A solution of methyl 3α-hydroxy-7-keto-5β-cholanoate (10 g, 24.72 mmoles) in methylene chloride (40 ml) was added with acetic anhydride (3.79 g, 37.1 mmoles), triethylamine (3.63 g, 36 mmoles) and 4-pyrrolidino-pyridine (0.11 g, 0.74 mmoles) and the reaction mixture was left to react for 3 hours under strong magnetic stirring, then it was diluted with methylene chloride (50 ml) and water (50 ml) and acidified with 10% hydrochloric acid. The organic phase was separated and the aqueous phase was extracted with methylene chloride (3×20 ml). The combined organic phases were washed with brine (1×30 ml), dried over sodium sulfate and concentrated under vacuum. The residue (11.0 g) was subjected to flash chromatography on silica, eluting with methylene chloride/methanol 99:1, to obtain 10.5 g of a pure product (95%).
