exo-2-(pyridazin-4-yl)-7-azabicyclo[2.2.1]heptanes: Syntheses and nicotinic acetylcholine receptor agonist activity of potent pyridazine analogues of (±)-epibatidine

Article abstract of DOI:10.1021/jm000949w

A new strategy for the straightforward synthesis of novel racemic epibatidine analogues is presented, in which the 2-chloropyridinyl moiety of epibatidine is bioisosterically replaced by differently substituted pyridazine rings. A key step of the new synt

Full text of DOI:10.1021/jm000949w

J . Med. Chem. 2001, 44, 47-57
47
exo-2-(P yr id a zin -4-yl)-7-a za bicyclo[2.2.1]h ep ta n es: Syn th eses a n d Nicotin ic
Acetylch olin e Recep tor Agon ist Activity of P oten t P yr id a zin e An a logu es of
(()-Ep iba tid in e
Daqing Che,† Thomas Wegge,† Milton T. Stubbs,† Gunther Seitz,*,† Heinrich Meier,‡ and Christoph Methfessel#
Institut fu¨r Pharmazeutische Chemie, Universita¨t Marburg, Marbacher Weg 6, D-35032 Marburg/ Lahn, Germany, Bayer AG,
PH-R-CR MC IV, Gescha¨ftsbereich Pharma, Chemische Forschung, Medizinische Chemie IV, D-42096 Wuppertal, Germany,
and Bayer AG, Central Research Biophysics Department, D-51368 Leverkusen, Germany
Received March 22, 2000
A new strategy for the straightforward synthesis of novel racemic epibatidine analogues is
presented, in which the 2-chloropyridinyl moiety of epibatidine is bioisosterically replaced by
differently substituted pyridazine rings. A key step of the new syntheses is the inverse type
Diels-Alder reaction of the electron-rich enol ether 13 with the electron-deficient diazadiene
systems of the 1,2,4,5-tetrazines 14a -d to yield the novel pyridazine analogues of (()-
epibatidine 18, 19, 22, and 24. In addition preparation of the N-substituted derivatives, such
as 26 and 28, is described. The structures of the novel epibatidine analogues were assigned on
the basis of spectral data, that of compound 24 being additionally verified by X-ray
crystallography exhibiting two racemic solid-state conformations in the crystal lattice and
representing the first X-ray structure of an unprotected 7-azabicyclo[2.2.1]heptane moiety. The
nAChR agonist activity of the racemic compounds 18, 19, 22, 24, and 28 was assayed in vitro
by whole-cell current recordings from Xenopus oocytes expressing different recombinant nicotinic
receptors from the rat. Among the compounds synthesized and tested, the pyridazine analogue
24 of (()-epibatidine and its N-methyl derivative 28 were found to be the most active ones
retaining much of the potency of natural epibatidine but with a substantially improved
selectivity ratio between the R4â2 and R3â4 subtypes.
Sch em e 1. Epibatidine (1), Nicotine (2), and Selected
Novel nAChR Ligands with Replacement of the
Azabicycloheptane Moiety or 2-Chloropyridinyl Unit of 1
In tr od u ction
Epibatidine (1) (Scheme 1), a pyridylazabicyclo[2.2.1]-
heptane, was discovered by Daly1 as a trace alkaloid in
skin extracts of an Ecuadorian frog. It was reported to
be a non-opioid analgesic agent with a potency far
greater than that of morphine in mice.1 The absolute
configuration of the natural product was shown to be
1R,2R,4S.2 Its structural similarity to (-)-nicotine (2)
suggested that epibatidine would have activity at nico-
tinic receptors. Indeed, epibatidine (1) was shown to
exert its antinociceptive actions through neuronal nico-
tinic acetylcholine receptors (nAChR).3-7 Due to its
unique structure2,8 and its remarkable pharmacological
activity as a non-opioid antinociceptive agent, several
approaches to the synthesis9-22 of the alkaloid have been
reported to make compound 1 available in sufficient
amounts for extensive pharmacological evalutations.
These have demonstrated that (+)- and (-)-epibatidine
(1) are highly potent agonists at several nAChR sub-
types yet, in contrast to the nicotines, have little
enantioselectivity. In addition, (()-epibatidine (1) itself
is much too toxic to be a useful therapeutic agent and
it was hoped that novel analogues with better selectivity
between different nAChR subtypes could be of greater
promise. In particular, it has been suggested4 that an
enhanced selectivity for the R4â2 receptor subtype over
the R3â4 variant could lead to an improved antinocice-
ptive effect with reduced toxic liability. Thus, based
upon the approach that bioisosteric alteration in the
ring systems of 1 might provide compounds with better
ratios of pharmacological to toxicological activity, a
series of novel neuronal nAChR ligands has been
developed recently: one series involves replacement of
the azabicycloheptane moiety of 1 by similar azabicy-
* To whom correspondence should be addressed. Tel: 0049-6421-
2825809. Fax: 0049-6421-2826652. E-mail: seitzg@mailer.uni-mar-
burg.de.
† Universita¨t Marburg.
‡ Bayer AG, Gescha¨ftsbereich Pharma.
#
Bayer AG, Central Research Biophysics Department.
10.1021/jm000949w CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/30/2000

48 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1
Che et al.
Sch em e 2a
a
Reagents and conditions: (a) lit. ref 25; (b) LiCl, NaBH4, EtOH/THF, 12 h, rt, 96%; (c) DMSO/(COCl2) then N(C2H5)3, 94%; (d)
[(Ph)3PCH2OCH3]+Cl-, LDA, toluene, 2 h, 0°C, 83%; (e) + 14a , toluene, 12 h, reflux, 88%; (f) + 14b, toluene, 2 days, 120 °C, pressure
bottle, 32% and 59%; (g) + 14c, toluene, 18 h, reflux, 74%; (h) (CH3)3SiI, CHCl3, 4 h, reflux, then CH3OH.
cloalkanes. Among others, (()-homoepibatidine (3),23-25
the 8-azabicyclo[3.2.1]octane derivative 4,26 and UB-165
(5)27 have been synthesized and reported to show
significant nicotinic receptor binding affinity and stimu-
lant activity.
A second approach replaces the 2-chloropyridinyl unit
of 1 with various heteroaryl groups, providing novel
nAChR ligands with quite promising properties. Thus
epiboxidine (6),28 in which the 2-chloropyridinyl ring has
been replaced by the methylisoxazolyl moiety, proved
to be a potent antinociceptive agent with a better
activity/toxicity ratio compared to epibatidine (1). Simi-
lar results were obtained with the 5-pyrimidinylepiba-
tidine analogue 729 exhibiting high affinities for the
[3H]cytisine rat brain nicotinic acetylcholine binding
sites.
To develop new ligands selective for distinct nAChR
substypes with potent analgesic activity, reduced toxic-
ity, and a satisfactory safety profile,30 we have started
to synthesize a new series of hitherto unknown epiba-
tidine analogues, in which the 2-chloropyridinyl moiety
is replaced by other nitrogen heterocycles such as 1,2-,
1,3-, or 1,4-diazines and 1,2,4-triazines. Herein we
report a new strategy for the synthesis of novel epiba-
tidine analogues which evokes a conceptually simple
route to novel exo-2-(pyridazin-4-yl)-7-azabicyclo[2.2.1]-
heptanes such as 8, utilizing as the key step the inverse
electron demand Diels-Alder reaction of the enol ether
13 with the π-electron-deficient 1,2,4,5-tetrazines
14a -d .31,32
Resu lts a n d Discu ssion
Ch em istr y. An attractive synthetic route to novel
epibatidine analogues such as 8 originates from the
commercially available 3-tropanone 9 which could easily
be converted in a three-step procedure utilizing Bai’s
methodology25 to the racemic ester 10 with the requisite
exo configuration. As outlined in Scheme 2, the ester
10 could be transformed by sequences of conventional
reactions to yield the desired electron-rich dienophilic
enol ether 13. This reacted with the electron-deficient
diazadiene systems of the 1,2,4,5-tetrazines 14a -c in
a LUMOdiene/HOMOdienophile-controlled Diels-Alder re-
action31,32 to yield the N-protected epibatidine analogues
15-18 in good yields after inverse [4+2] cycloaddition
and elimination of nitrogen. Removal of the protecting

exo-2-(Pyridazin-4-yl)-7-azabicyclo[2.2.1]heptanes
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1 49
Sch em e 4a
Sch em e 3a
a
Reagents and conditions: (a) toluene, 2 days, reflux, 74%; (b)
(CH3)3SiI, CHCl3, 4 h, 80 °C, then CH3OH, -CO2; (c) + concd aq
ammonia f pH ) 10; (d) dioxane/aq HCl (37%), 20 h, reflux, 28%.
a
Reagents and conditions: (a) pentafluorophenyl acetate, DMF,
14 h, rt, 13% and 69%.
33
groups from 15 and 16 by (CH3)3SiI in boiling CHCl3
afforded the desired pyridazine analogues 19 and 20 of
epibatidine, which could be isolated in high yields as
air-stable pale yellow oils. To our disappointment, all
attempts to deprotect carbamate 18 by conventional
methods furnished complex mixtures of unidentified
products.
Sch em e 5a
Fortunately the enol ether 13 was reactive enough
to cycloadd also to the less activated parent tetrazine
14d ,31 affording the N-protected epibatidine analogue
22 in more than 70% yield. Surprisingly, an attempt to
deprotect the carbamate 22, performed under the same
conditions as described above with e.g. 15, produced
an unexpected species in fair yield, identified as the
pyridazine-substituted cyclohex-3-enylamine 23 by its
spectroscopic properties (MS, 1H and 13C NMR). A
comprehensive and plausible mechanism for the un-
precedented formation of 23 via ring opening is outlined
in Scheme 3. We suppose that - in contrast to the
behavior for instance of the carbamate 15 - the car-
bamate 22 reacts under the conditions employed (excess
of trimethylsilyl iodide, refluxing in chloroform followed
by methanolyses) to yield the 2-fold protonated inter-
mediate 22a . On treatment with concentrated aqueous
ammonia, this suffers a heterolytic cleavage of the C-N
linkage of the bicyclic cationic system as indicated,
giving rise to the formation of the unexpected cyclohex-
ene derivative 23 in addition to small amounts of the
desired epibatidine analogue 24. Therefore, an alterna-
tive synthetic approach was required in order to gain
24 with satisfactory yield. Treatment of carbamate 22
with dioxane/aqueous hydrochloric acid (37%) seemed
to be a promising approach. Indeed the target compound
24 could be isolated in ca. 30% yield as a white solid
after careful chromatography on silica gel, by which the
byproduct 23 could be separated. Spectroscopic data
analysis (MS, 1H and 13C NMR) conclusively proved the
expected structure of 24.
a
Reagents and conditions: (a) LiAlH4/Et2O, 12 h, reflux, 87%;
(b) toluene, 12 h, reflux, 19%.
