Design and synthesis of a novel series of (1′S,2R,4′S)-3H-4′-azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2′-bicyclo[2.2.2]octanes] with high affinity for the α7 neuronal nicotinic receptor

Article abstract of DOI:10.1016/j.bmcl.2017.10.009

We describe an efficient and convergent synthesis of a series of (1′S,2R,4′S)-3H-4′-azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2′-bicyclo[2.2.2]octanes] displaying potency for the α7 nicotinic acetylcholine receptor (nAChR) and good selectivity vs. the re

Full text of DOI:10.1016/j.bmcl.2017.10.009

Accepted Manuscript
Design and synthesis of a novel series of (1'S,2R,4'S)-3H-4'-azaspiro[ben-
zo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] with high affinity for
the α7 neuronal nicotinic receptor
James Cook, F. Christopher Zusi, Matthew D. Hill, Haiquan Fang, Bradley
Pearce, Hyunsoo Park, Lizbeth Gallagher, Ivar M. McDonald, Linda Bristow,
John E. Macor, Richard E. Olson
PII:
S0960-894X(17)30983-6
https://doi.org/10.1016/j.bmcl.2017.10.009
BMCL 25338
DOI:
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 August 2017
1 October 2017
6 October 2017
Please cite this article as: Cook, J., Zusi, F.C., Hill, M.D., Fang, H., Pearce, B., Park, H., Gallagher, L., McDonald,
I.M., Bristow, L., Macor, J.E., Olson, R.E., Design and synthesis of a novel series of (1'S,2R,4'S)-3H-4'-
azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] with high affinity for the α7 neuronal
nicotinic receptor, Bioorganic & Medicinal Chemistry Letters (2017), doi: https://doi.org/10.1016/j.bmcl.
2017.10.009
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Design and synthesis of a novel series of
(1'S,2R,4'S)-3H-4'-
Leave this area blank for abstract info.
azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-
bicyclo[2.2.2]octanes] with high affinity for the
α7 neuronal nicotinic receptor
James Cook*, F. Christopher Zusi, Matthew D. Hill, Haiquan Fang, Bradley Pearce, Hyunsoo Park,
Lizbeth Gallagher, Ivar M. McDonald, Linda Bristow, John E. Macor and Richard E. Olson

Bioorganic & Medicinal Chemistry Letters
journal homepage: www.els evier.com
Design and synthesis of a novel series of (1'S,2R,4'S)-3H-4'-
azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] with high
affinity for the α7 neuronal nicotinic receptor
James Cook*, F. Christopher Zusi, Matthew D. Hill, Haiquan Fang, Bradley Pearce, Hyunsoo Park, Lizbeth
Gallagher, Ivar M. McDonald, Linda Bristow, John E. Macor, Richard E. Olson
Bristol-Myers Squibb Pharmaceutical Research Institute, 5 Research Parkway, Wallingford, CT 06492-7660, USA
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
We describe an efficient and convergent synthesis of a series of (1'S,2R,4'S)-3H-4'-
azaspiro[benzo[4,5]imidazo[2,1-b]oxazole-2,2'-bicyclo[2.2.2]octanes] displaying potency for
the α7 nicotinic acetylcholine receptor (nAChR) and good selectivity vs. the related 5-HT3A
receptor.
Accepted
Available online
Keywords:
2017 Elsevier Ltd. All rights reserved
.
α7 nicotinic acetylcholine receptor
schizophrenia
5-HT3A receptor
spirooxazolines
Cognitive impairments in patients suffering from
schizophrenia continue to manifest despite treatment with
potential for adverse side effects related to reduced
gastrointestinal motility (constipation) associated with
inhibition of the 5-HT3A receptor,14 selectivity versus this
receptor is desirable.
current antipsychotic medications.1,2
Reduction in
cognitive function in schizophrenics has been linked to
impaired community and social function,3-5 thus, a
treatment for cognitive dysfunction would treat an
important unmet medical need in this disease.6-8
Aromatic Group
H-Bond Acceptor
HN
N
N
Several studies have implicated nicotinic acetylcholine
receptors (nAChRs) in schizophrenia, cognition, sensory
O
N
O
N
N
H
gating, and anxiety.9,10
nAChRs are pentameric
N
N
acetylcholine-gated ion channels expressed throughout the
hippocampus, cortex, and subcortical limbic brain
regions.11 Neuronal nAChRs are formed by several
combinations of α and β subunits or, in the case of the α7
nAChR, as a homopentamer.11 The distribution and
abundance of the α7 and α4β2 subtypes in the brain
regions suggest these subtypes play a role in cognition and
sensory processing deficits.12
1
1a
Basic Amine
Figure 1. Lead spiroimidate from Reference 15. Compound 1 exists in two
tautomeric forms.
Recently, we disclosed a series of conformationally
restricted spirocyclic α7 nAChR partial agonists
exemplified by 115, consisting of a novel spiro-fused
aminooxazoline (spiroimidate) attached to a quinuclidine.
These spiroimidates may exist in two possible tautomeric
forms: one with an endocyclic imine double bond (1,
Figure 1), and one with the exocyclic double bond (1a).
Tautomer 1a was more consistent with the generally
accepted pharmacophoric model for α7 nAChR agonists: a
The α7 nicotinic receptor shares a high sequence
homology with the 5-HT3A serotonergic receptor (5-HT3R),
another ligand-gated ion channel. Many α7 nicotinic full
and partial agonists demonstrate antagonist activity at the
5-HT3A receptor with varying selectivity.13 Due to the

