Synthesis and structure-activity data of some new epibatidine analogues

Article abstract of DOI:10.1016/S0968-0896(98)00163-1

The high-pressure Diels-Alder reaction of N-carbomethoxypyrroles and phenyl vinyl sulfone affords versatile intermediates for the palladium-catalyzed preparation of new epibatidine analogues. Structure-activity relationships of new epibatidine analogues are presented. High affinities of K(i)=0.81 and 2.6nM for the [³H]-cytisine rat brain nicotinic acetylcholine binding sites were found for the 5-pyrimidinyl and the 5-(2-amino)-pyrimidinyl epibatidine analogues, respectively. Copyright (C) 1997 Elsevier Science Ltd.

Full text of DOI:10.1016/S0968-0896(98)00163-1

BIOORGANIC &
MEDICINAL
CHEMISTRY
Bioorganic & Medicinal Chemistry 6 (1998) 2103±2110
Synthesis and Structure±Activity Data of Some New
Epibatidine Analogues
Jean-Paul G. Seerden,a,* Martin Th. M. Tulp,b Hans W. Scheerena,y
and Chris G. Kruseb
aDepartmentof OrganicChemistry, NSRCenterforMolecularStructure, Design, andSynthesis, University ofNijmegen, 6525DENijmegen,
The Netherlands
bCNS Research, Solvay Pharmaceuticals, PO Box 900, 1380 DA Weesp, The Netherlands
Received 26 March 1998; accepted 19 June 1998
AbstractÐThe high-pressure Diels±Alder reaction of N-carbomethoxypyrroles and phenyl vinyl sulfone a€ords versa-
tile intermediates for the palladium-catalyzed preparation of new epibatidine analogues. Structure±activity relation-
ships of new epibatidine analogues are presented. High anities of Ki=0.81 and 2.6 nM for the [3H]-cytisine rat brain
nicotinic acetylcholine binding sites were found for the 5-pyrimidinyl and the 5-(2-amino)-pyrimidinyl epibatidine
analogues, respectively. # 1998 Elsevier Science Ltd. All rights reserved.
Introduction
e€ects.2a±d Despite the fact that 1 is too toxic for clinical
applications, active synthetic analogues have the poten-
tial for improved pharmacological pro®les. This may
open applications based on their nicotinic activity, such
as the treatment of Parkinson's and Alzheimer's disease,
schizophrenia, Tourette's syndrome, ulcerative colitis,
cognitive disorders, and tobacco dependence.2e Struc-
ture±activity studies of epibatidine 1 and analogues, as
reported in the literature, have shown that the chloro
substituent of the pyridyl ring of epibatidine is not cri-
tical to nAChR anity, since the deschloro analogue
has slightly greater anity than epibatidine.2 Very
recently, Daly et al.3 reported that replacement of the
chloropyridyl ring by an isoxazole ring gives epiboxidine
2, a compound that is less toxic than epibatidine but has
similar nAChR binding anity (Fig. 1).
Epibatidine 1, exo-2-(6-chloro-3-pyridyl)-7-azabicyclo-
[2.2.1]heptane, an alkaloid isolated from the skin of the
Ecuadorian poison frog, Epipedobates tricolor, was
recently reported as a novel, highly potent, nonopioid
analgesic agent and a speci®c agonist of central nicotinic
acetylcholine receptors (nAChRs).1 Neuronal nAChRs
occur as di€erent subtypes and are widely distributed in
the brain. It might be hypothesized that each subtype is
involved in mediating speci®c functions/behaviors with
a de®ned pharmacology that might be selectively tar-
geted. In vitro studies have shown 1 to be a very potent
activator of the nAChRs mediating cation ¯ux, current
activation, and neurotransmitter release. In vivo studies
have provided converging evidence that the potent
analgesic activity of this agent is accompanied by a
spectrum of nAChR-mediated nonanalgesic liabilities,
including motoric, thermoregulatory, and cardiovascular
These data suggest that various heteroaryl groups, other
than chloropyridyl, are allowed. From quantitative
structure±anity relationship (QSAR) studies of nico-
tinic ligands by Glennon et al.2b it was stated that the
distance between the pyrrolidine and pyridine nitrogen
atoms in epibatidine, nicotine, and its analogues is an
important parameter for high binding anity to the
nicotinic receptor. Because epibatidine binds with higher
anity at nicotinic receptors than nicotine and the
internitrogen distance is longer in epibatidine than in
Key words: Epibatidine analogues; structure±activity data;
high pressure; hydroarylation.
*Present address: University of Groningen, Department of
Organic and Molecular Inorganic Chemistry, Nijenborgh 4,
9747 AG Groningen, The Netherlands; e-mail: J.P.G.Seerden
@chem.rug.nl
yCorresponding author. Tel.: 0031 24 365 2331; fax: 31 24 365
2929; e-mail: jsch@sci.kun.nl
0968-0896/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved
PII: S0968-0896(98)00163-1

2104
J.-P. G. Seerden et al./Bioorg. Med. Chem. 6 (1998) 2103±2110
approach to 1. In this article we report: 1) an ecient
synthesis of the intermediate N-carbomethoxy 7-azabi-
cyclo[2.2.1]hept-2-ene (4) applying the high-pressure
Diels±Alder reaction of N-carbomethoxypyrrole to
phenyl vinyl sulfone (Scheme 1); 2) the synthesis of new
epibatidine analogues with variations in the aryl ring via
palladium catalyzed hydroarylation reactions of
4
(Scheme 2); and 3) structure±activity data of these pro-
ducts through receptor binding studies to nAChRs in
rat brain preparations.
Figure 1.
nicotine the introduction of aminoaryl groups becomes
an interesting issue. Therefore, we decided to explore
new epibatidine derivatives with variations in the het-
erocyclic system and the N-N distance.
