Synthesis and evaluation of a novel series of 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)- 3-(2-(4-pyridinyl)vinyl)pyridine analogues as potential positron emission tomography imaging agents for nicotinic acetylcholine receptors

Article abstract of DOI:10.1021/jm010550n

Reportedly, 2-[¹⁸F]fluoro-A-85380, 1, a promising radiotracer for imaging the nicotinic acetylcholine receptor (nAChR) by positron emission tomography (PET) in humans, exhibits slow penetration through the blood-brain barrier (BBB) due to its low lipophilicity. A ligand for nAChRs with greater lipophilicity than that of 1 would be potentially more favorable for PET imaging of nAChR due to its faster penetration through the BBB. Herein, a novel series of compounds has been developed based on the high affinity ligand for nAChRs, 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)- 3-(2-(4-pyridinyl)vinyl)pyridine, 3b. The in vitro binding affinities for the new series were found to be in the range of K_i = 9-331 pM. A molecular modeling study showed differences in the comformational profiles and the electronic properties of these compounds, which provides further insight into the structure-activity relationships at nAChR. Lipophilicities of the compounds 3b-6b have been found to be substantially higher than that of 1. As a result, compounds 3b-6b might exhibit a faster penetration through the BBB than the less lipophilic 1. The N-methyl derivatives 3b and 6b demonstrated very high affinities at nAChRs (K_i = 28 and 23 pM, respectively) and will be targets for development of ¹¹CH₃-labeled derivatives as radiotracers for PET imaging of nAChRs.

Full text of DOI:10.1021/jm010550n

J . Med. Chem. 2002, 45, 2841-2849
2841
Syn th esis a n d Eva lu a tion of a Novel Ser ies of
2-Ch lor o-5-((1-m eth yl-2-(S)-p yr r olid in yl)m eth oxy)-3-(2-(4-p yr id in yl)vin yl)p yr id in e
An a logu es a s P oten tia l P ositr on Em ission Tom ogr a p h y Im a gin g Agen ts for
Nicotin ic Acetylch olin e Recep tor s
LaVerne L. Brown,† Santosh Kulkarni,‡ Olga A. Pavlova, Andrei O. Koren,† Alexey G. Mukhin,†
Amy H. Newman,‡ and Andrew G. Horti*,†
Neuroimaging Research Branch and Medicinal Chemistry Section, Intramural Research Program, National Institute on Drug
Abuse, 5500 Nathan Shock Drive, Baltimore, Maryland 21224
Received December 4, 2001
Reportedly, 2-[18F]fluoro-A-85380, 1, a promising radiotracer for imaging the nicotinic
acetylcholine receptor (nAChR) by positron emission tomography (PET) in humans, exhibits
slow penetration through the blood-brain barrier (BBB) due to its low lipophilicity. A ligand
for nAChRs with greater lipophilicity than that of 1 would be potentially more favorable for
PET imaging of nAChR due to its faster penetration through the BBB. Herein, a novel series
of compounds has been developed based on the high affinity ligand for nAChRs, 2-chloro-5-
((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine, 3b. The in vitro binding
affinities for the new series were found to be in the range of Ki ) 9-331 pM. A molecular
modeling study showed differences in the comformational profiles and the electronic properties
of these compounds, which provides further insight into the structure-activity relationships
at nAChR. Lipophilicities of the compounds 3b-6b have been found to be substantially higher
than that of 1. As a result, compounds 3b-6b might exhibit a faster penetration through the
BBB than the less lipophilic 1. The N-methyl derivatives 3b and 6b demonstrated very high
affinities at nAChRs (Ki ) 28 and 23 pM, respectively) and will be targets for development of
11CH3-labeled derivatives as radiotracers for PET imaging of nAChRs.
In tr od u ction
brain. Soon after the discovery of the high affinity
nAChR agonist epibatidine,7 several of its analogues
were radiolabeled with PET isotopes C-11 and F-18.8-10
However, despite the success of [18F]fluorine-labeled
epibatidine ([18F]FPH) in animal studies,12-14 its use in
PET studies with humans is prohibited due to high
levels of toxicity.10,11 In the late 1990s, Abbott Labora-
tories reported break-through publications on the 3-py-
ridyl ether series of nAChR ligands.15 Some of these
derivatives were equipotent to epibatidine at R4â2 type
nAChRs in the in vitro binding assay, but unlike
epibatidine, the compounds of the Abbott series exhib-
ited low functional activity at R3âx subtype nAChRs.15
The latter receptors are located in the peripheral
nervous system and adrenals, and potentially, they are
responsible for adverse side effects of nAChR ligands.
We speculated that the high affinity for the R4â2
combined with a low functional activity at the R3âx
subtypes of the 3-pyridyl ether series would be advanta-
geous for the development of high affinity radiotracers
with low side effects in vivo. Several radiolabeled
analogues of A-85380 have been synthesized.16-20 2-[18F]-
fluoro-A-85380 (1) has been used in animal studies21,22
to image nAChRs, and it is currently the most promising
candidate for PET studies in the human brain. Unlike
[18F]FPH, 1 shows no adverse side effects at doses
substantially exceeding the tracer dose range. Unfor-
tunately, in PET studies with nonhuman primates, 1
exhibited very slow kinetics in the brain.22,23 Conse-
quently, 1 was deemed less than ideal for occupancy
studies of endogenous and exogenous ligands in humans
Positron emission tomography (PET) is a three-
dimensional (3D) imaging technique for the noninvasive
measurement of positron-emitting radioisotopes in vivo.
The technique requires tracer compounds of physiologi-
cal interest that are labeled with positron-emitting
isotopes. A wide variety of positron-emitting pharma-
ceuticals incorporating carbon-11 and fluorine-18 have
been developed to image brain receptors.1
The R4â2 type nicotinic acetylcholine receptors
(nAChRs) are the most prominent subtype of nAChRs
in mammalian brain.2 These receptors have been linked
to many central nervous system disorders, including
Alzheimer’s disease, Parkinson’s disease, Tourette’s
syndrome, schizophrenia and attention deficit hyperac-
tivity disorder.2 For example, decreased density of
nAChRs in the hippocampus of patients with schizo-
phrenia and in the cortex of patients with Alzheimer’s
and Parkinson’s diseases has been reported.3-5 Also,
studies show increased density of nAChR in smoking
patients when compared to nonsmoking patients.4,5
To date, there have been no reports of high specific
uptake PET tracers for the study of nAChRs in human
subjects. The 11C-labeled nicotine6 was not satisfactory
for such studies due to its high level of nonspecific
binding, rapid metabolism, and rapid clearance from the
* To whom correspondence should be addressed. Tel.: 410-550-2916.
