text: Another accepted explanation for the anomeric effect is the equatorial configuration has the dipoles involving both heteroatoms partially aligned, and therefore repelling each other. By contrast the axial configuration has these dipoles roughly opposing, thus representing a more stable and lower energy state.
  Both the hyperconjugation and the dipole minimization contribute to the preferred (Z)-conformation of esters over the (E)-conformation. In the (Z) conformation the lone pair of electrons in the alpha oxygen can donate into the neighboring σ* C-O orbital. In addition, the dipole is minimized in the (Z)-conformation and maximized in the (E)-conformation.