## 2 Results
The CD spectrum shows that the positive signal at 285 nm and the negative signal at 318 nm are similar to those of (+)-virantumycin [19], and the calculated ECD curve for 9R and 10R is similar to the measured curve (Figure 4), indicating that the configurations of position C-9 and C-10 are R and R. Thus, the absolute configuration of **1** is assigned to be 9R, 10R, 13S, and it is named virantmycin D.  
The molecular formula of compound **2** is determined to be C\({}_{19}\)H\({}_{27}\)NO\({}_{5}\) by using HRESIMS (m/z 372.1787 [M + Na] +, Figure S16), suggesting that **2** has seven degrees of unsaturation. The \({}^{1}\)H NMR signals of **2** show three aromatic protons \(\delta\) 7.83 (d, \(J\) = 8.4 Hz), 7.63 (s), and 7.61 (d, \(J\) = 8.4 Hz); one meth  
Figure 4: Comparison between the calculated and experimental ECD spectra of **1**. Figure 3: The key ROESY correlations of compounds **1–2**. Figure 2: The key \({}^{1}\)H-\({}^{1}\)H COSY and HMBC of compounds **1–3**. group; one methoxyl group \(\delta\) 3.29 (s); and three methyl groups \(\delta\) 1.39 (s), \(\delta\) 1.28 (s), and \(\delta\) 1.24 (s). Compound **2** is regarded as an analogue of virantmycin because of similar UV characteristics (Figure S17). The coupling constants of H-9 (t, \(J\) = 8.4 Hz) and the chemical shift value of C-9 (\(\delta\) 63.4) and C-10 (\(\delta\) 71.0) reveal that **2** has a benzopyrrole structure [20]. The molecular weight of **2** is 16 amu more than that of A-503451 B, and the main difference in the \({}^{13}\)C-NMR data between **2** and A-503451 B is the loss of two olefinic signals and the addition of two oxygenated signals (\(\delta\) 75.4 and \(\delta\) 76.5), indicating the existence of epoxy group at position C-13 and C-14. In the HMBC spectrum, the correlations between H-15 and C-13, between H-15 and C-14, between H-11 and C-13, between H-12 and C-13, and between H-12 and C-14 confirm the epoxy moiety at C-13 and C-14. The planar structure of **2** was elucidated from the \({}^{1}\)H-\({}^{1}\)H COSY, HSQC, and HMBC spectra (Figure 2). In the ROESY spectrum, the correlations between H-3 and H-15, between H-3 and H-16, and between H-3 and H-17 support the spatial proximity between H-3 and the epoxy group (Figure 3), which is consistent with the upshift of H-3 (\(\delta\) 7.83 for 2, and \(\delta\) 6.56 for A-503451 B). The correlations from H-9 to H-11b, from H-9 to H-12b, from H-8 to H-18, and from H-8 to H-9 in the ROESY spectrum suggest that the relative configuration of **2** is 9R\({}^{\star}\), 10R\({}^{\star}\)[20]. The absolute configuration at C-9 is speculated to be R, based on the different chemical shift values of H-8 and H-9 between **2** (H-9 (\(\delta\) 3.60) and H-8 (\(\delta\) 2.88)) and A-503451 B (H-9 (\(\delta\) 4.17), H-8a (\(\delta\) 3.17), and H-8b (\(\delta\) 2.98)). In order to confirm the absolute configuration of C-9, a theoretical ECD calculation of 9R, 10R and 9S, 10S was applied. The calculated ECD curve for 9R, 10R is similar to the measured curve, indicating the configuration of C-9 is R (Figure 5). After failing to cultivate single crystal, the stereochemistry of C-13 was ambiguity. Thus, the absolute configuration of compound **2** is determined to be 9R, 10R, and it is named virantmycin E.