S‑Bridged Thioether and Structure-Diversified Angucyclinone Derivatives from the South China Sea-Derived Micromonospora echinospora SCSIO 04089

Angucyclinces belong to the class of aromatic polyketides and display a wide variety of structure diversity and pharmaceutical significance. Herein we report the isolation, structure elucidation, and bioactivity evaluation of structure-diversified angucyclinone derivatives and anthracene from the South China Sea-derived Micromonospora echinospora SCSIO 04089, including a thioether, gephysulfuromycin (1), two new benzo­[b]­phenanthridines, homophenanthroviridone (2) and homophenanthridonamide (3), a new benzo­[b]­fluorene, homostealthin D (4), a new naphtho­[2,3-b]­benzofuran, nenesfuran (5), a new naphthoquinone, WS-5995 D (6) and a new anthracene, nenesophanol (7), together with three known compounds (8–10). Their structures were elucidated by extensive spectroscopic analyses. The structures of 1–3 and 5–8 were confirmed by X-ray crystallographic analyses. Gephysulfuromycin (1) featured a rare single S-bridged 3,12a-epithiotetraphene skeleton. Homophenanthroviridone (2) was found to be cytotoxic to SF-268, MCF-7, and HepG2 cell lines with IC50 values of 5.4 ± 0.4, 6.8 ± 0.3, and 1.4 ± 0.1 μM, respectively. Compound 2 was also active against Gram-positive bacteria with MIC (minimal inhibition concentration) values ranging 2–4 μg mL–1.