Fax: 410-550-2724. E-mail: ahorti@intra.nida.nih.gov.
† Neuroimaging Research Branch.
‡ Medicinal Chemistry Section.
10.1021/jm010550n This article not subject to U.S. Copyright. Published 2002 by the American Chemical Society
Published on Web 05/29/2002

2842 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 13
Brown et al.
F igu r e 1.
Sch em e 1a
a
Reagents: (a) I2, K2CO3, DMF; (b) POCl3, quinoline; (c) iron powder, H2O, AcOH; (d) HBF4, NaNO2/H2O; (e) Ac2O; (f) 2 N KOH.
with respect to other nAChR ligands such as nicotine.
A radiotracer exhibiting faster penetration through the
blood-brain barrier (BBB) should be favored for these
PET studies over the currently available radiotracers.
Assuming that nAChR ligands penetrate the BBB via
a simple diffusion mechanism, we proposed that ligands
that are more lipophilic than 1 should cross the BBB
at a faster rate. However, radiotracers with higher
lipophilicities display increased levels of nonspecific
binding and, as a result, produce noisy PET images. For
this reason, lipophilic PET radiotracers are not useful
unless they exhibit very high binding affinity and
correspondingly high specific binding, revealing PET
images with lower noise. Therefore, in our investigation
of novel PET tracers for nAChRs with more favorable
kinetics than that of 1, we searched for compound(s)
with higher lipophilicity and higher binding affinity.
affinity relationships of the series were examined using
molecular modeling calculations.
Resu lts a n d Discu ssion
Ch em istr y. The syntheses of 3-7 are outlined in
Schemes 1-4. In our initial experiments, the inter-
mediate 3-bromo-2-chloro-5-((1-(tert-butoxycarbonyl)-2-
(S)-pyrrolidinyl)methoxy)pyridine25 gave low yields in
the subsequent palladium-assisted Heck coupling with
4-vinylpyridine. Therefore, in anticipation of higher
yields and fewer byproducts in the Heck reaction, we
synthesized more reactive iodo intermediates (7a ,b).
The iodo intermediates (7a ,b) were obtained in high
yields via Mitsunobu coupling (Scheme 2) of 2-chloro-
5-hydroxy-3-iodopyridine (6) with 1-(tert-butoxycarbo-
nyl)-2(S)-azetidinemethanol or 1-(tert-butoxycarbonyl)-
2(S)-pyrrolidinemethanol. A simple five step synthesis
of 6 is outlined in Scheme 1.
Heck coupling of 7a with 4-vinylpyridine gave 3c with
20, 70, and 95% yields in the presence of triethylamine,
diisopropylethylamine, or 1,2,2,6,6-pentamethylpiperi-
dine (PMP), respectively (data not shown). Therefore,
PMP was used in subsequent syntheses of 4c, 5c, and
7c. Syntheses of 3b-7b were carried out by reductive
methylation of 3a ,c-7a ,c with paraformaldehyde in a
solution of formic acid.
The compound exhibiting the highest affinity for
nAChRs in the Abbott series, 2-chloro-5-((1-methyl-2-
(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyri-
dine, 3b (Ki ) 4.4 pM vs [3H]cytisine24), was chosen as
a lead molecule in our studies. Because 3b is signifi-
cantly more lipophilic than 1 (as estimated by ACD/
LogD Suite) and exhibited a higher affinity for nAChRs,
we speculated that the radiolabeling of 3b with 11C
would yield a PET radiotracer with faster penetration
through the BBB than that of 1. Here, we report the
synthesis of several novel analogues of 3b. The in vitro
binding affinities for compounds 3a ,b-7a ,b (see Figure
1) have been determined, and the structure-binding
Hydrogenation of 3c in ethyl acetate in the presence
of platinum oxide gave 6c. Noteworthy attempts to
obtain 6c via the hydrogenation of 3c using Pd/C in
methanol or in ethyl acetate were unsuccessful. Instead,

Agents for Nicotininc Acetylcholine Receptors
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 13 2843
Sch em e 2
Sch em e 3
Ta ble 1. In Vitro Inhibition of 5-[125I]A-85380 Binding to
nAChRs in Rat Brain
Sch em e 4
mean Ki (pM)
( SEM, 23 °C
mean Ki (pM)
( SEM, 23 °C
compd
compd
1
2
61 ( 5
10 ( 1
5b
6a
6b
7a
7b
8
265 ( 5
11 ( 1
23 ( 1
9 ( 1
72 ( 6
10 ( 1
3a
3b
4a
4b
5a
15 ( 1
28 ( 3
28 ( 1
88 ( 11
331 ( 43
suitable ligand for competition assays at room temper-
ature.27 All of the compounds in our series, 3b-7b,
competitively inhibited 2 in rat P2 membranes at room
temperature (23 °C) (Table 1).
the norchloro derivative (8) was obtained in high yields
(Scheme 4). The BOC groups of 3c-7c were readily
removed by treatment with trifluoroacetic acid (TFA)
yielding the TFA salts of 3a -7a .
Structure-activity relationships, in the N-methyl
series, 3b-5b, revealed that the para isomer (3b)
exhibited the highest binding affinity at the R4â2
subtype of nAChRs followed by the meta isomer (4b)
and then the ortho isomer (5b). Reportedly, the phenyl-
substituted analogue of 3b binds with high affinity to
nAChRs.28 These data suggest that the pyridyl nitrogen
in the para position may not necessarily enhance
affinity. However, the pyridyl nitrogen in the ortho and
meta positions may actually decrease optimal binding
affinity. Saturation of the olefin bond in the vinylpyridyl
moiety of 3a ,b resulted in corresponding ethylpyridyl
derivatives 6a ,b and did not affect affinity for nAChRs.
The binding affinities of the N-normethyl derivatives
of the pyrrolidine analogues (3a -6a ) were within a
3-fold range of those exhibited by the corresponding
N-methyl derivatives (3b-6b). With the exception of
5a ,b, the N-normethyl analogues were more potent than
the corresponding N-methylated analogues. The binding
affinity exhibited by the azetidinyl analogue, 7a , was
equivalent to that exhibited by epibatidine. Further-
Lip op h ilicity. All of the compounds in our series
(3b-7b) are substantially more lipophilic than 1. Com-
putation of the dissociative partition coefficients in
octanol-water system at pH 7.4 (ElogDoct7.4) with ap-
propriate software26 gave for compounds 1, 3b, 4b, 5b,
6b, and 7b the ElogDoct7.4 values as follow: -2.09, 1.59,
1.82, 1.55, 1.08, and 1.09. The partition coefficients
between octanol and water at pH 7.4 of compounds 1
7.4
7.4
(logDoct ) -2.06) and 3b (logDoct ) 1.60) were also
measured by a conventional shake-flask method. The
results of the shake-flask method were in remarkably
good agreement with calculated results.
