Practical and cost-effective manufacturing route for the synthesis of a β-lactamase inhibitor

Article abstract of DOI:10.1021/ol4031606

Compound 1, a potent and irreversible inhibitor of β-lactamases, is in clinical trials with β-lactam antibiotics for the treatment of serious and antibiotic-resistant bacterial infections. A short, scalable, and cost-effective route for the production of this densely functionalized polycyclic molecule is described.

Full text of DOI:10.1021/ol4031606

Letter
pubs.acs.org/OrgLett
Practical and Cost-Effective Manufacturing Route for the Synthesis of
a β‑Lactamase Inhibitor
Steven P. Miller,*,† Yong-Li Zhong,*,† Zhijian Liu,† Michael Simeone,‡ Nobuyoshi Yasuda,†
John Limanto,† Zheng Chen,‡ Joseph Lynch,† and Vincent Capodanno‡
†Process Chemistry, Merck Research Laboratories, P.O. Box 2000, Rahway, New Jersey 07065, United States
‡Chemical Process Development and Commercialization, Merck Manufacturing Division, P.O. Box 2000, Rahway, New Jersey 07065,
United States
S
* Supporting Information
ABSTRACT: Compound 1, a potent and irreversible
inhibitor of β-lactamases, is in clinical trials with β-lactam
antibiotics for the treatment of serious and antibiotic-resistant
bacterial infections. A short, scalable, and cost-effective route
for the production of this densely functionalized polycyclic
molecule is described.
he emergence of multidrug resistant Gram-negative
Tbacteria has created an increasingly unmet medical need
for new therapies to combat infectious diseases.1 Not only are
these bacteria associated with severe morbidity and mortality,2
treatment of such infections has also become more difficult as
resistance strengthens.3 That being said, β-lactamase inhibitors
have been demonstrated to restore the efficacy of antibacterials
against pathogens that have evolved immunity to treatment
over time by disrupting the central mode of bacterial resistance,
namely the production of β-lactamases.4 In this regard, MK-
76555 (1), a potent and irreversible inhibitor of β-lactamases, is
being clinically evaluated in conjunction with β-lactam
antibiotics against severe bacterial infections. Although
syntheses of this compound have been reported previously by
our laboratories,6 a short, cost-effective, safe, and scalable
manufacturing route to support the late-stage drug-develop-
ment program is desired.
In order to develop a desired commercial manufacturing
route, we identified key aspects of the prior synthetic route that
we sought to address (Figure 1). First, the conversion of 5 to 6
required optimization (Figure 1, eq 1).7 Second, we needed to
access intermediate 5 with the desired nitrogen protecting
group in a more expedient and efficient manner (Figure 1, eq
Figure 1. Issues to address in the synthesis of 1.
2). Third, the final deprotection step involves a hazardous
solvent and generated a significant amount of inorganic
with the inexpensive, commercially available amine 11. Lactone
3 can be prepared in a straightforward manner by taking
advantage of the recent availability of optically pure cis-5-
hydroxypipecolic acid 4 from enzymatic oxidation of pipecolic
acid.9
We began our studies by optimizing the hydroxyamine
nucleophile (Table 1). When this partner bore a Boc group, as
it had in the previous route, high conversion was observed but
byproducts that afforded only a moderate yield of desired
product (Figure 1, eq 3).8
Toward this end, we envisioned a synthetic process that
continued to leverage the formation of the bicyclic urea 9 at a
late stage but installed the desired Boc protecting group at the
beginning (Figure 2). This would avoid the need for a
cumbersome protecting group switch during the hydrogenative
removal of the benzyl protecting group. Intermediate 10 would
be derived from displacement of activated alcohol 2 with an
appropriately functionalized hydroxyamine, where 2 would
originate from the nucleophilic opening of protected lactone 3
Received: November 1, 2013
Published: December 13, 2013
© 2013 American Chemical Society
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Letter
Our next task was to determine the best way to remove both
the 2-Ns and 4-Ns protecting groups from compound 13
(Table 2). Initially, we attempted a global deprotection using
Table 2. Optimization of Global Deprotection Conditions
a
entry
Nu
base
solvent
temp (°C) 10:10A
Figure 2. Retrosynthesis of 1.