Results
The known 7-azabicyclo[2.2.1]hept-2-ene 4 is generally
prepared via a thermal Diels±Alder reaction of a N-
acylpyrrole to p-toluenesulfonylacetylene followed by
two reduction steps; i.e. regioselective hydrogenation of
a carbon±carbon double bond and subsequent reductive
desulfonylation with sodium amalgam.5e,7
Several approaches to the preparation of the 7-azabicy-
clo[2.2.1]heptane system and the total synthesis of 1 and
analogues have been reported.4,5 The Diels±Alder reac-
tion of N-protected pyrroles with activated dienophiles
is a straightforward route to prepare this ring system.
Subsequent exo-stereoselective palladium-catalyzed
hydroarylation reaction of N-protected 7-azabicyclo-
[2.2.1]-hept-2-ene cycloadducts with aryl halides has
proven to be an attractive, short, and ecient synthetic
We found that N-carbomethoxypyrrole reacted readily
with phenyl vinyl sulfone in acetonitrile at 12 kbar
pressure and 50 ꢀC to a€ord after 20 h, quantitatively, a
Scheme 1.
Scheme 2.

J.-P. G. Seerden et al./Bioorg. Med. Chem. 6 (1998) 2103±2110
2105
Scheme 3.
1:1 mixture of endo- and exo-5-phenylsulfonyl-7-meth-
oxycarbonyl 7-azabicyclo[2.2.1]hept-2-ene (3a and 3b,
respectively).9 The mixture of endo-3a and exo-3b was
subsequently desulfonylated by reduction with 6%
sodium amalgam to a€ord the desired alkene 4 in 30%
isolated yield after chromatography.10 The high-pres-
sure promoted synthesis of 4 displays some advantages:
a shorter sequence starting from commercially available
phenyl vinyl sulfone and a higher overall yield of 4. In
order to ®nd active synthetic epibatidine analogues, the
presence of an external hydrogen bond acceptor atom
attached to the 7-azabicycloalkane skeleton is presumed
to be a prerequisite.2 Our experiments in this ®eld were
therefore focused on the use of nitrogen-containing
(hetero)aryl halides (5±10)11 in the palladium(0) cata-
lyzed hydroarylation reaction12 with alkene 4 (Scheme
2). Three di€erent catalytic methods have been applied:
Method A uses Je€ery±Larock conditions, i.e. 1 equiv
nBu4N+Cl , 3 equiv KOOCH, 0.05 equiv Pd(OAc)2;
Method B uses 2.5 equiv piperidine, 3.5 equiv HCOOH,
0.08 equiv Pd(PPh3)4; and Method C is similar to
Method B, but applies 0.08 equiv Pd(OAc)2(PPh3)2 as
the catalyst. The experimental conditions and the results
of the reactions of alkene 4 with aryl halides 5±10 are
summarized in Table 1. From the results in Table 1, it
follows that although aryl iodides are known to be more
reactive than aryl bromides, the bromide 5 reacts well
with alkene 4 in the presence of one equiv of tetra-
butylammonium chloride, using Method A. A major
side reaction with 8a is the palladium-catalyzed forma-
analogues 12a±12f were isolated as their hydriodic acid
salts in a nearly quantitative yield (Figure 2).
Biological data
The compounds 12a±12f were evaluated for their bind-
ing anities to neuronal nAChRs by measuring the
displacement of [3H]-cytisine from a preparation of
whole rat brain according to the procedure described
by Anderson et al.14 The binding results of the test
compounds have been normalized to their equilibrium
dissociation constants (Ki) and are presented in Table 2.
The results from Table 2 show that high binding for the
nicotinic receptor is found for epibatidine 1 and the 5-
pyrimidinyl and 2-amino-5-pyrimidinyl epibatidine
analogues, resp. 12a and 12b. Although deschloro±epi-
batidine has a slightly greater anity than epibatidine,
the potent subnanomolar anity of epibatidine 1 to
Table 1. Reductive Heck reactions of alkene 4 with aryl
halides 5±10
Ar-X
Prep.
methoda
Time (h) Temp. (ꢀC) Product Yield
(%)b
5
6
A
Ac
B
24
16
60
11a
11b
11b
11c
Ð
42
74
60
24
75
20
18d
7a
7b
A
20
60
B
24
75
Ð
A
15
60
11c
11c
Ð
21
75e
tion of isoquinoline by
a
reductive mechanism.13
B
24
60
Replacement of halide in the ArPdL2X intermediate
(L=ligand; X=halide) by formate and decomposition
of the formed ArPdL2OCOH to arene through deinser-
tion of carbon dioxide is probably faster than its oxida-
tive addition to the double bond of alkene 4. N-
arylation of piperidine with aryl halides was another
side reaction observed when using method B to give, for
example, 1-piperidinyl-2-thiazole from 9.
8a
8b
9
A,B,C
B
days
17
60±75
70
Ð
11d
11e
11f
55
15f
A
16
60
10
B
24
75
30
a
Preparation methods: (A) 1 equiv aryl halide, 0.05 equiv
Pd(OAc)2, 3 equiv KOOCH, 1 equiv nBu4N+Cl ; (B) 2 equiv
aryl halide, 0.08 equiv Pd(PPh3)4, 3.5 equiv piperidine, 2.5 equiv
HCOOH; (C) 0.08 equiv Pd(OAc)2(PPh3)2, 3.5 equiv piperidine,
2.5 equiv HCOOH.
The exo-stereochemistry of the products 11 was
assigned by the NMR-signal multiplicity (a doublet of
doublets) for proton H-2, which is located at the endo-
side of the molecule.4 After treatment of the prepared
N-carbomethoxy exo-2-aryl-7-azabicyclo[2.2.1]heptanes
11 with trimethylsilyl iodide the new epibatidine
b
Isolated yield of pure compounds after column chromato-
graphy.
c
0.35 equiv Pd(OAc)2.
ca. 50% conversion of 7a.
d
When performed 24 h at 75 ꢀC, the yield drops to 60%.
e
f ca. 50% conversion of 4, 1-piperidinyl-2-thiazole as by-product.

