Citrinin Monomer, Trimer, and Tetracyclic Alkaloid Derivatives from the Hydrothermal Vent-Associated Fungus Penicillium citrinum TW132-59

Five new unusual citrinin-derived alkaloids with a tetracyclic core, citrinidines A-E (1-5), two new amide alkaloids, methyl (2S,8E)-1'-(2-methyl-3-oxodec-8-enamido) butanoate (6) and (2S,8E)-2-methyl-3-oxodec-8-enamide (7), a new unusual citrinin trimer, tricitrinol C (8), a new citrinin acetal-ketal derivative, citrininol (9), together with four known citrinin monomers (10-13), and three known citrinin dimers (14-16), were isolated from the fermentation of hydrothermal vent-associated fungus Penicillium citrinum TW132-59. Their structures were unambiguously determined by nuclear magnetic resonance (NMR), mass spectrometry, Mosher's method, ¹³C NMR calculation in combination with DP4+, and ECD calculations. A plausible biosynthetic pathway of all new compounds (1-9) was proposed. Citrinin trimer (8) exhibited potent cytotoxicity activity with an IC₅₀ value of 1.34 ± 0.11 μM, and compounds 1 and 15 showed moderate cytotoxicity with IC₅₀ values of 17.50 ± 1.43 and 9.45 ± 0.55 μM, respectively, against A549 cell line.