Highly Oxygenated Grayanane Diterpenoids from Flowers of Pieris japonica and Structure-Activity Relationships of Antifeedant Activity against Pieris brassicae.

Six new highly oxygenated grayanane diterpenoids, neopierisoids G-L, 1-6, together with 10 known related compounds, 7-16, were identified from the flowers of the poisonous plant Pieris japonica. The structures were elucidated on the basis of comprehensive NMR spectroscopy and mass analysis. The relative configurations of 1-6 were elucidated by analysis of ROESY spectra and comparison of NMR data with the analogues. The absolute configurations of 1-6 were established by the X-ray diffraction analysis of 1 and comparison of the CD spectra of 1-6. Compared with the skeleton of the normal grayanane diterpenoids, compounds 1-6 shared an unusual seco A ring moiety. The antifeedant activities of compounds 1-16 against Pieris brassicae were evaluated by using a dual-choice bioassay, and compounds 1-10 with a normal grayanane skeleton showed potent antifeedant activity against P. brassicae. The structure-activity relationships of antifeedant activities of 1-16 against P. brassicae are discussed.