Bioisosteric replacement strategy for the synthesis of 1-azacyclic compounds with high affinity for the central nicotinic cholinergic receptors

Article abstract of DOI:10.1016/S0968-0896(00)00063-8

Bioisosteric replacement of the isoxazole heterocycle in (3-methyl-5-isoxazolyl)methylene-azacyclic compounds with pyridine, oxadiazole, or an acyl group resulted in ligands with high to moderate affinity for the central nicotinic cholinergic receptors (IC₅₀=2.0 to IC₅₀>1000nM) labeled by [³H]methylcarbamylcholine. Additionally, further support of an important distance parameter for high-affinity nicotinic compounds has been provided. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0968-0896(00)00063-8

Bioorganic & Medicinal Chemistry 8 (2000) 1443±1450
Bioisosteric Replacement Strategy for the Synthesis of 1-Azacyclic
Compounds With High Anity for the Central Nicotinic
Cholinergic Receptors
Preben H. Olesen,* Janne E. Tùnder, John Bondo Hansen, Holger C. Hansen
and Karin Rimvall
Ê
Health Care Discovery, Novo Nordisk A/S, Novo Nordisk Park, DK-2760 Malùv, Denmark
Received 13 December 1999; accepted 18 February 2000
AbstractÐBioisosteric replacement of the isoxazole heterocycle in (3-methyl-5-isoxazolyl)methylene-azacyclic compounds with
pyridine, oxadiazole, or an acyl group resulted in ligands with high to moderate anity for the central nicotinic cholinergic recep-
tors (IC50=2.0 to IC50>1000 nM) labeled by [3H]methylcarbamylcholine. Additionally, further support of an important distance
parameter for high-anity nicotinic compounds has been provided. # 2000 Elsevier Science Ltd. All rights reserved.
Introduction
In this communication we report on the changes in the
binding anity observed after replacement of the iso-
xazole heterocycle present in the previously published
series with the pyridine, oxadiazole, or acyl group.
Additionally, further support for the important a±b
distance parameter for high-anity nicotinic com-
pounds has been provided.
Replacement of bioisosterically equivalent moieties is an
important and well recognized approach to analogue
design. A bioisosteric replacement may help improve
stability, optimize activity, and/or remove unwanted
side e€ects. It has been shown that the isoxazole het-
erocycle can substitute the pyridine heterocycle present
in nicotine and epibatidine, giving new nicotinic ago-
nists, e.g., ABT 418 (Fig. 1).1,2 In several cases the acetyl
moiety is a structural part of ligands with high anity
for nicotinic receptors, e.g., in anatoxin-a,3 and it has
recently been shown that it is possible to replace the
acetyl substituent in anatoxin-a with a pyridine unit, as
in UB-165.4
In the following sections, the di€erent azacycles are
designated in this manner: 1-azabicyclo[2.2.1]heptane
(a), 1-azabicyclo[2.2.2]octane (b), 1-azabicyclo[3.2.1]-
octane (c), and piperidine (d) (Fig. 2).
Results and Discussion
Chemistry
We have previously described a novel series of 3-(iso-
xazolyl)methylene-1-azacycles (i.e., NNC 90-0270 and
1d) as potent agonists for the central nicotinic acetyl-
choline receptors,5,6 which are considered a thera-
peutically important drug target.7,8 In addition, an
improved nicotinic pharmacophore was developed by
considering the site points a and b, where a is the site
point corresponding to a protonated nitrogen atom and
b is the site point corresponding to an electronegative
atom capable of forming a hydrogen bond. For com-
pounds possessing high anity (IC50<10 nM) for the
nicotinic receptors, these site points were separated by a
distance of 7.0±8.0 A.6
Focusing on the 3-substituted pyridine-ring, which is
present in the natural compounds nicotine and epibati-
dine, the compounds 5b±d were prepared by the Peter-
son reaction of the appropriate azacyclic ketone (2)
with 3-[(tert-butyldimethylsilanyl)methyl]-pyridine (3)
(Scheme 1). Yields were generally high, giving a mixture
of the (Z)- and (E)-isomers, which were separated by
column chromatography. The protected piperidine ana-
logues (Z)-5d and (E)-5d were subsequently debenzyl-
ated by treatment with 1-chloroethyl chloroformate in
toluene followed by re¯ux in methanol to give (Z)-9d
and (E)-9d in moderate yield.
