Glycosyl trifluoroacetimidates. 2. Synthesis of dioscin and xiebai saponin I

Chemical approaches for the stereocontrolled synthesis of 1,2-cis-β-D-rhamnosides

In carbohydrate chemistry, the stereoselective synthesis of 1,2-cis-glycosides remains a formidable challenge. This complexity is comparable to the synthesis of 1,2-cis-β-D-mannosides, primarily due to the adverse anomeric and Δ-2 effects. Over the past decades, to attain β-stereoselectivity in D-rhamnosylation, researchers have devised numerous direct and indirect methodologies, including the hydrogen-bond-mediated aglycone delivery (HAD) method, the synthesis of β-D-mannoside paired with C6 deoxygenation, and the combined approach of 1,2-trans-glycosylation and C2 epimerization. This review elaborates on the advancements in β-D-rhamnosylation and its implications for the total synthesis of tiacumicin B and other physiologically relevant glycans.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Pseudo-glycoconjugates with a C-glycoside linkage

Work by the author and colleagues has been focused on the development of pseudo-glycans (pseudo-glycoconjugates), in which the O-glycosidic linkage of the natural-type glycan structure is replaced by a C-glycosidic linkage. These analogs are not degraded by cellular glycoside hydrolases and are thus expected to be useful molecular tools that may maintain the original biological activity for a long period in the cell. However, their biological potential is not yet well understood because only a few pseudo glycans have so far been synthesized. This article aims to provide a bird's-eye view of our recent studies on the creation of C-glycoside analogs of ganglioside GM3 based on the CHF-sialoside linkage, and summarizes the chemical insights acquired during our stereoselective synthesis of the C-sialoside bond, ultimately leading to pseudo-GM3. Conformational analysis of the synthesized CHF-sialoside disaccharides confirmed that the anticipated conformational control by F-atom introduction was successful, and furthermore, enhanced the biological activity. In order to improve access to C-glycoside analogs based on pseudo-GM3, it is still important to streamline the synthesis process. With this in mind, we designed and developed a direct C-glycosylation method using atom-transfer radical coupling, and employed it in syntheses of pseudo-isomaltose and pseudo-KRN7000.

Chemical Synthesis of Saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Chemical synthesis of saponins: An update

Saponins, as secondary metabolites in terrestrial plants and marine invertebrate, constitute one of the largest families of natural products. The long history of folk medicinal applications of saponins makes them attractive candidates for innovative drug design and development. Chemical synthesis has become a practical alternative to the availability of the natural saponins and their modified analogs, so as to facilitate SAR studies and the discovery of optimal structures for clinical applications. The recent achievements in the synthesis of these complex saponins reflect the advancements of both steroid/triterpene chemistry and carbohydrate chemistry. This chapter provides an updated review on the chemical synthesis of natural saponins, covering the literature from 2014 to 2020.

A Stereoselective Glycosylation Approach to the Construction of 1,2-trans-β-d-Glycosidic Linkages and Convergent Synthesis of Saponins

Conventional syntheses of 1,2-trans-β-d- or α-l-glycosidic linkages rely mainly on neighboring group participation in the glycosylation reactions. The requirement for a neighboring participation group (NPG) excludes direct glycosylation with (1→2)-linked glycan donors, thus only allowing stepwise assembly of glycans and glycoconjugates containing this type of common motif. Here, a robust glycosylation protocol for the synthesis of 1,2-trans-β-d- or α-l-glycosidic linkages without resorting to NPG is disclosed; it employs an optimal combination of glycosyl N-phenyltrifluroacetimidates as donors, FeCl₃ as promoter, and CH₂ Cl₂ /nitrile as solvent. A broad substrate scope has been demonstrated by glycosylations with 12 (1→2)-linked di- and trisaccharide donors and 13 alcoholic acceptors including eight complex triterpene derivatives. Most of the glycosylation reactions are high yielding and exclusively 1,2-trans selective. Ten representative, naturally occurring triterpene saponins were thus synthesized in a convergent manner after deprotection of the coupled glycosides. Intensive mechanistic studies indicated that this glycosylation proceeds by S_N 2-type substitution of the glycosyl α-nitrilium intermediates. Importantly, FeCl₃ dissociates and coordinates with nitrile into [Fe(RCN)_n Cl₂ ]⁺ and [FeCl₄ ]^- , and the ferric cationic species coordinates with the alcoholic acceptor to provide a protic species that activates the imidate, meanwhile the poor nucleophilicity of [FeCl₄ ]^- ensures an uninterruptive role for the glycosidation.

