A method for synthesizing .DELTA..sup.3 -7-substituted amino desacetoxy cephalosporanic acid from a penicillin sulfoxide or its alkylsilylated ester derivative is provided. According to the method, penicillin sulfoxide is heated in the presence of an organic ammonium salt catalyst and a copolymer composed of dimethylsilane and urea units until formation of the cephalosporanic acid occurs by ring expansion reaction. The copolymer functions both as a dehydrating agent for removing the water by-product generated from the reaction as well as an esterifying agent for converting penicillin sulfoxide into its dimethylsilyl ester derivative. The method of the invention produces high yields of the cephalosporanic acid and avoids the need for excess dimethylsilyating agents.