Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 2′-fluoro-3′-(substituted pyridinyl)-7- deschloroepibatidine analogues

Article abstract of DOI:10.1021/jm401602p

2′-Fluoro-3-(substituted pyridine)epibatidine analogues 7a-e and 8a-e were synthesized, and their in vitro and in vivo nAChR properties were determined. 2′-Fluoro-3′-(4″-pyridinyl)deschloroepibatidine (7a) and 2′-fluoro-3′-(3″-pyridinyl)deschloroepibatidi

Full text of DOI:10.1021/jm401602p

Article
pubs.acs.org/jmc
Synthesis, Nicotinic Acetylcholine Receptor Binding, and
Antinociceptive Properties of 2′-Fluoro-3′-(substituted pyridinyl)-7-
deschloroepibatidine Analogues
Pauline W. Ondachi, Ana H. Castro, Jakub M. Bartkowiak, Charles W. Luetje, M. Imad Damaj,§
†
‡
‡,⊥
‡
†
†
,†
́
S. Wayne Mascarella, Hernan A. Navarro, and F. Ivy Carroll*
†Center for Organic and Medicinal Chemistry, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina
7709, United States
2
‡
Department of Molecular and Cellular Pharmacology, Miller School of Medicine, University of Miami, Miami, Florida 33101, United
States
§
Department of Pharmacology and Toxicology, Virginia Commonwealth University Medical Campus, P.O. Box 980615, Richmond,
Virginia 23298, United States
*
S Supporting Information
ABSTRACT: 2′-Fluoro-3-(substituted pyridine)epibatidine analogues 7a−e and 8a−e
were synthesized, and their in vitro and in vivo nAChR properties were determined. 2′-
Fluoro-3′-(4″-pyridinyl)deschloroepibatidine (7a) and 2′-fluoro-3′-(3″-pyridinyl)-
deschloroepibatidine (8a) were synthesized as bioisosteres of the 4′-nitrophenyl lead
compounds 5a and 5g. Comparison of the in vitro nAChR properties of 7a and 8a to
those of 5a and 5g showed that 7a and 8a had in vitro nAChR properties similar to those
of 5a and 5g but both were more selective for the α4β2-nAChR relative to the α3β4- and
α7-nAChRs than 5a and 5g. The in vivo nAChR properties in mice of 7a were similar to
those of 5a. In contrast, 8a was an agonist in all four mouse acute tests, whereas 5g was active only in a spontaneous activity test.
In addition, 5g was a nicotine antagonist in both the tail-flick and hot-plate tests, whereas 8a was an antagonist only in the tail-
flick test.
INTRODUCTION
example, nicotine interacts with α4β2-, α4β2α6*-, α4β2α5*,
and α7-nAChR in the dopaminergic mesolimbic pathway, a
brain system thought to mediate the pleasurable and rewarding
■
Tobacco use continues to be the leading cause of preventable
deaths in the United States as well as globally. Current statistics
reveal that smoking-related diseases are responsible for nearly 6
million premature deaths globally annually. In the United
States, tobacco use is responsible for nearly 1 in 5 deaths that is
5
effects of most substances of abuse, including nicotine. In
1
addition, currently, the few treatments for nicotine dependence
include nicotine replacement therapies (NRT); the antide-
6
,7
2
pressant buproprion (2), which acts as a dopamine uptake
approximately 443 000 premature deaths. In 2010, an
inhibitor in addition to its properties as a nicotinic antagonist of
estimated 23% (58.3 million) of U.S. adults were current
8
3
α3β4- and α4β2-nAChRs; and the FDA-approved vareni-
cigarette smokers. Tobacco use increases the risk of multiple
9
,10
cline (3), which acts as a partial nicotine agonist at the α4β2
cancers such as cancers of the lung, mouth, nasal cavities,
larynx, pharynx, esophagus, stomach, colorectum, liver,
pancreas, kidney, bladder, uterine, cervix, ovary, and myeloid
cells. Therefore, smoking accounts for at least 30% of all cancer
11
and a full agonist at the α3β4- and α7-nAChRs. In addition,
varenicline has affinity for α6β2*-nAChR equal to that at α4β2-
nAChR, but functionally varenicline was more potent in
3
4
stimulating α6β2* versus α4β2* mediated [ H]dopamine
deaths and 87% of lung cancer deaths.
12
release from rat striatal synaptosomes. However, side effects
such as gastrointestinal disturbances (nausea and vomiting) and
neuropsychiatric effects (trouble sleeping, unusual dreams,
violent or suicidal ideation) were frequently reported with the
use of varenicline. In addition, recent evidence suggests that
varenicline produces increased risk of heart attack, stroke, and/
Due to the well-documented negative heath consequences,
approximately 70% of smokers want to quit and about 40% try
to quit every year. Of those who try to quit, only about 7% stay
off nicotine for more than a year. The vast majority do not
make it even a week without cigarettes. The major factor that is
attributed to the initiation and sustaining of smoking is the
presence of nicotine (1), the addictive substance in tobacco.
Nicotine can produce a myriad of behavioral effects and is
unquestionably one of the most popular and powerful
reinforcing agents. Both the psychological and physiological
effects of tobacco smoke are a result of nicotine’s activation of
various nicotinic acetylcholine receptor (nAChR) subtypes. For
13
or other cardiovascular problems. Therefore, there is need for
development of new and improved pharmacotherapies for
smoking cessation.
Received: October 15, 2013
©
XXXX American Chemical Society
A
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Chart 1. Structures of Compounds 1−4, 5a−g, 6, 7a−e, and 8a−e
a
Scheme 1
a
Reagents and conditions: (a) Pd(OAc) , P(o-tolyl) , pyridinyl boronic acid, Na CO , DME, H O, 80 °C, 5 h for the preparation of 11b, 11c, 12b,
2
3
2
3
2
and 12c; (b) pyridinyl boronic acid, Pd(PPh ) , K CO , toluene, EtOH, H O, reflux, 24 h for the preparation of 11a, 11d and 12a; (c) 70% HF−
3
4
2
3
2
pyridine, NaNO ; (d) 2-aminopyridine-5-pinacol boronic ester, Pd(PPh ) , K CO , 1,4-dioxane, H O, 110 °C, 18 h.
2
3
4
2
3
2
The natural alkaloid epibatidine (4, exo-2-(2′-chloro-5′-
pyridinyl)-7-azabicyclo[2.2.1]heptane) is an important lead
structure in the development of pharmacotherapies for treating
nicotine addiction as well as other central nervous system
lot of interest because of its very high affinity for the α4β2*-
16,17
nAChRs.
In previous studies, we reported the synthesis,
nAChR binding affinity, and pharmacological properties of a
1
8,19
number of epibatidine analogues.
Interestingly, some
(
CNS) disorders including Alzheimers and Parkinson’s
analogues retained high affinity for nAChR but unlike
diseases, pain, schizophrenia, anxiety, depression, and Tour-
epibatidine showed no agonist activity in the acute mouse
14
ette’s syndrome among others. Since its isolation and
antinociception test and were antagonists of nicotine-induced
15
18−20
structural determination in 1992, epibatidine has drawn a
antinociception in these assays.
For example, we identified
B
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a
Scheme 2
a
Reagents and conditions: (a) bis(pinacolato)diboron, Pd dba (3 mol %), XPhos (16 mol %), 1,4-dioxane, 110 °C, 4 h; (b) Pd dba (3 mol %),
2
3
2
3
K PO (2.3 equiv), 1,4-dioxane, 110 °C, 18 h.
3
4
2
′-fluoro-3′-(4-nitrophenyl)deschloroepibatidine (5a), also re-
fluoropyridine-5-boronic acid, 2-fluoropyridine-4-boronic acid,
2-chloropyridine-5-boronic acid, and 2-chloropyridine-4-bor-
onic acid with the 2′-amino-3′-bromo compound 9, carried out
in the presence of palladium diacetate, tri-(o-tolyl)phosphine,
and sodium carbonate, heated at 80 °C in 1,2-dimethoxyethane
and water for 5 h furnished the bipyridine intermediates 11b,
ferred to as RTI-7527-102 and 4-nitro-PFEB, as an nAChR
ligand with a K value of 0.009 nM for inhibition of
i
3
[
H]epibatidine binding. This compound also showed potent
antagonism of nicotine-induced antinociception in the tail-flick
21
and hot-plate tests in mice. In a separate study, we showed
that 5a was a competitive antagonist of human α4β2-nAChRs
with a potency 17-fold higher than that of dihydro-β-
erythroidine (6) with very low efficacy at α3β4- and α7-
27
11c, 12b, and 12c. Suzuki cross-coupling of pyridine-4-
boronic acid, pyridine-3-boronic acid, and 2-methoxypyridine-
5-boronic acids with 9 in the presence of tetrakis-
(triphenylphosphine) palladium(0) as the catalyst, potassium
carbonate as the base, and toluene (15 mL), ethanol (1.5 mL),
and water (1.5 mL) as solvents and heating at reflux for 24 h in
a sealed tube provided the cross-coupled products 11a, 11d,
and 12a in good yields. Conversion of the amino group to the
fluoro group along with a concomitant removal of the tert-
butyloxycarbonyl protecting group in the intermediates 11a−d
and 12a−c performed through the diazotization reaction with
sodium nitrite in the presence of hydrogen fluoride in pyridine
(70%) furnished the products 8a−e and 7a−c. Compound 8d
was synthesized by subjecting 2-exo-(2′-fluoro-3′-bromo-5′-
2
2
nAChRs. In a more recent study, the α4β2-nAChR antagonist
a attenuated the discriminative stimulus effects of nicotine,
5
reduced nicotine’s ability to facilitate intracranial self-
stimulation (ICSS), blocked conditioned place preference
(CPP) produced by nicotine in mice, and dose-dependently
23
blocked nicotine self-administration in rats. Thus, 5a has both
in vitro and in vivo properties thought to be favorable for a
potential pharmacotherapy to treat smokers. However, the
presence of a nitro-substituted phenyl group, a system that is
associated with toxicity via partial reduction in vivo to the
hydroxylamine, which can undergo metabolic activation to an
electrophilic nitroso species of 5a, raises concern about its
future development. In a recent study, we reported that
replacement of the 4-nitro group in 5a by other strong
electron-withdrawing groups led to compounds 5b−g that
retained high affinity for α4β2-nAChRs and potent antagonist
2
1
pyridinyl)-7-azabicyclo[2.2.1]heptane 10 to a Suzuki−
Miyaura cross-coupling with 2-aminopyridine-5-pinacol boronic
ester in the presence of tetrakis(triphenylphosphine) palla-
dium(0), potassium carbonate, 1,4-dioxane, and water, heated
at 110 °C in a sealed tube overnight. The reaction furnished the
diamine 8d in a 67% yield (Scheme 1).
24
activity in the tail-flick test.
In this study, we report the synthesis, nAChR binding, and
pharmacological properties of compounds 7a−e and 8a−e.
Compound 7a is a bioisosteric analogue of 5a where the
nitrophenyl group has been replaced by a pyridine nitrogen.
Compound 8a is a similar bioisosteric analogue of 5g, a
The synthesis of the 2-exo-[2′-fluoro-3′-(2-aminopyridin-4-
yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptane 7d, was accom-
plished in a “one-pot” reaction that combined the borylation
and the Suzuki−Miyaura steps (Scheme 2). The borylation
reaction was accomplished using Buchwald’s dialkylphosphino-
biphenyl ligand, 2-dicyclohexylphosphino-2′,4′,6′-triisopropyl-
compound that has a K value of 0.053 nM of affinity for
i
3
inhibition of [ H]epibatidine binding and AD values of 0.5
biphenyl (XPhos), and tris(dibenzylideneacetone)dipalladium
5
0
24
28,29
and 130 μg/kg in the tail-flick and hot-plate tests. The
syntheses and evaluation of analogues 7b−e and 8b−e allowed
a determination of the effects of electron-withdrawing and
(0) as the catalytic system.
Cross-coupling of 2-amino-4-
bromopryidine (13) and bis(pinacolato)diborane in the
presence of XPhos, tris(dibenzylideneacetone)dipalladium
(0), and potassium acetate heated at 110 °C in 1,4-dioxane
converted 13 to the boronic ester, which was carried on to the
next step directly by addition of 2′-fluoro-3′-bromo inter-
mediate 10, tribasic potassium phosphate as base, and an
additional 3 mol % of tris(dibenzylideneacetone)dipalladium
(0) and heating at 110 °C for 18 h to provide 7d. Compound
7e was synthesized as shown in Scheme 3. Borylation of 14 was
achieved by cross-coupling with bis(pinacolato)diborane heated
-
donating groups on the pyridine ring. See Chart 1 for the
structures of the compounds described in the above paragraphs.
