Reactions of temazepam in acidic aqueous solution via oxygen-18-labeled product analysis by mass spectrometry

Temazepam, a hypnotic and anxiolytic drug, is inactivated by hydrolysis in acidic aqueous solutions including simulated gastric fluid. Reaction products formed in acidic oxygen-18 water were characterized by electron impact and/or chemical ionization mass spectral analysis. Temazepam remaining in the reaction mixture contained either one or two oxygen-18 atoms. The most abundant hydrolysis product (2ʹ-benzoyl-4ʹ-chloro-N-methyl-2-amino-2-hydroxyacetanilide) was found to contain either one or two oxygen-18 atoms and the minor hydrolysis product (2-methylamino-5-chlorobenzophenone) had one oxygen-18 atom. The benzophenone oxygens of the hydrolysis products underwent both acid- and base-catalyzed oxygen exchange with solvent water. The C2 carbonyl carbon of temazepam underwent an essentially irreversible nucleophilic attack by water in strongly acidic aqueous media to form a hydrate. The results not only provided additional evidence for the mechanism of acid-catalyzed hydrolysis, but also revealed some heretofore unknown acid-catalyzed reactions of temazepam.