Methylation of pyrido[3,4-b]homotropane (PHT) or (-)-
cytisine results in decreased affinity. With this in mind,
we began to prepare the N-acetylated and N-methylated
derivatives of the epibatidine analogue 24 as outlined
in Schemes 4 and 5.
Because acetylation of the secondary amine 24 with
acetic anhydride according to a method published for
epibatidine1 failed, the highly reactive and more selec-
tive acetylating agent pentafluorophenyl acetate40,41 was
used successfully to furnish the acetylated derivative
26a ,b (as a pair of rotamers about the N-CO bond) in
69% yield. The byproduct 25 (13% yield) could be
separated by careful column chromatography. As with
N-acetylepibatidine, the 1H NMR spectrum of 26a ,b was
recognized as a series of doubled signals, stemming from
two rotamers 26a ,b in a 1.6:1 ratio. Because of the
partial double bond character of the N-acetyl bond,
interconversion between the rotamers is very slow,
giving rise to the observed series of doubled signals.25,42
Our attempts to alkylate the secondary amine 24
directly with methyl iodide in solutions of various
anhydrous solvents and in the presence of a variety of
basic catalysts were not successful in contrast to similar
attempts with (()-epibatidine (1).43 Since reductive
methylation of 24 with trioxane/NaBH3CN or HCO2H/
To gain greater insight into structural features im-
portant for nAChR binding34-39 we varied the nitrogen
substituents at the bicyclic system of 24. It is well-
known that in some cases introduction of an N-methyl
group increases affinity (as with (-)-nornicotine/(-)-
nicotine) whereas in other cases it has little to no effect
(as with (-)-epibatidine/(-)-N-methylepibatidine).35 N-

50 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1
Che et al.
CH2O also failed due to instability of the starting
material, we tried an alternative procedure to obtain
the N-methyl derivative 28. The synthesis of 27 was
accomplished utilizing the N-protected enol ether 13 as
starting material. This could be transformed to the
corresponding N-methylated enol ether 27 upon treat-
ment with LiAlH4/ether in good yields. 27 entered into
the Diels-Alder reaction with the 1,2,4,5-tetrazine 14d
affording the desired N-methylated species 28 however
in low yield. All compounds described were obtained and
tested as racemic mixtures.
X-r a y Cr yst a llogr a p h ic An a lysis of t h e Ta r get
Liga n d 24. Because compounds such as (()-epibatidine
(1),1 (()-epiboxidine (6),28 or the 5-pyrimidinylepibati-
dine analogue 729 with the unusual 7-azabicyclo[2.2.1]-
heptane ring system as the typical pharmacophoric
element exhibit such exciting biological activities, it was
deemed of value to subject appropriate single crystals
of the structurally similar pyridazine analogue 24 to an
X-ray diffraction analysis. An advantage of having
available the crystal structure of 24 was the possibility
to obtain detailed information of the three-dimensional
arrangement and to check the calculated geometries of
epibatidine and its analogues, beneficial for the defini-
tion of relevant distances between putative pharma-
cophoric elements and thus for the improvement of
SARs. Figure 1 shows the ORTEP diagram of the
structure of two racemic solid-state conformations of 24,
solved by X-ray diffraction methods as detailed in the
Experimental Section. Thus the structure of the bioi-
sosteric variant 24 of epibatidine was unambiguously
verified and represents the first X-ray structure of a
N-unprotected 7-azabicyclo[2.2.1]heptane ring system,
in this case attached to a 4-pyridazine substituent at
C-2 in a pseudoequatorial position and in an exo
orientation, probably crucial for biological activity (see
Figure 1a). Interestingly, in the crystal lattice of the
pyridazine analogue 24 two different, obviously lowest-
energy conformations are fixed with the same prob-
ability.
F igu r e 1. (a) ORTEP diagram (50% probability ellipsoids)
showing the two solid-state conformations of one enantiomer
of 24a (top) and one of 24b (below). (b) Conformational
differences between the racemic species 24a ,b with numbering
through the text.
With regard to the configuration of the three stereo-
genic centers (see Figure 1b) these are recognized as
two racemic forms: 24a ,b. In addition, the conforma-
tional feature of these two preferred species can be
defined with two parameters: first, the disposition of
the N-H moiety, syn or anti to the pyridazine substitu-
ent, and second, as the only degree of freedom the
rotation around the bond between the two putative
pharmacophore elements, the 7-azabicyclo[2.2.1]heptane
and the pyridazine system. Thus, ligand 24a differs
from 24b with respect to an approximate 200° rotation
of the bond connecting the two heterocyclic systems.
Interestingly, the C-1a-C-2a-C-4′a-C-3′a dihedral
angle in 24a exhibits a value of 176.7° (see atom labeling
in Figure 1), whereas the corresponding C-1b-C-2b-
C-4′b-C-3′b dihedral angle in 24b is -26.3°.
In compound 24a the disposition of the N-H moiety
is anti to the pyridazine substituent, whereas in 24b
the N-H group, pointing toward the pyridazine ring,
is syn positioned. As the nitrogen inversion barrier of
7-azabicyclo[2.2.1]heptane has been calculated to be
11.4-12.0 kcal‚mol-1, syn/ anti conversion can easily
occur at room temperature.34 A second interesting
aspect is the inter-nitrogen distances observed for 24a ,b.
N-N distances are used in different pharmacophoric
models for various nAChR ligands due to the close
relationship between nAChR affinity and inter-nitrogen
distances.35-39 In the case of (()-epibatidine (1) the N-N
distance showed two extreme values in the conforma-
tions obtained by molecular modeling studies of ca. 4.5
and 5.5 Å.38
Comparatively, in conformation 24a , where N-7 and
N-2′ are on opposite sides of the molecule, the N-N
distance between N-7 and N-2′ is 5.38 Å (that between
N-7 and N-1′ is 5.54 Å), similar to the calculated inter-
nitrogen distance of the lowest-energy conformation of
(()-epibatidine (1) with 5.51 Å.35,36,38 In contrast, for
conformation 24b, in which the pyridazine ring is
rotated so that N-2′ is proximal to the sp3 nitrogen N-7,
the relevant N-7-N-2′ distance of 4.75 Å is obviously
shorter (the inter-nitrogen distance N-7-N-1′ is 5.74 Å)
and fits perfectly into Sheridan’s nicotinic pharmacoph-
ore model,39 similar to the calculated inter-nitrogen
distance in (-)-nicotine (2) with 4.87 Å35,39 and another
stable conformer of (()-epibatidine (1) with a relative
energy of only 0.18 kcal/mol, featuring the N-N dis-
tance of 4.47 Å.38 It is well-established that the acceptor-

exo-2-(Pyridazin-4-yl)-7-azabicyclo[2.2.1]heptanes
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1 51
Ta ble 1. Agonist Activity of the Test Compounds at Four
Representative Nicotinic Receptor Subtypesa
activity (µM)
compd
R3â4
3.2
R4â2
0.32
R7
rat muscle
acetylcholine
50
0.1
(-)-nicotine bitartrate 4.0
0.1
0.005
12.6
0.5
6.3
0.03
(()-epibatidine
0.005
18
19
22
24
28
not tested inactive inactive not tested
inactive
inactive
0.16
inactive inactive not tested
>100
0.02
inactive .100
40.0
25.0
0.16
0.20
0.32
0.013
a
To account for the different intrinsic agonist sensitivities of
the receptor subtypes, a reference concentration of ACh was first
defined for each subtype (first line). The activity of each test
compound is expressed by giving a concentration which elicits a
current signal of the same amplitude as the reference dose of ACh.
All compounds were initially tested at 100 µM; thus, for inactive
compounds this was the highest dose tested. Since the concentra-
tions given in the table were read from a plot of the dose-response
curve, it is not meaningful to assign statistical parameters such
as SD or SEM to them; all dose-response curves were triplicates,
from at least two different batches of frog oocytes.
F igu r e 2. Current response of rat R4â2 nicotinic receptors
expressed in Xenopus oocytes plotted against the concentration
of the respective agonist for epibatidine (Epi), 24, 28, nicotine
(Nic), and acetylcholine (ACh). All current responses were
normalized to the current elicited by 0,32 µM acetylcholine.
The dose-response curves are plotted in a double-logarithmic
fashion to emphasize the low-concentration regime. In this
representation, the dose-response curve is approximately
linear with a slope corresponding to the Hill coefficient.
bound conformations of ligands may not be the same
as the lowest-energy conformations either in vacuo, in
solution, or in a crystal. From a conformational point
of view the bioisosteric analogue 24, conformationally
mobile like the parent alkaloid 1, can be considered as
a mixture of several conformational minima and some
population of the intermediate structures. Thus, the
small nitrogen inversion barriers and the small rota-
tional barriers34 indicate that ligand 24 at room tem-
perature in solution should be a mixture of several
minimum conformations with different inter-nitrogen
distances more or less fit for a ligand/nAChR interac-
tion.
In addition, the agonistic potencies of the newly
synthesized ligands were compared with those of (-)-
nicotine and (()-epibatidine. The different standard
concentrations of acetylcholine were empirically fixed
to give a robust, nonsaturating current response for that
respective receptor subtype. Due to the different intrin-
sic sensitivities of the subtypes to agonists, these values
are necessarily also different for each one. The concen-
tration values given in Table 1 are not the same as the
EC50 values. However, when comparing the activity of
different compounds they fulfill a similar role. Indeed,
it is one of the main advantage of our “relative concen-
tration” parameters that they avoid many of the dif-
ficulties and pitfalls of determining EC50 values cor-
rectly. To determine an EC50 value from a dose-
response curve, obviously one first needs the correct
value of the maximum current that can be elicited. This
can be surprisingly difficult to determine for a variety
of reasons, including partial agonism, receptor desen-
sitization, open-channel block by the agonist itself,
kinetic parameters, and, in the case of the oocytes, the
simple fact that the maximum current is often much
too big to measure with the available recording elec-
tronics. By concentrating our measurements on the low-
dose regime of the dose-response curve, we effectively
avoid all of these problems and obtain results which
reflect the intrinsic activity of the compound at the
receptor much more reliably than the more commonly
used EC50 values.
