Organocatalysis: A recent development on stereoselective synthesis of o-glycosides

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.

Green Catalysis: Water as a Sustainable Medium in Organocatalyzed Reactions

The use of organocatalysts has increased significantly in recent years due to their tremendous applications in green solvents. Thus, using water as a solvent has evolved as a critical factor. Organocatalysts are efficient and sustainable agents for promoting chemical reactions in water. The literature has been extensively reviewed, and the use of various organocatalysts for three fundamental C-C bond-forming processes─the Aldol, Michael, and Mannich reactions in aqueous media─have been compiled in this study. Organocatalysts can overcome the limitations of conventional organic solvents by achieving high reaction rates and regioselectivity in water. This Review highlights the advantages of organocatalysts in aqueous media for these key reactions. It discusses the principles behind designing effective organocatalysts, focusing on their impact on selectivity, sustainability, and reaction efficiency. This study also summarizes the most significant advancements in sustainable organic reactions over the past decade.

Recent Advances in Organic Photocatalyst-Promoted Carbohydrate Synthesis and Modification under Light Irradiation

Photoredox catalysis has been developed as a sustainable and eco-friendly catalytic strategy, which might provide innovative solutions to solve the current synthetic challenges and barriers in carbohydrate chemistry. During the last few decades, the study of organic photocatalyst-promoted carbohydrate synthesis and modification has received significant attention, which provides an excellent and inexpensive metal-free alternative to photoredox catalysis as well as introduces a new fastest-growing era to access complex carbohydrates simply. In this review, we aim to provide an overview of organic photocatalyst-promoted carbohydrate synthesis and modification under light irradiation, which is expected to provide new directions for further investigation.

Cooperative Catalysis in Stereoselective O- and N-Glycosylations with Glycosyl Trichloroacetimidates Mediated by Singly Protonated Phenanthrolinium Salt and Trichloroacetamide

The development of small-molecule catalysts that can effectively activate both reacting partners simultaneously represents a pivotal pursuit in advancing the field of stereoselective glycosylation reactions. We report herein the development of the singly protonated form of readily available phenanthroline as an effective cooperative catalyst that facilitates the coupling of a wide variety of aliphatic alcohols, phenols, and aromatic amines with α-glycosyl trichloroacetimidate donors. The glycosylation reaction likely proceeds via an SN2-like mechanism, generating β-selective glycoside products. The developed protocol provides access to O- and N-glycosides in good yields with excellent levels of β-selectivity and enables late-stage functionalization of O- and N-glycosides via cross-coupling reactions. Importantly, this method exhibits excellent β-selectivity that is unattainable through a C2-O-acyl neighboring group participation strategy, especially in the case of glycosyl donors already containing a C2 heteroatom or sugar unit. Kinetic studies demonstrate that the byproduct trichloroacetamide group plays a previously undiscovered pivotal role in influencing the reactivity and selectivity of the reaction. A proposed mechanism involving simultaneous activation of the glycosyl donor and acceptor by the singly protonated phenanthrolinium salt catalyst with the assistance of the trichloroacetamide group is supported by kinetic analysis and preliminary computational studies. This cooperative catalysis process involves four consecutive hydrogen bond interactions. The first interaction occurs between the carbonyl oxygen of the trichloroacetamide group and the hydroxyl group of alcohol nucleophile (C═O···HO). The second involves the trichloroacetamide-NH2 forming a hydrogen bond with the nitrogen atom of the phenanthroline (NH···N). The third involves the donor trichloroacetimidate (═NH) engaging in a hydrogen bond interaction with the phenanthrolinium-NH (NH···N═H). Lastly, the protonated trichloroacetimidate-NH2 forms a hydrogen bond with the fluorine atom of the tetrafluoroborate ion.

Synthesis of C-Alkyl Glycosides from Alkyl Bromides and Glycosyl Carboxylic Acids via Ni/Photoredox Dual Catalysis

C-Alkyl glycosides, an important class of C-glycosides, are widely found in various drugs and natural products. The synthesis of C-alkyl glycosides has attracted considerable attention. Herein, we developed a Ni/photoredox catalyzed decarboxylative C(sp3)-C(sp3) coupling reaction of stable glycosylcarboxylic acids with simple aliphatic bromides to generate C-alkyl glycosides. The method successfully linked several functional molecular fragments (natural products or drugs) to a sugar moiety, showing the extensive application prospects of this transformation. Controlled experiments and DFT calculations demonstrated that the reaction pathway contains a free radical process, and a possible mechanism is proposed.

Recent advances in stereoselective 1,2-cis-O-glycosylations

For the stereoselective assembly of bioactive glycans with various functions, 1,2-cis-O-glycosylation is one of the most essential issues in synthetic carbohydrate chemistry. The cis-configured O-glycosidic linkages to the substituents at two positions of the non-reducing side residue of the glycosides such as α-glucopyranoside, α-galactopyranoside, β-mannopyranoside, β-arabinofuranoside, and other rather rare glycosides are found in natural glycans, including glycoconjugate (glycoproteins, glycolipids, proteoglycans, and microbial polysaccharides) and glycoside natural products. The way to 1,2-trans isomers is well sophisticated by using the effect of neighboring group participation from the most effective and kinetically favored C-2 substituent such as an acyl group, although high stereoselective synthesis of 1,2-cis glycosides without formation of 1,2-trans isomers is far less straightforward. Although the key factors that control the stereoselectivity of glycosylation are largely understood since chemical glycosylation was considered to be one of the useful methods to obtain glycosidic linkages as the alternative way of isolation from natural sources, strictly controlled formation of these 1,2-cis glycosides is generally difficult. This minireview introduces some of the recent advances in the development of 1,2-cis selective glycosylations, including the quite recent developments in glycosyl donor modification, reaction conditions, and methods for activation of intermolecular glycosylation, including the bimodal glycosylation strategy for 1,2-cis and 1,2-trans glycosides, as well as intramolecular glycosylations, including recent applications of NAP-ether-mediated intramolecular aglycon delivery.