**5. Conclusions**

In conclusion, nine polyketides, including two new chlorinated metabolites 8-chlorine-5-hydroxy-2,3-dimethyl-7-methoxychromone (**1**) and 3,4-dichloro-1 *<sup>H</sup>*-pyrrole-2,5-dione (**3**), were isolated from the mangrove-sediment-derived fungus *Mollisia* sp. SCSIO41409. The X-ray single-crystal diffraction analysis and absolute configuration of (4*S*, 5*S*, 13 *R*, 14 *R*, 17 *R*, 18 *R*, 21 *R*)-stemphone C (**4**) were described for the first time. Compounds **3** and **4** showed different intensities of antimicrobial activities and antiproliferative activities. Further experiments revealed that **4** could significantly reduce PC-3 cells colony formation, induce apoptosis, and block the cell cycle at the S phase in a dose-dependent manner. Stemphone C (**4**) could be considered as a potential antiproliferative agen<sup>t</sup> and a promising anti-prostate cancer lead compound.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/md20100637/s1. Figures S1–S14: The spectroscopic data of **2** and **4**–**9**; The NMR, HRESIMS, UV, and IR spectra of **1** and **3**; ITS sequence data of the strain.

**Author Contributions:** Conceptualization, X.Z.; investigation, J.C., X.W., X.L. and B.Y.; methodology, J.C., X.W., X.G. and Q.Z.; project administration, Y.L., D.R. and X.Z.; supervision, D.R. and X.Z.; writing—original draft, J.C. and X.W.; Writing—review and editing, D.R. and X.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Guangdong Local Innovation Team Program (2019BT02Y262), Guangdong Basic and Applied Basic Research Foundation (2019B151502042), National Natural Science Foundation of China (U20A20101, 81973235), Guangzhou Science and Technology Project (202102080478), and K. C. Wong Education Foundation (GJTD-2020-12).

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** We are grateful to Z. Xiao, X. Zheng, A. Sun, Y. Zhang, and X. Ma in the analytical facility at SCSIO for recording spectroscopic data.

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