Chemoenzymatic Synthesis of Complex N-Glycans of the Parasite S. mansoni to Examine the Importance of Epitope Presentation on DC-SIGN recognition

The importance of multivalency for N-glycan-protein interactions has primarily been studied by attachment of minimal epitopes to artificial multivalent scaffold and not in the context of multi-antennary glycans. N-glycans can be modified by bisecting GlcNAc, core xylosides and fucosides, and extended N-acetyl lactosamine moieties. The impact of such modifications on glycan recognition are also not well understood. We describe here a chemoenzymatic methodology that can provide N-glycans expressed by the parasitic worm S. mansoni having unique epitopes at each antenna and containing core xyloside. NMR, computational and electron microscopy were employed to investigate recognition of the glycans by the human lectin DC-SIGN. It revealed that core xyloside does not influence terminal epitope recognition. The multi-antennary glycans bound with higher affinity to DC-SIGN compared to mono-valent counterparts, which was attributed to proximity-induced effective concentration. The multi-antennary glycans cross-linked DC-SIGN into a dense network, which likely is relevant for antigen uptake and intracellular routing.

C-glycosylated pyrroles and their application in dipyrromethane and porphyrin synthesis

Pyrrole C-glycosylated in either the 2- or the 3-position could be prepared by the acid-catalyzed reaction between trichloroacetimidate glycosyl donors and pyrrole, or [Formula: see text]-phenyl-tri?uoroacetimidate glucosyl donor and [Formula: see text]-TIPS pyrrole, respectively. Pyrroles carrying glucose, mannose, galactose and lactose in the 2-position, and glucose in the 3-position were obtained. The configurations of the products could be assigned using a combination of 1D and 2D NMR spectroscopy. A number of undesired background reactions yielding a variety of stereo- and regioisomers were identified; in several cases these could be eliminated. Glycosylpyrroles could be incorporated into mono- and diglycosylated dipyrromethanes, a diglycosylated BODIPY dye, and a monoglycosylated Zn(II) porphyrin without damaging the sugar unit.

Exploratory N-Protecting Group Manipulation for the Total Synthesis of Zwitterionic Shigella sonnei Oligosaccharides

Shigella sonnei surface polysaccharides are well-established protective antigens against this major cause of diarrhoeal disease. They also qualify as unique zwitterionic polysaccharides (ZPSs) featuring a disaccharide repeating unit made of two 1,2-trans linked rare aminodeoxy sugars, a 2-acetamido-2-deoxy-l-altruronic acid (l-AltpNAcA) and a 2-acetamido-4-amino-2,4,6-trideoxy-d-galactopyranose (AAT). Herein, the stereoselective synthesis of S. sonnei oligosaccharides comprising two, three and four repeating units is reported for the first time. Several sets of up to seven protecting groups were explored, shedding light on the singular conformational behavior of protected altrosamine and altruronic residues. A disaccharide building block equipped with three distinct N-protecting groups and featuring the uronate moiety already in place was designed to accomplish the iterative high yielding glycosylation at the axial 4-OH of the altruronate component and achieve the challenging full deprotection step. Key to the successful route was the use of a diacetyl strategy whereby the N-acetamido group of the l-AltpNAcA is masked in the form of an imide.

Regioselective benzoylation of unprotected β-glycopyranosides with benzoyl cyanide and an amine catalyst – application to saponin synthesis

Regioselective protection of trans-trans triol and tetrol moieties in carbohydrates was achieved with BzCN as the benzoylating agent and amine catalysts. The protocols are useful for the chemical synthesis of oligosaccharides and saponins.

Chemical Synthesis of Fucosylated Chondroitin Sulfate Oligosaccharides

Fucosylated chondroitin sulfates (FuCSs) are a unique type of polysaccharides occurring in sea cucumber that show a variety of biological activities. In particular, well-defined FuCS oligosaccharides, consisting of a trisaccharide repeating unit of β-d-GalNAc(4,6-diS)-(1→4)-[α-l-Fuc(2,4-diS)-(1→3)]-β-d-GlcUA, display potent anticoagulant activity via selective inhibition of the intrinsic tenase, which could be developed into anticoagulant drugs without bleeding risk. Herein, we report an effective approach to the synthesis of FuCS oligosaccharides, as demonstrated by the successful elaboration of FuCS tri-, hexa-, and nonasaccharides. The syntheses employ an orthogonally protected trisaccharide as a pivotal building block that can be readily converted into the donor and acceptor for glycosidic coupling. In addition, the internal patterns of protecting groups, involving N-trichloroacetyl for N-acetyl group, benzylidene and benzyl groups for sulfonated hydroxyl groups, and benzoyl and methyl esters for free hydroxyl and carboxylic acid, respectively, ensure stereoselective formation of the glycosidic linkages and sequential transformation into the desired FuCS oligosaccharides.