The pyridyl positional isomers, compounds 6b, 7b and
8b (Scheme 1), were prepared from 1-azabicyclo[2.2.2]-
*Corresponding author. Tel.: +45-4443-4886; fax: +45-4466-3939;
e-mail: phol@novo.dk
0968-0896/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(00)00063-8

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P. H. Olesen et al. / Bioorg. Med. Chem. 8 (2000) 1443±1450
Scheme 2. For m and R see Table 2. (a) Dimethyl 2-oxoalkylphos-
phonate/KOH in water.
solutions on prolonged standing, and no isomerization
back to the (E)-isomers was detected.
Figure 1. Ligands for the central nicotinic cholinergic receptors.
In order to test if the oxadiazole showed bioisosterism
with the isoxazole on the nicotinic receptor, (Z)-14b and
(Z)-15b were prepared by reaction of the ester (Z)-13b10
with sodium ethoxide and N-hydroxy-acetamidine or
N-hydroxy-propionamidine (Scheme 3).
Figure 2. The azacyclic core of the compounds.
The relative con®guration of the compounds was
1
assigned on the basis of NOE H NMR analysis.
octan-3-one (2b) and the appropriate 2- or 4-[(tri-
methylsilanyl)methyl]-pyridines9 or 2-[(trimethylsila-
nyl)methyl]-pyrazine (4), by a procedure similar to that
described for compounds 5b±d. The a,b-unsaturated
ketones 10±12 were prepared by a Horner±Emmons±
Wadsworth reaction of the appropriate 1-azabicyclic
ketones (2a,b) and dimethyl 2-oxoalkylphosphonate
(Scheme 2).6
For the (Z)-isomers a strong interaction between the
bridgehead hydrogen and the methine hydrogen was
present, whereas a strong interaction between the
methine hydrogen and the C2-hydrogens for the (E)-
isomers was observed.
All compounds a and c were obtained and tested as
racemic mixtures, and 8b was tested as a (Z/E)-mixture.
Crystallization of the mixture of free bases 10±12 with
an acid addition salt, e.g., hydrochloric acid or oxalic
acid, resulted in isomerization to the thermodynamically
more stable (Z)-isomers, and it has not been possible to
obtain the (E)-isomers of 10±12 in pure form. Com-
pounds (Z)-10±12 were stable to neutral and acidic
Biology
The ability of the compounds to displace the nicotinic
agonist [3H]methylcarbamylcholine (MCC) from native
cholinergic binding sites in rat brain cortex was deter-
mined as previously described (Tables 1 and 2).6,11,12 All
Scheme 1. For X, Y, n see Figure 2 and Table 1. (a) 5: 3-[(tert-Butyldimethylsilanyl)methyl]-pyridine (3)/LDA in THF, 78 ꢀC. 6: 2-[(Trimethyl-
silanyl)methyl]-pyridine/LDA in THF, 78 ꢀC. 7: 2-[(Trimethylsilanyl)methyl]-pyrazine (4)/LDA in THF, 78 ꢀC. 8: 4-[(Trimethylsilanyl)methyl]-
pyridine/LDA in THF, 78 ꢀC. (b) 5d: (i) 1-Chloroethyl chloroformate in toluene, re¯ux. (ii) Re¯ux in methanol.

P. H. Olesen et al. / Bioorg. Med. Chem. 8 (2000) 1443±1450
1445
the compounds, except (Z/E)-8b and (Z)-6b, bound to
the nicotinic binding site with moderate to high anity.
whereas both isomers with the [3.2.1]azabicyclic ring
system had anities in the same range, (E)-5c (IC50=
35 nM) and (Z)-5c (IC50=24 nM). For the positional
isomers (Z)-6b and (E/Z)-8b a dramatic decrease in
nicotinic anity for the compounds was observed with
IC50 values higher than 1000 nM. Adding an additional
nitrogen to the aromatic ring such as the pyrazinyl
analogues (Z)-7b and (E)-7b also resulted in a sig-
ni®cant drop in anity for the nicotinic receptors by a
factor of 80. The same order of decrease in anity was
observed by addition of a nitrogen to the aromatic ring
of the NNC 90-0270, giving the oxadiazole-analogue
((Z)-14b). Extending the R substituent from methyl,
(Z)-14b to ethyl, (Z)-15b leads to a decrease in anity
just as described for the analogues of NNC 90-0270.
