60.0 g of 15α-acetoxy-18-methyl-4-estrene-3,17-dione in 500 ml of dimethylformamide is stirred with 500 ml of 2,2-dimethoxypropane, 20 ml of methanol and 6.0 g of pyridinium tosylate at 110° C. under argon. After 3.5 hours, 9.0 g of solid sodium bicarbonate is added, the reaction mixture is stirred into 10 liters of ice/water, the precipitated product is suctioned off, washed repeatedly with water, and dried under vacuum at 70° C. The crude product is suspended in 375 ml of methanol and stirred under reflux for 15 minutes. After gradual cooling down to room temperature, the colorless crystallized product is suctioned off and washed with cold methanol. Yield: 56 g of 15α-acetoxy-3-methoxy-18-methyl-3,5-estradien-17-one. Melting point 190° C.
