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A new phytotoxic unrearranged ent-pimaradiene diterpene, named chenopodolin, was isolated from the liquid culture of Phoma chenopodicola, a fungal pathogen proposed for the biological control of Chenopodium album, a common worldwide weed of arable crops such as sugar beet and maize. The structure of chenopodolin was established by spectroscopic, X-ray, and chemical methods as (1S,2S,3S,4S,5S,9R,10S,12S,13S)-1,12-acetoxy-2,3-hydroxy-6-oxopimara-7(8),15-dien-18-oic acid 2,18-lactone. At a concentration of 2 mg/mL, the toxin caused necrotic lesions on Mercurialis annua, Cirsium arvense, and Setaria viride. Five derivatives were prepared by chemical modification of chenopodolin functionalities, and some structure–activity relationships are discussed.

Two phytotoxins were isolated from the liquid culture of Phoma chenopodiicola , a fungal pathogen proposed for the biological control of Chenopodium album , a common worldwide weed of arable crops. The two phytotoxins appeared to be a new tetrasubstituted furopyran and a new ent- pimaradiene. From the same culture a new tetrasubstituted isocoumarin was also isolated. These compounds were characterized by using spectroscopic (essentially 1D and 2D NMR and HR ESI MS) and chemical methods as 3-(3-methoxy-2,6-dimethyl-7a H -furo[2,3- b ]pyran-4-yl)-but-2-en-1-ol (chenopodolan D, 1 ) (1 S ,2 S ,3 S ,4 S ,5 S ,9 R ,10 S ,12 S ,13 S )-1,3,12-triacetoxy-2,hydroxy-6-oxo- ent -pimara-7(8),15-dien-18-oic acid 2,18-lactone (chenopodolin B, 3 ), and, 4,5,7-trihydroxy-3-methyl-isochroman-1-one (chenisocoumarin, 2 ) The absolute configuration of chenisocoumarin was assigned by applying an advanced Mosher’s method through the derivatization of its secondary hydroxylated carbon C-4, while that of chenopodolan D by application of quantum mechanical calculations of chiroptical (ECD and ORD) properties. When assayed by leaf puncture against non-host weeds, chenopodolan D and chenopodolin B showed phytotoxicity while chenisocoumarin and the 9- O -acetyl derivative of chenopodolan D were inactive. These results confirm that the nature of the side chain at C-4 in chenopodolans, and in particular its hydroxylation, are important features for activity. The activity of chenopodolin B could also be explained by its possible hydrolysis to chenopodolin.