C-Glycosylmethylene carbenes: synthesis of anhydro-aldose tosylhydrazones as precursors; generation and a new synthetic route to exo-glycals

C-Glycopyranosyl aldehydes: emerging chiral synthons in organic synthesis

Herein, we have summarized the vast array of synthetic processes that have been developed for the synthesis of C-glycopyranosyl aldehydes and diverse C-glycoconjugates derived from them by covering the literature reported from 1979 to 2023. Notwithstanding its challenging chemistry, C-glycosides are considered stable pharmacophores and are used as important bioactive molecules. The discussed synthetic methodologies to access C-glycopyranosyl aldehydes take advantage of seven key intermediates, viz. allene, thiazole, dithiane, cyanide, alkene, and nitromethane. Furthermore, the integration of complex C-glycoconjugates derived from varied C-glycopyranosyl aldehydes involves nucleophilic addition/substitution, reduction, condensation, oxidation, cyclo condensation, coupling, and Wittig reactions. In this review, we have categorized the synthesis of C-glycopyranosyl aldehydes and C-glycoconjugates on the basis of the methodology used for their synthesis and on types of C-glycoconjugates, respectively.

Coupling Reactions of Anhydro-Aldose Tosylhydrazones with Boronic Acids

A catalyst-free coupling reaction between O-peracetylated, O-perbenzoylated, O-permethylated, and O-permethoxymethylated 2,6-anhydro-aldose tosylhydrazones (C-(β-d-glycopyranosyl)formaldehyde tosylhydrazones) and aromatic boronic acids is reported. The base-promoted reaction is operationally simple and exhibits a broad substrate scope. The main products in most of the transformations were open-chain 1-C-aryl-hept-1-enitol type compounds while the expected β-d-glycopyranosylmethyl arenes (benzyl C-glycosides) were formed in subordinate yields only. A mechanistic rationale is provided to explain how a complex substrate may change the well-established course of the reaction.

Coupling of N-tosylhydrazones with tetrazoles: synthesis of 2-β-D-glycopyranosylmethyl-5-substituted-2H-tetrazole type glycomimetics

Coupling reactions of O-peracylated 2,6-anhydro-aldose tosylhydrazones (C-(β-d-glycopyranosyl)formaldehyde tosylhydrazones) with tetrazoles were studied under metal-free conditions using thermic or microwave activation in the presence of different bases. The reactions proved highly regioselective and gave the corresponding, up-to-now unknown 2-β-d-glycopyranosylmethyl-2H-tetrazoles in 7-67% yields. The method can be applied to get new types of disaccharide mimetics, 5-glycosyl-2-glycopyranosylmethyl-2H-tetrazoles, as well. Galectin binding studies with C-(β-d-galactopyranosyl)formaldehyde tosylhydrazone and 2-(β-d-galactopyranosylmethyl)-5-phenyl-2H-tetrazole revealed no significant inhibition of any of these lectins.

Stereoselective Synthesis of Ribofuranoid exo-Glycals by One-Pot Julia Olefination Using Ribofuranosyl Sulfones

One-pot Julia olefination using ribofuranosyl sulfones is described. The α-anomers of the ribofuranosyl sulfones were synthesized with complete α-selectivity via the glycosylation of heteroarylthiols using ribofuranosyl iodides as glycosyl donors and the subsequent oxidation of the resulting heteroaryl 1-thioribofuranosides with magnesium monoperphthalate (MMPP). The Julia olefination of the α-ribofuranosyl sulfones with aldehydes proceeded smoothly in one pot to afford the thermodynamically less stable (E)-exo-glycals with modest-to-excellent stereoselectivity (up to E/Z = 94:6) under the optimized conditions. The E selectivity was especially high for aromatic aldehydes. In contrast, the (Z)-exo-glycal was obtained as the main product with low stereoselectivity when the corresponding β-ribofuranosyl sulfone was used (E/Z = 41:59). The remarkable impact of the anomeric configuration of the ribofuranosyl sulfones on the stereoselectivity of the Julia olefination has been rationalized using density functional theory (DFT) calculations. The protected ribose moiety of the resulting exo-glycals induced completely α-selective cyclopropanation on the exocyclic carbon-carbon double bond via the Simmons-Smith-Furukawa reaction. The 2-cyanoethyl group was found to be useful for the protection of the exo-glycals, as it could be removed without affecting the exocyclic C═C bond.

Stereoselective Synthesis of Carbon-Sulfur-Bridged Glycomimetics by Photoinitiated Thiol-Ene Coupling Reactions

Oligosaccharides and glycoconjugates are abundant in all living organisms, taking part in a multitude of biological processes. The application of natural O-glycosides in biological studies and drug development is limited by their sensitivity to enzymatic hydrolysis. This issue made it necessary to design hydrolytically stable carbohydrate mimetics, where sulfur, carbon, or longer interglycosidic connections comprising two or three atoms replace the glycosidic oxygen. However, the formation of the interglycosidic linkages between the sugar residues in high diastereoslectivity poses a major challenge. Here, we report on stereoselective synthesis of carbon-sulfur-bridged disaccharide mimetics by the free radical addition of carbohydrate thiols onto the exo-cyclic double bond of unsaturated sugars. A systematic study on UV-light initiated radical mediated hydrothiolation reactions of enoses bearing an exocyclic double bond at C1, C2, C3, C4, C5, and C6 positions of the pyranosyl ring with various sugar thiols was performed. The effect of temperature and structural variations of the alkenes and thiols on the efficacy and stereoselectivity of the reactions was systematically studied and optimized. The reactions proceeded with high efficacy and, in most cases, with complete diastereoselectivity producing a broad array of disaccharide mimetics coupling through an equatorially oriented methylensulfide bridge.

