Kinetics and Mechanism of Hydrolysis of N-Acyloxymethyl Derivatives of Azetidin-2-one

Article abstract of DOI:10.1021/jo0358123

The pH-independent, acid-catalyzed and base-catalyzed hydrolyses of N-acyloxymethylazetidin-2-ones all occur at the ester function. The pH-independent hydrolysis involves rate-limiting alkyl C-O fission and formation of an exocyclic β-lactam iminum ion. This iminium ion is then trapped by water at the exocyclic iminium carbon atom, rather than at the β-lactam carbonyl carbon atom, to form the corresponding N-hydroxymethylazetidin-2-ones. Calculations carried out at the B3LYP/6-31+G(d) level of theory also support that nucleophilic attack by water takes place at the exocyclic carbon rather than at the β-lactam carbonyl carbon of the iminium ion. The mechanism for the acid-catalyzed pathway involves a preequilibrium protonation, probably at the β-lactam nitrogen, followed by rate-limiting alkyl C-O fission with formation of an exocyclic iminum ion. The base-catalyzed hydrolysis involves rate-limiting hydroxide attack at the ester carbonyl carbon. These results imply formation of a β-lactam system containing a positively charged amide nitrogen atom that hydrolyzes via a pathway that preserves the β-lactam structure in the product and provide further evidence that cleavage of the β-lactam C-N bond is not as facile as is commonly imagined.

Full text of DOI:10.1021/jo0358123

Kin etics a n d Mech a n ism of Hyd r olysis of N-Acyloxym eth yl
Der iva tives of Azetid in -2-on e
Em´ılia Valente,† J ose´ R. B. Gomes,‡ Rui Moreira,† and J im Iley*,§
CECF, Faculdade de Farma´cia, Universidade de Lisboa, Av. Forc¸as Armadas,
1600-083 Lisboa, Portugal, CIQ, Departamento de Quı´mica, Faculdade de Cieˆncias,
Universidade do Porto, R. Campo Alegre, 687, 4169-007 Porto, Portugal, and
Department of Chemistry, The Open University, Milton Keynes MK7 6AA, U.K.
j.n.iley@open.ac.uk
Received December 12, 2003
The pH-independent, acid-catalyzed and base-catalyzed hydrolyses of N-acyloxymethylazetidin-2-
ones all occur at the ester function. The pH-independent hydrolysis involves rate-limiting alkyl
C-O fission and formation of an exocyclic â-lactam iminum ion. This iminium ion is then trapped
by water at the exocyclic iminium carbon atom, rather than at the â-lactam carbonyl carbon atom,
to form the corresponding N-hydroxymethylazetidin-2-ones. Calculations carried out at the B3LYP/
6-31+G(d) level of theory also support that nucleophilic attack by water takes place at the exocyclic
carbon rather than at the â-lactam carbonyl carbon of the iminium ion. The mechanism for the
acid-catalyzed pathway involves a preequilibrium protonation, probably at the â-lactam nitrogen,
followed by rate-limiting alkyl C-O fission with formation of an exocyclic iminum ion. The base-
catalyzed hydrolysis involves rate-limiting hydroxide attack at the ester carbonyl carbon. These
results imply formation of a â-lactam system containing a positively charged amide nitrogen atom
that hydrolyzes via a pathway that preserves the â-lactam structure in the product and provide
further evidence that cleavage of the â-lactam C-N bond is not as facile as is commonly imagined.
SCHEME 1
In tr od u ction
â-Lactams are effective inhibitors of several bacterial
enzymes such as D,D-transpeptidases1 and â-lactamases2
that contain serine as the catalytic residue. Though these
are the most widely studied therapeutic targets for
â-lactams, other serine proteases, such as mammalian3
or viral proteases,4 have received increased attention,
giving rise to further optimization of the â-lactam lead
structure to obtain potent and selective inhibitors.5
Recently, we became interested in N-acyloxymethyl (1,
LG ) OCOR) and N-aminocarbonyloxymethylazetidin-
2-ones (1, LG ) OCONHR) (see Scheme 1) as potential
mechanism-based or suicide inhibitors of human leuko-
cyte elastase (HLE, EC 3.4.21.37).6 HLE is a serine
protease that very efficiently degrades various tissue
† Faculdade de Farma´cia, Universidade de Lisboa.
‡ Faculdade de Cieˆncias, Universidade do Porto, Portugal.
§ The Open University.
(1) Newall, C. E.; Hallam, P. D. In Comprehensive Medicinal
Chemistry; Hansch, C., Ed.; Pergamon Press: Oxford, 1990; pp 609-
653.
(2) Brown, A. G.; Pearson, M. J .; Southgate, R. In Comprehensive
Medicinal Chemistry; Hansch, C., Ed.; Pergamon Press: Oxford, 1990;
pp 655-702
(3) (a) Babine, R. E.; Bender, S. L. Chem. Rev. 1997, 97, 1359. (b)
Leung, D.; Abbenante, G.; Fairlie, D. P. J . Med. Chem. 2000, 43, 305.
(4) LaPlante, S. R.; Bonneau, P. R.; Aubry, N.; Cameron, D. R.;
De´ziel, R.; Grand-Maˆıtre, C.; Plouffe, C.; Tong, L.; Kaway, S. H. J .
Am. Chem. Soc. 1999, 121, 2974.
matrix proteins such as elastin.7 The imbalance between
HLE and its endogenous inhibitors and the subsequent
excessive elastin proteolysis has been implicated in acute
and chronic inflammatory diseases of the lungs.8,9 Thus,
the use of specific inhibitors of HLE to restore the
protease/antiprotease imbalance represents an important
strategy to treat such pathologies.10,11
(5) Firestone, R. A.; Barker, P. L.; Pisano, J . M.; Ashe, B. M.;
Dahlgren, M. E. Tetrahedron 1990, 46, 2255.
(6) Clemente, A.; Domingos, A.; Grancho, A. P.; Iley, J .; Moreira,
R.; Neres, J .; Palma, N.; Santana, A. B.; Valente, E. Bioorg. Med. Chem.
Lett. 2001, 11, 1065.
(7) J anoff, A.; Scherer, J . J . Exp. Med. 1968, 128, 1137.
(8) Snider, G. L.; Ciccolella, D. F.; Morris, S. M. Ann. NY Acad. Sci.
1991, 624, 45.
