A double regio- and stereoselective glycosylation strategy for the synthesis of N-glycans

A building block approach for biantennary N-linked oligosaccharides from glycoproteins (N-glycans) has been developed. Starting from a core trisaccharide (beta-mannosyl chitobiose) containing a benzylidene-protected beta-mannoside, the attachment of the disaccharide building blocks for the antennae can be performed in a double regio- and stereoselective manner. A short synthesis of a GlcNPhtbeta1,2Man donor was developed. The benzylidene acetal moiety, as a minimal protection of the beta-mannoside, allows selective alpha-glycosylation at OH-3 of the 2,3-diol with GlcNbeta1,2Man trichloroacetimidate donors. Subsequent debenzylidenation leads to a 4,6-diol, which can be selectively extended at OH-6. Overreaction at OH-4 was generally low when phthalimido-protected donors were used. This general strategy represents a modular synthesis of N-glycans and their glycoconjugates.     

Combinatorial synthesis of an oligosaccharide library by using beta-bromoglycoside-mediated iterative glycosylation of selenoglycosides: rapid expansion of molecular diversity with simple building blocks

A new method for constructing an oligosaccharide library composed of structurally defined oligosaccharides is presented based on an iterative glycosylation of selenoglycosides. Treatment of 2-acyl-protected selenoglycosides with bromine selectively generates beta-bromoglycosides, which serve as glycosyl cation equivalents in the oligosaccharide synthesis. Thus, the coupling of the bromoglycosides with another selenoglycoside affords the corresponding glycosylated selenoglycosides, which can be directly used to next glycosylation. The iteration of this sequence allows the synthesis of a variety of oligosaccharides including an elicitor active heptasaccharide. A characteristic feature of the iterative glycosylation is that glycosyl donors and acceptors with the same anomeric reactivity can be selectively coupled by activation of the glycosyl donor prior to coupling with the glycosyl acceptor. Therefore, same selenoglycosides can be used for both the glycosyl donors and the acceptors. This feature has been exemplified by a construction of an oligosaccharide library directed to elicitor-active oligosaccharides. The library composed of stereochemically defined oligoglucosides with considerable structural diversity can be constructed starting from simple selenoglycosides.     

Innate immune responses of synthetic lipid A derivatives of Neisseria meningitidis

Differences in the pattern and chemical nature of fatty acids of lipid A of Neisseria meningitides lipooligosaccharides (LOS) and Escherichia coli lipopolysaccharides (LPS) may account for differences in inflammatory properties. Furthermore, there are indications that dimeric 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) moieties of LOS and LPS enhance biological activities. Heterogeneity in the structure of lipid A and possible contaminations with other inflammatory components have made it difficult to confirm these observations. To address these problems, a highly convergent approach for the synthesis of a lipid A derivative containing KDO has been developed, which relies on the ability to selectively remove or unmask in a sequential manner an isopropylidene acetal, 9-fluorenylmethoxycarbonyl (Fmoc), allyloxycarbonate (Alloc), azide, and thexyldimethylsilyl (TDS) ether. The strategy was employed for the synthesis of N. meningitidis lipid A containing KDO (3). Mouse macrophages were exposed to the synthetic compound and its parent LOS, E. coli lipid A (2), and a hybrid derivative (4) that has the asymmetrical acylation pattern of E. coli lipid A, but the shorter lipids of meningococcal lipid A. The resulting supernatants were examined for tumor necrosis factor alpha (TNF-alpha) and interferon beta (IFN-beta) production. The lipid A derivative containing KDO was much more active than lipid A alone and just slightly less active than its parent LOS, indicating that one KDO moiety is sufficient for full activity of TNF-alpha and IFN-beta induction. The lipid A of N. meningitidis was a significantly more potent inducer of TNF-alpha and IFN-beta than E. coli lipid A, which is due to a number of shorter fatty acids. The compounds did not demonstrate a bias towards a MyD88- or TRIF-dependent response.     

Extended applications of di-tert-butylsilylene-directed alpha-predominant galactosylation compatible with C2-participating groups toward the assembly of various glycosides

The high versatility of di-tert-butylsilylene(DTBS)-directed alpha-predominant galactosylation have been extended to the construction of difficult glycan sequences. First, to investigate the compatibility of the alpha-predominant reaction with various glycosylation systems a variety of 4,6-O-DTBS-tethered galactosaminyl or galactosyl donors were synthesized efficiently, which have C2-participating groups with a wide variety of leaving groups such as alkylsulfenyl, halide, trichloroacetimidate groups. The results of the detailed examination of the glycosylation reaction using the glycosyl donors showed the wide scope of the 4,6-DTBS-directed alpha-galactosylation. In the next step, the stereoselective construction of alpha-GalN-Ser/Thr sequences was examined by employing the DTBS-directed glycosylation. As a result, various types of serine and threonine derivatives were glycosylated alpha-selectively, producing alpha-GalN-Ser/Thr sequences in high yields. Moreover, the DTBS-directed galactosylation was successfully applied for the synthesis of alpha-tetrasaccharyl-Ser segment of glycophorin A.     

