Gram-scale syntheses of the (1→3)-linked and (1→4)-linked hyaluronan disaccharides

Detailed Investigation of the Immunodominant Role of O-Antigen Stoichiometric O-Acetylation as Revealed by Chemical Synthesis, Immunochemistry, Solution Conformation and STD-NMR Spectroscopy for Shigella flexneri 3a

Shigella flexneri 3a causes bacillary dysentery. Its O-antigen has the {2)-[α-d-Glcp-(1→3)]-α-l-Rhap-(1→2)-α-l-Rhap-(1→3)-[Ac→2]-α-l-Rhap-(1→3)-[Ac→6]≈40 % -β-d-GlcpNAc-(1→} ([(E)ABAc CAc D]) repeating unit, and the non-O-acetylated equivalent defines S. flexneri X. Propyl hepta-, octa-, and decasaccharides sharing the (E')A'BAc CD(E)A sequence, and their non-O-acetylated analogues were synthesized from a fully protected BAc CD(E)A allyl glycoside. The stepwise introduction of orthogonally protected mono- and disaccharide imidate donors was followed by a two-step deprotection process. Monoclonal antibody binding to twenty-six S. flexneri types 3a and X di- to decasaccharides was studied by an inhibition enzyme-linked immunosorbent assay (ELISA) and STD-NMR spectroscopy. Epitope mapping revealed that the 2C -acetate dominated the recognition by monoclonal IgG and IgM antibodies and that the BAc CD segment was essential for binding. The glucosyl side chain contributed to a lesser extent, albeit increasingly with the chain length. Moreover, tr-NOESY analysis also showed interaction but did not reveal any meaningful conformational change upon antibody binding.

Regioselective Benzylation of 2-Deoxy-2-Aminosugars Using Crown Ethers: Application to a Shortened Synthesis of Hyaluronic Acid Oligomers

The combination of benzyl bromide, sodium hydroxide and 15-crown-5 in tetrahydrofuran is shown to be an efficient method for installing benzyl groups at both the 4- and 6-positions regioselectively directly from peracetylated N-trichloroacetyl-protected glucosamine and galactosamine. Application of this benzylation strategy proved to significantly shorten the synthetic route to hyaluronic acid tetra- and hexamers.

Synthesis and cellular uptake of fluorescently labeled multivalent hyaluronan disaccharide conjugates of oligonucleotide phosphorothioates

Clustered hyaluronan disaccharides were studied as mediators of cellular delivery of antisense oligonucleotides through receptor-mediated endocytosis. For this purpose, a synthetic route for preparation of an appropriately protected hyaluronic acid dimer bearing an aldehyde tether (1) was devised. Up to three non-nucleosidic phosphoramidite building blocks (2), each bearing two phthaloyl protected aminooxy groups, were then inserted into the 3'-terminus of the desired phosphorothioate oligodeoxyribonucleotide, and 6-FAM phosphoramidite was introduced into the 5'-terminus. After completion of the chain assembly, the aldehyde-tethered sugar ligands were attached to the deprotected aminooxy functions by on-support oximation. Three fluorescein-labeled phosphorothioate oligonucleotide glycoconjugates (28-30) containing two, four, or six hyaluronan disaccharides were prepared. The influence of the hyaluronan moieties on the cellular uptake of the thioated oligonucleotides was tested in a cell line expressing the hyaluronan receptor CD44. Specific uptake was not detected with this combination of multiple hyaluronan disaccharides.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Enzymatic and chemical methods for the generation of pure hyaluronan oligosaccharides with both odd and even numbers of monosaccharide units

Hyaluronan oligosaccharides display physiological activities not associated with the polymer and are widely used to characterize hyaluronan-binding proteins. They can also be used as biocompatible starting blocks for chemical derivatization. Here we present methods for generating milligram quantities of unusual odd- and even-numbered oligosaccharides, greatly increasing the diversity of reagents for use in such studies. These methods are based upon protocols from the 1960s, at which time it was very difficult to assess the stereochemical purity of the products. To address this, products were analyzed with modern high-field nuclear magnetic resonance spectroscopy. Alkaline beta-elimination conditions previously used to remove reducing-terminal N-acetylglucosamine residues in fact introduce a significant ( approximately 30%) level of stereoisomerism in the products by alkali-catalyzed keto-enol tautomerizations. Milder alkaline conditions were used to overcome this problem, reducing the contamination to <5%. The elimination by-products from this reaction were isolated and characterized, allowing the mechanism of alkaline degradation of hyaluronan to be investigated for the first time. beta-Glucuronidase was used to remove nonreducing-terminal glucuronic acid residues from oligosaccharides. Odd-numbered oligosaccharides with terminal glucuronic acid residues isolated from hyaluronidase digests are shown to originate from acid-catalyzed acetal hydrolysis during boiling denaturation and also have significant levels of stereochemical impurities.