## 2 Results and Discussion
The IR spectrum showed absorption bands for amine (3214 cm\({}^{-1}\)), amide carbonyls (1688, 1654 cm\({}^{-1}\)), aromatic (3057, 1579 cm\({}^{-1}\)) and olefin (1607, 1468 cm\({}^{-1}\)) groups. The \({}^{13}\)C NMR, DEPTs and HSQC spectra (Table 2, Supplementary Figures S9 and S10) revealed the presence of two amide carbonyls (\(\delta\)c172.0 and 168.9), six quaternary sp\({}^{2}\) (\(\delta\)c 136.4, 136.3, 135.7, 127.9, 125.5, 108.0), ten methine sp\({}^{2}\) (\(\delta\)c 133.1, 131.4, 127.3, 125.2, 121.7, 121.0, 119.9, 119.2, 118.4, 109.9), one methine sp\({}^{3}\) (\(\delta\)c 52.4), two methylene sp\({}^{3}\) (\(\delta\)c 44.2 and 22.4) and two methyl (\(\delta\)c 25.6 and 18.1) carbons. The coupling system of the aromatic protons, observed in the COSY spectrum (Table 2, Supplementary Figure S8), indicated the presence of two 1,2-disubstituted benzene rings. That one of the 1,2-disubstituted benzene rings was part of the 3,4-dihydro-1_H_-1,4-benzodiazepepine-2,5-dione was supported by the HMBC cross peaks (Table 2, Supplementary Figure S11) of the singlet of the amine proton at \(\delta\)\({}_{\rm H}\) 9.03 (NH-16) to C-11 (\(\delta\)c 52.4) and C-14 (\(\delta\)c 125.5), of H-21 (\(\delta\)\({}_{\rm H}\) 7.91, dd, \(J\) = 8.0, 1.5 Hz) to C-13 (\(\delta\)c 168.9), as well as by the COSY cross peaks (Table 2, Supplementary Figure S8) observed between NH-12 (\(\delta\)\({}_{\rm H}\) 7.03, brd, \(J\) = 5.5 Hz) and H-11 (\(\delta\)\({}_{\rm H}\) 4.12, dt, \(J\) = 8.3, 5.5 Hz). That another 1,2-disubstituted benzene ring belonged to the indole moiety of the molecule was substantiated by the HMBC cross peaks (Table 2, Supplementary Figure S11) of H-2 (\(\delta\)\({}_{\rm H}\) 7.15, s) to C-3 (\(\delta\)c 108.0), C-8 (\(\delta\)c 136.3) and C-9 (\(\delta\)c 127.9). The presence of the 3-methylbuten-2-yl moiety was corroborated by cross peaks of H-1' (\(\delta\)\({}_{\rm H}\) 4.63, d, \(J\) = 6.8 Hz) to H-2' (\(\delta\)\({}_{\rm H}\) 5.35, m), CH\({}_{3}\)-4' (\(\delta\)\({}_{\rm H}\) 1.74, s) and CH\({}_{3}\)-5' (\(\delta\)\({}_{\rm H}\) 1.80, s) protons in the COSY spectrum, as well as by the HMBC cross peaks of CH\({}_{3}\)-4' (1.74, s) and CH\({}_{3}\)-5' (1.80, s) protons to C-2' (\(\delta\)c 119.9) and C-3' (\(\delta\)c 136.4). Since the HMBC spectrum showed cross peaks of H-1' to C-2 (\(\delta\)c 127.3) and C-8 (\(\delta\)c 136.3), the 3-methylbuten-2-yl moiety was linked to the indole nitrogen. That the indole moiety was linked to the 3,4-dihydro-1_H_-1,4-benzodiazepepine-2,5-dione by a methylene bridge, through C-3 of the former and C-11 of the latter, was evidenced by the COSY correlations of H\({}_{2}\)-10 (\(\delta\)\({}_{\rm H}\) 3.57, dd, \(J\) = 15.2, 5.5 Hz and 3.26, dd, \(J\) = 15.2, 8.3 Hz) to H-11 (\(\delta\)\({}_{\rm H}\) 4.12, dt, \(J\) = 8.3, 5.5 Hz), as well as by the HMBC cross peaks of H-10 to C-2, C-3 (\(\delta\)c 108.0) and C-17 (\(\delta\)c 172.0). A literature search revealed that the compound (3_S_)-3-[1-(3-methylbut-2-enyl)indol-3-yl]-3, 4-dihydro-1_H_-1,4-benzodiazepepine-2,5-dione (PubChem SID 185030170), whose flat structure is the same as that of compound **2**, was reported as a product of Angene Chemical (AGN-PC-069E9V) [11]. Although the absolute configuration of its C-11 is reported as \(S\), there is neither \({}^{1}\)H/\({}^{13}\)C NMR nor optical rotation data available for this compound in the PubChem Substance website. Since compound **2** did not provide suitable crystals for X-ray diffraction, it was not possible to determine the absolute configuration of C-11 with certainty. Thus, an attempt was made to combine the data from the NOESY spectrum, scalar coupling constants and molecular mechanics simulations. The NOESY spectrum (Supplementary Figure S12) exhibited correlations of H-11 to H-4, NH-12 and NH-16.