Stoechanones A and B, Phytotoxic Copaane Sesquiterpenoids Isolated from Lavandula stoechas with Potential Herbicidal Activity against Amaranthus retroflexus .

From the organic extract of Lavandula stoechas , a Mediterranean native plant species, two new phytotoxic copaane sesquiterpenoids were isolated and named stoechanones A and B ( 1 and 2 ). They were obtained together with the methyl esters of caffeic and p -coumaric acids and the flavonoid apigenin ( 3 - 5 , respectively). The structures of stoechanones A and B were determined by spectroscopic (essentially 1D and 2D ¹ H and ¹³ C NMR and HRESIMS) and chemical methods, and they were characterized as 9,10-dihydroxy-8-isopropyl-1,5-dimethyltricyclo[4.4.0.0 ^2.7 ]dec-4-en-3-one and its 9- O -acetyl derivative. Their relative configurations were assigned by NOESY experiments, and the absolute configurations by comparison of the experimental and DFT-computed ECD spectra. When assayed through Petri dish bioassays, both stoechanones A and B showed phytotoxic effects against seed germination and seedling growth of Amaranthus retroflexus , strongly inhibiting seed germination percentage and radicle and hypocotyl lengths of seedlings. Owing to the herbicidal activity toward A. retroflexus , these two new tricyclic sesquiterpenoids could be proposed and developed as natural bioherbicides in order to increase the control of this problematic weed in the future.