Evidence based both on nuclear magnetic resonance and rotation data primarily obtained from methyl 3-deoxy-13-L-erythro-pentopyranoside and a number of its derivatives is interpreted to show that the electrostatic repulsion between the oxygen atoms at the 2 and 4 positions is substantially less when these oxygens are linked to acyl groups than when in the form of either methyl ethers or as hydroxyl groups hydrogen bonded to solvent. Also, experimental evidence is presented which requires the hydrogen bridge between two axially disposed hydroxyl groups to be s~~bstantially strengthened by hydrogen bonding of the free hydroxyl by solvent.Canadian Journal of Chemistry, 47, 4441 (1969) The DurDose of this communication is to L L report on substitutional and solvation effects on chair-chair conformational equilibria which involve in one conformation an interaction between two oxygen atoms in opposing axial orientation. Profound effects are noted which have an important bearing on the notion that so-called non-bonded interaction free energies can be assigned to such interactions without reference to the kind of compound or to the conditions of the experiment.As seen in Table I, the coupling constants estimated for the di-0-acyl derivatives (16, lc, and ICE) of methyl 3-deoxy-P-L-erythro-pentopyranoside (la) require these compounds to exist largely in the C-1 conformation which has both the acyloxy groups in axial and opposing orien-e R = R' = CH, f R = Ac; R' = CH, g R = H ; R ' = CH3 'Presented by R. U. L. at the Karl Pfister Lecture,