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 H-1 and C-13 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 retrof lexus, strongly inhibiting seed germination percentage and radicle and hypocotyl lengths of seedlings. Owing to the herbicidal activity toward A. retrof lexus, 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.