New antifeedant C20 terpenoids from Leucosceptrum canum

Three novel C(20) terpenoids, norleucosceptroids A-C (1-3), were isolated from the leaves and flowers of Leucosceptrum canum (Labiatae) and were identified by comprehensive spectroscopic analysis and, in the case of 1, single-crystal X-ray diffraction. Structurally, compounds 1-3 should be categorized as pentanor-sesterterpenoids rather than diterpenoids. Moderate antifeedant activity of 1-3 against the generalist plant-feeding insect Helicoverpa armigera was detected, suggesting that they might also be involved in the plant defense against insect herbivores.