In Vitr o Tests. Agonist activity of the compounds
was assayed in vitro by whole-cell current recordings
from Xenopus oocytes expressing different recombinant
nicotinic receptors from the rat44 after injection with
appropriate cDNA plasmids of one of the following
subtypes: R3â4, R4â2, R7 (all neuronal) or R1â1γδ
(embryonic muscle subtype). It is common to test for the
interaction of novel compounds with a designated recep-
tor initially with competition binding assays that cannot
distinguish between agonists and antagonists of the
receptor. We have chosen the electrophysiological ap-
proach because it gives a direct functional measure of
the activation of ligand-gated receptor currents by the
test compound, which is the primary relevant parameter
for biological activity. Moreover, the use of recombinant
receptor subunits allows us to perform these measure-
ments with defined molecular targets representing a set
of the putative major subtypes of nAChR in the organ-
ism.
To account for the different sensitivities of the sub-
types toward acetylcholine, and for the different ef-
ficiencies of ligands, the agonistic potency at each
receptor subtype is expressed as the concentration found
to be equipotent with a standard reference dose of
acetylcholine (Table 1). [We describe these concentra-
tions as “equipotent” rather than “equi-efficacious”
because the values given were obtained as points of
intersection of the measured dose-response curves with
the ACh reference response rather then measured
directly (see Figures 2 and 3).]
As shown in Table 1, replacement of the chloropyri-
dine moiety in epibatidine with an unsubstituted py-
ridazine ring providing 24 led to a potent and highly
effective agonist on neuronal and muscle nAChRs,
especially on the R4â2 subtype. Thus much of the
biological activity of the most potent natural compound
(()-epibatidine was retained. This could be anticipated
for a bioisosteric analogue, in which the inter-nitrogen
distances, crucial for a highly potent ligand, are nearly
equivalent to those of the parent alkaloid. Indeed 24
turned out to be the most active compound. The drastic

52 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1
Che et al.
tive of the inter-nitrogen distance, which remains ap-
proximately equal to that of (()-epibatidine. A similar
significant loss of potency is also observed with the
structurally varied ligands 18 and 22, characterized by
a carbamate group at N-7 of the bicyclic moiety. In
comparison, 28, the N-7-methylated bioisostere of (()-
epibatidine, just like 24, exhibited significant agonistic
potencies, whereas the corresponding carbamate 22
revealed complete loss of activity.
Most probably due to the different basicity of the
pyridazine ring, 24 proved to be about 4-fold less potent
at the R4â2 subtype and ca. 30-fold less potent at the
R3â4 subtype than (()-epibatidine but about 5- and 25-
fold, respectively, more potent than (-)-nicotine. At the
R1â1γδ subtype 24 was about 5-fold less potent than
the frog poison but nearly 40-fold more potent than (-)-
nicotine. Remarkably, 24 and its N-7-methyl derivative
28 both differentiate somewhat better between nAChR
subtypes than (()-epibatidine, in particular between
R3â4 and R4â2. While (()-epibatidine in our system is
equipotent on both receptors, 28 is 25-fold selective
toward the R4â2 subtype, which is more comparable to
(-)-nicotine (40-fold).
Con clu sion
Bioisosterism is an important concept in medicinal
chemistry and serves as a valuable aid in SAR studies
and new drug design. Following the idea that bioisos-
teric alterations in the ring systems of (()-epibatidine
might provide compounds with better ratios of phar-
macological to toxicological activity, we successfully
replaced the chloropyridinyl pharmacophore of (()-
epibatidine by differently substituted pyridazines. A key
step of our new and efficient synthetic approach to novel
pyridazine analogues of (()-epibatidine was the inverse
type Diels-Alder reaction of the electron-rich enol ether
13 with the electron-deficient diazadiene system of
various 1,2,4,5-tetrazines. The most active compound
described was 24, the structure of which could be
verified by X-ray crystallography revealing a N-N
distance close to that of (()-epibatidine. Though some-
what less potent, 24 was found to retain much of the
agonist activity of the most potent frog poison (()-
epibatidine and proved to be a highly effective agonist
on neuronal and muscle nAChRs, especially on the R4â2
subtype. Interestingly 24 as well as its N-7-methyl
derivative 28 both differentiate somewhat better be-
tween nAChR subtypes, in particular between R3â4 and
R4â2.
Exp er im en ta l Section
Gen er a l P r oced u r es. Standard vacuum techniques were
used in handling of air-sensitive materials. Melting points are
uncorrected: “Leitz-Heiztischmikroskop” HM-Lux. Solvents
were dried and freshly distilled before use according to
literature procedures. IR: Perkin-Elmer 257, 398 and FT-IR
spectrometer 510-P (Nicolet). Liquids were run as films, solids
as KBr pellets. 1H and 13C NMR: J EOL J NM-GX 400 and LA
500; δ/ppm ) 0 for tetramethylsilane, 7.26 for chloroform.
MS: Vacuum Generators 7070 (70 eV; 11B). Column chroma-
tography: purifications were carried out on Merck silica gel
60 [70-260 (flash chromatography) or (200-400 mesh)].
Reactions were monitored by thin-layer chromatography (TLC)
using plates of silica gel (0.063-0.200 mm; Merck) or silica
gel 60-F254 microcards (Riedel de Haen).
F igu r e 3. Dose-dependent current responses evoked by
epibatidine (A), 24 (B), and 28 (C) on different subunit
combinations of rat nicotinic receptors. The responses are
normalized to the signal elicited by a reference concentration
of ACh which, for each receptor subtype, was chosen to give a
robust activation of current. Note that there is a clear
preference for the R4â2 over the R3â4 subtype with compounds
24 and 28, whereas epibatidine is about equipotent at these
receptor subtypes.
drop in agonistic potency for the 2-fold trifluoromethyl-
substituted pyridazine analogue such as 19 indicates
that 3,6-substitution in the pyridazine ring significantly
lowers the potency, mainly for steric reasons, irrespec-
exo-2-Hydr oxym eth yl-7-azabicyclo[2.2.1]h eptan e-7-car -
boxylic Acid Eth yl Ester (11). To a solution of 2.27 g (10.0

exo-2-(Pyridazin-4-yl)-7-azabicyclo[2.2.1]heptanes
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1 53
mmol) of ester 10 in 30 mL of dry tetrahydrofuran and 50 mL
of dry ethanol were added 0.86 g (20 mmol) of LiCl and 0.76
g (20 mmol) of NaBH4. The reaction mixture was stirred for
12 h at room temperature then a solution of citric acid was
added until pH ) 4 was reached. After evaporation of the
solvent in vacuo, 50 mL of water were added and the resulting
mixture extracted with CH2Cl2 (3 × 40 mL). The combined
organic layers were washed with saturated aqueous NaHCO3
and NaCl solutions, dried over MgSO4 and filtered. After
removal of solvent in vacuo the residue was purified by flash
chromatography over silica gel eluting with ethyl acetate:n-
hexane ) 2:1, to give 11 (1.96 g, 98%, Rf ) 0.43) as a colorless
oil. IR (film): v (cm-1) ) 3439, 2954, 2876, 1700, 1380, 1317,
1270, 1196, 1170, 1140, 1105, 1019, 918, 855. 1H NMR (400
MHz, CDCl3): δ ) 1.22 (t, J ) 7.1 Hz, 3H, 9-H), 1.28 (m, 1H,
6-H), 1.41 (m, 2H, 5-H and 6-H), 1.51 (dd, J ) 12.1 Hz, J )
8.5 Hz, 1H, 3R-H), 1.75 (m, 2H, 5-H and 3â-H), 1.92 (m, 1H,
2-H), 3.32-3.43 (m, 2H, 10-H), 4.08 (q, J ) 7.1 Hz, 2H, 8-H),
4.24 (m, 2H, 1-H and 4-H). 13C NMR (100 MHz, CDCl3): δ )
14.7 (C-9), 29.1 (C-5), 29.3 (C-6), 33.4 (C-3), 45.6 (C-2), 55.9
(C-4), 57.5 (C-1), 61.1 (C-8), 65.1 (C-10), 156.1 (C-7). MS (70
eV): m/z (%) 199 (23, M+). HRMS: calcd for C10H17NO3
199.1208, found 199.1207.
s, overlapping of 1-H and 10-H, 1.48H, 1-H and 10-H), 4.59-
4.71 (m, 0.52H, 10-H), 5.57 (d, J ) 6.0 Hz, 0.48H, 11-H,
Z-isomer), 6.31 (d, J ) 12.6 Hz, 0.52H, 11-H, E-isomer). 13C
NMR (100 MHz, CDCl3): δ ) 14.7 (C-9), 29.0 (C-5), 38.2 (C-
6), 39.4 (C-3), 42.3 (C-2), 55.8 and 56.0 (C-4), 59.6 (C-1), 60.9
(C-12), 61.8 (C-1), 62.3 (C-8), 107.3 and 111.4 (C-10), 145.0 and
146.6 (C-11), 156.1 (C-7). MS (70 eV): m/z (%) ) 225 (63, M+),
141 (100). HRMS: calcd for C12H19NO3 225.1365, found
225.1365.
e xo-2-(3,6-B i s t r i flu o r o m e t h y lp y r i d a z i n -4-y l)-7-
a za bicyclo[2.2.1]h ep ta n e-7-ca r boxylic Acid Eth yl Ester
(15). To a solution of 0.45 g (2.0 mmol) of the enol ether 13 in
15 mL of dry toluene was added dropwise a solution of 0.43 g
(2.0 mmol) of the tetrazine 14a in 10 mL of dry toluene at
room temperature. After stirring for 30 min at room temper-
ature the mixture was refluxed under argon for 12 h and cooled
to room temperature. The solvent was evaporated in vacuo and
the residue purified by flash chromatography over silica gel
eluting with diethyl ether:petroleum ether (40/60) 1:1 (column
20 × 1 cm) to give 15 (0.67 g, 88%) as a colorless oil, which
crystallized after storing at 4 °C for 3 days; mp 61-63 °C (n-
hexane). IR (film): v (cm-1) ) 2975, 2896, 1703, 1632, 1619,
1
1546, 1501, 1288, 1174, 1054. H NMR (400 MHz, CDCl3): δ
) 1.24 (t, J ) 7.1 Hz, 3H, 9-H), 1.62-1.71 (m, 2H, 5-H and
6-H), 1.79-1.85 (m, 1H, 5-H), 1.88-1.94 (m, 2H, 6-H and 3â-
H), 2.12 (dd, J ) 12.6 Hz, J ) 9.1 Hz, 1H, 3R-H), 3.32 (d, 1H,
J ) 9.0 Hz, J ) 5.2 Hz, 2-H), 4.15 (q, J ) 7.1 Hz, 2H, 8-H),
4.35 (s, br, 1H, 4-H), 4.50 (s, br, 1H, 1-H), 8.15 (s, 1H, 11-H).