(9) Stockley, R. A.; Hill, S. L.; Burnett D. Ann. NY Acad. Sci. 1991,
624, 257.
10.1021/jo0358123 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/20/2004
J . Org. Chem. 2004, 69, 3359-3367
3359

Valente et al.
Compounds 1, in addition to the ethyl groups at C-3
required for molecular recognition by the enzyme,12
incorporate the following structural features. First, a
good leaving group LG (e.g., a carboxylic or carbamic acid)
that unmasks an electrophilic Schiff base, 2, following
the initial serine attack at the â-lactam carbonyl atom
(Scheme 1). This reactive functionality has the potential
of reacting with a second amino acid residue within the
active site, leading to a doubly modified inactivated
enzyme. Second, an electron-withdrawing substituent, X,
at C-4 was required for increasing the acylating power
of the â-lactam. Strong evidence has been put forward
suggesting that the driving force for â-lactam reactivity
toward nucleophiles lies on the leaving group ability of
the amine formed from the decomposition of the tetra-
hedral intermediate rather than on the strain energy in
the four-membered ring or on the reduced amide reso-
nance.13,14 The C-4 substituent and the acyloxymethyl
moiety reduce the pKa of the amine leaving group to ca.
2,15 a value in the region of those reported for other
â-lactam inhibitors (e.g., penicillins 3 and cephalosporins
416). An approach similar to that depicted in Scheme 1
has been used to design inhibitors 5 of â-lactamases using
other leaving groups such as sulfinic acids, thiols, and
fluoride.17
nitrogen finds parallel in the chemistry of N-acyloxy-
methylamides, which undergo an SN1-type hydrolysis in
neutral buffers.18,19 To obtain further insight to the
mechanism of hydrolysis of N-acyloxymethylazetidin-2-
ones and its implication in the design of more potent
inhibitors of this class, herein we report a kinetic study
on the reactivity of simple N-acyloxymethylazetidin-2-
ones 6.
Resu lts a n d Discu ssion
Both N-acyloxymethyl- and N-aminocarbonyloxy-
methylazetidin-2-ones, 1, are time-dependent irreversible
inhibitors of HLE but, remarkably, are very stable at 37
°C and pH 7.4, with pseudo-first-order rate constants for
hydrolysis ranging from 10-8 to 10-7 s-1.6 In contrast, the
C-4-unsubstituted analogue is hydrolyzed at least 103
times more rapidly than any C-4-substituted N-acyl-
oxymethylazetidin-2-one. Such a dramatic effect on chemi-
cal reactivity exerted by substituents close to the lactam
Kin etics. Typical pH-rate profiles for the hydrolysis
of N-acyloxymethylazetidin-2-ones 6a , 6h , and 6i are
presented in Figure 1. For comparison purposes, the pH-
rate profile for an acyclic counterpart, N-benzoyloxy-
methyl-N-methylacetamide, 7, is also included. Both
(10) Hastla, D. J .; Pagani, E. D. Ann. Rep. Med. Chem. 1994, 29,
195.
(11) Bernstein, P. R.; Edwards, P. D.; Williams, J . C. Prog. Med.
Chem. 1994, 31, 59.
(12) Shah, S. K.; Dorn, C. P.; Finke, P. E.; Hale, J . J .; Hagmann,
W. K.; Brause, K. A.; Chandler, G. O.; Kissinger, A. L.; Ashe, B. M.;
Weston, H.; Knight, W. B.; Maycock, A. L.; Dellea, P. S.; Fletcher, D.
S.; Hand, K. M.; Mumford, R. A.; Underwood, D. J .; Doherty, J . B. J .
Med. Chem. 1992, 35, 3745.
N-acyloxymethylazetidin-2-ones 6 and theN-acyloxymethyl-
amide 7 undergo acid- and base-catalyzed hydrolysis, as
well as pH-independent hydrolysis. These three processes
conform to eq 1. The corresponding pseudo-first-order
rate constants for the pH-independent hydrolytic path-
way, k0, and the second-order rate constants for the acid,
(13) Page, M. I. Adv. Phys. Org. Chem. 1987, 23, 165.
(14) Page, M. I.; Laws, A. P. Tetrahedron 2000, 56, 5631.
(15) Perrin, D. D.; Dempsey, B.; Serjeant, E. P. pKa Prediction for
Organic Acids and Bases; Chapman and Hall: London, 1981. A pKa
value of 2.03 for the conjugated acid of HO2C(CH2)CH(SO2Ph)NHCH2-
OCOMe was calculated from the equation pKa ) 10.59-3.23Σσ*,
+
-
kH , and alkaline, kOH , hydrolyses are presented in Table
1.
+
derived for the dissociation of R′R′′NH2 and using the assumptions
kobs ) k0 + kH+ [H+] + k - [OH-]
(1)
set out on pp 38-40 of the above reference.
(16) Proctor, P.; Gensmantel, N. P.; Page, M. I. J . Chem. Soc., Perkin
Trans. 2 1982, 1185.
(17) Beauve, C.; Bouchet, M.; Touillaux, R.; Fastrez, J .; Marchand-
Brynaer, J . Tetrahedron 1999, 55, 13301.
OH
p H-In d ep en d en t Rea ction . The most striking fea-
ture of the pH-rate profiles of N-acyloxymethylazetidin-
3360 J . Org. Chem., Vol. 69, No. 10, 2004

Hydrolysis of N-Acyloxymethyl Derivatives of Azetidin-2-one
TABLE 2. Dep en d en ce of th e P seu d o-F ir st-Or d er Ra te
Con sta n ts, ko, on th e Bu ffer Con cen tr a tion for th e
Hyd r olysis of 6a a t 25 °C a n d µ ) 0.5 m ol d m -1
kobs/10-5
s-1
compd
buffer
[buffer]/10-2
M
pH
6a
CH2ClCO2-/CH2ClCO2H
1.0
2.0
5.0
10.0
1.0
2.0
5.0
10.0
1.0
2.0
5.0
10.0
1.0
2.0
5.0
10.0
2.59 2.87
2.39 2.88
2.17 3.17
2.06 3.36
5.06 2.22
5.01 2.38
4.98 1.85
4.98 2.10
6.85 2.53
6.87 2.35
6.87 2.43
6.90 2.35
4.83 0.134
4.84 0.121
4.80 0.122
4.80 0.135
AcO-/AcOH
-
HPO42-/H2PO4
6h
AcO-/AcOH
Cephalosporins also exhibit a significant pH-independent
hydrolysis, usually ranging from pH 2 to pH 7.21,22
However, the spontaneous hydrolysis of cephalosporins
has been ascribed to the intramolecular general-base-
catalyzed attack of water at the â-lactam carbonyl carbon
atom by the amide side chain,21 a neighboring group
effect that cannot be present in the N-acyloxymethylaze-
tidin-2-ones 6. The pH-rate profiles of 6a -g also differ
significantly from those for simple ester hydrolysis, which
occurs via an addition-elimination mechanism and usu-
ally exhibits the incursion of specific base catalysis at
pH 6-7.23 In contrast, N-acyloxymethylazetidin-2-ones
6i-k , which contain strongly electron-withdrawing sub-
stituents in C-4, are at least 102 less reactive than their
unsubstituted counterparts (Table 1). Four potential
mechanisms could be considered for the pH-independent
hydrolysis of N-acyloxymethylazetidin-2-ones 6: nucleo-
philic attack of water to the ester carbonyl atom (A, path
a), SN1 ionization with departure of the carboxylate anion
(A, path b), SN2 nucleophilic attack of water to the
acyloxymethyl carbon atom (A, path c), and â-lactam ring
opening via nucleophilic attack of water to the lactam
carbonyl atom (A, path d).
