Synthesis and nicotinic acetylcholine receptor binding properties of exo-2-(2'-fluoro-5'-pyridinyl)-7-azabicyclo-[2.2.1]heptane: A new positron emission tomography ligand for nicotinic receptors

Full text of DOI:10.1021/jm970187d

J . Med. Chem. 1997, 40, 2293-2295
2293
compare the nAChR binding properties of 1a ,b to that
of nicotine. We also report the synthesis of 7-(tert-
butyloxycarbonyl)-2-exo-[2′-(N,N,N-trimethylammoni-
umyl)-5′-pyridinyl]-7-azabicyclo[2.2.1]heptane iodide (2),
which is a more efficient precursor7,8 for the synthesis
of [18F]-1b (70% radiochemical yield)7 than exo-2-(2′-
bromo-5′-pyridinyl)-7-azabicyclo[2.2.1]heptane (1d ) or
N-protected analogs.9-11
A number of different chemical syntheses3,12-23 of
epibatidine (1a ) have been reported, including one from
our laboratory.15 A review of these methods suggested
that 1b might best be prepared by appropriate modifi-
cation of the synthesis of epibatidine (1a ) reported by
Clayton and Regan.13 Their synthesis involved subjec-
tion of the olefin (3) to a reductive Heck arylation using
2-chloro-5-iodopyridine (4a ) followed by removal of the
N-methyloxycarbonyl protecting group with hydrogen
bromide in acetic acid. This is the shortest synthesis
of epibatidine (1a ) thus far reported and gives only the
desired exo product (1a ).
Syn th esis a n d Nicotin ic Acetylch olin e
Recep tor Bin d in g P r op er ties of
exo-2-(2′-F lu or o-5′-p yr id in yl)-7-a za bicyclo-
[2.2.1]h ep ta n e: A New P ositr on Em ission
Tom ogr a p h y Liga n d for Nicotin ic
Recep tor s
Feng Liang,† Herna´n A. Navarro,† Philip Abraham,†
Pravin Kotian,† Yu-Shin Ding,‡ J oanna Fowler,‡
Nora Volkow,‡ Michael J . Kuhar,§ and
F. Ivy Carroll*,†
Chemistry and Life Sciences, Research
Triangle Institute, P.O. Box 12194,
Research Triangle Park, North Carolina 27709,
Brookhaven National Laboratory, Upton, New York 11973,
and National Institute on Drug Abuse, Addiction Research
Center, P.O. Box 5180, Baltimore, Maryland 21224
Received March 18, 1997
Epibatidine, a unique alkaloid, was originally isolated
from the skin of the Ecuadoran poison frog, Epipe-
dobates tricolor, by Daly and co-workers and has been
shown to have the structure exo-2-(2′-chloro-5′-pyridi-
nyl)-7-azabicyclo[2.2.1]heptane (1a ).1,2 Later reports
showed that the natural alkaloid possessed the 1R,2R,4S
stereochemistry.3 Numerous biological studies have
demonstrated that epibatidine (1a ) is a potent analgesic
agent.4 Its effects appear to be mediated via neuronal
nicotinic receptors (nAChRs); however, details of this
action at the molecular level have not been fully
elucidated. Nevertheless, epibatidine (1a ) has been a
very useful tool for gaining new information concerning
the pharmacological properties of neuronal nAChRs.4
Recently, we synthesized and characterized the in
vivo binding properties of (+)- and (-)-exo-2-(3-[6-3H]-
pyridinyl)-7-azabicyclo[2.2.1]heptane (1c, [3H]norchlo-
roepibatidine, [3H]NCEPB) in rats.5 Both (+)- and (-)-
[3H]NCEPB bind with high affinity but with higher
levels of specific binding in vivo for nAChR as compared
to epibatidine (1a ).5 Other investigators observed simi-
lar results with [3H]epibatidine.6 These studies sug-
gested that exo-2-(2′-fluoro-5′-pyridinyl)-7-azabicyclo-
[2.2.1]heptane (1b, norchlorofluoroepibatidine, NFEP)
might also have favorable binding properties and that
the fluorine-18 analog would be useful as a positron
emission tomography (PET) ligand for further charac-
terization of the nAChRs.
