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Fluostatins belong to the atypical angucyclinone aromatic polyketides featuring a distinctive tetracyclic benzo[a]fluorene skeleton. To understand the formation of the heavily oxidized A-ring in fluostatins, a flavin adenine dinucleotide-binding oxidoreductase-encoding gene flsP was inactivated, leading to the production of an unprecedented 1,4-oxazepine-linked seco-fluostatin heterodimer difluostatin I (7) and five new fluostatin-related derivatives, fluostatins T-X (8-12). Their structures were elucidated by mass spectrometry, nuclear magnetic resonance, X-ray diffraction analysis, and biosynthetic considerations. Difluostatin I (7) represents the first example with an A-ring-cleaved 3',4'-seco-fluostatin skeleton. The absolute configuration of fluostatin T (8) was determined by X-ray diffraction analysis. Fluostatin W (11) contains an uncommon isoxazolinone ring. These findings highlight the structural diversity of fluostatins.

Inactivation of the flavoenzyme-encoding gene flsO1 in fluostatin biosynthesis led to the isolation of four new angucyclinone derivatives (11, 12, 14, and 15), among which fluostarenes A (14) and B (15) featured the unprecedented 6/6/5/6/6 pentacyclic skeleton with fusion of a benzo[b]fluorene and a six-membered lactone ring. Both 14 and 15 were putatively generated via quinone methide-mediated nonenzymatic reactions. Fluostarene B (15) exhibited cytotoxicity against several cancer cell lines with IC50 values ranging from 7 to 10 μM. Fluostarenes A (14), B (15), and PK1 (16) showed α-glucosidase inhibition activity with IC50 of 0.89, 1.58, and 0.13 μM, respectively. Successful complementation of the ΔflsO1 mutant with alpK from kinamycin biosynthesis suggests that FlsO1 should function equivalently to AlpK as a putative C-5 hydroxylase.

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.

Pyrazolofluostatins A–C (1–3), three new benzo[a]fluorenes with an unprecedented carbon skeleton, were obtained from the South China Sea-derived Micromonospora rosaria SCSIO N160. Their structures were elucidated by extensive spectroscopic analyses. The structure of pyrazolofluostatin A (1) was confirmed by X-ray crystallographic analysis. Notably, 1–3 possessed a benzo[cd]indeno[2,1-f]indazol skeleton with a pyrazole-fused 6/5/6/6/5 pentacyclic ring system. Pyrazolofluostatin A (1) showed moderate antioxidation activity (EC50 48.6 μM).

The identification of a lobophorin biosynthetic gene cluster from the deep-sea derived Streptomyces sp. SCSIO 01127 reveals a paradigm of three glycosyltransferases (GTs) LobG1-LobG3 being responsible for appending four sugars. Characterization of five differentially glycosylated metabolites from three GT gene-inactivation mutants allowed the assignment of GT functions and the implication of LobG3 as an iterative GT to attach two digitoxoses.