Chemical reactivity of penicillins and cephalosporins. Intramolecular involvement of the acyl-amido side chain

The rate of degradation of B-epi-ampicillin in acidic, neutral, and alkalin e aqueous solutions was followed at 35 degrees C and an ionic strength of 0 .5 mol dm(-3) (KCl) by high-performance liquid chromatography (HPLC) and sp ectrophotometric assays. Pseudo-first-order rate constants were determined in a variety of buffer solutions, and the overall pH-rate profile was obtai ned by extrapolation to zero buffer concentration. The hydrolysis of 6-epi- ampicillin is subject to acid and hydroxide-ion catalysis and, for a penici llin, an unusual pH-independent reaction. Intramolecular general base-catal yzed hydrolysis by the side chain amido group is proposed to explain the en hanced rate of neutral hydrolysis of 6-epi-ampicillin and cephalosporins. T he beta-lactam of 6-epi-ampicillin also undergoes intramolecular aminolysis by nucleophilic attack of the 6-alpha side chain amino group to give a sta ble piperazine-2,5-dione derivative. The low effective molarity for intramo lecular aminolysis of only 40 M is partly attributed to the unfavorable tra ns to cis isomerization about the B-amide side chain required for ring clos ure. Theoretical calculations show that the intramolecular aminolysis of 6- epi-ampicillin nucleophilic attack occurs from the alpha-face of the beta-l actam ring with an activation energy of 14.4 kcal/mol.