Synthesis of monophosphoryl lipid A using 2-naphtylmethyl ethers as permanent protecting groups

Lipid A, which is a conserved component of lipopolysaccharides of gram-negative bacteria, has attracted considerable interest for the development of immuno-adjuvants. Most approaches for lipid A synthesis rely on the use of benzyl ethers as permanent protecting groups. Due to the amphiphilic character of lipid A, these compounds aggregate during the hydrogenation step to remove benzyl ethers, resulting in a sluggish reaction and by-product formation. To address this problem, we have developed a synthetic approach based on the use of 2-naphtylmethyl ether (Nap) ethers as permanent protecting group for hydroxyls. At the end of a synthetic sequence, multiple of these protecting groups can readily be removed by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Di-allyl N,N-diisopropylphosphoramidite was employed to install the phosphate ester and the resulting allyl esters were cleaved using palladium tetrakistriphenylphosphine. The synthetic strategy allows late stage introduction of different fatty acids at the amines of the target compound, which is facilitated by Troc and Fmoc as orthogonal amino-protecting groups.

α-Selective glycosylations using glycosyl N-(ortho-methoxyphenyl)trifluoroacetimidates

Six N-(o-methoxyphenyl)trifluoroacetimidate glycosyl donors have been synthesized and their role as glycosyl donors has been investigated. The donors were synthesized with complete β-selectivity, except in one case, and were found to be stable. When Bi(OTf)3, Fe(OTf)2, and Zn(OTf)2 were employed as catalysts, the glycosylations were found to be highly α-selective in Et2O. The selectivity and reaction rate changed with a change in the acceptor reactivity.

Glycosylation with ulosonates under Mitsunobu conditions: scope and limitations

Mitsunobu reactions on highly hindered tertiary type hydroxy groups of ulosonates: from 47 NuH compounds O-, N-, and S-nucleophiles gave the corresponding ulosidonates in 30–78% yields, while C-nucleophiles were unreactive.

Are Brønsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate

Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amounts of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Experimental evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temperature NMR (1H and 13C) experiments. These observations, together with density functional theory calculations and a kinetic study, corroborate a mechanism involving triflic acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the production of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counter ion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal triflate-catalyzed glycosylation reactions.

Chemical synthesis of culmorin metabolites and their biologic role in culmorin and acetyl-culmorin treated wheat cells

The Fusarium metabolite culmorin (1) is receiving increased attention as an "emerging mycotoxin". It co-occurs with trichothecene mycotoxins and potentially influences their toxicity. Its ecological role and fate in plants is unknown. We synthesized sulfated and glucosylated culmorin conjugates as potential metabolites, which are expected to be formed in planta, and used them as reference compounds. An efficient procedure for the synthesis of culmorin sulfates was developed. Diastereo- and regioselective glucosylation of culmorin (1) was achieved by exploiting or preventing unexpected acyl transfer when using different glucosyl donors. The treatment of a wheat suspension culture with culmorin (1) revealed an in planta conversion of culmorin into culmorin-8-glucoside (6) and culmorin acetate, but no sulfates or culmorin-11-glucoside (7) was found. The treatment of wheat cells with the fungal metabolite 11-acetylculmorin (2) revealed its rapid deacetylation, but also showed the formation of 11-acetylculmorin-8-glucoside (8). These results show that plants are capable of extensively metabolizing culmorin.

A Semi-Synthetic Glycoconjugate Vaccine Candidate for Carbapenem-Resistant Klebsiella pneumoniae

Hospital-acquired infections are an increasingly serious health concern. Infections caused by carpabenem-resistant Klebsiella pneumoniae (CR-Kp) are especially problematic, with a 50 % average survival rate. CR-Kp are isolated from patients with ever greater frequency, 7 % within the EU but 62 % in Greece. At a time when antibiotics are becoming less effective, no vaccines to protect from this severe bacterial infection exist. Herein, we describe the convergent [3+3] synthesis of the hexasaccharide repeating unit from its capsular polysaccharide and related sequences. Immunization with the synthetic hexasaccharide 1 glycoconjugate resulted in high titers of cross-reactive antibodies against CR-Kp CPS in mice and rabbits. Whole-cell ELISA was used to establish the surface staining of CR-Kp strains. The antibodies raised were found to promote phagocytosis. Thus, this semi-synthetic glycoconjugate is a lead for the development of a vaccine against a rapidly progressing, deadly bacterium.