F IGURE 1. pH-rate profiles for compounds 6a (b), 6h (9),
6i (O), and 7 (2) at 25 °C in aqueous buffers containing 20%
(v/v) acetonitrile.
TABLE 1. P seu d o-F ir st-Or d er Ra te Con sta n ts, ko, for
th e p H-In d ep en d en t Hyd r olysis a n d Secon d -Or d er Ra te
+
-
Con sta n ts, kH a n d kOH , for th e Acid - a n d
Ba se-Ca ta lyzed Hyd r olyses of N-Acyloxym eth yl-
a zetid in -2-on es 6 a n d Com p ou n d 7 a t 25 °C a n d in
Aqu eou s Med ia Con ta in in g 20% (v/v) Aceton itr ile
103kH
102kOH
+
-
compd
105ko (s-1
)
(L mol-1 s-1
)
(L mol-1 s-1
)
6a
2.35; 2.25;a 5.11;b
7.88;c 15.1;d 28.1;e
49.1f
1.93; 2.86;a 3.77;b
6.50;c 10.5;d 21.5e
10.3; 12.0;a 14.2;b
18.7;c 22.9d
6b
6c
6d
6e
6f
0.954
8.94
18.0
1.46
1.95
1.48
1.57
8.67
1.84
31.2
193
3.32
1.71
10.7
61.2
6g
6h
4.83
15.7
7.83
0.121; 0.0906;a
0276;b 0.626;g
1.30;e 3.09f
0.0226h
0.0888; 0.111;a
0.147;b 0.244;c
0.488;d 0.763e
0.0204
971; 1196a
6i
6j
6k
7
5.84
23.2
8.62
7.75
0.0120h
0.000598
0.000419
35.2
0.0197h
77.3
a
b
d
g
In D2O. 30 °C. c 35 °C. 40 °C. e 45 °C. f 50 °C. 36.9 °C.
h
Reaction followed up to ca. 30%.
2-ones 6a -g (exemplified in Figure 1 by that for 6a ) is
the unusually broad plateau extending from pH ca. 2 to
pH ca. 10 corresponding to the pH-independent pathway,
which is characterized by the absence of both general-
base and nucleophilic catalysis (Table 2). This contrasts
markedly with the hydrolysis of penicillin â-lactam
antibiotics, which have no significant spontaneous reac-
tion (e.g., benzylpenicillin)13 or have only small plateaus
around neutral pH (e.g., cyclacillin or ampicillin).20,21
The analysis of reaction mixtures of 6a -k by HPLC
and by HPLC-MS, together with spiking experiments
with authentic samples, shows that at pH 3-5 the
corresponding N-hydroxymethylazetidin-2-one, 8, and
(20) Yamana, T.; Tsuji, A.; Mizukami, Y. Chem. Pharm. Bull. 1974,
22, 1186.
(21) Llina´s, A.; Vilanova, B.; Frau, J .; Mun˜oz, F.; Donoso, J .; Page,
M. I. J . Org. Chem. 1998, 63, 9052.
(22) Yamana, T.; Tsuji, A. J . Pharm. Sci. 1976, 65, 1563.
(23) Euranto, E. K. In The Chemistry of Carboxylic Acids and Esters;
Patai, S., Ed.; Wiley: Chichester, 1969; p 505.
(18) Iley, J .; Moreira, R.; Rosa, E. J . Chem. Soc., Perkin Trans. 2
1991, 563.
(19) Iley, J .; Moreira, R.; Calheiros, T.; Mendes, E. Pharm. Res. 1997,
14, 1634.
J . Org. Chem, Vol. 69, No. 10, 2004 3361

Valente et al.
SCHEME 3
carbon atom that is attached, but it conforms with path
b, i.e. the unimolecular ionization presented in Scheme
3, in which the developing positive charge on nitrogen is
destabilized by electron-withdrawing substituents. In
contrast, conventional ester hydrolysis would be en-
hanced by electron-withdrawing substituents in the
alcohol leaving group, which further implies that path a
is not followed by the compounds under study.23,26 The
proposal of an SN1-type mechanism in the pH-indepen-
dent region receives additional support from the sub-
stituent effect at C-4 of the azetidin-2-one moiety. The
electron-withdrawing substituents SPh, 6i, and SO2Ph,
6j, lead to a dramatic decrease in reactivity, with less
than 30% of both compounds being hydrolyzed after 20
days in pH 5.0 acetate buffer (Table 1). This decrease in
reactivity is inconsistent with A, path c, in which
electron-withdrawing groups would be expected to in-
crease the rate of reaction. The order of reactivity is 6a
. 6i > 6j, which suggests the development of a positive
charge in the transition state is consistent with path b
above.
F IGURE 2. Brønsted plot for the pH-independent hydrolysis
of compounds 6a -g at 25 °C.
