New diterpenes from Plectranthus ernstii

The Lamiaceae herb Plectranthus ernstii L.E.Codd was phytochemically and biologically studied as part of a project to assess plants belonging to the genus Plectranthus for antibacterial activity. This genus is known to be a rich source of diterpenes, particularly of the abietane and labdane class [1,2]. The phytochemical investigation of this herb led to the characterisation of a new pimarane (1) and a labdane diterpene with an unusual 8,13-ether moiety (2). The chemical structures were elucidated by 1D and 2D NMR experiments as 15,16-epoxy-7α-hydroxy-pimar-8,14-ene (1) and 1β,11β-dihydroxy-8,13-epoxy-labd-14-ene (2) and are reported here for the first time. Compound 1 also exhibited moderate antibacterial activity, using the broth microtitre dilution assay, with a minimum inhibitory concentration of 32µg/ml recorded against a panel of Staphylococcus aureus strains, including methicillin-resistant and multidrug resistant strains.

Acknowledgement: The University of London School of Pharmacy and EPSRC are thanked for a post-doctoral scholarship to M. Stavri.

References: [1] Dellar, J.E. et al. (1996) Phytochemistry 735–738. [2] Rijo, P. et al. (2005) Magn Reson Chem 595–598.