To a mixture of betulinic acid (2) (500 mg; 1.1 mmol) and potassium carbonate (276 mg; 2.0 mmol) in N,N-dimethylformamide, hereinafter DMF (20 ml), benzylbromide (178 μl; 1.5 mmol) was added and the reaction mixture was stirred at room temperature for 24 h. The course of the reaction was monitored by TLC (toluene/diethyl ether 10:1). The reaction mixture was then diluted with ten-fold excess of water and extracted into ethyl acetate. Combined organic phases were shaken three times with 5% HCl, three times with water, dried with magnesium sulphate and the solvents were evaporated at a rotary vacuum evaporator. The evaporate was purified by column chromatography on silica gel, toluene being the mobile phase. The obtained crude benzyl betulinate (517 mg; 86%) was used without purification in the next step.