For pairs of regio-isomers the highest nicotinic receptor
anity for the [2.2.2]azabicyclic ring system was
obtained with the (Z)-isomer (Z)-5b (IC50=2.0 nM),
Scheme 3. (a) NaOEt in EtOH, N2, 80 ꢀC.
Table 1. In vitro binding data for pyridyl (5, 6, 8, 9) and 2-pyrazinyl (7) methylene-1-azacycles
Compound
n
X
Y
N-position
Isomeric puritya (%)
[3H]MCC cortexb IC50 (nM)
(Z)-5b
(E)-5b
(Z)-5c
(E)-5c
(Z)-6b
(Z)-7b
(E)-7b
(E/Z)-8b
(Z)-9d
(E)-9d
1
1
2
2
1
1
1
1
1
-CH2CH2-
-CH2CH2-
-CH2-
Meta
Meta
Meta
Meta
Ortho
98
90
>99
>99
91
92
75
57:43
>99
>99
Ð
2.0Æ1.2
66Æ21
35Æ16
24Æ21
>1000*
169Æ106
339Æ1.4
>1000*
227Æ119
160Æ32
3.9
-CH2-
-CH2CH2-
-CH2CH2-
-CH2CH2-
-CH2CH2-
Ortho and meta
Ortho and meta
Para
H
H
H
H
Meta
Meta
Ð
Meta
Meta
1
NNC 90-0270
(Æ)-Epibatidine
(S)-Nicotine
Ð
Ð
Ð
Ð
Ð
Ð
Ð
Ð
0.30
6.6
aMeasured by GC.
bValues represent mean Æ S.E.M. nꢁ3, except *n=2.
Table 2. In vitro binding data for the 3-acyl (10, 11, 12) and 3-oxadiazolyl (14, 15) methylene-1-azabicycles
Compound
m
R
Isomeric puritya (%)
[3H]-MCC cortexb IC50 (nM)
(Z)-10a
(Z)-11a
(Z)-10b
(Z)-11b
(Z)-12b
(Z)-13b
1
1
2
2
2
2
CH3
CH2CH3
CH3
CH2CH3
CH2CH2CH3
OCH2CH3
96
95
97
96
86
95
45Æ11
81Æ23
13Æ8.5
12Æ0.7*
103Æ41
490Æ150
(Z)-14b
(Z)-15b
Ð
Ð
CH3
CH2CH3
93
>99
92Æ8.0
590Æ270
aMeasured by GC.
bValues represent mean Æ S.E.M. nꢁ3, except *n=2.

1446
P. H. Olesen et al. / Bioorg. Med. Chem. 8 (2000) 1443±1450
Table 3. Results from the molecular modellinga
Substitution of the 3-pyridyl moiety to an acyl sub-
stituent results in a minor reduction in anity for the
nicotinic receptor, by a factor 7 when comparing (Z)-5b
and (Z)-10b. Increasing the carbon chain length in the
ketone-[2.2.2] series from methyl, (Z)-10b to propyl,
(Z)-12b decreases the anity for nicotinic receptors by a
factor 10. Interestingly, the ester analogue, (Z)-13b, was
not substantially less active than (Z)-12b.
Compound
Torsionb
ÁE (kcal/mol)
RMSc
a±b distanced
(Z)-5b
(E)-5b
(R)-(Z)-5c
(R)-(E)-5c
(Z)-6b
Æ54
Æ53
52
63
0
0
0
0
0
0
0
4.0
1.8
0
0
0
0
0.4
0.110
1.800
0.156
1.450
2.441
1.696
0.096
0.829
0.110
0.746
0.547
0.277
0.187
7.4
11.0
Ð
10.5
2.7
(Z)-7b
0
11.2
Æ130
Æ130
Æ54
Æ128
0
(E)-7b
(Z)-9d
(E)-9b
(R)-(Z)-10a
(Z)-10b
(Z)-14b
8.5
Ð
Ð
8.1
7.5
Ð
Changing the azabicyclo[2.2.2]-system in (Z)-5b to
piperidine in (Z)-14d also results in a signi®cant drop in
anity for the nicotinic receptors.
