AbstractTrichocladinols A–C (1–3), three new metabolites, and the known massarigenin A (4), have been isolated from cultures of a Cordyceps‐colonizing ascomycete Trichocladium opacum. Their structures were elucidated by NMR spectroscopy and X‐ray crystallography. The absolute configuration of 1 was assigned by using the modified Mosher method and that of 3 was determined by X‐ray crystallographic analysis of its (S)‐MTPA ester. Compounds 1–3 showed modest cytotoxic effects against the human tumor cell lines HeLa and MCF‐7. Structurally, compounds 1 and 2 possess a previously undescribed 2,9‐dioxatricyclo[5.2.1.03,8]dec‐4‐ene skeleton.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)