Rapid and novel discrimination and quantification of oleanolic and ursolic acids in complex plant extracts using two-dimensional nuclear magnetic resonance spectroscopy—Comparison with HPLC methods

A novel strategy for NMR analysis of mixtures of oleanolic and ursolic acids that occur in natural products is described. These important phytochemicals have similar structure and their discrimination and quantification is rather difficult. We report herein the combined use of proton–carbon heteronuclear single-quantum coherence (1H–13C HSQC) and proton–carbon heteronuclear multiple-bond correlation (1H–13C HMBC) NMR spectroscopy, in the identification and quantitation of oleanolic acid (OA) and ursolic acid (UA)in plant extracts of the Lamiaceae and Oleaceae family. The combination of 1H–13C HSQC and 1H–13C HMBC techniques allows the connection of the proton and carbon-13 spins across the molecular backbone resulting in the identification and, thus, discrimination of oleanolic and ursolic acid without resorting to physicochemical separation of the components. The quantitative results provided by 2D 1H–13C HSQC NMR data were obtained within a short period of time (∼14 min) and are in excellent agreement with those obtained by HPLC, which support the efficiency of the suggested methodology.