Chemoenzymatic Approach for the Preparation of Asymmetric Bi-, Tri-, and Tetra-Antennary N-Glycans from a Common Precursor

Progress in glycoscience is hampered by a lack of well-defined complex oligosaccharide standards that are needed to fabricate the next generation of microarrays, to develop analytical protocols to determine exact structures of isolated glycans, and to elucidate pathways of glycan biosynthesis. We describe here a chemoenzymatic methodology that makes it possible, for the first time, to prepare any bi-, tri-, and tetra-antennary asymmetric N-glycan from a single precursor. It is based on the chemical synthesis of a tetra-antennary glycan that has N-acetylglucosamine (GlcNAc), N-acetyllactosamine (LacNAc), and unnatural Galα(1,4)-GlcNAc and Manβ(1,4)-GlcNAc appendages. Mammalian glycosyltransferases recognize only the terminal LacNAc moiety as a substrate, and thus this structure can be uniquely extended. Next, the β-GlcNAc terminating antenna can be converted into LacNAc by galactosylation and can then be enzymatically modified into a complex structure. The unnatural α-Gal and β-Man terminating antennae can sequentially be decaged by an appropriate glycosidase to liberate a terminal β-GlcNAc moiety, which can be converted into LacNAc and then elaborated by a panel of glycosyltransferases. Asymmetric bi- and triantennary glycans could be obtained by removal of a terminal β-GlcNAc moiety by treatment with β-N-acetylglucosaminidase and selective extension of the other arms. The power of the methodology is demonstrated by the preparation of an asymmetric tetra-antennary N-glycan found in human breast carcinoma tissue, which represents the most complex N-glycan ever synthesized. Multistage mass spectrometry of the two isomeric triantennary glycans uncovered unique fragment ions that will facilitate identification of exact structures of glycans in biological samples.

Synthesis of Peptidoglycan Fragments from Enterococcus faecalis with Fmoc-Strategy for Glycan Elongation

Peptidoglycan (PGN) is an essential structural component of the bacterial cell wall conferring cell shape, which can be recognized by host-recognition proteins and receptors as well as bacterial surface proteins. In this work, the PGN partial structures from Enterococcus faecalis that contain a tetrasaccharide and an octasaccharide with a unique heptapeptide were synthesized via an Fmoc-strategy for elongation of the glycan chains. Namely, a 4'-O-Fmoc-protected disaccharide was utilized as the key intermediate in this efficient synthetic pathway for preparing various PGN fragments. Both the tetrasaccharide and octasaccharide with the unique heptapeptide were successfully synthesized for the first time.

Synthesis of Ocotillol-Type Ginsenosides

A total of 14 ocotillol-type ginsenosides were conveniently synthesized employing glycosylation of ocotillol sapogenin derivatives with glucosyl ortho-alkynylbenzoate donors under the promotion of a gold(I) catalyst as the key step. Relying on a rational protecting group strategy and the unexpected regioselectivity of the glycosylation of the 3,25-diol sapogenins (2a/2b, 5a/5b) for the tertiary 25-OH, mono 3-O-glucosyl ocotillol-PPD, 6-O-glucosyl ocotillol-PPT, 25-O-glucosyl ocotillol-PPD/PPT and 3,25-di-O-glucosyl ocotillol-PPD/PPT ginsenosides were prepared in which the configuration at the C-24 is either R or S.

Chemical synthesis of saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

N-Alkyl derivatives of diosgenyl 2-amino-2-deoxy-β-D-glucopyranoside; synthesis and antimicrobial activity

Diosgenyl 2-amino-2-deoxy-β-D-glucopyranoside is a synthetic saponin exhibiting attractive pharmacological properties. Different pathways tested by us to obtain this glycoside are summarized here. Moreover, the synthesis of N-alkyl and N,N-dialkyl derivatives of the glucopyranoside is presented. Evaluation of antibacterial and antifungal activities of these derivatives indicates that they have no inhibitory activity against Gram-negative bacteria, whereas many of the tested N-alkyl saponins were found to inhibit the growth of Gram-positive bacteria and human pathogenic fungi.

Efficient synthesis of a library of heparin tri- and tetrasaccharides relevant to the substrate of heparanase

A robust glycosylation protocol was fixed to construct the GlcN–(1α→4)-GlcA/IdoA linkagesen routeto heparin oligosaccharides.