Str u ctu r e-Activity Rela tion sh ip s. It has been
reported that compounds 3a (Ki 39 pM), and, especially,
3b (Ki 4.4 pM) are extremely high affinity ligands at
central nAChRs vs [3H]cytisine at 4 °C.25 In our studies,
competition binding assays were performed using 5-[125I]-
iodo-A-85380 (2) at 23 °C. Compound 2 has been shown
to selectively bind at the R4â2 subtype of nAChRs with
high affinity and with high specificity, and it is a

2844 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 13
Brown et al.
Ta ble 2. Number of Superimposable Conformers, Distances
between Pharmacophore Elements, and RMSD for
(-)-Epibatidine and nAChR Ligands
more, 7a showed the largest difference in binding
affinity as compared to its corresponding N-methyl
derivative (7b). Compound 7a exhibited an 8-fold higher
affinity for R4â2 type nAChRs at 23 °C than 7b.
no. of superimposed conformers
internitrogen RMSDc
compds
0.0-1.0a
1.0-2.0a
distanceb (Å)
(Å)
Molecu la r Mod elin g. The development of
a
8
5.5
nAChR pharmacophore has been the target of many
papers.24,29-33 These studies have proposed a model for
nAChR ligands that contains three essential groups: (i)
a cationic center (e.g., a basic or quaternary sp3 nitrogen
atom), (ii) an electronegative atom capable of forming
a hydrogen bond (e.g., a pyridine-like nitrogen or a
carbonyl oxygen), and (iii) a dummy point or an atom
to define a line along which the hydrogen bond may
form. Optimal distances between these groups have
been proposed. Synthesis of a novel series of high
affinity nAChR ligands has necessitated the develop-
ment of a more-refined pharmacophore. Because the
affinity in the present series of compounds appeared to
be influenced by structural and electronic effects, these
approaches were used to explain the structure-activity
relationship.
3a
3b
7a
7b
1
16(40)d
14(36)
7(20)
4(15)
11(37)
11(32)
20(81)
22(75)
20(76)
23(67)
27(85)
25(90)
6.82
6.82
6.82
6.82
6.66
6.80
0.768
0.768
0.781
0.780
0.784
0.781
2
a
The energy window in kcal/mol calculated from the lowest
energy conformer. Distance between the sp3 nitrogen and the
b
nitrogen of the chloropyridine ring. c RMSD of the three pharma-
d
cophore elements from those of (-)-epibatidine. Values in pa-
rentheses indicate the total number of conformers in that par-
ticular energy window.
ropyridine ring.35 Therefore, while comparing the com-
formational profiles of these compounds, the chloropy-
ridine ring was always held coplanar with that of
epibatidine. A visual analysis was also included while
selecting the conformer for superimposition. This analy-
sis was performed on compounds 3a ,b and 7a ,b. The
results from compounds 3a ,b should be applicable to
compounds 4a ,b-6a ,b. Two compounds, 1 and 2, from
our previous studies were used for comparison.23 The
number of conformers of these compounds, which can
be superimposed on epibatidine, was partitioned into
different energy windows (Table 2). Compounds 3a ,b
had nearly identical conformational profiles. However,
the conformational profiles of 7a ,b were quite different.
Only 26% (4 out of 15) of the conformers of 7b were
superimposable within the 0-1.0 kcal/mol window, but
for compound 7a , 35% (7 out of 20) of the conformers
were superimposable. In addition, for compound 7b,
34% (23 out of 67) of the conformers in the 1.0-2.0 kcal/
mol window were superimposed as compared to 26% (20
out of 76) for compound 7a . Therefore, one explanation
for the 8-fold decrease in affinity of 7b relative to 7a
could be due to the fact that the conformational en-
semble contains many higher energy conformers that
can be superimposed on epibatidine.
A similar analysis was performed on 2-fluoro-A-85380
and 5-iodo-A-85380 from a previous study.23 The sub-
stitution in the 2-position of the pyridine ring of A-85830
reduced binding affinity to a greater degree than that
of a compound substituted at the 5-position. A molecular
modeling study on these compounds showed that bulky
substitutions on the 2-position affect the geometry of
the molecules. As shown in Table 2, a higher number
of conformers can be superimposed from the 1.0-2.0
kcal/mol window for 1 than that from 0 to 1.0 kcal/mol
window. Thus, for these nicotinic ligands, it appears
that the location of binding elements around the nico-
tinic pharmacophore, as defined by epibatidine, in the
conformational ensembles is important for optimal
binding affinity.
(-)-Epibatidine (8) was used as a template for super-
imposition studies. Epibatidine possesses a rigid 7-aza-
bicyclo[2.2.1]heptane skeleton and a rotatable bond
connecting the chloropyridine ring. A systematic con-
formational search on epibatidine yielded two low
energy conformers. Both conformers showed small con-
formational energy barriers but differed by a 180°
rotation of the chloropyridine ring resulting in an
internitrogen distance of 4.4 Å for one conformer and
5.5 Å for the other conformer. Thus, it was difficult to
be sure of which low energy conformer of epibatidine
was responsible for high affinity binding to nAChRs. A
conformational profile analysis of epibatidine (8) with
molecular mechanics, semiempirical, and ab initio meth-
ods indicated that a mixture of all of the conformational
minima and some population of the intermediate struc-
tures are responsible for the binding of epibatidine to
nAChRs.34 Previous modeling studies have shown that
the internitrogen distance for epibatidine was longer
than that which was found in nicotine (4.87 Å) and
exceeds that of a proposed nAChR pharmacophore.30
Therefore, the conformer with the internitrogen distance
of 5.5 Å was initially selected as a template.
Compounds (3a ,b-7a ,b) in this series contain flexible
ether linkages connecting two pharmacophoric ele-
ments. Thus, it is possible that many conformers of
these compounds can be superimposed onto epibatidine.
