Conformational Analysis of Cyclohexane-1,2-diol Derivatives and MM3 Parameter Improvement

Abstract

The positions of the equilibria between the diequatorial and diaxial conformers of trans-1,2-dimethoxycyclohexane (2) and trans-2-methoxycyclohexanol (3) have been measured accurately by 13C n.m.r. spectroscopy at -80°C in a series of solvents ranging from non-polar pentane to highly polar methanol. The equilibria favour the diequatorial conformers under all conditions but the extent increases with solvent polarity and is greater for (3). Improved parameters for the OCCO torsional term in MM3 (94) have been developed (V1, V2, and V3, 3.0, -2.5, 1.25, respectively) by comparison with conformational data for (2) and 1,2-dimethoxyethane (1). Application of the modified parameters to a number of examples demonstrates marked improvement for ethers. These examples include 1,2-dimethoxypropane, for which n.m.r. spectra in (D12) cyclohexane have been analysed and those from the gas phase reevaluated. Lesser improvement was achieved for systems having intramolecular hydrogen bonding. Ab initio results for rotation about the C 5-C 6 bond in pyranosides are satisfactorily reproduced but experimental results favour the gg and tg rotamers over the tg rotamer considerably more than calculated.