**4. Conclusions**

A new aspyrone-related polyketide, aspilactonol G (**2**), a new meroterpenoid, 12-*epi*-aspertetranone D (**4**), two new drimane derivatives (**7**,**9**), together with six known metabolites were isolated from the Vietnamese marine sediment-derived fungus *A. flocculosus*. The structures of compounds **1**–**10** were established using spectroscopic methods. The absolute configurations of chiral centers were determined using either a modified Mosher's method (for compounds **1** and **2**) or a combination of ROESY data, coupling constants analysis and biogenetic considerations for compounds **4**, **7** and **9**. Drimane sesquiterpenoid derivatives **7** and **8** showed cytotoxicity toward human prostate cancer 22Rv1, human breast cancer MCF-7, and murine neuroblastoma Neuro-2a cells. The analysis of structure–activity relationships of compounds **7**–**10** together with literature data showed that these compounds have three sites in their structures related to cytotoxicity, i.e., a double bond at C7=C8, a hydroxyl group at C-9, and a *p*-nitrobenzoyl moiety.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/17/10/579/s1, Figures S1–S57: 1D and 2D NMR spectra and ECD spectra of compounds **1**–**10**.

**Author Contributions:** Conceptualization, A.N.Y.; Data curation, P.T.H.T. and S.A.D.; Formal analysis, E.V.G., O.F.S., A.B.R., R.S.P., S.A.D. and E.S.M.; Funding acquisition, G.v.A., T.T.T.V. and S.S.A.; Investigation, P.T.H.T., E.V.G., O.F.S., A.B.R., R.S.P., S.A.D. and E.S.M.; Methodology, A.N.Y.; Project administration, T.T.T.V. and S.S.A.; Resources, A.N.Y., G.v.A. and S.S.A.; Supervision, G.v.A., T.T.T.V. and S.S.A.; Validation, A.N.Y. and S.A.D.; Visualization, A.N.Y.; Writing—original draft, A.N.Y., P.T.H.T. and S.A.D.; Writing—review ' editing, A.N.Y., G.v.A., T.T.T.V. and S.S.A.

**Funding:** The study was supported by the Russian Science Foundation (grant No 19-74-10014 for the chemical study of compounds **<sup>7</sup>**–**10**), by the Russian Foundation of Basic Research (grants No 18-34-00737 for the cytotoxicity study on MCF-7 and Neuro-2A, No 19-53-54002 for the chemical study of compounds **1**–**6** and cytotoxicity study), and by the Vietnam Academy of Science and Technology (grant No QTRU01.03/19-20) for microbiology.

**Acknowledgments:** The study was carried out on the equipment of the Collective Facilities Center "The Far Eastern Center for Structural Molecular Research (NMR/MS) PIBOC FEB RAS". The authors thank Ekaterina A. Yurchenko (PIBOC FEB RAS, Vladivostok, Russia) for valuable comments in the work and writing of the article and Natalya Yu. Kim (PIBOC FEB RAS, Vladivostok, Russia) for CD spectra acquisition. The authors are grateful to Andrea Speckmann and Dipl.-Ing. Jessica Hauschild (Universitätsklinikum Hamburg-Eppendorf, Hamburg, Germany) for assistance in the performance of the biological experiments and data analysis

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