Streptoglycerides A–D with a Rare 6/5/5 Tricyclic Ring Skeleton from a Marine Actinomycete Streptomyces species

Four novel secondary metabolites possessing a unique 6/5/5 tricyclic ring system, streptoglycerides A–D (1–4), were isolated from a marine actinomycete Streptomyces sp. derived from a mangrove sample collected on Kosrae Island. Their structures and absolute configurations were elucidated by spectroscopic data and electronic circular dichroism data. Streptoglyceride C (3) showed a weak inhibitory effect on the production of nitric oxide in BV-2 microglia cells.