SCHEME 2
azetidin-2-one, 9 (Scheme 2), are the products of these
reactions and that the two products together account for
all the substrate lost. This implies that pH-independent
hydrolysis of N-acyloxymethylazetidin-2-ones does not
involve â-lactam ring opening (A, path d). Further
evidence that enables us to discount this pathway is the
observation that compound 6k , which is 3,3-diethyl
substituted, is slightly more reactive than its unsubsti-
tuted counterpart 6j. If â-lactam hydrolysis were occur-
ring then such substitution should hinder the reaction,
as suggested by the 2-fold reduction in the HO--induced
hydrolysis of 1,3-dimethylazetidin-2-one when compared
with 1-methylazetidin-2-one.24
A plot of the apparent first-order rate constants of
N-acyloxymethylazetidin-2-ones 6a -g versus the pKa of
the carboxylic acid leaving group generates a Brønsted
âlg value of -1.20 (r2 ) 0.96) (Figure 2). Significantly,
the 2-methoxybenzoate 6e is included in this plot,
indicating that the 2-MeO substituent does not exert any
retarding effect on reactivity and that the source of
reactivity of N-acyloxymethylazetidin-2-ones 6a -g is not
conventional ester hydrolysis (A, path a). Similar Brøn-
sted plots, also including sterically hindered carboxylate
groups, have been reported for the pH-independent
hydrolysis of N-acyloxymethylamides19 and N-acyloxy-
methylsulfonamides,25 which undergo SN1 ionization with
departure of the carboxylate anion.
According to the mechanism presented in Scheme 3 for
the pH-independent process, the hydrolysis of compounds
6 involves the formation of an iminium ion, 10, and a
carboxylate anion; the corresponding rate law, as derived
using the steady-state assumption for intermediate 10,
is given by eqs 2 and 3.
k1kH O
2
kobs
)
(2)
-
kH O + k-1[RCO2
]
2
k-1
1
1
k1
)
[RCO2-] +
(3)
kobs k1kH O
2
Accordingly, the rate of hydrolysis of compound 6a was
found to decrease in the presence of increasing concen-
trations of added benzoate anion (Figure 3). This is
consistent with generation of a free, solvent-equilibrated
iminium ion with a lifetime that is long enough to allow
trapping by added benzoate. The plot of 1/kobs versus
[PhCO2-] for these data (insert in Figure 3) gives a slope,
k-1/k1kH O, of 6.37 × 107 s L mol-1 and an intercept, 1/k1,
2
3.25 × 104 s, from which a value for k-1/kH of 1960 L
2O
Compound 6h , which contains a CO2Et substituent in
the linker between the â-lactam and ester moieties, is
hydrolyzed 25 times slower than its unsubstituted coun-
terpart 6a . This rate retardation is consistent with an
apparent Taft F* value for R-substitution of ca. -1. This
is inconsistent with path c, as electron-withdrawing
groups would enhance the electrophilic nature of the
mol-1 is obtained. That is, the benzoate anion is ca. 2000
times more effective at trapping the iminium ion than
water. Assuming that the value of k-1 is diffusion
controlled, i.e., k-1 ≈ 5 × 109 L mol-1 s-1 27
,
a lifetime for
the iminium ion, 1/kH O, of 3.9 × 10-7 s can be estimated
2
from the k-1/kH ratio. The assumption that the rate
2O
constant for the reaction of the iminium ion with the
(24) Washkuhn, R. J .; Robinson, J . R. J . Pharm. Sci. 1971, 60, 1168.
(25) Lopes, F.; Moreira, R.; Iley, J . J . Chem. Soc., Perkin Trans. 2
1999, 431.
(26) Nielsen, N. M.; Bundgaard, H. Int. J . Pharm. 1987, 39, 75.
(27) Eldin, S.; J encks, W. P. J . Am. Chem. Soc. 1995, 117, 4851.
3362 J . Org. Chem., Vol. 69, No. 10, 2004

Hydrolysis of N-Acyloxymethyl Derivatives of Azetidin-2-one
F IGURE 3. Inhibition of the pH-independent hydrolysis of
6a at 25 °C as a function of [PhCO2-]. The solid line shows
the least-squares fit of the data to eq 4. The inset is a plot of
1/kobs vs [PhCO2-].
F IGURE 4. Hammett plots for the acid-catalyzed hydrolysis
of 6a -d (b) and base-catalyzed hydrolysis of 6a -e (9) at 25
°C.
benzoate ion is diffusion controlled is supported by the
dependence of log kobs values on the pKa of the benzoate
leaving group. The âlg value of -1.20 for the hydrolysis
of 6a -g indicates that the reaction has a very late
transition state in which the C-O bond is almost
completely broken. By the principle of microscopic re-
versibility,28 the reverse reaction, i.e., the combination
of the benzoate anion with the iminium ion, must proceed
via a transition state with almost no bond formation.
Additional evidence for the SN1-type mechanism de-
picted in Scheme 3 comes from the following observa-
tions. First, the solvolysis of compounds 6a and 6h
+
catalyzed hydrolysis with kH values ca. 2 orders of
magnitude higher than those reported for the acid-
catalyzed hydrolysis of other azetidin-2-ones (e.g., for
nocardicin A, 11, kH is 7.1 × 10-5 L mol-1 s-1 at 30 °C16),
+
which undergo â-lactam ring opening via an A-1 mech-
anism. The products of degradation of 6a -h are those
resulting from ester hydrolysis, i.e., N-hydroxymethyl-
azetidin-2-one and the corresponding carboxylic acid. In
contrast, penicillins16 are more reactive toward acid than
N-acyloxymethylazetidin-2-ones 6a -h by a factor of 102,
but this high reactivity as been ascribed to the neighbor-
ing group participation by the 6-acylamido moiety,16,21
a
proceeds slightly faster in H2O than in D2O, giving a
pathway not available to compounds 6. Moreover, N-acyl-
oxymethylazetidin-2-ones 6a -h are at least 103 times
more reactive than simple alkyl benzoates, which un-
dergo acid-catalyzed ester hydrolysis via an A-2 mecha-
nism.23
H2
kinetic solvent isotope effect (KSIE), k0 O/k0D O, of 1.2.
