Synthesis of Protected 3-Deoxy-3-fluoro- and 4-Deoxy-4-fluoro-d-galactopyranosides from Levoglucosan

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position. There are not many applications involving polyfluorinated carbohydrates, here defined as monosaccharides in which more than one carbon has at least one fluorine substituent directly attached to it, with the notable exception of their use as mechanism-based inhibitors. The increasing attention to carbohydrate physical properties, especially around lipophilicity, has resulted in a surge of interest for this class of compounds. This review covers the considerable body of work toward the synthesis of polyfluorinated hexoses, pentoses, ketosugars, and aminosugars including sialic acids and nucleosides. An overview of the current state of the art of their glycosidation is also provided.

Synthesis of fluorinated thiodigalactoside analogues

In this work, we report the first synthesis of fluorinated thiodigalactoside analogues. We used tri-isopropylsilyl thioglycosides as masked glycosyl thiol nucleophiles for the elaboration of two monofluorinated heterodimers, one difluorinated homodimer, and one difluorinated heterodimer. Moreover, we also present an alternative synthesis of 3-deoxy-3-fluorogalactose and 4-deoxy-4-fluorogalactose from a common precursor. Finally, this small set of more stable thiodigalactoside analogues could be interesting inhibitors of galactose-specific lectins.

Fluorine effect in nucleophilic fluorination at C4 of 1,6-anhydro-2,3-dideoxy-2,3-difluoro-β-D-hexopyranose

In this work, we have developed a simple synthetic approach using Et3N·3HF as an alternative to the DAST reagent. We controlled the stereochemistry of the nucleophilic fluorination at C4 of 1,6-anhydro-2,3-dideoxy-2,3-difluoro-4-O-triflate-β-ᴅ-talopyranose using Et3N·3HF or in situ generated Et3N·1HF. The influence of the fluorine atom at C2 on reactivity at C4 could contribute to a new fluorine effect in nucleophilic substitution. Finally, with the continuous objective of synthesizing novel multi-vicinal fluorosugars, we prepared one difluorinated and one trifluorinated alditol analogue.

Addressing the Structural Complexity of Fluorinated Glucose Analogues: Insight into Lipophilicities and Solvation Effects

In this work, we synthesized all mono-, di-, and trifluorinated glucopyranose analogues at positions C-2, C-3, C-4, and C-6. This systematic investigation allowed us to perform direct comparison of ¹⁹ F resonances of fluorinated glucose analogues and also to determine their lipophilicities. Compounds with a fluorine atom at C-6 are usually the most hydrophilic, whereas those with vicinal polyfluorinated motifs are the most lipophilic. Finally, the solvation energies of fluorinated glucose analogues were assessed for the first time by using density functional theory. This method allowed the log P prediction of fluoroglucose analogues, which was comparable to the C log P values obtained from various web-based programs.

Stereoselective Synthesis of Fluorinated Galactopyranosides as Potential Molecular Probes for Galactophilic Proteins: Assessment of Monofluorogalactoside-LecA Interactions

The replacement of hydroxyl groups by fluorine atoms on hexopyranoside scaffolds may allow access to invaluable tools for studying various biochemical processes. As part of ongoing activities toward the preparation of fluorinated carbohydrates, a systematic investigation involving the synthesis and biological evaluation of a series of mono- and polyfluorinated galactopyranosides is described. Various monofluorogalactopyranosides, a trifluorinated, and a tetrafluorinated galactopyranoside have been prepared using a Chiron approach. Given the scarcity of these compounds in the literature, in addition to their synthesis, their biological profiles were evaluated. Firstly, the fluorinated compounds were investigated as antiproliferative agents using normal human and mouse cells in comparison with cancerous cells. Most of the fluorinated compounds showed no antiproliferative activity. Secondly, these carbohydrate probes were used as potential inhibitors of galactophilic lectins. The first transverse relaxation-optimized spectroscopy (TROSY) NMR experiments were performed on these interactions, examining chemical shift perturbations of the backbone resonances of LecA, a virulence factor from Pseudomonas aeruginosa. Moreover, taking advantage of the fluorine atom, the ¹⁹ F NMR resonances of the monofluorogalactopyranosides were directly monitored in the presence and absence of LecA to assess ligand binding. Lastly, these results were corroborated with the binding potencies of the monofluorinated galactopyranoside derivatives by isothermal titration calorimetry experiments. Analogues with fluorine atoms at C-3 and C-4 showed weaker affinities with LecA as compared to those with the fluorine atom at C-2 or C-6. This research has focused on the chemical synthesis of "drug-like" low-molecular-weight inhibitors that circumvent drawbacks typically associated with natural oligosaccharides.

Synthetic strategies for fluorination of carbohydrates

Different synthetic strategies for accomplishing regio- and stereoselective fluorinations of carbohydrate scaffolds are discussed in light of the biological implications arising from such substitutions.

Acyl Glycosides through Stereospecific Glycosyl Cross-Coupling: Rapid Access to C(sp3)-Linked Glycomimetics

Replacement of a glycosidic bond with hydrolytically stable C-C surrogates is an efficient strategy to access glycomimetics with improved physicochemical and pharmacological properties. We describe here a stereoretentive cross-coupling reaction of glycosyl stannanes with C(sp2)- and C(sp3)-thio(seleno)esters suitable for the preparation C-acyl glycosides as synthetic building blocks to obtain C(sp3)-linked and fluorinated glycomimetics. First, we identified a set of standardized conditions employing a Pd(0) precatalyst, CuCl additive, and phosphite ligand that provided access to C-acyl glycosides without deterioration of anomeric integrity and decarbonylation of the acyl donors (>40 examples). Second, we demonstrated that C(sp3)-glycomimetics could be introduced into the anomeric position via a direct conversion of C1 ketones. Specifically, the conversion of the carbonyl group into a CF2 mimetic is an appealing method to access valuable fluorinated analogues. We also illustrate that the introduction of other carbonyl-based groups into the C1 position of mono- and oligosaccharides can be accomplished using the corresponding acyl donors. This protocol is amenable to late-stage glycodiversification and programmed mutation of the C-O bond into hydrolytically stable C-C bonds. Taken together, stereoretentive anomeric acylation represents a convenient method to prepare a diverse set of glycan mimetics with minimal synthetic manipulations and with absolute control of anomeric configuration.

A Chiron approach towards the stereoselective synthesis of polyfluorinated carbohydrates

The replacement of hydroxyl groups by fluorine atoms on hexopyranose scaffolds may allow access to the discovery of new chemical entities possessing unique physical, chemical and ultimately even biological properties. The prospect of significant effects generated by such multiple and controlled substitutions encouraged us to develop diverse synthetic routes towards the stereoselective synthesis of polyfluorinated hexopyranoses, six of which are unprecedented. Hence, we report the synthesis of heavily fluorinated galactose, glucose, mannose, talose, allose, fucose, and galacturonic acid methyl ester using a Chiron approach from inexpensive levoglucosan. Structural analysis of single-crystal X-ray diffractions and NMR studies confirm the conservation of favored ⁴C₁ conformation for fluorinated carbohydrate analogs, while a slightly distorted conformation due to repulsive 1,3-diaxial F···F interaction is observed for the trifluorinated talose derivative. Finally, the relative stereochemistry of multi-vicinal fluorine atoms has a strong effect on the lipophilicities (logP).

Polyfluorinated hexopyranoses display unique physical, chemical and biological properties, however their stereoselective synthesis is highly challenging. Here, the authors report a synthetic approach based on the chemical manipulation of inexpensive levoglucosan to obtain heavily fluorinated monosaccharides stereoselectively.