Structural properties of D-mannopyranosyl rings containing O-acetyl side-chains

Abstract

The crystal structures of 1,2,3,4,6-penta-O-acetyl-α-D-mannopyranose, C16H22O11, and 2,3,4,6-tetra-O-acetyl-α-D-mannopyranosyl-(1→2)-3,4,6-tri-O-acetyl-α-D-mannopyranosyl-(1→3)-1,2,4,6-tetra-O-acetyl-α-D-mannopyranose, C40H54O27, were determined and compared to those of methyl 2,3,4,6-tetra-O-acetyl-α-D-mannopyranoside, methyl α-D-mannopyranoside and methyl α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside to evaluate the effects of O-acetylation on bond lengths, bond angles and torsion angles. In general, O-acetylation exerts little effect on the exo- and endocyclic C—C and endocyclic C—O bond lengths, but the exocyclic C—O bonds involved in O-acetylation are lengthened by ∼0.02 Å. The conformation of the O-acetyl side-chains is highly conserved, with the carbonyl O atom either eclipsing the H atom attached to a 2°-alcoholic C atom or bisecting the H—C—H bond angle of a 1°-alcoholic C atom. Of the two C—O bonds that determine O-acetyl side-chain conformation, that involving the alcoholic C atom exhibits greater rotational variability than that involving the carbonyl C atom. These findings are in good agreement with recent solution NMR studies of O-acetyl side-chain conformations in saccharides. Experimental evidence was also obtained to confirm density functional theory (DFT) predictions of C—O and O—H bond-length behavior in a C—O—H fragment involved in hydrogen bonding.