Phyllostictines A–D, oxazatricycloalkenones produced by Phyllosticta cirsii, a potential mycoherbicide for Cirsium arvense biocontrol

Phyllosticta cirsii, a fungal pathogen isolated from Cirsium arvense and proposed as biocontrol agent of this noxious perennial weed, produces in liquid cultures different phytotoxic metabolites with potential herbicidal activity. Four new oxazatricycloalkenones, named phyllostictines A–D, were isolated and characterized using essentially spectroscopic and chemical methods. Tested by leaf-puncture assay on the fungal host plant phyllostictine A proved to be highly toxic. The phytotoxicity decreases when both the dimension and the conformational freedom of the macrocyclic ring change, as in phyllostictines B and D, and it is totally lost when also the functionalization of the same ring is modified, as in phyllostictine C. Beside its phytotoxic properties, phyllostictine A has no antifungal activity, an interesting antibiotic activity only against Gram+ bacteria, and a noticeable zootoxic activity when tested at high concentrations. The integrity of the oxazatricycloalkenone system appears to be an important feature to preserve these activities.