KK01. TfOH-catalyzed glycosylation of N-Troc-protected d-galactosaminyl N-phenyl trifluoroacetimidate with a sterically hindered 5-hydroxyl group of the β-Kdo building block in toluene proceeded smoothly to provide the desired disaccharide in excellent yield with satisfactory β-selectivity. An optimal sequence for the deprotection of the disaccharide skeleton was found to access the disaccharide antigen of Kingella kingae KK01 for further immunological studies.

Divergent Synthesis of 3-Deoxy-d-manno-oct-2-ulosonic Acid (Kdo) Glycosides Containing α-(2→4)-Linked Kdo-Kdo Unit

A convenient and divergent approach was developed to prepare diverse bacterial 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) oligosaccharides containing a Kdo-α-(2→4)-Kdo fragment. The orthogonal protected α-(2→4) linked Kdo-Kdo disaccharide 3, serving as a common precursor, was divergently transformed into the corresponding 8-, 8'-, and 4'-hydroxy disaccharides 5, 7, and 14, respectively. Then, these alcohols were glycosylated, respectively, with the 5,7-O-di-tert-butylsilylene (DTBS) protected Kdo thioglycoside donors 1 or 2 in an α-stereoselective and high-yielding manner to afford a range of Kdo oligosaccharides. Finally, removal of all protecting groups of the newly formed glycosides resulted in the desired free Kdo oligomer.

Progress in Kdo-glycoside chemistry

Glycosylation chemistry of 3-deoxy-D-manno-oct-2-ulosonic acid units has been considerably developed within the last decade. This review covers major achievements with respect to improved yields and anomeric selectivity as well as suppression of the elimination side reaction via selection of dedicated protecting groups and appropriate activation of the anomeric center.

Scope and Limitations of 3-Iodo-Kdo Fluoride-Based Glycosylation Chemistry using N-Acetyl Glucosamine Acceptors

The ketosidic linkage of 3-deoxy-d-manno-octulosonic acid (Kdo) to lipid A constitutes a general structural feature of the bacterial lipopolysaccharide core. Glycosylation reactions of Kdo donors, however, are challenging due to the absence of a directing group at C-3 and elimination reactions resulting in low yields and anomeric selectivities of the glycosides. While 3-iodo-Kdo fluoride donors showed excellent glycosyl donor properties for the assembly of Kdo oligomers, glycosylation of N-acetyl-glucosamine derivatives was not straightforward. Specifically, oxazoline formation of a β-anomeric methyl glycoside, as well as iodonium ion transfer to an allylic aglycon was found. In addition, dehalogenation of the directing group by hydrogen atom transfer proved to be incompatible with free hydroxyl groups next to benzyl groups. In contrast, glycosylation of a suitably protected methyl 2-acetamido-2-deoxy-α-d-glucopyranoside derivative and subsequent deiodination proceeded in excellent yields and α-specificity, and allowed for subsequent 4-O-phosphorylation. This way, the disaccharides α-Kdo-(2→6)-α-GlcNAcOMe and α-Kdo-(2→6)-α-GlcNAcOMe-4-phosphate were obtained in good overall yields.

Synthesis of chlamydia lipopolysaccharide haptens through the use of α-specific 3-iodo-Kdo fluoride glycosyl donors

A scalable approach towards high-yielding and (stereo)selective glycosyl donors of the 2-ulosonic acid Kdo (3-deoxy-D-manno-oct-2-ulosonic acid) is a fundamental requirement for the development of vaccines against Gram-negative bacteria. Herein, we disclose a short synthetic route to 3-iodo Kdo fluoride donors from Kdo glycal esters that enable efficient α-specific glycosylations and significantly suppress the elimination side reaction. The potency of these donors is demonstrated in a straightforward, six-step synthesis of a branched Chlamydia-related Kdo-trisaccharide ligand without the need for protecting groups at the Kdo glycosyl acceptor. The approach was further extended to include sequential iteration of the basic concept to produce the linear Chlamydia-specific α-Kdo-(2→8)-α-Kdo-(2→4)-α-Kdo trisaccharide in a good overall yield.

