Cheng-Chung Wang

Regioselective one-pot protection of carbohydrates

Carbohydrates are involved in a wide range of biological processes. These structurally diverse compounds are more complex than other biological polymers, and are often present as heterogeneous mixtures in nature. The chemical synthesis of carbohydrates is one way to obtain pure oligosaccharides, but it is hampered by difficulties associated with the regioselective protection of polyhydroxyls and challenges related to the stereoselective assembly of glycosidic linkages. Here we describe a combinatorial, and highly-regioselective, method that can be used to protect individual hydroxy groups of a monosaccharide. This approach can be used to install an orthogonal protecting group pattern in a single reaction vessel (a ‘one-pot’ reaction), which removes the need to carry out the time-consuming isolation and purification of intermediates. Hundreds of building blocks have been efficiently prepared starting from d-glucose, and the iterative coupling of these building blocks enabled us to assemble β-1,6-glucans and a library of oligosaccharides based on the influenza-virus-binding trisaccharide.

Regioselective one-pot protection of glucose

Detailed protocols for the regioselective protection of individual hydroxyls in monosaccharide units are described here. This expedient methodology incorporates up to seven reaction sequences, obviating the necessity to carry out intermittent tedious work-ups and time-consuming purifications. Using this TMSOTf-catalyzed one-pot protocol, the 2,3,4,6-tetra-O-trimethylsilylated hexopyranosides bearing an anomeric group could be transformed into a whole set of differentially protected 2-alcohols, 3-alcohols, 4-alcohols, 6-alcohols and fully protected monosaccharides in high yields. These tailor-made glycosyl donors and acceptors can then be used for stereoselective one-pot glycosylation for oligosaccharide synthesis. The total time for the preparation of a purified protected sugar unit ranges between 1 and 2 d. This process would otherwise take 1–2 weeks.

Divergent synthesis of 48 heparan sulfate-based disaccharides and probing the specific sugar-fibroblast growth factor-1 interaction.

Several biological processes involve glycans, yet understanding their ligand specificities is impeded by their inherent diversity and difficult acquisition. Generating broad synthetic sugar libraries for bioevaluations is a powerful tool in unraveling glycan structural information. In the case of the widely distributed heparan sulfate (HS), however, the 48 theoretical possibilities for its repeating disaccharide call for synthetic approaches that should minimize the effort in an undoubtedly huge undertaking. Here we employed a divergent strategy to afford all 48 HS-based disaccharides from just two orthogonally protected disaccharide precursors. Different combinations and sequence of transformation steps were applied with many downstream intermediates leading up to multiple target products. With the full disaccharide library in hand, affinity screening with fibroblast growth factor-1 (FGF-1) revealed that four of the synthetic sugars bind to FGF-1. The molecular details of the interaction were further clarified through X-ray analysis of the sugar-protein cocrystals. The capability of comprehensive sugar libraries in providing key insights in glycan-ligand interaction is, thus, highlighted.

Imaging early endothelial inflammation following stroke by core shell silica superparamagnetic glyconanoparticles that target selectin

Activation of the endothelium is a pivotal first step for leukocyte migration into the diseased brain. Consequently, imaging this activation process is highly desirable. We synthesized carbohydrate-functionalized magnetic nanoparticles that bind specifically to the endothelial transmembrane inflammatory proteins E and P selectin. Magnetic resonance imaging revealed that the targeted nanoparticles accumulated in the brain vasculature following acute administration into a clinically relevant animal model of stroke, though increases in selectin expression were observed in both brain hemispheres. Nonfunctionalized naked particles also appear to be a plausible agent to target the ischemic vasculature. The importance of these findings is discussed regarding the potential for translation into the clinic.

Highly Stereoselective Glycosyl‐Chloride‐Mediated Synthesis of 2‐Deoxyglucosides

Cl intermediates: The glycosylation of per-O-benzylated 2-deoxy- and 2,6-dideoxythioglycosides, promoted by the combination of para-toluenesulfenyl chloride (p-TolSCl) and silver triflate (AgOTf), furnished the products in high yields and high stereoselectivity. The glycosyl chloride was the intermediate (see scheme).