2106
J.-P. G. Seerden et al./Bioorg. Med. Chem. 6 (1998) 2103±2110
Figure 2. Prepared epibatidine analogues.
brain nAChRs is still ®vefold higher than 12a and 16-
fold higher than 12b. The lower basicity of the pyr-
imidinyl group in 12a and to a lesser extent in 12b can
explain this di€erence. In addition, the NH2 substituent
in 12b probably acts as a sterically demanding group
which slightly decreases anity. For comparison, sub-
stitution at the 5-position of the pyridine ring in nicotine
with a bromo or a ethynyl group has led to derivatives
with attractive pharmacological pro®les and are eval-
uated as novel anti-Parkinson agents.14 The drop in
binding potency for 4-isoquinolyl analogue 12d further
indicates that any other substitution in the pyridyl ring
in epibatidine signi®cantly lowers the binding, irrespec-
tive of the internitrogen distance, which is for 12a, 12b,
and 12d approximately equal to epibatidine. Novel
cholinergic channel activators (ChCAs) of neuronal
nicotinic acetylcholine receptors (nAChRs) have been
described in the literature in which a substituted iso-
xazole ring has been incorporated as a bioisosteric
replacement for the pyridine ring found in (S)-nicotine,
(R)-nornicotine and ( )-epibatidine.16
analogue, the moderate binding potency of the 4-
pyrazole analogue 12f, is somewhat disappointing. The
presence of the N-H functionality in 4-pyrazole analo-
gue 12f, might hinder ecient hydrogen bond formation
with the receptor. The lower potencies of N,N-dime-
thylanilino analogue 12c and 2-thiazole analogue 12e
suggest that the position of the nitrogen atom in the
heteroaryl ring is still very important. This distance
seems to be too short in 12e and too long in 12c.
Experimental
DMF was distilled and dried on 4 A molecular sieves.
Piperidine was stored on potassium hydroxide pellets.
Dichloromethane was dried and distilled on CaH2 and
1
stored over 4 A molecular sieves. H NMR spectra and
13C NMR were recorded on
a Bruker AM-100
(100 MHz, FT), AM-300 (300 MHz, FT) or AM-400
(400 MHz, FT) spectrometer with TMS as internal
standard. IR spectra were run on a Perkin±Elmer 298
spectrophotometer. Mass spectra were measured with a
Varian SM1-B double focusing mass spectrometer or
with a VG 7070E mass spectrometer. Gas chromato-
graphy was performed on a Hewlett±Packard 5710A
GC instrument equipped with a capillary HP cross-
linked methyl silicone (25 mÂ0.31 mm) column. Pur-
i®cation of products was done by normal or ¯ash chro-
matography on silica gel, using Merck silica gel 60 or
60H as stationary phase. The aryl halides 5, 7a, 8a, 9,
and 10 were commercially purchased. The aryl halides
5-iodo-2-aminopyrimidine 6, 4-iodo-N,N-dimethylani-
line 7b and 4-iodoisoquinoline 8b were prepared
according to literature procedures.11
As compared with the recently reported high binding
potency of epiboxidine 2, an isoxazole epibatidine
Table 2. Binding anities of epibatidine analogues for [3H]-
cytisine-labeled nicotinic acetylcholine receptorsa
nAChR ligand
Ki (nM)b
Epibatidine 1
12a
12b
0.16‹0.08 (3)
0.81‹0.058 (3)
2.6‹0.78 (3)
18‹1.0 (2)
12f
12d
53‹15 (3)
12e
12c
5,800‹3,700 (3)
29,500‹3,500 (2)
3.6‹0.11 (5)
0.82‹0.24 (3)
Nicotine
Cytisine
High pressure Diels±Alder reaction of methyl
pyrrole-1-carboxylate and phenyl vinyl sulfone
mixture of methyl pyrrole-1-carboxylate8 (7.5 g,
60 mmol) and 1.1 equiv phenyl vinyl sulfone (9.9 g,
a
A
Ref. 14.
b Mean Ki values from (x) determinations.

J.-P. G. Seerden et al./Bioorg. Med. Chem. 6 (1998) 2103±2110
2107
58.9 mmol) is dissolved in acetonitrile and kept at
12 kbar high pressure overnight at 50 ꢀC. The pyrrole 2
is completely converted and the formed product appears
to consist of an equimolar mixture of endo- and exo-5-
phenylsulfonyl-N-carbomethoxy-7-azabicyclo[2.2.1]hept-
2-ene (3a and 3b, respectively) which can be separated
by chromatography using ethyl acetate/n-hexane mix-
tures and are isolated in almost quantitative yield.
3 mmol), and Pd(OAc)2 (11.8 mg, 0.05 mmol) were
added. The vial was then sealed and placed in an oil
bath of 60 ꢀC and left stirring for 16 h. The crude pro-
duct was washed with 5 mL water and extracted three
times with 25 mL ethyl acetate. After drying with
sodium sulfate and evaporation of the solvent, the resi-
due was subjected to silica gel chromatography using
ethyl acetate/n-hexane mixtures as the eluent. Carba-
mates 11a±11f were isolated as pure compounds and
were analyzed and characterized by MS and NMR.
endo-5-Phenylsulfonyl-N-carbomethoxy-7-azabicyclo-
[2.2.1]hept-2-ene (3a). 1H NMR (400 MHz, CDCl3) d
1.71 (1H, m, H-6a), 2.32 (1H, ddd, J=4.3, 9.1, and 12.4,
H-6b), 3.62 (3H, s, OCH3), 3.74 (1H, ddd, appears as
quintet, J=4.3 and 8.6, H-5), 4.83 (1H, br s, H-4), 4.86
(1H, br s, H-1), 6.46 (1H, m, H-2), 6.54 (1H, m, H-3),
7.59 (2H, t, ArH), 7.68 (1H, t, ArH), 7.86 (2H, d, ArH);
EIMS 293 (rel intensity) 168 (18), 141 (3), 125 (100);
Anal. calcd for C14H15NO4S (Mass=293.343): C, 57.32;
H, 5.15; N, 4.77; S, 10.93. Found: C, 57.13; H, 5.01; N,
4.85; S, 10.69.
