Circular dichroism spectroscopy and DFT calculations in determining absolute configuration and E/Z isomers of conjugated oximes

The primary purpose of this work was to demonstrate the suitability of circular dichroism (CD) spectroscopy in stereochemical studies of α,β-unsaturated oximes, with particular emphasis on determination of E and Z geometry of the oxime double bond. As models for this study, O-phenyl and O-triphenylmethyl (trityl) oximes of 4-hydroxy-2-methylcyclopent-2-en-1-one were selected. These model compounds differ in both absolute configuration at C4 carbon atom and E-Z configuration of the oxime double bond. The basic dichroic technique applied was electronic circular dichroism (ECD) assisted by quantum-chemical calculations and vibrational circular dichroism (VCD) for selected cases. Such an approach enabled effective implementation of both goals. Thus, we were able to associate the signs of Cotton effects in the range of 190-240 nm with the absolute configuration at C4 and within 240-300 nm with the E- or Z-geometry of the oxime double bond. Within this work, optical activity of the protecting trityl group was also studied towards formation of the propeller-shaped conformations by using the same combined CD/DFT methodology. As shown, the helical structure of the trityl group has a considerable influence on the ECD spectra. However, the MPM and PMP conformers of the trityl group are in fact almost equally populated in the conformational equilibrium, making it impossible to distinguish them. On the other hand, rotamers of the hydroxyl group at C4 show a decisive impact on the VCD spectra in both phenoxy and trityl oximes.