0
180
Basically, the same SAR is observed as those described
for analogues of NNC 90-0270, with the (Z)-isomers of
compounds containing the [2.2.2]azabicyclic ring system
having the highest anity. For the [3.2.1]azabicycle equal
anity between the (Z)- and (E)-isomers was obtained,
again the same pattern as previously observed for the NNC
90-0270 analogues. The data show that the 3-pyridyl and
acyl substituents are excellent bioisosters to the isoxazole
heterocycle but that even a small change in the aromatic
ring, pyridyl to pyrazinyl or isoxazolyl to oxadiazolyl,
results in compounds with signi®cantly less anity.
aOnly the supposed binding conformations are listed.
bFor chiral compounds only the results for the (R)-enantiomers are
shown.
cSuperimposing with 16.
dThe a±b distance was measured only for the molecules in which elec-
trostatic and substituent e€ects could be ruled out.
same good overlap between the piperidine compounds
((Z)- and (E)-9d), the (Z)-azabicyclo[3.2.1]octane com-
pound ((Z)-5c), and the oxadiazole compound (14b)
with 16 is seen. Thus, in view of their moderate a-
nities, these compounds probably also possess unfavor-
able steric or electrostatic properties. The pyrazine
compound ((Z)-7b) would be expected to bind to the
nicotinic receptors in the same conformation as the
meta-pyridine (Z)-5b. However, the conformational
analysis shows that this conformation is energetically
unfavored by 4.0 kcal/mol. Hence, due to conforma-
tional changes the pyrazine ring cannot be used as a
bioisoster for the isoxazole ring in this system.
Computational studies
For a further investigation of the SAR, 12 compounds
were chosen for computational studies: seven com-
pounds from the pyridyl-series (5, 6, 9), two pyrazinyl
compounds ((Z)- and (E)-7b), two acyl (10a,b), and one
oxadiazolyl compound (14b).
As the conformational freedom in the compounds con-
taining an azabicycle is very limited, a conformational
search was performed by rotation around the single
bond connecting the azabicycle and the aromatic het-
erocycle or the acyl group.
To test whether these compounds display the correla-
tion between the a±b distance and anity as previously
reported for the isoxazole compounds,6 the a±b distance
was measured for the molecules in which electrostatic
and substituent e€ects could be ruled out (Table 3). The
molecules were represented by their supposed binding
conformations. The a±b distance of the most active
compounds ((Z)-5b and (Z)-10b) corresponds to the
previously reported6 optimal distance of 7.0±8.0 A.
The more ¯exible piperidine compounds were investigated
using the molecular dynamics option in Sybyl.13 The
results from the conformational study are summarized in
Table 3. For the further studies only low-energy con-
formers with a ÁE<3.0 kcal/mol were considered relevant.
The compounds were superimposed on 16 (Fig. 3),
which previously has been shown to possess only one
low-energy conformation.6 The compounds were super-
imposed using the sp3 hybridized nitrogen atom (N+),
its complementary anionic site point (a), and the
hydrogen bond donor site point (b) complementary to a
nitrogen atom in the heteroaromatic ring or the oxygen
atom in the acyl group. The site points were placed 2.9
A away from the nitrogen atoms (Fig. 3).
Conclusion
The compounds described here comprise a series of
compounds with a unique structure compared to other
nicotinic agonists described previously. We have shown
that it is possible to use a pyridine or an acyl moiety as
bioisosteric replacements for the isoxazole heterocycle
in this series of nicotine analogues. Additional studies
are needed to substantiate the therapeutic potential for
these compounds with respect to treatment of neurode-
generative diseases such as Alzheimer's disease and
Parkinson's disease.