Method B. The alkene 4 (153 mg, 1 mmol) was weighed
into a 10 mL vial, containing a magnetic stirrer and aryl
halide 5±10 (1±3 mmol). Subsequently, DMF (2 mL),
piperidine (2.5 mmol), Pd(PPh3)4 (0.08 mmol) and for-
mic acid (3.5 mmol) were added. The vial was then
sealed and placed in an oil bath of 70 ꢀC and left stirring
overnight for at least 16 h. The crude product was
worked up as described in Method A.
Method C is essentially the same as Method B except for
the fact that 8 mol% (Ph3P)2Pd(OAc)2 was used as cat-
alyst instead of Pd(PPh3)4.
exo-5-Phenylsulfonyl-N-carbomethoxy-7-azabicyclo[2.2.1]-
hept-2-ene (3b). Solid; mp 127 ꢀC; H NMR (400 MHz,
1
315 K, CDCl3) 1.61 (1H, dd, J=8.4 and 11.9, H-6), 2.32
(1H, br d, J=12.1, H-6), 3.03 (1H, dd, J=4.3 and 8.4,
H-5), 3.60 (3H, s, OCH3), 4.79 (1H, br s, H-4), 5.07 (1H,
br s, H-1), 6.32 (1H, d, J=4.6, H-2), 6.43 (1H, d, J=4.6,
H-3), 7.55 (2H, t, ArH), 7.64 (1H, t, ArH), 7.92 (2H, d,
ArH); EIMS 293 (rel, int.) 168 (20), 141 (4), 125 (100);
Anal. calcd for C14H15NO4S (Mass=293.343): C, 57.32;
H, 5.15; N, 4.77; S, 10.93. Found: C, 57.20; H, 5.02; N,
4.87; S, 10.83.
General procedure for deprotection of carbamates
11a±11f with trimethylsilyl iodide
Carbamates 11 were dissolved in chloroform or acet-
onitrile (ca. 100 mg/25 mL solvent). Trimethylsilyliodide
(2 mL) is added via a syringe and the mixture is re¯uxed
for 3 h. After cooling, methanol (50 mL) is added to the
brown solution and the mixture is re¯uxed again for 2 h.
Evaporation of the solvent in vacuo gave a brown oily
residue which was treated with methanol/diisopropyl-
ether mixtures (ca. 1/5±1/20). A white precipitate was
formed which was subsequently ®ltered over a glass ®l-
ter, washed with small amounts of diisopropylether and
dried to give the epibatidine analogues 12 as their
hydriodic acid salts.
Reduction of 3a/3b with 6% sodium amalgam. Preparation
of 7-carbomethoxy-7-azabicyclo[2.2.1]hept-2-ene (4). 7
The crude cycloadduct 3a/3b (11.05 g, 37.5 mmol) is
treated with 6% sodium amalgam according to litera-
ture.6 After column chromatography on silica gel with
ethylacetate/n-hexane (1/8), the product was isolated
and characterized as 7-carbomethoxy-7-azabicyclo-
[2.2.1]heptene 4 (oil, 1.72 g, 30%). All physical data of 4
were identical7 as reported. 1H NMR (300 MHz,
CDCl3) d 1.12 (2H, m, H-5, H-6), 1.86 (2H, m, H-5, H-
6), 3.63 (3H, s, OCH3), 4.73 (2H, br s, H-1, H-4), 6.23
(2H, br s, H-2, H-3); 13C NMR (75 MHz, CDCl3) d
23.1, 24.1, 53.0, 59.2, 134.0, 134.9, 156.0; GC±MS (%
rel intensity, ion): calcd for C8H11NO2, 153; found, 153
(M+, 3), 125 (100), 80 (50).
exo-2-(Pyrimidin-5-yl)-7-carbomethoxy-7-azabicyclo[2.2.1]-
heptane (11a). The reductive Heck reaction was per-
formed according to method A, starting from 153 mg
(1.0 mmol) alkene 4 and 159 mg (1.0 mmol) 5-bromo-
pyrimidine 5. After 24 h at 60 ꢀC, the reaction mixture
was separated by silica gel chromatography using ethyl
acetate/n-hexane (1/3) as the eluent to give 98 mg 11a
(42% isolated yield, Rf=0.1) as an oil: 1H NMR
(300 MHz, CDCl3) d 1.56±1.69 (2H, m, H-5, H-6),
1.86±1.93 (3H, m, exo H-3, H-5, H-6), 2.06 (1H, dd,
JH3endo,H2endo=8.9 and J=12.5 Hz, endo H-3), 2.89 (1H,
dd, J=4.7 and JH2endo,H3endo=8.9 Hz, H-2), 3.68 (3H, s,
OCH3), 4.28 (1H, br s, H-4), 4.49 (1H, br s, H-1), 8.66
(2H, s, ArH), 9.08 (1H, s, ArH); 13C NMR (75 MHz,
CDCl3) d 28.8, 29.5, 39.8, 43.7, 52.5, 56.1, 56.2, 61.7,
138.2, 155.5, 157.0, 213.0; Mass calcd. C12H15N3O2
General procedures for reductive Heck-reaction with
alkene 4
Method A. The alkene 4 (153 mg, 1 mmol) was weighed
into a 10 mL vial, containing a magnetic stirrer and aryl
halide 5±10 (1 mmol). Subsequently, nBu4N+Cl
(278 mg, 1 mmol), DMF (2 mL), KOOCH (252.4 mg,

2108
J.-P. G. Seerden et al./Bioorg. Med. Chem. 6 (1998) 2103±2110
M=233; found GC/MS (CI, Mass, rel int.) 233 (M+,
10), 201 (M+ OMe, 16), 159 (6), 127 (100).
