Zinc(II) Iodide-Directed ß-Mannosylation: Reaction Selectivity, Mode, and Application.

A direct, efficient, and versatile glycosylation methodology promises the systematic synthesis of oligosaccharides and glycoconjugates in a streamlined fashion like the synthesis of medium to long-chain nucleotides and peptides. The development of a generally applicable approach for the construction of 1,2-cis-glycosidic bond with controlled stereoselectivity remains a major challenge, especially for the synthesis of ß-mannosides. Here, we report a direct mannosylation strategy mediated by ZnI2, a mild Lewis acid, for the highly stereoselective construction of 1,2-cis-ß linkages employing easily accessible 4,6-O-tethered mannosyl trichloroacetimidate donors. The versatility and effectiveness of this strategy were demonstrated with successful ß-mannosylation of a wide variety of alcohol acceptors, including complex natural products, amino acids, and glycosides. Through iteratively performing ZnI2-mediated mannosylation with the chitobiosyl azide acceptor followed by site-selective deprotection of the mannosylation product, the novel methodology enables the modular synthesis of the key intermediate trisaccharide with Man-ß-(1 → 4)-GlcNAc-ß-(1 → 4)-GlcNAc linkage for N-glycan synthesis. Theoretical investigations with density functional theory calculations delved into the mechanistic details of this ß-selective mannosylation and elucidated two zinc cations' essential roles as the activating agent of the donor and the principal mediator of the cis-directing intermolecular interaction.

Unified Strategy toward Stereocontrolled Assembly of Various Glucans Based on Bimodal Glycosyl Donors.

Polymers of glucose, the most abundant and one of the biologically important natural products, named glucans are widely present in fungi, bacteria, mammals, and plants with various anomeric configurations and glycosidic linkages. Because of their structural diversity, the unified strategy for the assembly of pure glucans is yet to be developed. Herein, we describe a general strategy that is applicable to construction of all types of glucans by exploiting a bimodal glycosyl donor equipped with C2-o-TsNHbenzyl ether (TAB), which enables stereocontrolled synthesis of both α- and ß-glycosides by switching reaction conditions.

Total synthesis of the hexasaccharide arabinan domain of mycobacterial arabinogalactan.

The hexasaccharide arabinan domain of Mycobacterial Arabinogalactan was provided with the versatile methodology toward ß-selective arabinofuranosylation directed by B(C6F5)3, demonstrating the effectiveness of the ß-arabinofuranosylation strategy. Derivatization of the amino moiety at the reducing end are essential prerequisites for elucidating the biosynthetic pathway and conjugating of this compound to a protein carrier for vaccine generation.

Zinc(ii)-mediated stereoselective construction of 1,2-cis 2-azido-2-deoxy glycosidic linkage: assembly of Acinetobacter baumannii K48 capsular pentasaccharide derivative.

The capsular polysaccharide (CPS) is a major virulence factor of the pathogenic Acinetobacter baumannii and a promising target for vaccine development. However, the synthesis of the 1,2-cis-2-amino-2-deoxyglycoside core of CPS remains challenging to date. Here we develop a highly α-selective ZnI2-mediated 1,2-cis 2-azido-2-deoxy chemical glycosylation strategy using 2-azido-2-deoxy glucosyl donors equipped with various 4,6-O-tethered groups. Among them the tetraisopropyldisiloxane (TIPDS)-protected 2-azido-2-deoxy-d-glucosyl donor afforded predominantly α-glycoside (α : ß = >20 : 1) in maximum yield. This novel approach applies to a wide acceptor substrate scope, including various aliphatic alcohols, sugar alcohols, and natural products. We demonstrated the versatility and effectiveness of this strategy by the synthesis of A. baumannii K48 capsular pentasaccharide repeating fragments, employing the developed reaction as the key step for constructing the 1,2-cis 2-azido-2-deoxy glycosidic linkage. The reaction mechanism was explored with combined experimental variable-temperature NMR (VT-NMR) studies and mass spectroscopy (MS) analysis, and theoretical density functional theory calculations, which suggested the formation of covalent α-C1GlcN-iodide intermediate in equilibrium with separated oxocarbenium-counter ion pair, followed by an SN1-like α-nucleophilic attack most likely from separated ion pairs by the ZnI2-activated acceptor complex under the influence of the 2-azido gauche effect.

Immunomodulatory Effect and Biological Significance of ß-Glucans.

ß-glucan, one of the homopolysaccharides composed of D-glucose, exists widely in cereals and microorganisms and possesses various biological activities, including anti-inflammatory, antioxidant, and anti-tumor properties. More recently, there has been mounting proof that ß-glucan functions as a physiologically active "biological response modulator (BRM)", promoting dendritic cell maturation, cytokine secretion, and regulating adaptive immune responses-all of which are directly connected with ß-glucan-regulated glucan receptors. This review focuses on the sources, structures, immune regulation, and receptor recognition mechanisms of ß-glucan.

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans.

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as ß-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.

ZnI2-Mediated ß-Galactosylation of C2-Ether-Type Donor.

Conventional glycosylation with galactosyl donors having C-2 benzyl (Bn) ether-type functionality often leads to anomeric mixtures, due to the anomeric and steric effects that stabilize the 1,2-cis-α- and 1,2-trans-ß-glycosides, respectively. Herein we report a versatile ZnI2-directed ß-galactosylation approach employing a 4,6-O-tethered and 2-O-Bn galactosyl donor for the stereoselective and efficient synthesis of ß-O-galactosides. With a broad substrate scope, the reaction tolerates a wide range of functional groups and complex molecular architectures, providing stereocontrolled ß-galactosides in moderate to excellent yields. The practicality of this transformation is demonstrated through the synthesis of a tetrasaccharide arabinogalactan fragment with high stereoselectivity.

ZnI2-Directed Stereocontrolled α-Glucosylation.

Here we report a glucosylation strategy mediated by ZnI2, a cheap and mild Lewis acid, for the highly stereoselective construction of 1,2-cis-O-glycosidic linkages using easily accessible and common 4,6-O-tethered glucosyl donors. The versatility and effectiveness of the α-glucosylation strategy were demonstrated successfully with various acceptors, including complex alcohols. This approach demonstrates the feasibility of the modular synthesis of various α-glucans with both linear and branched backbone structures.