Phenethyl angelate - a new ester from immortelle essential oil?

Esters of angelic, senecioic and tiglic acids with various saturated/ unsaturated/aromatic alcohols contribute to the aroma of many essential oils. However, mass spectrometry with electron-impact ionization sometimes fails to distinguish these regio-/geometric isomers and this was the case with the minor constituent of Helichrysum italicum (immortelle) essential oil that was tentatively identified as the ester of 2- phenyl-1-ethanol with one of the mentioned acids. Our efforts to identify this phenethyl ester were also hampered by the inconsistency or by the lack of appropriate RI data in the literature. Therefore, we prepared and fully spectrally characterized (1D- and 2DNMR, IR, MS) synthetic samples of all three isomeric esters. Subsequent GC analyses of immortelle oil samples with spiked synthetic phenethyl esters unambiguously confirmed that the compound in question was phenethyl angelate. This rare plant secondary metabolite has been previously reported only twice as a constituent of samples of natural origin. However, the outcomes of our study strongly imply that this molecule was misidentified in these earlier studies with the corresponding senecioate/tiglate. Thus, the existing libraries of RI/MS data for tiglates and angelates have to be upgraded with appropriate data for senecioates to avoid these kinds of errors in the future.