Chemistry. The synthetic route to the 7a−c and 8a−e
analogues commenced with the intermediate 7-tert-butoxycar-
bonyl-2-exo-(2′-amino-3′-bromo-5′-pyridinyl)-7-azabicyclo-
[
2.2.1]heptane (9) prepared in several steps from N-Boc
25,26
pyrrole as reported in earlier work.
As outlined in Scheme
1, the Suzuki cross-couplings of the haloboronic acids, that is, 2-
C
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a
Scheme 3
fluoro-3′-(2-methoxypyridin-4-yl)-5′-pyridinyl]-7-azabicyclo-
[2.2.1]heptane (7e).
3
Biology. The inhibition of [ H]epibatidine binding at
21
α4β2*-nAChRs were conducted as previously reported. The
binding assays were performed using tissue homogenates
prepared from freshly collected cerebral cortices from adult
male Sprague−Dawley rats. These homogenates were frozen at
−
80 °C until use. It should be noted that, although this brain
region contains a variety of nAChRs, α4β2 is the predominant
>90%) subtype. The epibatidine analogues were tested for
(
agonist and antagonist activity at rat α4β2-, α3β4-, and α7-
nAChRs in an in vitro electrophysiology assay as previously
24
described. The epibatidine analogues were tested in mice for
their effects on body temperature and two pain models (tail-
flick and hot-plate assays) after acute administration as
21
previously described. For the antagonist experiments, male
ICR adult mice were pretreated subcutaneously (sc) with either
saline or epibatidine analogues 10 min before nicotine. Nicotine
was administered at a dose of 2.5 mg/kg, sc (an ED84 dose),
and mice were tested 5 min later. ED50 and AD50 values with
95% confidence limits were determined.
a
Reagents and conditions: (a) bis(pinacolato)diboron, KOAc,
PdCl (dppf), DMF, 80 °C; (b) Pd(PPh ) , K CO , 1,4-dioxane,
H O, 110 °C, 18 h.
2
3
4
2
3
2
RESULTS AND DISCUSSION
at 80 °C in the presence of 1,1′-bis(diphenylphosphino)-
ferrocene-palladium(II)dichloride, potassium acetate, and
dimethylformamide as the solvent to provide the pinacol
boronic ester 15 (Scheme 3). Compound 15 was cross-coupled
with 2′-fluoro-3′-bromo intermediate 10 to furnish 2-exo-[2′-
■
The nAChR binding affinities and the functional nicotinic
pharmacological properties of 2′-fluoro-3′-(substituted
pyridine)deschloroepibatidine analogues 7a−e and 8a−e were
3
determined. The K values for the inhibition of [ H]epibatidine
i
Table 1. Radioligand Binding and Efficacy Profile Data for 2′-Fluoro-3′-(substituted pyridine)deschloroepibatidine Analogues
agonist activity at 100 μM (% of max
antagonist activity at 100 μM (%
d
e
ACh activity)
ACh response remaining)
3
b
α4β2* [ H]epibatidine (K , nM) (Hill
i
a
compd
X
Y
slope)
α4β2
α3β4
α7
α4β2
nd
α3β4
nd
α7
c
nicotine
1.50 ± 0.30
0.026 ± 0.002
0.12 ± 0.02
0.009 ± 0.001
0.053 ± 0.004
0.12 ± 0.03
0.067 ± 0.01
1.18 ± 0.14
0.13 ± 0.005
0.04 ± 0.012
0.35 ± 0.038
0.049 ± 0.02
0.063 ± 0.08
0.25 ± 0.033
0.13 ± 0.027
40 ± 1
131 ± 13
13 ± 0.4
0
37 ± 3
97 ± 4
66 ± 4
4 ± 1
1.3 ± 0.2
7 ± 1
5 ± 0.4
4 ± 0.4
0
43 ± 5
150 ± 8
74 ± 5
6 ± 1
6 ± 1
8 ± 2
2 ± 0.5
3 ± 0.3
10 ± 4
2.1 ± 0.8
7 ± 2
nd
nd
nd
nat-epibatidine
nd
nd
varenicline
38 ± 2
6 ± 1
5 ± 1
6 ± 1
10 ± 1
7 ± 1
8 ± 1
4 ± 1
7 ± 1
24 ± 4
8 ± 1
7 ± 1
14 ± 1
nd
f
5
5
7
7
7
7
7
8
8
8
8
8
a
g
a
NO2
H
H
9 ± 2
2.1 ± 0.4
34 ± 4
27 ± 2
20 ± 1
9 ± 1
6 ± 1
15 ± 2
73 ± 8
23 ± 2
9 ± 1
23 ± 6
55 ± 6
17 ± 3
45 ± 4
44 ± 4
31 ± 3
32 ± 10
12 ± 4
54 ± 9
75 ± 18
32 ± 4
50 ± 6
41 ± 2
f
NO2
1.0 ± 0.1
1.5 ± 0.3
3 ± 0.3
1.2 ± 0.2
3.9 ± 0.5
0
H
b
c
F
Cl
d
e
a
b
c
NH2
CH3
H
1.5 ± 0.2
3 ± 0.5
9 ± 1
14 ± 1
1.3 ± 0.3
9 ± 2
2 ± 0.3
9 ± 1
1 ± 0.2
2.7 ± 0.3
5 ± 0.1
F
8 ± 0.7
0
Cl
d
e
NH2
5 ± 2
22 ± 4
CH O
3
a
b
3
c
d
All compounds were tested as their (±)-isomers. The K for (±)-[ H]epibatidine is 0.02 nM. Data taken from ref 21. Assessed by comparing the
d
current response to 100 μM of each compound to the mean current response of three preceding applications of ACh, applied at an EC20
concentration (20 μM for α4β2, 110 μM for α3β4) or an EC50 concentration (300 μM for α7) and expressed as a percentage of the maximal
e
response to ACh. Assessed by comparing the current response to an EC50 concentration of ACh (70 μM for α4β2, 200 μM for α3β4, 300 μM for
f
α7) in the presence of 100 μM of each compound to the mean current response of three preceding applications of ACh alone. Data taken from ref
2
4.
D
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Journal of Medicinal Chemistry
Article
binding at the α4β2*-nAChRs for compounds 7a−e and 8a−e
along with reference compounds nat-epibatidine (4), vareni-
cline (3), and the lead compounds 5a and 5g are listed in Table
antagonize α4β2 receptors but is a full agonist at α3β4- and α7-
nAChRs. The ability of 8e to both activate and antagonize the
α7-nAChR subtype indicates that 8e is a partial agonist at this
receptor. The results from this initial screen suggested that
some compounds in this series may be selective α4β2
antagonists, and 7a, 7c, and 8a were selected for more detailed
studies.
We examined the subtype selectivity of antagonist activity of
compounds 7a, 7c, and 8a in more detail by generating
concentration−inhibition curves (Table 2) and compared the
1. The reference standards nat-epibatidine and varenicline and
lead compounds 5a and 5g have K values of 0.026, 0.12, 0.009,
i
24
and 0.053 nM for the α4β2*-nAChR, respectively. The
pyridine bioisosteres 7a and 8a of 5a and 5b, respectively, had
Ki values of 0.12 and 0.35 nM, respectively, for α4β2*-nAChR.
Even though 7a and 8a have K values slightly less than 5a and
i
5
g, respectively, their K values are still subnanomolar and thus
i
3
have potent affinity for nAChRs labeled by [ H]epibatidine.
Substitution of the 4′-pyridyl and 3′-pyridyl rings of
analogues 7a and 8a, respectively, with 3′- and 4′-substituents,
respectively, had only small effects on α4β2*-nAChR binding
Table 2. Comparison of Antagonist Potency (IC50 Values)
for Several Epibatidine Analogues at α4β2-, α3β4-, and α7-
nAChRs
affinity. The K values varied from 0.04 to 1.18 nM. With the
i
a
antagonist activity IC50 (μM)
exception of the 3′-chloro analogue 7c, all the 3′-substituted 4′-
compd
α4β2
α3β4
α7
pyridyl analogues 7b−e had very high affinity for α4β2*-
b
d
d
nAChRs. The K values ranged from 0.04 for 7e to 0.13 nM for
varenicline
0.20 ± 0.03
3.2 ± 0.2
4.3 ± 0.6
1.4 ± 0.1
2.0 ± 0.4
1.7 ± 0.2
i
c
c
c
7
d. Even the 3′-chloro substituted analogue had a K value of
5a
5g
7a
7c
8a
7.9 ± 0.5
3.9 ± 0.3
8 ± 1
8.8 ± 0.9
18 ± 3
32 ± 12
23 ± 5
75 ± 16
56 ± 10
99 ± 24
i
1
.18 nM. All the 4′-substituted 3′-pyridyl analogues had K
i
values of 0.04−0.35 nM and thus very high affinity for α4β2*-
nAChR. The presence of 3′-substituents on the 4′-pyridyl
analogues 7b−e or 4′-substituents on the 3′-pyridyl analogues
a
8
b−e did not show any clear structure affinity patterns. In the
Antagonist activity of 7a, 7c, and 8a was assessed in the in vitro
case of the 7a−e series, the two highest affinity compounds
electrophysiology assay at a range of concentrations to generate
were the electron-withdrawing 3′-fluoro analogue 7b (K =
concentration−inhibition curves. Data were fit to the following
i
n
equation: I = I /[1+(IC /X) ], where I is the current response at
0
0
.067) and the electron-donating methoxy analogue 7e (K =
.04). For the 4′-substituted 3′-pyridyl analogues, the 4′-fluoro
max
50
i
a compound concentration (X), Imax is the maximum current, IC50 is
the compound concentration producing half-maximal inhibition of the
and 4′-chloro electron-withdrawing analogues 8b and 8c (K =
0
i
b
current response, and n is the Hill coefficient. Data taken from ref 11.
.049 and 0.063 nM) had higher α4β2*-nAChR affinity than
c
d
Data taken from ref 24. Varenicline is an agonist at α3β4- and α7-
the 3′-amino and 3′-methoxy electron-donating analogues 8d
nAChRs, with an EC50 of 55 ± 8 and 18 ± 6 μM, respectively (ref 11).
and 8e (K = 0.25 and 0.13 nM). However, all four compounds
i
have subnanomolar K values.
i
The receptor subtype selectivity of 7a−e and 8a−e was
assessed in an electrophysiological assay using rat α4β2-, α3β4-,
and α7-nAChRs expressed in Xenopus oocytes and assayed by a
two-electrode voltage clamp. Compounds were compared to
previously determined values for nicotine, nat-epibatidine,
varenicline, and compounds 5a and 5g (Table 1). Current
responses to a high concentration (100 μM) of each compound
were compared to the maximum response that can be achieved
with acetylcholine. All compounds differed dramatically from
nat-epibatidine (a full agonist at α4β2-, α3β4-, and α7-
nAChRs). Compounds 7a, 7c, 7e, 8a, and 8c displayed little
or no agonist activity at α4β2 in this initial screen, while 7b, 7d,
IC values to the lead nitro compounds 5a and 5g. Compound
50
7a, the bioisosteric analogue of 5a, where the nitro group has
been replaced by a pyridine nitrogen, displayed an improved
α4β2 selectivity. While 5a was 2.5-fold selective for α4β2 over
α3β4 and 10-fold selective for α4β2 over α7, compound 7a was
5.7-fold selective for α4β2 over α3β4 and 54-fold selective for
α4β2 over α7. Similarly, compound 8a, the bioisosteric
analogue of 5g, where the nitro group has been replaced by a
pyridine nitrogen, displayed an improved α4β2 selectivity.
While 5g was nonselective between α4β2 and α3β4 and 5-fold
selective for α4β2 over α7, compound 8a was 11-fold selective
for α4β2 over α3β4 and 58-fold selective for α4β2 over α7. We
also examined 7c but found it to be less selective for α4β2 than
was 7a. None of these compounds was as potent an antagonist
as varenicline at α4β2-nAChRs. The ability of these compounds
to antagonize α3β4- and α7-nAChRs differs markedly from
varenicline, which is a full agonist at these subtypes.