In the previously published series the compounds con-
taining a piperidine ring or a (Z)-compound containing
an azabicyclo[3.2.1]octane ring possessed lower anities
than expected, as the compounds displayed good over-
lap with the reference compound (16).6 These ®ndings
were ascribed to unfavorable steric and electrostatic
properties of the compounds.6 When the molecules in
the present pyridine series are superimposed on 16, the
Experimental
1H NMR spectra were recorded at 300 MHz on a Bru-
ker AC-300 MHz FT-NMR instrument, and COESY,

P. H. Olesen et al. / Bioorg. Med. Chem. 8 (2000) 1443±1450
1447
Figure 3. The position of the elements (N+, a, b) used for superpositioning the compounds is shown in 16. (a) and (b) are placed 2.9 A away from
the nitrogen or oxygen atoms. The numbering refers to the atoms used for measuring the torsional angles.
NOESY and ROESY spectra were recorded at
400 MHz on a Bruker AC-400 MHz FT-NMR instru-
ment. Mass spectra were recorded on a Finnigan 5100
mass spectrometer, and melting points (uncorrected)
were determined on a Buchi capillary melting point
apparatus. Column chromatography was performed on
silica gel 60 (70±230 mesh, ASTM, Merck). Elemental
analyses were performed by Novo Nordisk Micro Ana-
lytical Laboratory, Denmark, and except for (Z)-7b,
they were within 0.4% of the calculated values. All the
reactions described are non-optimized, therefore the
yields are not always as high as could be otherwise
obtained. The (Z/E)-purity of the compounds subjected
to biological testing was measured by gas chromato-
graphy (Chrompack CP9001).
2,2,6,6-tetramethylpiperidine (6.7 mL, 60 mmol) in dry
THF (75 mL) under N2. The mixture was stirred for
45 min at 0 ꢀC, upon which 2-methylpyrazine (5.5 mL,
60 mmol) dissolved in dry THF (20 mL) was added over
a period of 5 min. The reaction mixture was then stirred
for another 45 min at 0 ꢀC, whereupon trimethylsilyl
chloride (15 mL, 120 mmol) dissolved in THF (50 mL)
was added at 78 ꢀC. Stirring was continued at 78 ꢀC
for 30min, followed by an additional 1.5 h, during which
the temperature was allowed to rise up to 20 ꢀC. The
reaction mixture was poured on ice (600 mL) and stirred
until all the ice had melted. Extraction with diethyl ether
(2Â300 mL), drying (Na2SO4) of the combined organic
phases, and concentration in vacuo gave 4.8 g (48%) of
the title compound as a red oil. The product was used
without further puri®cation. 1H NMR (300 MHz,
CDCl3) d 8.36 (m, 1H, H-meta); 8.24 (m, 1H, H-ortho);
8.21 (m, 1H, H-para); 2.30 (s, 2H, CH2); 0.09 (s, 9H,
CH3).
(Z)-3-(1-Azabicyclo-[2.2.2]oct-3-ylidene)-3-acetic acid
ethyl ester ((Z)-13b),10 2- or 4-(trimethylsilanyl)methyl-
pyridines,9 and 2-[(tert-butyldimethylsilanyl)methyl]-
pyridine14 were synthesized according to modi®ed litera-
ture procedures. 3-Acetonylidene-1-azabicyclo[2.2.2]-
octane ((Z)-10b), 3-propionylidene-1-azabicyclo[2.2.1]-
heptane ((Z)-11a), and 3-propionylidene-1-azabicyclo-
[2.2.2]octane ((Z)-11b) were synthesized following a
published procedure.6
3-(3-Pyridyl)methylene-1-azabicyclo[2.2.2]octane 5b. n-
Butyllithium (1.6 M, 5 mL, 8 mmol) was added to a
solution of diisopropylamine (1.15 mL, 8.0 mmol) in
THF (25 mL). The mixture was stirred for 45 min at
0 ꢀC, and 3-[(tert-butyldimethylsilanyl)methyl]-pyridine
(3) (1.7 g, 8 mmol) dissolved in THF (5 mL) was added.
The reaction mixture was stirred for another 45 min
at 0 ꢀC, then cooled to 78 ꢀC, and 1-azabicyclo-
[2.2.2]octan-3-one (0.85 g, 7.6 mmol) dissolved in
THF (10 mL) was added. The reaction mixture was
stirred at 78 ꢀC for 1.5 h, then quenched with water
(100 mL) and made alkaline (K2CO3). The aqueous
phase was extracted with diethyl ether (3Â75 mL). The
combined organic extracts were dried (MgSO4) and
concentrated in vacuo. The resulting oil was puri®ed by
column chromatography on silica (eluent ethanol:ethyl
acetate:ammonium hydroxide, 25% in water: 1:2:0.02).
