A highly efficient asymmetric synthesis of vernakalant

Article abstract of DOI:10.1021/ol501002a

A novel synthesis of vernakalant is described. Using inexpensive and readily available reagents, the key transformations involve (1) an efficient zinc-amine-promoted etherification, (2) a highly stereoselective enzyme-catalyzed dynamic asymmetric transamination to set up the two contiguous chiral centers in the cyclohexane ring, and (3) a pyrrolidine ring formation via alkyl-B(OH)₂-catalyzed amidation and subsequent imide reduction.

Full text of DOI:10.1021/ol501002a

Letter
pubs.acs.org/OrgLett
A Highly Efficient Asymmetric Synthesis of Vernakalant
John Limanto,* Eric R. Ashley,* Jingjun Yin,* Gregory L. Beutner, Brendan T. Grau, Amude M. Kassim,
Mary M. Kim, Artis Klapars, Zhijian Liu, Hallena R. Strotman, and Matthew D. Truppo
Department of Process Chemistry, Merck Research Laboratories, Merck & Co., Inc., Rahway, New Jersey 07065, United States
S
* Supporting Information
ABSTRACT: A novel synthesis of vernakalant is described. Using
inexpensive and readily available reagents, the key transformations
involve (1) an efficient zinc-amine-promoted etherification, (2) a
highly stereoselective enzyme-catalyzed dynamic asymmetric
transamination to set up the two contiguous chiral centers in the
cyclohexane ring, and (3) a pyrrolidine ring formation via alkyl-
B(OH)2-catalyzed amidation and subsequent imide reduction.
trial fibrillation (AF), which affects 1−2% of the
of Cs2CO3 gave mostly Favorskii rearrangement product 3
(Table 1, entry 1).7
A
population, is the most common cardiac arrhythmia
leading to 20% of all strokes.1 While the exact mechanism of AF
remains unknown, significant development efforts on antiar-
rhythmic drugs for AF have focused on achieving high atrial
selectivity.2 Vernakalant, discovered by Cardiome Pharma
Corp., was developed as a novel antiarrhythmic agent for the
treatment of recent-onset AF via atrial-selective inhibition of
the Na- and K-channel currents.3 The compound has been
demonstrated to be effective, safe, and well-tolerated in various
clinical studies and has gained approval by the European agency
(EMEA) for the intravenous conversion of AF to sinus rhythm
under the trade name of Brinavess.
While various routes to the chiral aminocyclohexyl ether core
of vernakalant have previously been reported, most employ a
classical resolution/separation of a racemic intermediate or
utilize an expensive chiral starting material.4 Our asymmetric
approach to vernakalant is illustrated in Scheme 1. We
a
Table 1. α-Etherification Optimization
c
temp
(°C)
yield
(%)
b
entry
base
additives
solvent
1:3 ratio
1
2
3
4
5
6
7
Cs2CO3 none
THF
THF
THF
THF
PhCF3
PhCF3
THF
60
60
60
45
85
85
45
<1:100
45
<1
27
14
23
46
66
Et3N
none
d
Et2Zn
none
16:1
>30:1
>30:1
>30:1
>30:1
e
DIPEA
none
Zn(OTf)2
e
e
pyrrolidine
pyrrolidine
e
Et3N
e
DIPEA
DIPEA
DIPEA
Zn(OTf)2,
d
pyrrolidine
Scheme 1. Retrosynthetic Analysis of Vernakalant
e
g
8
9
Zn(OTf)2,
PhCF3
45
>30:1
>30:1
68
(S)-MMP ,
f d
d
d
ZnCl2,
pyrrolidine
PhCH3
100
94
h
a
b
c
All reactions were run for 16−24 h. 2 equiv. Estimated assay yields
d
e
f
by HPLC. 1 equiv. 0.4 equiv. MMP = 2-(methoxymethyl)-
pyrrolidine. Achieved 53% ee. 0.2 equiv.
g
h
A more comprehensive screening of inorganic bases such as
KOAc, KHCO3, KF, K2CO3, and K3PO4 and solvents such as
THF, toluene, and dichloromethane failed to improve the
reaction. Organic bases, such as Et3N, afforded <1% conversion
(Table 1, entry 2). Interestingly, the use of Et2Zn as a base8
gave the desired ether 1 as the major product in THF (Table 1,
envisioned accessing the active pharmaceutical ingredient
(API) through hydroxypyrrolidine ring formation from chiral
amine 2, which could be, in turn, derived from racemic ketone
1 via an enzyme-catalyzed dynamic asymmetric−transamina-
tion (DA-TA) process. The ketone 1 could then be prepared
from inexpensive and readily available α-chlorocyclohexanone.
