The Degradation Mechanism of an Oral Cephalosporin: Cefaclor

The degradation of cefaclor ( 1 ), an oral cephalosporin antibiotic, was studied at 37° in a neutral aqueous medium by HPLC and 1 H‐NMR. Under these conditions, 1 underwent intramolecular aminolysis by the 7‐side‐chain NH 2 group on the β‐lactam moiety to give a piperazine‐2,5‐dione. The most prominent peak in the HPLC profile of a degradation solution from 1 was isolated by prep. HPLC. Mechanistically, the formation of this degradation product cis ‐ 11 from 1 involves the contraction from a six‐membered cephem ring to a five‐membered ring, which presumably takes place via a common episulfonium ion intermediate 9 (see Scheme ). Loss of the Cl‐atom from 3‐chloro‐3‐cephem is a general reaction subsequent to β‐lactam ring opening.