Rhytidenones A–F, Spirobisnaphthalenes from Rhytidhysteron sp. AS21B, an Endophytic Fungus

Rhytidenone A (1), a unique spirobisnaphthalene with a 1,7-dioxaspiro­[4,4]­nonan-2-one motif, and five new spirobisnaphthalenes, rhytidenones B–F (2–6), were isolated from the extract of a cultured fungal endophyte, Rhytidhysteron sp. AS21B. Their structures were elucidated mainly by analysis of NMR spectroscopic data. The structure and configuration of 1 were further confirmed by single-crystal X-ray diffraction analysis. Compounds 3 and 4 exhibited significant inhibitory activity against nitric oxide production from activated macrophages with IC50 values of 0.31 and 3.60 μM, respectively.