2
This value is consistent with an unimolecular ionization
process being the rate-limiting step29,30 and is similar to
the KSIE reported for the pH-independent hydrolyses of
N-acyloxymethyl derivatives of amides18,19 and sulfona-
mides,25 which also hydrolyze via a SN1 mechanism. The
The acid-catalyzed pathway is very sensitive to the
electronic effects of the substituent X at C-4 of the
k0H O/k0D O value of 1.2 for compounds 6 contrasts sharply
with the primary KSIE of 4.5 reported for the pH-
independent hydrolysis of penicillins, which involves
â-lactam hydrolysis.13 Second, the temperature depen-
dence for the reaction of compounds 6a and 6h gave rise
to entropies of activation, ∆Sq, of -20.1 ((9.2) and -38.9
((18.9) J K-1 mol-1, respectively. These values are
slightly negative but are well within the range observed
for unimolecular ionization reactions carried out in the
presence of organic cosolvents.31
2
2
+
â-lactam. The kH value for compound 6j (X ) PhSO2) is
3000-fold less than that for its counterpart 6a (X ) H),
+
while 6i (X ) PhS) has a 100-fold reduction in the kH
value when compared with the unsubstituted derivative
6a . Compounds 6a , 6i, and 6j conform to the Taft
equation (eq 4). The Taft F* value of -1.3 is indicative of
the development of a positive charge (in the lactam) in
the transition state. The second-order rate constant for
the acid-catalyzed hydrolysis of the N-acyloxymethylaze-
tidin-2-one 6h is 20-fold less than that for the unsubsti-
tuted compound 6a , indicative of an apparent Taft F*
value of -0.7 for the substituent at the C-1′ in these
compounds. This value is similar to that observed for the
pH-independent hydrolysis and is also consistent with
the development of a positive charge in the transition-
state. In contrast with these Taft F* values, the Hammett
plot for the esters 6a -d gives rise to a F value of almost
zero (Figure 4).
Compound 6a is ca. 25 times less reactive than its
acyclic counterpart 7. The decreased reactivity of 6a ,
when compared to 7, may be ascribed to an increase in
ring strain resulting from the presence of two exocyclic
double bonds in the iminium ion 10. This observation is
supported by theoretical calculations as described in the
section “Fate of â-Lactam Iminium Ion”.
Acid -Ca ta lyzed Hyd r olysis. N-Acyloxymethylazeti-
din-2-ones unsubstituted in C-4 (6a -h ) undergo acid-
log kH+ ) -1.3σ* - 2.2 (n ) 3, r2 ) 1.0)
(4)
(28) Page, M. I.; Williams, A. Organic and Bio-organic Mechanisms;
Longman: Harlow, 1997; p 42.
(29) Thornton, E. K.; Thornton, E. R. In Isotope Effects In Chemical
Reactions; Collins, C. J ., Bowman, N. S., Eds.; Van Nostrand Rein-
hold: New York, 1970; p 213.
(30) J ohnson, S. L. Adv. Phys. Org. Chem. 1967, 5, 273.
(31) Winstein, S.; Fainberg, A. H. J . Am. Chem. Soc. 1957, 79, 5937.
Activation parameters (∆Sq) for the acid-catalyzed
hydrolyses of 6a and 6h were determined to be +3.6
((11.7) and -40.7 ((11.3) J K-1 mol-1, respectively, from
the kH+ values in the range of 25-45 °C (Table 1). These
J . Org. Chem, Vol. 69, No. 10, 2004 3363

Valente et al.
SCHEME 4
ester O-protonated species C (Scheme 4) cannot be ruled
out, particularly for compounds 6i-k , which contain
electron-withdrawing substituents on the carbon next to
the â-lactam nitrogen. In fact, the relative magnitudes
+
of kH for compounds 6e-g may well reflect propensities
toward protonation of the ester groups in these molecules,
though this is somewhat speculative in the absence of
appropriate confirmatory data.36
The F/F* values determined for the acid-catalyzed
pathway are a combination of the F value for protonation
and that for the decomposition of the protonated species.
Protonation of compounds 6 to give the N-protonated
intermediate B would be expected to be retarded by
electron-withdrawing substituents at C-4 in the â-lactam,
whereas decomposition of B would not be expected to
greatly affected by such substituents since, on going from
B to the iminium ion, a significant proportion of the
positive charge would remain on the N-atom. Conversely,
if hydrolysis were to proceed via C, then substituents at
C-4 in the â-lactam would be expected to have little effect
on the protonation and the final F* value would be
expected to reflect the dissociation step. The estimated
F* value observed for 6a , 6i, and 6j is -1.3, consistent
with either species B or C.
The substituent effect in the ester group can also be
accommodated by a mechanism involving either site of
protonation. N-Protonation, leading to B, should be
unaffected by substituents in the benzoyl moiety of the
ester group (for example, σ* values vary from 2.50 for
the 4-methoxybenzoyloxy substituent to 2.73 for the
4-nitrobenzoyloxy substituent37), nor would the decom-
position of B (as one oxygen acts as electron acceptor and
the other acts as electron donor), resulting in an overall
F of ca. zero, as observed (Figure 4). However, an identical
outcome would be expected for C, as F values for the
protonation and decomposition steps are likely to be of
similar magnitude but opposite sign. The solvent isotope
effect, ca. 0.8, is higher than the value of ca. 0.4 usually
found for typical fast preequilibrium processes29 and may
be an argument in favor of the formation of species B,
followed by a rate-limiting intramolecular proton transfer
from nitrogen to the carbonyl oxygen.25
values provide strong evidence for a dissociative mech-
anism,32 rather than one involving nucleophilic attack by
solvent, and are similar to those of N-acyloxymethyl-
amides18 and N-acyloxymethylsulfonamides.25
Taken together, these results suggest that N-acyl-
oxymethylazetidin-2-ones 6 undergo acid-catalyzed hy-
drolysis via a dissociative mechanism identical to that
for the pH-independent pathway, but involving preequi-
librium protonation of the substrate prior to the dissocia-
tive step (Scheme 4). Additional evidence comes from the
absence of general-acid catalysis (Table 2) and from the
+
+
KSIE, kH /kD , of 0.7 and 0.8 for compounds 6a and 6h ,
respectively (Table 1). Moreover, the relative rates of
hydrolysis of 6a and its acyclic counterpart, 7, differ by
less than a factor of 2 between the acid-catalyzed (k7/k6a
) 18) and pH-independent (k7/k6a ) 33) pathways,
suggesting that the two reactions follow similar path-
ways.
