Stereoselective Synthesis of the O-antigen of A. baumannii ATCC 17961 Using Long-Range Levulinoyl Group Participation

Herein, we described the first synthesis of the pentasaccharide and decasaccharide of the A. baumannii ATCC 17961 O-antigen for developing a synthetic carbohydrate-based vaccine against A. baumannii infection. The efficient synthesis of the rare sugar 2,3-diacetamido-glucuronate was achieved using our recently introduced organocatalytic glycosylation method. We found, for the first time, that long-range levulinoyl group participation via a hydrogen bond can result in a significantly improved β-selectivity in glycosylations. This solves the stereoselectivity problem of highly branched galactose acceptors. The proposed mechanism was supported by control experiments and DFT computations. Benefiting from the long-range levulinoyl group participation strategy, the pentasaccharide donor and acceptor were obtained via an efficient [2+1+2] one-pot glycosylation method and were used for the target decasaccharide synthesis.     

Synthesis of a pentasaccharide repeating unit of the O-antigen of enteroadherent Escherichia coli O154 strain

A straightforward linear synthetic strategy has been developed for the synthesis of the pentasaccharide repeating unit of the cell wall O-antigenic polysaccharide of enteroadherent Escherichia coli O154 strain. Newly developed glycosylation conditions using glycosyl trichloroacetimidate derivatives as glycosyl donors and nitrosyl tetrafluoroborate as the glycosylation activator have been used in all of the glycosylation reactions throughout the synthetic scheme. The stereochemical outcomes of the glycosylations were excellent and the yields were very good.     

Straightforward sequential and one-pot synthesis of a pentasaccharide repeating unit corresponding to the cell wall O-antigen of Shigella boydii type 18

Synthesis of a pentasaccharide repeating unit corresponding to the cell of wall O-antigen Shigella boydii type 18 has been achieved by sequential as well as iterative glycosylations in one-pot. Use of p-methoxybenzyl group (PMB) as an in situ removable protecting group allowed obtaining the desired pentasaccharide derivative in a generalized glycosylation condition and in one-pot condition. Synthesis of a beta-L-rhamnosidic linkage present in the molecule has been successfully achieved using l-rhamnosyl thioglycoside donor having a picoloyl group at remote C-3 position influencing beta selectivity in the glycosylation. A combination of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO₄–SiO₂) has been used as thiophilic glycosylation promoter in all glycosylation reactions. TEMPO mediated selective oxidation of the primary hydroxyl group has been carried out at the late stage of the synthetic strategy.     

Synthetic directions towards capsular polysaccharide of Streptococcus pneumoniae serotype 18C

An efficient synthetic strategy has been developed for the synthesis of the tri, tetrasaccharide block and pentasaccharide corresponding to the capsular polysaccharide of Streptococcus pneumonia serotype 18C as their 2-aminoethyl glycosides. A one-pot glycosylation-deprotection, sequential glycosylations strategy has been adopted for the construction of the fragments and pentasaccharide derivative, which were then transformed into target compound after a series of functional group transformations. The synthetic method relies on the use of p-methoxybenzyl ether as an in situ-removable protecting group to reduce the number of reaction steps significantly. Here H₂SO₄-silica has been used successfully as a promoter for all glycosylation reaction. In addition, the synthetic target also contained a free amino group at its reducing end, facilitating its conjugation with other molecules for various biological studies and applications.     

Synthesis of the Tolerance-Inducing Oligosaccharide Lacto-N-Fucopentaose III Bearing an Activated Linker

A concise synthetic route to an immunomodulatory pentasaccharide, lacto-N-fucopentaose III (1) and its corresponding human serum albumin conjugate, is described. Key transformations of the strategy include two highly regio- and stereoselective glycosylations for the construction of disaccharide 10 and pentasaccharide 12, a Birch reduction for deprotection of benzyl ethers, and a UV-promoted radical addition of a thiol to an alkene for modification of the aglycone.     

Concise synthesis of a pentasaccharide repeating unit corresponding to the O-antigen of Escherichia coli O102

An efficient synthetic strategy has been developed for the synthesis of a pentasaccharide repeating unit of the O-antigen of Escherichia coli O102 strain. The target pentasaccharide 1 has been synthesized using a [2+3] block glycosylation strategy. All glycosylation steps are highly stereoselective and high yielding. Concept of armed-disarmed and orthogonal glycosylation strategies has been applied during the synthesis. The target compound has been synthesized using the minimum number of steps.     