13C NMR (100 MHz, CDCl3): δ ) 14.5 (C-9), 28.7 (C-5), 29.7
(C-6), 40.3 (C-3), 41.8 (C-2), 56.1 (C-4), 61.5 (C-1), 61.8 (C-8),
120.9 (q, J C,F ) 275.3 Hz, C-14), 121.6 (q, J C,F ) 276.8 Hz,
C-12), 123.3 (C-15), 146.2 (C-10), 150.7 (q, J C,F ) 33.0 Hz,
C-13), 153.9 (q, J C,F ) 35.3 Hz, C-11), 155.3 (C-7). MS (70 eV):
m/z (%) 383 (41), 141 (100). Anal. (C15H15F6N3O2) C, H, N.
exo-2-(3,6-Dip yr id in -2-ylp yr id a zin -4-yl)-7-a za b icyclo-
[2.2.1]h ep ta n e-7-ca r boxylic Acid Eth yl Ester (16) a n d
exo-2-(5-Meth oxy-3,6-dipyr idin -2-yl-4,5-dih ydr opyr idazin -
4-yl)-7-a za bicyclo[2.2.1]h ep ta n e-7-ca r boxylic Acid Eth yl
Ester (17). A solution of 0.47 g (2.0 mmol) of the vinyl ether
13 and 0.35 g (1.5 mmol) of the tetrazine 14b in 15 mL of dry
toluene was heated for 2 days at 120 °C in a pressure bottle.
After cooling to room temperature the solvent was evaporated
in vacuo and the residue was purified by column chromatog-
raphy (silica gel, column 30 × 2 cm), eluting with ethyl acetate:
petroleum ether (40/60) 6:1. Fraction 1 contained 0.38 g (59%,
Rf ) 0.48) of compound 17 as yellow crystals; fraction 2
contained 0.19 g (32%, Rf ) 0.45) of compound 16 as colorless
crystals. 16: mp 154-156 °C. IR (KBr): v (cm-1) ) 2979, 2921,
1706, 1568, 1466, 1386, 1089, 822, 743. 1H NMR (400 MHz,
CDCl3): δ ) 1.20 (t, 3H, J ) 7.1 Hz, 9-H), 1.49-1.53 (m, 1H,
5-H), 1.60-1.64 (m, 1H, 6-H), 1.83-1.87 (m, 4H, 4-H, 5-H and
6-H), 3.73 (dd, 1H, J ) 8.8 Hz, J ) 5.2 Hz, 2-H), 4.10 (q, 2H,
J ) 7.1 Hz, 8-H), 4.49-4.50 (m, 2H, 1-H and 4-H, overlapped),
7.35-7.40 (m, 2H, 20-H and 15-H), 7.84-7.92 (m, 2H, 22-H
and 13-H), 8.14-8.16 (m, 1H, 23-H), 8.68-8.71 (m, 4H, 14-H,
16-H, 19-H and 21-H). 13C NMR (100 MHz, CDCl3): δ ) 14.4
(C-9), 28.8 (C-5), 29.7 (C-6), 38.8 (C-3), 43.2 (C-2), 56.4 (C-4),
61.2 (C-1), 62.0 (C-8), 121.7, 122.6, 123.5, 124.5, 125.5, 136.9,
144.5, 148.5, 149.4, 155.6, 156.6, 157.8, 158.4, 153.7 (C-7). MS
(70 eV): m/z (%) 401 (M+, 45), 261 (100). Anal. (C23H23N5O2),
C, H, N.
exo-2-F or m yl-7-a za b icyclo[2.2.1]h ep t a n e-7-ca r b oxyl-
ic Acid Eth yl Ester (12). To a solution of 1.0 mL (11 mmol)
of oxalyl chloride in 30 mL of dry CH2Cl2 was slowly added a
solution of 1.7 mL (24 mmol) of DMSO in 5 mL of dry CH2Cl2
at -78 °C. After 5 min a solution of 1.59 g (8.0 mmol) of 11 in
10 mL of dry CH2Cl2 was added. The reaction mixture was
stirred for 30 min at -78 °C, than 9 mL of triethylamine was
added. After the mixture was warmed to room temperature
100 mL of water was added and the organic layer separated.
The aqueous layer was extracted with CH2Cl2 (3 × 40 mL)
and the combined organic layers washed with saturated
aqueous NaHCO3 and NaCl solutions, dried over MgSO4 and
filtered. After removal of solvent in vacuo the residue was
purified by flash chromatography over silica gel eluting with
ethyl acetate:petroleum ether (40/60) ) 3:1 to give 1.56 g (98%,
Rf ) 0.54) 12 as a colorless oil. IR (film): v (cm-1) ) 2980,
1699, 1412, 1379, 1345, 1315, 1271, 1194, 1171, 1102, 1010,
958, 778. 1H NMR (400 MHz, CDCl3): δ ) 1.19 (t, J ) 7.1 Hz,
3H, 9-H), 1.42-1.58 (m, 3H, 6R-H and 5-H), 1.72-1.88 (m, 2H,
6â-H and 3R-H), 2.16-2.22 (m, 1H, 3â-H), 2.49 (ddd, J ) 8.8
Hz, J ) 4.6 Hz, J ) 1.7 Hz, 1H, 2-H), 4.04 (q, J ) 7.1 Hz, 2H,
8-H), 4.34 (t, J ) J ) 4.5 Hz, 1H, 4-H), 4.55 (d, J ) 4.5 Hz,
1H, 1-H), 9.58 (d, 1H, J ) 1.7 Hz, 10-H). 13C NMR (100 MHz,
CDCl3): δ ) 14.5 (C-9), 29.1 (C-5), 29.3 (C-6), 30.4 (C-3), 55.1
(C-2), 56.0 (C-4), 57.0 (C-1), 61.2 (C-8), 155.4 (C-7), 200.8 (C-
10). MS (70 eV): m/z (%) 197 (6, M+), 140 (100). HRMS: calcd
for C10H15NO3 197.1052, found 197.1046.
exo-2-(2-Meth oxyeth en yl)-7-a za bicyclo[2.2.1]h ep ta n e-
7-ca r boxylic Acid Eth yl Ester (13). To a suspension of 5.15
g (15.0 mmol) of methoxymethyltriphenylphosphonium chlo-
ride in 30 mL of dry toluene was added dropwise at 0 °C under
argon 7.5 mL of a freshly prepared LDA solution (2 M in THF).
The deep red reaction mixture was stirred at 0 °C for 15 min
and then a solution of 0.99 g (5 mmol) of 12 in 8 mL of dry
toluene was added slowly. After 2 h at 0 °C the reaction
mixture was partitioned between 50 mL of ice-water and 100
mL of Et2O and stirred for 30 min. The organic layer was
separated, the aqueous layer extracted with Et2O (3 × 30 mL),
the combined organic layers washed with 100 mL of saturated
aqueous NaHCO3 and NaCl solutions, dried over MgSO4 and
filtered. After removal of the solvent in vacuo the oily residue
was purified by flash chromatography over silica gel eluting
with petroleum ether (40/60):Et2O ) 1:1 to give 0.89 g (78%,
Rf ) 0.70) enol ether 13 as colorless oil. IR (film): v (cm-1) )
2955, 2875, 2836, 1699, 1376, 1345, 1311, 1252, 1208, 1176,
1104, 1014, 977, 936, 902, 875, 780. 1H NMR (400 MHz,
CDCl3): δ ) 1.21-1.27 (m, overlapped by t, 3H, 9-H), 1.33-
1.51 (m, 3H, 5-H and 6-H), 1.68-1.78 (m, 3H, 6-H and 3-H),
2.21-2.28 (m, 0.52H, 2-H), 2.71-2.78 (m, 0.48H, 2-H), 3.48
(s, 1.56H, 12-H), 3.58 (s, 1.44H, 12-H), 3.96-4.03 (br s, 1H,
4-H), 4.08-4.16 (m, overlapped by q, 2H, 8-H), 4.26-4.34 (br
17: IR (KBr): v (cm-1) ) 2988, 2941, 1696, 1585, 1463, 1376,
1
1088, 802, 746. H NMR (400 MHz, CDCl3): δ ) 1.15 (t, J )
7.1 Hz, 3H, 9-H), 1.19-1.28 (m, 3H, 5-H and 6-H), 1.39 (m,
3H, 3-H and 6-H), 1.71-1.78 (m, 1H, 2-H), 3.32 (s, 3H, 24-H),
3.75 (d, 1H, J ) 7.0 Hz, 10-H), 4.01 (q, 2H, J ) 7.1 Hz, 8-H),
4.24 (s, br, 1H, 4-H), 4.27 (s, br, 1H, 1-H), 5.13 (s, 1H, 23-H),
7.36 (m, 2H, 20-H and 15-H), 7.82 (m, 2H, 22-H and 13-H),
8.44 (m, 2H, 21-H and 14-H), 8.70 (m, 2H, 19-H and 16-H).
13C NMR (100 MHz, CDCl3): δ ) 14.6 (C-9), 28.6 (C-5), 29.2
(C-6), 36.5 (C-3), 36.9 (C-2), 40.6 (C-10), 56.1 (C-4), 57.2 (C-1),
58.5 (C-24), 60.8 (C-8), 64.4 (C-23), 122.5-164.5, 153.8 (C-7).
