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

Phomanolides A (1) and B (2), unique meroterpenoids with new pentacyclic and tetracyclic skeletons, respectively, and phomanoxide (3), the double-epoxidation product of a putative biosynthetic precursor of 1 and 2, were isolated from the solid substrate fermentation cultures of the fungus Phoma sp., along with the known compound eupenifeldin (4). The structures of 1–3 were elucidated based on NMR spectroscopic data and electronic circular dichroism calculations and further secured by X-ray crystallography. Biogenetically, compounds 1 and 2 could be derived from a hypothetical monotropolonic sesquiterpene intermediate via hetero-Diels–Alder reactions. Compound 4 showed potent antiproliferative effects against three human glioma cell lines, with IC50 values of 0.08–0.13 μM.