Crystal structure of penta-O-acetyl-beta-D-galactopyranose with modeling of the conformation of the acetate groups.

The crystal structure of penta-O-acetyl-beta-D-galactopyranose was determined with Mo K(alpha) radiation at 150 K to R = 0.029. The space group is P2(1)2(1)2(1), and the unit cell dimensions are, a = 8.348, b = 9.021 and c = 25.418 A. The ring has the usual 4C(1) shape and O-6 is in the tg position as frequently observed for sugars having the axial galacto configuration at C-4. Conformations of the acetate groups were compared with those from the literature. Nearly eclipsed, 'Z' conformations are found for the ester moieties, and the torsion angles for the sequence involving the ring hydrogen, carbon, alkoxy oxygen and carbonyl carbon for both 1 and related compounds are eclipsed-to-gauche. Orientations and conformations of the acetate substituents were modeled with both MM3 molecular mechanics and various levels of quantum mechanics theory. Higher levels of theory and more complete models provided better prediction of the experimental observations.