MS (70 eV): m/z (%) 433 (M+, 0.1), 401 (28, M+ - CH3OH),
265 (100). Anal. (C24H27N5O3), C, H, N.

54 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1
Che et al.
exo-2-(3,6-Dich lor op yr id a zin -4-yl)-7-a za bicyclo[2.2.1]-
h ep ta n e-7-ca r boxylic Acid Eth yl Ester (18). To a solution
of 302 mg (1.35 mmol) of the enol ether 13 in 10 mL of dry
toluene was added a solution of 204 mg (1.35 mmol) of 3,6-
dichlorotetrazine 14c in 5 mL of dry toluene. The mixture was
refluxed under an atmosphere of Ar for 60 h, cooled to room
temperature and the solvent evaporated in vacuo. The residue
was purified by flash chromatography over silica gel eluting
with ethyl acetate:n-hexane 1:2 (column 15 × 2.5 cm) to give
) 2979, 1704, 1562, 1377, 1326, 1134, 1102. 1H NMR (500
MHz, CDCl3): δ ) 1.24 (t, J ) 7.1 Hz, 3H, 9-H), 1.58-1.78
(m, 4H, 5-H and 6-H), 1.86 (m, 1H, 3â-H), 2.11 (dd, J ) 12.6
Hz, J ) 9.4 Hz, 1H, 3R-H), 3.17 (dd, J ) 5.0 Hz, J ) 9.4 Hz,
1H, 2-H), 4.11 (d, J ) 7.1 Hz, 2H, 8-H), 4.38 (br s, 1H, 4-H),
4.45 (br s, 1H, 1-H), 7.55 (s, 1H, 11-H). 13C NMR (125 MHz,
CDCl3): δ ) 14.6 (C-9), 28.9 (C-5), 29.3 (C-6), 38.2 (C-3), 44.1
(C-2), 56.2 (C-4), 60.0 (C-1), 61.6 (C-8), 127.1 (C-11), 146.3 (C-
10), 155.2 (C-7), 156.2 (C-12), 156.3 (C-13). MS (70 eV): m/z
(%) ) 315 (21, M+), 141 (100). HRMS: calcd for C13H15Cl2N3O2
315.0541, found 315.0530.
(D): δ ) 25.5 (C-5), 27.8 (C-6), 34.9 (C-3), 41.5 (C-2), 58.6 (C-
4), 62.8 (C-1), 121.8, 123.3, 124.4, 125.3, 125.5, 137.8, 137.9,
140.9, 148.9, 153.4, 156.0, 157.5, 158.8, 164.9. MS (70 eV): m/z
(%) 329 (M+ - oxalic acid, 20), 261 (100). Anal. (C23H23N5O2)
C, H, N.
exo-2-(P yr idazin -4-yl)-7-azabicyclo[2.2.1]h eptan e-7-car -
boxylic Acid Eth yl Ester (22). Following the procedure
described for the preparation of carbamate 15 0.36 g (74%) of
22 was obtained as a colorless oil (Rf ) 0.47) from 0.45 g (2.0
mmol) of the enol ether 13 and 0.12 g (2.0 mmol) of the
tetrazine 14d . IR (film): v (cm-1) ) 2978, 2912, 1705, 1584,
1374. 1H NMR (400 MHz, CDCl3): δ ) 1.21 (t, 3H, J ) 7.1
Hz, 9-H), 1.50-1.62 (m, 2H, 5-H), 1.73-1.91 (m, 3H, 6-H and
3-H), 2.01-2.07 (m, 1H, 3-H), 2.86 (dd, 1H, J ) 9.0 Hz, J )
4.8 Hz, 2-H), 4.10 (q, 2H, J ) 7.1 Hz, 8-H), 4.30 (br s, 1H,
1-H), 4.45 (br s, 1H, 4-H), 7.37 (br s, 1H, J not determinable,
13-H), 9.03 (dd, 1H, J ) 5.5 Hz, J ) 1.1 Hz, 12-H), 9.06
(pseudo-t, overlapped by dd, J ) 1.2 Hz, 11-H). 13C NMR (100
MHz, CDCl3): δ ) 14.5 (C-9), 28.8 (C-5), 29.6 (C-6), 39.5 (C-
3), 45.2 (C-2), 56.1 (C-1), 61.2 (C-4), 61.4 (C-8), 124.0 (C-13),
144.3 (C-10), 151.2 (C-12), 151.7 (C-11), 155.5 (C-7). MS (70
eV): m/z (%) 247 (M+, 41), 141 (100). HRMS: calcd for
18 (331 mg, 78%, Rf ) 0.27) as a colorless oil. IR (film): v (cm-1
)
e xo-2-(3,6-B i s t r i flu o r o m e t h y lp y r i d a z i n -4-y l)-7-
a za bicyclo[2.2.1]h ep ta n e (19). To a solution of 308 mg (0.80
mmol) of the carbamate 15 in 4 mL of dry chloroform was
added dropwise under argon at room temperature a solution
of 0.33 mL (2.41 mmol) of trimethylsilyl iodide and the reaction
mixture refluxed for 4 h. After cooling 1.5 mL of methanol was
added dropwise and the mixture kept at room temperature
for 30 min, the solvent was removed in vacuo and the residue
resolved in 8 mL of water. Concentrated aqueous ammonia
was added until pH ) 10 was reached. The resulting mixture
was extracted with chloroform (3 × 15 mL). The combined
organic layers were washed with saturated aqueous NaHCO3
and NaCl solutions, dried over MgSO4 and filtered. After
removal of solvent in vacuo the residue was purified by flash
chromatography over silica gel eluting with CH2Cl2:CH3OH:
concentrated aqueous ammonia 97:3:1, to give 206 mg (91%)
19 as a pale yellow oil. IR (film): v (cm-1) ) 3412, 2956, 1569,
1412, 1367, 1291, 1109. 1H NMR (400 MHz, CDCl3): δ ) 1.48-
1.70 (m, 5H, 5-H, 6-H and 3-H), 1.97 (dd, J ) 12.4 Hz, J ) 9.0
Hz, 1H, 3R-H), 3.10 (m, 1H, 2-H), 3.64 (s, br, 1H, 4-H), 3.87 (s,
br, 1H, 1-H), 8.72 (s, 1H, 8-H). 13C NMR (100 MHz, CDCl3): δ
) 30.8 (C-5), 31.8 (C-6), 40.7 (C-3), 40.9 (C-2), 56.2 (C-4), 62.7
(C-1), 121.0 (q, J C,F ) 275.3 Hz, C-9), 121.9 (q, J C,F ) 276.0
Hz, C-11), 124.8 (C-7), 148.3 (C-12), 150.4 (q, J C,F ) 32.2 Hz,
C-8), 153.7 (q, J C,F ) 35.3 Hz, C-10). MS (70 eV): m/z (%) 311
(100, M). HRMS: calcd for C12H11F6N3 311.0895, found
311.0879.
The free base (180 mg, 0.58 mmol) was converted to the
corresponding hydrogen oxalate monohydrate salt in acetone
(6 mL) by reaction with oxalic acid monohydrate (64 mg, 0.59
mmol) to yield colorless crystals (209 mg, 90%), mp 167-168
°C (acetone). 1H NMR (400 MHz, CD3OD): δ ) 1.75-1.84 (m,
2H, 5-H), 1.85-1.98 (m, 2H, 6-H), 2.01-2.10 (m, 1H, 3â-H),
2.25 (dd, 2J ) 12.4 Hz, 3J ) 9 Hz, 1H, 3R-H), 3.49 (m, 1H,
2-H), 4.19-4.21 (m, 1H, 1-H), 4.43 (s, broad, 1H, 4-H), 8.64 (s,
1H, 12H). MS (70 eV): m/z (%) ) 311 (M+ - oxalic acid, 100).
Anal. (C14H15F6N3O5) C, H, N.
exo-2-(3,6-Dip yr id in -2-ylp yr id a zin -4-yl)-7-a za bicyclo-
[2.2.1]h ep ta n e (20). Using the same procedure as described
for 19 87 mg (88%) 20 was obtained from 0.12 g (0.30 mmol)
of 16, which was characterized as the corresponding hydrogen
oxalate: to a solution of the obtained free base 16 in 3 mL of
acetone was added a solution of 30 mg (0.28 mmol) of the
monohydrate of oxalic acid in 2 mL of acetone. The mixture
was refluxed for 10 min, filtered and the filtrate stored for 12
h at 4 °C to give, after drying in vacuo, 98 mg (90%, related to
the free base) of colorless crystals, mp 177-178 °C. IR (film):
v (cm-1) ) 3346, 2946, 1639, 1438. 1H NMR (400 MHz, CD3O-
(D): δ ) 1.64-1.70 (m, 1H, 5-H), 1.75-1.82 (m, 1H, 6-H),
1.86-1.93 (m, 2H, 6-H and 5-H), 1.96 (dd, J ) 13.3 Hz, J )
9.5 Hz, 3R-H), 2.07-2.16 (m, 1H, 3â-H), 3.93 (dd, J ) 9.3 Hz,
J ) 6.6 Hz, 2-H), 4.10 (br s, 2H, NH2), 4.20 (t, 1H, J ) 4.3 Hz,
4-H), 4.60 (d, 1H, J ) 4.4 Hz, 1-H). 13C NMR (100 MHz, CD3O-
C
13H17N3O2 247.1312, found 247.1320.
3-P yr id a zin -4-ylcycloh ex-3-en yla m in e (23). To a solu-
tion of 272 mg (1.10 mmol) of the carbamate 22 in 5 mL of dry
chloroform was added dropwise under argon at room temper-
ature a solution of 0.45 mL (3.31 mmol) of trimethylsilyl iodide.
