Diterpenoid Constituents of and Evaluation of Their Antimicrobial Activity.

Sixteen diterpenes (-), along with 10 previously described compounds, including four flavonoids and six diterpenes, were isolated from the aerial parts of growing in Saudi Arabia. The diterpene structures were elucidated using NMR spectroscopy and mass spectrometry data. Furthermore, a DFT/NMR procedure was used to suggest the relative configuration of several compounds. The labdane-derived skeletons, namely, -atisane, -beyerene, -trachylobane, and -kaurene, were identified. The extracts, fractions, and pure compounds were then tested against , , , and . One diterpenoid, namely, psiadin, showed an additive effect with the antiseptic chlorhexidine, with a fractional inhibitory concentration index of less than 1. Additionally, psiadin showed a prospective inhibition activity for bacterial efflux pumps.