The ®rst fractions contained the (Z)-isomer con-
taminated with small amounts of the (E)-isomer. The
crude material was crystallized with oxalic acid from
methanol to give (Z)-3-(3-pyridyl)methylene-1-azabicy-
clo[2.2.2]octane ((Z)-5b) in 635 mg (22%) yield as the
oxalate salt. The later fractions contained (E)-3-(3-pyr-
idyl)methylene-1-azabicyclo[2.2.2]octane ((E)-5b), which
was crystallized as the oxalate salt from acetone in
430 mg (15%) yield.
3-[(tert-Butyldimethylsilanyl)methyl]-pyridine 3. n-Butyl-
lithium (1.6 M, 15.6 mL, 25 mmol) was added to a solu-
tion of diisopropylamine (3.6 mL, 25 mmol) in THF
(50 mL). The mixture was stirred for 45 min at 0 ꢀC and
3-picoline (2.6 mL, 25 mmol) dissolved in THF (10 mL)
was added. The reaction mixture was stirred for 30 min
at 0 ꢀC, then cooled to 78 ꢀC and transferred via a
cannula to a 78 ꢀC solution of tert-butyldimethylsilyl
chloride (7.5 g, 50 mmol) in THF (50 mL). The reaction
mixture was stirred for another 30 min at 78 ꢀC, then
allowed to warm to 20 ꢀC over a 90 min period.
Quenching with ice (250 mL) and basi®cation (K2CO3)
was followed by extraction with diethyl ether
(3Â150 mL). The combined organic extracts were
washed with brine, dried (MgSO4), and concentrated in
vacuo. The impurities present in the crude product were
distilled o€, and the residue (4.4 g, 85%) was used
without further puri®cation. MS (EI) m/z 207 (M+). 1H
NMR (300 MHz, CDCl3) d 8.41 (m, 2H, ArH-2, ArH-
6); 7.42 (m, 1H, ArH-5); 7.23 (m, 1H, ArH-4); 2.18 (s,
2H, CH2); 1.00 (s, 9H, CH3); 0.00 (s, 6H, CH3).
(Z)-5b. Mp 197±200 ꢀC. MS (EI) m/z 200 (M+). GC:
98% Z. 1H NMR (DMSO-d6) d 1.85 (m, 2H, CH2), 2.05
(m, 2H, CH2), 2.78 (m, 1H, CH), 3.28 (m, 4H, NCH2),
2-[(Trimethylsilanyl)methyl]-pyrazine 4. n-Butyllithium
(2.5 M, 24 mL, 60 mmol) was added to a 0 ꢀC solution of

1448
P. H. Olesen et al. / Bioorg. Med. Chem. 8 (2000) 1443±1450
4.39 (s, 2H, NCH2C), 6.50 (s, 1H, CHˆ), 7.40 (dd, 1H,
H-meta), 7.63 (dd, 1H, H-ortho), 8.40 (d, 1H, H-para),
8.47 (s, 1H, H-ortho). Anal. (C13H16N2, 212 C2H2O4) C,
H, N.
of dry THF under nitrogen at 78 ꢀC. The mixture was
stirred for 10 min, whereupon 2-[(trimethylsilanyl)-
methyl]-pyridine9 (1.4 g, 8.5 mmol) was added drop wise
over a period of 10 min to the solution of lithium tetra-
methylpiperidine. After stirring for 10 min a solution of
1-azabicyclo[2.2.2]octan-3-one (1.8 g, 14 mmol) in 5 mL
of THF was added over 15 min. Stirring was continued
at 78 ꢀC for 1 h. Then the mixture was allowed to
warm to room temperature and 25 mL of water was
added. The mixture was extracted with diethyl ether
(3Â25 mL). The extracts were combined and dried
(Na2SO4). Removal of the solvent in vacuo gave 1.55 g
of a slowly crystallizing oil. The crystals were collected
by ®ltration yielding 0.64 g (38%) of (Z)-3-(2-pyridyl-
methylene)-1-azabicyclo[2.2.2]-octane ((Z)-6b), which
was crystallized as the oxalate salt from acetone. The
mother liquor was concentrated in vacuo and puri®ed
by column chromatography (eluent dichloromethane:
methanol:ammonium hydroxide, 25 % in water: 80:20:
0.5%). The last fractions gave 13 mg (0.8%) of a (Z/E)-
mixture.
