Trichorenins A–C, Algicidal Tetracyclic Metabolites from the Marine-Alga-Epiphytic Fungus <i>Trichoderma virens</i> Y13‑3

Three novel polyketide-like metabolites, trichorenins A–C (1–3), with a unique tetracyclic carbon skeleton were obtained from the culture of Trichoderma virens Y13-3, an epiphyte of the marine red alga Gracilaria vermiculophylla. Their structures and relative configurations were established by analysis of 1D/2D NMR and MS data, and their absolute configurations were unequivocally assigned by X-ray diffraction and ECD spectra aided by quantum chemical calculations. Compounds 1–3 exhibited potent inhibition against two marine phytoplankton species, Chattonella marina and Karlodinium veneficum.