A solution of 5.0 g of 17,21-diacetoxy-4-pregnene-3,20-dione in 35 ml of anhydrous methylene chloride and 22.5 ml of methylal is combined in incremental portions at room temperature with a mixture of 1.0 g of phosphorus pentoxide and 7.5 g of silica gel. The mixture is agitated for 3 days at room temperature and then another 11 ml of methylal, 500 mg of phosphorus pentoxide, and 3.75 g of silica gel are added. After another 2 days, the reaction mixture is vacuum-filtered, the residue is washed thoroughly with methylene chloride, and evaporated to dryness under vacuum. The crude product is purified on 350 g of silica gel with a hexane/ethyl acetate gradient (0-50% ethyl acetate), thus obtaining 3.6 g of 17α,21-diacetoxy-6-methylene-4-pregnene-3,20-dione, mp 227°-228° C. [α]D25 =+165° (chloroform).
