# Naphthoquinone-Based Meroterpenoids from Marine-Derived _Streptomyces_ sp. B9173
## 3 Results  
### Structural Elucidation of Naphthoquinone Family Metabolites  
Meroterpenoids are hybrid natural products harboring structurally various terpenoid and non-terpenoid moieties which diversify the chemical structures and biological activities. Flaviogeranin is a prenylated naphthoquinone derivative featuring a polykite-derived naphthoquinone unit and a geranyl side chain [10]. Generally, the secondary metabolite profile of the certain strain is medium-dependent. We had previously investigated marine-derived _Streptomyces_ sp. B9173 and foundthat it produced a variety of metabolites under different culture conditions. The growth and sporulation of _Streptomyces_ sp. B9173 occurs extremely well on MS medium plate, encouraging us to isolate new natural products in this medium (Figure S1). The strain was cultivated in MS medium at 28 \({}^{\circ}\)C for 7 days, extracted with a half volume of EtOAC three times, and subsequent column chromatography resulted in the isolation of seven compounds (Figure 2). Among these naphthoquinone-containing compounds, three were new, with various modifications including methylations and prenylations or an unusual animation on the benzene moiety (ring A). Each of the new compounds is named \"flaviogeranin\" after the prenylated THN scaffold. Flaviogeranin B1 (**1**) was obtained as a violet powder. UV-vis (CH\({}_{3}\)Cl) \(\lambda_{\rm max}\) (log \(\varepsilon\)): 242 (3.71), 304 (3.38), 504 (3.34), 536 (3.54), 576 (3.5) nm; high-resolution electrospray ionization mass (HRESIMS) analysis for **1** afforded an [M + H]\({}^{+}\) ion at _m_/2 264.0868, and the molecular formula of **1** can be deduced as C\({}_{13}\)H\({}_{13}\)NO\({}_{5}\) (calcd. 264.0866 for [M + H]\({}^{+}\)). Interpretation of the \({}^{1}\)H, \({}^{13}\)C, DEPT, and HSQC NMR spectra of **1** in CDCl\({}_{3}\) revealed one aromatic methine signal (\(\delta_{C}\) 104.3, C-6 and \(\delta_{H}\), 6.48, H-6), two methoxy groups (\(\delta_{C}\) 56.4, C-9 and \(\delta_{H}\) 3.97, H-9; \(\delta_{C}\) 61.0, C-11, and \(\delta_{H}\) 4.07, H-11), and one methyl group (\(\delta_{C}\) 8.83, C-10 and \(\delta_{H}\), 2.09, H-10). Further analysis of the 1D NMR data revealed the existence of one hydroxyl group at \(\delta_{H}\) 14.41 that should form a hydrogen bond with a carbonyl group, two carbonyl carbons (\(\delta_{C}\) 186.8, C-4; \(\delta_{C}\) 180.1, C-1), and seven nonprotonated double-bond carbons (\(\delta_{C}\) 159.4, C-2; \(\delta_{C}\) 131.6, C-3; \(\delta_{C}\) 106.1, C-4; \(\delta_{C}\) 160.3, C-5; \(\delta_{C}\) 155.4, C-7; \(\delta_{C}\) 139.9, C-8; \(\delta_{C}\) 107.5, C-8a). Thus, **1** was identified as a naphthoquinone bearing a hydroxy group, an amino group, one methyl group, and two methoxy groups. The analysis of the HMBC spectrum revealed the correlations from H-6 to C-4a and C-8; OCH\({}_{3}\)-9 to C-7; CH\({}_{3}\)-10 to C-2 and C-4; OCH\({}_{3}\)-11 to C-2. The positions of other substituent groups have been established by NMR, expect the NH\({}_{2}\) at C8. According to the furaquinocin biosynthesis, 8-amino flavin has been confirmed as a cryptic intermediate [23]. Therefore, the amino group is speculated to attach at the C8. Finally, the structure of **1** was established as 8-amino-5-hydroxy-2,7-dimethoxy-3-methylnaphthalene-1,4-dione (Figure 3 and Figure S3 and Table 1). Flaviogeranin B (**2**) was obtained as a violet powder. UV-vis (CH\({}_{3}\)Cl) \(\lambda_{\rm max}\) (log \(\varepsilon\)): 242 (3.40), 304 (3.07), 504 (3.06), 536(3.27), 576(3.23) nm; HRESIMS analysis afforded an [M + H]\({}^{+}\) ion at _m_/2 386.1969, giving the molecular formula of **2** as C\({}_{22}\)H\({}_{27}\)NO\({}_{5}\) (calcd. 386.1962 for [M + H]\({}^{+}\)).