The 3′-substituted 4′-pyridyl analogues 7a−e and the 4′-
substituted 3-pyridyl analogues (8a−e) were evaluated for their
in vivo nAChR properties in mice, and the results were
compared to the properties of lead compounds 5a and 5g and
varenicline (Table 3). Similar to 5a, the pyridine bioisostere 7a
does not have agonist activity in the tail-flick and hot-plate tests
but like 5a and varenicline did show activity in the hypothermia
and spontaneous-activity tests. The ED50 values for 7a in the
hypothermia and spontaneous-activity tests were 1.69 and 0.38
mg/kg compared to 0.21 and 0.22 mg/kg for 5a. Similar to 5a,
7a antagonized nicotine-induced antinociception in the tail-flick
and hot-plate tests with AD50 values of 12 and 290 μg/kg,
respectively, compared to 3 and 120 μg/kg for 5a. Thus, 7a is a
8
b, and 8d−e had a low level of agonist activity at this subtype.
Compounds 7d and 8d had little or no agonist activity at α3β4,
while 7a−c, 8a−c, and 8e had a low level of agonist activity at
this subtype. At α7-nAChRs, compounds 7b, 7e, and 8c had
little or no agonist activity, compounds 7a, 7c−d, 8a−b, and 8d
displayed low levels of agonist activity, and compound 8e
showed a moderate level of agonist activity (22 ± 4% of the
maximal acetylcholine response). Compounds 7a−e and 8a−e
all showed lower agonist activity at α4β2-, α3β4-, and α7-
nAChR than did varenicline (a partial agonist at α4β2- and a
full agonist at α3β4- and α7-nAChRs).
As an initial screen of antagonist properties, we measured the
current response to an EC50 concentration of acetylcholine in
the presence of 100 μM of each compound and compared this
to a preceding current response to acetylcholine alone (Table
1
). Compounds 7a−e and 8a−e antagonized, to varying
extents, the α4β2-, α3β4-, and α7-nAChR subtypes in this
preliminary screen. This contrasts with varenicline, which can
E
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Article
very good bioisostere analogue of 5a. In contrast, the 3′-pyridyl
compound 8a, which is the pyridine bioisostere of 5g, was an
agonist in all four mice acute tests, whereas 5g was active only
in the spontaneous-activity test. In addition, whereas 5g was an
antagonist of nicotine-induced antinociception in the tail-flick
and hot-plate tests with AD50 values of 0.5 and 130 μg/kg,
respectively, 8a was an antagonist in only the tail-flick test with
an AD50 value of 3 μg/kg. Somewhat surprisingly, the in vivo
properties of 8a are quite different from those of 5g, and thus,
even though it is an interesting partial agonist, it is not a good
bioisosteric analogue of 5g in the mice in vivo test. These
discrepancies between the in vivo and in vitro effects of 5g and
Table 4. Calculated Physiochemical Properties of 5a, 5g, 7a−
e, 8a−e, Nicotine, Nat Epibatidine, and Varenicline
a
a
b
compd
logP
TPSA
logBB
nicotine
1.16
1.84
1.01
3.14
3.14
1.99
2.52
1.75
2.81
2.12
1.99
2.52
2.81
1.75
2.42
16.13
24.92
37.81
70.74
70.74
37.81
37.81
63.83
37.81
37.81
37.81
37.81
37.81
63.83
47.04
0.08
0.05
epibatidine
varenicline
−0.27
−0.43
−0.43
−0.12
−0.04
−0.54
0.01
5a
5g
7a
7b
7d
7c
7e
8a
8b
8c
8d
8e
8
a could also be related to many factors such as differences in
−0.10
−0.12
−0.04
0.01
the metabolic profile and brain penetrability of these two
analogues as well as the fact that we used expressed receptors
systems and not native receptors preparations. Furthermore,
since the agonistic response of nicotine in these two tests is
largely mediated by α4β2* nAChR subtypes, it is possible that
−0.54
−0.19
5
g and 8a differ in their affinity/activity at the various α4β2*
a
b
ChemAxon Calculator Plugins, Marvin 6.1.0, 2013. logBB =
0.0148 × TPSA + 0.152 × clogP + 0.139 (from ref 34).
nAChR subtypes mediating their pharmacological responses. In
addition, in vivo regulation of nicotinic receptors such as α4β2*
nAChR subtypes by these two analogues may differ also. For
example, it is possible that, since 5g was more potent than 8a as
a functional blocker in the tail-flick and hot-plate tests, in vivo
desensitization/blockade of α4β2* receptors by 5g is more
pronounced. We recently reported that varenicline (3) and
sazetidine, two α4β2* nicotinic partial agonists that differ in
their desensitization properties, differ in their potency to act as
−
the corresponding values of 3.14, 70.74, and −0.43 for lead
compounds 5a and 5g show that these two bioisosteric
analogues (7a and 8a) have at least as favorable if not better
calculated physicochemical properties than lead compounds 5a
and 5g. In addition, both 7a and 8a have calculated logBB
values somewhat better than that of varenicline.
3
0,31
functional antagonists of nicotine in these tests.
In summary, 2′-fluoro-3-(substituted pyridine)epibatidine
Similar to the unsubstituted analogue 7a, none of the 3′-
substituted 4-pyridyl analogues 7b−e had any agonist activity in
the tail-flick and hot-plate tests (Table 3). In the case of the 4′-
substituted 3′-pyridyl analogues, the 4′-fluoro and 4′-methoxy
analogues 8b and 8e had agonist activity in the tail-flick and
hot-plate tests. Similar to 5a, 5g, and varenicline, 7b−e and
analogues 7a−e and 8a−e were synthesized and evaluated for
3
the ability to inhibit [ H]epibatidine binding to nAChR, tested
for agonist and antagonist activity at α4β2-, α3β4-, and α7-
nAChR in an electrophysiology assay, and evaluated for agonist
effects in the tail-flick, hot-plate, spontaneous-activity, and
hypothermia tests in the mouse and as antagonists of nicotine-
induced antinociception in the tail-flick and hot-plate tests in
the mouse. A comparison of the nAChR binding and
electrophysiology of bioisosteres 7a and 8a to those of the
nitrophenyl lead compounds 5a and 5g, respectively, showed
that 7a and 8a had in vitro nAChR properties similar to those
of 5a and 5g but were more selective for the α4β2-nAChR
relative to the α3β4- and α7-nAChRs than 5a and 5g. Similar to
8
tests.
b−e had activity in the hypothermia and spontaneous-activity
Analogues 7b−e and 8c−e antagonized nicotine-induced
antinociception in the tail-flick test (Table 3). Analogues 7b−c
and 8c also antagonized nicotine-induced antinociception in
the hot-plate test with AD50 values ranging from 117 to 1370
μg/kg. Compounds 7a and 7b, which have AD values of 290
5
0
and 117 μg/kg, respectively, in the hot-plate test, compared to
70 μg/kg for varenicline strongly suggest that these
5a, 7a did not have agonist activity in the tail-flick and hot-plate
4
tests and like 5a was a potent antagonist of nicotine-induced
antinociception in these two tests. Thus, 7a is a very good
bioisosteric analogue of 5a. In contrast, 8a unlike 5g was an
agonist in both the tail-flick and hot-plate tests and was an
antagonist of nicotine-induced antinociception only in the tail-
flick test, whereas 5g was an antagonist in both tests. Even
though 8a is not a good bioisosteric analogue of 5g in the mice
test, it is an interesting partial agonist. A comparison of the
AD50 value of 7a to that of varenicline in the hot-plate test,
strongly suggest that this compound penetrates the brain in
mice. Calculated logBB values for 7a and 8a also suggest that
this compound will have good blood−brain barrier penetration.
Since both nAChR antagonists and partial agonists are of
interest as possible pharmacotherapies for treating smokers,
both 7a and 8a are candidates for development as
pharmacotherapies to treat nicotine addiction.
compounds have good brain penetration. Unlike any of the
other pyridine substituted analogues, the 3′-fluoro-4′-pyridyl
analogue 8b was an agonist in all four acute mouse tests and
had no antagonist properties.
Calculated physicochemical properties such as lipophilicity
(clogP), topological polar surface area (TPSA), and derived
values such as logBB can be used as an indication of the
potential of a compound for development as a CNS drug.
These molecular descriptors were calculated for lead com-
pounds 5a and 5g, bioisosteric analogues 7a and 8a,
respectively, as well as compounds 7b−e, 8b−e and reference
compounds nicotine, epibatidine, and varenicline (Table 4). In
32
general, CNS drugs have a clogP in the range 2−4, TPSA less
3
3
34
than 76 Å, and logBB greater than −1. All of the
compounds have clogP values within or close to the desirable
range and logBB values between −0.54 and +0.08. In addition,
all of the compounds have TPSA values of less than 76 Å. A
comparison of the logP, TPSA, and logBB values of 1.99, 37.81,
and −0.12, respectively, for bioisosteric analogues 7a and 8a to
EXPERIMENTAL SECTION
Melting points were determined on a Mel-temp (Laboratory Devices,
Inc.) capillary tube apparatus. NMR spectra were recorded on a Bruker
■
G
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
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1
Avance 300 or AMX 500 spectrometer using tetramethylsilane as
internal standard. Mass spectra were determined on a Perkin−Elmer
Sciex API 150EX mass spectrometer outfitted with APCI and ESI
sources. Melting point was determined on a Laboratory Devices MEL-
TEMP II. Elemental analyses were carried out by Atlantic Microlab,
Inc., Norcross GA. The purity of the compounds (>95%) was
established by elemental analysis. Analytical thin-layer chromatography
mp 192−195 °C. H NMR (300 MHz, CD OD) δ 1.86−2.22 (m,
3
6H), 2.44−2.51 (dd, J = 9.0, 11.0 Hz, 1H), 3.50−3.55 (m, 1H), 4.35
(br s, 1H), 4.56 (d, J = 3.9 Hz, 1H), 6.63 (s, 1H), 7.72−7.75 (m, 2H),
8.20 (dd, J = 2.4, 9.0 Hz, 1H), 8.27 (d, J = 2.4 Hz, 1H), 8.67 (m, 2H);
C NMR (CD OD) δ 27.0, 29.0, 37.7, 43.4, 60.2, 64.1, 121.6, 125.1,
136.2, 137.6, 141.3, 143.9, 147.8, 148.0, 150.7, 159.0, 162.2, 171.4; MS
13
3
+
(ESI) m/z 270.1 [(M − fumaric) , M = C H FN ·C H O ]. Anal.
16
16
3
4
4
4
(
TLC) was carried out on plates precoated with silica gel (60 F254).
(C H FN O ·0.25 H O) C, H, N.
20 20 3 4 2
TLC visualization was accomplished with a UV lamp or in an iodine
chamber. Purifications by flash chromatography were performed on a
Combiflash Teledyne ISCO instrument.
2-exo-[2′-Fluoro-3′-(2-fluoropyridin-4-yl)-5′-pyridinyl]-7-
azabicyclo[2.2.1]heptane (7b) Fumarate. A solution of 12b (230
mg, 0.60 mmol, 1.0 equiv) in 70% HF in pyridine (1.5 mL) was stirred
at 0 °C for 30 min. Sodium nitrite (413 mg, 10 equiv) was added in
small portions, and the reaction mixture was stirred at room
temperature. After 1 h, the mixture was poured into a 1:1 aqueous
solution of NH OH−H O (100 mL) and extracted with EtOAc (3 ×
Suzuki Cross-Coupling Reaction: General Procedure (Meth-
od A). To a resealable reaction vessel under nitrogen was added 1.0
equiv of 7-tert-butoxycarbonyl-2-exo-(2′-amino-3′-bromo-5′-pyridin-
yl)-7-azabicyclo[2.2.1]heptane (9), Pd(OAc) (0.1 equiv), P(o-tolyl)
2
3
4
2
(
0.2 equiv), sodium carbonate (2.0 equiv) and the respective pyridinyl
100 mL). The combined organic layers were dried over anhydrous
boronic acid (1.6 equiv), DME (6 mL), and water (0.7 mL). The
mixture was degassed through bubbling nitrogen, sealed, and heated
on a sand bath at 80 °C for 5 h. The mixture was cooled, poured into
Na SO , filtered through Celite, and concentrated in vacuo. The
resultant residue was purified by flash chromatography on a silica gel
2
4
column using CHCl −MeOH as the eluent to provide 121 mg (70%)
3
1
2
0 mL of a saturated aqueous solution of NaHCO , and extracted with
of 7b as a colorless oil. H NMR (300 MHz, CDCl ) δ 1.56−1.68 (m,
3
3
EtOAc (3 × 30 mL). The combined organic layers were dried over
6H), 1.92−1.98 (dd, J = 9.1, 11.2 Hz, 1H), 2.81−2.86 (m, 1H), 3.60
(s, 1H), 3.83 (br s, 1H), 7.17 (d, J = 1.0 Hz, 1H), 7.43 (ddd, J = 1.6,
MgSO and filtered through Celite, and the solvent was removed
4
1
3
under reduced pressure. The resultant residue was purified on silica gel
4.9, 6.9 Hz, 1H), 8.15−8.19 (m, 2H), 8.23 (d, J = 5.3 Hz, 1H); C
by flash chromatography eluted with CHCl −MeOH (50:1 to 10:1).
