An eco-friendly N-benzoylglycine/thiourea cooperative catalyzed stereoselective synthesis of β-L-rhamnopyranosides

Organocatalyzed O-glycosylation of glycosyl trichloroacetimidates donors: l-prolinethioamide as brønsted acid catalyst

In this study, we present the utilization of l-proline-derived thioamide small organic molecules as an effective organocatalyst for the O-glycosylation of various glycosyl trichloroacetimidate donors, eliminating the need for any cocatalysts or additives. The catalytic process achieves high yields with a wide array of alcohol and sugar nucleophiles, demonstrating a broad substrate scope and operational simplicity under mild reaction conditions. Preliminary mechanistic investigations indicate that l-prolinethioamide facilitates the glycosylation reaction via Brønsted acid/base catalysis, involving the formation of a catalyst-acceptor adduct.

Thiourea-catalyzed glycosylation: a breakthrough in stereoselective synthesis of oligosaccharides

Oligosaccharides are vital biomolecules that play an essential role in various biological functions. For studying biochemical processes, it is essential to have access to pure as well as structurally well-defined oligosaccharides. However, achieving high yields with excellent anomeric stereoselectivity through chemical glycosylation remains a challenging task for chemists. In the last few years, thiourea-catalyzed glycosylation has offered promising progress in carbohydrate chemistry due to its ability to catalytically activate glycosyl donors through non-covalent interactions, particularly hydrogen bonding. These thiourea catalysts have emerged as powerful organocatalysts, gaining popularity for facilitating highly stereoselective glycosylation reactions. This review highlights recent developments in thiourea-catalyzed glycosylation, a field still in its early stages but showing significant potential.

Fe(III)-catalyzed stereoselective synthesis of deoxyglycosides using stable bifunctional deoxy-phenylpropiolate glycoside donors

Herein, we report a mild and economical route for the stereoselective synthesis of 2-deoxy and 2,6-dideoxyglycosides via FeCl3-catalyzed activation of bench stable deoxy-phenylpropiolate glycosyl donors (D-PPGs). Optimized reaction conditions work well under additive-free conditions to afford the corresponding 2-deoxy and 2,6-dideoxyglycosides in good yields with high α-anomeric selectivity by reacting with sugar and non-sugar-based acceptors. The optimized conditions were also extended for disarmed D-PPG donors. In addition, the developed strategy is amenable to high-scale-up synthesis.

Glycosyl 3-Phenyl-4-pentenoates as Versatile Glycosyl Donors: Reactivity and Their Application in One-Pot Oligosaccharide Assemblies

Both glycoconjugates and oligosaccharides are important biomolecules having significant roles in several biological processes, and a new strategy for their synthesis is crucial. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf) promoted glycosidation approach with shelf-stable 3-phenyl-4-pentenoate glycosyl as a donor for the efficient synthesis of O/C-glycosides with free alcohols, silylated alcohols, and C-type nucleophile acceptors in good to excellent yields. The mild activation conditions and outstanding reactivity of phenyl substituted pentenoate donors analogous to 4-pentenoate glycosyl donors enhance their applicability to various one-pot strategies for the synthesis of oligosaccharides, such as single-catalyst one-pot and acceptor reactivity-controlled one-pot strategies.

Organocatalysis applied to carbohydrates: from roots to current developments

Organocatalysis emerged in the last decade as a powerful tool for the synthesis of complex molecules. In the field of carbohydrates, it found widespread use in the synthesis of rare and non-natural carbohydrate derivatives. Additionally, it has also found important application in the stereoselective functionalization of the anomeric carbon in glycosylation reactions. These efforts culminated in the development of different types of catalysts operating through distinct activation modes that allow the selective synthesis of α- or β-glycosides even on daunting substrates. All these advances starting from its first examples in carbohydrate synthesis to the current developments in glycosylation reactions are reviewed.

Exploiting non-covalent interactions in selective carbohydrate synthesis

Non-covalent interactions (NCIs) are a vital component of biological bond-forming events, and have found important applications in multiple branches of chemistry. In recent years, the biomimetic exploitation of NCIs in challenging glycosidic bond formation and glycofunctionalizations has attracted significant interest across diverse communities of organic and carbohydrate chemists. This emerging theme is a major new direction in contemporary carbohydrate chemistry, and is rapidly gaining traction as a robust strategy to tackle long-standing issues such as anomeric and site selectivity. This Review thus seeks to provide a bird's-eye view of wide-ranging advances in harnessing NCIs within the broad field of synthetic carbohydrate chemistry. These include the exploitation of NCIs in non-covalent catalysed glycosylations, in non-covalent catalysed glycofunctionalizations, in aglycone delivery, in stabilization of intermediates and transition states, in the existence of intramolecular hydrogen bonding networks and in aggregation by hydrogen bonds. In addition, recent emerging opportunities in exploiting halogen bonding and other unconventional NCIs, such as CH-π, cation-π and cation-n interactions, in various aspects of carbohydrate chemistry are also examined.

Recent advances in β-l-rhamnosylation

l-Rhamnose forms the key components of important antigenic oligo- and polysaccharides of a variety of pathogens. Obtaining 1,2-cis stereoselectivity in the glycosylation of l-rhamnoside is quite challenging due to the unavailability of neighboring group participation and disfavoring of the anomeric effect and stereoelectronic effect of the substituents on the C-2 axial position. Nevertheless, various methodologies have been developed exploiting diverse pathways for obtaining β-stereoselectivity in the glycosylation of l-rhamnose. This review describes the recent advances in β-l-rhamnosylation and its applications in the total synthesis of β-l-rhamnose-containing biologically important oligosaccharides.

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.

Picoloyl protecting group in synthesis: focus on a highly chemoselective catalytic removal

The picoloyl ester (Pico) has proven to be a versatile protecting group in carbohydrate chemistry. It can be used for the purpose of stereocontrolling glycosylations via an H-bond-mediated Aglycone Delivery (HAD) method. It can also be used as a temporary protecting group that can be efficiently introduced and chemoselectively cleaved in the presence of practically all other common protecting groups used in synthesis. Herein, we will describe a new method for rapid, catalytic, and highly chemoselective removal of the picoloyl group using inexpensive copper(ii) or iron(iii) salts.