Eringiacetal A, 5,6‐ seco ‐(5 S ,6 R ,7 R ,9 S )‐5,6:5,7:6,9‐Triepoxyergosta‐8(14),22‐diene‐3β,7β‐diol, an Unusual Ergostane Sterol from the Fruiting Bodies of Pleurotus eryngii

A new rearranged ergostane‐type steroid, named eringiacetal A ( 1 ), together with known compound 2 , were isolated from the fruiting bodies of Pleurotus eryngii (Pleurotaceae). Its structure was fully elucidated by NMR spectroscopy and X‐ray crystallography. Eringiacetal A ( 1 ) features an unprecedented 5,6‐ seco ‐(5 S ,6 R ,7 R ,9 S )‐5,6:5,7:6,9‐triepoxyergosta‐8(14),22‐diene‐3β,7β‐diol. This is the first report of a 5,6‐ seco ‐ergostane‐type steroid. A plausible biogenetic pathway of 1 from 2 is also described. Furthermore, isolated compounds were evaluated for inhibitory effects on nitric oxide (NO) production induced by lipopolysaccharide in macrophages. Compound 1 exhibited inhibitory activity on NO production (IC 50 = 19.9 μ M ) accompanied by modest cytotoxicity (IC 50 = 25.6 μ M ). Compound 2 did not have inhibitory effects on NO production or cytotoxicity (IC 50 > 100 μ M each).