Lanthanide-induced shift reagents enable the structural elucidation of natural products in inseparable complex mixtures – the case of elemenal from Inula helenium L. (Asteraceae)

Abstract

The use of lanthanide complexes for resolving intricate NMR signals and, in the case of chiral ligands, for determining enantiomeric excess has progressively decreased in the last 30 years. Recently, a sesquiterpene aldehyde from Inula helenium with a possible potent antistaphylococcal activity remained unidentified due to the impossibility of separating the compound from its complex matrix available in very low amounts (ca. 5 mg). Detailed analyses of 1D and 2D NMR spectra of this original complex sample allowed access to a very limited amount of structural data for the unknown aldehyde. We decided to investigate the potential usefulness of lanthanide-induced shift reagents for the resolution and assigning of overlapped ¹H NMR signals originating from different components of this complex mixture (i.e. for a qualitative analysis). The incremental addition of tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium(III) (Eu(fod)₃) led to a simplification of the NMR spectra in terms of signal overlap and removal of chemical shift degeneracy, allowing the mining of crucial data from the shifted NMR spectra. 2D NMR spectra (¹H–¹H–COSY, NOESY, HSQC and HMBC) of the sample mixed with Eu(fod)₃ proved to be particularly valuable in this respect. The obtained additional information revealed that the compound in question was a rare sesquiterpene – elemenal (elema-1,3,11(13)-trien-12-al). Therefore, herein we report on a new chromatography-free methodology that could be of value in structure elucidation of unknown compounds even if they are not available in a pure state.