Phytotoxic ent-Isopimarane-Type Diterpenoids from Euphorbia hylonoma

Thirteen new ent-isopimarane-type diterpenoids, 1–10 and 14–16, and seven known diterpenoids, 11–13 and 17–20, were isolated from the roots of Euphorbia hylonoma. Among these compounds, four pairs of C-12 epimers (1 vs 2, 4 vs 5, 12 vs 13, and 14 vs 15) were identified. The structures of the new diterpenoids were elucidated using spectroscopic data analyses, electronic circular dichroism, and single-crystal X-ray diffraction data. The phytotoxic effects of compounds 1–20 on the growth of the roots and shoots of Poa annua and Festuca arundinacea seedlings were evaluated. Among the tested diterpenoids, 18 was the most active for inhibiting the growth of P. annua seedlings, and this compound was as active as glyphosate.