The reaction mixture was heated for 4 h under argon to 75-
80 °C (pressure bottle), cooled to room temperature and 1.8
mL of methanol added. After gas evolution has ceased, the
solvent was evaporated in vacuo. The residue dissolved in 8
mL of water and concentrated aqueous ammonia added until
pH ) 12 was reached. The resulting reaction mixture was
extracted with chloroform (3 × 15 mL). The combined organic
layers were washed with saturated aqueous NaHCO3 and
NaCl solutions, dried over MgSO4 and filtered. After removal
of solvent in vacuo the residue was purified by flash chroma-
tography over silica gel (column 20 × 2 cm) eluting with CH2-
Cl2:CH3OH:concentrated aqueous ammonia 85:15:1, to give 23,
148 mg (77%), as a pale yellow oil. IR (Film): v (cm-1) ) 3397,
2973, 2911, 1602, 1587. 1H NMR (400 MHz, CDCl3): δ ) 1.38
(m, 1H, 6R-H), 1.61 (br s, 2H, NH2), 1.82-1.89 (m, 1H, 6â-H),
2.07-2.16 (m, 1H, 5R-H), 2.23-2.32 (m, 1H, 5â-H), 2.33-2.42
(m, 1H, 2R-H), 2.52-2.61 (m, 1H, 2â-H), 3.13 (br s, 1H, 1-H),
6.45 (br s, 1H, 4-H), 7.29 (dd, 1H, J ) 5.5 Hz, J ) 2.5 Hz,
8-H), 9.01 (dd, 1H, J ) 5.5 Hz, J ) 1.1 Hz, 9-H), 9.17 (dd, 1H,
J ) 2.5 Hz, J ) 1.1 Hz, 10-H). 13C NMR (100 MHz, CDCl3): δ
) 25.0 (C-6), 31.1 (C-5), 35.6 (C-2), 46.5 (C-1), 120.9 (C-4), 130.1
(C-3), 130.5 (C-8), 138.6 (C-7), 148.3 (C-9), 151.1 (C-10). MS
(70 eV): m/z (%) 175 (M+, 54), 133 (100). HRMS: calcd for
C10H13N3 175.1109, found 175.1102.
exo-2-(P yr id a zin -4-yl)-7-a za bicyclo[2.2.1]h ep ta n e (24).
To a solution of 198 mg (0.80 mmol) of compound 22 in 0.5
mL of dioxane was added 2 mL of aqueous hydrochloric acid
(37%) and argon was bubbled through the solution for 10 min.
The mixture was refluxed for 20 h under argon, and cooled to
room temperature before 2 mL of water was added. The solvent
was evaporated in vacuo, the residue dissolved in 1 mL of
water and concentrated aqueous ammonia was added until pH
) 12 was reached. After removal of the solvent in vacuo the
residue was purified by column chromatography (silica gel,
column 30 × 1.5 cm, eluting with CH2Cl2:MeOH:concentrated
aqueous ammonia ) 95:15:0.1). Fraction 1 contained 9 mg (7%,
Rf ) 0.16) of 23 as colorless oil; fraction 2 contained compound
24: yield 39 mg (28%, Rf ) 0.09) as colorless crystals, mp 158
°C. IR (KBr): v (cm-1) ) 3389, 2924, 1606, 1467, 1369. 1H NMR
(500 MHz, CDCl3) δ ) 1.50-1.72 (m, 5H, 3â-H, 5-H and 6-H),
1.93 (dd, J ) 12.4 Hz, J ) 9.1 Hz, 1H, 3R-H), 2.75 (dd, J ) 9.1
Hz, J ) 5.0 Hz, 1H, 2-H), 3.66 (m, 1H, 4-H), 3.85 (m, 1H, 1-H),
7.53 (dd, J ) 5.2 Hz, J ) 2.3 Hz, 1H, 8-H), 9.02 (dd, J ) 5.2
Hz, J ) 1.1 Hz, 1H, 9-H), 9.14 (dd, J ) 2.3 Hz, J ) 1.1 Hz,
1H, 10-H). 13C NMR (125 MHz, CDCl3): δ ) 30.2 (C-5), 31.3
(C-6), 39.5 (C-3), 44.8 (C-2), 56.4 (C-4), 62.2 (C-1), 124.5 (C-8),
145.7 (C-7), 151.0 (C-9), 152.2 (C-10). MS (70 eV): m/z (%) )

exo-2-(Pyridazin-4-yl)-7-azabicyclo[2.2.1]heptanes
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1 55
176 (4, M+ + 1), 175 (25, M+), 69 (100). HRMS: calcd for
1H, 5â-H), 2.41-2.47 (m, 1H, 2R-H), 2.78-2.84 (m, 1H, 2â-
H), 4.21-4.32 (m, 1H, 1-H), 5.72 (br d, 1H, NH), 6.52 (dd, J )
4.9 Hz, J ) 3.1 Hz, 1H, 4-H), 7.32 (d, J ) 5.4 Hz, 1H, 12-H),
9.07 (d, J ) 5.4 Hz, 1H, 11-H), 9.21 (br s, 1H, 10-H). 13C NMR
(125 MHz, CDCl3): δ ) 24.6 (C-8), 27.1 (C-6), 29.7 (C-5), 32.3
(C-2), 44.7 (C-1), 121.0 (C-4), 129.8 (C-3), 130.4 (C-12), 138.2
(C-9), 148.2 (C-11), 151.1 (C-10), 169.6 (C-7). MS (70 eV): m/z
(%) ) 217 (M+, 16), 158 (100). HRMS: calcd for C12H15N3O
217.1215 found, 217.1186.
C10H13N3 175.1128, found 175.1110.
Cr ysta l Str u ctu r e Deter m in a tion of 24. A brownish
crystal (ca. 0.25 × 0.25 × 0.10 mm3), obtained by recrystalli-
zation from dichloromethane, was mounted on a glass fiber
and investigated on a Rigaku AFC5R diffractometer with
graphite monochromated Cu KR radiation and a rotating
anode generator (Rigaku). Empirical formula C20H12N3Cl,
molecular mass 209.68 au. Cell constants and an orientation
matrix for data collection, obtained from a least-squares
refinement using the setting angles of 25 carefully centered
reflections in the range 41.18° < 2Θ < 57.13°, corresponded
to a primitive orthorhombic cell with dimensions: a ) 22.634-
(3) Å, b ) 20.097(5) Å, c ) 9.418(5) Å, V ) 4284(4) Å3. For Z
) 16, the calculated density is 1.30 g/cm3. The systematic
absences of 0kl: k * 2n, h0l: l * 2n, hk0: h * 2n uniquely
determine the space group to be Pbca (No. 61). The data were
collected at a temperature of 23 ( 1 °C using the ω-2Θ scan
technique to a maximum 2Θ value of 120.1°. Omega scans of
several intense reflections, made prior to data collection, had
an average width at half-height of 0.30° with a takeoff angle
of 6.0°. Scans of (1.10 ( 0.30 tan Θ)° were made at a speed of
16.0°/min (in omega). The weak reflections (I < 10.0σ(I)) were
rescanned (maximum of 4 scans) and the counts were ac-
cumulated to ensure good counting statistics. Stationary
background counts were recorded on each side of the reflection.
The ratio of peak counting time to background counting time
was 2:1. The diameter of the incident beam collimator was 1.0
mm, the crystal to detector distance was 400 mm, and the
detector aperture was 9.0 × 13.0 mm (horizontal × vertical).
A total of 3624 reflections was collected. The intensities of
three representative reflections were measured after every 150
reflections. No decay correction was applied. The linear
absorption coefficient, µ, for Cu KR radiation is 28.6 cm-1. An
empirical absorption correction based on azimuthal scans of
several reflections was applied which resulted in transmission
factors ranging from 0.79-1.00. The data were corrected for
Lorentz and polarization effects.
26a ,b (rotamers): IR (film): v (cm-1) ) 2965, 1634, 1420,
1
1052, 885. H NMR (500 MHz, CDCl3) 2 rotamers (ratio 1.6:
1): δ ) 1.51-1.77 (m, 2H, 5R-H and 6R-H), 1.77-1.93 (m, 3H,
3â-H, 5â-H and 6â-H), 1.83 (s, 1.14H, 8-H), 1.97-2.05 (0.38
H, 3R-H), 2.03 (s, 1.86H, 8-H), 2.16 (dd, J ) 9.3 Hz, J ) 12.2
Hz, 0.62H, 3R-H), 2.91 (dd, J ) 4.9 Hz, J ) 9.0 Hz, 0.62H,
2-H), 2.95 (dd, J ) 4.5 Hz, J ) 9.0 Hz, 0.38H, 2-H), 3.95 (d, J
) 4.2 Hz, 0.38H, 1-H), 4.27 (t, J ) 4.6 Hz, 0.62H, 4-H), 4.73
(d, J ) 4.6 Hz, 0.62H, 1-H), 4.82 (t, J ) J ) 4.8 Hz, 0.38 H,
4-H), 7.24 (br s, 0.62H, 12-H), 7.39 (br s, 0.38 H, 12-H), 8.94-
9.13 (m, 2H, 10-H and 11-H). 13C NMR (125 MHz, CDCl3): 2
rotamers, δ ) 21.3 and 22.9 (C-8), 28.2 and 28.9 (C-5), 29.8
and 31.1 (C-6), 37.7 and 40.9 (C-3), 43.1 and 44.4 (C-2), 53.2
and 57.1 (C-1), 56.7 and 62.9 (C-4), 123.5 and 123.8 (C-12),
143.7 and 144.0 (C-9), 150.9 and 151.0 (C-11), 151.3 and 151.5
(C-10), 167.1 and 167.8 (C-7). MS (70 eV): m/z (%) ) 217 (M+,
38), 106 (100). HRMS: calcd for C12H15N3O 217.1215, found
217.1214.