NMR (CDCl ) δ 30.4, 31.5, 40.7, 44.2, 56.4, 62.9, 109.0, 119.4, 121.1,
3
3
Suzuki Cross-Coupling Reaction: General Procedure (Meth-
od B). To a resealable reaction vessel under nitrogen was added 1.0
equiv of the 3′-bromo compound 9, Pd(PPh ) (10 mol %), K CO
139.6, 141.5, 147.5, 157.1, 160.3, 162.6, 162.7; MS (ESI) m/z 288.3
+
(M + H) .
3
4
2
3
A solution of 7b (141 mg, 0.49 mmol) in CH Cl in a vial was
2
2
(
2.0 equiv) and the respective pyridinyl boronic acid (1.3 equiv),
treated with 1.2 equiv of fumaric acid (0.65 M) in MeOH, and the vial
was allowed to stand in a refrigerator overnight. The excess solvent
was then removed in vacuo from the salt and then redissolved in a
minimal amount of MeOH, and the fumarate salt was recrystallized
from MeOH using diethyl ether to provide 110 mg (55%) of the salt
toluene (12 mL), ethanol (1.5 mL), and water (1.5 mL). The mixture
was degassed through bubbling nitrogen, sealed, and heated on a sand
bath at 110 °C. After 24 h, the mixture was cooled, poured into 30 mL
of H O, and extracted with EtOAc (3 × 30 mL). The combined
2
1
organic layers were dried over MgSO and filtered through Celite, and
7b·C H O as a white crystalline solid: mp 203−205 °C. H NMR
4
4
4
4
the solvent was removed in vacuo. The resultant residue was purified
by flash chromatography on a silica gel column using hexanes−
(500 MHz, CD OD) δ 1.87−2.20 (m, 5H), 2.45−2.50 (dd, J = 9.3,
3
13.2 Hz, 1H), 3.50−3.53 (m, 1H), 4.34−4.35 (br s, 1H), 4.56 (d, J =
3.9 Hz, 1H), 6.64 (s, 2H), 7.41 (s, 1H), 7.61−7.63 (m, 1H), 8.21 (dd,
J = 2.4, 9.3 Hz, 1H), 8.28 (d, J = 1.0 Hz, 1H), 8.32 (d, J = 5.3 Hz, 1H);
isopropanol (80:20 to 25:75) or CHCl −MeOH (30:1 to 10:1) as the
3
eluent.
13
General Procedure C: Removal of the Boc-Protecting Group.
A solution of the Boc-protected compound in methylene chloride (5
mL) was treated with TFA (1.5 mL) and stirred at room temperature
overnight. In some cases the solution was heated at 40 °C for 2 h and
then stirred at room temperature overnight. The solvent was then
removed in vacuo, and the residue was treated with a solution of
NH Cl (20 mL) and extracted with CHCl −MeOH (10%) (3 × 30
C NMR (CD OD) δ 25.8, 27.8, 36.5, 42.2, 59.0, 62.8, 109.4, 121.6,
3
135.0, 136.5, 140.1, 147.2, 147.8, 158.3, 160.2, 163.4, 165.3, 170.2; MS
+
(ESI) m/z 288.3 [(M − fumaric) , M = C H F N ·C H O ]. Anal.
16
15
2
3
4
4
4
(C H F N O ) C, H, N.
20
19
2
3
4
2-exo-[2′-Fluoro-3′-(2-chloropyridin-4-yl)-5′-pyridinyl]-7-
azabicyclo[2.2.1]heptane (7c) Fumarate. A solution of 12c (130
mg, 0.32 mmol, 1.0 equiv) in 70% HF in pyridine (1.5 mL) was stirred
at 0 °C for 30 min. Sodium nitrite (224 mg, 10 equiv) was added in
small portions, and the reaction mixture was stirred at room
temperature. After 1 h, the mixture was poured into a 1:1 aqueous
solution of NH OH−H O (100 mL) and extracted with EtOAc (3 ×
4
3
mL). The combined organic layers were dried over anhydrous
Na SO , filtered, and concentrated in vacuo, and the residue was
2
4
purified by flash chromatography using a silica gel column eluted with
CHCl −MeOH (10%) to provide the respective amine.
3
4
2
2
-exo-[2′-Fluoro-3′-(pyridin-4-yl)-5′-pyridinyl]-7-azabicyclo-
100 mL). The combined organic layers were dried over anhydrous
[
2.2.1]heptane (7a) Fumarate. A solution of 12a (378 mg, 1.03
Na SO , filtered through Celite, and concentrated in vacuo. The
2
4
mmol, 1.0 equiv) in 70% HF in pyridine (1.5 mL) was stirred at 0 °C
for 30 min. Sodium nitrite (806 mg, 10 equiv) was added in small
portions, and the reaction mixture was stirred at room temperature.
After 1 h, the mixture was poured into a 1:1 aqueous solution of
NH OH−H O (100 mL) and extracted with EtOAc (3 × 100 mL).
resultant residue was purified by flash chromatography on a silica gel
column using CHCl −MeOH as the eluent to provide 86 mg (87%) of
3
1
7c as a colorless oil. H NMR (300 MHz, CDCl ) δ 1.54−1.67 (m,
3
6H), 1.92−1.98 (dd, J = 9.1, 11.2 Hz, 1H), 2.81−2.86 (m, 1H), 3.60
(s, 1H), 3.83 (br s, 1H), 7.46 (dd, J = 1.2, 5.2 Hz, 1H), 7.56 (s, 1H),
4
2
1
3
The combined organic layers were dried over anhydrous Na SO ,
8.12−8.15 (m, 2H), 8.47 (d, J = 5.2 Hz, 1H); C NMR (CDCl ) δ
2
4
3
filtered through Celite, and concentrated in vacuo. The resultant
residue was purified by flash chromatography on a silica gel column
30.4, 31.6, 40.7, 44.3, 56.4, 62.9, 119.2, 122.1, 139.6, 141.5, 145.1,
147.2, 149.9, 152.1, 157.1, 160.3; MS (ESI) m/z 304.3 (M + H) .
+
using CHCl −MeOH as the eluent to provide 192 mg (69%) of 7a as
A solution of 7c (106 mg, 0.35 mmol) in CH Cl in a vial was
3
2
2
1
a colorless oil. H NMR (300 MHz, CD OD) δ 1.50−1.78 (m, 6H),
treated with 1.2 equiv of fumaric acid (0.65 M) in MeOH, and the vial
was allowed to stand in a refrigerator overnight. The excess solvent
was then removed in vacuo from the salt and then redissolved in a
minimal amount of MeOH, and the fumarate salt was recrystallized
from MeOH using diethyl ether to provide 62 mg (42%) of the salt 7c·
3
2
1
2
.01−2.08 (dd, J = 9.0, 11.2 Hz, 1H), 3.02−3.07 (dd, J = 8.7, 5.2 Hz,
H), 3.66 (s, 1H), 3.77 (br s, 1H), 7.70−7.73 (m, 2H), 8.13 (dd, J =
.4, 9.4 Hz, 1H), 8.18 (s, 1H), 8.64 (d, J = 1.5 Hz, 1H), 8.65 (d, J = 1.5
13
Hz, 1H); C NMR (CD OD) δ 30.0, 31.8, 41.1, 45.7, 57.9, 63.7,
3
1
1
21.2, 125.1, 141.3, 142.1, 144.2, 147.8, 148.0, 150.6, 158.6, 161.7; MS
C H O as a white crystalline solid: mp 193−194 °C. H NMR (500
4
4
4
+
(
ESI) m/z 270.2 (M + H) .
MHz, CD OD) δ 1.87−2.21 (m, 5H), 2.45−2.50 (dd, J = 9.2, 13.2 Hz,
3
A solution of 7a (302 mg, 1.12 mmol) in chloroform (2 mL) was
1H), 3.50−3.53 (m, 1H), 4.34−4.35 (br s, 1H), 4.56 (d, J = 3.9 Hz,
1H), 6.63 (s, 2H), 7.67 (dd, J = 1.4, 9.3 Hz, 1H), 7.80 (s, 1H), 8.21
(dd, J = 2.4, 9.3 Hz, 1H), 8.28 (d, J = 2.4 Hz, 1H), 8.48 (d, J = 4.9 Hz,
placed in vial and treated with 1.1 equiv of fumaric acid (0.65 M in
MeOH). After 24 h, the white solid obtained was recrystallized from a
MeOH−Et O mixture to provide the salt 7a·C H O as a white solid:
1H); 13C NMR (CD OD) δ 25.7, 27.8, 36.5, 42.2, 59.0, 62.9, 119.4,
2
4
4
4
3
H
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Article
1
1
22.7, 135.0, 136.6, 140.1, 145.4, 147.3, 149.9, 151.8, 158.3, 160.3,
3.48−3.53 (m, 1H), 3.96 (s, 3H), 4.34 (br s, 1H), 4.55 (s, 1H), 6.65
(s, 2H), 7.07 (s, 1H), 7.22 (dd, J = 1.2, 4.1 Hz, 1H), 8.12 (d, J = 9.2
+
70.1; MS (ESI) m/z 304.0 [(M − fumaric) , M = C H ClFN ·
16
15
3
13
C H O ]. Anal. (C H ClFN O ·0.25 H O) C, H, N.
Hz, 1H), 8.22−8.23 (m, 2H); C NMR (CD OD) δ 26.9, 29.0, 37.7,
4
4
4
20 19
3
4
2
3
2-Fluoro-3-(2′-amino-4′-pyridinyl)deschloroepibatidine (7d)
43.4, 54.2, 60.2, 64.1, 111.6, 117.9, 136.1, 137.5, 141.2, 145.9, 147.5,
Hydrochloride. A solution of 2-amino-4-bromopyridine (200 mg,
147.6, 148.3, 162.2, 166.2, 171.1; MS (ESI) m/z 300.3 [(M −
+
1
1
0
.16 mmol, 1.0 equiv), bispinacolato diborane (307 mg, 1.21 mmol,
.05 equiv), Pd dba (36 mg, 0.035 mmol, 3 mol %), Xphos (88 mg,
fumaric) , M = C H FN O·C H O ]. Anal. (C H FN O ) C, H,
17
18
3
4
4
4
21 22
3
5
2
3
N.
.185 mmol, 16 mol %), and KOAc (272 mg, 2.77 mmol, 2.4 mmol)
2-exo-[2′-Fluoro-3′-(pyridin-3-yl)-5′-pyridinyl]-7-azabicyclo-
in dioxane placed in a resealable pressure vessel was degassed through
bubbling nitrogen for 40 min then heated at 110 °C for 4 h. A TLC
check revealed that all the bromopyridine had been converted to the
boronic ester. The reaction was allowed to cool to room temperature,
and K PO (613 mg, 2.89 mmol, 2.5 equiv), a solution of 10 (270 mg,
[2.2.1]heptane (8a) Hemifumarate. A solution of 11a (394 mg,
1.08 mmol, 1.0 equiv) in 70% HF in pyridine (1.5 mL) was stirred at 0
°C for 30 min. Sodium nitrite (742 mg, 10 equiv) was added in small
portions, and the reaction mixture was stirred at room temperature.
