Bipoladisins A–G, Uncommon Sesterterpenoids with Three Types of Carbon Skeletons from Bipolaris maydis and Their Reversal Activity on Paclitaxel Resistance

Bipoladisins A–G (1–7), seven sesterterpenoids featuring three distinct carbon skeletons, were isolated from the fungus Bipolaris maydis. Their structures were elucidated by spectroscopic data, HRESIMS, ECD/ORD calculations, and single-crystal X-ray diffraction analyses. Bipoladisin C (3) contained a unique C-5–C-14 oxygen bridge that formed a complex caged structure. Bipoladisins D (4) and E (5) were a pair of C-17 stereoisomers, both decorated by an oxygen bridge. Bipoladisins F (6) and G (7), possessing a 4/9 bicyclic ring system, were first reported from the genus Bipolaris. Compound 4 could reverse the resistance of cancer cells to paclitaxel (PTX) with the reverse fold of 24 and inhibit the efflux function of P-gp.