Phomanolides A and B from the Fungus <i>Phoma</i> sp.: Meroterpenoids Derived from a Putative Tropolonic Sesquiterpene via Hetero-Diels–Alder Reactions

Phomanolides A (<b>1</b>) and B (<b>2</b>), unique meroterpenoids with new pentacyclic and tetracyclic skeletons, respectively, and phomanoxide (<b>3</b>), the double-epoxidation product of a putative biosynthetic precursor of <b>1</b> and <b>2</b>, were isolated from the solid substrate fermentation cultures of the fungus <i>Phoma</i> sp., along with the known compound eupenifeldin (<b>4</b>). The structures of <b>1</b>–<b>3</b> were elucidated based on NMR spectroscopic data and electronic circular dichroism calculations and further secured by X-ray crystallography. Biogenetically, compounds <b>1</b> and <b>2</b> could be derived from a hypothetical monotropolonic sesquiterpene intermediate via hetero-Diels–Alder reactions. Compound <b>4</b> showed potent antiproliferative effects against three human glioma cell lines, with IC<sub>50</sub> values of 0.08–0.13 μM.