## 2 Results and Discussion
Interestingly, both CH\({}_{3}\)-27 (&c 26.5) and CH\({}_{3}\)-28 (&c 26.9) resonated at higher chemical shift values than their counterparts in tryptoquivaline L (&c 16.4 and 22.8) while C-15 exhibited lower chemical shift value (&c 64.6) than the corresponding carbon (&c 70.0) of tryptoquivaline L [9]. Thus, the only difference between the structure  
Figure 4: Conformations of C-11\(R\) stereoisomer of compound **2** obtained by simulation performed with ChemBio3D Ultra 14.0; (**a**) conformer with H-11 in equatorial position; (**b**) conformer with H-11 in axial position. of compound **3** and that of tryptoquivaline L is the presence of a hydrogen atom on N-16 instead of a hydroxyl group. This was supported by the molecular formula of compound **3** (C\({}_{23}\)H\({}_{20}\)N\(4\)O\({}_{4}\)), which has one oxygen atom less than that of tryptoquivaline L. In order to verify if the stereochemistry of compound **3** is the same as that of tryptoquivaline L, X-ray analysis of compound **3** was performed. The ORTEP diagram of compound **3** (Figure 5) showed unambiguously that the absolute configurations of C-2, C-3 and C-12 are \(S\), \(S\) and \(R\), the same as that of the corresponding carbons of tryptoquivaline L. Since compound **3** is a new analog of tryptoquivalines, and in accordance with the names given to the tryptoquivaline series, we have named compound **3** tryptoquivaline U. Since we have previously found that the meroditerpenes aszonapyrone A and sartorypyrone A, isolated from the culture of \(N\). _fischeri_ KUFC 6344, exhibited potent antibacterial activity as well as synergism with antibiotics against the Gram-positive multidrug-resistant strains [7], we also evaluated sartorendol **(1)**, takakiamide **(2)** and tryptoquivaline U **(3)** for their antibacterial activity against four reference strains (_Staphylococcus aureus_, _Bacillus subtilis_, _Escherichia coli_ and _Pseudomonas aeruginosa_), as well as the environmental multidrug-resistant isolates, according to the previously described method [7]. The results showed that none of the tested compounds exhibited relevant antibacterial activity, _i.e._, their \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Position** & **\(\delta\)c, Type** & **\(\delta\)n, (\(J\) in Hz)** & **COSY** & **HMBC** \\ \hline
2 & 82.0, CH & 5.55, d (8.4) & NH-16 & C-13, 14 \\
3 & 84.7, C & - & & \\
4 & 132.0, C & - & & \\
5 & 125.7, CH & 7.71, d (7.3) & H-6 & C-7, 9 \\
6 & 125.7, CH & 7.38, ddd (7.5, 7.5, 1.2) & H-5, 7 & C-4, 8 \\
7 & 131.6, CH & 7.57, ddd (8.1, 7.7, 1.2) & H-6, 8 & C-5, 9 \\
8 & 116.2, CH & 7.49, d (7.2) & H-7 & C-4, 6 \\
9 & 139.8, C & - & & \\
11 & 170.7, CO & - & & \\
12 & 56.9, CH & 5.58, dd (10.8, 9.1 & H-13 & C-3, 11, 13, 18, 26 \\
13 & 31.6, CH\({}_{2}\) & 2.86, dd (12.9, 9.1) & H-12 & C-2, 4, 11, 12 \\  & & 3.45, dd (12.9, 11.2) & H-12 & C-2, 3, 4, 12 \\
14 & 176.0, CO & - & & \\
15 & 64.6, C & - & & \\
16 & - & 3.76, d (8.4) & H-2 & C-2, 3, 14, 15, 26, 27 \\
18 & 159.6, CO & - & & \\
19 & 121.4, C & - & & \\
20 & 126.1, CH & 8.23, dd (8.0, 1.2) & H-21 & C-18, 22, 24 \\
21 & 127.6, CH & 7.63, ddd (7.6, 7.6, 1.0) & H-20, 22 & C-19, 23 \\
22 & 135.0, CH & 7.92, ddd (8.2, 8.2, 1.5) & H-21, 23 & C-20, 24 \\
23 & 127.3, CH & 7.76, d (7.7) & H-22 & C-19, 21 \\
24 & 147.5, C & - & & \\
26 & 147.4, CH & 8.49, s & C-12, 18, 24 \\
27 & 26.5, CH\({}_{3}\) & 1.45, s & C-14, 15, 28 \\
28 & 26.9, CH\({}_{3}\) & 1.24, s & C-14, 15, 27 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: \({}^{1}\)H and \({}^{13}\)C NMR (DMSO, 300.13 MHz and 75.47 MHz) and HMBC assignment for tryptoquivaline U **(3)**. MIC values are higher than 256 mg/mL. These compounds were also tested for their capacity to inhibit a quorum sensing by the screening protocol based on the pigment production by _Chromobacterium violaceum_ ATCC 31532 [13] and none of them showed a quorum sensing inhibitory activity.