Unusual antifeedant spiro-sesterterpenoid from the flowers of Leucosceptrum canum

Leucosceptroid O (1), an unusual sesterterpenoid possessing a spiro α,β-unsaturated γ-lactone moiety, was discovered from the flowers of a large woody Labiatae, Leucosceptrum canum. Its structure including absolute stereochemistry was determined by comprehensive NMR, MS, and single-crystal X-ray diffraction (with copper radiation) analyses. The obvious antifeedant activity of 1 against the generalist plant-feeding insect Helicoverpa armigera suggested that it to be a new defensive sesterterpenoid of Leucosceptrum canum.