Accumulating evidence demonstrates that mifepristone (RU486) exhibits potent anti-proliferative effects on various cancer cell lines. Our recent work shows that its major metabolite metapristone (RU42633) is a good drug candidate for cancer metastatic chemoprevention. However, lack of an efficient method for synthesizing metapristone limited its further clinical development. Herein, an improved and efficient condition of N-demethylation of mifepristone with an excellent yield is described. The procedure presented here has several advantages including one-pot preparation, ease of synthesis, and large-scale feasibility.