Also, it is well-known that the receptor-bound confor-
mation of the ligands may not be the same as the low
energy conformation. Thus, it is important to consider
the conformational profiles of the flexible compounds
while superimposing onto the template molecule. Gen-
erally, it is assumed that for flexible molecules the
differences in affinities could be explained based on the
superimposibility of the conformers on the template
structure. The conformational ensemble within the 2
kcal/mol of the lowest energy conformer for each com-
pound was analyzed by superimposition onto epibati-
dine, using the sp3 nitrogen, the nitrogen in the chlo-
ropyridine ring, and the centroid of the chloropyridine
ring. Each conformer was scored by its root mean square
of deviation (RMSD) values. The nAChR pharmacophore
elements are approximately in a plane with the chlo-
Once these compounds were superimposed on epiba-
tidine, a difference in the orientation of the N-methyl
group of compounds 3b and 7b was observed (see Figure
2). The measurement of the angle between the direction
of the N-methyl group of compounds 3b and 7b shows
a difference of approximately 25°. It should be remem-
bered that the N-methyl group in nicotine is essential
for activity,36 whereas N-methylation of epibatidine

Agents for Nicotininc Acetylcholine Receptors
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 13 2845
F igu r e 3. Dipole moments are shown for each compound after
superimposition on (-)-epibatidine.
obtained in the present study are consistent with the
ligand binding domain of the acetylcholine binding
protein.
F igu r e 2. Orientation of the substituent on the sp3 nitrogen
of nicotinic compounds. Hydrogens are eliminated from the
display for clarity.
The compounds 3a ,b-5a ,b contain similar ether
linkages; however, the substitution at position three of
the chloropyridine ring was varied systematically. A
preliminary analysis of these compounds showed that
the electronic nature of the pendant pyridyl substitution
affects binding affinity. The dipole moments of the
compounds were visualized and compared with nicotine
and epibatidine (Figure 3). A clear trend was observed
wherein the more active compound 3a shows a dipole
moment in a similar direction to epibatidine, whereas
for compounds 4a and 5a the dipole moment was
oriented in different directions. Hence, the electron
density and its localization also influence nAChR bind-
ing affinity. Lack of uniform biological data on a larger
scale precluded a quantitative correlation of the elec-
tronic descriptors with activity. However, this analysis
shows the importance of electrostatic interactions for
activity on the nAChR.
reduces activity.37 This differential effect of the N-
methyl group suggests that the nAChR contains a
narrow binding pocket that requires proper orientation
of the groups for optimal binding affinity. Thus, the
N-methyl group in the present series of compounds
binds in a different environment, and because of con-
formational flexibility of the pyridyl ether compounds,
the sp3 nitrogen could orient in a different direction.
Furthermore, the internitrogen distance in these
conformers was found to be much larger than that which
has been proposed for the nicotinic pharmacophore
(Table 2).29 There has been disagreement on this
parameter in the literature,30-32 and the flexibility of
these compounds makes the task of identifying a com-
mon binding pharmacophore difficult. Therefore, in the
absence of a conformationally restricted analogue hav-
ing a longer internitrogen distance, it is difficult to
suggest an optimum internitrogen distance. Overall, the
nAChR appears to have wide flexibility with respect to
binding of the ligands. The identification of specific
types of interactions of the pharmacophore elements
with the receptor will help to refine the current under-
standing of the nAChR pharmacophore.
For example, structure-activity data have shown that
the pyridine nitrogen is involved in hydrogen-bonding
types of interactions.38 The sp3 nitrogen was proposed
to form an ionic interaction with the anionic site in the
receptor.39 It has also been suggested that the sp3
nitrogen may form cation-π interactions with the
aromatic residues in the receptor.40,41 Recently, Sixma
and co-workers reported the X-ray crystal structure of
acetylcholine binding protein from a snail, which has
high sequence similarity with the N-terminal domain
of an R-subunit of nAChRs.42 This crystal structure may
provide clues toward mapping the topology of the ligand
binding domain of nAChRs. The binding domain in this
protein is lined with hydrophobic residues, and tryp-
tophan-143 was found to be involved in cation-π
interactions with the cationic center of the ligand. Also,
the cationic binding site was surrounded with aromatic
residues, which may interact with the sp3 nitrogen of
the ligands. The walls of the binding domain are formed
by flat aromatic residues such as tyrosines and tryp-
tophans suggesting that the aromatic ring of the ligands
may bind in this region, which is coplanar with the
cationic binding site. The molecular modeling data
Con clu sion s
Compound 1 is, to date, the most promising candidate
for the quantitative imaging of nAChRs in human
studies. However, the kinetics exhibited by 1 are likely
too slow for occupancy studies of nAChR ligands due to
7.4
its high hydrophilicity (logDoct ) -2.1).
A series of novel analogues of 3a ,b were synthesized
and evaluated for binding at R4â2 type nAChRs. All of
the compounds in our series (3a ,b-7a ,b) exhibited a
wide range of affinities for nAChRs (Ki ) 9-331 pM)
measured at room temperature. Compound 7a was
discovered to bind with exceptionally high affinity to
R4â2 type nAChRs (Ki ) 9 pM). The lipophilicities of
7.4
compounds 3b-7b were within the range of logDoct
) 1-2.
Molecular modeling performed with the conforma-
tional profile analysis provides further insight into the
structure-activity relationship of the present series.
The N-methyl group in these compounds influences the
activity by altering the conformational mobility and
interactions within the receptor pocket. Furthermore,
replacement of the vinyl-4-pyridyl substituent of com-
pounds 3a ,b with ethyl-4-pyridyl moiety of 6a ,b had no
substantial effect on binding affinities for nAChRs.
However, the position of the nitrogen atom in the
vinylpyridyl substituent dramatically affects the binding
affinity within the series 3a ,b-5a ,b, perhaps due to the
changes in the dipole moment orientation.
Because the N-methyl derivatives 3b and 6b demon-
strated very high affinities at nAChRs (Ki ) 28 and 23

2846 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 13
Brown et al.
Compound 5 was obtained as a dark nonviscous liquid (8.32
g, 0.028mol, 72% yield). 1H NMR (CDCl3/TMS): δ 2.35 (s, 3H),
8.0 (d, 1H), 8.21 (d, 1H).
pM, respectively), they will be targets for development
of 11CH3-labeled derivatives as radiotracers for PET
imaging of nAChRs. High lipophilicity of 3b and 6b
suggests that their 11C-radiolabeled analogues might
exhibit a faster penetration through the BBB than the
less lipophilic 1.
2-Ch lor o-5-h yd r oxy-3-iod op yr id in e (6). Compound 5 (8.4
g, 2.8 mmol) was added to KOH (50 mL, 2 N) at 5 °C. After it
was mixed for 24 h in an ice bath, the unreacted starting
material (1.38 g) was removed via filtration. The solution was
made acidic, pH 5, with glacial acetic acid, and the resulting
solid was filtered. Compound 6 was obtained as a tan powder
(4.48 g, 0.018mol, 63% yield); mp 150-160 °C. 1H NMR
(acetone-d6/TMS): δ 7.81 (d, 1H,), 8.05 (d, 1H).
