Dehydropeptidase I catalyzed hydrolysis of the carbapenem antibiotics imipenem (1) and the 3-methylthio analogue 14 was examined by spectroscopic methods. The principal product in both cmes was a mixture of p-lactam ring-opened 1-pyrrolines epimeric a t C3. A transient 2-pyrroline intermediate was observed by 'H NMR in the methylthio carbapenem case. Nonenzymatic acid-catalyzed hydrolysis of each antibiotic under dilute conditions rapidly afforded a protonated 2-pyrroline product that isomerized to the diastereomeric 1-pyrroline mixture on neutralization. At higher carbapenem concentrations, the acid-catalyzed process also produced a diketopiperazine structure resulting from initial bimolecular attack of the carboxyl group of one carbapenem molecule on the p-lactam group of a second molecule. The analysis of the imipenem-derived products was complicated by the observation of side-chain formamidinium rotational isomers. Under mildly acidic conditions, the imipenem side-chain isomers were separable by high pressure liquid chromatography and the major form was identified as the 2 isomer by X-ray crystallography. (6) (a) Ashton, W. T.; Barash, L.; Brown, J. E.; Brown, R. D.; Canning, L. F.; Chen, A.; Graham, D. W.; Kahan, K. M.; Kropp, H.; Sundelof, J. G.; Rogers, E.

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