We found that 1b could be prepared by the route
shown in Scheme 1. Since we knew that the 2-fluoro-
pyridinyl group of 1b would be more reactive than the
2-chloropyridinyl group of epibatidine, we chose to use
an N-tert-butyloxycarbonyl protecting group which could
be removed under milder conditions in place of the
N-methyloxycarbonyl group used by Clayton and
Regan.13 Thus, heating a solution of (p-tolylsulfonyl)-
acetylene13 (5) and N-(tert-butoxycarbonyl)pyrrole24 (6)
provided 65% of the diene 7. Nickel boride reduction
of 7 gives 86% of the olefin 8. Desulfonation of 8 using
2.5% sodium amalgam in a 1:1 mixture of ethyl acetate
and tert-butyl alcohol yielded 55% of the desired 7-(tert-
butoxycarbonyl)-7-azabicyclo[2.2.1]hept-2-ene (9). Cou-
pling of 9 with 2-amino-5-iodopyridine (4b) using pal-
ladium acetate as catalyst in dimethylformamide
containing tetrabutylammonium chloride and potassium
formate provided 68% of 7-(tert-butoxycarbonyl)-exo-2-
(2′-amino-5′-pyridinyl)-7-azabicyclo[2.2.1]heptane (10).
The 2-exo stereochemistry assigned to 10 was based on
an analysis of the 1H NMR (CDCl3) spectrum. The
spectrum showed a doublet of doublets at 2.74 ppm for
the H-2 proton with J 2R,3â ) 5.0 Hz, J 2R,3R ) 8.8 Hz, and
J 2R,1 ) 0 Hz, which is characteristic of the 2-exo
stereochemistry.15 Diazotization25 of 10 in pyridine
containing 70% hydrogen fluoride effected conversion
of the 2-amino group to a fluoro group and deprotection
of the N-Boc group to give 46% of 1b. Compound 1b
could also be obtained by direct reductive Heck coupling
of 2-fluoro-5-iodopyridine (4c, obtained by diazotization
of 2-amino-5-iodopyridine (4b)13 in pyridine‚HF) with
9 to give 11 which on removal of the N-Boc protecting
group with trifluoroacetic acid gave the desired 1b.
However, this route was less desirable since the overall
yield was 39% and the coupling of 2-fluoro-5-iodopyri-
+
In this report, we present a synthesis of 1b which can
be adapted to provide a new synthesis of 1a and
† Research Triangle Institute.
‡ Brookhaven National Laboratory.
§ National Institute on Drug Abuse.
S0022-2623(97)00187-8 CCC: $14.00 © 1997 American Chemical Society

2294 J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15
Communications to the Editor
Sch em e 1
Ta ble 1. Kapp Values for 1a ,b and Nicotine at the nAChRa,b
compd
1a
1b
nicotine
X
Kapp (nM) (SEM)
Cl
F
0.024 ( 0.001
0.020 ( 0.002
6.2 ( 0.5
a
The Kapp values are reported as mean ( SEM and were
calculated from 3-4 separate competition binding experiments.
Compound 1b was characterized as its hydrochloride salt. C, H,
b
and N analyses were within 0.4% of theoretical values.
F igu r e 1. [3H]Norchloroepibatidine competition binding ex-
periments. The data represent the mean ( SEM from 3-4
binding experiments using 11-14 concentrations of test
compound run in triplicate with different rat forebrain
homogenate preparations for each experiment. The final assay
concentration of [3H]norchloroepibatidine (Kd ) 0.026 ( 0.002
nM; Bmax ) 5.7 ( 0.05 fmol/mg of tissue) was 30 pM.
dine (4c) with 9 took 4 days, whereas the coupling of
2-amino-5-iodopyridine (4b) with 9 was complete in 12
h. We also found that diazotization of 10 in hydrochloric
acid in the presence or absence of cuprous chloride gave
a 76% yield of epibatidine (1a ) which was identical to
an authentic sample.15 This provided additional support
for the 2-exo structural assignment of 1a ,b.
