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232 Chemistry and Biology of Beta-Lactams
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SCHEME 7.11 (A) AND 12 (B) Optimized structure of transition-state TSia: isomerization of the imine. Adapted with
permission from Royal Society Advance 2022, 12, 10 4 –117.
isomeric structural intermediates, and the total activation energy for the entire isomerization process
were depicted in the following diagram (Schemes 7.9 and 7.10).
The isomerization route for the imine was in complete accordance with an in-plane inversion of the
starting structure. The out-of-plane rotation and inversion were impossible since the C4–N1–C5 bond
angle of TSia was too high and it was about 180 degree. But the naphthyl group in TSia was located in
a perpendicular position with respect to the C7–C4–N1 plane. This suggested an aza-allenyl structural
feature for this saddle point. This cumulenic structure was stabilized better compared to N-phenyl imine
through a complete delocalization of the negative charge of the TSia. This delocalization was obviously
observed by a direct examination of the bond index of the N1–C5 part. The overall outcome and result
were that the activation energy for the isomerization of (E)-isomer is about 1.1 kcal/mol lower than that
obtained for its related N-phenyl isomeric compound. On the other hand, notably, the (Z)-isomer was
about 5 kcal/mol less stable than its (E)-isomer (Scheme 7.11).
However, the isomerization process of the methoxyphenyl imine was different in nature. The calculations demonstrated that TSib exhibits a Cs symmetry and no isomerization is feasible through in-plane
inversion with no rotation about the N-p-methoxyphenyl bond. The valence bond resonance forms were
233Mechanism of Beta-Lactam Synthesis by Cycloaddition Reaction
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SCHEME 7.13 (A) AND 14 (B) Transition structure TSib during the isomerization of the imine. Adapted with permission
from Royal Society Advance 2022, 12, 104 –117.
shown. The aromatic system of the indole in TSib determined the major contribution of the polar intermediates. In these forms, the simultaneous aza-allenic delocalization process as seen in TSia was unnecessary. This fact allowed a symmetry feature, and so, it lowered the activation energy (Scheme 7.12).
The transition state structures, intermediates of the reactions, and products of the processes between
methoxyketene and both isomers of the imines were analyzed systematically. The calculations showed
that the (E)-isomer is less nucleophilic in nature than the (Z)-isomer. The activation energy required for
the production of the (E) -I N Ta a via the (E)-TSaa was 4.8 kcal/mol higher than that of (Z)-INTaa. At the
saddle points in the transition state, TSaa had a relationship with the noncoplanar attack of the nitrogen
of the imine on the sp-hybridized carbon of the methoxyketene compound. It was noted that the activation energy gaps for the production of the N1–C2 bonds in both of these unstable intermediates were
higher. But interestingly, these were very close to those calculated values for the isomerization pathways
between the (E)- and (Z)-isomers. Interestingly, the activation free energy required for the generation of
(Z)-INTaa from the (Z)-isomeric structure and ketene was 0.6 kcal/mol higher than that necessary for
the transformation of the (Z)- into the (E)-isomer. Moreover, the (Z)-INTaa transition state was about
5.7 kcal/mol more stable than the (E)-INTaa transition state due to the lower steric interaction associated
with the phenyl group at C4 position (Scheme 7.13).
234 Chemistry and Biology of Beta-Lactams
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A conrotatory cyclization pathway of the (E)-INTaa transition state to form the cis-isomer had 1.4
kcal/mol activation energy barrier. This pathway, therefore, became more fast than that of the isomerization process to produce the (Z)-INTaa. In a same manner, the electrocyclization of the (Z)-INTaa had
a lower activation barrier difference. In fact, the trans-TSaa was 0.6 kcal/mol lower in energy than that
of the cis-TSaa. This was due to the torquoelectronic force, which shows a 3-outward and a 4-outward
disposition of the aliphatic methoxy and aromatic phenyl groups in the rst conrotatory saddle point.
Therefore, the trans-product was 0.8 kcal/mol more stable than the corresponding cis-isomer (Scheme
7.14).
This method was followed with other examples, and thus, the reaction between acetoxyketene and
Schiff base was analyzed. The energies and the geometries of the possible transition-state structures are
given in Scheme 7.15.
The isomers of the imines obtained from the indole were less nucleophilic than the isomeric naphthalene compounds. Moreover, acetoxyketene was less electrophilic than methoxyketene. Therefore, the
activation energy leading to INTbb was higher than that calculated for the previous reaction pathway.
