16.beta.-Methyl-17.alpha.-hydroxy-1,4,9(11)-pregnatriene-3,20-dione was treated with AcNHBr in dil. HClO4-THF to yield pregnadienedione I (R = Br, R1 = H), which underwent successive epoxidn. by NaOH and ring opening by HF in DMF to give I (R = F, R1 = H). The latter was treated with iodine, CaCl2, and CaO in MeOH at 25.degree. to give I (R = F, R1 = iodo), which was refluxed with KOAc in Me2Co to give betamethasone acetate in 63.3% overall yield.