Stereoselective Synthesis of 2-C-Branched (Acetylmethyl) Oligosaccharides and Glycoconjugates: Lewis Acid-Catalyzed Glycosylation from 1,2-Cyclopropaneacetylated Sugars

One-Pot Synthesis of 2-C-Branched Glycosyl Triazoles by Integrating 1,2-Cyclopropanated Sugar Ring-Opening Azidation and CuAAC Reaction

A series of 2-C-branched glycosyl triazoles including triazole-tethered oligosaccharides and glycopeptides were synthesized in one pot from 1,2-cyclopropanated sugars or 2'-acetonyl-2-O-Ts-C-furanosides, NaN₃, and alkynes using PEG-400 as a single solvent. Nucleophilic ring-opening azidation of 1,2-cyclopropanated sugars (or 2'-acetonyl group 1,2-migration-azidation of C-furanosides) obtained glycosyl azides, which upon reaction with alkynes under CuAAC conditions achieved glycosyl triazoles in good yields and high stereoselectivity without the need to change the solvent and isolate any intermediates.

Phosphotungstic acid as a novel acidic catalyst for carbohydrate protection and glycosylation

This work demonstrates the utilization of phosphotungstic acid (PTA) as a novel acidic catalyst for carbohydrate reactions, such as per-O-acetylation, regioselective O-4,6 benzylidene acetal formation, regioselective O-4 ring-opening, and glycosylation. These reactions are basic and salient during the synthesis of carbohydrate-based bioactive oligomers. Phosphotungstic acid's high acidity and eco-friendly character make it a tempting alternative to corrosive homogeneous acids. The various homogenous acid catalysts were replaced by the phosphotungstic acid solely for different carbohydrate reactions. It can be widely used as a catalyst for organic reactions as it is thermally stable and easy to handle. In our work, the reactions are operated smoothly under ambient conditions; the temperature varies from 0 °C to room temperature. Good to excellent yields were obtained in all four kinds of reactions.

An ultra-low thiourea catalyzed strain-release glycosylation and a multicatalytic diversification strategy

The utility of thiourea catalysis in selective glycosylation strategies has gained significant momentum lately due to its versatility in hydrogen bonding or anionic recognition activation modes. The use of these non-covalent interactions constitute a powerful means to construct glycosidic linkages as it mimics physiologically occurring glycosyltransferases. However, glycosyl donor activation through the currently employed catalysts is moderate such that, in general, catalyst loadings are rather high in these transformations. In addition, thiourea catalysis has not been well explored for the synthesis of furanosides. Herein, we demonstrate an ultra-low loadings stereoselective and stereospecific thiourea catalyzed strain-release furanosylation and pyranosylation strategy. Our ultra-low organocatalyzed furanosylation enables a multicatalytic strategy, which opens up a unique avenue towards rapid diversification of synthetic glycosides. In-situ NMR monitoring unravel insights into unknown reaction intermediates and initial rate kinetic studies reveal a plausible synergistic hydrogen bonding/Brønsted acid activation mode.

Non-covalent glycosyl donor activation often requires high organocatalyst loadings. Here, the authors demonstrate that strain-release glycosylations can take place at very low thiourea catalyst loadings. In addition, the authors developed a one-pot multicatalytic strategy that can diversify glycosides rapidly.

Recent developments in β-C-glycosides: synthesis and applications

In the last few years, considerable progress has been made in the synthesis of C-glycosides. Despite its challenging chemistry, due to its versatility, C-glycosides play a pivotal role in developing novel materials, surfactants and bioactive molecules. In this review, we present snapshots of various synthetic methodologies developed for C-glycosides in the recent years and the potential application of C-glycosides derived from β-C-glycosidic ketones.

Stereospecific [3+2] cycloaddition of 1,2-cyclopropanated sugars and ketones catalyzed by SnCl4: an efficient synthesis of multi-substituted perhydrofuro[2,3-b]furans and perhydrofuro[2,3-b]pyrans

An efficient stereospecific [3+2] cycloaddition of 1,2-cyclopropanated sugars and ketones catalyzed by SnCl₄ is described.

Extending the scope of the Ferrier reaction: fragmentation-rearrangement reactions of selectively substituted 1,2-cyclopropanated glucose derivatives

Two further variations of the Ferrier-type allylic rearrangements of 1,2-cyclopropanated glucose derivatives bearing an acetoxylated carbon at the 1'-position are described. In the first, treatment of the cyclopropanated sugar with a nucleophile (ROH, PhSH, azide) and Lewis acid (BF(3)·Et(2)O or Al(OTf)(3)), gives 2-C-vinyl glucosides in good yields and α-selectivities. Alternatively, treatment with a combination of Lewis acid and acetic acid leads to a novel fragmentation-rearrangement to form a 2,3-dehydro-2-formyl-C-glycoside.

Recent advances in the stereoselective synthesis of carbohydrate 2-C-analogs

C-branched carbohydrates are of current interest for glycochemistry, are widely found in nature and serve as important subunits in many antibiotics, bacterial polysaccharides and macrolides. Among C-functionalized saccharides, 2-C-branched carbohydrates represent challenging structures for synthetic chemists, since in contrast to C-glycosides they are not easily accessible from glycosyl bromides or other simple precursors. In this perspective we want to summarize recent approaches to 2-C-branched carbohydrates over the past fifteen years. The two main strategies are based on ring-opening of 1,2-cyclopropanated carbohydrates by various reagents, as well as radical additions to glycals and further transformations, developed in our group. Both methods are characterized by high stereoselectivities and good yields and give access to a broad variety of functionalized carbohydrate 2-C-analogs.