Vibrational Circular Dichroism Study of Pipitzols and of Their Inverse Epimer Constituents α- and β-Pipitzol

Inverse epimers are uncommon in nature and a lack of study of their vibrational circular dichroism (VCD) behavior seems evident. This is relevant, among other facts, since in the case of epimers, VCD has been capable of determining the absolute configuration (AC) of 1 stereogenic center in a molecule having 10 stereogenic centers. The case of pipitzol is unique since it took 80 years from its first preparation by thermal intramolecular cycloaddition of perezone (1) to know that the reaction outcome is an equimolar mixture of the 2 inverse epimers α-pipitzol (2: 3R, 3aR, 7R, and 8aS) and β-pipitzol (3: 3R, 3aS, 7S, and 8aR). Evaluation of 2 and 3 reveals that some VCD bands have an opposite phase while other bands show the same phase. The VCD spectrum of the naturally occurring equimolecular mixture was also measured and the 3 experimental spectra were contrasted with density functional theory (DFT) calculated spectra allowing individual band assignments. The comparisons were made in the 1800 to 950 and 1550 to 950 cm⁻¹ ranges and the numerical differences are highlighted thereby showing that carbonyl bands influence such comparisons. The 2 carbonyl absorption bands of 2 and 3 show weak VCD bands and the conjugated double bond band provides an intense AC-dependent VCD band.