The α-etherification of α-chloroketones with an aliphatic
alcohol nucleophile has not been reported,5 largely due to the
tendency of such compounds to undergo a Favorskii rearrange-
ment.6 Not surprisingly, our initial experiment showed that
exposure of α-chlorocyclohexanone to alcohol 4 in the presence
entry 3). Similarly, a combination of Zn(OTf) and Hunig’s
̈
2
base also afforded the desired product, albeit in low yields
Received: April 4, 2014
Published: May 1, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
(Table 1, entry 4). Further attempts to improve the reactivity
and selectivity failed with other metal salts, including Li, Mg,
and Cu salts with or without Pd, Ir, Ni, or Cu catalysts.
aminase.16 After only one round of evolution, a variant was
identified that exhibited a reverse in diastereoselectivity (entry
6). The desired trans-isomer product 2 was obtained with
excellent enantioselectivity and a >95:1 trans:cis ratio after
three rounds of evolution (entries 6−8). The enzyme was
simultaneously evolved to increase its reactivity/selectivity and
tolerance to high pH.
Subsequent studies revealed that the enzymatic DA-TA step
is sensitive to certain impurities present in the starting material.
The presence of either α-hydroxy- or α-chlorocyclohexanone
(>5 wt %) in crude 1 gave lower conversions (50−79%)
relative to purer materials (<1 wt %).17 After screening
solvents, pH buffers, and enzyme loadings, the optimal
conditions were to treat a solution of the substrate in a 1:9
mixture of DMSO/H2O with 5 wt % of ATA-303, 1 wt % of
PLP cofactor, and 1 M of i-PrNH2 at 45 °C, while maintaining
a basic pH of 10. Under these conditions, the desired amine 2
was obtained in 85% assay yield, 95:1 dr, and >99.5% ee. While
several crystalline salts of the product were identified, isolation
of the product as the crystalline D-malate salt is preferred
because it offers chemical and physical stability and allows for a
purity upgrade to 99.6:0.4 dr in 81% yield from ketone 1
(Scheme 2). Additionally, the D-malate salt could also be
conveniently and directly used in subsequent pyrrolidine ring
formation.
Concurrently, in an attempt to promote this reaction via
activation of the ketone with an organoamine catalyst, we
screened various primary (1°), secondary (2°), and tertiary
(3°) amines and found that pyrrolidine can provide the desired
product with little rearrangement (Table 1, entries 5 and 6).9
Additionally, the combination of Zn and pyrrolidine also
provided the product in good yields in THF while preserving
high chemoselectivities (Table 1, entry 7). Interestingly, using
(S)-2-methoxymethylpyrrolidine as a chiral catalyst in PhCF3
gave the desired product in 53% ee (Table 1, entry 8). As the
subsequent step does not require an optically pure product, we
focused our optimization efforts on nonstereoselective
conditions. Our optimal conditions were to perform the
reaction in toluene at 100 °C in the presence of 20 mol % of
pyrrolidine and stoichiometric amounts of both ZnCl2 and
Hunig’s base. Under this protocol, compound 1 was obtained
̈
in 94% assay yield (Table 1, entry 9) and was used directly in
the next step without isolation.
With keto ether 1 in hand, we next evaluated generation of
the pyrrolidinol ring. Unfortunately, direct reductive amination
of 1 with 3-hydroxypyrrolidine under various reaction
conditions afforded poor trans/cis diastereoselectivity.10 We
hypothesized that a dynamic asymmetric-transamination (DA-
TA), on the other hand, would allow for a stereoselective
introduction of an amino functionality that could be further
elaborated. While TA of ketones using selectively engineered
transaminases has been well-established,11 no literature on the
dynamic variant,12 involving α-substituted ketones, where two
contiguous stereocenters could be simultaneously set, had been
reported at the start of our work in 2009.13
a
Scheme 2. Optimized DA-TA Conditions
The initial DA-TA screening was performed with available
transaminase variants14 under known reaction conditions
(using pyridoxal 5′-phosphate (PLP) cofactor and i-PrNH2 as
the nitrogen source), while maintaining basic pH (>9) to
promote equilibration of starting enantiomers.15 Unfortunately,
while high enantioselectivities were obtained, the reactions
afforded predominantly the cis-diastereomer 5 (entries 1−5,
Table 2). A combination of in silico design and directed
evolution was subsequently carried out on ATA-013 trans-
a
The reaction typically takes 24 h, and partial removal of acetone
byproduct might be necessary to drive the conversion further.
Our initial efforts to generate the pyrrolidine ring relied on
double alkylation of amine 2 with an appropriate bis-
electrophile. Subjection of amine 2 to either (R)-4-bromo-
1,2-butylene epoxide or (R)-1,4-dibromo-2-butanol18 under
basic conditions afforded the API in 78−84% assay yields
(Scheme 3). While this approach is convergent, the high risk of
a
Table 2. Enzyme-Catalyzed Dynamic-TA Optimization
a
Scheme 3. Bis-alkylation Approach to API
b
entry
transaminase variant
ATA-007
2:5 ratio
1
2
3
4
5
6
7
8
1:5
1:1.2
1:8
ATA-013
ATA-018
a
The reaction typically takes 24 h with 1.1 equiv of bis-electrophiles
ATA-024
1:9
and 2.0 equiv of base at 80 °C.
