In 200 ml of methylene chloride were dissolved 5.00 g (14.6 mmoles) of 7α-hydroxypregna-1,4-dien-3-one-20-carbaldehyde and 4.56 g (43.8 mmoles) of 2,2-dimethyl-1,3-propanediol, followed by addition of 100 mg of p-toluenesulfonic acid and 20 g of molecular sieve 4A 1/16. The mixture was stirred-gently at room temperature for 12 hours. From the reaction mixture thus obtained, the molecular sieve was filtered off and the solvent in the filtrate was distilled off under reduced pressure. Finally the residue was purified by silica gel chromatography (eluent:ethyl acetate-hexane=1:10, v/v) to give 3.14 g of 20-(5,5-dimethyl-1,3-dioxan-2-yl)-7α-hydroxypregna-1,4-dien-3-one (yield: 50%).
