13C NMR as a tool for identification of individual components of essential oils from Labiatae - A review

The analysis of natural mixtures is commonly carried out by GC(RI), GC-MS, GC-IRFT, HPLC-MS or a combination of these techniques. In a different approach, 13C NMR spectroscopy could be used for the non separative identification of the individual components of complex mixtures (essential oils, resins, extracts, vegetable oils, ...). Following the pioneering work of Formácek and Kubeczka, we developed an experimental procedure, based on the computer-aided analysis of the 13C-NMR spectrum of the mixture, which allows the identification of the main components of essential oils and extracts. We analysed a large number of essential oils containing components bearing diverse skeletons and functions. This method allowed an accurate identification of terpenes (including stereoisomers, diastereoisomers and other compounds which exhibit insufficiently resolved mass spectral patterns or which co-eluate on GC as well as thermolabile compounds)in essential oils and solvent extracts. In this review we will report on the utilisation of C-NMR for the unambiguous identification of unusual terpenes present in essential oils from Labiatae, its efficiency and its complementarity with conventional techniques. The complete analysis, by GC(RI), GC-MS and 13C-NMR spectroscopy, of some complex essential oils is reported. Examples are chosen among the following species: Calamintha nepeta, Clinopodium ascendens, Lavandula dentata, L. stoechas, Rosmarinus officinalis, Salvia guaranitica, Stachys glutinosa, Teucrium polium ssp capitatum, Thymus herba barona, T. carnosus, T. camphoratus, T. caespititius.