1H NMR study of degradation mechanisms of oxacephem derivatives with various 3'-substituents in alkaline solution.

The degradation process of oxacephems with various 3'-substituents in alkaline solution was examined by 1H NMR spectroscopy, and the structures of two types of degradation products were determined: the hydrolysis products having the cleaved beta-lactam ring and the remaining 3'-substituents, and the exo-methylene compounds having the cleaved beta-lactam ring and the expelled 3'-substituents. The oxacephems were found to decompose, giving the former compounds that subsequently decomposed to the latter compound. Although the ratios of the formation of the exo-methylene compound 15 relative to the other degradation products depended on the leavability of the 3'-substituents, there was little correlation between the relative yields and the beta-lactam reactivity. Thus, the expulsion of the leaving group at the 3'-position was concluded to be not involved in the nucleophilic attack on the beta-lactam carbonyl.