NMR (75 MHz, CDCl3) d 28.5, 29.6, 40.0, 40.5, 47.0,
62.1, 51.9, 55.7, 112.5, 127.3, 133.5, 148.9, 212.9; Mass
calcd for C16H22N2O2, 274; found: GC±MS (CI, Mass,
rel intensity) 274 (M+, 20), 199 (64), 172 (100), 157 (26).
exo-2-(Pyrimidin-5-ylꢀ)-7-azabicyclo[2.2.1]heptane,
1
.
(12a 2HI). Mp 215±220 C uncor; H NMR (300 MHz,
CD3OD/CDCl3, d 1.78±2.16 (5H, m, 2ÂH-5, 2ÂH-6,
exo H-3), 2.59 (1H, m, endo H-3), 3.73 (1H, m, H-2),
4.32 (1H, br s, H-4), 4.47 (1H, br s, H-1), 9.16 (2H, s,
ArH), 9.30 (1H, s, ArH); 13C NMR (75 MHz, CDCl3) d
31.3, 34.6, 39.9, 40.9, 58.4, 61.3, 148.8, 152.7, 155.5;
.
Anal. calcd for C10H13N3 2HI (Mass=431.058): C,
27.86; H, 3.51; N, 9.75. Found: C, 27.86; H, 3.48; N,
9.69.
exo-2-(4-N,N-Dimethylanilino)-7-azabicyclo[2.2.1]heptane
.
(12c 2HI). From 200 mg carbamate 11c is prepared
334 mg (97%) of 12c as its hydriodic acid salt following
the standard procedure: Mp 210±215 ꢀC dec; H NMR
1
(300 MHz, CD3OD/CDCl3) d 1.9±2.5 (6H, m, H-3, H-5
en H-6), 3.31 (6H, br s, 2ÂN(CH3)2), 3.48 (1H, m, H-
2a), 4.39 (1H, br s, H-4), 4.52 (1H, br s, H-1), 7.60±7.73
(4H, m, Ar-H); 13C NMR (CD3OD/CDCl3) d 25.1
(CH2, C-5), 27.1 (CH2, C-6), 35.9 (C-2 en C-3), 43.9 (N-
CH3), 45.9 (N-CH3), 58.5 (C-4), 62.5 (C-1), 120.3 (C-40),
exo-2-(2-Aminopyrimidin-5-yl)-7-carbomethoxy-7-aza-
bicyclo[2.2.1]heptane (11b). The reductive Heck reaction
was performed according to method A, starting from
153 mg (1.0 mmol) 4 and 221 mg (1.0 mmol) 2-amino-5-
iodopyrimidine 12 and using 0.35 equiv Pd(OAc)2. After
16 h at 60 ꢀC, the reaction mixture was separated with
silica gel chromatography using dichloromethane/
methanol/ammonia (80/20/1) as eluent to give 184 mg of
0
128.6 (C-30 en C-5 ); Anal. calcd for C14H20N2 2HI
(Mass=472.15): C, 35.61; H, 4.70; N, 5.93. Found: C,
35.25; H, 4.67; N, 5.84.
.
exo-2-(4-Isoquinolyl)-7-carbomethoxy-7-azabicyclo[2.2.1]-
heptane (11d). The reductive Heck reaction was per-
formed according to method B, starting from 153 mg
(1.0 mmol) 4 and 638 mg (2.5 mmol) 8b. After 17 h at
70 ꢀC analysis by GC showed that the alkene was com-
pletely converted and the reaction products were sepa-
rated by silica gel chromatography using ethyl acetate/n-
hexane (1/2) as the eluent, to yield 155 mg of 11d (55%
isolated yield) as an oil: Rf=0.14; Mass calcd for
C17H18N2O2, 282; found, GC/MS (CI) (Mass, rel inten-
sity) 282 (M+, 28), 180 (20), 156 (100), 127 (91).
1
21c (74% isolated yield, Rf=0.45) as an oil. H NMR
(300 MHz, CDCl3) 1.43±1.83 (5H, m, 2ÂH-5, 2ÂH-6,
H-3 exo), 1.97 (1H, dd, JH3endo,H2endo=8.9 and
J=12.5 Hz, endo H-3), 2.73 (1H, dd, J=4.8 and
JH2endo,H3endo=8.9 Hz, H-2), 3.66 (3H, s, OCH3), 4.16
(1H, br s, H-4), 4.44 (1H, br s, H-1), 5.49 (2H, br s,
NH2), 8.19 (2H, s, ArH); 13C NMR (75 MHz, CDCl3) d
28.8, 29.5, 39.9, 43.2, 52.6, 56.3, 59.3, 62.5, 128.6, 157.7,
162.9, 213.5; Mass calcd for C12H26N4O2, 248; found
GC/MS (CI, Mass, rel intensity) 248 (M+, 22), 216
(M+ OMe, 9), 146 (8), 127 (100).
exo-2-(4-Isoquinolyl)-7-azabicyclo[2.2.1]heptane (12d.2HI).
Mp 220±225 ꢀC uncor; 1H NMR (300 MHz, CDCl3) d
0.89±1.13 (5H, m, 2ÂH-5, 2ÂH-6, H-3), 1.73 (1H, dd,
J=9.7 and 11.8, H-3), 3.15 (1H, dd, J=6.0 and 9.7, H-
2), 3.29 (1H, br s, H-1), 3.84 (1H, br s, H-4), 6.96 (1H, t,
J=7.3, ArH), 7.19 (1H, t, J=7.3, ArH), 7.34 (1H, d,
J=8.4, ArH), 7.44 (1H, d, J=8.4, ArH), 7.61 (1H, s,
.