exo-2-(Met h oxyet h en yl)-7-m et h yl-7-a za bicyclo[2.2.1]-
h ep ta n e (27). To a suspension of LiAlH4 (380 mg, 10.0 mmol)
in dry diethyl ether (40 mL) was added dropwise a solution of
enol ether 13 (525 mg, 2.33 mmol). The reaction mixture was
refluxed under an atmosphere of Ar for 12 h and after cooling
to 0 °C a saturated aqueous solution of potassium sodium
tartrate tetrahydrate (20 mL) was slowly added at 0 °C. The
suspension was vigorously stirred for 30 min at 10 °C, then
the aqueous phase was separated and extracted with CH2Cl2
(3 × 20 mL). The combined organic extracts were washed with
brine (10 mL) dried over MgSO4 and filtered. After removal
of the solvent in vacuo the oily residue was purified by flash
chromatography over silica gel eluting with CH2Cl2:MeOH:
diethylamine ) 95:15:0.2 (column 25 × 1.5 cm) to give 27 (390
mg, 87%, Rf ) 0.26) as a colorless oil. IR (film): v (cm-1) )
2958, 2872, 2792, 1685, 1652, 1464, 1129, 1106. 1H NMR (400
MHz, CDCl3, E/ Z-isomers, ratio 3:2): δ ) 1.15-1.42 (m, 3H,
5-H, 6-H), 1.71-1.90 (m, 3H, 6-H, 3-H), 1.92-2.00 (m, 0.6H,
2-H, E-isomer), 2.17 (s, 1.2H, N-CH3, Z-isomer), 2.23 (s, 1.8H,
N-CH3, E-Isomer), 2.42-2.50 (m, 0.4H, 2-H, Z-isomer), 3.07-
3.15 (m, 2H, 1-H, 4-H, E-isomer), 3.12-3.17 (0.8H, 1-H, 4-H,
Z-isomer), 3.46 (s, 1.8H, OCH3, E-isomer), 3.50 (s, 1.2H, OCH3,
Z-isomer), 4.42 (dd, J ) 9.0 Hz, J ) 6.2 Hz, 0.4H, 8-H,
Z-isomer), 4.76 (dd, J ) 9.1, J ) 12.6 Hz, 0.6H, 8-H, E-isomer),
5.70 (d, J ) 6.2 Hz, 0.4H, 9-H, Z-isomer), 6.22 (d, J ) 12.6
Hz, 0.6H, 9-H, E-isomer). 13C NMR (100 MHz, CDCl3, E/ Z-
isomers): δ ) 29.7 (C-5), 31.5 (C-6), 35.1 (N-CH3), 35.5 (C-3),
40.6 (C-2), 59.3 and 59.5 (C-4, E/ Z), 61.1 and 61.5 (OCH3,
E/ Z), 67.6 and 68.1 (C-1, E/ Z), 108.6 and 113.2 (C-8, E/ Z),
144.3 and 145.9 (C-9, E/ Z). MS (70 eV): m/z ) 167 (31, M+),
83 (100). HRMS: calcd for C10H17NO 167.1310, found 167.1298.
exo-7-Me t h yl-2-(p yr id a zin -4-yl)-7-a za b icyclo[2.2.1]-
h ep ta n e (28). To a solution of the enol ether 27 (380 mg, 2.28
mmol) in dry toluene (30 mL) was added dropwise a solution
of the tetrazine 14d in dry toluene (20 mL) at room temper-
ature under an argon atmosphere. The mixture was heated
at reflux and stirred under Argon for 16 h and cooled to room
temperature. The solvent was evaporated in vacuo and the
residue purified twice by flash chromatography over silica gel
(eluting with CH2Cl2:MeOH:diethylamine ) 95:15:0.2, column
25 × 1,5 cm) to give 28 (82 mg, 19%, Rf ) 0.23) as a slightly
brownish-yellow oil. IR (film): v (cm-1) ) 2961, 2882, 2804,
1586. 1H NMR (400 MHz, CDCl3): δ ) 1.41-1.44 (m, 2H, 5-H),
1.62-1.69 (m, 1H, 3â-H), 1.83 (dd, J ) 12.0 Hz, J ) 9.2 Hz,
1H, 3R-H), 1.89-1.99 (m, 2H, 6-H), 2.23 (s, 3H, N-CH3), 2.60
(dd, J ) 9.2 Hz, J ) 4.9 Hz, 1H, 2R-H), 3.17 (d, J ) 4.0 Hz,
1H, 1-H), 3.32-3.33 (m, 1H, 4H), 7.56-7.57 (dd, J ) 2.3 Hz,
The structure was solved by direct methods45 and expanded
using Fourier techniques.46 The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included but not
refined. The final cycle of full-matrix least-squares refinement
was based on 1917 observed reflections (I > 3.00σ(I)) and 253
variable parameters and converged (largest parameter shift
was 0.01× its ESD) with unweighted and weighted agreement
factors of: R ) ∑||Fo| - |Fc||/∑|Fo| ) 0.054; Rw ) x(∑$(|Fo| -
|Fc|)2/∑$Fo2)] ) 0.064. The standard deviation of an observa-
tion of unit weight was 1.80. The weighting scheme was based
on counting statistics and included a factor (p ) 0.029) to
2
downweight the insense reflections. Plots of ∑$(|Fo| - |Fc|
versus |Fo|, reflection order in data collection, sin θ/λ and
various classes of indices showed no unusual trends. The
maximum and minimum peaks on the final difference Fourier
map corresponded to 0.48 and -0.36 e-/Å3, respectively.
Neutral atom scattering factors were taken from Cromer
and Waber.47 The values for the mass attenuation coefficients
are those of Creagh and Hubbell.48 All calculations were
performed using the teXsan49 crystallographic software pack-
age of Molecular Structure Corp.
N-(3-P yr ida zin -4-ylcycloh ex-3-en yl)a ceta m id e (25) a n d
exo-7-Ac e t y l-2-(p y r id a zin -4-y l)-7-a za b ic y c lo [2.2.1]-
h ep ta n e (26a ,b). To a solution of 35 mg (0.20 mmol) of amine
22 in 1 mL of dry DMF was added 68 mg (0.30 mmol) of
pentafluorophenyl acetate. The mixture was stirred under an
atmosphere of argon for 14 h at room temperature, then the
solvent was evaporated in vacuo at room temperature and the
residue purified by flash chromatography over silica gel eluting
with ethyl acetate: CH3OH ) 19:1 (column 25 × 1.5 cm).
Fraction 1 contained 6 mg (13%, Rf ) 0.14) of compound 25 as
a colorless oil; fraction 2 contained 30 mg (68%, Rf ) 0.09)
26a ,b as a colorless oil. 25: IR (film): v (cm-1) ) 3278, 3070,
2933, 1649, 1558, 1374, 919, 836, 733. 1H NMR (500 MHz,
CDCl3): δ ) 1.59-1.72 (m, 1H, 6R-H), 1.73-1.84 (m, 1H, 6â-
H), 1.89-1.99 (m, 1H, 5R-H), 2.01 (s, 3H, 8-H), 2.19-2.28 (m,

56 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1
Che et al.
(14) Sirisoma, S. N.; J ohnson, C. R. R-Iodocycloalkenones: Synthesis
of (()-Epibatidine. Tetrahedron Lett. 1998, 39, 2059-2062.
(15) Avenoza, A.; Busto, J . H.; Cativiela, C.; Peregrina, J . M. A
Versatile and Stereoselective Synthesis of (()-Epibatidine.
Synthesis 1998, 1335.
(16) Clive, D. J . L.; Yeh, V. S. C. Formal Synthesis of Natural
Epibatidine and of its Enantiomer: Use of Radical Cyclization
in an Enantiospecific Route. Tetrahedron Lett. 1998, 39, 4789-
4792.
J ) 5.2 Hz, 1H, 5′-H), 8.99 (dd, J ) 5.2 Hz, J ) 1.1 Hz, 1H,
6′-H), 9.17 (dd, J ) 2.3 Hz, J ) 1.1 Hz, 1H, 3′-H). 13C NMR
(100 MHz, CDCl3): δ ) 25.5 (C-5), 26.4 (C-6), 34.5 (N-CH3),
40.7 (C-3), 45.8 (C-2), 61.2 (C-4), 67.0 (C-1), 124.8 (C-5′′), 146.3
(C-4′), 151.0 (C-6′), 152.3 (C-3′). MS (70 eV): m/z (%) ) 189
(24, M+), 83 (100). HRMS: calcd for C11H15N3 189.1266, found
189.1257.
Biologica l Ch a r a cter iza tion . Whole-cell current record-
ing from Xenopus oocytes expressing recombinant nAChR from
the rat was performed as described elsewhere.44 Oocytes were
injected with appropriate cDNA plasmids to express one of the
following nAChR subypes: R3â4, R4â2, R7 (all neuronal) or
R1â1γδ (embryonic muscle subtype). Oocytes were voltage-
clamped to -80 mV membrane potential and total membrane
currents were recorded with a GeneClamp 500 Amplifier (Axon
Instruments). For initial testing, compounds were applied by
superfusion at a concentration of 100 µM in frog Ringer
solution (in mM: 115 NaCl, 2.5 KCl, 1.8 CaCl2, 10 HEPE, pH
7.2). If a current response indicating the activation of receptors
resulted, then the compounds were further characterized by
the determination of a dose-response curve. The agonistic
potency of the active compounds at each receptor subtype is
expressed as the concentration which was found to be equi-
potent with a standard reference dose of acetylcholine (Table
1).
(17) Lee, J . R.; Carrol, F. I. New Synthesis of 7-(tert-Butoxycarbonyl)-
7-azabicyclo[2.2.1]hept-2-ene. A key Intermediate in the Syn-
thesis of Epibatidine and Analogues. Tetrahedron Lett. 1998,
39, 5321-5322.
(18) Node, M.; Nishide, K.; Fujiwara, T.; Ichihashi, S. New asym-
metric transformation of optically active allene-1,3-dicarboxylate
and its application to the formal asymmetric synthesis of (-)-
epibatidine. Chem. Commun. 1998, 2363-2365.
(19) Aoyagi, S.; Tanaka, R.; Naruse, M.; Kibayashi, C. Total Synthesis
of (-)-Epibatidine Using an Asymmetric Diels-Alder-Reaction
with a Chiral N-Acylnitroso Dienophile. J . Org. Chem. 1998, 63,
8397-8406.
(20) Otten, A.; Namyslo, J . C.; Stoermer, M.; Kaufmann, D. E.
2-(Het)aryl-substituted 7-Azabicyclo[2.2.1]-heptane Systems. Eur.
J . Org. Chem. 1998, 1997-2001.
(21) Palmgren, A.; Larsson, L. E.; Backvall, J .-E., Palladium(II)-
Catalyzed 1,4-Oxidation of 2-Aryl-1,3-cyclohexadienes. Applica-
tion to the Synthesis of (()-Epibatidine and Analogues. J . Org.
Chem. 1999, 64, 836.
(22) Chunming, Z.; Ballay, J . C., II; Trudell, M. L. 2-Bromoethynyl
aryl sulfones as versatile dienophiles: a formal synthesis of
epibatidine. J . Chem. Soc., Perkin Trans 1 1999, 675-676.
(23) Xu, R.; Bai, D.; Chu, G.; Tao, J .; Zhu, X. Synthesis and Analgesic
Activity of Epibatidine Analogues. Bioorg. Med. Chem. Lett.