After 1 h, the mixture was poured into a 1:1 aqueous solution of
NH OH−H O (40 mL) and extracted with EtOAc (3 × 40 mL). The
3
4
1
.0 mmol, 0.87 equiv) in dioxanes, an additional 3 mol % of Pd dba ,
2 3
4
2
and H O (1 mL) were added to the reaction. The mixture was
combined organic layers were dried over anhydrous Na SO , filtered
2
2 4
degassed for 30 min and heated for 18 h at 110 °C. The reaction was
through Celite, and concentrated in vacuo. The resultant residue was
cooled to room temperature and extracted with EtOAC (3 × 30 mL).
purified by flash chromatography on silica gel using CHCl −MeOH as
3
1
The combined organic layers were dried over MgSO and filtered
the eluent to provide 203 mg (70%) of 8a as a colorless oil. H NMR
4
through Celite, and the solvent was removed in vacuo. Two
purifications of the residue by flash chromatography through an
(300 MHz, CD OD) δ 1.49−1.79 (m, 6H), 2.01−2.08 (dd, J = 9.1,
3
11.2 Hz, 1H), 3.02−3.07 (dd, J = 3.3, 5.4 Hz, 1H), 3.67 (s, 1H), 3.77
(br s, 1H), 7.54 (dd, J = 2.6, 7.8 Hz, 1H), 8.08−8.15 (m, 3H), 8.58−
ISCO column using CHCl −MeOH (10:1) as the eluent provided 60
3
1
13
mg (21%) of 7d as a colorless oil. H NMR (300 MHz, CDCl ) δ
8.60 (d, 2H), 8.58 (d, J = 1.4 Hz, 1H), 8.80 (s, 1H); C NMR
3
1
.51−1.71 (m, 5H), 1.90−1.97 (m, 1H), 2.36 (br s, 1H), 2.80−2.85
(CD OD) δ 29.9, 31.8, 40.6, 41.1, 45.7, 57.8, 63.9, 121.1, 125.2, 138.4,
3
(
(
dd, J = 3.8, 5.0 Hz, 1H), 3.61 (s, 1H), 3.81 (d, J = 2.7 Hz, 1H), 4.66
br s, 2H), 6.72 (s, 1H), 6.84 (d, J = 5.3 Hz, 1H), 8.02 (dd, J = 2.3, 9.5
141.4, 142.0, 147.0, 150.1, 158.7, 161.8; MS (ESI) m/z 270.3 (M +
+
H) .
13
Hz, 1H), 8.11 (s, 1H), 8.13 (d, J = 5.7 Hz, 1H); C NMR (CDCl ) δ
A solution of 8a (246 mg, 0.91 mmol) in chloroform (2 mL) was
placed in vial and treated with 1.1 equiv of fumaric acid (0.65 M in
MeOH). After 24 h, the white solid obtained was recrystallized from
MeOH using Et O to provide the salt 8a·0.5C H O as a white solid:
3
3
1
0.2, 31.4, 40.5, 44.3, 56.5, 62.9, 108.1, 113.9, 139.4, 140.6, 143.7,
45.9, 148.5, 157.4, 158.8, 160.6; MS (ESI) m/z 285.5 (M + H) .
+
A solution of 7d (122 mg, 0.43 mmol) in chloroform in a vial was
2
4
4
4
1
treated with a 2.0 equiv solution of HCl in diethyl ether and allowed to
stand at room temperature. The excess solvent was filtered off, and the
obtained salt was washed with ether and then dried to provide 94 mg
of the salt 7d·HCl as a white solid: mp 205−208 °C. H NMR (300
MHz, CD OD) δ 1.83−2.28 (m, 5H), 2.46−2.53 (dd, J = 3.8, 9.6 Hz,
mp 155−159 °C. H NMR (300 MHz, CD OD) δ 1.86−2.22 (m,
3
6H), 2.44−2.51 (dd, J = 9.0, 11.0 Hz, 1H), 3.49−3.54 (dd, J = 3.0, 5.1
Hz, 1H), 4.35 (br s, 1H), 4.56 (d, J = 3.9 Hz, 1H), 6.63 (s, 1H), 7.56−
7.60 (dd, J = 2.3, 7.5 Hz, 1H), 8.12−8.16 (m, 2H), 8.23 (s, 1H), 8.61
1
(dd, J = 1.4, 6.0 Hz, 1H), 8.81 (s, 1H); 13C NMR (CD OD) δ 27.4,
3
3
1
4
1
1
1
H), 3.52−3.57 (dd, J = 3.1, 5.5 Hz, 1H), 4.37 (d, J = 3.6 Hz, 1H),
.59 (d, J = 2.7 Hz, 1H), 7.02−7.05 (dd, J = 1.6, 6.1 Hz, 1H), 7.10 (s,
H) 7.98 (d, J = 6.1 Hz, 1H) 8.16 (dd, J = 2.3, 9.2 Hz, 1H) 8.28 (s,
29.4, 38.2, 43.8, 59.8, 64.0, 121.0, 125.4, 136.8, 138.2, 138.5, 141.4,
147.0, 147.2, 150.0, 159.1, 162.3, 171.5; MS (ESI) m/z 270.2 [(M −
+
fumaric) , M = C H FN ·0.5C H O ]. Anal. (C H FN O ·0.5
2
16
16
3
4
4
4
18 18
3
2
H); 13CNMR (CD OD) δ 26.8, 28.9, 37.5, 43.3, 60.5, 64.2, 112.0,
3
H O) C, H, N.
13.8, 137.4, 141.3, 143.2, 148.0, 148.2, 158.8, 158.9, 162.1; MS (ESI)
2-exo-[2′-Fluoro-3′-(6-fluoropyridin-3-yl)-5′-pyridinyl]-7-
azabicyclo[2.2.1]heptane (8b) Hemifumarate. Compound 11b
(250 mg, 0.65 mmol, 1.0 equiv) was placed in a plastic vessel and was
treated dropwise with 1.5 mL of 70% HF in pyridine, and the mixture
was stirred at 0 °C for 30 min. Sodium nitrite (449 mg, 10 equiv) was
added in small portions, and the reaction mixture was stirred at room
temperature. After 1 h, the mixture was poured into a 1:1 aqueous
solution of NH OH−H O (40 mL) and extracted with EtOAc (3 × 40
+
m/z 285.7 [(M − HCl) , M = C H FN ·2HCl]. Anal.
1
6
17
4
(
C H Cl FN ) C, H, N.
1
6
19
2
4
2
-exo-[2′-Fluoro-3′-(2-methoxypyridin-4-yl)-5′-pyridinyl]-7-
azabicyclo[2.2.1]heptane (7e) Fumarate. To a resealable reaction
pressure vessel under nitrogen was added compound 10 (180 mg, 0.66
mmol, 1.0 equiv), compound 16 (188 mg, 0.80 mmol, 1.2 equiv),
(
Pd(PPh ) (38 mg, 0.03 mmol, 5 mol %), K CO (184 mg, 1.33
3
4
2
3
4
2
mmol, 2.0 equiv), 1,4-dioxane (10 mL), and water (0.80 mL). The
reaction mixture was degassed through bubbling nitrogen for 40 min,
sealed, and heated over a sand bath at 110 °C for 18 h. After cooling,
the solvent was removed under reduced pressure, and to the residue
was added 20 mL of H O. The organic product was extracted using
EtOAc (3 × 30 mL). The combined organic layers were dried over
MgSO and filtered through Celite, and the solvent was removed in
vacuo. Purification by flash chromatography on silica gel using
MeOH−CHCl as the eluent provided 100 mg (50%) of 7e as a
colorless oil. H NMR (300 MHz, CDCl ) δ 1.51−1.68 (m, 5H),
mL). The combined organic layers were dried over anhydrous
Na SO , filtered through Celite, and concentrated in vacuo. The
2
4
resultant residue was purified by flash chromatography on silica gel
using CHCl −MeOH as the eluent to provide 170 mg (91%) of 8b as
3
1
2
a colorless oil. H NMR (300 MHz, CDCl ) δ 1.54−1.70 (m, 6H),
3
1.92−1.99 (dd, J = 9.0, 11.2 Hz, 1H), 2.82−2.87 (m, 1H), 3.61 (s,
1H), 3.83 (br s, 1H), 7.04 (dd, J = 3.0, 8.4 Hz, 1H), 7.99−8.09 (m,
4
2H), 8.14 (br s, 1H), 8.42 (d, J = 0.8 Hz, 1H); 13C NMR (CDCl ) δ
3
3
30.3, 31.5, 40.6, 44.3, 56.4, 62.9, 109.3, 118.5, 139.5, 141.3, 145.8,
1
+
3
147.5, 157.3, 160.4, 161.7, 164.9; MS (ESI) m/z 288.3 (M + H) .
1
.89−1.96 (dd, 3.8, 9.6 Hz, 1H), 1.98 (broad signal 1H), 2.79−2.84
A solution of 8b (198 mg, 0.69 mmol) in chloroform (2 mL) was
placed in a vial and treated with 1.1 equiv of fumaric acid (0.65 M in
MeOH). After 24 h, the white solid obtained was recrystallized from
MeOH using Et O to provide 200 mg (84%) of the salt 8b·0.5C H O
4
(
(
1
3
1
dd, J = 3.4, 5.5 Hz, 1H), 3.59 (s, 1H), 3.81 (s, 1H), 3.96 (s, 3 H), 6.96
s, 1H), 7.07−7.10 (dt, J = 5.3, 1.5 Hz, 1H), 8.06 (dd, J = 2.4, 9.6 Hz,
H), 8.11 (s, 1H), 8.21 (d, J = 5.3 Hz, 1H); 13C NMR (CDCl ) δ 30.2.
3
2
4
4
1
1.4, 40.5, 44.3, 53.5, 56.4, 62.9, 110.5, 116.6, 139.6, 140.8, 144.6,
as a white crystalline solid: mp 197−199 °C. H NMR (500 MHz,
+
46.2, 146.4, 147.2, 160.5, 164.6; MS (ESI) m/z (300.4) (M + H) .
CD OD) δ 1.81−2.15 (m, 5H), 2.38−2.43 (dd, J = 9.3, 13.2 Hz, 1H),
3
A solution of 7e (156 mg, 0.52 mmol) in CH Cl in a vial was
3.42−3.46 (m, 1H), 4.43 (br s, 1H), 6.57 (s, 1H), 7.21 (dd, J = 2.4, 8.3
2
2
treated with 1.2 equiv of fumaric acid (0.65 M) in MeOH, and the vial
was allowed to stand in a refrigerator overnight. The excess solvent
was then removed in vacuo from the salt that was then redissolved in a
minimal amount of MeOH, and the fumarate salt was recrystallized
from MeOH using diethyl ether to provide 164 mg (74%) of the salt
7
Hz, 1H), 8.14 (dd, J = 2.4, 8.2 Hz, 1H), 8.21−8.25 (m, 2H), 8.48 (br s,
1H); 13C NMR (CD OD) δ 27.5, 29.5, 38.3, 43.8, 59.9, 64.1, 111.0,
3
120.5, 137.0, 141.4, 143.8, 147.2, 148.8, 159.7, 161.6, 164.1, 166.0,
+
174.0; MS (ESI) m/z 288.3 [(M − fumaric) , M = C H F N ·
1
6
15
2
3
0.5C H O ]. Anal. (C H F N O ) C, H, N.
4 4 4 18 17 2 3 2
1
e·C H O as a white solid: mp 160−164 °C. H NMR (300 MHz,
2-exo-[2′-Fluoro-3′-(6-chloropyridin-3-yl)-5′-pyridinyl]-7-
azabicyclo[2.2.1]heptane (8c) Fumarate. Compound 11c (300
4
4
4
CD OD) δ 1.85−2.19 (m, 5H), 2.43−2.50 (dd, J = 9.3, 13.2 Hz, 1H),
3
I
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Article
mg, 0.75 mmol, 1.0 equiv) was placed in a plastic vessel and was
treated dropwise with 3 mL of 70% HF in pyridine, and the mixture
was stirred at 0 °C for 30 min. Sodium nitrite (559 mg, 10 equiv) was
added in small portions, and the reaction mixture was stirred at room
temperature. After 1 h, the mixture was poured into a 1:1 aqueous
solution of NH OH−H O (40 mL) and extracted with EtOAc (3 × 40
solution of NH OH−H O (100 mL) and extracted with EtOAc (3 ×
4
2
100 mL). The combined organic layers were dried over anhydrous
Na SO , filtered through Celite, and concentrated in vacuo. The
2
4
resultant residue was purified by flash chromatography on silica gel
using CHCl −MeOH as the eluent to provide 227 mg (94%) of 8e as
3
1
4
2
a colorless oil. H NMR (300 MHz, CD OD) δ 1.48−1.76 (m, 6H),
3
mL). The combined organic layers were dried over anhydrous
1.94−2.05 (m, 2H), 2.96−3.01 (dd, J = 3.4, 5.5 Hz, 1H), 3.65 (s, 3H),
3.77 (br s, 1H), 6.83 (d, J = 8.7 Hz, 1H), 7.88 (tt, J = 0.7, 1.7, 8.7 Hz,
1H), 7.99 (dd, J = 2.4, 9.6 Hz, 1H), 8.04 (d, J = 0.8 Hz, 1H), 8.34 (d, J
Na SO , filtered through Celite, and concentrated in vacuo. The
2
4
resultant residue was purified by flash chromatography on silica gel
= 1.6 Hz, 1H); 13C NMR (CD OD) δ 29.9, 31.7, 40.9, 45.7, 54.3, 57.7,
using CHCl −MeOH as the eluent to provide 142 mg (62%) of 8c as
3
3
1
a colorless oil. H NMR (300 MHz, CDCl ) δ 1.54−1.71 (m, 6H),
3
63.7, 111.7, 121.3, 124.5, 140.8, 141.6, 145.8, 147.9, 158.6, 161.8,
+
1
1
4
.92−1.98 (dd, J = 9.1, 11.2 Hz, 1H), 2.81−2.86 (m, 1H), 3.61 (s,
H), 3.81 (br s, 1H), 7.42 (dd, J = 0.6, 8.3 Hz, 1H), 7.88 (ddd, J = 0.8,
.1, 8.3 Hz, 1H), 8.06 (dd, J = 2.4, 9.6 Hz, 1H), 8.15 (br s, 1H), 8.58
1
65.5; MS (ESI) m/z 300.3 (M + H) .