Convergent synthesis and cytotoxic activities of 26-thio- and selenodioscin

Convergent block syntheses of 26-thio- and selenodioscin have been achieved by developing the highly stereoselective 1,2-trans glycosylations of chacotriosyl imidate without recourse to neighboring group assistance. Both thiodioscin and selenodioscin possess cytotoxic activities similar to dioscin, a natural spirostanol glycoside.

A divergent approach to the synthesis of simplexides and congeners via a late-stage olefin cross-metathesis reaction

Simplexides constitute a unique group of immunosuppressive glycolipids that demonstrate antiproliferative activities against activated T-cell lymphocytes via a unique non-cytotoxic inhibition. To investigate the structure-activity relationship of the varied long-chain secondary alcohols on simplexides, we developed an efficient and divergent route to the synthesis of simplexides and congeners, taking advantage of a late-stage olefin cross-metathesis reaction.

Synthesis of oligosaccharide fragments of the rhamnogalacturonan of Nerium indicum

Three trisaccharides, one pentasaccharide, and one heptasaccharide, namely α-D-GalA-(1→2)-α-L-Rha-(1→4)-β-D-GalA-OC3H7 (1), α-L-Rha-(1→4)-α-D-GalA-(1→4)-β-D-GalA-OC3H7 (2), α-D-GalA-(1→4)-α-D-GalA-(1→2)-α-L-Rha-OC3H7 (3), α-D-GalA-(1→2)-α-L-Rha-(1→4)-α-D-GalA-(1→2)-α-L-Rha-(1→4)-β-D-GalA-OC3H7 (4), and α-D-GalA-(1→2)-α-L-Rha-(1→4)-α-D-GalA-(1→2)-α-L-Rha-(1→4)-α-D-GalA-(1→2)-α-L-Rha-(1→4)-β-D-GalA-OC3H7 (5), which are relevant to the fragments of the rhamnogalacturonan of Nerium indicum, were concisely synthesized. The syntheses feature highly stereoselective formation of the α-D-GalA-linkage with GalA N-phenyltrifluoroacetimidates as donors.

Synthesis of a chlorogenin glycoside library using an orthogonal protecting group strategy

Naturally occurring spirostanol saponins bear a chacotriose, α-L-rhamnopyranosyl-(1→2)-[α-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranose residue as the oligosaccharide moiety which is believed to be important for biological activity. Herein the development of a concise, combinatorial method for the synthesis of two series of glycan variants at the 2' and/or 4' positions of chacotriose is described and the structure-activity relationships of the glycone part at 3-OH of chlorogenin investigated. These compounds were found to be weakly-cytotoxic toward leukemia cell lines CCRF and HL-20, indicating that the chacotriose moiety is important for anticancer activity.

Diversity-oriented synthesis of inner core oligosaccharides of the lipopolysaccharide of pathogenic Gram-negative bacteria

Lipopolysaccharide (LPS) is a potent virulence factor of pathogenic Gram-negative bacteria. To better understand the role of LPS in host-pathogen interactions and to elucidate the antigenic and immunogenic properties of LPS inner core region, a collection of well-defined L-glycero-D-manno-heptose (Hep) and 3-deoxy-α-D-manno-oct-2-ulosonic acid (Kdo)-containing inner core oligosaccharides is required. To address this need, we developed a diversity-oriented approach based on a common orthogonal protected disaccharide Hep-Kdo. Utilizing this new approach, we synthesized a range of LPS inner core oligosaccharides from a variety of pathogenic bacteria including Y. pestis, H. influenzae, and Proteus that cause plague, meningitis, and severe wound infections, respectively. Rapid access to these highly branched core oligosaccharides relied on elaboration of the disaccharide Hep-Kdo core as basis for the elongation with various flexible modules including unique Hep and 4-amino-4-deoxy-β-L-arabinose (Ara4N) monosaccharides and branched Hep-Hep disaccharides. A regio- and stereoselective glycosylation of Kdo 7,8-diol was key to selective installation of the Ara4N moiety at the 8-hydroxyl group of Kdo moiety of the Hep-Kdo disaccharide. The structure of the LPS inner core oligosaccharides was confirmed by comparison of (1)H NMR spectra of synthetic antigens and isolated fragments. These synthetic LPS core oligosaccharides can be covalently bound to carrier proteins via the reducing end pentyl amine linker, to explore their antigenic and immunogenic properties as well as potential applications such as diagnostic tools and vaccines.