Ulrich Schopfer

Conformation Design of Hydrocarbon Backbones: A Modular Approach

Full conformational flexibility and yet a high preference for a single backbone conformation is displayed by certain hydrocarbon skeletons such as the one on the right (calculated conformational preference >99%). The principles behind the design of such monoconformational structures are discussed here.

Calculation of 13C 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. 13C NMR properties of molecules containing such segments can be reproduced well by a combination of molecular mechanics and density functional (SOSDFPT/IGLO) calculations based on MM3 geometries. 13C NMR chemical shifts and 13C–13C 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.

Conformation Induction Between Neighboring Dimethylpentane Segments

A 2,4-dimethylpentane unit can be rendered monoconformational by the presence of a conformation-inducing group (an inductor group) at C-1 (cf. 6). The resulting entity may serve as an inductor group to control in turn the conformation of a neighboring dimethylpentane segment (cf. 7). This holds if the inducing dimethylpentane segment is isotactic (cf. 15, 25), but not when it is syndiotactic (cf. 28).

Z- and U-shaped open chain molecular backbones by conformation design

Abstract Derivatives 5 and 7 of 1,10-decanediol have been synthesized that exist in an intramolecularly hydrogen-bonded U-shaped conformation (5), or in a non-hydrogen-bonded Z-shaped conformation (7). Conformation design of these compounds rests on the introduction of four methyl groups, the relative configuration of the methyl bearing stereocenters and the presence of a configurationally defined double bond respectively.

Understanding conformer equilibria in solution with the aid ofcalculated 13C NMR spectra.1 A density functionaland molecular mechanics study

MM3 conformational analyses were performed for a series of diastereomeric 2,4,6-trimethylated alcohols. Using MM3 geometries and energies, 13C NMR chemical shifts of 15–40 low energy conformers were calculated by means of an SOS–DFPT/IGLO approach and Boltzmann weighted. A comparison of calculated and experimental shifts allows us to analyse the conformational equilibria in solution and to estimate the importance of intramolecular hydrogen bonds in CDCl3 solution.