Antinociceptive Diterpenoids from the Leaves and Twigs of Rhododendron decorum

Three new leucothane-type (1–3), two new micrathane-type (4, 5), eight new grayanane-type diterpenoids (6–13), and four known compounds were obtained from the ethanol extract of the leaves and twigs of Rhododendron decorum. The structures were determined based on NMR spectra, quantum chemical calculations, and X-ray crystallography. The antinociceptive activities of compounds 1, 3, 4, 6, 8, 10–13, and 15–17 were evaluated via the acetic acid-induced writhing test. Compounds 1, 8, 11–13, and 15 exhibited significant antinociceptive activities. In particular, 12 and 15 were found to be effective at doses of 0.8 and 0.08 mg/kg, respectively.