A solution of 8e (169 mg, 0.53 mmol) in CH Cl in a vial was
2
2
treated with a 1.2 equiv of fumaric acid (0.65 M) in MeOH, and the
vial was allowed to stand in a refrigerator overnight. The excess solvent
was removed in vacuo from the salt that was then redissolved in a
minimal amount of MeOH, and the fumarate salt was recrystallized
(
br s, 1H); 13C NMR (CDCl ) δ 30.4, 31.5, 40.6, 44.3, 56.4, 62.9,
3
1
18.5, 124.1, 129.2, 139.5, 141.3, 146.1, 149.2, 151.2, 157.3, 160.5; MS
+
(
ESI) m/z 304.3 (M + H) .
A solution of 8c (138 mg, 0.46 mmol) in chloroform (2 mL) was
placed in a vial and treated with 1.1 equiv of fumaric acid (0.65 M in
MeOH). After 24 h, the white solid obtained was recrystallized from
from MeOH using diethyl ether to provide 159 mg of the salt 8e·
1
0
1
.5C H O : mp 193−195 °C. H NMR (300 MHz, methanol-d ) δ
4
4
4
4
.80−2.15 (m, 6H), 2.36−2.43 (dd, J = 9.3, 13.2 Hz, 1H), 3.40−3.45
(m, 1H), 3.96 (s, 3H), 4.27 (br s, 1H), 4.42 (s, 1H), 6.61 (s, 1H), 6.91
MeOH using Et O to provide 105 mg (55%) of the salt of 8c·
2
1
0
(
1
4
8
.5C H O as a white crystalline solid: mp 194−195 °C. H NMR
4
4
4
(dd, J = 0.7, 7.6 Hz 1H), 7.95 (dt, J = 0.8, 2.4, 8.8 Hz, 1H), 8.06 (dd, J
500 MHz, CD OD) δ 1.89−2.20 (m, 5H), 2.45−2.49 (dd, J = 9.2,
13
3
= 1.9, 8.8 Hz, 1H), 8.14 (d, J = 1.9 Hz, 1H), 8.41 (br s, 1H); C NMR
methanol-d ) δ 26.9, 29.0, 37.7, 43.4, 54.3, 60.2, 64.1, 111.7, 124.2,
3.2 Hz, 1H), 3.49−3.52 (dd, J = 3.5, 9.5 Hz, 1H), 4.34 (br s, 1H),
.56 (d, J = 3.5 Hz, 1H), 6.63 (s, 2H), 7.60 (d, J = 8.5 Hz, 1H), 8.09−
.15 (m, 2H), 8.23 (d, J = 2.4 Hz, 1H), 8.64 (br s, 1H); 13C NMR
(
4
1
36.2, 137.4, 140.6, 140.8, 145.8, 148.0, 159.1, 162.3, 165.8, 171.3; MS
+
(
ESI) m/z 300.5 [(M − fumaric) , M = C H FN O·0.5C H O ].
17
18
3
4
4
4
(
CD OD) δ 27.1, 29.1, 37.8, 43.5, 60.3, 64.2, 120.4, 125.8, 130.6,
3
Anal. (C H FN O ·0.25 H O) C, H, N.
19
20
3
3
2
1
36.3, 137.8, 141.3, 147.4, 150.6, 152.6, 159.8, 161.7, 171.5; MS (ESI)
7
-tert-Butoxycarbonyl-2-exo-[2′-amino-3′-(pyridin-3-yl)-5′-
+
m/z 304.5 [(M − fumaric) , M = C H ClFN ·C H O ]. Anal.
16
15
3
4
4
4
pyridinyl]-7-azabicyclo[2.2.1]heptane (11a). A solution of
compound 9 (322 mg, 0.87 mmol, 1.0 equiv), pyridine-3-boronic
acid (140 mg, 1.14 mmol, 1.3 equiv), Pd(PPh ) (50 mg, 0.044 mmol,
(
C H ClFN O ) C, H, N.
2
0
19
3
4
2
′-Fluoro-3′-(2″-amino-5″-pyridinyl)deschloroepibatidine
3
4
(
8d) Hydrochloride. To a resealable reaction pressure vessel under
5
mol %), and K CO (242 mg, 1.75 mmol, 2.0 equiv) in toluene (10
2 3
nitrogen was added 2-exo-(2′-fluoro-3′-bromo)-7-azabicyclo[2.2.1]-
mL), EtOH (2 mL), and water (2 mL) was degassed through bubbling
nitrogen for 20 min. The mixture was sealed and heated over a sand
bath at 110 °C for 22 h. After cooling to room temperature, 20 mL of
heptane (10) (125 mg, 0.46 mmol, 1.0 equiv), Pd(PPh ) (27 mg, 5
3
4
mol %), K CO (128 mg, 0.92 mmol, 2.0 equiv), 1,4-dioxane (10 mL),
2
3
water (0.80 mL), and 2-aminopyridine-5-pinacolate boronic ester (122
mg, 0.55 mmol, 1.2 equiv). The mixture was degassed through
bubbling nitrogen for 40 min and heated at 110 °C for 18 h. After
cooling, the solvent was removed under reduced pressure, and to the
H O was added, and the organic product was extracted with EtOAc (3
2
×
30 mL). The combined organic layers were dried over MgSO4,
filtered through Celite, and concentrated in vacuo. The resultant
residue was purified by flash chromatography on silica gel using i-
residue was added 20 mL of H O. The organic product was extracted
2
1
PrOH−hexanes as the eluent to provide 263 mg (82%) of 11a. H
using EtOAc (3 × 30 mL). The combined organic layers were dried
NMR (300 MHz, CDCl ) δ 1.41 (br s, 9H), 1.48−1.61 (m, 2H),
3
over Na SO and filtered through Celite, and the solvent was removed
2
4
1
.75−1.86 (m, 3H), 1.96−2.04 (m, 1H), 2.79−2.83 (dd, J = 3.8, 5.0
in vacuo. Purification by flash chromatography on silica gel using
Hz, 1H), 4.16 (s, 1H), 4.35 (br s, 1H), 4.66 (s, 2 NH), 7.34 (d, J = 2.5
Hz, 1H), 7.38 (d, J = 4.9 Hz, 1H), 7.80 (dt, J = 7.9, 1.9 Hz, 1H), 7.96
MeOH−CHCl as the eluent provided 88 mg (67%) of the desired
3
1
product 8d as a colorless oil. H NMR (300 MHz, CDCl ) δ 1.47−
3
(
1
6
d, J = 2.2 Hz, 1H), 8.59 (dd, J = 4.9, 1.6 Hz, 1H), 8.69 (d, J = 1.8 Hz,
1
1
8
1
4
1
.67 (m, 5H), 1.85−1.92 (m, 2H), 2.76−2.80 (dd, J = 3.8, 5.0 Hz,
H), 3.56 (s, 1H), 3.75 (d, J = 2.7 Hz, 1H), 4.82 (s, 2H), 6.53 (d, J =
.6 Hz, 1H), 7.63 (dt, J = 1.9, 8.6 Hz, 1H), 7.87 (dd, J = 2.3, 9.5 Hz,
H); 13C NMR (CDCl ) δ 28.3 (3 C), 28.8, 29.7, 40.3, 44.9, 55.9,
3
2.2, 79.5, 118.0, 123.6, 132.1, 134.1, 136.2, 136.9, 146.6, 148.9, 149.7,
+
H), 7.98 (s, 1H), 8.23 (s, 1H); 13CNMR (CDCl ) δ 30.2, 31.4, 40.5,
154.5, 154.9; MS (ESI) m/z 367.6 (M + H) .
3
7
-tert-Butoxycarbonyl-2-exo-[2′-amino-3′-(6-fluoropyridin-
4.5, 56.4, 62.8, 108.2, 120.2, 138.0, 138.7, 140.7, 144.2, 147.9, 157.5,
58.3, 160.6; MS (ESI) m/z 285.7 (M + H) .
+
3-yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptane (11b). A solution
of compound 9 (241 mg, 0.65 mmol, 1.0 equiv), 5-fluoropyridine-4-
A solution of the diamine 8d (217 mg, 0.76 mmol) in chloroform in
a vial was treated with a 2.0 equiv solution of HCl in diethyl ether and
allowed to stand at room temperature. The excess solvent was filtered
off, and the obtained salt washed with ether and then dried to provide
boronic acid (148 mg, 1.05 mmol, 1.6 equiv), Pd(OAc) (15 mg, 0.065
2
mmol, 10 mol %), P(o-tolyl) (40 mg, 0.131 mmol, 20 mol %), and
3
Na CO (139 mg, 1.31 mmol, 2.0 equiv) in DME (8 mL) and water
2
3
1
(0.9 mL) was degassed through bubbling nitrogen for 20 min. The
mixture was sealed and heated over a sand bath at 80 °C for 5 h. After
cooling to room temperature, the mixture was poured into a saturated
2
(
46 mg (90%) of 8d·HCl as a white solid: mp 202−205 °C. H NMR
300 MHz, CD OD) δ 1.88−2.24 (m, 5H), 2.44−2.52 (dd, J = 3.8, 9.6
3
Hz, 1H), 3.51−3.56 (dd, J = 3.1, 5.5 Hz, 1H), 4.37 (d, J = 3.4 Hz, 1H),
aqueous solution of NaHCO (20 mL) and extracted with EtOAc (3 ×
4
4
1
.58 (d, J = 2.7 Hz, 1H), 7.11 (dd, J = 1.9, 8.2 Hz, 1H), 8.18−8.28 (m,
3
H); 13CNMR (CD OD) δ 26.8, 28.9, 37.6, 43.3, 60.5, 64.4, 114.7,
30 mL). The combined organic layers were dried over MgSO , filtered
4
3
through Celite, and concentrated in vacuo. The resultant residue was
19.3, 120.4, 137.6, 140.6, 145.1, 147.2, 155.8, 158.9, 162.1; MS (ESI)
m/z 285.6 [(M − HCl) , M = C H FN ·2HCl]. Anal.
+
purified by flash chromatography on silica gel using EtOAc−hexanes as
1
6
17
4
1
the eluent to provide 250 mg (99%) of 11b. H NMR (300 MHz,
(
C H Cl FN ·1.25 H O) C, H, N.
1
6
19
2
4
2
CDCl ) δ 1.39 (br s, 9H), 1.51−1.59 (m, 2H), 1.81−1.85 (m, 3H),
2
-exo-[2′-Fluoro-3′-(6-methoxypyridin-3-yl)-5′-pyridinyl]-7-
3
azabicyclo[2.2.1]heptane (8e) Hemifumarate. Compound 11d
1.94−2.00 (m, 1H), 2.79−2.84 (m, 1H), 4.16 (s, 1H), 4.35 (br s, 1H),
4.70 (s, 2 NH), 7.02 (dd, J = 2.9, 8.4 Hz, 1H), 7.34 (d, J = 2.25 Hz,
1H), 7.91 (ddd, J = 2.5, 8.4, 16 Hz, 1H), 7.96 (d, J = 2.25 Hz, 1H),
(
480 mg, 1.21 mmol, 1.0 equiv) was placed in a plastic vessel and was
treated dropwise with 3 mL of 70% HF in pyridine, and the mixture
was stirred at 0 °C for 30 min. Sodium nitrite (835 mg, 10 equiv) was
added in small portions, and the reaction mixture was stirred at room
temperature. After 1 h, the mixture was poured into a 1:1 aqueous
8.28 (d, J = 2.4 Hz, 1H); 13C NMR (CDCl
) δ 28.2 (3 C), 28.8, 29.7,
3
40.3, 44.8, 55.9, 62.1, 79.5, 109.5, 116.8, 132.0, 136.9, 141.5, 146.8,
+
147.5, 154.6, 154.9, 161.3, 164.5; MS (ESI) m/z 385.5 (M + H) .
