Crystal structures of methyl 6-O-acyl-α-d-galactopyranosides

The crystal structures of methyl 6-O-n-octanoyl-α-d-galactopyranoside (1) and methyl 6-O-n-decanoyl-α-d-galactopyranoside (2) were investigated by X-ray analysis. Anhydrous crystals obtained from methanol solution by slow evaporation are monoclinic, and the space group is P21 with Z = 2. The cell dimensions are as follows, 1: a = 5.774(1), b = 8.013(1), c = 19.183(1) Å, β = 98.50(1)°; 2: a = 5.762(1), b = 8.003(1), c = 21.227(2) Å, β = 93.93(1)°. The galactopyranoside ring is nearly in a 4C1 chair conformation but slightly distorted to twist boat and the alkyl chain is in all-trans conformation. In these crystals, molecules are arranged in a bilayer structure with interdigitated alkyl chains. The hydrogen bonding linkages between sugar moieties, which are found only between the layers, form an infinite chain through the crystal. 1 and 2 show higher melting points than those of corresponding β-d-glucopyranosides. The results indicate the important role of the sugar in the crystal.