Determination of the absolute configuration of a novel tetrasubstituted isoindolinone by vibrational circular dichroism

The absolute configuration of a recently prepared asymmetric 3,3-disubstituted isoindolinone (ethyl 2-benzyl-3-oxo-1-(3-oxobutyl)isoindoline-1-carboxylate), possessing highly promising pharmaceutical activity, has been determined by means of VCD spectroscopy and DFT calculations. The great flexibility of the molecule reduces to a few relevant conformers, all contributing in the same way to the shape of the VCD spectrum for the carbonyl stretching region. Two of the three CO groups of the molecule interact with each other during the stretching vibration, thus providing a non-conservative VCD couplet whose signature, together with the VCD sign of the third CO stretching mode, unequivocally determines the absolute configuration of the molecule, which is found to be (S) for the (-) optical isomer.