**4. Conclusions**

In summary, chemical studies on the marine-derived *Streptomyces* sp. BTBU20218885 resulted in the characterization of three cyclized thiolopyrrolones, including a unique unsymmetrical thiolopyrrolone (**1**), 2,2-dioxidothiolutin (**2**), and the previously reported thiolutin (**3**). Dithiolopyrrolones are a class of structurally intriguing natural products with broad antibacterial spectrum [34]. Most of the analogues are characterized by a unique bicyclic pyrrolinonodithiole, with the differences in the substitution groups on *N*-4 and *N*-7 positions of the holothin core [35,36]; however, thiolopyrrolone A is the first sample of analogues with a macrocyclic skeleton. Compound **1** exhibited antibacterial activities against BCG, *M. tuberculosis*, and *S. aureus* with MIC values of 10, 10, and 100 μg/mL, respectively. Thiolutin (**3**) displayed potential antibacterial activities against *E. coli*, BCG, *M. tuberculosis*, and *S. aureus* with MIC values of 6.25, 0.3125, 0.625, and 3.125 μg/mL, respectively.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/md20030214/s1: Figures S1–S16: HRESIMS, 1D, and 2D NMR for compounds **1**–**3**; Figure S17: Colony characteristics of BTBU20211089; Figure S18: Neighbor-joining phylogenetic tree of strain BTBU20211089.

**Author Contributions:** Data curation, F.S.; funding acquisition, F.S.; investigation, F.S., J.H., X.Z., W.X., J.Y. and S.L.; supervision, X.X.; writing—original draft, F.S. and X.X.; writing—review and editing, F.S., J.H., X.Z. and X.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by grants from the National Natural Science Foundation of China (81973204), the Key Lab of Marine Bioactive Substance and Modern Analytical Technique, SOA (MBSMAT-2019-06), and Research Foundation for Advanced Talents of Beijing Technology and Business University (19008021176).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the text.

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