## 3 Results and Discussion  
### Hydrogenation of Limonin  
Seven different hydrogenated limonin products (compounds 2-8) were obtained using the various reagents. Through structural analysis, the reaction pathways of different reducing agents with limonin were determined. Compound **2** was obtained by hydrogenation of limonin using sodium cyanoborohydride. It was a white amorphous powder, slightly soluble in dichloromethane and easily soluble in methanol. The NMR spectral data of compound 2 are shown in Figures S1 and S2. After data comparison with the literature [3], compound **2** was confirmed to be epi-limonol, a known compound. The structure of compound **2** and the reaction equation for the hydrogenation of limonin by sodium cyanoborohydride is illustrated in Figure 3. In summary, sodium cyanoborohydride only reduced the ketone carbonyl at the C-7 position to a secondary alcohol. Compound **3** was obtained by hydrogenating limonin with lithium aluminum hydride. It was also a white amorphous powder, which was not easily soluble in dichloromethane and easily soluble in methanol. According to HR-ESI-MS (Figure S3) ([M + Na]\({}^{+}\), \(m\)/z 505.2777, the calculated value was 505.2772) and the NMR spectra (Figures S4 and S5), its molecular formula could be inferred to be C\({}_{26}\)H\({}_{2}\)O\({}_{8}\), and the degree of unsaturation was six. Through the analysis of \({}^{13}\)C-NMR, Distortionless Enhancement by Polarization Transfer (DEPT), and Heteronuclear Single Quantum Coherence Spectroscopy (HSQC) spectra (Figure S6), it was confirmed that compound **3** had six quaternary carbons, eight methylene groups, eight methine groups, and four methyl groups. Combined analysis of HSQC and Heteronuclear Multiple Bond Correlation (HMBC) spectra (Figure S7) exhibited  
Figure 3: Hydrogenation of limonin by sodium cyanoborohydride. the following: C-20 (\(\delta_{\rm C}\) 128.15), together with H-17 (\(\delta_{\rm H}\) 5.81) and H-18 (\(\delta_{\rm H}\) 1.07), had HMBC correlations; H-17, together with C-13 (\(\delta_{\rm C}\) 46.40), C-12 (\(\delta_{\rm C}\) 34.88), and C-14 (\(\delta_{\rm C}\) 86.55), together with C-18 (\(\delta_{\rm C}\) 17.22), had HMBC correlations; C-14, together with H-15 (\(\delta_{\rm H}\) 2.19) and C-16 (\(\delta_{\rm H}\) 3.62, 3.74), had HMBC correlations. Both C15 and C16 were methylene groups, indicating that the ternary epoxy was opened. During the opening of the ternary epoxy, the hydroxyl group attached at the C-14 position with the \(\beta\) configuration because the generated tertiary alcohol was more stable. The hydroxyl group of compound **3,** at the C-7 position, was examined with Nuclear Overhauser Effect Spectroscopy (NOESY) spectra (Figure S8). This showed that H-7 (\(\delta_{\rm H}\) 4.31), together with H-5 (\(\delta_{\rm H}\) 1.60) and H-9 (\(\delta_{\rm H}\) 1.53), were correlated with NOEs (Figure S9), confirming that the hydroxyl group at the C-7 position had the \(\beta\) configuration. The structure of **3** and the reaction equation of limonin hydrogenation by lithium aluminum hydride is illustrated in Figure 4. This reaction was a deep hydrogenation reaction, and **3** was in a highly reduced state. Compounds **4-8** were all white amorphous powders, which were not easily soluble in dichloromethane and easily soluble in methanol. They were produced in the reaction between limonin and sodium borohydride, with compound **2** as the intermediate product. Therefore, they were supposed to be the hydrogenation products of compound **2**. The HR-ESI-MS characterization of compounds **4** and **5** showed the same _m/z_ signal of 475.2336. According to the \({}^{1}\)H NMR and \({}^{13}\)C NMR data (Figures S10-S13), it could be inferred that they were isomers, having the same molecular formula as C\({}_{26}\)H\({}_{34}\)O\({}_{8}\), with the corresponding degree of unsaturation of 10. The IR (Figures S14 and S15) showed the signals of an ester carbonyl group (1733 cm\({}^{-1}\)) and C=C bond (1680 and 960 cm\({}^{-1}\)).