The 3,3-diethyl compound 6k and its C-3 unsubstituted
counterpart 6j are hydrolyzed at comparable rates. This
result is consistent with either N- or O-protonation but
rules out the A-1 â-lactam ring-opening pathway that
operates in monocyclic and bicyclic â-lactams.13 Such an
A-1 mechanism involves formation of an acylium cation,
12, and therefore would be enhanced by two electron-
donating ethyl groups at C-3, i.e., 6k , as expected from
the FI value of 5 determined for the acid-catalyzed
hydrolysis of â-lactams.13
Unfortunately, is not possible to determine the exact
site of protonation that leads to hydrolysis. Despite
amides being well-known oxygen bases,33 Page has sug-
gested that the acid-catalyzed hydrolysis of bicyclic
â-lactams involves N-protonation and that the pKa for
such protonated species must be <-5.13 However, Wan
et al. reported a pKa value of -1 for azetidin-2-one,34
which is structurally similar to the unsubstituted â-lac-
tam compounds 6a -g. This indicates that the expected
pKa values for â-lactam protonation in compounds 6
should be e-1. Esters are known to be very weak bases,
with pKa ranging from ca. -6.5 to -7.5.35 Thus, extensive
protonation of the ester moiety is likely to be relatively
unimportant for the unsubstituted compounds 6a -g,
which would imply that the most likely corresponding
protonated species is B (Scheme 4). Nevertheless, the
(32) Kohnstam, G. Adv. Phys. Org. Chem. 1967, 5, 121.
(33) Homer, R. B.; J ohnson, C. D. In The Chemistry of Amides; Patai,
S., Ed.; Wiley: Chichester, 1970; pp 187.
(34) Wan, P.; Modro, T. A.; Yates, K. Can. J . Chem. 1981, 58, 2423.
(35) March, J . Advanced Organic Chemistry; J ohn Wiley: New York,
1992.
Alk a lin e H yd r olysis. N-Acyloxymethylazetidin-2-
ones 6a -h undergo base-catalyzed hydrolysis to azetidin-
2-one and the corresponding carboxylic acid. In contrast
to the acid-catalyzed pathway, the base-catalyzed path-
3364 J . Org. Chem., Vol. 69, No. 10, 2004

Hydrolysis of N-Acyloxymethyl Derivatives of Azetidin-2-one
SCHEME 5
way is sensitive to the electronic effects of the substitu-
ents in the ester moiety (Table 1). Electron-withdrawing
substituents in the ester moiety (6a -d ) increase the rate
of hydroxide ion hydrolysis, with a Hammett F value of
1.6 (Figure 4), which is similar to the value of ca. 2.2
reported for the alkaline hydrolysis of methyl and ethyl
benzoates.38 Significantly, compound 6h is hydrolyzed ca.
100 times faster than 6a . This rate enhancement can be
ascribed to the improved leaving group ability of the
alcohol that results from the decomposition of the tetra-
hedral intermediate 13 (Scheme 5) due to the electron-
withdrawing effect of the CO2Et group. Indeed, the pKa
of the alcohol generated from ester hydrolysis of 6a -g
is 12.7,39 while that for the alcohol from 6h is 9.5,39 which
corresponds to a good leaving alcohol. For compounds 6a
and 6h , the KSIE, kOH /kOD , of 0.9 and 0.87, respectively,
and the ∆Sq value of -134 ((7) J K-1 mol-1 for 6a , are
also consistent with a BAc2-type mechanism.32 Further
evidence for a change in mechanism comes from the
comparison of the k7/k6a ratio of 0.8 for the alkaline
hydrolysis with the values of ca. 20 for the pH-indepen-
dent and acid-catalyzed hydrolysis.
2.0,15 a difference sufficiently large that should be
reflected in the corresponding kOH values.16 Second, the
-
3,3-diethyl compound 6k is only three times less reactive
toward hydroxide than the C-3 unsubstituted counterpart
6j. Third, the reaction of 6i,j in alkaline solutions leads
to an absorbance increase at 265 nm, the magnitude of
which is pH dependent (maximum ꢀ at pH 13). This peak
disappears on the addition of HCl and is subsequently
regenerated by addition of NaOH. This pH-dependent
pattern of UV changes has also been reported for the
alkaline hydrolysis of 4-aryloxyazetidin-2-ones (14, X )
OAr)40 and arylthioazetidin-2-ones (14, X ) SAr)41 and
is consistent with the enolate of 3-oxopropanamide 15
(Scheme 5). Upon acidification, the enolate of 15 reverts
to its neutral form.40 In contrast, the alkaline hydrolysis
of the 3,3-diethyl-substituted N-acyloxymethylazetidin-
2-one 6k , which cannot form an enolate, shows no
increase in absorbance at 265 nm. Signficantly, for the
alkaline hydrolysis of both 6j and 6k , the benzenesulfinic
acid coproduct was also detected in the reaction mixtures.
These results strongly suggest that compounds 6i-k
also undergo hydroxide attack exclusively at the ester
carbonyl carbon (Scheme 5). The resulting intermediate
is rapidly converted to the corresponding azetidin-2-one
anion, which undergoes a fast E1cB elimination and
hydrolytic ring opening to form 15.
-
-
In contrast to compounds 6a -h , the azetidin-2-ones
from the corresponding compounds 6i-k were not de-
tected in the alkaline region. From this result, nucleo-
philic attack of hydroxide ion at the â-lactam carbonyl
could be envisaged. However, the following observations
-
suggest that this is not the case. First, the kOH value for
6a (X ) H) is of the same order of magnitude as that for
6j (X ) SO2Ph). If nucleophilic attack of hydroxide ion
were occurring at the â-lactam carbonyl it would be
expected to be significantly enhanced by electron-
withdrawing substituents in C-4, as they would reduce
the pKa of the amine leaving group. Indeed, the pKa of
the protonated amine from 6a is 6.0, while that of 6j is
F a te of th e â-La cta m Im in iu m Ion . One of the
outcomes of the present work is information regarding
the fate of â-lactam iminium ion 10. The solvolysis of
penicillins and cephalosporins involves decomposition of
the tetrahedral intermediate via general-acid-catalyzed
N-protonation, e.g., 16,13 or complexation with a metal
ion, to effect C-N bond cleavage.13 The iminium ion 10,
however, has a formal positive charge at the nitrogen
atom; even so, it does not react with water by cleavage
of the â-lactam acyl C-N bond. Rather, reaction proceeds
by attack at the carbon atom of the iminium to afford
N-hydroxymethylazetidin-2-one. This is another mani-
festation of the observation that reactivity of â-lactam
antibiotics lies not in the relief of ring strain but in the
pKa of the leaving group amine.
