# Synthesis, Structure, and In Vitro Biological Evaluation of Semi-Synthetic Derivatives of Betulin
## 2 Materials and Methods
### Synthesis
#### 2.1.2 General Procedure for the Synthesis of the \(\alpha\)-Lipoic Esters of Triterpenoids 6-9  
An appropriate triterpenoid like betulin, 3-acetylbetulin 2,3,28-diacetyl-30-hydroxy betulin 3, or 28-acetylbetulin 5 (0.5 mmol) was dissolved in dry dichloromethane (2.5 mL), and \(\alpha\)-lipoic acid (0.122 g, 0.59 mmol) was added. The reaction mixture was cooled to \(-\)10 \({}^{\circ}\)C, and was then added dropwise to a solution of 0.5 mL dichloromethane containing \(N\),\(N^{\prime}\)-dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) in amounts of 0.14 g (0.70 mmol) and 0.015 g (0.12 mmol), respectively. The reaction mixture was slowly brought to room temperature and then stirred for 24 h. The resulting precipitate was filtered off and washed with 6 mL of dichloromethane. The solvent was distilled from the filtrate using a rotary evaporator. The obtained crude product was purified by column chromatography (SiO2; dichloromethane-ethanol, 60:1, \(v/v\)). 28-(5-(1,2-Dithiolan-3-yl)pentanoyl)]b betulin 6 Yield 75%; mp 82-86 \({}^{\circ}\)C; R\({}_{\text{f}}\) 0.27 (dichloromethane/ethanol, 60:1, \(v/v\)); \({}^{1}\)H NMR (600 MHz, CDCl\({}_{3}\)) 6: 0.78 (s, 3H, CH\({}_{3}\)), 0.84 (s, 3H, CH\({}_{3}\)), 0.98 (s, 3H, CH\({}_{3}\)), 0.99 (s, 3H, CH\({}_{3}\)), 1.05 (s, 3H, CH\({}_{3}\)), 1.39-1.45 (m, 6H, CH\({}_{2}\)), 1.71 (s, 3H, CH\({}_{3}\)), 1.94 (m, 1H, CH\({}_{2}\)CHS), 0.69-2.02 (m, 25H, CH\({}_{2}\)), 2.37 (t, 2H, \(J\) = 7.2 Hz, CH\({}_{2}\)S), 2.48 (m, 2H, CH\({}_{2}\)CHS, H-19), 3.13 (m, H, H-3), 3.19 (m, 2H, CH\({}_{2}\)C=O), 3.59 (m, 1H, CHS), 3.87 (m, 1H, H-28), 4.29 (m, 1H, H-28), 4.61 (m, 1H, H-29), 4.71 (m, 1H, H-29) (Figure S1, Supplementary Materials); \({}^{13}\)C NMR (150 MHz, CDCl\({}_{3}\)) 6: 14.1, 14.8, 15.4, 16.0, 16.1, 18.3, 19.1, 20.8, 24.8, 25.2, 27.1, 27.4, 28.0, 28.8, 29.6, 29.8, 34.2, 34.3, 34.6, 37.2, 37.6, 38.5, 38.7, 38.9, 40.2, 40.9, 42.7, 46.4, 47.7, 48.8, 50.4, 55.3, 56.4, 62.6, 78.9, 109.9, 150.2, 174.0 (Figure S2, Supplementary Materials); IR (v max cm\({}^{-1}\), KBr): 3419, 2939, 1734, 1456, 1259, 881 (Figure S3, Supplementary Materials); HRMS (APCI) \(m/z\) (neg): 629.4052; C\({}_{38}\)H\({}_{61}\)O\({}_{5}\)S\({}_{2}\) (Calculated 629.4062) (Figure S4, Supplementary Materials). 3-Acetyl-28-(5-(1,2-dithiolan-3-yl)pentanoyl)) betulin 7 Yield 88%; mp 99-102 \({}^{\circ}\)C; R\({}_{\text{f}}\) 0.61 (dichloromethane/ethanol, 60:1, \(v/v\)); \({}^{1}\)H NMR (600 MHz, CDCl\({}_{3}\)) 6: 0.85 (s, 3H, CH\({}_{3}\)), 0.87 (s, 3H, CH\({}_{3}\)), 0.99 (s, 3H, CH\({}_{3}\)), 1.05 (s, 3H, CH\({}_{3}\)), 1.39-1.45 (m, 6H, CH\({}_{2}\)), 1.72 (s, 3H, CH\({}_{3}\)), 1.94 (m, 1H, CH\({}_{2}\)CHS), 0.79-1.95 (m, 24, CH, CH\({}_{2}\)), 2.06 (s, 3H, CH\({}_{3}\)C=O), 2.37 (t, 2H, \(J\) = 7.2 Hz, CH\({}_{2}\)S), 2.46 (m, 2H, CH\({}_{2}\)CHS, H-19), 3.14-3.23 (m, 2H, CH\({}_{2}\)C=O), 3.59 (m, 1H, CHS), 3.87 (d, 1H, \(J\) = 10.8 Hz, H-28), 4.29 (d, 1H, \(J\) = 10.8 Hz, H-28), 4.48 (m, 1H, H-3), 4.61 (m, 1H, H-29), 4.71 (m, 1H, H-29) (Figure S5, Supplementary Materials); \({}^{13}\)C NMR (150 MHz, CDCl\({}_{3}\)) 6: 14.7, 16.0, 16.2, 16.5, 18.2, 19.1, 20.8, 21.3, 23.7, 23.7, 24.8, 25.2, 27.1, 27.9, 28.8, 29.6, 29.8, 34.1, 34.3, 34.6, 37.1, 37.6, 37.8, 38.4, 38.5, 40.2, 40.9, 42.7, 46.4, 47.7, 48.8, 50.3, 55.4, 56.4, 62.7, 80.9, 109.9, 150.2, 171.1, 174.0 (Figure S6, Supplementary Materials); IR (v max cm\({}^{-1}\), KBr): 2941, 1732, 1246 (Figure S7, Supplementary Materials); HRMS (APCI) \(m/z\) (neg): 671.4119; C\({}_{40}\)H\({}_{63}\)O\({}_{5}\)S\({}_{2}\) (Calculated 671.4167) (Figure S8, Supplementary Materials).