2-Ch lor o-3-iod o-5-((1-(ter t-bu toxyca r bon yl)-2-(S)-p yr -
r olid in yl)m eth oxy)p yr id in e (7a ). Diethyl azodicarboxylate
(0.72 mL, 4.6 mmol) and triphenylphosphine (1.2 g, 4.6 mmol)
were mixed in THF (11.4 mL) at 0 °C, under argon for 30 min.
N-BOC-prolinol (0.92 g, 4.6 mmol) and 6 (0.98 g, 3.83 mmol)
were then added to the reaction flask, and the mixture was
stirred at room temperature for 3 h. The solvent was evapo-
rated under reduced pressure at 55 °C, and the crude oil was
purified via flash chromatography (80:20 hexane:ethyl acetate,
300 mL). Compound 7a was obtained as a yellow oil (1.19 g,
2.7 mmol, 71% yield). 1H NMR (CDCl3/TMS): δ 1.49 (s, 9H,
BOC), 1.87-2.05 (m, 4H), 3.35-3.50 (m, 2H), 4.09-4.22 (m,
3H), 7.7 (d, 1H), 8.1 (d, 1H).
Exp er im en ta l Section
Ch em istr y Gen er a l. All chemicals were purchased from
Aldrich, Fluka, or Sigma. Analytical thin-layer chromatogra-
phies (TLCs) were run on precoated plates of silica gel (0.25
mm, F254, Alltech). Flash chromatography was conducted
using silica gel (230-400 mesh, Merck). Nuclear magnetic
resonance (NMR) spectra were recorded on a Bruker (300
MHz) instrument using deuterated solvents and tetrameth-
ylsilane (TMS) as an internal standard. Galbraith Laborato-
ries, Inc. performed elemental analyses, and all novel com-
pounds gave satisfactory elemental results (C, H, and N,
(0.4%).
2-Hyd r oxy-3-iod o-5-n itr op yr id in e (2). 2-Hydroxy-5-ni-
tropyridine, 1, (14 g, 0.1mol) and potassium carbonate (17 g,
0.1mol) were added to dimethylformamide (DMF; 100 mL, 1.3
mol). Iodine (25.3 g, 0.1mol) was added to the reaction mixture
over a 3 min period. After the addition of iodine was complete,
the mixture was heated to 80-95 °C for 8 h. The dark reaction
mixture was rotary-evaporated to one-half of its initial volume,
then cooled to room temperature, and diluted with water (150
mL), and the solid precipitate was filtered. The filtered solution
was then made acidic (pH ) 5) with glacial acetic acid. The
resulting yellow solid was filtered and dried under vacuum to
give 2 (11.3 g, 0.043mol, 45% yield). 1H NMR (acetone-d6/
TMS): δ 8.76 (t, 1H), 8.82 (t, 1H). Anal. calcd for C5H3N2IO2.
2-Ch lor o-3-iod o-5-n it r op yr id in e (3). 2-Hydroxy-3-iodo-
5-nitropyridine, 2, (70.5 g, 0.27mol) was added to quinoline
(16 mL, 0.133mol). The reaction flask was cooled to 5 °C, and
phosphoryl chloride (25 mL, 0.27mol) was added dropwise. The
mixture was blanketed with argon and heated to 120 °C for 2
h. Upon complete consumption of the precursor, as indicated
by TLC, the mixture was cooled to room temperature and 100
mL of H2O was added. The mixture was then cooled to 0 °C,
and the resulting brown solid was filtered. Recrystallization
from ethanol gave sand-colored crystals, 3 (60 g, 0.21mol, 78%
2-Ch lor o-3-iod o-5-((1-(ter t-bu toxyca r bon yl)-2-(S)-a ze-
tid in yl)m eth oxy)p yr id in e (7b). Diethyl azodicarboxylate
(0.16 mL, 1.08 mmol) and triphenylphosphine (0.28 g, 1.08
mmol) were mixed in THF (3 mL) at 0 °C, under argon, for 15
min. N-BOC-azetidinylmethanol (0.2 g, 0.88 mmol) and 6 (0.32
g, 0.88 mmol) were then added to the reaction flask in a
solution of THF (5 mL). The mixture was stirred at room
temperature for 3 h. The solvent was evaporated under
reduced pressure at 40 °C, and the crude oil was purified via
flash chromatography (50:50 petroleum ether:ethyl acetate,
200 mL). Compound 7b was obtained as a yellow oil (0.351 g,
1
0.82 mmol, 94% yield). H NMR (CDCl3/TMS): δ 1.50 (s, 9H,
BOC), 2.25-2.4 (m, 2H), 3.8 (m, 2H), 4.15 (m, 1H), 4.35 (m,
1H), 4.55 (m, 1H), 7.7 (d, 1H), 8.1 (d, 1H).
2-Ch lor o-5-((1-(ter t-bu toxycar bon yl)-2-(S)-pyr r olidin yl)-
m eth oxy)-3-(2-(4-p yr id in yl)vin yl)p yr id in e (3c) a n d Gen -
er a l P r oced u r e for 3c-5c. Compound 7a (80 mg, 1.83 mmol)
was dissolved in acetonitrile (6.4 mL). The solution was
blanketed with argon as vinylpyridine (0.22 mL, 2.08 mmol),
PMP (0.36 mL, 2.08 mmol), palladium acetate (0.04 g, 0.18
mmol), and o-tritolylphosphine (0.054 g, 0.18 mmol) were
added to the reaction flask. The mixture was stirred for 2-24
h at 95 °C until the hihg-performance liquid chromatography
(HPLC) monitoring of the reaction indicated complete con-
sumption of the precursor. The solvent was then evaporated
at 60 °C. The crude oil was purified via flash chromatography
using a short silica gel column (100% diethyl ether, 600 mL).
1
yield); mp 92-97 °C. H NMR (CDCl3/TMS): δ 8.88 (d, 1H),
9.15 (d, 1H).
5-Am in o-2-ch lor o-3-iod op yr id in e (4). Compound 3 was
added to a solution of H2O (616 mL) and glacial acetic acid
(816 mL). Iron powder (49 g, 0.88mol) was then added to the
reaction flask, and the mixture was heated to 65-70 °C. TLC
monitoring of the reaction after 5 h indicated no reaction. After
6 h, the exothermic reaction was apparent by a sudden
increase in temperature to 85 °C and bubbling in the flask.
The bubbling ceased, after approximately 10 min, and the
reaction temperature returned to 70 °C. TLC monitoring of
the reaction indicated complete consumption of the precursor.