1
2
3
4
HSCH2CO2H
PhSH
LiOH
K2CO3
LiOH
LiOH
LiOH
LiOH
K2CO3
Cs2CO3
DBU
DMF
DMF
20
50
20
50
20
50
50
50
50
20
1.6:1
1.8:1
Table 1. Optimization of Sulfonate Group on the
Nucleophile
PhSH
DMF
1:4.3
HSCH2CO2H
HSCH2CO2H
HSCH2CO2H
HSCH2CO2H
HSCH2CO2H
HSCH2CO2H
HSCH2CO2H
THF
1.6:1
b
5
MeCN
MeOH
MeOH
MeOH
MeOH
MeOH
6
10:1
97:3
97:3
4:1
7
8
a
b
9
entry
R
time (h)
temp (°C)
conversion (%)
yield
10
K2CO3
99:1
b
1
2
3
4
Boc
Ts
6
24
72
72
40
40
60
60
>98
>98
63
70
68
55
88
a
The ratio was measured by HPLC at 210 nm wavelength. No
desired product was detected.
2-Ns
4-Ns
>98
a
conditions reported by Fukuyama11 (entry 1) and Maligres
(entry 2).12 Unfortunately, significant amounts of a byproduct
(10A, Table 2)13 formed from displacement of the 4-nitro
group with the thiol were observed, which was consistent with
Wuts’ prior observation.14 A small survey of solvent, base,
nucleophile, and temperature showed no improvement with
regards to byproduct formation (entries 3−5). However, when
methanol was used as the solvent in combination with LiOH
and thioglycolic acid, we achieved a much improved ratio of
desired product to byproduct (entry 6). A subsequent screen of
bases and temperature (entries 7−10) identified the optimal
reaction conditions that provided a 99:1 ratio of desired
product to byproduct with very clean HPLC profile (entry 10).
An important observation from the new protocol is that it
requires anhydrous conditions as contamination with water
generated significant amount of byproduct 10A.
The scope of this new protocol was investigated and cleavage
of 4-nitrobenzenesulfonamides under the described conditions
was found to be both chemoselective and high yielding (Table
3). Sulfonamides of acyclic secondary amines (entries 1 and 2,
Table 3), cyclic secondary amines (entries 3−6, Table 3), and a
primary amine (entry 7, Table 3) performed well, yielding the
corresponding amines in greater than 95% isolated yield and
greater than 97:3 selectivity versus the byproduct. These results
are notable given the reported observation that cyclic amines
are prone to give the thiolate addition byproduct in up to 34.5%
yield and the reported amount of byproduct for entries 1 and 2
were 3.4% and 7.5%, respectively.14
Unless otherwise noted, reactions carried out with 1.2 molar equiv of
b
base and nucleophile and a reaction concentration of 0.35 M. Yields
determined by HPLC.
only marginal formation of the desired material (entry 1) was
observed, as a significant amount of an elimination byproduct
was formed. The tosyl group provided similar results (entry 2).
We postulated that using an electron-poor group like 2-
nitrobenzene-1-sulfonyl (2-Ns) could reduce the basicity of the
nucleophile, and indeed, a more selective, albeit slower,
reaction was observed in this case (entry 3). Reducing the
steric bulk of the nucleophile by choosing the 4-nitrobenzene-
1-sulfonyl (4-Ns) group provided the optimal balance between
reactivity and basicity, as the desired product was obtained in
88% assay yield, although the reaction required 72 h to
complete (entry 4).
Given the success of the 2-Ns and 4-Ns groups10 on the
nucleophile used in the displacement reaction, we hypothesized
that sulfonate groups of this type might also be beneficial as
both the leaving group and nitrogen protecting group on the
piperidine in the displacement reaction. This strategy would
hopefully enhance the speed of the reaction and unify the
protecting group strategy for a potential one-step deprotection
downstream. Pleasingly, compound 2 underwent the desired
nucleophilic displacement in 90% HPLC assay yield in 24 h (eq
4).
