Regioselective and 1,2-cis-α-Stereoselective Glycosylation Utilizing Glycosyl-Acceptor-Derived Boronic Ester Catalyst

Designed Mannosylerythritol Lipid Analogues Exhibiting Both Selective Cytotoxicity Against Human Skin Cancer Cells and Recovery Effects on Damaged Skin Cells

Mannosylerythritol lipids (MELs) are a class of amphipathic molecules bearing a hydrophilic 4-O-β-D-mannopyranosyl-D-erythritol skeleton. Here, we designed and synthesized four kinds of MEL analogues R-MEL-A ([2R,3S]-erythritol type), S-mannosylthreitol lipid (MTL)-A ([2S,3S]-threitol type), R-MTL-A ([2R,3R]-threitol type), and α-S-MEL-A ([2S,3R]-erythritol type) using our previously reported boron-mediated aglycon delivery (BMAD) method and a neighboring group assisted glycosylation method. The selective cytotoxicity of the target compounds against cancer cells was evaluated, with R-MTL-A showing the highest selective cytotoxicity against human skin squamous carcinoma HSC-5 cells. Our findings suggest that R-MTL-A induces necrosis-like cell death against HSC-5 cells by decreasing cell membrane fluidity. R-MTL-A also exhibits an efficient recovery effect on damaged skin cells, indicating that R-MTL-A has potential as a lead compound for new cosmeceuticals with both cancer cell-selective toxicity and recovery effects on damaged skin cells.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Stereoselective O-Glycosylations by Pyrylium Salt Organocatalysis

Despite many years of invention, the field of carbohydrate chemistry remains rather inaccessible to non-specialists, which limits the scientific impact and reach of the discoveries made in the field. Aiming to increase the availability of stereoselective glycosylation chemistry for non-specialists, we have discovered that several commercially available pyrylium salts catalyze stereoselective O-glycosylations of a wide range of phenols and alkyl alcohols. This catalytic reaction utilizes trichloroacetimidates, an easily accessible and synthetically proven electrophile, takes place under air and only initiates when all three reagents are mixed, which should provide better reproducibility by non-specialists. The reaction exhibits varying degrees of stereospecificity, resulting in β-selective glycosylations from α-trichloroacetimidates, whilst an α-selective glycosylation proceeds from β-trichloroacetimidates. A mechanistic study revealed that the reaction likely proceeds via an S_N 2-like substitution on the protonated electrophile.

Self-Assembling Properties and Recovery Effects on Damaged Skin Cells of Chemically Synthesized Mannosylerythritol Lipids

Mannosylerythritol lipids (MELs), which are one of the representative sugar-based biosurfactants (BSs) produced by microorganisms, have attracted much attention in various fields in the sustainable development goals (SDGs) era. However, they are inseparable mixtures with respect to the chain length of the fatty acids. In this study, self-assembling properties and structure-activity relationship (SAR) studies of recovery effects on damaged skin cells using chemically synthesized MELs were investigated. It was revealed, for the first time, that synthetic and homogeneous MELs exhibited significant self-assembling properties to form droplets or giant vesicles. In addition, a small difference in the length of the fatty acid chains of the MELs significantly affected their recovery effects on the damaged skin cells. MELs with medium or longer length alkyl chains exhibited much higher recovery effects than that of C18-ceramide NP.

Glycan Assembly Strategy: From Concept to Application

Glycans have been hot topics in recent years due to their exhibition of numerous biological activities. However, the heterogeneity of their natural source and the complexity of their chemical synthesis impede the progress in their biological research. Thus, the development of glycan assembly strategies to acquire plenty of structurally well-defined glycans is an important issue in carbohydrate chemistry. In this review, the latest advances in glycan assembly strategies from concepts to their applications in carbohydrate synthesis, including chemical and enzymatic/chemo-enzymatic approaches, as well as solution-phase and solid-phase/tag-assisted synthesis, are summarized. Furthermore, the automated glycan assembly techniques are also outlined.