H2O (500 mL) was added to the reaction flask, and the mixture
was made basic, pH g8, with potassium hydroxide pellets. The
mixture was then extracted with ethyl acetate (3 × 250 mL)
and evaporated to a yellow solid, 4 (30.55 g, 0.12mol, 68%
1
3c. 95% yield. H NMR (CDCl3/TMS): δ 1.5 (s, 9H), 2.0 (m,
4H), 3.45 (m, 2H), 3.9 (m, 1H), 4.1 (m, 2H), 7.43 (d, 2H), 7.58
(d, 1H), 7.95 (m, 1H), 8.05 (d, 1H), 8.62 (d, 2H).
1
4c. 52% yield. H NMR (CDCl3/TMS): δ 1.5 (s, 9H), 2.0 (m,
4H), 3.45 (m, 2H), 3.9 (m, 1H), 4.1-4.3 (m, 2H), 7.4-7.6 (m,
3H), 8.0-8.2 (m, 3H), 8.5 (d, 1H), 8.8 (d, 1H).
1
5c. 95% yield. H NMR (CDCl3/TMS): δ 1.5 (s, 9H), 2.0 (m,
4H), 3.3-3.45 (m, 2H), 3.9 (m, 1H), 4.1 (m, 2H), 7.2 (m, 2H),
7.5 (m, 1H), 7.7 (m, 1H), 7.8 (m, 2H), 8.05 (d, 1H), 8.65 (d,
1H).
1
yield); mp 105-107 °C. H NMR (CDCl3/TMS): δ 3.75 (br s,
2H), 7.51 (d, 1H), 7.85 (d, 1H). Anal. calcd for C5H4N2ICl‚
0.12CH3CO2H.
2-Ch lor o-5-((1-(ter t-bu toxycar bon yl)-2-(S)-pyr r olidin yl)-
m eth oxy)-3-(2-(4-p yr id in yl)vin yl)p yr id in e (7c). Compound
7b (100 mg, 0.24 mmol) was dissolved in acetonitrile (8 mL).
The solution was blanketed with argon as 4-vinylpyridine (0.29
mL, 2.67 mmol), diisopropylethylamine (0.46 mL, 2.64 mmol),
palladium acetate (0.05 g, 0.23 mmol), and o-tritolylphosphine
(0.07 g, 0.23 mmol) were added to the reaction flask. The
mixture was stirred for 3 h at 95 °C. TLC monitoring of the
reaction indicated complete consumption of the precursor. The
solvent was then evaporated via rotary evaporation at 60 °C.
The crude oil was purified via flash chromatography on a short
silica gel column (100% ethyl acetate, 400 mL) to give a yellow
5-Acetoxy-2-ch lor o-3-iod op yr id in e (5). Compound 4 (10
g, 39 mmol) was added to HBF4 (59 mL, 50%) and cooled to 0
°C. A solution of sodium nitrite (2.85 g, 41.8 mmol) and H2O
(39 mL) was added dropwise over a period of 1 h. After the
addition of sodium nitrite was complete, the mixture was
stirred for 1 h at 0 °C. The resulting solid was filtered, washed
with diethyl ether (3 × 16 mL), and air-dried for 15 min. The
solid was then dissolved in acetic anhydride (47 mL) and
heated for 1 h to 70-85 °C. The solvent was evaporated, and
the resulting residue was dissolved in diethyl ether (35 mL).
The diethyl ether solution was washed with H2O (4 × 20 mL),
dried over MgSO4, and evaporated under reduced pressure.
1
oil (41 mg, 0.102 mmol, 44% yield). H NMR (CDCl3/TMS): δ

Agents for Nicotininc Acetylcholine Receptors
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 13 2847
1.4 (s, 9H), 2.4 (m, 2H), 3.9 (m, 2H), 4.2 (m, 1H), 4.55 (m, 1H),
4.65 (m, 1H), 7.05 (d, 1H), 7.4 (m, 2H), 7.55 (d, 1H), 7.65 (m,
1H), 8.10 (d, 1H), 8.65 (m, 2H).
precursor was indicated by HPLC. The mixture was filtered
through a C18 Sep-Pak, first with ethyl acetate and then with
methanol, into two flasks. A norchloro analogue (8) was
obtained from the methanol fraction (20 mg). Compound 6c
was obtained from the ethyl acetate fraction as a viscous oil
(43 mg, 0.1 mmol, 25% yield). 1H NMR (CDCl3/TMS): δ 1.5
(s, 9H), 1.9-2.09 (m, 4H), 2.98 (m, 4H), 3.39 (m, 2H), 4.05-
4.2 (m, 3H), 7.12 (m, 3H), 7.86 (d, 1H), 8.49 (d, 2H).
2-Ch lor o-5-(2-(S)-p yr r olid in yl)m eth oxy)-3-(2-(4-p yr id i-
n yl)vin yl)p yr id in e (3a ) a n d Gen er a l P r oced u r e for 4a ,
5a , a n d 7a . TFA (2.5 mL, 0.032mol) was added to a solution
of 3c (410 mg, 0.99 mmol) and dichloromethane (10 mL). The
reaction was stirred at room temperature overnight. After the
reaction was complete, as monitored by TLC, the solvent was
evaporated via rotary evaporation at 50 °C. Compound 3a was
obtained as a viscous oil in the form of a TFA salt. Attempts
to purify the salt, via flash chromatography on a short silica
gel column (600 mL of 90% CH2Cl2, 10% CH3OH, 0.4% NH4-
OH), resulted in significant degradation. Purification via
HPLC (82:9:9 0.1% TFA in H2O:CH3CN:CH3OH, Preparative
PRP-1 Column, 7 mL/min) gave 3a ‚TFA (130 mg, 0.41 mmol,
42% yield). For NMR analysis, a portion of 3a ‚TFA was
dissolved in H2O (basified with NaHCO3 to pH 8.5) and
extracted with diethyl ether to give 3a in the form of a free
base (all other isomers were saved as TFA salts to maintain
stability). 1H NMR (acetone-d6/TMS): δ 2.08-2.2 (m, 4H), 3.55
(m, 2H), 4.25 (m, 1H), 4.6 (m, 2H), 7.47 (d, 1H), 7.6 (d, 2H),
7.65 (d, 1H), 8.0 (d, 1H), 8.15 (d, 1H), 8.62 (d, 2H). Anal. calcd
for C17H18N3ClO‚2.8TFA‚1.5H2O.
1
8. H NMR (CDCl3/TMS): δ 1.5 (s, 9H), 1.9-2.09 (m, 4H),
2.98 (m, 4H), 3.39 (m, 2H), 4.05-4.2 (m, 3H), 7.12 (m, 3H),
7.85 (d, 1H), 7.86 (d, 1H), 8.49 (d, 2H).
2-Ch lor o-5-(2-(S)-p yr r olid in yl)m eth oxy)-3-(2-(4-p yr id i-
n yl)eth yl)p yr id in e (6a ). TFA (0.9 mL) was added to a
solution of 6c (167 mg, 0.4 mmol) and dichloromethane (10
mL). The mixture was stirred overnight at room temperature.
