Ochrolines A-C, three new indole diketopiperazines from cultures of endophytic fungi Bionectria ochroleuca SLJB-2.

Three new indole diketopiperazines, ochrolines A-C (1-3), along with three known compounds (4-6), were isolated and identified from the EtOAc extract of the solid fermentation of Bionectria ochroleuca SLJB-2. Notably, compound 1 featured a natural rarely-occurring caged skeleton with a 6/5/6/7 heterotetracyclic bridged ring system. The structures including absolute configurations of 1-3 were fully accomplished by extensive spectroscopic analyses, DFT GIAO C NMR and electronic circular dichroism (ECD) calculations. The plausible biogenetic pathways of these new indole diketopiperazines were also proposed. Moreover, the cytotoxic activity screening revealed that compound 2 exhibited moderate inhibitory effect against A549 with inhibition rate of 57.44% at the concentration of 50 μM and compound 1 exhibited mild inhibitory activities against A549, Hela and MCF-7.