Scheme 1. Synthesis of betulinic acid derivatives. Reagent and conditions: a 1,4-dibromobutane, K2CO3, MeCN, DMF, 50 °C, 3h; b PPh3, CH3CN, reflux, Ar, 20–35 h; c Cl2COOH, DMAP, DCC, CH2Cl2, 2–7 h, rt; d AcCI, Py, DMAP, THF, 20 °C; e tri(ethylene glycol) dibromide, K2CO3, DMF, 50 °C, 3 h; f tri(ethylene glycol) ditosylate, K2CO3, MeCN, 70 °C, 7 h.
