Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Protecting group migrations in carbohydrate chemistry

Protecting groups are valuable in chemo- and regioselective synthetic manipulations. In particular, they are indispensable in carbohydrate chemistry. Although a wide array of protecting groups are available at the disposal of carbohydrate chemists, their stability and orthogonality make the choice of protecting groups challenging. Another important factor is the migratory aptitude of different protecting groups used in carbohydrate chemistry. Migration of commonly used groups like silyl, acetal and acyl groups under various reaction conditions are discussed. Synthetic application of predicted migrations, alternate protecting groups to avoid migration and conditions favoring and disfavoring migrations are discussed in this review.

Chemical synthesis of human syndecan-4 glycopeptide bearing O-, N-sulfation and multiple aspartic acids for probing impacts of the glycan chain and the core peptide on biological functions

Proteoglycans are a family of complex glycoproteins with glycosaminoglycan chains such as heparan sulfate (HS) attached to the core protein backbone. Due to the high structural heterogeneity of HS in nature, it is challenging to decipher the respective roles of the HS chain and the core protein on proteoglycan functions. While the sulfation patterns of HS dictate many activities, the core protein can potentially impact HS functions. In order to decipher this, homogeneous proteoglycan glycopeptides are needed. Herein, we report the first successful synthesis of proteoglycan glycopeptides bearing multiple aspartic acids in the core peptide and O- and N-sulfations in the glycan chain, as exemplified by the syndecan-4 glycopeptides. To overcome the high acid sensitivities of sulfates and base sensitivities of the glycopeptide during synthesis, a new synthetic approach has been developed to produce a sulfated glycan chain on a peptide sequence prone to the formation of aspartimide side products. The availability of the structurally well-defined synthetic glycopeptide enabled the investigation of their biological functions including cytokine, growth factor binding and heparanase inhibition. Interestingly, the glycopeptide exhibited context dependent enhancement or decrease of biological activities compared to the peptide or the glycan alone. The results presented herein suggest that besides varying the sulfation patterns of HS, linking the HS chain to core proteins as in proteoglycans may be an additional approach to modulate biological functions of HS in nature.

2- O- N-Benzylcarbamoyl as a Protecting Group To Promote β-Selective Glycosylation and Its Applications in the Stereoselective Synthesis of Oligosaccharides

This study examines the utility of the N-benzylcarbamoyl (BnCar) protecting group in glycosylation reactions of the parent O-2 protected carbohydrate donor. It was found that the BnCar group imparted exclusively β-selectivity with primary and secondary alcohols. A mechanistic study revealed the activated intermediate to be the glycosyl triflate in a skew conformation, which results in β-selective glycosylation via an S_N2-like pathway. The BnCar group can be readily cleaved using tetrabutylammonium nitrite, without affecting ester and ether protecting groups. Taken together, these results show BnCar to be useful for the synthesis of complex oligosaccharides, an undertaking that requires delicate chemical differentiation of various protecting groups.

Mimicking/extracting structure and functions of natural products: synthetic approaches that address unexplored needs in chemical biology

Natural products are often attractive and challenging targets for synthetic chemists, and many have interesting biological activities. However, synthetic chemists need to be more than simply suppliers of compounds to biologists. Therefore, we have been seeking ways to actively apply organic synthetic methods to chemical biology studies of natural products and their activities. In this personal review, I would like to introduce our work on the development of new biologically active compounds inspired by, or extracted from, the structures of natural products, focusing on enhancement of functional activity and specificity and overcoming various drawbacks of the parent natural products.

Large-scale synthesis of 6-deoxy-6-sulfonatomethyl glycosides and their application for novel synthesis of a heparinoid pentasaccharide trisulfonic acid of anticoagulant activity

Multigram-scale syntheses of three 6-deoxy-6-sulfonatomethyl α-glucosides were accomplished via reactions of the corresponding primary triflate derivatives with the lithiated ethyl methanesulfonate. Chemoselective glycosylation reactions of different 6-C-sulfonatomethyl glucoside donors were studied. The sulfonic acid-containing building blocks were utilised in a novel [2+3] block synthesis of a trisulfonic acid isoster of the anticoagulant pentasaccharide idraparinux.

Chemical synthesis of a heparan sulfate glycopeptide: syndecan-1

The highly complex structure of syndecan-1 heparan sulfate glycopeptide was successfully assembled. The protective groups utilized and sequences for glycosyl linkage formation and protective group removal are critical for the success of synthesis. This is the first time this type of sulfated glycopeptides has been prepared, which lays the foundation to access other members of this important class of molecules.

Studies on the selectivity between nickel-catalyzed 1,2-cis-2-amino glycosylation of hydroxyl groups of thioglycoside acceptors with C2-substituted benzylidene N-phenyl trifluoroacetimidates and intermolecular aglycon transfer of the sulfide group

The stereoselective synthesis of saccharide thioglycosides containing 1,2-cis-2-amino glycosidic linkages is challenging. In addition to the difficulties associated with achieving high α-selectivity in the formation of 1,2-cis-2-amino glycosidic bonds, the glycosylation reaction is hampered by undesired transfer of the anomeric sulfide group from the glycosyl acceptor to the glycosyl donor. Overcoming these obstacles will pave the way for the preparation of oligosaccharides and glycoconjugates bearing the 1,2-cis-2-amino glycosidic linkages because the saccharide thioglycosides obtained can serve as donors for another coupling iteration. This approach streamlines selective deprotection and anomeric derivatization steps prior to the subsequent coupling event. We have developed an efficient approach for the synthesis of highly yielding and α-selective saccharide thioglycosides containing 1,2-cis-2-amino glycosidic bonds, via cationic nickel-catalyzed glycosylation of thioglycoside acceptors bearing the 2-trifluoromethylphenyl aglycon with N-phenyl trifluoroacetimidate donors. The 2-trifluoromethylphenyl group effectively blocks transfer of the anomeric sulfide group from the glycosyl acceptor to the C(2)-benzylidene donor and can be easily installed and activated. The current method also highlights the efficacy of the nickel catalyst selectively activating the C(2)-benzylidene imidate group in the presence of the anomeric sulfide group on the glycosyl acceptors.