9056 J . Org. Chem., Vol. 63, No. 24, 1998
Llina´s et al.
cephalosporins which are ca. 104 fold less reactive than
penicillins toward acid-catalyzed hydrolysis.1,28 The
second-order rate constants, kH, for the acid-catalyzed
hydrolysis of the two epimers of ampicillin are similar
(Table 1). However, that for 6-epi-ampicillin is 103 fold
less than that for benzylpenicillin and is similar to the
values observed for cephalosporins. The lack of neigh-
boring group participation in the acid-catalyzed degrada-
tion of the two ampicillins is almost certainly due to the
reduced nucleophilicity of the amide side chain because
of the adjacent protonated amine residue (AH2+, Scheme
3).
Ta ble 1. Ra te Con sta n ts for Degr a d a tion of Som e
Cep h a losp or in s a n d P en icillin s a t 35 °C a n d a n Ion ic
Str en gth of 0.5 m ol d m -3
105kH
(dm3 mol-1 107k0 106ka (dm3 mol-1
s-1 (s-1 (s-1 s-1
kOH
antibiotic
)
)
)
)
6-epi-ampicillin (3)
ampicillin (7)
13.90
38.30
4.17
41.70 61.10
e2.78
13.90 37.50
0.250
0.540
0.364
0.073
0.110
0.070
1.078
0.154
cephaloglycina (9)
cephalexina (10)
cephradinea (11)
cephadroxilb (12)
cephaloridinea (15)
benzylpenicillinc (16)
3.19
3.05
2.61
2.80
2.05
4.47
3.72
13500
12.22
Studies on cephalosporins with an amino group in the
7-â side chain (cephaloglycin (9),6-9 cephalexin (10),6,9,11
cephradine (11),6,9,12 and cephadroxil (12),14) show in-
tramolecular attack on the â-lactam by the primary
amino group to yield the piperazine-2,5-dione derivatives,
but this intramolecular aminolysis does not occur in
similarly 6-â-substituted penicillins.8,31-35 Intermolecular
nucleophilic attack on the â-lactam of penicillins takes
place preferentially from the R-side, and so, presumably,
intramolecular attack from the sterically hindered â-side
is unfavorable.20-22 For example, ampicillin (8) with an
amino substituent in the 6-â-acyl-amido side chain does
not show intramolecular aminolysis. The crystal data
for ampicillin from the Cambridge Structural Database36
(CSD code, Amcill)37 indicates that the 2-â-methyl group
and the 3-hydrogen could block nucleophilic attack at the
â-lactam carbonyl carbon from the â-face.
a
b
Data from ref 7. Data from ref 14. c Data from ref 28.
values obtained are given in Table 1 together with
corresponding values for some other cephalosporins and
penicillins. The good agreement between experimental
and theoretical data indicates that eq 1 describes ad-
equately the rate constant of 6-epi-ampicillin degradation
as a function of pH (Figure 2).
The pathways for the degradation of 6-epi-ampicillin
in aqueous solution from pH 0.5 to 13 are shown in
Scheme 3. At pH less than 2, 6-epi-ampicillin exists
predominantly as the cationic species (AH2+) and specific
hydrogen-ion-catalyzed hydrolysis occurs. The principal
reaction in the pH-independent region extending from
pH 2.5 to 6 is the hydrolysis of the zwitterion (AH().
Between pH 6 and 10, where the kinetic term ka is
dominant, the major product is the diketopiperazine
derivative as a result of intramolecular aminolysis by the
attack of the unprotonated side chain amino group on
the â-carbonyl carbon. Above pH 10, the main reaction
is hydroxide-ion-catalyzed hydrolysis of (A-).
