Meroantarctines A–C, Meroterpenoids with Rearranged Skeletons from the Alga-Derived Fungus Penicillium antarcticum KMM 4685 with Potent p‑Glycoprotein Inhibitory Activity

New meroterpenoids, meroantarctines A–C (1–3), with unique 6/5/6/6, 6/5/6/5/6, and 6/5/6/5 polycyclic systems were isolated from the alga-derived fungus Penicillium antarcticum KMM 4685. Their structures were elucidated by spectroscopic methods, X-ray diffraction, and quantum chemical calculations. A biogenetic pathway for 1–3 was proposed. Meroantarctines A–C (1–3) inhibited p-glycoprotein activity and could resensitize drug-resistant cancer cells to docetaxel.