Automated synthesis of sialylated oligosaccharides

Summary Sialic acid-containing glycans play a major role in cell-surface interactions with external partners such as cells and viruses. Straightforward access to sialosides is required in order to study their biological functions on a molecular level. Here, automated oligosaccharide synthesis was used to facilitate the preparation of this class of biomolecules. Our strategy relies on novel sialyl α-(2→3) and α-(2→6) galactosyl imidates, which, used in combination with the automated platform, provided rapid access to a small library of conjugation-ready sialosides of biological relevance.

1,6-Anhydro-β- l -hexopyranoses as valuable building blocks toward the synthesis of l -gulosamine and l -altrose derivatives

1,6-Anhydro-β-l-hexopyranoses as valuable building blocks toward the synthesis of l-gulosamine and l-altrose derivatives via the regioselective triflation and benzoylation of 1,6-anhydro-β-l-idopyranose followed by SN2 substitution with various nucleophiles as key steps is described here.

Interactions that influence the binding of synthetic heparan sulfate based disaccharides to fibroblast growth factor-2.

Heparan sulfate (HS) is a linear sulfated polysaccharide that mediates protein activities at the cell-extracellular interface. Its interactions with proteins depend on the complex patterns of sulfonations and sugar residues. Previously, we synthesized all 48 potential disaccharides found in HS and used them for affinity screening and X-ray structural analysis with fibroblast growth factor-1 (FGF1). Herein, we evaluated the affinities of the same sugars against FGF2 and determined the crystal structures of FGF2 in complex with three disaccharides carrying N-sulfonated glucosamine and 2-O-sulfonated iduronic acid as basic backbones. The crystal structures show that water molecules mediate different interactions between the 3-O-sulfonate group and Lys125. Moreover, the 6-O-sulfonate group forms intermolecular interactions with another FGF2 unit apart from the main binding site. These findings suggest that the water-mediated interactions and the intermolecular interactions influence the binding affinity of different disaccharides with FGF2, correlating with their respective dissociation constants in solution.

Microwave-Assisted One-Pot Synthesis of 1,6-Anhydrosugars and Orthogonally Protected Thioglycosides

Living organisms employ glycans as recognition elements because of their large structural information density. Well-defined sugar structures are needed to fully understand and take advantage of glycan functions, but sufficient quantities of these compounds cannot be readily obtained from natural sources and have to be synthesized. Among the bottlenecks in the chemical synthesis of complex glycans is the preparation of suitably protected monosaccharide building blocks. Thus, easy, rapid, and efficient methods for building-block acquisition are desirable. Herein, we describe routes directly starting from the free sugars toward notable monosaccharide derivatives through microwave-assisted one-pot synthesis. The procedure followed the in situ generation of per-O-trimethylsilylated monosaccharide intermediates, which provided 1,6-anhydrosugars or thioglycosides upon treatment with either trimethylsilyl trifluoromethanesulfonate or trimethyl(4-methylphenylthio)silane and ZnI2, respectively, under microwave irradiation. We successfully extended the methodology to regioselective protecting group installation and manipulation toward a number of thioglucosides and the glycosylation of persilylated derivatives, all of which were conducted in a single vessel. These developed approaches open the possibility for generating arrays of suitably protected building blocks for oligosaccharide assembly in a short period with minimal number of purification stages.

Regioselective and stereoselective benzoylation of 2-N-protected 4,6-O-ketal derivatives of d-glucosamines with 1-(benzoyloxy)benzotriazole

A highly regioelective and stereoselective benzoylation of the 2-N-protected 4,6-O-ketal derivatives of D-glucosamines with 1-(benzoyloxy)benzotriazole that affords the corresponding beta anomeric benzoates in excellent yields is described herein.