Enantiomeric meroterpenoids from Rhododendron capitatum and their anti-inflammatory activity

Ten enantiomeric pairs of meroterpenoids were isolated from Rhododendron capitatum Maxim. Among them, (+/-)-capitanoids A-F, (+)-ranhuadujuanine B, (-)-rubiginosin G, and (-)-anthopogochromene B are undescribed compounds. Their structures were fully established by spectroscopic techniques, X-ray crystallographic analyses, modified Mosher's method, and ECD calculation. (+/-)-Capitanoid A are the first pair of meroterpenoids possessing an unusual 6/7/5 heterocyclic system characterized by a 1,2,3,3a, 10,10a-hexahydrobenzo [b]cyclopenta[e][1,4]dioxepine motif. (+/-)-Capitanoid F are a pair of meroterpenoids with a tris-normonoterpene moiety that are discovered from the genus Rhododendron for the first time. (+/-)-Rubiginosin G, (+/-)-anthopogochromene B, and (-)-rhodonoid B showed inhibition in hypoxia-inducible factor-1α (HIF-1α) responsive element (HRE) luciferase assay. Furthermore, (+)-anthopogochromene B and (-)-rhodonoid B exhibited suppressive effects on macrophage-mediated inflammatory response by downregulating the pro-inflammatory factor IL-1β expression level and secretion.