Rotational barriers. 4. Dimethoxymethane. The anomeric effect revisited

The conformational energy map for the rotamers of dimethoxymethane has been calculated using both the 3-21G and 6-31G* basis sets with complete geometry optimization in each case. Higher level calculations including electron correlation also have been performed on selected rotamers. At larger torsional angles, the map has approximate 4-fold symmetry, and this symmetry may be used to quantify the difference in the interactions in the (+sc, {minus}sc) and (+sc, +sc) regions. Large changes in bond angles were found, and the dipole moment also changed considerably on C-O bond rotation. Calculations on equatorial and axial 2-methoxytetrahydropyran show that the axial form is favored by 1.33 kcal/mol, in fair agreement with experiment. Calculations also were carried out on the two lowest energy forms of 1,1-dimethoxyethane and on five forms of methyl propyl ether. Consideration of all these data indicates that the anomeric stabilization in dimethoxymethane is much greater than in sterically more congested systems, but the stabilization of 1,1-dimethoxyethane is similar to that of 2-methoxytetrahydropyran and other acetals.