Koningipyridines A and B, two nitrogen-containing polyketides from the fungus Trichoderma koningiopsis SC-5

Two novel koninginin derivatives, koningipyridines A and B (1 and 2), along with four known compounds (3-6) were isolated from the EtOAc extract of the endophytic fungus Trichoderma koningiopsis SC-5. Among them, koningipyridine A featured an unprecedented pentacyclic ketal skeleton with the formation of a fascinating 6/6/5/6/5 fused ring system and shared a characteristic pyridine core, which represents the first example of nitrogen-containing koninginin-type natural product. Moreover, koningipyridine B was the first member in the koninginin family sharing a unique 6/6/5 dihydropyridine skeleton, and it was suggested to be the critical biosynthetic precursor of koningipyridine A. The structures of 1 and 2 were elucidated by the interpretation of 1D and 2D NMR spectroscopy, HRESIMS data, as well as theoretical calculations of ¹³C NMR and electronic circular dichroism (ECD). Moreover, all isolates were screened for antimicrobial activities against Staphylococcus aureus, MRSA, and Escherichia coli as well as the cytotoxic effects against three cancer cell lines (A549, Hela, and HepG2).