Abstract: The highly selective conversion of the side chain in diosgenin and tigogenin into 26-chloro-16β-acetoxy-22-one-framework was achieved in good yields using the new E/F ring-opening system,namely acetyl chloride and CH2Cl2 in the presence of catalytic amount of anhydrous AlCl3.The corresponding ring-opened products have been prepared and characterized.Mechanism has been proposed for this new reaction.These ring-opened products were particularly suitable as new starting material for synthesizing bioactive steroids with side chain,because of having the intact skeleton of cholestanol alkanes and several functional groups at the positions of C16,C22 and C26.