First and stereoselective synthesis of an α-(2→5)-linked disaccharide of 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo)

Resistance of bacterial pathogens toward antibiotics has revived interest in lipopolysaccharide (LPS) motifs as potential therapeutic targets. The LPS of several pathogenic Acinetobacter strains comprises a 4,5-branched Kdo trisaccharide containing an uncommon (2→5)-linkage. In this contribution the first stereoselective glycosylation method for obtaining an α-Kdo-(2→5)-α-Kdo disaccharide in good yield is highlighted. The synthetic approach used for accessing this linkage type will allow for future studies of the immunoreactivity associated with this unique bacterial Kdo inner core structure.

Preparation of a protected 3-deoxy-D-manno-oct-2-ulosonate glycal donor for the synthesis of β-KDO-containing oligosaccharides

A practical method for the synthesis of KDO glycal donors was developed. The prepared KDO donors exhibited excellent disastereoselectivity of glycosylation in a CH2Cl2-CH3CN solvent mixture, which was found to be associated with the isopropylidene protection at the C-4 and C-5 hydroxyls. The synthetic use of the KDO donor was demonstrated in the preparation of β-KDO-containing oligosaccharides.

Expanded potential of seleno-carbohydrates as a molecular tool for X-ray structural determination of a carbohydrate-protein complex with single/multi-wavelength anomalous dispersion phasing

Seleno-lactoses have been successfully synthesized as candidates for mimicking carbohydrate ligands for human galectin-9 N-terminal carbohydrate recognition domain (NCRD). Selenium was introduced into the mono- or di-saccharides using p-methylselenobenzoic anhydride (Tol2Se) as a novel selenating reagent. The TolSe-substituted monosaccharides were converted into selenoglycosyl donors or acceptors, which were reacted with coupling partners to afford seleno-lactoses. The seleno-lactoses were converted to the target compounds. The structure of human galectin-9 NCRD co-crystallized with 6-MeSe-lactose was determined with single/multi-wavelength anomalous dispersion (SAD/MAD) phasing and was similar to that of the co-crystal with natural lactose.

Chemical synthesis and immunological evaluation of the inner core oligosaccharide of Francisella tularensis

Francisella tularensis, which is a Gram negative bacterium that causes tularemia, has been classified by the Center for Disease Control and Prevention (CDC) as a category A bioweapon. The development of vaccines, immunotherapeutics, and diagnostics for F. tularensis requires a detailed knowledge of the saccharide structures that can be recognized by protective antibodies. We have synthesized the inner core region of the lipopolysaccharide (LPS) of F. tularensis to probe antigenic responses elicited by a live and subunit vaccine. The successful preparation of the target compound relied on the use of a disaccharide which was modified by the orthogonal protecting groups diethylisopropylsilyl (DEIPS), 2-naphthylmethyl (Nap), allyl ether (All), and levulinoyl (Lev) ester. The ability to remove the protecting groups in different orders made it possible to establish the optimal glycosylations sequence to prepare a highly crowded 1,2,3-cis configured branching point. A variety of different methods were exploited to control anomeric selectivities of the glycosylations. A comparison of the (1)H NMR spectra of isolated material and the synthetic derivative confirmed the reported structural assignment of the inner core oligosaccharide of F. tularensis . The observation that immunizations with LPS lead to antibody responses to the inner core saccharides provides an impetus to further explore this compound as a vaccine candidate.

Expeditious chemoenzymatic synthesis of homogeneous N-glycoproteins carrying defined oligosaccharide ligands

An efficient chemoenzymatic method for the construction of homogeneous N-glycoproteins was described that explores the transglycosylation activity of the endo-beta-N-acetylglucosaminidase from Arthrobacter protophormiae (Endo-A) with synthetic sugar oxazolines as the donor substrates. First, an array of large oligosaccharide oxazolines were synthesized and evaluated as substrates for the Endo-A-catalyzed transglycosylation by use of ribonuclease B as a model system. The experimental results showed that Endo-A could tolerate modifications at the outer mannose residues of the Man3GlcNAc-oxazoline core, thus allowing introduction of large oligosaccharide ligands into a protein and meanwhile preserving the natural, core N-pentasaccharide (Man3GlcNAc2) structure in the resulting glycoprotein upon transglycosylation. In addition to ligands for galectins and mannose-binding lectins, azido functionality could be readily introduced at the N-pentasaccharide (Man3GlcNAc2) core by use of azido-containing Man3GlcNAc oxazoline as the donor substrate. The introduction of azido functionality permits further site-specific modifications of the resulting glycoproteins, as demonstrated by the successful attachment of two copies of alphaGal epitopes to ribonuclease B. This study reveals a broad substrate specificity of Endo-A for transglycosylation, and the chemoenzymatic method described here points to a new avenue for quick access to various homogeneous N-glycoproteins for structure-activity relationship studies and for biomedical applications.