Merging Reagent Modulation and Remote Anchimeric Assistance for Glycosylation: Highly Stereoselective Synthesis of α-Glycans up to a 30-mer

The efficient synthesis of long, branched, and complex carbohydrates containing multiple 1,2-cis glycosidic linkages is a long-standing challenge. Here, we report a merging reagent modulation and 6-O-levulinoyl remote anchimeric assistance glycosylation strategy, which is successfully applied to the first highly stereoselective synthesis of the branched Dendrobium Huoshanense glycans and the linear Longan glycans containing up to 30 contiguous 1,2-cis glucosidic bonds. DFT calculations shed light on the origin of the much higher stereoselectivities of 1,2-cis glucosylation with 6-O-levulinoyl group than 6-O-acetyl or 6-O-benzoyl groups. Orthogonal one-pot glycosylation strategy based on glycosyl ortho-alkynylbenzoates and ortho-(1-phenylvinyl)benzoates has been demonstrated in the efficient synthesis of complex glycans, precluding such issues as aglycon transfer inherent to orthogonal one-pot synthesis based on thioglycosides.

Synthesis of a Pentasaccharide Repeating Unit of Lipopolysaccharide Derived from Virulent E. coli O1 and Identification of a Glycotope Candidate of Avian Pathogenic E. coli O1

Avian pathogenic Escherichia coli (APEC) is a common bacterial pathogen infecting chickens, resulting in economic losses to the poultry industry worldwide. In particular, APEC O1, one of the most common serotypes of APEC, is considered problematic due to its zoonotic potential. Therefore, many attempts have been made to develop an effective vaccine against APEC O1. In fact, the lipopolysaccharide (LPS) O-antigen of APEC O1 has been shown to be a potent antigen for inducing specific protective immune responses. However, the detailed structure of the O-antigen of APEC O1 is not clear. The present study demonstrates the first synthesis of a pentasaccharide repeating unit of LPS derived from virulent E. coli O1 and its conjugate with BSA. ELISA tests using the semi-synthetic glycoconjugate and the APEC O1 immune chicken serum revealed that the pentasaccharide is a glycotope candidate of APEC O1, with great potential as an antigen for vaccine development.

Recent Developments in Stereoselective Chemical Glycosylation

Because of their pivotal biological functions, attention to sugars and glycobiology has grown rapidly in recent decades, leading to increased demand for homogeneous oligosaccharides. The stereoselective preparation of oligosaccharides by chemical means remains challenging and continues to be a vivid research area for organic chemists. In the past decade, new approaches and reinvestigated traditional methods have transformed the field. These developments include novel catalyses, various types of glycosylation modulators and the use of photochemical energy to facilitate glycosylation. This Minireview presents a brief overview of the latest trends in chemical glycosylation, with emphasis on the stereoselective synthetic protocols developed in the past decade.

Synthesis of Disaccharide Nucleosides Utilizing the Temporary Protection of the 2',3'-cis-Diol of Ribonucleosides by a Boronic Ester

Disaccharide nucleosides are an important class of natural compounds that have a variety of biological activities. In this study, we report on the synthesis of disaccharide nucleosides utilizing the temporary protection of the 2',3'-cis-diol of ribonucleosides, such as adenosine, guanosine, uridine, 5-metyluridine, 5-fluorouridine and cytidine, by a boronic ester. The temporary protection of the above ribonucleosides permits the regioselective O-glycosylation of the 5'-hydroxyl group with thioglycosides using a p-toluenesulfenyl chloride (p-TolSCl)/silver triflate (AgOTf) promoter system to afford the corresponding disaccharide nucleosides in fairly good chemical yields. The formation of a boronic ester prepared from uridine and 4-(trifluoromethyl)phenylboronic acid was examined by ¹H, 11B and 19F NMR spectroscopy.