ArH), 8.66 (1H, s, ArH); Anal. calcd for C15H16N2 2HI
(Mass=480.130): C, 37.52; H, 3.78; N, 5.84. Found: C,
37.50; H, 3.73; N, 5.90.
exo-2-(2-Aminopyrimidin-5-yl)-7-azabicyclo[2.2.1]heptane
(12b.2.2HI). Mp 215±220 ꢀC uncor; 1H NMR
(300 MHz, CD3OD/CDCl3) d 1.91±2.17 (5H, m, 2ÂH-5,
2ÂH-6, H-3 exo), 2.44 (1H, m, endo H-3), 3.46 (1H, m,
H-2), 4.38 (1H, br s, H-4), 4.58 (1H, br s, H-1), 4.87
(2H, br s, NH2), 8.63 (2H, s, ArH); Anal. calcd for
.
C10H14N4 2.2 HI (Mass=471.66): C, 25.47; H, 3.46; N,
11.88. Found: C, 25.47; H, 3.48; N, 11.80.
exo-2-(2-Thiazolyl)-7-carbomethoxy-7-azabicyclo[2.2.1]-
heptane (11e). The reductive Heck reaction was per-
formed according to method A, starting from 153 mg
(1.0 mmol) 4 and 164 mg (1.0 mmol) 9. After 16 h at
60 ꢀC, analysis by GC showed that the 9 was completely
converted, but ca. 50% of 4 was still present. Silica gel
chromatography using ethyl acetate/n-hexane (1/1) as
the eluent gave 36 mg of 11e (15% isolated yield, 30%
based on converted alkene): Rf=0.20; 1H NMR
(300 MHz, CDCl3) d 1.52±1.69 (2H, m, H-5 and H-
6), 1.87 (2H, m, H-5 and H-6), 2.05 (1H, dd,
JH3endo,H2endo=8.8 and J=12.6 Hz, endo H-3), 2.19 (1H,
m, exo H-3), 3.43 (1H, dd, J=4.6 and JH2endo,H3endo
=8.8 Hz, H-2), 3.63 (3H, s, OCH3), 4.40±4.47 (2H, m,
exo-2-(4-N,N-Dimethylanilino)-7-carbomethoxy-7-aza-
bicyclo[2.2.1]heptane (11c). The reductive Heck reaction
of 4 (223 mg, 1.46 mmol) with 7b (2 equiv) was catalyzed
by 8 mol% Pd(PPh3)4 in the presence of 3.5 equiv
piperidine and 2.5 equiv formic acid at 60 ꢀC in 2 mL
DMF. After 24 h, the hydroarylated product 11c
(300 mg, 75%) was isolated as an oil by chromato-
graphy using ethyl acetate/n-hexane as (1/4) eluent:
Rf=0.16; 1H NMR (300 MHz, CDCl3) d 1.47±1.97 (6H,
m, 2ÂH-3, 2ÂH-5, 2ÂH-6), 2.80 (1H, dd, J=6.0 and
7.9, H-2), 2.90 (6H, s, N(CH3)2), 3.64 (3H, s, OCH3),
4.20 (1H, br s, H-4), 4.41 (1H, br s, H-1), 6.68 (2H, d,
J=8.7, 2ÂArH), 7.11 (2H, d, J=8.7, 2ÂArH); 13C

J.-P. G. Seerden et al./Bioorg. Med. Chem. 6 (1998) 2103±2110
2109
H-1 and H-4), 7.21 (1H, d, J=3.3, ArH), 7.65 (1H, d,
J=3.3, ArH); 13C NMR (75 MHz, CDCl3) d 28.8, 39.2,
46.9, 52.3, 55.9, 62.5, 118.5, 141.6, 156.0, 174.4, 213.8;
Calculated mass for C11H14N2O2S, 238; found by GC/
MS (Mass, rel intensity) 238 (M+, 2), 207 (5), 136 (11),
127 (34), 112 (100).
samples were counted for [3H] in a Liquid Scintillation
Counter (Packard). Concentrations of the unlabelled
drug, causing 50% displacement of the speci®c binding
of a labeled compound (IC50 values), were obtained by
computerized log-probit linear regression analysis of
data obtained in experiments in which four to six dif-
ferent concentrations of the test compound were used.
Inhibition constant (Ki) values were calculated using the
Cheng±Pruso€ equation; Ki=IC50/(1+S/Kd) in which
S represents the concentration of the label. Mean Ki
values were calculated from at least three values
obtained from independent experiments, that is, in
experiments performed on di€erent days with di€erent
membrane preparations. All incubations were done in
triplicate. Results are shown in Table 2.
.
exo-2-(2-Thiazolyl)-7-azabicyclo[2.2.1]heptane (12e 2HI).
Carbamate 11e (30 mg, 0.13 mmol) was cleaved with
trimethylsilyl iodide to give, after quenching with
methanol and precipitation in diisopropyl ether, 49 mg
12e (89%) as its hydriodic acid salt: Mp 215±220 ꢀC dec;
1H NMR (300 MHz, CD3OD/CDCl3) d 1.9±2.2 (4H, m,
H-5 en H-6), 2.30 (1H, m, H-3b), 2.53 (1H, dd, H-3a),
3.93 (1H, dd, J2a,3a=12.5 Hz, J2a,3b=6.6 Hz, J2a,1<1,
H-2a), 4.44 (1H, m, H-4), 4.58 (1H, m, H-1), 7.64 (1H,
d, J=4.6 Hz, H-30), 7.91 (1H, d, J=4.6 Hz, H-40); 13C
NMR (CD3OD/CDCl3) d 25.4 (CH2, C-5), 25.5 (CH2,
C-6)0, 35.8 (C-3), 410.5 (C-2), 58.0 (C-4), 63.2 (C-1), 119.9
References and Notes
1. (a) Spande, T. F.; Garra€o, H. M.; Edwards, M. W.; Yeh,
H. J. C.; Pannell, L.; Daly, J. W. J. Am. Chem. Soc. 1992, 114,
3475; (b) Badio, B.; Daly, J. W. Mol. Pharmacol. 1994, 45, 563.
