Complete ¹H NMR Assignment of 3-Oxo-γ-Costic Acid from Ageratina glabrata

Nuclear Magnetic Resonance (NMR) is a powerful technique for elucidating the structure of organic molecules, and it is especially useful in determining the structure of natural products, including terpenes. 3-Oxo-γ-costic acid is an eudesmene-type sesquiterpene that has been isolated from different vegetable species of the Asteraceae and Lauraceae families. Its ¹H NMR has been reported with some discrepancies in the chemical shift assignments as well as in J values. In the present work, 3-oxo-γ-costic acid was isolated from the dichloromethane extract of the aerial parts of Ageratina glabrata by chromatographic means and the complete assignment of the ¹H NMR spectrum of 3-oxo-γ-costic acid is reported using a hybrid protocol that involves a DFT-calculated spectrum and the experimental ¹H spectrum. The calculated spectrum was constructed based on the DFT-calculated J values for each hydrogen, and the respective chemical shifts are those recorded by experimental means, which are supported by 2D NMR. Then, the completely calculated ¹H spectrum was simulated in the Mnova software and compared with the experimental spectrum. This methodology is especially useful for the detection of low magnitude NMR couplings (<2 Hz), which are complicated to measure experimentally; then, this hybrid protocol, experimental-DFT, could be applied to NMR determination of more complex sesquiterpenes.