(36) See the NIST Standard Reference Database No. 69, March
2003, in the NIST Chemistry WebBook available at http://
webbook.nist.gov/chemistry (accessed 3rd Mar 2004). (Neither pKa
values nor proton affinities (PAs) are available for esters of 2-meth-
oxybenzoic, phenylacetic, or cinnamic acids. However, methyl esters
of 3- and 4-methoxybenzoic acids have higher PAs than methyl
benzoate, as do the esters of all methyl-substituted benzoic acids, which
leads us to assume that the methyl ester of 2-methoxybenzoic acid will
have a higher PA than methyl benzoate. Similarly, the PA of methyl
2-butenoate is greater than that for methyl butanoate, suggesting that
the PA for methyl cinnamate will be greater than that for methyl
benzoate.)
(37) See ref 15, p 109.
(38) Ritchie, C. D.; Uschold, R. E. J . Am. Chem. Soc. 1968, 90, 2821.
(39) The pKa for the alcohol from 6a was calculated using the
equation pKa ) 15.9 - 1.42σ*, derived for the dissociation of alcohols
RCH2OH, assuming that azetidin-2-one exerts the same electronic
effect that of N-methylacetamido moiety (σ* ) 2.25). See ref 15.
(40) Fedor, L. R. J . Org. Chem. 1984, 49, 5094.
(41) Gu, H.; Fedor, L. R. J . Org. Chem. 1990, 55, 5655.
J . Org. Chem, Vol. 69, No. 10, 2004 3365

Valente et al.
of corresponding N-hydroxymethylazetidin-2-one 8 (3 mmol)
and triethylamine (3.6 mmol) in dichloromethane (3 mL) at 0
°C. When the reaction was complete (as assessed by TLC using
ethyl ether/petroleum ether 7:3 as eluant), the reaction
mixture was filtered and the dichloromethane solution sub-
sequently dried evaporated. The residue was purified by
column chromatography (silica gel 60) using diethyl ether as
eluant.
(2-Oxoa zetid in -1-yl)m eth yl Ben zoa te 6a . Prepared from
8a and benzoyl chloride (16%): mp 64-65 °C; 1H NMR (CDCl3)
δ 3.10 (2H, t, J ) 4.2 Hz, C3-H), 3.60 (2H, t, J ) 4.0 Hz, C4-H),
5.52 (2H, s, NCH2O), 7.43-8.35 (5H, m, ArH); IR (film) 1780,
1735 cm-1. Anal. Calcd for C11H11NO3: C, 64.38; H, 5.40;
N,6.83. Found: C, 64.19; H, 5.31; N, 7.02.
Another major finding is that both N-acyloxymethyl-
azetidin-2-ones 6 and their acyclic amide counterparts
(e.g., 7) appear to undergo the same SN1-type mechanism
in the pH-independent region. To gain further insight
into the structural effects on iminium ion formation, ab
initio calculations were carried out at G3MP2B3 and
B3LYP/6-31+G(d) levels of theory.42,43 Heats of formation,
∆Hf, for the fully geometry-optimized structures of the
E and Z rotamers of N-acetoxymethyl-N-methylaceta-
mide, and also for the N-acetoxymethylazetidin-2-one,
together with the differences, ∆∆Hf, between each mol-
ecule and its corresponding iminium ion, are contained
in the Supporting Information (Table SI2). These data
indicate that iminium ion formation is disfavored (∆∆Hf
1182 kJ mol-1) from N-acetoxymethylazetidin-2-one by
13-18 kJ mol-1 as compared with the acyclic counterpart
(∆∆Hf 1164 (Z rotamer) and 1169 (E rotamer) kJ mol-1).
This is consistent with the experimental observation that
compound 6a is ca. 25 times less reactive than the acyclic
ester 7 in the pH-independent region. Interestingly,
calculation for the iminium ion 10 computed at the
B3LYP/6-31+G(d) level of theory shows that in the
LUMO the pz orbital of the exocyclic carbon atom
represents the highest coefficient of electron probability
(Supporting Information, Figure SI1). Its value is of about
0.46, while the coefficients in the other atoms are all
smaller than 0.30. Moreover, the LUMO reveals signifi-
cant π-bonding overlap between the â-lactam N and
carbonyl C atoms. While attack of the nucleophile at the
exocyclic C atom preserves this N-C π-bonding overlap
in the amide product, attack at the â-lactam carbonyl C
atom to form a tetrahedral intermediate destroys such
overlap. Thus, nucleophilic attack is predicted to take
place at the exocyclic carbon and, despite the presence
of a positively charged iminium ion leaving group, is
consistent with the observation that the pH-independent
hydrolysis of 6a -g occurs via cleavage of the ester C-O
bond rather than cleavage of the â-lactam acyl C-N
bond.
Data for compounds 6b-e can be found in the Supporting
Information
Kin etic P r oced u r es. (a ) Bu ffer s. All kinetic measure-
ments were carried out at 25.0 ( 0.1 °C, except where stated,
and with an ionic strength adjusted to 0.5 M by addition of
NaClO4. Sodium hydroxide and hydrochloric acid solutions
were prepared by dilution of standardized Titrisol stock
solutions. Sodium deuterioxide and deuterium chloride solu-
tions were prepared from dilution of NaOD 40% in D2O
(Aldrich) and DCl 37% in D2O stock solutions. Buffer materials
(chloroacetic acid, acetic acid, NaH2PO4, Na2HPO4, and Na2B4O7)
for kinetics were of analytical reagent grade, and the solvents
for HPLC were HPLC grade. Due to substrate solubility
problems, all buffers contained 20% (v/v) acetonitrile.
(b) Kin etics by UV Sp ectr op h otom etr y. Rate constants
were determined using UV spectrophotometry by recording the
decrease of substrate absorbance at fixed wavelength, using
a spectrophotometer equipped with a temperature controller.
In a typical run, reaction was initiated by adding a 15 µL
aliquot of a 10-2 M stock solution of substrate in acetonitrile
to a cuvette containing 3 mL3 of the buffer solution. The
pseudo-first-order rate constants were obtained by least-
squares treatment of log(At - A∞) data, where At and A∞
represent the absorbance at time t and at time infinity,
respectively. Rate constants derived using this method were
reproducible to (5%.
(c) Kin etics by HP LC. For those compounds that gave
small differences between the initial and final absorbances,
rate constants were determined by HPLC using an isocratic
system. In this method, a 15 µL aliquot of 10-2 M stock solution
of substrate was added to a reaction flask containing 3 mL of
the buffer solution. At regular intervals, samples of the
reaction mixture were analyzed on a Lichrosorb RP-8 column
(5 µm particles, 4 × 250 mm; Merck) with an eluant consisting
of acetonitrile/water (30:70 to 60:40, depending on the sub-
strate) at a flow rate of 1 mL/min. Rate constants derived using
this method were reproducible to (10%.