After the reaction was complete, the solvent was rotary-
evaporated at 50 °C. Compound 6a ‚TFA was obtained as a
1
viscous oil (218 mg). H NMR (CDCl3/TMS): δ 1.25-1.65 (m,
4H), 1.7-2.1 (m, 4H), 3.5 (m, 2H), 3.8-3.95 (m, 3H), 6.98 (d,
1H), 7.12 (d, 2H), 7.95 (d, 1H), 8.56 (d, 2H). Anal. calcd for
C
17H20N3ClO‚2.9TFA‚1.5H2O.
2-Ch lor o-5-((1-m et h yl-2-(S)-p yr r olid in yl)m et h oxy)-3-
(2-(4-p yr id in yl)eth yl)p yr id in e (6b). Compound 6a ‚TFA (40
mg, 0.126 mmol) was dissolved in formic acid (1 mL). Paraform-
aldehyde (28 mg) was added, and the reaction mixture was
stirred for 5 h at 105 °C. After the reaction was complete, the
solution was made basic (pH g8) with anhydrous potassium
carbonate. The solution was then extracted with ethyl acetate
and dried over sodium sulfate. Rotary evaporation at 60 °C
gave 6b as a yellow oil (10 mg, 0.03 mmol, 24% yield). 1H NMR
(CDCl3/TMS): δ 1.7-1.81 (m, 4H), 1.9-2.05 (m, 4H), 2.49 (s,
3H), 2.6-2.7 (m, 2H), 3.85-4.0 (m, 3H), 6.95 (d, 1H), 7.05 (d,
2H), 7.98 (d, 1H), 8.5 (d, 2H). Anal. calcd for C18H22N3ClO‚
2.5TFA‚MeOH‚1.3H2O.
Com p etition Bin d in g Assa y. Competition assays with 2
were performed as described previously.27 Briefly, the rat brain
P2 membrane fractions (5-10 µg of protein) were incubated
at 23 °C for 3 h in 0.4 mL of HEPES-salt solution (pH 7.4) in
the presence of 10 pM 2 and 9-11 concentrations of test
compounds. Nonspecific binding was determined in the pres-
ence of (-)-nicotine (300 µM). The binding was terminated by
a vacuum filtration through GF/B filters pretreated with 1%
polyethyleneimine. Radioactivity was measured using LKB
Wallac 1277 γ-counter (efficiency 67%). All assays were carried
out in duplicate.
4a ‚3.5TF A‚2H2O. 58% yield. 1H NMR (acetone-d6/TMS): δ
1.9-2.9 (m, 5H), 3.5 (m, 1H), 4.1-4.3 (m, 2H), 4.6 (m, 1H),
7.6-7.8 (m, 2H), 8.01-8.16 (m, 3H), 8.8 (m, 2H), 9.13 (d, 1H).
Anal. calcd for C17H18N3ClO‚3.5TFA‚2H2O.
1
5a ‚2.9TF A‚1.5H2O. 47% yield. H NMR (acetone-d6/TMS):
δ 2.2-3.0 (m, 5H), 3.56 (m, 2H), 4.3 (m, 1H), 4.56 (m, 2H),
7.7-8.4 (m, 7H), 8.77 (d, 1H).
7a ‚3.5TF A. 50% yield. 1H NMR (acetone-d6/TMS): δ 1.4 (s,
9H), 2.4 (m, 2H), 3.9 (m, 2H), 4.2 (m, 1H), 4.55 (m, 1H), 4.65
(m, 1H), 7.05 (d, 1H), 7.4 (m, 2H), 7.55 (d, 1H), 7.65 (m, 1H),
8.10 (d, 1H), 8.65 (m, 2H).
2-Ch lor o-5-((1-m et h yl-2-(S)-p yr r olid in yl)m et h oxy)-3-
(2-(4-p yr id in yl)vin yl)p yr id in e (3b) a n d Gen er a l P r oce-
d u r e for 4b, 5b, a n d 7b. Compound 3a (420 mg, 1.33 mmol)
was dissolved in formic acid (2.1 mL). Paraformaldehyde (220
mg) was added, and the reaction mixture was stirred for 5 h
at 105 °C. After the reaction was complete, the solvent was
evaporated via rotary evaporation at 60 °C. Hydrochloric acid
(4.2 mL, 36%) was added to the residue and extracted with
diethyl ether (3 × 8 mL). The extract was washed with
NaHCO3, dried over MgSO4, and rotary-evaporated at 60 °C
to a yellow oil. The oil was rapidly purified by preparative
HPLC (PRP-1 column, 84:8:8 0.1% TFA in H2O:ACN:CH3OH)
forming 3b‚TFA as a viscous oil (81 mg, 0.25 mmol, 19% yield).
For NMR analysis, a portion of 3b‚TFA was dissolved in H2O
and extracted with diethyl ether to give 3 in the form of a free
base. 1H NMR (CDCl3/TMS): δ 1.72-1.98 (m, 4H), 2.35-2.45
(m, 1H), 2.51 (s, 3H), 2.71-2.8 (m, 1H), 3.19 (m, 1H), 3.98-
4.05 (m, 2H), 6.9 (d, 1H), 7.41 (d, 2H), 7.52 (d, 1H), 7.57 (d,
1H), 8.06 (d, 1H), 8.62 (d, 2H). Anal. calcd for C18H20N3ClO‚
3TFA‚2H2O‚0.3HCl.
The Ki values were calculated by the Cheng-Prusoff equa-
tion based on the measured IC50 values and Kd ) 10 pM for
binding of 2. These Kd values were determined in direct
binding assays as described previously.27 The direct binding
assays were performed on the same membrane preparations
that were used for the competition assay.
Lip op h ilicity Estim a tion . Dissociative partition coef-
ficient (LogDoct7.4) determination for 1 and 3b. Flask-shake
method: 0.6 mg of compound 123 or 3b were added to a vial
containing 1 mL of octanol and 1 mL of saline (pH 7.4 was set
by addition of 0.1 M sodium phosphate buffer). The vial was
shaken for 2.5 h, the mixture was allowed to equilibrate for
48 h at room temperature, and the ligand concentrations in
4b. Yield 15%. 1H NMR (CDCl3/TMS): δ 1.72-1.98 (m, 6H),
2.35-2.45 (m, 1H), 2.51 (s, 3H), 2.71-2.8 (m, 1H), 3.19 (m,
1H), 3.98-4.05 (m, 2H), 7.1 (d, 1H), 7.35 (m, 1H), 7.41 (d, 1H),
7.52 (d, 1H), 7.9 (m, 1H), 8.06 (d, 1H), 8.55 (m, 1H), 8.75 (d,
1H). Anal. calcd for C18H20N3ClO‚0.7C3H6O‚0.3H2O.
