Interconvertible Pyridone Alkaloids from the Marine-Derived Fungus <i>Penicillium oxalicum</i> QDU1

Eleven new pyridone alkaloids, penicipyridones A–K (1–11), and three new tetramic acids, tolypocladenols D–F (12–14), were isolated from rice media cultures of the marine-derived fungus Penicillium oxalicum QDU1. Their structures, including absolute configurations, were determined by comprehensive analyses of spectroscopic data, electronic circular dichroism (ECD) calculations, and single-crystal X-ray diffraction data. Interestingly, several of the penicipyridones undergo interconversions between hydroxy and methoxy groups at C-4 in acidic MeOH solution. Furthermore, in an acidic aqueous solution, OH-4 could be replaced by diverse substituent groups. Compounds 1, 4, 5, 8, 10, 11, and 14 exhibited moderate inhibitory effects on NO production in the LPS-induced RAW264.7 macrophages, with IC50 values ranging from 9.2 to 19 μM.