Open-Close Strategy toward the Organocatalytic Generation of 2-Deoxyribosyl Oxocarbenium Ions: Pyrrolidine-Salt-Catalyzed Synthesis of 2-Deoxyribofuranosides

Strained Ion Pair Interactions-Driven Anion-Assisted Concerted Addition of Ketoximes/Aldoximes and Hydroxamic Acids to Glycals

Oximes and hydroxylamides are notable for their role as coupling partners in organic synthesis. However, their direct application as acceptors in O-glycosylation with glycal donors remains largely unexplored. Herein, we introduce a novel 2-deoxy glycosylation method for synthesizing N-O linked glycosides facilitated by sterically strained 2,4,6-tri-tert-butylpyridinium salts. This approach offers a broad substrate range, high tolerance for functional groups, and easy scalability, resulting in glycosyl oximes and glycosyloxyamines with exclusive α-selectivity and excellent yields. The effectiveness of this method is showcased through functionalization of glycosylated products, late-stage modification of bioactive drug molecules, and disaccharide synthesis. This innovative strategy offers an alternative route and holds promise for wide-ranging applications in the construction of bioactive N-O-linked glycosides in the future. The unique catalytic mechanism by the sterically hindered pyridinium salt has been studied via 1H NMR, IR, UV-vis, and fluorescence studies.

Supramolecular Capsule-Catalyzed Highly β-Selective Furanosylation Independent of the SN1/SN2 Reaction Pathway

The resorcin[4]arene capsule was found to catalyze β-selective furanosylation reactions for a variety of different furanosyl donors: α-d- and α-l-arabinosyl-, α-l-fucosyl-, α-d-ribosyl-, α-d-xylosyl-, and even α-d-lyxosyl fluorides. The scope is only limited by the inherently finite volume inside the closed capsular catalyst. The catalyst is readily available on a multi-100 g scale and can be recycled for at least seven rounds without significant loss in activity, yield, and selectivity. The mechanistic investigations indicated that the furanosylation mechanism is shifted toward an SN1 reaction on the mechanistic continuum between the prototypical SN1 and SN2 substitution types, as compared to the pyranosylation reaction inside the same catalyst. This is especially true for the lyxosyl donor, as indicated by the nucleophile reaction order of 0.26, and supported by metadynamics calculations. The mechanistic shift toward SN1 is of high interest as it indicates that this catalyst not only enables β-selective furanosylations and pyranoslyations independently of the substrate configuration but in addition also independently of the operating mechanism. To our knowledge, there is no alternative catalyst available that displays such properties.

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.

Direct Dehydrative Glycosylation Catalyzed by Diphenylammonium Triflate

Methods for direct dehydrative glycosylations of carbohydrate hemiacetals catalyzed by diphenylammonium triflate under microwave irradiation are described. Both armed and disarmed glycosyl-C1-hemiacetal donors were efficiently glycosylated in moderate to excellent yields without the need for any drying agents and stoichiometric additives. This method has been successfully applied to a solid-phase glycosylation.

Stereospecific Furanosylations Catalyzed by Bis-thiourea Hydrogen-Bond Donors

We report a new method for stereoselective O-furanosylation reactions promoted by a precisely tailored bis-thiourea hydrogen-bond-donor catalyst. Furanosyl donors outfitted with an anomeric dialkylphosphate leaving group undergo substitution with high anomeric selectivity, providing access to the challenging 1,2-cis substitution pattern with a range of alcohol acceptors. A variety of stereochemically distinct, benzyl-protected glycosyl donors were engaged successfully as substrates. Mechanistic studies support a stereospecific mechanism in which rate-determining substitution occurs from a catalyst-donor resting-state complex.