DETERMINATION OF THE ABSOLUTE-CONFIGURATION OF BIS(TETRAHYDROPYRAN-2-YL)METHANE BY COMPARISON OF MEASURED AND CALCULATED CD-SPECTRA

The absolute configuration of bis(tetrahydropyran-2-yl) methane (1) wa s determined by comparison of measured and calculated CD spectra. The theoretical CD spectra were obtained by means of the CNDO/2S method. T he five presumably lowest local minima on the energy hypersurface of t he title compound were used to describe the conformer equilibrium mixt ure. The geometries of these conformers were calculated employing the MM3 force field, the semiempirical AM1 method and one-determinant ab i nitio calculations employing the 6-31G basis set, Boltzmann factors w ere then obtained using relative energies calculated with three differ ent basis sets and including correlation- and zero point vibrational e nergy. Based on the sign of the observed and calculated longest wavele ngth Cotton effect we assign an absolute configuration to the compound which is in keeping with the chirality expected from the assumed reac tion mechanism. The results of force field and ab initio calculations converge to the point that the conformer equilibrium if dominated (85 - 96%) by one single conformer which is energetically separated from t he other conformers by about 2 - 3 kcal/mol. This result agrees with p revious experimental data.