Evaluation of thioglycosides of Kdo as glycosyl donors

The use of Kdo thioglycosides as glycosyl donors using DMTST, IBr/AgOTf and NIS/AgOTf as promoters has been evaluated. Activation at low temperature allowed to escape the formation of 2,3-glycal byproducts to give glycosides in high yield and with good beta-anomeric selectivity. The use of diethyl ether as solvent and (especially) isopropylidene acetals as protecting groups improved the alpha-anomeric selectivity. NIS/AgOTf as promoter surprisingly yielded the 3-iodo-product via the glycal intermediate.

Anomeric O-acylation of Kdo using alkyl and aryl isocyanates

To develop a convenient method for the preparation of an alpha-Kdo derivative carrying a functional spacer at the reducing end, we examined anomeric O-acylation using Kdo and halogenated alkyl/aryl isocyanates as nucleophile and electrophiles, respectively. Reaction of a Kdo derivative with 2-chloroethyl isocyanate in the presence of DMAP gave an alpha-spiro product (82%) and an alpha-Kdo derivative of a dimeric isocyanate adduct (10%). Similar reaction with 4-(chloromethyl)phenyl isocyanate gave only the corresponding alpha-spiro product (81%). The NMR data show that the pyranose rings of both the alkyl and aryl spiro products adopt the 5C2 conformation. Thus, we accomplished alpha-selective anomeric O-acylation by coupling the Kdo derivative with alkyl and aryl isocyanates.

Synthesis of bidesmosidic dihydrodiosgenin saponins bearing a 3-O-β-chacotriosyl moiety

3-O-beta-Chacotriosyl-26-O-beta-D-glucopyranosyl-(25R)-furost-5-en (1), a mimic of the antitumor active proto-dioscin, was concisely synthesized from diosgenin in a linear nine steps and in 17% overall yield. Its congeners with a alpha-l-rhamnopyranosyl, beta-lactosyl, or without a substituent at the 26-OH (13-15) were also prepared. Compound 1, as well as 13-15, did not show any inhibition against tumor cells, implying that proto-dioscin might be also inactive, but readily converted into the antitumor active dioscin.

Synthesis of dimeric lactose and dimeric (sialyl) Lewis(X) glycolipids

To investigate structural requirements for the homophilic interaction between carbohydrates on planar model membranes, divalent derivatives with enforced proximity between the two carbohydrate epitopes (lactose, Lewis(X), and sialyl Lewis(X)) were synthesized by use of a dimeric membrane anchor as scaffold.

Chemoenzymatic synthesis of a trimeric ganglioside GM3 analogue

A trimeric beta-lactosyl cluster based on 2-nitro-2-(hydroxymethyl)propane-1,3-diol was prepared using the trichloroacetimidate method. Kinetic studies showed that this cluster was an effective acceptor for rat-liver alpha-(2-->3)-sialyltransferase. Its KM was comparable to those for monomeric lactose and N-acetyllactosamine acceptors, and its Vmax was 1% of that measured for the LacNAc acceptor. Preparative-scale sialylation using this enzyme afforded a trimeric cluster of the ganglioside GM3 oligosaccharide in good yield. The NMR spectra of the trimeric GM3 analogue suggest that the oligosaccharide conformation is not significantly perturbed by this level of clustering.

Synthesis of L-glycero-D-manno-heptopyranose-containing oligosaccharide structures found in lipopolysaccharides from Haemophilus influenzae