1996, 6, 279-282.
Ack n ow led gm en t. Financial support from Deut-
sche Forschungsgemeinschaft and the Fonds der Che-
mischen Industrie is gratefully acknowledged. We thank
Bayer AG, Merck AG, and Degussa AG for supplying
us with valuable chemicals.
(24) Malpass, J . R.; Hemmings, D. A.; Wallis, A. L. Synthesis of
Epibatidine Homologues: Homoepibatidine and Bis-Homoepi-
batidine. Tetrahedron Lett. 1996, 37, 3911-3914.
(25) Bai, D.; Xu, R.; Chu, G.¸ Zhu, X. Synthesis of (()-Epibatidine
and its Analogues. J . Org. Chem. 1996, 61, 4600-4606.
(26) Chunming, Z.; Gyermek, L.; Trudell, M. L. Synthesis of Optically
Pure Epibatidine Analogues: (1R,2R,5S)-2â-(2-Chloro-5-pyridi-
nyl)-8-azabicyclo[3.2.1]octane and (1R,2S,5S)-2R-(2-chloro-5-py-
ridinyl)-8-azabicyclo[3.2.1]octane from (-)-cocaine. Tetrahedron
Lett. 1997, 38, 5619-5622.
(27) Wright, E.; Gallagher, T.; Sharples, C. G. V.; Wonnacott, S.
Synthesis of UB-165: A Novel Nicotinic Ligand and Anatoxin-
a/Epibatidine Hybrid. Bioorg. Med. Chem. Lett. 1997, 7, 2867-
2870.
Su p p or tin g In for m a tion Ava ila ble: Elemental analysis
for compounds 15-17, 19, and 20 (Table 2); 1H and 13C NMR
spectra of compounds 18, 22, 24, and 28; crystal data, final
atomic positional parameters, atomic thermal parameters, and
bond distances and angles for compound 24 (Tables 3-7). This
material is available free of charge via the Internet at http://
pubs.acs.org.
(28) Badio, B.; Garaffo, H. M.; Plummer, C. V.; Padgett, W. L.; Daly,
J . W. Synthesis and nicotinic activity of epiboxidine: an isoxazole
analogue of epibatidine. Eur. J Pharmacol. 1997, 321, 189-194.
(29) Seerden, J . G.; Tulp, M. Th. M.; Scheeren, H. W.; Kruse, C. G.
Synthesis and Structure-Activity Data of Some new Epibatidine
Analogues. Bioorg. Med. Chem. 1998, 6, 2103-2110.
(30) Che, D.; Siegl, J .; Seitz, G. Synthesis of chiral 2-(2′-pyrrolidinyl)-
pyridines from (S)- and (R)-proline: potential ligands of the
neuronal nicotinic acetylcholine receptors. Tetrahedron: Asym-
metry 1999, 10, 573-585.
(31) Sauer, J . 1,2,4,5-Tetrazines. In Comprehensive Heterocyclic
Chemistry II; Katritzky, A. R., Rees, C. W., Eds.; Pergamon
Press: Oxford, U.K., 1997; Vol. 6, pp 901-957.
(32) Boger, D. L.; Weinreb, S. M. Hetero Diels-Alder Methodology
in Organic Synthesis; Academic Press: New York, 1987; pp 335-
357.
(33) Lott, R. S.; Chauhan, V. S.; Stammer, C. H. Trimethylsilyl Iodide
as a Peptide Deblocking Agent. J . Chem. Soc., Chem. Commun.
1979, 495-496.
(34) Campillo, N.; Pa´ez, J . A.; Alkorta, I.; Goya, P. A theoretical study
of epibatidine. J . Chem. Soc., Perkin Trans. 2 1998, 2665-2670.
(35) Glennon, R. A.; Herndon, J . L.; Dukat, M. Epibatidine-aided
studies toward definition of a nicotine receptor pharmakophore.
Med. Chem. Res. 1994, 4, 461-473.
(36) Holladay, M. W.; Dart, M. J .; Lynch, J . K. Neuronal Nicotinic
Acetylcholine Receptors as Targets for Drug Discovery. J . Med.
Chem. 1997, 40, 4169-4194.
(37) Lin, N.-H.; Meyer, M. D. Recent developments in neuronal
nicotinic acetylcholine receptor modulators. Exp. Opin. Ther.
Patents 1998, 8, 991-1015.
Refer en ces
(1) Spande, T. F.; Garaffo, H. M.; Edwards, M. W.; Yeh, H. J . C.;
Pannell, L.; Daly, J . W. Epibatidine: A Novel (Chloropyridyl)-
azabicycloheptane with Potent Analgesic Activity from an Ec-
uadoran Poison Frog. J . Am. Chem. Soc. 1992, 114, 3475-3478.
(2) Fletcher, S. R.; Baker, R.; Chambers, M. S.; Herbert, R. H.;
Hobbs, S. C.; Thomas, S. R.; Verrier, H. M.; Watt, A. P.; Ball, R.
G. Total Synthesis and Determination of the Absolute Config-
uration of Epibatidine. J . Org. Chem. 1994, 59, 1771-1778.
(3) Badio, B.; Daly, J . W. Epibatidine, a potent analgetic and
nicotinic agonist. Mol. Pharmacol. 1994, 45, 563-569.
(4) Sullivan, J . P.; Bannon, A. W. Epibatidine: Pharmacological
Properties of a Novel Nicotinic Acetylcholine Receptor Agonist
and Analgesic Agent. CNS Drug Rev. 1996, 2, 21-39.
(5) Badio, B.; Garaffo, H. M.; Spande, T. F.; Daly, J . W. Epibati-
dine: Discovery and definition as a potent analgesic and nicotinic
agonist. Med. Chem. Res. 1994, 4, 440-448.
(6) Traynor, J . R. Epibatidine and pain. Br. J . Anaesthesiol. 1998,
81, 69-76.
(7) Decker, M. W.; Meyer, M. D. Therapeutic potential of neuronal
nicotinic acetylcholine receptor agonists as novel analgesics.
Biochem. Pharmacol. 1999, 58, 917-923.
(8) For a recent review on 7-azabicyclo[2.2.1]heptanes see: Chen,
Z.; Trudell, M. L. Chemistry of 7-Azabicyclo[2.2.1]hepta-2,5-
diens, 7-Azabicyclo[2.2.1]hept-2-enes, and 7-Azabicyclo[2.2.1]-
heptanes. Chem. Rev. 1996, 96, 1179-1194.
(9) Broka, C. A. Synthetic Approaches to Epibatidine. Med. Chem.
Res. 1994, 4, 449-460.
(10) Sza´ntay, C.; Kardos-Balogh, Z.; Sza´ntay, C., J r. Epibatidine. In
The Alkaloids; Cordell, G. A., Ed.; Academic Press: San Diego,
CA, 1995; Vol. 46, pp 95-125.
(38) Koren, A. O.; Horti, A. G.; Mukhin, A. G.; Gu¨ndisch, D.; Kimes,
A. S.; Dannals, R. F.; London, E. D. 2-, 5- and 6-Halo-3-(2(S)-
azetidinylmethoxy)pyridines: Synthesis, Affinity for Nicotinic
Acetylcholine Receptors, and Molecular Modeling. J . Med. Chem.
1998, 41, 3690-3698.
(39) Sheridan, R. P.; Nilakantan, R.; Dixon, J . S.; Venkataraghavan,
R. The Ensemble Approach to Distance Geometry: Application
to the Nicotinic Pharmacophore. J . Med. Chem. 1986, 29, 899-
906.
(11) Dehmlow, E. V. The Epibatidine Competition: Synthetic Work
on a Novel Natural Analgetic. J . Prakt. Chem. 1995, 337, 167-
174.
(12) Mu¨ller, C. E. Epibatidin
- ein nicotinartiges, analgetisch
wirksames Alkaloid aus Pfeilgiftfro¨schen. Pharmazie Zeit 1996,
25, 85-92.
(13) Bai, D.; Xu, R.; Zhu, X. Epibatidine. Drugs Future 1997, 22,
1210-1220.

exo-2-(Pyridazin-4-yl)-7-azabicyclo[2.2.1]heptanes
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 1 57
(40) Kisfaludy, L.; Mohacsi, T.; Low, M.; Drexler, F. Pentafluorophe-
nyl Acetate: A New, Highly selective Acetylating Agent. J . Org.
Chem. 1979, 44, 654-655.
(41) Imming, P.; J ung, M.-H. Pentafluorophenyl Esters of Dicarboxy-
lic Acids. Arch. Pharm. Med. Chem. 1995, 328, 87-91.
(42) Cox, C.; Lectka, T. Solvent Effects on the Barrier to Rotation in
Carbamates. J . Org. Chem. 1998, 63, 2426-2427.
(43) Horti, A. G.; Scheffel, U.; Kimes, A. S.; Musachio, J . L.; Ravert,
H. T.; Mathews, W. B.; Zhan, Y.; Finley, P. A.; London, E. D.;
Dannals, R. F. Synthesis and Evaluation of N-[11C]Methylated
Analogues of Epibatidine as Tracers for Positron Emission
Tomographic Studies of Nicotinic Acetylcholine Receptors. J .
Med. Chem. 1998, 41, 4199-4206.
(44) Cooper, J . C.; Gutbrod, O.; Witzemann, V.; Methfessel, C.
Pharmacology of the nicotinic acetylcholine receptor from fetal
rat muscle expressed in Xenopus oocytes. Eur. J . Pharmacol.
1996, 309, 287-298.
(45) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.
Completion and refinement of crystal structures with SIR92. J .
Appl. Crystallogr. 1993, 26, 343-350.
(46) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.;
Bosman, W. P.; de Gelder, R.; Israel, R.; Smits, J . M. M. The
DIRDIF-94 program system; Technical Report of the Crystal-
lography Laboratory; University of Nijmegen: The Netherlands,
1994.
(47) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England,
1974; Vol. IV, Table 2.2.A.
(48) Creagh, D. C.; Hubbell, J . H. International Tables for Crystal-
lography; Kluwer Academic Publishers: Boston, MA, 1992; Vol.
C, Table 4.2.4.3, pp 200-206.
(49) teXsan: Crystal Structure Analysis Package; Molecular Structure
Corp., 1985, 1992.
J M000949W