Kapp values for inhibition of [3H]NCEPB binding for
1a ,b, and nicotine were obtained using a modification
of previously reported procedures.2,26 The data are
presented in Table 1 and Figure 1. For each compound,
the IC50 values obtained from three to four independent
competition binding experiments were used to calculate
Kapp values.27,28 The 20 pM Kapp value for 1b makes
this compound more than 100 times more potent than
nicotine at the nAChR. A recent report has shown that
[18F]-1b is an excellent PET ligand for mapping nAChR
in vivo, and it provides the first evidence for using this

Communications to the Editor
J ournal of Medicinal Chemistry, 1997, Vol. 40, No. 15 2295
class of ligand to visualize nAChR in vivo with PET.7
The studies demonstrated a high brain uptake of [18F]-
1b in both baboon (12-15%) and mouse and high
specificity of its binding for nAChR in vivo. The high
thalamus to cerebellum ratio (4.0-4.5 in baboon at the
end of a 2-h study) used as the index for specific to
nonspecific binding provided a high signal-to-noise ratio
suggesting a new approach to investigate the nAChR
system and its role in neurodegeneration and addiction.
The synthesis of 2 and its conversion to 1b are shown
in Scheme 1. Reductive methylation of 10 using form-
aldehyde and sodium cyanoborohydride gives the 2-di-
methylamino compounds 12. Alkylation of 12 with
methyl iodide affords 7-(tert-butyloxycarbonyl)-exo-2-[2′-
(N,N,N-trimethylammoniumyl)-5′-pyridinyl]-7-azabicyclo-
[2.2.1]heptane iodide (2). Treatment of 2 with potas-
sium fluoride in dimethyl sulfoxide containing Kryptofix
gave a 75% yield of 11 which when treated with
hydrochloric acid provided 55% of 1b‚HCl. Details for
the synthesis of [18F]NFEP will be published elsewhere.
(8) Ding, Y.-S.; Gatley, S. J .; Fowler, J . S.; Volkow, N. D.; Aggarwal,
D.; Logan, J .; Dewey, S. L.; Liang, F.; Carroll, F. I.; Kuhar, M.
J . Mapping nicotinic acetylcholine receptors with PET. Society
for Neuroscience, Washington, DC, Nov. 16-21, 1996; Abstract
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(9) Horti, A.; Ravert, H. T.; London, E. D.; Dannals, R. F. Synthesis
of a radiotracer for studying nicotinic acetylcholine receptors:
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Labelled Compd. Radiopharm. 1996, 38 (4), 355-365.
(10) Horti, A.; Scheffel, U.; Dannals, R. F.; Stathis, M.; Finley, P. A.;
Ravert, H. T.; London, E. D. [18F] (()-exo-2-(2-Fluoro-5-pyridyl)-
7-azabicyclo[2.2.1]heptane. A radioligand for in vivo labeling and
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In summary, an efficient synthesis of exo-2-(2′-fluoro-
5′-pyridinyl)-7-azabicyclo[2.2.1]heptane (1b) and epiba-
tidine (1a ) was developed. 7-(tert-Butyloxycarbonyl)-2-
exo-[2′-(N,N,N-trimethylammoniumyl)-5′-pyridinyl]-7-
azabicyclo[2.2.1]heptane (2) was synthesized as an
excellent precursor for the synthesis of [18F]-1b.
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Ack n ow led gm en t. This research was supported in
part by a grant from Guilford Pharmaceuticals, Inc.
(Baltimore, MD), Fisons Pharmaceuticals (now Astra
Research Corp., Rochester, NY), Department of Energy,
Office of Health and Environmental Research, and
National Institutes of Health NS15380 and Grant
CTR402 from the Council of Tobacco Research.
Su p p or tin g In for m a tion Ava ila ble: Experimental data
for the synthesis of 1a , 2, 7-11, and 1b from 10, 11, or 2 and
the radioligand binding (7 pages). Ordering information is
found on any current masthead page.
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(28) Under our experimental conditions, the concentration of free
ligand was not much (>10×) greater than the number of
receptors, a requirement if the binding data were to be analyzed
as a pseudo-first-order bimolecular reaction. The result is the
free ligand concentration and nonspecific binding will increase
with increasing concentration of test compound leading to an
underestimation of the Ki. However, for the purpose of this
communication, a comparison of the Kapp is sufficient to dem-
onstrate the potency of 1b relative to 1a and nicotine. Thus, more
sophisticated data analysis techniques designed to correct for
changes in free ligand concentration were not employed.
(7) Ding, Y.-S.; Gatley, S. J .; Fowler, J . S.; Volkow, N. D.; Aggarwal,
D.; Logan, J .; Dewey, S. L.; Liang, F.; Carroll, F. I.; Kuhar, M.
J . Mapping nicotinic acetylcholine receptors with PET. Synapse
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J M970187D