Molecular Arrangement and Intermolecular Hydrogen Bonding in Crystals of Methyl 6-O-Acyl-d-glycopyranosides

Abstract

The crystal structures and thermal properties of methyl 6-O-n-decanoyl-α-D-glucopyranoside (1), methyl 6-O-n-dodecanoyl-α-D-glucopyranoside (2), methyl 6-O-n-dodecanoyl-β-D-glucopyranoside (3), and methyl 6-O-n-dodecanoyl-α-D-galactopyranoside (4) were studied by X-ray and thermal analysis. Anhydrous crystals obtained from methanol solution by slow evaporation are monoclinic and in space group P2 1 with Z = 2. Cell dimensions : 1, a = 4.967(1) A, b = 7.513(1) A, c = 25.917(2) A, β = 92.92(1)° ; 2, a = 4.968(1) A, b = 7.503(1) A, c = 28.443(2) A, β = 92.68(1)° ; 3, a = 7.722(1) A, b = 7.346(1) A,c = 18.626(2) A, β = 91.51(1)° ; 4, a = 5.760(1) A, b = 7.986(1) A, c = 23.339(1) A, β = 90.21(1)°. The sugar moieties of 1 and 2 are disordered. The pyranose rings of β-D-glucopyranoside and α-D-galactopyranoside in 3 and 4, respectively, are in 4 C 1 chair conformation, and the alkyl chains in 1-4 are all-trans. Molecules are arranged in a bilayer structure with interdigitated alkyl chains. The hydrogen bonds between sugar moieties in 3 and 4, which are found only between adjacent layers, form infinite and finite chains in respective crystals. The melting point of compounds which have the same alkyl chain length are 70.1, 93.5, and 137.8°C for 2, 3, and 4, respectively. Epimers of these molecules exhibit packing arrangements which explain the wide variety in their melting temperatures.