Orientations of carbohydrate substituents

The two-conformer hypothesis: 2,3,4,6-tetra-O-methyl-mannopyranosyl and -glucopyranosyl oxacarbenium ions

Computational chemistry can give information about the probable conformations of reactive intermediates that are difficult to determine experimentally. Based on density functional theory (DFT) calculations of tetra-O-methyl-D-mannopyranosyl and -glucopyranosyl oxacarbenium ions, two families of conformations, which we call B0 and B1, were found. For the manno configuration, a 4H3 and 3E almost isoenergetic pair were found, whereas for the gluco-configuration a 4H3 and 5S1 pair favouring 4H3 were calculated. These results corroborate earlier results and suggest that this two or more conformer hypothesis is general. Nucleophilic attack on these pairs of cations was modelled with methanol and led to four cases to consider namely alpha- or beta-attack on B0 or B1. The resulting complexes (G0, G1 and F0, F1) demonstrate facial selectivity. The relative energies of these complexes are dominated by intramolecular hydrogen bonding and the conformational consequences to the pyranose ring of changes in the C-5-O-5-C-1-C-2 torsion angle. Constrained variation of the nucleophilic oxygen (methanol) to C-1 distance shows that these ion dipole complexes are the only minima with this constraint.