Grayanane and leucothane diterpenoids from the leaves of Rhododendron micranthum

Abstract

Eleven grayanane diterpenoids, 1-epi-grayanotoxin IV, 1-epi-grayanotoxin II, 6-deoxy-1-epi-grayanotoxin XVII, 6-deoxygrayanotoxin XVII, 16-acetylgrayanotoxin II, 3-oxograyanotoxin IX, 14-deoxygrayanotoxin VIII, 14-acetylisograyanotoxin II, rhodomicranols C–E, and a leucothane diterpenoid, rhodomicranol F, together with eleven known diterpenoids were isolated from leaves of Rhododendron micranthum. Their structures were elucidated by spectroscopic analyses, with the absolute configurations of 1-epi-grayanotoxin IV and rhodomicranol C determined by single-crystal X-ray diffraction with Cu Kα radiation, and the structures of 14-acetylisograyanotoxin II and known grayanotoxins IX and X confirmed by single-crystal X-ray diffraction. All twenty-three diterpenoids were evaluated for their in vitro immunomodulatory activities, and none showed significant immunomodulatory activities in a dose-dependent manner. In addition, they are non-toxic to the murine lymphocytes in the general cytotoxicity assay.