7.4
each layer were determined by HPLC analysis. LogDoct
values for 1 and 3b were estimated by dividing the peak area
1
from the octanol layer by the peak area from the saline layer
5b. Yield 35%. H NMR (CDCl3/TMS): δ 1.72-2.1 (m, 4H),
7.4
7.4
and calculating the log. LogDoct (1) ) -2.06; logDoct (3b)
2.35-2.45 (m, 1H), 2.51 (s, 3H), 2.71-2.8 (m, 1H), 3.19 (m,
1H), 3.98-4.05 (m, 2H), 7.1-7.3 (m, 2H), 7.49 (d, 1H), 7.59 (d,
1H), 7.7 (m, 1H), 7.85 (d, 1H), 8.05 (d, 1H), 8.68 (d, 1H). Anal.
calcd. for C18H20N3ClO‚0.25TFA‚0.8CH3CN.
) 1.60.
Molecu la r Mod elin g. A molecular modeling study was
performed with Sybyl 6.7 running on a Silicon Graphics Octane
computer.43 The structures were built from the standard
fragment library in Sybyl. The structures were minimized in
1
7b. Yield 52%. H NMR (CDCl3/TMS): δ 2.1 (s, 6H), 2.5 (s,
3H), 3.0 (m, 1H), 3.5 (m, 3H), 4.1 (m, 2H), 4.4 (m, 1H), 7.05 (d,
1H), 7.3-7.7 (m, 4H), 8.10 (m, 1H), 8.65 (m, 2H). Anal. calcd
for C17H18ClN3O‚0.7EtOAc‚0.3HCOOH.
a
vacuum using a conjugate gradient algorithm until a
convergence gradient of 0.001 kcal/mol/Å was reached. The
rule-based Gasteiger Marsilli charges were assigned to these
molecules during minimization.
The conformational analysis was performed using the
systematic search routine. All rotatable single bonds were
rotated by 15° increments from 0 to 360°. Conformers having
2-Ch lor o-5-((1-(ter t-bu toxycar bon yl)-2-(S)-pyr r olidin yl)-
m eth oxy)-3-(2-(4-p yr id in yl)eth yl)p yr id in e (6c). Platinum
oxide (195 mg) was added to a solution of ethyl acetate (14
mL) and 3c (177 mg, 0.4 mmol) and hydrogenated at 50 psi
(Parr hydrogenator) for 3 h. Complete consumption of the

2848 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 13
Brown et al.
(15) Abreo, M. A.; Lin, N. H.; Garvey, D. S.; Gunn, D. E.; Hettinger,
A. M.; Wasicak, J . T.; Pavlik, P. A.; Martin, Y. C.; Donnelly-
Robert, D. L.; Anderson, D. J .; Sullivan, J . P.; Williams, M.;
Arneric, S. P.; Holladay, M. W. Novel 3-Pyridyl Ethers with
subnanomolar affinity for central neuronal nicotinic acetylcho-
line receptors. J . Med. Chem. 1996, 39, 817-825.
(16) Kassiou, M.; Ravert, H. T.; Mathews, W. B.; Musachio, J . L.;
London, E. D.; Dannals, R. F. Synthesis of 3-[(1-[11C]methyl-
2(S)-pyrrolidinyl)methoxy]pyridine and 3-[(1-[11C]methyl-2(R)-
pyrrolidinyl)methoxy]pyridine: radioligands for in vivo studies
of neuronal acetylcholine receptors. J . Labelled Compd. Radio-
pharm. 1997, 39, 425-431.
(17) Horti, A. G.; Koren, A. O.; Ravert, H. T.; Musachio, J . L.;
Mathews, W. B.; London, E. D.; Dannals, R. F. Synthesis of a
radiotracer for studying nicotinic acethylcholine receptors: 2-[18F]-
fluoro-3-(2(S)-azetidinylmethoxy)pyridine (2-[18F]A-85380). J .
Labelled Compd. Radiopharm. 1998, 41, 309-318.
(18) Dolle, F.; Valette, H.; Bottlaender, M.; Hinner, F.; Vaufrey, F.;
Guenter, I.; Crouzel, C. Synthesis of a radiotracer for studying
nicotinic acethylcholine receptors: 2-[18F]fluoro-3-[(2(S)-2-aze-
tidinylmethoxy)pyridine, a highly potent radioligands for in vivo
imaging central nicotinic acetylcholine receptors. J . Labelled
Compd. Radiopharm. 1998, 41, 451-463.
(19) Horti, A. G.; Chefer, S. I.; Mukhin, A. G.; Koren, A. O.; Gu¨ndisch,
D.; Links, J . M.; Kurian, V.; Dannals, R. F.; London, E. D. 6-[18F]-
fluoro-A-85380, a novel radioligand for in vivo imaging of central
nicotinic acetylcholine receptors. Life Sci. 2000, 67, 463-469.
(20) Sihver, T.; Fasth, K. J .; Horti, A. G.; Koren, A. O.; Bergstrom,
M.; Lu, L.; Hagberg, G.; Lundqvist, H.; Dannals, R. F.; London,
E. D.; Nordberg, A.; Langstrom, B. Synthesis and characteriza-
tion of binding of 5-[Br-76]bromo-3-[[2(S)-azetidinyl]methoxy]-
pyridine, a novel nicotinic acetylcholine receptor ligand, in rat
brain. J . Neurochem. 1999, 73, 1264-1272.
bad steric interactions were rejected automatically using the
van der Waals radii scaling factors of 0.95, 0.87, and 0.65 for
general, 1-4, and H-bond interactions. The conformational
profile of each compound was evaluated. (-)-Epibatidine (8)
was used as a template for superimposition, and a conforma-
tional search on it was performed analogously. Superimposi-
tion was performed by three point alignment using the sp3
nitrogen, the chloropyridine nitrogen, and the centroid of the
chloropyridine ring. The best conformer for each compound was
selected based on its RMSD value, distance parameters, and
visual analysis. It was observed that for all compounds, the
lowest energy conformer could not provide a better superim-
position on (-)-epibatidine (8); hence, conformers with higher
than the lowest energy were used for superimposition. The
selected conformers were minimized to local minima. The
dipole moments were computed after assigning the MOPAC
charges calculated from AM1 Hamiltonian.
Ack n ow led gm en t. We are grateful for discussions
with Dr. Alane Kimes. We also thank Mr. Robert Silvia
for administrative support and Ms. Alvina Walters and
Ms. Cindy Ambriz for secretarial assistance.
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