Convenient use of ortho-formylphenyl thioglycoside for regioselective conjugation with glycosyl acceptors towards regioselective 1,2-cis-glycosylation

A facile methodology is proposed for regioselective conjugation between glycosyl donors and acceptors towards the development of regioselective 1,2-cis-glycosylation method. ortho-Formylphenyl 1-thio-β-d-galactopyranoside was regioselectively tethered to methyl α-d-glucopyranoside under acidic condition to furnish an 4,6-O-arylidene acetal-linked conjugate. This conjugate can be readily converted to an ether-linked 4-O- or 6-O-derivative by regioselective cleavage of the acetal ring. In the glycosylation reaction, the ether-linked 4-OH conjugate was found to show excellent 1,2-cis selectivity via an intramolecular 1,9-transfer.     

From Polymer to Size‐Defined Oligomers: A Highly Divergent and Stereocontrolled Construction of Chondroitin Sulfate A,C, D,E, K,L, and M Oligomers from a Single Precursor: Part 2

An efficient, stereocontrolled, and highly divergent approach for the preparation of oligomers of chondroitin sulfate (CS) A, C, D, E, K, L, and M variants, starting from a single precursor easily obtained by semisynthesis from abundant natural polymer is reported for the first time. Common intermediates were designed that allowed the straightforward construction of O‐sulfonated species either on the D ‐galactosamine unit (CS‐A, ‐C, and ‐E) or on both D ‐glucuronic acid and D ‐galactosamine units (CS‐D and CS‐K, ‐L, and ‐M). This strategy represents a successful improvement and brings a definitive answer toward the synthesis of such complex molecules with numerous relevant biological functions.     

A study of polymer-supported bases for the solution phase synthesis of glycosyl trichloroacetimidates

Polystyrene-supported strong organic bases are highly efficient reagents for the solution-phase synthesis of glycosyl trichloroacetimidates, affording quantitative yields of pure products in short reaction times after simple filtration and evaporation. Although efficiency of the different bases varies with substrate structure, polymer-bound 1,8-diazabicyclo[5.4.0]undec-7-ene was found to give the best results for all the substrates tested.     

From Polymer to Size‐Defined Oligomers: A Step Economy Process for the Efficient and Stereocontrolled Construction of Chondroitin Oligosaccharides and Biotinylated Conjugates Thereof: Part 1

Controlled acid hydrolysis of polymeric chondroitin sulfate of bovine origin afforded in good yield a basic disaccharide fragment that was used for the first time as a starting material for the expeditious preparation of a set of building blocks that in turn act as versatile synthons for the efficient and stereocontrolled construction of a collection of size‐defined chondroitin oligomers (from di‐ to octasaccharides). This step economy process allows their preparation as reducing species, fitted with a fluorophore, or as biotinylated conjugates; all useful tools for the preparation of microarrays, or as probes for the study of the biosynthesis of chondroitin sulfate.     

An expedient synthesis of the repeating unit of the acidic polysaccharide of the bacteriolytic complex of lysoamidase

The first synthesis of the trisaccharide repeating unit of the acidic polysaccharide of the bacteriolytic complex of lysoamidase is presented. The construction is based on a linear glycosylation strategy that starts from the reducing end and employs thio- and selenoglycosides in a highly stereoselective manner by a single set of activation conditions. The thus-formed trisaccharide is selectively deprotected and oxidised, after which a final deprotection step furnishes the desired repeating unit.     

Efficient synthesis of core 2 class glycosyl amino acids by one-pot glycosylation approach

We describe the one-pot synthesis of core 2 class branched oligosaccharides initiated by chemo-selective glycosylation of silyl ether. Glycosylation of 6-O-silyl-4-benzyl-2-azido-thiogalactoside with glycosyl fluoride provided selectively 6-glycosylated thioglycoside without both O-glycosylation at the 3 position and S-glycosylation. Subsequent coupling of galactosyl fluoride and amino acids afforded the protected branched oligosaccharides in good yields.