For instance, the actual energy barrier associated with the formation of (E)-INTbb was 0.4 kcal/mol
higher than that for the (Z)-isomer. The activation barrier required for the formation of the N1–C2 bond
was higher than the barrier required for the isomerization. So, a pre-equilibration of the starting imine
before the cycloaddition reaction was feasible. The formation of the C3–C4 bond following a conrotatory route had also higher energy. The saddle point of the cis-TSbb was connected to a lower free energy
barrier with respect to the trans-TSbb. This was possible because the electron-withdrawing amide C=O
group occupied the 4-inward position in the conrotatory saddle point. In contrast, the electron-releasing
phenyl group was located in a 4-outward disposition. So, a rotation of the N1–C4 bond through TSRbb
required higher energy. Because of this, the trans-3bb was not produced from the (E)-INTbb through an
isomerization of TSRbb. The kinetic diagram was in agreement with the observed results on the stereoselectivity (Scheme 7.16 and Scheme 7.17).
Importantly, the precise data was highly sensitive and depended on many factors. The entire process
was able to alter because of minor changes of the reaction conditions or alteration of the substrates.
In short, an effective initial E/Z-imine isomerization process in N-polyaromatic imines can generate
trans-β-lactams adopting the zwitterion–conrotatory ring closure pathway. This explanation became
partially contradictory because these Schiff bases on reaction with chromium–carbene complexes under
photochemical conditions produced cis–trans-isomeric mixtures. The steric crowding of the imine was
responsible to slow down the ring closure process.
We summarized a few facts on the mechanism of Staudinger cycloaddition reaction (Scheme 7.18).
Some of the results and explanations were identical with the previous results as described above.25 The
stereochemistry of the products was explained on the basis of torquoelectronic effects. Infrared spectroscopy (IR) was also used at room temperature to get information on the unstable reactive intermediates.26
IR spectroscopy showed a peak at 2200 cm-1 when acetoxyacid chloride and Schiff base were reacted
using triethylamine or N-methylmorpholine.18 The experiments were conducted in an IR cell. This band
was noticed at the beginning of the process and not seen after a few hours. This observation indicated
the involvement of ketene species in the reaction mixture. This formation of trans-compound was also
possible with nitro-substituted imines. The nitro group exerted its electron-withdrawing power causing
an inversion of the conguration of the intermediate ion. The formation of a cis-β-lactam with a polyaromatic group at the -N and cinnamyl at the C4 indicated the concept of extended conjugation.26 Doyle et
al.27 hypothesized that an extended conjugation can stabilize the acyliminium ion much more efciently,
and because of this, super-stabilization inversion of the ion is not possible. It seemed that the cinnamyl or
polyaromatic group at the C4 position has superior contributing force than the N-polyaromatic system.
Therefore, cis-β-lactam was the only product when cinnamyl (extended conjugation) or polyaromatic
structures are occupied at the carbon part of the Schiff base. A proton release from a complex was facile to produce a cis-β-lactam through anion inversion. This observation was additionally supported by
the production of a donor-acceptor complex. Cossio et al.7 showed an SN2 intramolecular route for the
preferential or exclusive formation of trans-β-lactams. An explanation on the stereoselectivity based
upon the steric results was advanced. However, the formation of a mixture of cis- and trans-isomeric
β-lactams with naphthalenyl and anthracenyl imines was not explained adequately regardless of all
235Mechanism of Beta-Lactam Synthesis by Cycloaddition Reaction
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SCHEME 7.15 Transition structures to β-lactams from methoxyketene and imine derived from naphthalene derivative.
Adapted with permission from Royal Society Advance 2022, 12, 10 4 –117.
advanced mechanisms. The electron-withdrawing properties and/or the steric bulk of the N-polyaromatic
group should generate the trans-β-lactams with isomeric naphthalenyl and anthracenyl derivatives. The
N-polyaromatic structures that produced the trans-isomers have a structural identity with the naphthalene and anthracene derivatives. The N-polyaromatic Schiff base has a peri-hydrogen in the aromatic
236 Chemistry and Biology of Beta-Lactams
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SCHEME 7.16 Transition state for the formation of β-lactams from acetoxyketene and Schiff base.
system close to the C=N bond. These imines produced only the trans-β-lactams. The peri-hydrogen is
located away from the C=N bond in some naphthalene and anthracene imines that produced a mixture
of isomeric β-lactams. The role of the peri-hydrogen in controlling the conguration of β-lactam was
not described by any authors. The involvement of several ionic intermediates as observed indicated an
interaction with the suitably located peri-hydrogen with the transition states of these processes.