There is nothing unusual about the relative rates and
magnitude of the base-catalyzed hydrolysis of 6-epi-
ampicillin and ampicillin (Table 1). The rate constants
kOH are similar to each other and to that observed for
benzylpenicillin, which in turn is not, in general, signifi-
cantly different from the kOH for cephalosporins (Table
1). The expected steric effect of 6-R substituents retard-
ing nucleophilic attack on the â-lactam carbonyl carbon
from the R-face of penicillins is not large.1
The rate of alkaline hydrolysis of penicillins and
cephalosporins is influenced by the nature of the sub-
stituents in the two rings.1 For example, electron-
withdrawing substituents at C-6 in penicillins and at C-7
in cephalosporins facilitate nucleophilic attack of hydrox-
ide ion on the â-lactam carbonyl carbon.28 The 6-â-acyl-
amido side chain in penicillins fits the Hammett plot
generated by other substituents and the rate-enhancing
effect of the amide substituent compared with no sub-
stituent at C-6 is due entirely to an inductive effect, and
there is no evidence of neighboring group participation
by this group during alkaline hydrolysis.28
Intramolecular aminolysis does, however, occur from
the less-hindered exo face/R-side of 6-epi-ampicillin (3)
to give the diketopiperazine derivative (6) (Scheme 2).
The theory of stereoelectronic control17 predicts that the
preferred direction of attack would be from the endo face/
â-side, as the lone pair of electrons on the â-lactam
nitrogen is located primarily on the R-face and so would
be anti to the attacking nucleophile. Which direction is
actually followed depends on the balance between steric
and stereoelectronic effects. A measure of the effective-
ness of the intramolecular reaction is the ratio of the rate
constants for intra- to intermolecular aminolysissthe
effective molarity.38 For the 6-epi-ampicillin, the rate
constant for the uncatalyzed intramolecular aminolysis,
ka, is 6.11 × 10-5 s-1, compared with a second-order rate
constant of 1.6 × 10-6 mol-1 dm3 s-1 for the intermolecu-
lar aminolysis of benzylpenicillin with an amine of pKa
7.4, i.e., an effective molarity of only 40 mol dm-3. This
is similar to the effective molarity of about 20 mol dm-3
calculated for cephaloglycin (the rate constant for the
uncatalyzed intramolecular aminolysis of cephaloglycin
is 3.75 × 10-5 s-1, and the estimated rate constant for
the equivalent intermolecular reaction for an amine of
pKa 7.0 is 2.0 × 10-6 mol-1 dm3 s-1).39
If both the intramolecular and analogous intermolecu-
lar reactions are free of strain energy effects, then the
entropy difference between the two systems can give
The acid-catalyzed degradations of 6-acylamidopeni-
cillins show rate enhancements of ca. 103 compared with
that predicted from the Hammett plot for 6-substitu-
ents.28 This is the result of neighboring group participa-
tion by the amide side chain probably trapping the ring
opened acylium ion intermediate28,29 to give penicillenic
acid.1,30 Interestingly, this reaction does not occur with
(31) Hou, J . P.; Poole, J . W. J . Pharm. Sci. 1969, 58, 447.
(32) Saccani, F.; Pansera, F. Bull. Chim. Pharm. 1968, 107, 640.
(33) Hou, J . P.; Poole, J . W. J . Pharm. Sci. 1971, 60, 503.
(34) Schwartz, M. A. J . Pharm. Sci. 1969, 58, 643.
(35) Guindy, N. M.; Abdel Fattah, S.; Amer, M. M. Egypt. Pharm.
Sci. 1990, 31, 185.
(36) Botes, M. O.; Girven, R. J . Acta Crystallogr. 1976, B32, 2279.
(37) Allen, F. H.; Davies, I. E.; Galloy, J . J .; J ohnson, O.; Kennard,
O.; Macrae, C. F.; Mitchell, E. M.; Smith, J . M.; Watson, D. G. J . Chem.
Inf. Comput. Sci. 1991, 31, 187.
(28) Proctor, P.; Gensmantel, N. P.; Page, M. I. J . Chem. Soc., Perkin
Trans. 2 1982, 1185.
(29) Wan, P.; Modro, T. A.; Yates, K. Can. J . Chem. 1980, 58, 2423.
(30) Schwartz, M. A. J . Pharm. Sci. 1965, 54, 472.
(38) Page, M. I. Chem. Soc. Rev. 1973, 295.
(39) Proctor, P.; Page, M. I. J . Am. Chem. Soc. 1984, 106, 3820.