The three-dimensional structures of dioxocarbenium ions related to glycosyl cations were determined by an analysis of spectroscopic, computational, and reactivity data. Hypothetical low-energy structures of the dioxocarbenium ions were correlated with both experimentally determined (1)H NMR coupling constants and diastereoselectivity results from nucleophilic substitution reactions. This method confirmed the pseudoaxial preference of C-3 alkoxy-substituted systems and revealed the conformational preference of the C-5 alkoxymethyl group. Although the monosubstituted C-5 alkoxymethyl substituent preferred a pseudoequatorial orientation, the C-5-C-6 bond rotation was controlled by an electrostatic effect. The preferred diaxial conformer of the trans-4,5-disubstituted tetrahydropyranyl system underscored the importance of electrostatic effects in dictating conformational equilibria. In the 2-deoxymannose system, although steric effects influenced the orientation of the C-5 alkoxymethyl substituent, the all-axial conformer was favored because of electrostatic stabilization.

The effect of electrostatic interactions on conformational equilibria of multiply substituted tetrahydropyran oxocarbenium ions.

Chemically-modified polysaccharide extract derived from Leucaena leucocephala alters Raw 264.7 murine macrophage functions

α-C-glucopyranosides and mannopyranosides are obtained in 65–85% yields from 4,6-O-benzylidene-protected glucosyl and mannosyl thioglycosides bearing ester functionality at the 3-O-position by coupling reaction with C-nucleophiles on activation with diphenyl sulfoxide, 2,4,6-tri-tert-butylpyrimidine, and trifluoromethanesulfonic anhydride.

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Influence of the O3 protecting group on stereoselectivity in the preparation of C-mannopyranosides with 4,6-O-benzylidene protected donors.

InCl3 catalyzed highly diastereoselective [3 + 2] cycloaddition of 1,2-cyclopropanated sugars with aldehydes: a straightforward synthesis of persubstituted bis-tetrahydrofurans and perhydrofuro[2,3-b]pyrans.

New types of porphyrin derivatives bearing “C-glycoside” moieties, either in 5,10,15,20- or in 5,15-meso-positions, were prepared and fully characterized. The presence of the glycosidic groups imparts to the title macrocycles, besides an amphiphilic character, a clear tendency to form chiral suprastructures upon solvent-driven self-aggregation in different aqueous–organic solvent mixtures. Supra-assembly phenomena, in terms of the size and morphology of the resulting structures, as well as their kinetics of aggregation, were studied by UV-visible, fluorescence, resonance light scattering (RLS), and CD spectroscopy, indicating that the morphology of the aggregates depends strongly on the structure of the porphyrin rings, and on the bulk conditions of aggregation.

https://www.rsc.org/suppdata/ob/b6/b616096d/b616096d.pdf

Synthesis and solvent driven self-aggregation studies of meso-"C-glycoside"-porphyrin derivatives.

Treatment of 2'-carbonyl-alpha-C-glycopyranosides of gluco, galacto, manno, 2-deoxy, and 2-azido sugars with 4% NaOMe resulted in anomeric epimerization to give their respective beta-anomers in good to excellent yields. The epimerization of the 2'-aldehyde of alpha-C-galactopyranoside (10) in deuterium methanol, which afforded the beta-anomer with exclusive deuterium replacements at the 1'-position, excluded the possibility of the exo-glycal as being involved as an intermediate. When 2'-aldehyde (36) and 2'-ketone (41) of 2,3-di-O-benzyl-alpha/beta-l-C-arabinofuranoside were used as substrates we were able to obtain the respective equatorial alpha-C-arabinopyranosides (37 and 42). These observations confirmed that the epimerization involves an acyclic alpha,beta-unsaturated aldehyde or ketone, which is formed by the enolation of 2'-carbonyl-alpha-C-glycoside with subsequent beta-elimination. Thereafter an intramolecular hetero-Michael cycloaddition occurs, leading to the formation of thermodynamically controlled stable products, which were exclusively the equatorial C-glycopyranosides, except in the case of 2'-carbonyl-C-furanosides, where a mixture of two anomers was obtained.

Zerong Wang

Papers

Epimerization of 2'-carbonylalkyl-C-glycosides via enolation, beta-elimination and intramolecular cycloaddition.