Synthesis of penta- and hexasaccharide fragments corresponding to the O-antigen of Escherichia coli O150

The chemical synthesis of a pentasaccharide and a hexasaccharide corresponding to the O-antigen of Escherichia coli O150 has been achieved using sequential glycosylation and [3+3] block glycosylation strategies. Suitably protected monosaccharide synthons have been prepared from the commercially available reducing sugars and then stereoselectively coupled to give the pentasaccharide and a hexasaccharide in excellent yields. 4-Methoxyphenyl and 2-(4-methoxyphenoxy) ethyl groups have been used as the anomeric-protecting groups in the target pentasaccharide and a hexasaccharide, respectively.     

New synthesis of idraparinux,the non-glycosaminoglycan analogue of the antithrombin-binding domain of heparin

Idraparinux, the fully O-sulfated, O-methylated, heparin-related pentasaccharide possessing selective factor Xa inhibitory activity, was prepared by a new synthetic pathway. This route was based on a [2+3] block synthesis utilizing a 6-O-silyl-protected l-idose-containing trisaccharide acceptor, which was glycosylated with a disaccharide donor containing a non-oxidized precursor of the glucuronic acid. The unique strategy of multiple functionalizations at pentasaccharide levels, involving triple methylation followed by oxidation of the glucose and the idose precursors into d-glucuronic and l-iduronic acids in one step, proved to be highly efficient, providing the target pentasaccharide through a 39-step synthesis starting from d-glucose and methyl α-d-glucopyranoside.     

Synthetic directions of acidic hexasaccharide repeating unit of the O-antigen of Cronobacter sakazakii HPB 2855 using one pot glycosylation

Synthesis of a hexasaccharide repeating unit of the O-antigen of Cronobacter sakazakii HPB 2855 has been achieved by sequential glycosylations and one-pot glycosylation-deprotection techniques. The synthetic method relies on the use of a p-methoxybenzyl ether as an in situ-removable protecting group to reduce the number of reaction steps significantly. All the glycosylations have been accomplished by the activation of the only one class of simple and stable thioglycosyl donors using NIS in the presence of sulfuric acid immobilized on silica (H₂SO₄-silica) as a Brönsted acid catalyst to work as a promoter. The stereo outcomes of all the glycosylation steps were excellent with satisfactory yield. TEMPO mediated selective oxidation of the primary hydroxyl group has been carried out at the late stage of the synthetic strategy to achieve the required uronic acid motif.     

Propargyl mediated intramolecular aglycon delivery (IAD): applications to the synthesis of core N-glycan oligosaccharides

The use of propargyl mediated intramolecular aglycon delivery (IAD) for the synthesis of the key Manβ(1→4)GlcNAc linkage of N-glycan oligosaccharides, including the core N-glycan pentasaccharide, is investigated. Isomerisation of a 2-O-progargyl group of manno thioglycoside donors to an allene is followed by iodonium ion mediated mixed acetal formation with the 4-OH of protected GlcNAc acceptors, and subsequent intramolecular glycosylation occurs with complete control of anomeric stereochemistry to form the Manβ(1→4)GlcNAc linkage. A variety of linear and convergent approaches (1+2, 3+1, 3+2) to the core pentasaccharide are investigated as means of probing the generality and limitations of this type of intramolecular aglycon delivery for the formation of β-mannoside linkages in complex oligosaccharides.     

Expedient synthesis of a tetrasaccharide and a pentasaccharide corresponding to the cell wall O-antigen of Escherichia coli O77 and Escherichia coli O17

A concise chemical synthetic strategy has been developed for the synthesis of a tetrasaccharide and a pentasaccharide corresponding to the O-antigen of Escherichia coli O77 and E. coli O17 strains, respectively using [2+2] and [3+2] block glycosylation approaches from suitably functionalized common monosaccharide intermediates. All of the intermediate steps are high yielding while the glycosylation steps are highly stereoselective. A number of recently developed methodologies have been used in the synthesis.     

Synthesis of an N-Acetylated Heparin Pentasaccharide and its anticoagulant activity in comparison with the heparin pentasaccharide with high anti-factor-Xa activity

The synthesis of tri-N-acetylated heparin pentasaccharide 2 is described. It was assembled from five suitably blocked monosaccharide units ( 3 – 7 ). Glucuronic-acid building block 4 was prepared from glucose by direct Jones oxidation of the 6-O-trityl derivative 18 . The resulting acid 16 was esterified to 17 in large mounts using methyl chloroformate/base. Trimethylsilyl bromide proved to be an excellent reagent for the hydrolysis of a prop-1-enyl glycoside ( 19 →21 ). The pentasaccharide 29 was obtained by a [2 + 2] + 1 synthesis, the glycosylation reactions furnished good to very good yields. The identity of protected oligosaccharides was confirmed by ¹H-NMR spectroscopy. Sequential deblocking of the pentasaccharide, O-sulfation, and N-acetylation gave 2 which was shown to exhibit ca. 600 times lower anticoagulant activity than pentasaccharide 1 .     

