Detection of N-acetyl-p-benzoquinone imine produced during the hydrolysis of the model phenacetin metabolite N-(pivaloyloxy)phenacetin

N-Acetyl-p-benzoquinone imine (4) can be detected by UV spectrophotometry and HPLC during the hydrolysis of N-(pivaloyloxy)phenacetin (3c). This material serves as a model for the sulfate and glucuronide conjugates (3a and 3b) of N-hydroxyphenacetin (2). Direct detection of 4 during the hydrolysis of 3a and 3b is precluded by the extreme instability of 3a and the very slow hydrolysis of 3b. Our data show that greater than 90% of the hydrolysis of 3c proceeds through the intermediate 4 at all pH values in the range 1.0-8.0. All our results indicate that 4 is produced through a nitrenium ion mechanism. Many of the differences in the hydrolysis reactions of 3b and 3c can be explained in terms of a mechanism involving tight and solvent-separated nitrenium ion pairs. Other differences may be due to a homolysis pathway, which is more important for the material with the poorer leaving group (3b).