MOLECULAR ARRANGEMENT AND INTERMOLECULAR HYDROGEN-BONDING IN CRYSTALSOF METHYL 6-O-ACYL-D-GLYCOPYRANOSIDES

The crystal structures and thermal properties of methyl 6-O-n-decanoyl -alpha-D-glucopyranoside (1), methyl 6-O-n-dodecanoyl-alpha-D-glucopyr anoside (2), methyl 6-O-n-dodecanoyl-beta-D-glucopyranoside (3), and m ethyl 6-O-n-dodecanoyl-alpha-D-galactopyranoside (4) were studied by X -ray and thermal analysis. Anhydrous crystals obtained from methanol s olution by slow evaporation are monoclinic and in space group P2(1) wi th Z = 2. Cell dimensions: 1, a = 4.967(1) Angstrom, b = 7.513(1) Angs trom, c = 25.917(2) Angstrom, beta = 92.92(1)degrees; 2, a = 4.968(1) Angstrom, b = 7.503(1) Angstrom, c = 28.443(2) Angstrom, beta = 92.68( 1)degrees; 3, a = 7.722(1) Angstrom, b = 7.346(1) Angstrom, c = 18.626 (2) Angstrom, beta = 91.51(1)degrees; 4, a = 5.760(1) Angstrom, b = 7. 986(1) Angstrom, c = 23.339(1) Angstrom, beta = 90.21(1)degrees. The s ugar moieties of 1 and 2 are disordered. The pyranose rings of beta-D- glucopyranoside and alpha-D-galactopyranoside in 3 and 4, respectively , are in C-4(1) chair conformation, and the alkyl chains in 1-4 are al l-trans. Molecules are arranged in a bilayer structure with interdigit ated alkyl chains. The hydrogen bonds between sugar moieties in 3 and 4, which are found only between adjacent layers, form infinite and fin ite chains in respective crystals. The melting point of compounds whic h have the same alkyl chain length are 70.1, 93.5, and 137.8 degrees C for 2, 3, and 4, respectively. Epimers of these molecules exhibit pac king arrangements which explain the wide variety in their melting temp eratures.