.
(C-3 ), 139.8 (C-4 ); Anal. calcd for C9H12N2S 2HI
(Mass=436.098): C, 24.79; H, 3.24; N, 6.42; S, 7.35.
Found: C, 24.86; H, 3.17; N, 6.57; S, 7.18.
2. (a) Bai, D.; Xu, R.; Zhu, X. Drugs of the Future 1997, 22,
1210. (b) For a review about pharmacological properties of
epibatidine, see: Sullivan, J. P.; Bannon, A. W. CNS Drug Rev.
1996, 2, 21. (c) For QSAR studies see: Glennon, R. A.; Hern-
don, J. L.; Dukat, M. Med. Chem. Res. 1994, 4, 461. (d) Hol-
laday, M.; Lebold, S. A.; Lin, N.-H. Drug Dev. Res. 1995, 35,
191. (e) For pharmacological properties of nicotine see: Beno-
witz, N. L. Annu. Rev. Pharmacol. Toxicol. 1996, 36, 597.
.
exo-2-(4-Pyrazolyl)-7-azabicyclo[2.2.1]heptane (12f 2.25HI).
The reductive Heck reaction was performed according
to method B, starting from 153 mg (1.0 mmol) 4 and
388 mg (2.0 mmol) of 10. After 24 h at 75 ꢀC, the reac-
tion mixture was worked up and puri®cation silica gel
chromatography gave 66 mg exo-2-(4-pyrazolyl)-7-car-
bomethoxy-7-azabicyclo[2.2.1] heptane 11f (30% iso-
lated yield). Deprotection of the carbamate 11f with
trimethylsilyl iodide a€orded 12f as its hydriodic acid
salt: Mp 220±225 ꢀC uncor; 1H NMR (400 MHz,
CD3OD) d 1.84 (1H, m, H-6a), 2.0±2.2 (2H, m, H-5b en
H-6b), 2.25 (1H, m, H-3b), 2.41 (1H, dd, J3a,2a=9.3,
3. Badio, B.; Garra€o, H. M.; Plummer, C. V.; Padgett, W. L.;
Daly, J. W. Eur. J. Pharmacol. 1997, 321, 189.
4. (a) For reviews on the total synthesis of epibatidine, see:
Broka, C. A. Med. Chem. Res. 1994, 4, 449. Dehmlow, E. V. J.
Prakt. Chem. 1995, 167; Szantay, C.; Kardos-Balogh, Z.;
Szantay, Jr. The Alkaloids; Cordell, G. A., Ed.; Academic
Press: San Diego, 1995; Vol. 46, 95; (b) For a review on the
synthesis of the 7-azabicyclo[2.2.1]heptane system, see: Chen,
Z.; Trudell, M. L. Chem. Rev. 1996, 96, 1179.
J
J
J
3a,3b=13.6, H-3a), 3.41 (1H, dd,
2a,3b=5.4, J2a,1<1, H-2a), 4.41 (2H, t, J4,3b&J4,5b
1,6b&4.4, J4,3a&J1,2a<1, H-1 en H-4), 8.11 (2H, br s,
J2a,3a=9.3 Hz,
&
H-30 en H-50); 13C NMR (75 MHz, CD3OD) d 25.6
(CH2, C-5), 26.9 (CH2, C-6), 35.7 (C-2), 36.0 (CH2, C-
30), 58.7 (C-4), 64.3 (C-1), 122.5 (C-40), 132.4 (C-30 en C-
5. Synthesis of epibatidine and analogues via cycloaddition to
pyrrole derivatives: (a) Huang, D. F.; Shen, T. Y. Tetrahedron
Lett. 1993, 34, 4477; (b) Okabe, K.; Natsume, M. Chem.
Pharm. Bull. 1994, 42, 1432; (c) Kotian, P. L.; Carroll, F. I.
Synth. Commun. 1995, 25, 63; (d) Gonzalez, J.; Koontz, J. I.;
Hodges, L. M.; Nilsson, K. R.; Neely, L. K.; Myers, W. H.;
Sabat, M.; Harman, W. D. J. Am. Chem. Soc. 1995, 117, 3405;
palladium-catalyzed hydroarylations: (e) Clayton, S. C.; Regan,
A. C. Tetrahedron Lett. 1993, 34, 7493; (f) Xu, R.; Chu, G.;
Bai, D. Tetrahedron Lett. 1996, 37, 1463; (g) Xu, R.; Bai, D.;
Chu, G.; Tao, J.; Zhu, X. Bioorg. Med. Chem. Lett. 1996, 6,
279; (h) Bai, D.; Xu, R.; Chu, G.; Zhu, X. J. Org. Chem. 1996,
37, 4600; (i) Malpass, J. R.; Hemmings, D. A.; Wallis, A. L.
Tetrahedron Lett. 1996, 37, 3911; (j) Larock, R. C. U.S. Patent
5,565,573, 1996; Chem Abstr. 125, 301303; (k) Akasaka, K.;
Kimura, T.; Senaga, M.; Machida, Y. Jpn. Patent Kokai Tok-
kyo, JP 070 61,940, 1995, Chem. Abstr. 123, 132885; Akasaka,
K.; Kimura, T.; Senaga, M.; Machida, Y. Jpn. Patent Kokai
Tokkyo JP 070 10,878, 1995; Chem. Abstr. 122, 291257; (l)
Shen, T. Y.; Harman, W.; Dean, W.; Huang, D. F.; Gonzalez,
.
