Four phytotoxic bi- and tri-cyclic sesquiterpene lactones, named inuloxins A???D, were isolated together with the known a-costic acid, from the aerial parts of Inula viscosa (family Asteraceae), a widespread Mediterranean plant well known for its content of pharmacologically active metabolites. The structures of inuloxins A???D were established by spectroscopic and chemical methods and determined to be: (4E,7R*,8R*,10S*)-3-oxo-germacra-4,11(13)-dien-8b-12-olide (A), its 11,13-dihydro analogue (B), (5*, 7R*,8R*,10R*)-1,15-methylene-5b-hydroxy-eudesm-1(15),11(13)-dien-8b-12-olide (C), and (7R*,8R*)-1,4-dimethyl-4-hydroxy-secoeudesm-5(10),11(13)-dien-8b-12-olide (D). The S absolute stereochemistry at C-5 of 5-hydroxyhexan-2-yl side chain of inuloxin D was assigned by applying an advanced Mosher???s method. The phytotoxic activity of inuloxins A???D, that of the diazo and monoacetyl derivatives (of inuloxin A and C, respectively), as well as that of a-costic acid was evaluated against two parasitic plant species, i.e. crenate broomrape (Orobanche crenata) and field dodder (Cuscuta campestris). Inuloxins A, C and D were the most active on both parasites and caused up to 100% inhibition of the seed germination. Inuloxin B was less active on Cuscuta and completely inactive against Orobanche. The main metabolite a-costic acid had a suppressive effect on the dodder seed germination but had a stimulating action on the broomrape seed germination. These preliminary results allowed to suppose some structure???activity relationships
