**4. Conclusions**

In conclusion, ten diketopiperazine alkaloids including four new derivatives, 5'-hydroxy-6'-ene-epicoccin G (**1**), 7-methoxy-7'-hydroxyepicoccin G (**2**), 8'-acetoxyepicoccin D (**3**), and 7'-demethoxyrostratin C (**4**), and a pair of new enantiomeric diketopiperazines (±)-5-hydroxydiphenylalazine A ((±)-**5**), along with five known analogues (**6**–**10**) were characterized from the deep sea-derived fungus *E. nigrum* SD-388. The discovery of these compounds might provide further insight into the biosynthesis of the diketopiperazine family and provide new targets for synthetic or biosynthetic studies. Compounds **4** and **10** exhibited potent cytotoxic activities against Huh7.5 liver cancer cells and may provide useful candidates for further study as antitumor agents.

**Supplementary Materials:** Selected 1D and 2D NMR, HRESIMS, and ECD spectra of compounds **1**–**5** are available online at: http://www.mdpi.com/1660-3397/18/3/160/s1.

**Author Contributions:** L.-P.C. performed the experiments for the isolation, structure elucidation, and prepared the manuscript; X.-M.L. performed the 1D and 2D NMR experiments; L.L. participated in the isolation of the fungus *Epicoccum nigrum*. X.L. contributed to ECD calculations and revised the manuscript; B.-G.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22050401) and the Aoshan Scientific and Technological Innovation Project of Qingdao National Laboratory for Marine Science and Technology (2016ASKJ14). X.L. appreciates the China Postdoctoral Science Foundation (2017M612360) for project funding. B.-G.W. acknowledges the support of the Research Vessel KEXUE of the National Major Science and Technology Infrastructure from the Chinese Academy of Sciences (KEXUE2018G28) and the Taishan Scholar Project from Shandong Province.

**Acknowledgments:** The authors appreciate Chao-min Sun and Ge Liu at the Institute of Oceanology, Chinese Academy of Sciences, for their support in cytotoxic assays, and the High Performance Computing Environment Qingdao Branch of the Chinese Academy of Science (CAS)–High Performance Computing Center of Institute of Oceanology of CAS for CPU time.

**Conflicts of Interest:** The authors declare no conflicts of interest.