5 ); Anal calcd for C9H13N3 2.25HI (Mass=451.02): C,
23.97; H, 3.41; N, 9.32. Found: C, 23.97; H, 3.37; N,
9.30.
Binding assay
Brie¯y, binding assays have been carried out as follows:
drug solutions were prepared manually, and conse-
quently diluted by a Tecan dilution robot, which trans-
ferred the proper dilution series in triplicate in
individual incubation tubes. Those were placed in a Fil-
terprep-101 (Ismatec, Zurich, Switzerland) which was
programmed to add [3H]-cytisine solutions and tissue
suspensions to the test tubes, and to perform the actual
binding experiment automatically. Finally, the resulting

2110
J.-P. G. Seerden et al./Bioorg. Med. Chem. 6 (1998) 2103±2110
F. PCT Int. Appl., WO 9422868, 1995; Chem. Abstr. 123,
131780 and PCT Int. Appl. WO 9606093, 1996: Chem. Abstr.,
125, 58831; (m) Giblin, G. M. P.; Jones, C. D.; Simpkins, N. S.
Synlett 1997, 589; (n) Qian, C.; Li, T.; Biftu, T.; Shen, T.-Y.
PCT Int. Appl., WO 9507078, 1995; Chem. Abstr. 123, 25683.
batidine-type compounds will be described in a forthcoming
paper.
11. For preparation of some aryl halides see: (a) 5-iodo-2-ami-
nopyrimidine 6 from 2-aminopyrimidine: Shepherd, R. G.;
Fellows, C. E. Am. Soc. 1948, 70, 157; (b) 4-iodo-dimethylani-
line 7b from 4-bromo-dimethylaniline 7a: Gilman, H.; Sum-
mers, L. Am. Soc. 1950, 72, 2767; (c) 4-iodo-isoquinoline 8b
from 4-bromo-isoquinoline 8a via iododestannation of 4-tri-
methylstannylisoquinoline: Yamamoto, Y.; Yanagi, A. Chem.
Pharm. Bull. 1982, 30, 1731.
6. For a recent review about high-pressure synthesis see: Cio-
banu, M., Matsumoto, K. Liebigs Ann./Recueil 1997, 4, 623.
7. Altenbach, H. J.; Constant, D.; Martin, H.-D.; Mayer, B.;
Muller, M.; Vogel, E. Chem. Ber. 1991, 124, 791.
8. Wang, N.-C.; Anderson, H. J. Can. J. Chem. 1977, 67, 4103.
12. (a) For a recent review on Heck reactions, see: de Meijere,
A.; Meyer, F. E. Angew. Chem. 1994, 106, 2473; (b) Arcadi, A.;
Marinelli, F.; Bernochi, E.; Cacchi, S.; Ortar, G. J. Organomet.
Chem. 1989, 368, 249; (c) Larock, R. C.; Johnson, P. L. J.
Chem. Soc., Chem. Commun. 1989, 1368.
9. Similarly, N-carbomethoxy 2,4-dimethylpyrrole gave a high-
pressure Diels±Alder reaction with phenyl vinyl sulfone in
chloroform at 12 kbar and 50 ꢀC to yield the crude sulfonyl
compound after 48 h. Subsequent reductive desulfonylation
with 6% sodium amalgam gave 1,3-dimethyl-7-carbomethoxy-
7-azabicyclo[2.2.1]hept-2-ene in ca. 35% isolated yield.
13. (a) Cortese, N. A.; Heck, R. F. J. Org. Chem. 1977, 42,
3491; (b) Cacchi, S.; Arcadi, A. J. Org. Chem. 1983, 48, 4236.
10. The sodium amalgam reduction of the phenylsulfonyl
group in 3a/3b gave a side product in 7% isolated yield which
was tentatively characterized as N-carbomethoxy-6-azabicy-
clo[3.1.1]hept-2-ene 40 (based on NMR and mass spectra),
which probably arises from of a rearrangement of the inter-
mediate radical (Scheme 3). 1H NMR d 1.58 (2H, br s), 2.65
(1H, m), 3.12 (1H, br s), 3.36 (1H, br d), 3.66 (3H, s, OMe),
4.70 (1H, d), 6.29 (2H, br d). Formation of 6-azabicy-
clo[3.1.1]hept-2-ane derivatives have been described before in
literature, see for example: Corey, E. J.; Loh, T.-P.; Achyu-
thaRao, S.; Daley, D. C.; Sarshar, S. J. Org. Chem. 1993, 58,
5600. Structure elucidation and application of 40 towards epi-
14. Anderson, D.; Arneric, S. Eur. J. Pharm. 1994, 253, 261.
15. Cosford, N. D. P.; Bleicher, L.; Herbaut, A.; McCallum, J.
S.; Vernier, J.-M.; Dawson, H.; Whitten, J. P.; Adams, P.;
Chavez-Noriega, L.; Correa, L. D.; Crona, J. H.; Maha€y, L.
S.; Menzaghi, F.; Rao, T. S.; Reid, R.; Sacaan, A. I.; Santori,
E.; Stauderman, K. A.; Whelan, K.; Lloyd, G. K.; McDonald,
I. A. J. Med. Chem. 1996, 39, 3235.
16. Garvey, D. S.; Wasicak, J. T.; Elliot, R. L.; Lebold, S. A.;
Hettinger, A.-M.; Carrera, G. M.; Lin, N.-H.; He, Y.; Holla-
day, M. W.; Anderson, D. J.; Cadman, E. D.; Raszkiewicsz, J.
L; Sullivan, J. P.; Arneric, S. P. J. Med. Chem. 1994, 37, 4455.