Didymorenloids A and B, two polycyclic cyclopenta[b]fluorene-type alkaloids with anti-hepatoma activity from the mangrove endophytic fungus Didymella sp. CYSK-4

Didymorenloid A (1), a polycyclic cyclopenta[b]fluorene-type alkaloid with a new skeleton, and one new analogue didymorenloid B (2) were isolated from the mangrove endophytic fungus Didymella sp. CYSK-4 under the guidance of molecular networking. Their structures including absolute configurations were determined from extensive analysis of NMR, HRESIMS, ECD calculations and X-ray crystallographic data. Structurally, compound 1 features an unprecedented 6/5/6/5/13(6)/5/5/5-fused ring system based on a unique 6,13,16-trioxa-21-azole-tricyclo-[10.7.2.2(7,10).0(12,19).0(14,18)]tricosane core. Plausible biosynthetic pathways for 1 and 2 were proposed. Compound 1 exhibited potent cytotoxicity against Hepa1-6 and Huh7 cell lines, with IC50 values of 5.17 and 10.57 mu M, respectively.