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Table 3. Deprotection of 4-Nitrobenzenesulfonamides
Scheme 1. Synthesis of Intermediate 10
a
b
cyclization to form urea 9 in 92% yield followed by
hydrogenative removal of the benzyl group in 90% yield.
Previously, the Boc removal with HBF4 required aqueous
treatment for isolation of 1 due to the significant amount of
inorganic byproducts formed. To avoid this tedious handling
and troublesome isolation, TMS-I was used for the removal of
the Boc group following sulfation of 16. All byproducts and
impurities were soluble in the organic phase and 1 was
crystallized as the monohydrate upon direct addition of 3
equivalents of water to the reaction and isolated in 93% yield
over two steps.
In summary, we have developed a highly efficient and
scalable manufacturing route to the β-lactamase inhibitor MK-
7655 (1) and a practical protocol for the highly selective
removal of 4-nitrobenzenesulfonamide groups. This route
provides pure, crystalline 1 in 8 steps and 42% overall yield,
allowing for a more cost-effective and productive manufacture
The ratio was measured by HPLC at 210 nm wavelength. Byproduct
c
is thiolate addition to the nitro carbon of 4-nosylate. Isolated yields
based on an average of at least two experiments.
With the optimized nucleophilic displacement and depro-
tection conditions in hand, we began our synthetic route with
the protection of amine 4, which is commercially available on
large scale15 (Scheme 1). Utilizing 2-NsCl, the desired N-
protection, subsequent lactonization via activation of the
carboxylic acid with 2-NsCl, and crystallization from ethyl
acetate proceeded in 79% isolated yield to provide lactone 3 in
>99% purity. Given that input starting material 4 was only 77%
pure,16 this upgrade in purity was an important achievement.
The choice of 4 as the starting material obviated the need to
use an expensive iridium catalyst to cyclize the piperidine ring,
as in the previously reported synthesis,6 and avoided the
multiple resin purifications and recrystallizations needed on
scale to reduce residual iridium to pharmaceutically acceptable
levels.17 Lactone 3 was then treated with the commercially
available Boc-protected aminopiperidine 11 in THF, followed
by reaction with 2-NsCl and DMAP in one-pot. By choosing to
pass through intermediate lactone 3 to form the amide bond,
we did not need to use typical coupling reagents (e.g., EDC,
CDI, T3P, etc.), allowing for a cleaner reaction profile and a
more cost-effective and environmentally benign process.
Crystallization from methanol and water gave desired sulfonate
ester 2 in 98% overall isolated yield from 3.
Scheme 2. Completion of the Synthesis of 1
This process constitutes a two-step synthesis of 2 from
commercially available inputs in high yield and purity, while the
manufacture of the respective intermediate in the previous
route required six isolated steps. Combination of the
subsequent displacement and deprotection steps afforded a
through process from which crystalline OBn-hydroxyamine 10
was isolated in 70% yield with >97.5% HPLC purity.
Completion of the synthesis involved triphosgene-mediated
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Letter
of the desired compound. In fact, aspects of this route have
already been successfully demonstrated on multikilogram scale
allowing for the continued support of the development of 1 as a
cotherapy in the treatment of antibacterial resistant infections.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, compound characterization, and
1
copies of H and 13C NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: steven.miller@merck.com.
*E-mail: yongli_zhong@merck.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the following individuals from Merck
Manufacturing Division for analytic support: Keri McNamara,
Jason Kowalski, and Yadan Chen. We acknowledge the
following individuals from Merck Research Laboratories for
analytic support: Hao Luo, Li Zhang, Ping Zhuang, Tom
Novak, Billy Chen, and Leo Joyce. We also acknowledge Bob
Reamer for NMR support and Jeff Moore, Victoria Sprague,
and Birgit Kosjek for project support.
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