Stagonolides B-F, nonenolides produced by Stagonospora cirsii, a potential mycoherbicide of Cirsium arvense

Stagonospora cirsii, a fungal pathogen isolated from Cirsium arVense and proposed as a potential mycoherbicide of
this perennial noxious weed, produces phytotoxic metabolites in liquid and solid cultures. Recently, the main metabolite,
stagonolide (1), with interesting phytotoxic properties, was isolated from a liquid culture and characterized as a new
nonenolide. In the present work this same fungus, grown in solid culture, exhibited an increased capacity to produce
nonenolides. Five new nonenolides, named stagonolides B-F (2-6), were isolated and characterized using spectroscopic
methods. When tested by a leaf disk puncture assay at a concentration of 1 mg/mL, these compounds showed no toxicity
to C. arVense and Sonchus arVensis, whereas stagonolide (1) was highly toxic. Stagonolide (1) and stagonolide C (3)
were weakly toxic to Colpoda steinii, a protozoan, when tested at 0.05 mg/mL, with the other stagonolides nontoxic.
A number of structure-activity relationship observations were made.