Id en tifica tion of P r od u cts. When reactions followed by
UV spectrophotometry were complete they were injected into
an HPLC apparatus and analyzed using an RP-18 (Merck)
column (5 mm particles, 4 × 125 mm) with the UV detector
tuned at 200 nm for compounds 6a -h and 230 nm for
compounds 6i-k . Some of the reactions were also analyzed
by LC-MS. Mass spectra were recorded using an LC-MS
system comprising a binary gradient pump coupled to a UV-
vis detector, a 15 cm × 4.6 mm i.d. Phenomenex LUNA C18-
(2) 5 µm column at 40 °C, and an electrospray ionization mass
spectrometer.
Th eor etica l Ca lcu la tion s. The enthalpies of formation,
∆Hf, for iminium ions were computed at the G3MP2B3 level
of theory.42 This is a variation of the Gaussian-3 approach with
zero-point vibrational and internal energy corrections for T )
298.15 K computed at the B3LYP/6-31G(d) level of theory43
and use reduced perturbation orders. The ∆Hf value for the
iminium ion derived from N-acetoxymethylazetidin-2-one was
obtained via the calculation of the enthalpy for the reaction
depicted in eq 5 (at the G3MP2B3 level of theory, ∆rH°T)298.15
K is 969.2 kJ mol-1) and considering the enthalpies of formation
Exp er im en ta l Section
Gen er a l P r oced u r e for N-Hyd r oxym eth yla zetid in -2-
on es 8. A suspension of the appropriate azetidin-2-one (15
mmol) and potassium carbonate (0.22 g) in a 37% aqueous
formaldehyde solution (161.3 mmol, 12 mL) was stirred at
room temperature for 24 h. The water was removed under
reduced pressure and the residue washed with dichlo-
romethane. The resulting dichloromethane extract was fil-
tered, evaporated, and chromatographed on silica gel 60
(0.063-0.200 mm particles) to give the corresponding N-hy-
droxymethylazetidin-2-one, 8.
N-Hydroxymethylazetidin-2-one 8a : yellowish oil (79%); 1H
NMR (DMSO-d6) δ 3.05 (2H, t, J ) 4.0 Hz, C3-H), 3.43 (2H, t,
J ) 4.0 Hz, C4-H), 4.75 (1H, t, J ) 3.0 Hz, OH), 4.95 (2H, d, J
) 3.0 Hz, NCH2O); IR (film) 3450, 1769. Anal. Calcd for C4H7-
NO2: C, 47.52; H, 6.98; N, 13.85. Found: C, 47.39; H, 7.10; N,
14.13.
Data for compounds 8b-e can be found in the Supporting
Information.
Gen er a l P r oced u r e for N-Acyloxym eth yla zetid in -2-
on es 6. A solution of the appropriate acid chloride (3.6 mmol)
in dichloromethane (3 mL) was added dropwise to a solution
(42) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Pople, J . A. J .
Chem. Phys. 2002, 112, 7374 and references therein.
(43) Becke, A. D. J . Chem. Phys. 1993, 98, 5648.
3366 J . Org. Chem., Vol. 69, No. 10, 2004

Hydrolysis of N-Acyloxymethyl Derivatives of Azetidin-2-one
of the H- and of N-methylazetidin-2-one.
thylacetamide. Due to the size of N-acetoxymethylazetidin-2-
one and N-acetoxymethyl-N-methylacetamide, it is impossible
for us to compute their enthalpies of formation with the
G3MP2B3 approach. Therefore, we have used a new set of
isodesmic reactions and the B3LYP/6-31+G(d) enthalpies at
T ) 298.15 K, e.g., eq 6:
The experimental ∆Hf of H- is 145.2 kJ mol-1
.
A ∆Hf value
36
of -112.86 kJ mol-1 was computed for N-methylazetidin-2-
one from the G3MP2B3 enthalpy and the atomization reaction
since no experimental value is found for this compound.
Similarly, we have computed the ∆Hf values for the iminium
ions derived from both rotamers of N-acetoxymethyl-N-me-
and considering the experimental enthalpies of formation of
CH4 and MeCO2Me36 and the previously computed enthalpies
of formation for N-methylazetidin-2-one and N-methylaceta-
mide. The calculations were carried out using the GAMESS-
US44 and the GAUSSIAN9845 computer codes. Some illustra-
tions were obtained with the Molekel visualization software.46
(44) Schmidt, M. W.; Baldridge, K. K.; Boatz, J . A.; Elbert, S. T.;
Gordon, M. S.; J ensen, J . H.; Koseki, S.; Matsunaga, N.; Nguyen, K.
A.; Su, S. J .; Windus, T. L.; Dupuis, M.; Montgomery, J . A. J . Comput.
Chem. 1993, 14, 1347, GAMESS-US Version 14/01/2003.
Ack n ow led gm en t. This work was supported by the
project POCTI/QUI/10039/98/2001 (Fundac¸a˜o para a
Cieˆncia e Tecnologia, Portugal) and FEDER. J .R.B.G.
thanks the Fundac¸a˜o para a Cieˆncia e Tecnologia for a
postdoctoral grant (SFRH/BPD/11582/2002). We are
grateful to the reviewers for helpful comments.
(45) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millan, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adammo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, G.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Y. Peng, C.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.;
Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.; Pople,
J . A. Gaussian 98, Revisions A.9, Gaussian, Inc., Pittsburgh, PA, 1998.
(46) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J . Molekel 4.0,
Swiss Center for Scientific Computing, Manno, Switzerland, 2000.
(47) Moreira, R.; Mendes, E.; Calheiros, T.; Bacelo; M. J .; Iley, J .
Tetrahedron Lett. 1994, 35, 7107.
Su p p or tin g In for m a tion Ava ila ble: The spectral data
for compounds 6b-e, 7,47 and 8b-e, the wavelengths used to
monitor the kinetic reactions and the optimized structures,
energies of all compounds, and the molecular orbital details
of the â-lactam iminium ion 10. This material is available free
of charge via the Internet at http://pubs.acs.org.
J O0358123
J . Org. Chem, Vol. 69, No. 10, 2004 3367