A simple and rapid method for the differentiation of C-13 manoyl oxide epimers in biologically important samples using GC–MS analysis supported with NMR spectroscopy and computational chemistry results

There is confusion in the literature related to the characterization of biologically important C13 manoyl oxide epimers by using their mass spectrometric data. Furthermore a method for calculating the epimeric purity has not been established. In this work mixtures of manoyl oxide C13 epimers and pure ent-13-epi-manoyl oxide have been isolated from plant extracts. The GC–MS analysis allows the characterization of each stereoisomer in a sample of natural origin: the ratio of intensities of peaks m/z 275 : 257 is lower in ent-13-epi-manoyl oxide than in manoyl oxide. NMR spectroscopy is used to give experimental evidence and simple calculations were performed to support the MS data. On the basis of these results the characterization of the two stereoisomers and the calculation of their ratio in plant extracts and essential oils can be done in a routine basis. The biological activity evaluation of mixtures with different epimeric composition of manoyl oxide showed that the ratio of the two epimers is important for their antibacterial activity. Ent-13-epi-manoyl oxide seems to be more active than its epimeric congener against Gram-positive bacteria.