basic amine connected to a H-bond acceptor with a
pendant lipophilic substituent.16-20 We observed that α-aza
heterocycles, such as the isoquinoline in compound 1,
attached to the spiroimidate were required for potency
(Figure 2). Compound 2, which is isosteric but lacks an α-
aza group, showed a 750-fold loss in activity. This trend
was also observed in the case of the pyridine isomers 3
and 4, wherein the 2-aminopyridyl analog possessed
moderate potency while the 3-aminopyridyl isomer
resulted in a 110-fold loss of activity. We hypothesized
that α-aza heterocycles facilitated an intramolecular
hydrogen bond interaction involving the spiroimidate NH
and heterocyclic nitrogen atoms, in turn favoring the
tautomer with the requisite exocyclic double bond as
shown in Figure 1 (compound 1a).
human α7 nAChRs (Ki = 5 nM and 3 nM, respectively)
while being equipotent at the 5-HT3A receptor.21 A similar
motif, PSAB-OFP (8), had similar binding potency for
human α7 nAChR (Ki = 2 nM) with equipotent partial
agonism of the 5-HT3A receptor (Ki = 2 nM).22 The activity
of these spirofuropyridine systems gave us hope that the
simpler and more ligand efficient deannulated version of
benzimidazole 5 (i.e., benzimidazole 6) might provide a
sufficiently potent and novel α7 nAChR agonist.
Figure 4. Examples of known furopyridine α7 nAChR agonists22
Herein, we describe an efficient synthesis to access these
compounds (Scheme 1). Our efforts began with the
synthesis of deannulated spirocycle 6. Borane protected
racemic quinuclidine epoxide 9 was prepared from 3-
quinuclidinone according to literature procedures.22,23
Epoxide 9 was then reacted with 2-halobenzimidazole 10
via treatment with n-butyllithium, which presumably
proceeds via intermediate 11.24 Subsequent deprotection
with HCl afforded the racemic fused compounds.
Individual enantiomers (6, 13-25) were separated using
chiral chromatography.
Figure 2. Regioisomer impact on α7 nAChR activity.
We sought to test this hypothesis by installing a covalent
bond in place of the intramolecular hydrogen bond in our
series, thus locking the imine double bond in the required
exocyclic orientation, as exemplified in Figure 3.
Scheme 1a. Reagents and conditions (a) n-BuLi, THF, -78 oC to 75 oC, R = H,
64%. (b) 3M HCl, then 1N NaOH,
R = H, 92%. (c) chiral SFC
chromatography, R = H, 37% of 13 and 39% of 14. Refer to supplemental
for synthetic details.
We were pleased that in our first attempts, the
unsubstituted benzimidazole 6 (R=H) showed promising
α7 nAChR potency of 11 nM (Table 3) and inactive (>100
µM) at other nicotinic receptors (α1β1δε, α3β4, and α4β2).
Unfortunately, 6 was nearly equipotent at the 5-HT3A
receptor. Potencies for α7 nAChR agonism and 5-HT3A
receptor antagonism were determined by a functional
Figure 3. Replacing the proposed intramolecular hydrogen-bond with a
covalent bond.
The proposed novel imidazo[2,1-b]oxazole 5 shares some
structural similarities with known furopyridine α7 nAChR
agonists (Figure 4). Spirofuropyridine AZD-0328 (7) is
reported to exhibit strong binding potency for the rat and