Syntheses are described of the tetrasaccharide 2-(4-trifluoroacetamidophenyl)ethyl O-(beta-D-galactopyranosyl)-(1-->2)-O- (L-glycero-alpha-D-manno-heptopyranosyl)-(1-->2)-O-(L-glycero-alpha-D- manno-heptopyranosyl)-(1-->3)-L-glycero-alpha-D-manno-heptopyranoside (20) and the three trisaccharides 2-(4-trifluoroacetamidophenyl)ethyl O-(L-glycero-alpha-D-manno-heptopyranosyl)-(1-->2)-O-(L-glycero-alpha-D- manno-heptopyranosyl)-(1-->3)-L-glycero-alpha-D-manno-heptopyranoside (17), 2-(4-trifluoroacetamidophenyl)ethyl O-(beta-D-glucopyranosyl)-(1-->4)- O-(beta-D-glucopyranosyl)-(1-->4)-L-glycero-alpha-D-manno-heptopyrano side (5), and 2-(4-trifluoro-acetamidophenyl)ethyl O-(beta-D-galactopyranosyl)-(1-->4)-O-(beta-D-glucopyranosyl)-(1-->4)- L-glycero-alpha-D-manno-heptopyranoside (8), corresponding to structures found in the lipooligosaccharides of Haemophilus influenzae.

Synthesis of 2-(4-aminophenyl)ethyl 3-deoxy-5-O-(3,4,6-tri-O-beta-D- glucopyranosyl-alpha-D-glucopyranosyl)-alpha-D-manno-oct-2-ulopyrano sid onic acid, a highly branched pentasaccharide corresponding to structures found in lipopolysaccharides from Moraxella catarrhalis

Syntheses of the pentasaccharide 2-(4-aminophenyl)ethyl 3-deoxy-5-O-(3,4,6- tri-O-beta-D-glucopyranosyl-alpha-D-glucopyranosyl)-alpha-D-manno-oct-2- ulopyranosidonic acid and of the tetrasaccharide 3,4,6-tri-O-beta-D-glucopyranosyl-alpha-D-glucopyranoside, both as its methyl and 2-(4-trifluoro-acetamidophenyl)ethyl glycoside, are described. These oligosaccharides correspond to structures found in the lipopolysaccharide of Moraxella catarrhalis and were needed for biological experiments aimed at producing antibodies against the bacteria. The best way to introduce the glucopyranosyl groups into the 3-, 4-, and 6-positions of the branched target compounds was found to be a one-step reaction using a 3,4,6-triol as acceptor and 2,3,4,6-tetra-O-benzoyl-D-glucopyranosyl bromide as donor in a silver trifluoromethanesulfonate-promoted coupling. The spacer arm, necessary for the formation of immunoactive glycoconjugates, was introduced into the glucose moiety via a dimethyl(methylthio)sulfonium trifluoromethanesulfonate-promoted reaction using the ethyl thioglucoside as donor, whereas for Kdo, the acetylated glycal derivative, methyl 4,5,7,8-tetra-O-acetyl-2,6-anhydro-3-deoxy-D-manno-oct-2-enonate, was used as donor and phenylselenyl trifluoromethanesulfonate as a stereocontrolling promoter.

Synthesis and NMR spectroscopic investigation of oligosaccharides containing Kdo and L-glycero-D-manno-heptopyranosyl residues

The disaccharides O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->8)-sodium (allyl 3-deoxy-beta-D-manno-2-octulopyranosid)onate (8), O-L-glycero-alpha-D-manno-heptopyranosyl-(1-->7)-sodium (allyl 3-deoxy-beta-D-manno-2-octulopyranosid)onate (12), and O-alpha-D-mannopyranosyl-(1-->7)-sodium (allyl 3-deoxy-beta-D-manno-2-octulopyranosid)onate (21) and the branched trisaccharides O-L-glycero-alpha-D-manno-heptopyranosyl-(1-->7)-[O-(sodium 3-deoxy-alpha- and -beta-D-manno-2-octulopyranosylonate)-(2-->8)]-sodium (allyl 3-deoxy-beta-D-manno-2-octulopyranosid)onate (15 and 16) and O-alpha-D-mannopyranosyl-(1-->7)-[O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->8)]-sodium (allyl 3-deoxy-beta-D-manno-2-octulopyranosid)onate (24) were prepared. Per-O-acetylated mannopyranosyl or Kdo bromide derivatives were employed for the glycosylation steps under Helferich conditions, whereas the imidate derivative 9 was used for the coupling of the L-glycero-D-manno-heptopyranosyl residues. The oligosaccharides were fully characterized by NMR spectroscopic data. Their structures correspond to an artificial linkage pattern providing a potential cross-reactive epitope for antibodies directed against the inner-core-region of enterobacterial as well as chlamydial lipopolysaccharides.