(E)-5b. Mp 145±146 ꢀC. MS (EI) m/z 200 (M+). GC:
90% E. 1H NMR (DMSO-d6) d 1.90 (m, 2H, CH2), 2.00
(m, 2H, CH2), 3.12 (m, 1H, CH), 3.32 (m, 4H, NCH2),
4.05 (s, 2H, NCH2Cˆ), 6.45 (s, 1H, CHˆ), 7.40 (dd,
1H, H-meta), 7.70 (dd, 1H, H-ortho), 8.40 (m, 2H, H-
ortho/para). Anal. (C13H16N2, 2.25 C2H2O4) C, H, N.
Compounds 5c and 5d were prepared as described for
compound 5b with the variations stated.
(R/S)-6-(3-Pyridyl)methylene-1-azabicyclo[3.2.1]octane
5c. Starting from 1-azabicyclo[3.2.1]octane-3-one and
3-[(tert-butyldimethylsilanyl)methyl]-pyridine (3). (Z)-5c
was crystallized as the oxalate salt from acetone. Mp
178±180 ꢀC. MS (EI) m/z 200 (M+). GC: 100% Z. H
1
NMR (DMSO-d6) d 1.72 (m, 2H, CH2), 1.88 (m, 2H,
CH2), 3.18 (s, 1H, CH), 3.2±3.4 (m, 4H, NCH2), 4.35 (d,
1H, Jgem=14 Hz, NCH2Cˆ), 4.55 (d, 1H, Jgem=14 Hz,
NCH2Cˆ), 6.60 (s, 1H, CHˆ), 7.43 (dd, 1H, H-meta),
7.73 (d, 1H, H-ortho), 8.47 (d, 1H, H-para), 8.58 (s, 1H,
H-ortho). Anal. (C13H16N2, 2 C2H2O4) C, H, N.
(Z)-6b. Mp 162±164 ꢀC. MS (EI) m/z 200 (M+). GC:
1
91% Z. H NMR (CDCl3, 300 MHz) d 1.7±2.0 (m, 4H,
CH2), 2.45±2.60 (m, 1H, CH), 2.8±3.1 (m, 4H, NCH2),
4.00±4.15 (m, 2H, NCH2Cˆ), 6.20±6.35 (m, 1H, CHˆ),
7.00±7.20 (m, 2H, H-ortho/para), 7.5±7.7 (m, 1H, H-
meta), 8.55±8.65 (m, 1H, H-meta). Anal. (C13H16N2, 221
C2H2O4) C, H, N.
(E)-5c. This was crystallized as the hydrochloride salt
from ethyl acetate. Mp 254±255 ꢀC. MS (EI) m/z 200
(M+). GC: 100% E. 1H NMR (DMSO-d6) d 1.7±2.0 (m,
4H, CH2), 3.3±3.5 (m, 4H, NCH2), 3.55 (s, 1H, CH),
4.20 (m, 2H, NCH2Cˆ), 6.68 (s, 1H, CHˆ), 7.85 (dd,
1H, H-meta), 8.28 (d, 1H, H-ortho), 8.70 (d, 1H, H-
para), 8.80 (s, 1H, H-ortho). Anal. (C13H16N2, 212 HCl)
C, H, N.
Compounds 7b and 8b were prepared as described for
compound 6b with the variations stated.
3-(2-Pyrazinyl)methylene-1-azabicyclo[2.2.2]octane 7b.