ATA-032 (CDX-017)
ATA-013 Evolution 1
ATA-013 Evolution 2
ATA-013 Evolution 3 (ATA-303)
1:12
4:1
14:1
>95:1
potential genotoxic impurity (PGI) contamination from the
alkylating agents or reaction intermediates was not deemed
amenable for a commercial process.
We subsequently evaluated a stepwise approach by first
generating succinimide intermediate 6 and then performing an
exhaustive reduction to access vernakalant. Standard amide
a
Screening conditions: 1:9 DMSO/H2O, 10−100 wt % enzyme
loading, pH = 10.5, 45−50 °C, 1 mL scale, 5 g/L ketone, 1 g/L PLP, 1
M i-PrNH2 Ratio determined by HPLC; in all cases, >99.5% ee of 2
was obtained.
b
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Organic Letters
Letter
coupling conditions, including the use of catalytic boric acid19
Scheme 4. Endgame Chemistry: Exhaustive Reduction of
Imide
or aryl boronic acids (Table 3, entry 1),20 proved ineffective for
Table 3. Formation of Penultimate Imide Intermediate
TA of an α-substituted ketone (not β-keto esters), affording the
corresponding trans-amino product in high enantio- and
diastereoselectivities.
a
b
c
entry
R1
solvent
x
% conv to 7
% yield of 6
d
1
2
3
4
5
6
7
8
Ar
i-PrOAc
i-PrOAc
n-BuOAc
n-BuOAc
n-PrOAc
n-PrOAc
n-PrOAc
n-PrOAc
20
20
20
20
30
20
10
5
8−15
8
n.d.
n.d.
85
ASSOCIATED CONTENT
* Supporting Information
■
OH
e
S
1°-Alk
98−99
82
isopropyl
n-butyl
n-butyl
n-butyl
n-butyl
n.d.
n.d.
n.d.
n.d.
Experimental procedures and characterization of new com-
pounds, including their NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
f
82
f
98
f
99
AUTHOR INFORMATION
Corresponding Authors
99
85 (77)
■
a
b
c
Reaction volume = 10−20 mL/g. Conversions after 2 h. Assay
yields; the number in parentheses correspond to isolated yield; n.d. =
not determined. Ar = Ph, 2-Br-Ph, 2-I-Ph, 3,4,5-F3-Ph. 1°-Alk =
methyl, ethyl, n-butyl. 6 h data point.
*E-mail: john_limanto@merck.com.
*E-mail: eric_ashley@merck.com.
*E-mail: jingjun_yin@merck.com.
d
e
f
Notes
the amidation. However, excellent conversion was observed
when 1° alkyl-B(OH)2 catalysts were used (entry 3).21 Due to
its low cost and wide availability, n-butyl-B(OH)2 was selected
for further development. Interestingly, both turnover number
and frequency improved with lower catalyst loadings (entries
5−8), observations that are consistent with a trimeric boroxine
catalyst resting state22 and multiple order catalyst degrada-
tion.23 Further optimization led to the use of 5 mol % of n-
butyl-B(OH)2 in refluxing n-propyl acetate under azeotropic
drying, providing a 99% conversion to amides 7 in 2 h.
The cyclization of amides 7 to imide 6 could be effected by
using a combination of ZnCl2 and HMDS.24 Importantly, this
transformation was efficient even on the crude amidation
reaction mixture and therefore, we developed a one-pot process
for the conversion of 2-malate to 6. Once n-butyl-B(OH)2
catalyzed amidation was completed, the mixture was cooled to
70 °C, treated with HMDS and ZnCl2, and aged for 6 h to
afford penultimate 6 in 85% assay yield and 77% isolated yield
after crystallization (Table 3, entry 8).
With compound 6 in hand, we next investigated the
exhaustive imide reduction step. We found that the optimal
transformation involved subjecting imide 6 in THF to NaBH4
(3 equiv) and B(OMe)3 (1 equiv), followed by BF3·Et2O (4
equiv) to afford the product in high conversions.25 Upon
treatment with HCl in IPA, vernakalant was isolated as a
crystalline HCl salt in 97% yield and 99.5% purity (Scheme 4).
In summary, we have identified and developed a highly
efficient route to vernakalant starting from readily available and
inexpensive starting materials. This approach, which involves
three novel transformations, including a ZnCl2 and pyrrolidine-
mediated α-etherification, an enzymatic dynamic asymmetric-
transamination (DA-TA) of an α-substituted ketone, and an
alkyl-B(OH)2-promoted amidation, provides vernakalant in five
steps and 56% overall yield. To the best of our knowledge, this
work represents the first reported example of an enzymatic DA-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the following individuals from Merck: Bob Reamer,
Peter Dormer, and Lisa DiMichele for their continuous
assistance with NMR experiments, Yuri Bereznitski for
analytical support, Ian Mangion for optical rotation measure-
ment, and Cheol Chung, Paul Devine, Greg Hughes, and Dave
Tschaen. We also thank Cardiome Pharma and Codexis for
very helpful discussions.
REFERENCES
■
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Organic Letters
Letter
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