The exo-methylene group in esters of 7-acylamido- and 7-amino-3-methylenecephams was ozonized to give 3-hydroxy-3-cephems. Conditions are described to effect a selective N-acylation of a 3-hydroxy-3-cephem nucleus ester. Vilsmeier reagents converted 7-acylamido-3-hydroxy compounds to 3-halo-3-cephem derivatives. Diazomethane converted the 3-hydroxy compounds to 3-methoxy-3-cephem derivatives. Removal of the ester-protecting group at the C4-carboxyl afforded a select group of cephalosporins with direct halo and methoxy substitution at C3. A number of these compounds are potent antibiotics.