Concise synthesis of a hexasaccharide present in the cell wall lipopolysaccharide of Azospirillum lipoferum Sp59b

Concise Synthesis of the Hexasaccharide Repeating Unit of the Capsular Polysaccharide of Klebsiella K19 Strain

A convergent [4+2] stereoselective block glycosylation strategy has been developed for the synthesis of the hexasaccharide repeating unit of the capsular polysaccharide of Klebsiella K19 strain in very good yield. The p-methoxybenzyl (PMB) group was used as a temporary alkyl protecting group, which was removed by tuning the glycosylation conditions. A thioglycoside was used as a glycosyl acceptor in an orthogonal glycosylation reaction. A late-stage TEMPO-mediated selective oxidation of a primary hydroxyl group into carboxylic acid allowed incorporation of the d-glucuronic acid moiety in the hexasaccharide. A combination of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO4–SiO2) was used as a thiophilic promoter for the activation of thioglycosides. HClO4–SiO2 was also used as a solid acid activator for a glycosyl trichloroacetimidate derivative.

Synthesis of an Aminooxy Derivative of the Trisaccharide Globotriose Gb3

The synthesis of α-aminooxy trisaccharide moiety [α-d-Gal-(1,4)-β-d-Gal-(1,4)-β-d-Glc-α-aminooxy], related to the cell surface globotriaosylceramide (Gb3) receptor of the B subunit of the AB₅ Shiga toxin of Shigella dysenteriae, has been synthesized for the first time in 11 steps with a 15% overall isolated yield. A highlight of this work entails utilizing chemically compatible synthetic transformations, including those related to glycosylation, incorporative of the succinimidyl moiety as a precursor to the aminooxy Gb3 derivative. The fully deprotected trisaccharide aminooxy compound was reacted with a carbonyl compound leading to oxime formation in quantitative yield underscoring the importance for future glyco-conjugations.

Carbohydrate synthesis and biosynthesis technologies for cracking of the glycan code: recent advances

The glycan code of glycoproteins can be conceptually defined at molecular level by the sequence of well characterized glycans attached to evolutionarily predetermined amino acids along the polypeptide chain. Functional consequences of protein glycosylation are numerous, and include a hierarchy of properties from general physicochemical characteristics such as solubility, stability and protection of the polypeptide from the environment up to specific glycan interactions. Definition of the glycan code for glycoproteins has been so far hampered by the lack of chemically defined glycoprotein glycoforms that proved to be extremely difficult to purify from natural sources, and the total chemical synthesis of which has been hitherto possible only for very small molecular species. This review summarizes the recent progress in chemical and chemoenzymatic synthesis of complex glycans and their protein conjugates. Progress in our understanding of the ways in which a particular glycoprotein glycoform gives rise to a unique set of functional properties is now having far reaching implications for the biotechnology of important glycodrugs such as therapeutical monoclonal antibodies, glycoprotein hormones, carbohydrate conjugates used for vaccination and other practically important protein-carbohydrate conjugates.