Unexpected Anomeric Selectivity of a 1-C-Arylglycal Donor in Kdo Glycoside Synthesis

Additive-assisted synthesis of α-Kdo glycosides with peracetylated glycosyl ynenoate as a donor

A DMF-modulated glycosylation approach for the stereoselective synthesis of α-Kdo glycosides with readily accessible peracetylated Kdo ynenoate as a donor was described. By utilizing this approach, we completed the synthesis of various linkage types of Kdo-Kdo disaccharides and the α-Kdo-containing protected trisaccharide variant relevant to the lipopolysaccharide of Coxiella burnetii strain Nine Mile.

Hydroxamate-directed access to β-Kdo glycosides

The reaction repertoire for forming transient aziridinone or azaoxyallyl cations from α-halohydroxamate is conceptually extended to design Kdo-glycosyl donors by installing the hydroxamate moiety at an anomeric centre, which is shown to be highly effective for stereoselective access to β-Kdo glycosides. The pivotal roles of hydroxamate over amide are revealed in control experiments.

Preparation of glycosyl carboxylic acids via stereoselective synthesis and oxidative cleavage of C-vinyl glycosides

We have developed an improved cyanide-free strategy for the synthesis of glycosyl carboxylic acids, employing stereoselective C-vinyl glycosylation and oxidative cleavage of C-vinyl glycosides as key steps. Compared to our previous work, the amount of NaIO₄ required for the oxidative cleavage step is reduced significantly from 18 equivalents to 4.5 equivalents. This modification not only is advantageous in terms of operation and costs but also avoids the over-oxidation problem, thus greatly expanding the substrate scope, which is evidenced by the fact that 10 out of 21 glycosyl carboxylic acids synthesized are undocumented. With differently O5-protected furanosyl acids in hand, we demonstrate that an electron-rich protecting group is beneficial for the decarboxylative arylation of furanosyl carboxylic acids. This represents a rare example of protecting groups affecting the reaction efficiency in radical C-glycosylation. As C-vinyl glycosides can be prepared stereoselectively and the oxidative step is stereoretentive, the approach provides an effective means to access 1,2-trans or 1,2-cis glycosyl acids, which would be a valuable alternative to the cyanide-based synthesis of glycosyl carboxylic acids.

Stereoselective Synthesis of β-C-Glycosides of 3-Deoxy-d-manno-oct-2-ulosonic Acid (Kdo) via SmI2-Mediated Reformatsky Reactions

An efficient and simple approach for stereoselective synthesis of β-Kdo C-glycosides was described, which relies on easily available peracetylated anomeric acetate or anomeric 2-pyridyl sulfide to couple with carbonyl compounds via SmI₂-mediated Reformatsky reactions. The utility of this methodology is exemplified by the streamlined synthesis of a practical β-Kdo C-glycoside with an anomeric aminopropyl linker to conjugate with other biomolecules for further biological studies.

Cyanide-Free Synthesis of Glycosyl Carboxylic Acids and Application for the Synthesis of Scleropentaside A

We have developed a cyanide-free strategy for the synthesis of glycosyl carboxylic acids, which can provide 1,2-trans or 1,2-cis glycosyl carboxylic acids and is compatible with common protecting groups. The synthetic utility was demonstrated by the synthesis of 12 unreported glycosyl acids and the total synthesis of scleropentaside A.

Stereocontrolled Synthesis of the Equatorial Glycosides of 3-Deoxy-d-manno-oct-2-ulosonic Acid: Role of Side Chain Conformation

The pseudosymmetric relationship of the bacterial sialic acid, pseudaminic acid, and 3-deoxy-d-manno-oct-2-ulosonic acid (KDO) affords the hypothesis that suitably protected KDO donors will adopt the trans, gauche conformation of their side chain and consequently be highly equatorially selective in their coupling reactions conducted at low temperature. This hypothesis is borne out by the synthesis, conformational analysis, and excellent equatorial selectivity seen on coupling of per-O-acetyl or benzyl-protected KDO donors in dichloromethane at -78 °C. Mechanistic understanding of glycosylation reactions is advancing to a stage at which predictions of selectivity can be made. In this instance, predictions of selectivity provide the first highly selective entry into KDO equatorial glycosides such as are found in the capsular polysaccharides of numerous pathogenic bacteria.

