The conformation of tetrafluorinated methyl galactoside anomers: crystallographic and NMR studies

Abstract

The first single-crystal X-ray diffraction study of tetrafluorinated monosaccharide derivatives is presented. Both α- and β-methyl 2,3-dideoxy-2,2,3,3-tetrafluoro- d-galactopyranoside anomers adopt the 4 C 1 conformation. The values for the C1–O1 and C1–O5 bond lengths and the O5–C1–O1–CH 3 dihedral angles are in line with what can be expected from the anomeric and exo-anomeric effects. The chair conformations are slightly distorted, presumably due to repulsion between 1,3-diaxial C–O and C–F bonds. The asymmetric unit of both compounds contains up to three independent molecules, which differ in the conformation of the hydroxymethyl group (including in one case a ‘forbidden’ gg rotamer). The molecular packing of the β-anomer shows a clear segregation between fluorinated and hydrophilic domains, while for the α-anomer the regions of fluorine segregation are broken by interleafing of OMe groups. There is one close OH⋯F contact, which is likely to arise from the crystal packing. NMR studies show that the two anomers also adopt a 4 C 1 conformation in solution (D 2O, CDCl 3).