Molecular structures of glycal-based bolaamphiphiles: analysis of crystal packing and hydrogen-bond networks

The crystal structures for the glycal bolaamphiphiles, 1,12-bis-(2,3-α-d-erythro-hex-2-enopyranosyloxy)-dodecane (1) and 1,12-bis-(2,3-α-d-threo-hex-2-enopyranosyloxy)-dodecane (2), were determined by single-crystal X-ray analysis. The structure for 1 showed that the α:α and α:β diastereomers co-crystallized, with occupancy factors determining an isomeric ratio of 69:31. The pyranose rings for both structures are oriented away from each other and adopt a conventional glycal geometry. The head groups are nearly gauche to the hydrophobic chain, which adopts an all-trans zigzag conformation. Bolaamphiphile 1 packs in anti-parallel layers, while bolaamphiphile 2 displays a parallel arrangement of layers. Both structures display a three-dimensional hydrogen-bonding network involving the hydroxylic substituents on the head groups. The high similarity in large-scale solid state structures between 1 and glucosamide bolaamphiphile 3, and 2 and galactosamide bolaamphiphile 4 suggest a strong dependence on head group stereochemistry, and that only a few, key intermolecular interactions between head groups are necessary in controlling the ultimate structure observed. The solid state results may have implications for understanding the intermolecular forces directing nanoscale self-assembly in solution.