Scheme 2. Synthesis of ursolic acid derivatives. Reagents and conditions: a 1,4-dibromobutane, K₂CO₃, MeCN, DMF, 50 °C, 3 h; b PPh₃, CH₃CN, reflux, Ar, 20–35 h; c Cl₂COOH, DMAP, DCC, CH₂Cl₂, 2–7 h, rt; d tri(ethylene glycol) dibromide, K₂CO₃, DMF, 50 °C, 3 h.