Dimethylformamide-Modulated Kdo Glycosylation for Stereoselective Synthesis of α-Kdo Glycosides

A simple and direct DMF-modulated α-selective Kdo glycosylation approach for the stereoselective synthesis of the α-linked Kdo glycosides is developed. Glycosylation of the readily available peracetylated Kdo ortho-hexynylbenzoate with common acceptor alcohols using SPhosAuNTf₂ as a promoter and DMF as a modulating molecule afforded a range of Kdo glycosides with good α-selectivities. Furthermore, the present method is effectively applied in the latent-active synthesis of the α-linked di-Kdo glycoside bearing a linker at the reducing end. Finally, the first observation of a Kdo imidinium ion in the low-temperature NMR provides evidence for the plausible mechanism of the DMF-modulated α-selective Kdo glycosylation.

Synthesis of the β-linked GalNAc-Kdo disaccharide antigen of the capsular polysaccharide of Kingella kingae KK01

The first construction of the challenging β-(1 → 5)-linked GalNAc-Kdo skeleton is described for the synthesis of the disaccharide antigen of the capsular polysaccharide of Kingella kingae KK01. TfOH-catalyzed glycosylation of N-Troc-protected d-galactosaminyl N-phenyl trifluoroacetimidate with a sterically hindered 5-hydroxyl group of the β-Kdo building block in toluene proceeded smoothly to provide the desired disaccharide in excellent yield with satisfactory β-selectivity. An optimal sequence for the deprotection of the disaccharide skeleton was found to access the disaccharide antigen of Kingella kingae KK01 for further immunological studies.

C-Glycopyranosyl Arenes and Hetarenes: Synthetic Methods and Bioactivity Focused on Antidiabetic Potential

This Review summarizes close to 500 primary publications and surveys published since 2000 about the syntheses and diverse bioactivities of C-glycopyranosyl (het)arenes. A classification of the preparative routes to these synthetic targets according to methodologies and compound categories is provided. Several of these compounds, regardless of their natural or synthetic origin, display antidiabetic properties due to enzyme inhibition (glycogen phosphorylase, protein tyrosine phosphatase 1B) or by inhibiting renal sodium-dependent glucose cotransporter 2 (SGLT2). The latter class of synthetic inhibitors, very recently approved as antihyperglycemic drugs, opens new perspectives in the pharmacological treatment of type 2 diabetes. Various compounds with the C-glycopyranosyl (het)arene motif were subjected to biological studies displaying among others antioxidant, antiviral, antibiotic, antiadhesive, cytotoxic, and glycoenzyme inhibitory effects.

Progress in Kdo-glycoside chemistry

Glycosylation chemistry of 3-deoxy-D-manno-oct-2-ulosonic acid units has been considerably developed within the last decade. This review covers major achievements with respect to improved yields and anomeric selectivity as well as suppression of the elimination side reaction via selection of dedicated protecting groups and appropriate activation of the anomeric center.

Preparation of a protected 3-deoxy-D-manno-oct-2-ulosonate glycal donor for the synthesis of β-KDO-containing oligosaccharides

A practical method for the synthesis of KDO glycal donors was developed. The prepared KDO donors exhibited excellent disastereoselectivity of glycosylation in a CH2Cl2-CH3CN solvent mixture, which was found to be associated with the isopropylidene protection at the C-4 and C-5 hydroxyls. The synthetic use of the KDO donor was demonstrated in the preparation of β-KDO-containing oligosaccharides.