**4. Conclusions**

This study is the second case of isolation of melatonin-related compound from microfilamentous fungi. The neuroprotective activity in the 6-OHDA- and PQ-induced PD cell models of this and some other polyketides and alkaloids from marine-derived fungi were investigated. All of them protected Neuro2a cells against the damaging influence of 6-OHDA to varying degrees. We suppose that this effect is realized via a ROS scavenging pathway as one of the possibilities. The new melatonin analogue, 6-hydroxy-*N*-acetyl-β-oxotryptamine, protected Neuro2A cells more effectively against the 6-OHDA-induced neuronal death in comparison with melatonin. Moreover, 6-hydroxy-*N*-acetyl-β-oxotryptamine and melatonin protected Neuro2a cells against the damaging influence of PQ in a similar manner. It was shown that dehydroxylation at C-3" and C-4"significantly increases free radical scavenging and neuroprotective activity of candidusin-related *p*-terphenyl polyketides in both the 6-OHDA- and PQ-induced PD models.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/16/11/457/ s1, Figures S1–S15. The NMR and mass spectra of compounds **1**–**3** and **6**. Figure S16. ROS formation in the 6-OHDA- and PQ-treated Neuro2a cells.

**Author Contributions:** Conceptualization, E.A.Y. and A.N.Y.; Data curation, A.N.Y.; Formal analysis, E.A.Y. and E.A.P.; Funding acquisition, E.A.Y. and E.V.I.; Investigation, E.A.Y., E.S.M., E.A.P., P.T.H.T., E.V.I. and O.F.S.; Project administration, E.A.Y.; Resources, E.A.Y., P.T.H.T. and A.N.Y.; Supervision, E.A.Y.; Validation, E.A.Y. and O.F.S.; Visualization, E.A.Y., E.S.M. and E.A.P.; Writing—original draft, E.A.Y. and P.T.H.T.; Writing—review & editing, A.N.Y.

**Funding:** This research was funded by Russian Foundation for Basic Research, grant numbers 18-34-00621 (isolation and identification of compounds) and 18-34-00737 (investigation of biological activity).

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