Phytotoxic halimanes isolated from Baccharis salicifolia (Ruiz & Pad.) Pers.

From the EtOH extract of the medicinal native plant, Baccharis salicifolia, two novel halimane-type diterpenoids, salicifolic acid (1) and 5-hydroxy-6-hydro-salicifolic acid (2) together with the known compounds sakuranetin (3), apigenin (4) and scopoletin (5) were bioguided isolated against Panicum miliaceum (monocotyledonous). The structures of 1 and 2 were established by extensive spectroscopic analyses. The effective concentration for 50% inhibition of germination (ECg50) and the root (ECr50) and shoot (ECs50) elongations was determined for 1-5 against P. miliaceum and Raphanus sativus (dicotyledonous). Compound 2 was the most active in the inhibition of germination of P. miliaceum (ECg50 = 1 mM), followed by 1, 5 and 3, although 1 was the most effective in regulating the growth of P. miliaceum seedlings, with a ECr50 and ECs50 values of 1.8 and 6.6 mM, respectively. Compounds 1 and 3 were the only samples capable of inhibiting the germination of R. sativus, while seedling development was affected by 1, 2, and 3 with different effectiveness.

Highlights► Bio-guided isolation of an extract of Baccharis salicifolia yielded phytotoxic compounds. ► Two new halimanes together with 2 known flavonoids and a coumarin were isolated. ► Germination of Panicum milliaceum and Raphanus sativus were inhibited by halimanes.