## 3 Results and Discussion
### Hydrogenation of Limonin
From the perspective of structure stability, ring A and ring E, and the three-membered epoxy were more likely to be opened. By analyzing the HSQC and HMPC spectra (Figures S32 and S33), H-17 (\(\delta_{\rm H}\) 5.01) and C-16 (\(\delta_{\rm C}\) 89.10) had HMPC correlation, and C-15 (\(\delta_{\rm C}\) 59.2, \(\delta_{\rm H}\), 3.40) was still methane. Therefore, the following findings were affirmed: the ring E and ternary epoxy were not opened; C-1 (\(\delta_{\rm C}\) 86.8) and H-2 (\(\delta_{\rm H}\) 1.91, 2.14)/H-3 (\(\delta_{\rm H}\) 3.64, 3.74) had HMPC correlation; C-10 (\(\delta_{\rm C}\) 51.90) and H-1 (\(\delta_{\rm H}\) 3.62)/H-19 (\(\delta_{\rm H}\) 3.96, 4.01) had HMBC correlation. Based on this, the ester bond of ring A was broken to form two primary alcohols. The above information confirmed the planar structures of compounds **6** and **7**. Further efforts were paid to enclosing the configuration of the hydroxyl group at C-7 and C-16 positions, which might be the main difference between **6** and **7**. First, the comparison between the data of the \({}^{1}\)H and \({}^{13}\)C-NMR of compounds **5** and **6** showed that there was no noticeable difference. However, the NOESY spectrum (Figure S32) clearly showed NOE correlation between H-7 (\(\delta_{\rm H}\) 3.57) and H-24 (\(\delta_{\rm H}\) 1.09) and NOE correlation between H-16 (\(\delta_{\rm H}\) 5.30) and H-15/H-18 (\(\delta_{\rm H}\) 1.09), confirming that the hydroxyl group at the C-7 position of compound **6** was in \(\alpha\) configuration, and the hydroxyl group at the C-16 position was in \(\beta\) configuration. Combined with the identification of isomer compounds **4** and **5**, the difference between compounds **7** and **6** in \({}^{1}\)H and \({}^{13}\)C data was essentially near the C-7 position. The assumption was that the hydroxyl group at the C-7 position was reversed, turning into \(\beta\)-OH. The above assumption was confirmed by the NOESY spectrum (Figure S33) of compound **7**, which showed that H-7 (\(\delta_{\rm H}\) 4.02), H-5 (\(\delta_{\rm H}\) 1.70), and H-9 (\(\delta_{\rm H}\) 2.09) had NOE correlation, and H-16 (\(\delta_{\rm H}\) 4.93), together with H-15 (\(\delta_{\rm H}\) 3.83) and H-18 (\(\delta_{\rm H}\) 1.29), had NOE correlation, confirming that compound **7** was \(\beta\)-OH at both C-7 and C-16 positions. Compound **8** was shown to be relatively more polar when detected in TLC. According to HR-ESI-MS ([M + H]\({}^{+}\), \(m\)/\(z\) 481.2806, the calculated value was 481.2796) and, with the NMR spectra \({}^{1}\)H and \({}^{13}\)C data (Figures S34 and S35), it was deduced that its molecular formula was C\({}_{\rm 26}\)H\({}_{\rm 40}\)O\({}_{\rm s}\), and its degree of unsaturation was seven. According to the analysis of \({}^{13}\)C-NMR, DEPT, and HSQC spectra (Figure S36), compound **8** had six quaternary carbons, seven methylene groups, nine methine groups, and four methyl groups. In the infrared spectrum, no signal of ester carbonyl was observed either. Combined analysis of HSQC and HMBC spectra (Figure S37) showed the following: C-20 (\(\delta_{\rm C}\) 127.70), together with H-17 (\(\delta_{\rm H}\) 4.96) and H-18 (\(\delta_{\rm H}\) 1.30), had HMBC correlation; H-17, together with C-13 (\(\delta_{\rm C}\) 44.20), C-12 (\(\delta_{\rm C}\) 30.80), C-14 (\(\delta_{\rm C}\) 78.50), and C-18 (\(\delta_{\rm C}\) 23.81), had HMBC correlation; C-14, together with H-15 (\(\delta_{\rm H}\) 3.80) and C-16 (\(\delta_{\rm H}\) 3.76, 4.02), had HMBC correlation. C-16 was methylene, which meant that ring E was opened and the ester bond was reduced to two primary alcohols. Referring to compounds **6** and **7**, ring A was also opened. The only site where compound **8** had a configuration issue was the hydroxyl group at the C-7 position. The NOESY spectrum (Figure S38) clearly showed the NOE correlation between H-7 (\(\delta_{\text{H}}\) 3.57) and H-24 (\(\delta_{\text{H}}\) 1.09); H -16 (\(\delta_{\text{H}}\) 5.30) and H-15/H-18 (\(\delta_{\text{H}}\) 1.09) had NOE correlation.