In Depth Analysis of Chiroptical Properties of Enones Derived from Abietic Acid

With
the use of inexpensive commercially available abietic acid,
a whole series of abietane enones were prepared in high yields. The
structures of all the products obtained were determined by comprehensive
spectroscopic analysis with particular emphasis on the use of advanced
NMR techniques, comparison with previously reported data and, where
possible, by single crystal X-ray diffraction. However, in cases where
X-ray crystallography was not applicable or compounds tested were
unstable, a final stereochemical assignment could be inferred only
by electronic circular dichroism (ECD) supported by vibrational circular
dichroism to increase credibility. To reveal the relationship between
structure and chiroptical properties, we used combined experimental
and theoretical analysis of geometries, structural parameters, and
chiroptical properties of all enones synthesized. A thorough analysis
of their conformational flexibility by examining the effect of solvent
and temperature on the ECD spectra was also used to achieve desired
objectives. As a result, the impact of substituents adjacent to the
enone chromophore on the conformation was determined by demonstrating
that even slight changes in the position of hydroxyl and isopropyl
groups attached to carbon C13 may substantially affect ECD curves’
pattern, leading in some cases to Cotton effects sign reversal.