237Mechanism of Beta-Lactam Synthesis by Cycloaddition Reaction
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SCHEME 7.17 Simulated stereochemical results. Adapted with permission from Angew. Chem. Int. Ed. 2007, 46, 3028.
Recently, the mechanism of the reaction between t-butyl-cyano ketene and N-phenyl phenylimine
was investigated using DFT method.28 Like earlier studies, a two-step mechanism was advanced. It was
proposed that the trans- and cis-β-lactams are formed through the endo and exo stereoisomeric channels,
respectively. An electron localization function (ELF) quantum topological analysis was helpful to permit
a characterization of the pathways.
The formation of the N1–C4 single bond was possible through a nucleophilic reaction of the nitrogen
electron pair on the central carbon of the ketene. Then, a ring closure took place by the carbon-tocarbon coupling of the C2 and C3 pseudoradical centers generated in the previous process. The theoretical investigation conducted in this study raised issues in HOMO/LUMO interactions. This issue was
extended onto the nucleophilic reaction of the imines on the ketenes and a subsequent torquoelectronic
effect–conrotatory ring-closure step leading to the formation of the β-lactams.
The electronic effects of the substituents on the stereoselectivity of the Staudinger catalytic reaction
were investigated.29 The DFT was used to compare the role of N-tosyl (N-Ts) and N-triyl (N-Tf) imines
on the mechanism pathways. This was studied to know two aspects. For example, to understand which
one is formed rst: imine-rst or ketene-rst. Also, this was investigated to know the stereochemistry
of the products.30 These results demonstrated that the mechanistic route is not due to the switch from
ketene-rst to imine-rst by altering the group on the nitrogen of the Schiff base. The alteration in selectivity in the N-triyl imine system was due to the charge transfer and electron density reorganization in
238 Chemistry and Biology of Beta-Lactams
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SCHEME 7.18 Adapted with permission from Bioorg. Med. Chem. 2005, 13, 3 611.
the transition states as found from electronic analyses. So, the cis- and trans-selectivity of the Staudinger
reaction was due to the controlling effects by the electronic characteristics of the substituents in the
reacting components. An N-protecting group in the Schiff base with a high electron-withdrawing power
was able to accelerate the slowest step, ring closure, and improve the stabilization charge distribution in
the activated state, leading to a preference for a trans-β-lactam. An N-substituent with a better electronwithdrawing property was able to activate the Schiff base by the nucleophilic catalyst in a competition
with the ketene.
Following the methods of racemic β-lactam synthesis mentioned above, preparation of optical isomers
starting from suitable imines and ketenes was conducted.31 The conguration of the resulting products
was not predictable, and it depended on the structure of the reactive partners. The induction of chirality
in the β-lactam ring was possible by three different ways. In general, optically active carbonyl compounds with primary amines produced a single enantiomeric β-lactam. The chiral ketenes from the acid
chloride equivalent on reaction with nonchiral Schiff bases gave a mixture of β -lactams. On the other
hand, chiral amino components in the Schiff bases gave a mixture of isomeric β-lactam enantiomers.
Amino acids or chiral amines demonstrated low asymmetric induction than the chiral aldehyde components. Asymmetric synthesis of β-lactam was investigated by a number of prominent researchers.
It was postulated that the origin of the diastereoselection between the β-lactams must be within the
different energies of the transition structures that correspond to the ring closures. This result was in
1
O
R
Cu
Ph
2
Ar
2
1
i)
Pyridine,
https://t.me/med1917
Ar
+
SCHEME 7.19 The Kinugasa reaction for the synthesis of β-lactams.
N
Ar
O
i)
1
-
0.5
rt,
239Mechanism of Beta-Lactam Synthesis by Cycloaddition Reaction
Ar
Ph
h
1
N
O
SCHEME 7. 2 0 Mechanism as proposed by Ding and Irwin.
agreement to rationalize the excellent asymmetric induction observed in Staudinger cycloaddition
between achiral ketenes and chiral epoxy imines. This seemed to be valid for any Staudinger method
between ketenes and chiral imines derived from aldehydes.
