Computational Interpretation of Vibrational Optical Activity: The ROA Spectra of (4S)‐4‐Methylisochromane and the (4S)‐Isomers of Galaxolide®

Abstract

AbstractThe reliable computation of Raman‐optical‐activity (ROA) spectra of molecules of the size of the title compounds has, until now, not been possible. We show that our rarefied basis sets yield results in good agreement with the experimental data for (4S)‐4‐methylisochromane (=(4S)‐3,4‐dihydro‐4‐methyl‐1H‐2‐benzopyran; 1), provided the equilibrium between the pseudo‐equatorial and the pseudo‐axial conformers is taken into account. Comparison between the measured and the computed ROA back‐scattering spectra allows the unequivocal assignment of the absolute configuration of the molecule. Comparison with more‐approximate calculations for the larger (4S)‐isomers of Galaxolide® (2), which contain the (4S)‐4‐methylisochromane moiety, shows large‐scale group frequencies on the same chiral fragments of the two molecules. The data confirm that ROA can be generated by interactions extending over several bonds, i.e., over larger distances than can be probed by NMR spectroscopy. Thus, the absolute configuration at C(7) of Galaxolide® is assignable independently of that at C(4). The computation of ROA for forward‐scattering, which will soon be measurable for Galaxolide®, suggests that this scattering geometry provides additional stereochemical information that will be valuable in situations where absolute configurations at several stereogenic centers have to be assigned.