New syntheses towards C-glycosyl type glycomimetics

Glycomimetics are compounds that resemble carbohydrate molecules in their chemical structure and/or biological effect. A large variety of compounds can be designed and synthesized to get glycomimetics, however, C-glycosyl derivatives represent one of the most frequently studied subgroup. In the present survey syntheses of a range of five- and six membered C-glycopyranosyl heterocycles, anhydro-aldimine type compounds, exo-glycals, C-glycosyl styrenes, carbon-sulfur bonded oligosaccharide mimics are described. Some of the C-glycopyranosyl azoles, namely 1,2,4-triazoles and imidazoles belong to the most efficient glucose analog inhibitors of glycogen phosphorylase known to date. Biological studies revealed the therapeutical potential of such inhibitors. Other synthetic derivatives offer versatile possibilities to get further glycomimetics.

Thio-click reaction of 2-deoxy-exo-glycals towards new glycomimetics: stereoselective synthesis of C-2-deoxy-d-glycopyranosyl compounds

Photoinitiated addition of thiols to 2-deoxy-exo-glycals obtained from endo-glycals of d-arabino, d-lyxo, d-erythro and d-threo configurations resulted in highly regio- and stereoselective formation of glycosylmethyl sulfide type glycomimetics.

Synthesis of New C- and N-β-d-Glucopyranosyl Derivatives of Imidazole, 1,2,3-Triazole and Tetrazole, and Their Evaluation as Inhibitors of Glycogen Phosphorylase

The aim of the present study was to broaden the structure-activity relationships of C- and N-β-d-glucopyranosyl azole type inhibitors of glycogen phosphorylase. 1-Aryl-4-β-d-gluco-pyranosyl-1,2,3-triazoles were prepared by copper catalyzed azide-alkyne cycloadditions between O-perbenzylated or O-peracetylated β-d-glucopyranosyl ethynes and aryl azides. 1-β-d-Gluco-pyranosyl-4-phenyl imidazole was obtained in a glycosylation of 4(5)-phenylimidazole with O-peracetylated α-d-glucopyranosyl bromide. C-β-d-Glucopyranosyl-N-substituted-tetrazoles were synthesized by alkylation/arylation of O-perbenzoylated 5-β-d-glucopyranosyl-tetrazole or from a 2,6-anhydroheptose tosylhydrazone and arenediazonium salts. 5-Substituted tetrazoles were glycosylated by O-peracetylated α-d-glucopyranosyl bromide to give N-β-d-glucopyranosyl-C-substituted-tetrazoles. Standard deprotections gave test compounds which were assayed against rabbit muscle glycogen phosphorylase b. Most of the compounds proved inactive, the best inhibitor was 2-β-d-glucopyranosyl-5-phenyltetrazole (IC₅₀ 600 μM). These studies extended the structure-activity relationships of β-d-glucopyranosyl azole type inhibitors and revealed the extreme sensitivity of such type of inhibitors towards the structure of the azole moiety.

Pd-catalyzed coupling reactions of anhydro-aldose tosylhydrazones with aryl bromides to produce substituted exo-glycals

Palladium-catalyzed cross-couplings of O-peracylated and O-permethylated 2,6-anhydro-aldose tosylhydrazones with aryl halides were studied under thermic conditions in the presence of LiOtBu and phosphine ligands. The reactions gave the corresponding aryl substituted exo-glycals as mixtures of diastereomers in 11-75% yields. The transformations represent a new access to these types of glycomimetic compounds. The double bond of some aryl substituted exo-glycals was saturated to give good yields of benzylic C-glycosyl derivatives.

A new synthesis of C-β-d-glycopyranosylmethyl sulfides by metal-free coupling of anhydro-aldose tosylhydrazones with thiols

Cross-coupling of anhydro-aldose tosylhydrazones with thiols provided a new access to C-glycopyranosylmethyl sulfides.

Photoinitiated hydrothiolation of pyranoid exo-glycals: the d-galacto and d-xylo cases

Radical-mediated addition reactions of thiols to O-peracetylated exo-galactal and exo-xylal with 2,2-dimethoxy-2-phenylacetophenone as the photoinitiator resulted in high yielding formation of the corresponding β-d-glycopyranosylmethyl-sulfide derivatives (2,6-anhydro-1-deoxy-1-S-substituted-1-thio-alditols) with exclusive regio- and very high stereoselectivity, including disaccharide mimicks with Gly-CH2-S-Gly scaffolds.

Synthetic applications of hypophosphite derivatives in reduction

The purpose of this review is to collect the applications in fine synthesis of hypophosphite derivatives as reducing agents.