During the course of this study, a few related methods for the synthesis of β-lactams came to our
attention. Kinugasa reaction, rst reported in 1972, is one of the most interesting methods for synthesis
of β-lactams.32 It involves a [3+2] cycloaddition followed by contraction of the ve-membered intermediate into the four-membered azetidinone ring. The synthetic players in Kinugasa reaction are the copper
(I) acetylides and nitrones (Scheme 7.19). The original report provided an entry to the formation of
cis-azetidin-2-ones in about 60% yields within 1 h. The reaction was carried out in anhydrous pyridine
that acts as both solvent and base at room temperature under inert atmosphere. The role of copper (I)
acetylide is signicant; it helps to promote the cycloaddition reaction like the click reaction as well as
further transformation to isoxazolines (pyrrolidinediones) to azetidin-2-ones.
Two fundamental mechanisms have so far been accepted in favor of Kinugasa reaction. The rst one
was originally proposed by Ding and Irwin33 (Scheme 7.20). Preliminary mechanistic studies were performed by the authors who introduced isotopic labeling (D2O and H
carbonyl oxygen originated from the oxygen of the nitrone; rather, there was no incorporation of 18O in
the product. It was also conrmed that the deuterium incorporation to the C3 was derived from the solvent. However, the shortcoming of the mechanism is the involvement of a highly strained, fused bicyclic
system comprising a three-membered oxaziridine and a four-membered azetidine. They also established
that cis-β-lactams undergo base-catalyzed epimerization at C3 to the thermodynamically more stable
trans-isomer, providing a carbonyl substituent at C3.
The second mechanism involves an intramolecular cyclization via a ketene intermediate.34 This was
further supported by De Shong’s work35 of cycloaddition between a nitrone and trimethylsilylacetylene followed by desilylation with uoride. In both mechanisms, the stereochemical outcome relied on
the initial cycloaddition to form the isoxazoline derivative. This addition xes the conguration at C4,
which, in turn, inuences the stereochemistry at C3. The cis-β-lactam is generated rst as the major diastereomer in most cases. However, epimerization at C3 position leads to trans-isomer, a process dependent upon the reaction conditions, the nature of base used, and also the substituent at C3. For example,
Kinugasa reaction with ethyl prop-2-ynoate produces only the trans-isomer.
A very recent DFT-based theoretical study36 on the mechanism of Kinugasa reaction showed the
involvement of two copper ions leading to the formation of dicopper-acetylide (which acts as the starting
material) (Scheme 7.21). The study advocates the formation of ketene as an intermediate.
2
R
N
O
R
Cu
Epimerization
1
R
2
R
2
O
[3+2]
N
R
1
R
R
Cu
2
1
R
N
R
R
N
O
Protonation
less
from
hindered site
1
R
2
R
R
Cu
18
O) in order to demonstrate that the
2
R
N
O
240 Chemistry and Biology of Beta-Lactams
2
R
L
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H
R
2
O
R
H
N
1
R
Cu
L
R
2
L
R
O
H
N
1
R
R
C
O
HN
2
R
H
1
R
R
2
O
SCHEME 7. 2 1 Alternate mechanism involving ketene intermediate.
R
H
N
1
R
R
O
R
H
N
1
Ligands play a crucial role in determining the product composition. For example, phosphorus-containing ligands such as Ph3P, Bu3P, dppe, and dppp exclusively give trans-β-lactam albeit in poor yield
(<20%), while nitrogen-based ligands such as pyridine and 1,10-phenanthroline give both cis- and trans-
β-lactams (2:1 or 1.2:1) in high yield (55–71%).
37
7.4 Conclusion
Staudinger cycloaddition reaction and a few related processes are investigated extensively for the preparation of various β-lactams. Mechanistically, the pathways are extremely complex, and no generalization can be made. Different types of complex DFT calculations are advanced. These studies are very
carefully done. Based upon the information provided here, scientists may be able to identify the precise
mechanistic route involved in this crucial process in the future.
Acknowledgments
AD is grateful to CEA-Grenoble, Joseph Fourier University, University of Göttingen, and University of
California, Los Angeles, for their support. BKB is grateful to the US NIH, the US NCI, Texas Kleberg
Foundation, Stevens Institute of Technology, University of Texas MD Anderson Cancer Center, University
of Texas-Pan American, and Community Health Systems of Texas for their nancial and moral support
to his research. BKB is grateful to Professor Cossio for his tremendous contribution to this subject.
AD and BKB are also grateful to their current employer, Prince Mohammad Bin Fahd University. The
authors are grateful to Professor Fernando Cossio for his tremendous contribution in this subject.
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