Three new antinociceptive diterpenoids from the flowers of Rhododendron molle

Three new grayanane-type diterpenoids (1-3), together with eight known compounds, were isolated from the flowers of Rhododendron molle G. Don. The structures of new compounds were fully determined based on spectroscopic analysis, including HRESIMS, 1D, and 2D NMR data. The absolute configuration of the new compounds was confirmed by single-crystal X-ray diffraction. The analgesic activities of compounds 1-7 and 10-11 were evaluated by the acetic acid-induced writhing method in mice. Compounds 3 and 6 had significant analgesic effects at a low dose of 0.2 mg/kg, and the writhe inhibition rates were 66.3% and 82.9%, respectively. Compounds 3-4 and 10-11 also showed significant analgesic effects at a dose of 1 mg/kg (writhe inhibition rate was 66.8%-91.7%). Compounds 5, 7, and 10-11 showed significant analgesic effects at a dose of 5 mg/kg (writhe inhibition rate was 68.4%-83.4%).