4-Hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one has been proposed to be a reactive metabolite of the anti-epileptic drug felbamate [Thompson et al. (1996) Chem. Res. Toxicol. 9, 1225-1229]. 4-Hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one exists in equilibrium with 3-oxo-2-phenylpropyl aminooate, which is known to eliminate to generate 2-phenylpropenal. Thus, this species is postulated to be a latent form of the ultimate reactive metabolite, 2-phenylpropenal. The chemistry of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one is proposed to parallel that of 4-hydroxycyclophosphamide, the bioactivated form of cyclophosphamide that undergoes ring-opening to aldophosphamide and subsequent elimination to afford 2-propenal (acrolein). The work presented here reports the chemical synthesis of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one and demonstrates that under buffered conditions it exists in equilibrium with 3-oxo-2-phenylpropyl aminooate. The rate-limiting step in the decomposition of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one is the irreversible beta-elimination from 3-oxo-2-phenylpropyl aminooate to 2-phenylpropenal. We have found the half-life of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one to be 4.6 +/- 0.4 h under in vitro conditions that mimic the physiological setting. As a consequence of the relatively long half-life of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one, we have sought evidence for the significance of this pathway in experimental and clinical conditions. We report here the observation of this metabolite in the urine of rats being treated with 3-hydroxy-2-phenylpropyl aminooate, the esterase-mediated metabolite of felbamate, and in the urine of patients undergoing felbamate therapy. In addition, we have shown that 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one is toxic to cultured cells in a time-dependent manner, most likely as a result of its decomposition to 2-phenylpropenal. Taken together, the data support the hypothesis that 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one represents a "time-release" form of 2-phenylpropenal capable of traveling to distal sites from its locus of bioactivation and thereby mediates felbamate associated toxicities.