Cephamycin derivatives 16b, c which were synthesized from the naturally occurring cephamycin C (16a) were converted to the correspondingoxamic acid derivatives 17a, e respectively by the reaction with oxalyl chloride and successive treatment with water. The reaction of the oxamic acid 17a with diphenylcarbodiimide gave 7-aminocephamycinoic acid (7-ACMA) benzhydryl ester (21a) which was further converted