Ring- and side-group conformational properties of di-O-acylated xylopyranosides: A computational study

The conformational properties (4C1, 1C4, and skew ring structures, side chain torsion) of methyl 2,4- (1) and 3,4-O-diacetyl (2) as well as 2,4- (3) and 3,4-O-dibenzoyl (4)-ß-D-xylopyranoside are calculated by dispersion-corrected density functional theory (B3LYP-D3) and LPNO–CEPA/1-def2-TZVPP. Derivatives with acyl groups in positions 2 and 4 (1 and 3) prefer the 1C4 chair conformation of the pyranose chair; 3,4-di-O-acyl compounds 2 and 4 show nearly equal or even dominating amounts of the 4C1 chair. Intramolecular hydrogen bonding of the type O3H⋯O1 (1 and 3) and O2H⋯O4 are characteristic for the lowest energy conformations of 1C4 chairs. With increasing solvent polarity a shift towards 4C1 chair structures is calculated. B3LYP-D3 conformer populations show fairly good agreement with CEPA/1 results (R2 = 0.992).