Conformational features of C-glycosyl compounds: Crystal structure and molecular modelling of “methyl C-gentiobioside”

Abstract

The crystal of “methyl C-gentiobioside” (methyl 8,12-anhydro-6,7-dideoxy- d- glycero- d- gulo-α- d- gluco-tridecapyranoside) (C 14H 26O 10) is triclinic, space group P1, with a = 1.0181 (6) nm, b = 0.8093 (5) nm, c = 0.5066 (4) nm, α = 96.03 (5)°, β = 99.94 (5)°; γ = 90.85 (5)°. The two d-glucose residues have the 4 C 1 conformation. The orientation of the β-(1→6) linkage is characterized by torsion angles Φ = 55.9°, Ψ = 175.1°, and Ω = −63.9°. The orientation of the primary hydroxyl group at the non-reducing residue is gauche-trans (Ω′ = −53.6°). There is no intra-molecular hydrogen bond. Molecules are held together by a network of hydrogen bonds involving all of the hydroxyl groups. This crystal structure is the first experimental characterization of a “ C-disaccharide”. Unlike methyl gentiobioside, which has a high level of conformational flexibility, the “ C-disaccharide” has a restricted flexibility. Each of the low-energy conformers in vacuo has a value of Φ centered about 60°, in agreement with the solid state conformation, and the exo-anomeric effect is no longer predominant.