FLIPR assay as previously described.15
previously demonstrated that careful choice of
We have
C7 and C8 of compound 6 may project along a similar
vector to that occupied by the phenyl ring of isoquinoline
1. However, not all substituents at C7 and C8 afforded
selectivity. Only the 7-CF3 substituted compound (22) and
the bulky 8-p-MeOPh (25) substituent showed significantly
reduced 5-HT3A receptor activity. Although the reasons for
these differences were not clear, it is possible that
electronic effects may be playing a role in addition to steric
factors as the 7-methyl (19) and 7-methoxy (24)
substituents provided minimal selectivity versus the 5-
HT3A receptor.
a
substituents could improve α7 nAChR selectivity against
the 5-HT3A receptor within the our spiroimidate series.
Substituted compounds in our novel series could be
obtained by the same method employed for the
unsubstituted benzimidazole as described in Scheme 1,
however, the products were obtained as mixtures of
regioisomers. The regioisomers were often separated
simultaneously with the chiral separation, although in some
cases the regioisomers were separated by C18
chromatography prior to chiral separation.
Table
1.
α7
nAChR
FLIPR
data
for
substituted
2,3-
dihydrobenzo[4,5]imidazo[2,1-b]oxazole analogs.
8
N
O
7
N
R
N
6
5
FLIPR
Selectivity
Ratio
α7 nAChR
EC50 (nM)a,c
5-HT3A
IC50 (nM)c
Cmpda
R
6
H
H
11
15
1.4×
2.0×
13b
14
6400
268
13000
11
Figure 5. Overlay of 1 (cyan) and 22 (green) rmsd minimized oxazolidine
ring.
5-Cl
0.04×
15
16
17
18
19
20
21
22
23
24
6-Cl
7-Cl
14
31
16
33
5
9
6
0.6×
0.2×
0.3×
0.2×
2.6×
1.0×
0.25×
51×
The single crystal X-ray structure of 22 confirmed the
regiochemistry of the trifluoro substituent (Figure 6).
Additionally, X-ray analysis of
a related dibromo
8-Cl
4
compound (see supplemental information) suggests the R-
stereochemistry for the more active enantiomer.
6-CH3
7-CH3
8-CH3
6-CF3
7-CF3
8-CF3
7-OMe
6
13
7
7
69
13
22
7
17
660
11
11
0.5×
1.6×
25
8-p-OMePh
26
10400
400×
Figure 6. X-ray of 22.
a All compounds are homochiral. b Enantiomer of 6.
c All values are the geometric mean of at least two independent experiments.
In summary, this work supported our hypothesis that the
intramolecular hydrogen bond in our previous spiroimidate
series (compound 1a) could be replaced with a covalent
bond. The resultant novel fused ring system (compound 6)
possessed excellent α7 nAChR potency. Subsequent
optimization led to compound 22 and 25, in which both
demonstrated α7 nAChR potency and selectivity that
rivaled previous leads from our previous spiroimidate
chemotype.15 This novel system may be of interest for
further lead optimization in pursuit of potent and selective
α7 nAChR agonists.
The SAR trends with the 6, 7, and 8-chloro (15, 16, and
17), methyl (18, 19, and 20), and trifluoromethyl (21, 22,
and 23) regioisomers, for the most part, showed good α7
nAChR potency, while 5-chloro regioisomer 14 was
significantly less active (Table 1). These results also
suggested that the optimum α7 nAChR potency and
selectivity resides with substitution at the C7 and C8 of the
benzimidazole ring. This is consistent with our
observations in our spiroimidate series, wherein compound
1 (but not other isoquinoline isomers) showed good α7
nAChR activity and 5-HT3A receptor selectivity. A
computationally derived overlay of compounds 1 (cyan)
and 22 (green, Figure 5)25 suggests that substituents at the
Supporting information

The Supporting Information is available free of charge on
the ACS Publications website at DOI:
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Levin, E.; McClernon, F.; Rezvani, A.
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Gopalakrishnan, S.; Puttfarken, P.; Briggs, C.; Li,
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Jingfang Cutrone and Ed Kozlowski for help with chiral
and regioisomer separations.
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25. Structures 1 and 22 (green) were minimized to
clusters of global low energy conformations.
Conformations were investigated using a mixed
torsional/low mode sampling protocol within
Macromodel [Schrodinger, Maestro, version 2016-
2, OPLS3 FF, water solvent]. Structure 22 (green)
represents a global minimum. The structure 1
(cyan) having the closest conformation to structure
22 by rmsd fit [PE <0.06 kcal/mol from global
minimum] was then further geometry optimized
using Schrodinger’s Jaguar program [6-31G**,
DFT, default settings in water] and overlayed onto
22 [rmsd = 0.02 Å].