J
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Article
7
-tert-Butoxycarbonyl-2-exo-[2′-amino-3′-(6-chloropyridin-
mixture was sealed and heated over a sand bath at 80 °C for 5 h. After
3
-yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptane (11c). A solution
cooling to room temperature, the mixture was poured into a saturated
of compound 9 (304 mg, 0.83 mmol, 1.0 equiv), 5-chloropyridine-4-
boronic acid (208 mg, 1.32 mmol, 1.6 equiv), Pd(OAc) (19 mg, 0.083
mmol, 10 mol %), P(o-tolyl) (51 mg, 0.166 mmol, 20 mol %), and
aqueous solution of NaHCO (20 mL) and extracted with EtOAc (3 ×
3
2
30 mL). The combined organic layers were dried over MgSO , filtered
4
3
through Celite, and concentrated in vacuo. The resultant residue was
Na CO (176 mg, 1.66 mmol, 2.0 equiv) in DME (6 mL) and water
purified by flash chromatography on silica gel using EtOAc−hexanes as
2
3
1
(
0.7 mL) was degassed through bubbling nitrogen for 20 min. The
mixture was sealed and heated over a sand bath at 80 °C for 5 h. After
cooling to room temperature, the mixture was poured into a saturated
aqueous solution of NaHCO (20 mL) and extracted with EtOAc (3 ×
0 mL). The combined organic layers were dried over MgSO , filtered
through Celite, and concentrated in vacuo. The resultant residue was
purified by flash chromatography on silica gel using EtOAc−hexanes as
the eluent to provide 305 mg (99%) of 11c as a colorless oil. H NMR
300 MHz, CDCl ) δ 1.31 (br s, 9H), 1.43−1.50 (m, 2H), 1.72−1.76
m, 3H), 1.85−1.92 (m, 1H), 2.70−2.74 (m, 1H), 4.06 (s, 1H), 4.26
br s, 1H), 4.60 (s, 2 NH), 7.25 (d, J = 2.25 Hz, 1H), 7.30 (d, J = 8.2
the eluent to provide 300 mg (92%) of 12b as a colorless oil. H NMR
(300 MHz, CDCl ) δ 1.4 (br s, 9H), 1.52−1.59 (m, 2H), 1.82−1.84
3
(m, 3H), 1.94−1.98 (m, 1H), 2.79−2.84 (m, 1H), 4.16 (s, 1H), 4.36
(br s, 1H), 4.77 (s, 2 NH), 7.06 (s, 1H), 7.34 (ddd, J = 1.6, 5.13, 8.4
Hz, 1H), 7.41 (d, J = 2.3 Hz, 1H), 8.0 (d, J = 2.3 Hz, 1H), 8.26 (d, J =
3
3
4
5.16 Hz, 1H); 13C NMR (CDCl ) δ 28.3 (3 C), 28.8, 29.7, 40.5, 44.8,
3
55.9, 62.1, 79.7, 108.8, 121.1, 132.5, 136.5, 147.8, 148.3, 152.0, 153.7,
1
+
155.0, 162.8, 166.0. MS (ESI) m/z 385.3 (M + H) .
(
(
(
7-tert-Butoxycarbonyl-2-exo-[2′-amino-3′-(2-chloropyridin-
4-yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptane (12c). A solution
of compound 9 (192 mg, 0.52 mmol, 1.0 equiv), 2-chloropyridine-4-
3
Hz, 1H), 7.71 (dd, J = 2.5, 8.2 1H), 7.88 (d, J = 2.2 Hz, 1H), 8.38 (d, J
boronic acid (131 mg, 0.83 mmol, 1.6 equiv), Pd(OAc)
mmol, 10 mol %), P(o-tolyl) (32 mg, 0.104 mmol, 20 mol %), and
Na CO (111 mg, 1.04 mmol, 2.0 equiv) in DME (6 mL) and water
(0.7 mL) was degassed through bubbling nitrogen for 20 min. The
mixture was sealed and heated over a sand bath at 80 °C for 5 h. After
cooling to room temperature, the mixture was poured into a saturated
2
(12 mg, 0.052
=
2.2 Hz, 1H); 13C NMR (CDCl ) δ 28.3 (3 C), 28.8, 29.7, 40.3, 44.8,
3
3
5
1
5.9, 62.1, 79.5, 116.7, 124.2, 132.2, 133.1, 136.9, 139.0, 147.0, 149.5,
50.6, 154.4, 155.0; MS (ESI) m/z 401.5 (M + H) .
2
3
+
7
-tert-Butoxycarbonyl-2-exo-[2′-amino-3′-(6-methoxypyri-
din-3-yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptane (11d). A sol-
ution of 9 (337 mg, 0.92 mmol, 1.0 equiv), 2-methoxypyridine-5-
boronic acid (182 mg, 1.2 mmol, 1.3 equiv), Pd(PPh ) (53 mg, 0.046
aqueous solution of NaHCO (20 mL) and extracted with EtOAc (3 ×
3
3
4
30 mL). The combined organic layers were dried over MgSO , filtered
4
mmol, 5 mol %), and K CO (253 mg, 1.83 mmol, 2.0 equiv) in
through Celite, and concentrated in vacuo. The resultant residue was
2
3
toluene (12 mL), EtOH (2 mL), and H O (2 mL) was placed in a
resealable pressure vessel and degassed through bubbling nitrogen for
purified by flash chromatography on silica gel using EtOAc−hexanes as
2
1
the eluent to provide 112 mg (54%) of 12c as a colorless oil. H NMR
2
0 min. The vessel was sealed and placed on a sand bath that was
(300 MHz, CDCl ) δ 1.41 (br s, 9H), 1.49−1.61 (m, 2H), 1.77−1.83
3
heated at 110 °C overnight. After cooling to room temperature, H O
(m, 3H), 1.94−2.00 (m, 1H), 2.78−2.83 (m, 1H), 4.16 (s, 1H), 4.36
(br s, 1H), 4.54 (s, 2 NH), 7.37 (dd, J = 1.4, 5.13 Hz, 1H), 7.40 (d, J =
2.22 Hz, 1H), 7.45 (s, 1H), 8.0 (d, J = 2.22 Hz, 1H), 8.44 (d, J = 5.10
2
(
20 mL) was added, and the organic product was extracted with
EtOAc (3 × 30 mL). The combined organic layers were dried over
anhydrous MgSO , filtered through Celite, and concentrated in vacuo.
Hz, 1H); 13C NMR (CDCl ) δ 28.3 (3 C), 28.8, 29.7, 40.5, 44.8, 55.9,
4
3
The residue was purified by flash chromatography on silica gel using
62.1, 79.6, 117.4, 122.0, 123.8, 132.4, 136.5, 147.8, 149.6, 150.2, 152.4,
153.8, 154.9; MS (ESI) m/z 401.3 (M + H) .
Preparation of 2-Methoxypyidine-4-boronic Acid Pinacol
Ester (15). A solution of 4-bromo-2-methoxypyridine (14) (462 mg,
2.46 mmol, 1.0 equiv), bis(pinacolato)diboron (749 mg, 2.95 mmol,
+
EtOAc−hexanes as the eluent to furnish compound 11d (310 mg,
1
9
1
2
1
7
2%) as a colorless oil. H NMR (300 MHz, CDCl ) δ 1.39 (br s, 9H),
3
.50−1.59 (m, 2H), 1.76−1.88 (m, 3H), 1.91−1.98 (m, 1H), 2.77−
.81 (dd, J = 3.7, 5.0 Hz, 1H), 3.94 (s, 3H), 4.16 (s, 1H), 4.34 (br s,
H), 4.78 (s, 2 NH), 6.79 (d, J = 8.5 Hz, 1H), 7.30 (d, J = 2.3 Hz, 1H),
.65 (dd, J = 8.4, 2.4 Hz, 1H), 7.93 (d, J = 2.3 Hz, 1H) 8.22 (d, J = 2.3
1.2 equiv), PdCl (dppf) (54 mg, 0.074 mmol, 3 mol %), and KOAc
2
(724 mg, 7.37 mmol, 3.0 equiv) in DMF (6 mL) in a resealable
pressure vessel was degassed through bubbling nitrogen for 20 min.
The reaction mixture was sealed and heated over a sand bath at 85 °C
overnight. After cooling to room temperature, the mixture was diluted
with EtOAc and filtered through a plug of Celite and anhydrous
Na SO . The solvent was removed in vacuo, and the residue was
13
Hz, 1H); C NMR (CDCl ) δ 28.1 (3 C), 28.4, 28.6, 40.1, 44.8, 53.3,
3
5
1
5.3, 62.1, 79.3, 110.8, 118.1, 126.9, 128.4, 131.8, 136.7, 138.9, 145.8,
46.6, 154.8, 163.5; MS (ESI) m/z 397.5 (M + H) .
+
7
-tert-Butoxycarbonyl-2-exo-[2′-amino-3′-(pyridin-4-yl)-5′-
pyridinyl]-7-azabicyclo[2.2.1]heptane (12a). A solution of
compound 9 (354 mg, 0 0.96 mmol, 1.0 equiv), pyridine-4-boronic
acid (154 mg, 1.25 mmol, 1.3 equiv), Pd(PPh ) (56 mg, 0.048 mmol,
2
4
purified by flash chromatography on silica gel using EtOAc−MeOH as
1
the eluent to provide 427.4 mg (74%) of 15 as a brownish oil. H
3
4
5
mol %), and K CO (266 mg, 1.92 mmol, 2.0 equiv) in toluene (10
NMR (300 MHz, CDCl ) δ 1.34 (s, 12H), 3.93 (s, 3H), 7.13 (s, 1H),
2
3
3
mL), EtOH (2 mL), and water (2 mL) was degassed through bubbling
nitrogen for 20 min. The mixture was sealed and heated over a sand
bath at 110 °C for 22 h. After cooling to room temperature, 20 mL of
7.18 (d, J = 5.0 Hz, 1H), 8.18 (d, J = 5.0 Hz, 1H).
[ H]Epibatidine Binding Assay. The inhibition of [ H]-
3
3
epibatidine binding at rat brain α4β2*-nAChRs was conducted as
2
1
H O was added, and the organic product was extracted with EtOAc (3
previously reported.
Electrophysiology. The electrophysiology assays with rat α4β2-,
2
×
30 mL). The combined organic layers were dried over MgSO4,
filtered through Celite, and concentrated in vacuo. The resultant
2
4
α3β4, and α7-nAChRs were conducted as previously described.
residue was purified by flash chromatography on silica gel using i-
In Vivo Test. The antinociception (tail-flick and hot-plate),
1
PrOH−hexanes as the eluent to provide 340 mg (97%) of 12a. H
locomotor, and body temperature tests were all conducted in mice
2
1
NMR (300 MHz, CDCl ) δ 1.39 (br s, 9H), 1.44−1.59 (m, 2H),
as previously described.
3
1
.81−1.84 (m, 3H), 1.93−2.00 (m, 1H), 2.79−2.84 (dd, J = 3.8, 5.0
Hz, 1H), 4.16 (s, 1H), 4.36 (br s, 1H), 4.67 (s, 2 NH), 7.39−7.43 (m,
H), 7.99 (d, J = 2.3 Hz, 1H), 8.66 (dd, J = 6.0, 1.5 Hz, 1H); 13
NMR (CDCl ) δ 28.3 (3 C), 28.8, 29.7, 40.4, 44.8, 55.8, 62.1, 79.5,
ASSOCIATED CONTENT
Supporting Information
■
3
C
*
S
3
1
1
18.7, 123.4 (2 C), 132.2, 136.5, 146.4, 147.2, 150.5 (2 C), 153.9,
54.9; MS (ESI) m/z 367.6 (M + H) .
Elemental analysis data. This material is available free of charge
via the Internet at http://pubs.acs.org.
+
7
-tert-Butoxycarbonyl-2-exo-[2′-amino-3′-(2-fluoropyridin-
4
-yl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptane (12b). A solution
of compound 9 (319 mg, 0.87 mmol, 1.0 equiv), 2-fluoropyridine-4-
AUTHOR INFORMATION
■
boronic acid (196 mg, 1.39 mmol, 1.6 equiv), Pd(OAc) (20 mg, 0.087
2
Corresponding Author
mmol, 10 mol %), P(o-tolyl) (53 mg, 0.173 mmol, 20 mol %), and
3
Na CO (184 mg, 1.73 mmol, 2.0 equiv) in DME (6 mL) and water
*Phone: (919) 541-6679; fax: (919) 541-8868; e-mail: fic@rti.
org.
