Calculation of C-13 NMR chemical shifts and coupling constants for the analysis of conformer populations and relative configuration in flexible molecules

1,3-Dimethylated hydrocarbon segments are common as structural elements in conformationally flexible natural products. C-13 NMR properties of molecules containing such segments can be reproduced well by a combination of molecular mechanics and density functional (SOS-DFPT/IGLO) calculations based on MM3 geometries. C-13 NMR chemical shifts and C-13-C-13 coupling constants are calculated for the individual conformers and are Boltzmann weighted according to MM3 energies, and information about conformer equilibria in solution is obtained. The population averaged values of chemical shifts differ in a characteristic manner for diastereomeric compounds, and thus can help in making assignments of the relative configuration of such natural products.