Interconvertible Pyridone Alkaloids from the Marine-Derived Fungus Penicillium oxalicum 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 IC 50 values ranging from 9.2 to 19 μM.