2
3
(
0.7 mL) was degassed through bubbling nitrogen for 20 min. The
K
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Present Address
Jakub M. Bartkowiak: Virginia Commonwealth University
School of Medicine, Richmond, Virginia 23298, United States.
Article
receptors in rat and monkey striatum. J. Pharmacol. Exp. Ther. 2012,
⊥
3
(
42, 327−334.
13) U.S. Food and Drug Administration. FDA Drug Safety
Communication: Chantix (varenicline) may increase the risk of
certain cardiovascular adverse events in patients with cardiovascular
disease (Safety Announcement June 16, 2011). http://www.fda.gov/
Drugs/Drugsafety/ucm259161.htm (accessed September 2013).
Notes
The authors declare no competing financial interest.
(
14) Lloyd, G. K.; Williams, M. Neuronal nicotinic acetylcholine
receptors as novel drug targets. J. Pharmacol. Exp. Ther. 2000, 292,
61−467.
15) Spade, T. E.; Garraffo, H. M.; Edwards, M. W.; Yeh, H. J. C.;
ACKNOWLEDGMENTS
■
This research was supported by the National Institute on Drug
Abuse Grant DA12001.
4
(
Pannell, L.; Daly, J. W. Epibatidine: A novel (chloropyridyl)-
azabicycloheptane with potent analgesic activity from an Ecuadoran
poison frog. J. Am. Chem. Soc. 1992, 114, 3475−3478.
ABBREVIATIONS USED
■
NRT, nicotine replacement therapy; CDC, Centers for Disease
Control; nAChR, nicotinic acetylcholine receptor; ICSS,
intracranial self-stimulation; CPP, conditioned place prefer-
ence; DD, drug discrimination; SA, self-administration;
(16) Badio, B.; Daly, J. W. Epibatidine, a potent analgetic and
nicotinic agonist. Mol. Pharmacol. 1994, 45, 563−569.
(17) Badio, B.; Shi, D.; Garraffo, M.; Daly, J. W. Antinociceptive
effects of the alkaloid epibatidine: Further studies on involvement of
nicotinic receptors. Drug Dev. Res. 1995, 36, 46−59.
(18) Carroll, F. I. Epibatidine structure−activity relationships. Bioorg.
Med. Chem. Lett. 2004, 14, 1889−1896.
PdCl (dppf), 1,1′-bis(diphenylphosphino)ferrocene-palladium-
2
(
II) dichloride; MCPBA, meta-chloroperoxybenzoic acid; MPE,
maximum potential effect; DHβE, dihydro-β-erythroidine;
(
19) Carroll, F. I. Epibatidine analogs synthesized for characterization
of nicotinic pharmacophoresA review. Heterocycles 2009, 79, 99−
20.
20) Carroll, F. I.; Lee, J. R.; Navarro, H. A.; Brieaddy, L. E.;
AD , the antagonist dose that blocks 50% of the nicotine
5
0
response
1
(
REFERENCES
■
Abraham, P.; Damaj, M. I.; Martin, B. R. Synthesis, nicotinic
acetylcholine receptor binding, and antinociceptive properties of 2-
exo-2-(2′-substituted-3′-phenyl-5′-pyridinyl)-7-azabicyclo[2.2.1]-
heptanes. Novel nicotinic antagonist. J. Med. Chem. 2001, 44, 4039−
4041.
(21) Carroll, F. I.; Ware, R.; Brieaddy, L. E.; Navarro, H. A.; Damaj,
M. I.; Martin, B. R. Synthesis, nicotinic acetylcholine receptor binding,
and antinociceptive properties of 2′-fluoro-3′-(substituted phenyl)-
deschloroepibatidine analogs. Novel nicotinic antagonist. J. Med. Chem.
(
1) World Health Organization. WHO Report on the Global Tobacco
Epidemic, 2011; Warning about the Dangers of Tobacco; 2011.
2) Center for Disease Control and Prevention.. Vital Signs: Current
(
cigarette smoking among adults aged ≥18 years: United States, 2009.
Morbidity and Mortality Weekly Report 2010, 59, 1135−1140.
(
3) SAMHSA. (Substance Abuse and Mental Health Services
Administration) Results from the 2010 National Survey on Drug Use
and Health: Summary of National Findings; U. S. Department of Health
and Human Services: Washington, DC, 2011.
4) American Cancer Society. Cancer Facts & Figures 2011; American
Cancer Society: Atlanta, Georgia, 2011.
5) De Biasi, M.; Dani, J. A. Reward, addiction, withdrawal to
nicotine. Annu. Rev. Neurosci. 2011, 34, 105−130.
6) Hurt, R. D.; Sachs, D. P.; Glover, E. D.; Offord, K. P.; Johnston, J.
2
(
2
004, 47, 4588−4594.
22) Abdrakhmanova, G. R.; Damaj, M. I.; Carroll, F. I.; Martin, B. R.
-Fluoro-3-(4-nitro-phenyl)deschloroepibatidine is a novel potent
(
(
competitive antagonist of human neuronal α4β2 nAChRs. Mol.
Pharmacol. 2006, 69, 1945−1952.
(
(23) Tobey, K. M.; Walentiny, D. M.; Wiley, J. L.; Carroll, F. I.;
A.; Dale, L. C.; Khayrallah, M. A.; Schroeder, D. R.; Glover, P. N.;
Sullivan, C. R.; Croghan, I. T.; Sullivan, P. M. A comparison of
sustained-release bupropion and placebo for smoking cessation. N.
Engl. J. Med. 1997, 337, 1195−1202.
Damaj, M. I.; Azar, M. R.; Koob, G. F.; George, O.; Harris, L. S.; Vann,
R. E. Effects of the specific α4β2 nAChR antagonist, 2-fluoro-3-(4-
nitrophenyl)deschloroepibatidine, on nicotine reward-related behav-
iors. Psychopharmacology (Berlin, Ger.) 2012, 223, 159−168.
(
7) Hayford, K. E.; Patten, C. A.; Rummans, T. A.; Schroeder, D. R.;
(24) Ondachi, P.; Castro, A.; Luetje, C. W.; Damaj, M. I.; Mascarella,
Offord, K. P.; Croghan, I. T.; Glover, E. D.; Sachs, D. P.; Hurt, R. D.
Efficacy of bupropion for smoking cessation in smokers with a former
history of major depression or alcoholism. Br. J. Psychiatry 1999, 174,
73−178.
8) Slemmer, J. E.; Martin, B. R.; Damaj, M. I. Bupropion is a
nicotinic antagonist. J. Pharmacol. Exp. Ther. 2000, 295, 321−327.
9) Coe, J. W.; Brooks, P. R.; Vetelino, M. G.; Wirtz, M. C.; Arnold,
E. P.; Huang, J.; Sands, S. B.; Davis, T. I.; Lebel, L. A.; Fox, C. B.;
Shrikhande, A.; Heym, J. H.; Schaeffer, E.; Rollema, H.; Lu, Y.;
Mansbach, R. S.; Chambers, L. K.; Rovetti, C. C.; Schulz, D. W.;
Tingley, F. D., III; O’Neill, B. T. Varenicline: An α4β2 nicotinic
receptor partial agonist for smoking cessation. J. Med. Chem. 2005, 48,
S. W.; Navarro, H. A.; Carroll, F. I. Synthesis and nicotinic
acetylcholine receptor in vitro and in vivo pharmacological properties
of 2′-fluoro-3′-(substituted phenyl)deschloroepibatidine analogues of
1
(
2
5
(
′-fluoro-3′-(4-nitrophenyl)deschloroepibatidine. J. Med. Chem. 2012,
5, 6512−6522.
25) Brieaddy, L. E.; Liang, F.; Abraham, P.; Lee, J. R.; Carroll, F. I.
(
New synthesis of 7-(tert-butoxycarbonyl)-7-azabicyclo[2.2.1]hept-2-
ene. A key intermediate in the synthesis of epibatidine and analogs.
Tetrahedron Lett. 1998, 39, 5321−5322.
(26) Carroll, F. I.; Liang, F.; Navarro, H. A.; Brieaddy, L. E.;
Abraham, P.; Damaj, M. I.; Martin, B. R. Synthesis, nicotinic
acetylcholine receptor binding, and antinociceptive properties of 2-
exo-2-(2′-substituted 5′-pyridinyl)-7-azabicyclo[2.2.1]heptanes. Epiba-
tidine analogues. J. Med. Chem. 2001, 44, 2229−2237.
3
(
474−3477.
10) Coe, J. W.; Brooks, P. R.; Wirtz, M. C.; Bashore, C. G.; Bianco,
K. E.; Vetelino, M. G.; Arnold, E. P.; Lebel, L. A.; Fox, C. B.; Tingley,
F. D., III; Schulz, D. W.; Davis, T. I.; Sands, S. B.; Mansbach, R. S.;
Rollema, H.; O’Neill, B. T. 3,5-Bicyclic aryl piperidines: A novel class
of α4β2 neuronal nicotinic receptor partial agonists for smoking
cessation. Bioorg. Med. Chem. Lett. 2005, 15, 4889−4897.
(27) Gao, Y.; Horti, A. G.; Kuwabara, H.; Ravert, H. T.; Hilton, J.;
Holt, D. P.; Kumar, A.; Alexander, M.; Endres, C. J.; Wong, D. F.;
Dannals, R. F. Derivatives of (-)-7-methyl-2-(5-(pyridinyl)pyridin-3-
yl)-7-azabicyclo[2.2.1]heptane are potential ligands for positron
emission tomography imaging of extrathalamic nicotinic acetylcholine
receptors. J. Med. Chem. 2007, 50, 3814−3824.
(28) Billingsley, K. L.; Barder, T. E.; Buchwald, S. L. Palladium-
catalyzed borylation of aryl chlorides: Scope, applications, and
computational studies. Angew. Chem., Int. Ed. 2007, 46, 5359−5363.
(
11) Mihalak, K. B.; Carroll, F. I.; Luetje, C. W. Varenicline is a
partial agonist at α4β2 and a full agonist at α7 neuronal nicotinic
receptors. Mol. Pharmacol. 2006, 70, 801−805.
12) Bordia, T.; Hrachova, M.; Chin, M.; McIntosh, J. M.; Quik, M.
(
Varenicline is a potent partial agonist at α6β2* nicotinic acetylcholine
L
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Article
(
29) Martin, T.; Laguerre, C.; Hoarau, C.; Marsais, F. Highly efficient
borylation Suzuki coupling process for 4-bromo-2-ketothiazoles:
straightforward access to micrococcinate and saramycetate esters.
Org. Lett. 2009, 11, 3690−3693.
(
30) AlSharari, S. D.; Carroll, F. I.; McIntosh, J. M.; Damaj, M. I. The
antinociceptive effects of nicotinic partial agonists varenicline and
sazetidine-A in murine acute and tonic pain models. J. Pharmacol. Exp.
Ther. 2012, 342, 742−749.
(
31) Ortiz, N. C.; O’Neill, H. C.; Marks, M. J.; Grady, S. R.
Varenicline blocks β2*-nAChR-mediated response and activates β4*-
nAChR-mediated responses in mice in vivo. Nicotine Tob. Res. 2012,
1
(
4, 711−719.
32) Summerfield, S. G.; Read, K.; Begley, D. J.; Obradovic, T.;
Hidalgo, I. J.; Coggon, S.; Lewis, A. V.; Porter, R. A.; Jeffrey, P. Central
nervous system drug disposition: The relationship between in situ
brain permeability and brain free fraction. J. Pharmacol. Exp. Ther.
2
(
007, 322, 205−213.
33) Ghose, A. K.; Herbertz, T.; Hudkins, R. L.; Dorsey, B. D.;
Mallamo, J. P. Knowledge-based, central nervous system (CNS) lead
selection and lead optimization for CNS drug discovery. ACS Chem.
Neurosci. 2012, 3, 50−68.
(
34) Clark, D. E. Rapid calculation of polar molecular surface area
and its application to the prediction of transport phenomena. 2.
Prediction of blood−brain barrier penetration. J. Pharm. Sci. 1999, 88,
8
15−821.
M
dx.doi.org/10.1021/jm401602p | J. Med. Chem. XXXX, XXX, XXX−XXX