Synthesis of three regioisomers of the pentasaccharide part of the Skp1 glycoprotein of Dictyostelium discoideum

Three regioisomers of the linear pentasaccharide part of the Skp1 glycoprotein, found in Dictyostelium discoideum, were prepared in the form of (2-trimethylsilyl)ethyl glycosides by means of 2+3 block syntheses using the disaccharide donor at the non-reducing end, and three different trisaccharide acceptors at the reducing end. Fucosylation of (2-trimethylsilyl)ethyl 3,4,6-tri-O-benzyl-β-d-galactopyranosyl-(1→3)-4,6-O-benzylidene-2-deoxy-2-NPhth-β-d-glucopyranoside with different fucosyl donors carrying an O-(2-naphthyl)methyl ether as a temporary-protecting group at positions C2, C3 or C4 gave rise to the protected core trisaccharides. After selective removal of the (2-naphthyl)methyl group, the resulting acceptors were glycosylated with the α(1→6) linked digalactosyl donor to yield the respective three regioisomeric pentasaccharides. Transformation of the phthalimido moiety into an N-acetyl group, followed by catalytic hydrogenation of the reducible-protecting groups furnished the free target pentasaccharides, which should be able to assist during the elucidation of the exact structure of the natural pentasaccharide.     

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.     

Convergent Synthesis of a Bisecting N‐Acetylglucosamine (GlcNAc)‐Containing N‐Glycan

The chemical synthesis of a bisecting N‐acetylglucosamine (GlcNAc)‐containing N‐glycan was achieved by a convergent synthetic route through [4+2] and [6+2] glycosylations. This synthetic route reduced the number of reaction steps, although the key glycosylations were challenging in terms of yields and selectivities owing to steric hindrance at the glycosylation site and a lack of neighboring group participation. The yields of these glycosylations were enhanced by stabilizing the oxocarbenium ion intermediate through ether coordination. Glycosyl donor protecting groups were explored in an effort to realize perfect α selectivity by manipulating remote participation. The simultaneous glycosylations of a tetrasaccharide with two disaccharides was investigated to efficiently construct a bisecting GlcNAc‐containing N‐glycan.     

First preparative synthesis of a 3-acetamido-3,6-dideoxy-d-galactopyranose glycosyl donor via intramolecular cyclization of an epoxytrichloroacetimidate

The preparative synthesis of a 3-acetamido-3,6-dideoxy-d-galactopyranose N-phenyl-trifluoroacetimidate donor has been accomplished using as key step a silica gel mediated cyclization of an epoxytrichloroacetimidate, while other more conventional routes to aminosugars failed. Test glycosylations with the N-phenyl-trifluoroacetimidate donor are also reported.     

Synthesis of a N-glycan nonasaccharide of the bisecting type with additional core-fucose

A chemical synthesis was developed for N-glycans substituted with core-fucose and an additional bisecting GlcNAc (LEC10 motif). The synthesis was conducted using a suitably functionalized N-glycan pentasaccharide assembled from modular building blocks. Selective introduction of the bisecting GlcNAc residue was followed by attachment of the α1,6-arm and core-fucose. After introduction of an aminohexanoyl spacer the nonasaccharide was completely deprotected providing a substrate for enzymatic elongation and conjugation to proteins for further biological studies.     

Synthesis of an N-glycan decasaccharide of the hybrid type

A hybrid-type N-glycan decasaccharide GlcNAcMan₇GlcNAc₂ was synthesized from the pentasaccharide GlcNAcMan₂GlcNAc₂ as an advanced intermediate and an acyl-protected pentamannosyl donor. Benzyl mannoside was regioselectively benzoylated and glycosylated at OH-3 and OH-6 with a dimannoside to give the 3,6-branched pentamannoside. Coupling of the two pentasaccharides furnished the target decasaccharide in 60% yield. Deprotection of the base labile functions furnished a hybrid-type N-glycan decasaccharide functionalized for the conjugation with peptides or proteins.     

Convergent synthesis of a pentasaccharide repeating unit corresponding to the cell wall O-antigen of Salmonella enterica O44

A convergent synthetic strategy has been developed for the synthesis of a pentasaccharide fragment corresponding to the O-antigen of Salmonella enterica O44 strain. An intermediate tetrasaccharide derivative was prepared by a [2+2] block glycosylation of two disaccharide derivatives. The p-methoxybenzyl (PMB) group has been used as the in situ temporary protecting group minimizing the number of functional group manipulation steps. The application of the armed–disarmed glycosylation concept reduced the number of steps in the synthetic strategy. The glycosylation steps were highly stereoselective and high yielding.