Chemical Reactivity of Penicillins and Cephalosporins. Intramolecular Involvement of the Acyl-Amido Side Chain Antonio Llina ´s, †,‡ Bartolome ´ Vilanova, † Juan Frau, † Francisco Mun ˜ oz, † Josefa Donoso, † and Michael I. Page* ,‡ Departament de Quı ´mica, Facultat de Cie ` ncies, Universitat de les Illes Balears, E-07071 Palma de Mallorca, Spain, and Department of Chemical and Biological Sciences, The University of Huddersfield, Huddersfield, HD1 3DH, U.K. Received August 11, 1998 The rate of degradation of 6-epi-ampicillin in acidic, neutral, and alkaline aqueous solutions was followed at 35 °C and an ionic strength of 0.5 mol dm -3 (KCl) by high-performance liquid chromatography (HPLC) and spectrophotometric assays. Pseudo-first-order rate constants were determined in a variety of buffer solutions, and the overall pH-rate profile was obtained by extrapolation to zero buffer concentration. The hydrolysis of 6-epi-ampicillin is subject to acid and hydroxide-ion catalysis and, for a penicillin, an unusual pH-independent reaction. Intramolecular general base-catalyzed hydrolysis by the side chain amido group is proposed to explain the enhanced rate of neutral hydrolysis of 6-epi-ampicillin and cephalosporins. The -lactam of 6-epi-ampicillin also undergoes intramolecular aminolysis by nucleophilic attack of the 6-R side chain amino group to give a stable piperazine-2,5-dione derivative. The low effective molarity for intramolecular aminolysis of only 40 M is partly attributed to the unfavorable trans to cis isomerization about the 6-amide side chain required for ring closure. Theoretical calculations show that the intramolecular aminolysis of 6-epi-ampicillin nucleophilic attack occurs from the R-face of the -lactam ring with an activation energy of 14.4 kcal/mol. Introduction Penicillins (1) and cephalosporins (2) are widely used -lactam antibiotics. Both compounds contain a -lactam ring fused to another ring, a thiazolidine in penicillins and a dihydrothiazine in cephalosporins. 1 The antibacte- rial activity of these compounds is, to some degree, related to their chemical reactivity, and there have been many studies on the mechanisms of their reactions. 2-16 In aqueous solution, both classes undergo acid- and base- catalyzed hydrolysis. Despite the structural similarity of penicillins and cephalosporins, their pH log-rate profiles show clear differences. While in penicillins these plots are “V” shaped, cephalosporins show “U” behavior (Figure 1) because cephalosporins, but not penicillins, show a “spontaneous” or water-catalyzed pH-independent reaction. 6,7 † Universitat de les Illes Balears. ‡ The University of Huddersfield. (1) Page, M. I. The Chemistry of -Lactams; Blackie: Glasgow, 1992; p 129. Page, M. I. Adv. Phys. Org. Chem. 1987, 23, 165. (2) Gensmantel, N. P.; Page, M. I. J. Chem. Soc., Perkin Trans. 2 1979, 137. (3) Martin, A. F.; Morris, J. J.; Page, M. I. J. Chem. Soc., Chem. Commun. 1979, 298. (4) Morris, J. J.; Page, M. I. J. Chem. Soc., Perkin Trans. 2 1980, 212. (5) Gensmantel, N. P.; Page, M. I. J. Chem. Soc., Perkin Trans. 2 1982, 147. (6) Yamana, T.; Tsuji, A.; Kamayama, K.; Nakano, O. J. Antibiot. 1974, 27, 1000. (7) Yamana, T.; Tsuji, A. J. Pharm. Sci. 1976, 65, 1563. (8) Indelicato, J. M.; Norvilas, T. T.; Pfeiffer, R. R.; Wheeler, W. J.; Wilham, W. L. J. Med. Chem. 1974, 17, 523. (9) Bundgaard, H. Arch. Pharm. Chem. Sci. 1976, 25. (10) Bundgaard, H. Arch. Pharm. Chem. Sci. 1977, 149. (11) Barbhaiya, H.; Brown, R. C.; Payling, D. W.; Turner, P. J. Pharm. Pharmacol. 1977, 30, 224. (12) Cohen, A. I.; Funke, P. T.; Puar, M. S. J. Pharm. Sci. 1973, 62, 1559. (13) Tsuji, A.; Nakashima, E.; Nishide, K.; Yamana, T. Chem. Pharm. Bull. 1983, 31, 4057. (14) Tsuji, A.; Nakashima, E.; Deguchi, Y.; Nishide, K.; Shimizu, T. J. Pharm. Sci. 1981, 70, 1120. (15) Dinner, A. J. Med. Chem. 1977, 20, 963. (16) Boyd, D. J. Med. Chem. 1983, 26, 1010. Figure 1. Plots of log kint-pH profiles for benzylpenicillin and cephaloridine degradation in aqueous solution at 35 °C and an ionic strength of 0.5 mol dm -3 , where kint is the apparent first-order rate constant (s -1 ) for the degradation in buffer- free solutions or in buffers showing no effect on the degrada- tion rate. 9052 J. Org. Chem. 1998, 63, 9052-9060 10.1021/jo981628j CCC: $15.00 © 1998 American Chemical Society Published on Web 11/03/1998