Starting from 2-[(trimethylsilanyl)methyl]-pyrazine (4)
and 1-azabicyclo[2.2.2]octan-3-one. The reaction mix-
ture was poured on ice (400 mL) and stirred until all
the ice had melted. Extraction with diethyl ether
(2Â250 mL), drying (Na2SO4) of the combined organic
phases, and concentration in vacuo gave 0.90 g (37%) of
the crude product. The product was puri®ed by column
chromatography on silica (eluent dichloromethane:
methanol:ammonium hydroxide, 25% in water: 80:20:
0.5%). The ®rst fractions contained 80 mg (3.3%) pure
(Z)-3-(2-pyrazinyl)methylene-1-azabicyclo[2.2.2]octane
((Z)-7b). The following fractions contained 250 mg
(10%) of a mixture of the two isomers, and the last
fractions contained 90 mg (3.7%) (E)-3-(2-pyrazinyl)-
methylene-1-azabicyclo[2.2.2]octane ((E)-7b) con-
taminated with some (Z)-isomer.
1-Benzyl-3(3-pyridylmethylene)-piperidine 5d. Starting
from 1-benzyl-3-piperidone and 3-[(tert-butyldimethyl-
silanyl)methyl]-pyridine (3). Column chromatography
was performed in ethyl acetate and gave 25% (Z)-3-(1-
benzyl-3-pyridylmethylene)-piperidine ((Z)-5d) and 23%
(E)-3-(1-benzyl-3-pyridylmethylene)-piperidine ((E)-5d).
The compounds were used without further puri®cation.
1
(Z)-5d. H NMR (300 MHz, CDCl3) d 8.35 (m, 2H, H-
ortho/para); 7.25 (m, 6H, Ar-H + H-ortho); 7.03 (dd,
1H, J=9 Hz, H-meta); 6.22 (s, 1H, CHˆ); 3.50 (s, 2H,
NCH2); 3.10 (s, 2H, Ar-CH2); 2.59 (t, 2H, J=6 Hz,
NCH2); 2.30 (t, 2H, J=6 Hz, CH2Cˆ); 1.76 (quintet,
2H, J=6 Hz, CH2).
Crystallization with oxalic acid from acetone gave the
title compounds as their oxalate salts. The compounds
were recrystallized from methanol/acetone.
(E)-5d. 1H NMR (300 MHz, CDCl3) d 8.49±8.38 (m,
2H, H-ortho/para); 7.48 (d, 1H, J=9 Hz, H-ortho);
7.38±7.14 (m, 6H, Ar-H+H-meta); 6.22 (s, 1H, CHˆ);
3.58 (s, 2H, NCH2); 3.08 (s, 2H, Ar-CH2); 2.56 (t, 2H,
J=6 Hz, NCH2); 2.39 (t, 2H, J=6 Hz, CH2Cˆ); 1.64
(quintet, 2H, J=6 Hz, CH2).
(Z)-7b. Mp 147±152 ꢀC. MS (EI) m/z 201 (M+). GC:
1
92% Z. H NMR (300 MHz, CDCl3) d 8.51 (d, 1H,
J=2 Hz, H-para); 8.40 (s, 1H, H-ortho); 8.28 (d, 1H,
J=2 Hz, H-meta); 6.29 (s, 1H, CHˆ); 4.05 (s, 2H,
NCH2Cˆ); 3.10±2.77 (m, 4H, NCH2); 2.59 (m, 1H,
CH); 1.93±1.68 (m, 4H, CH2). Anal. (C12H15N3, 2
C2H2O4, 0.50 CH3OH) Calcd: C, 49.87; H, 5.33; N,
10.57. Found: C, 50.13; H, 4.92; N, 10.32.
3-(2-Pyridylmethylene)-1-azabicyclo[2.2.2]octane 6b. A
2.5 M solution of n-butyllithium in hexanes (3.4 ml,
8.5 mmol) was added over 5 min to a stirred solution of
2,2,6,6-tetramethylpiperidine (1.19 g, 8.5 mmol) in 10 ml

P. H. Olesen et al. / Bioorg. Med. Chem. 8 (2000) 1443±1450
1449
(E)-7b. Mp 149±150 ꢀC. MS (EI) m/z 201 (M+). GC:
75% E. H NMR (300 MHz, DMSO) d 8.63 (s, 1H, H-
ortho); 8.58 (s, 1H, H-para); 8.47 (s, 1H, H-meta); 6.51
(s, 1H, CHˆ); 4.24 (m, 1H, CH); 4.11 (s, 2H,
NCH2Cˆ); 3.49±3.18 (m, 4H, NCH2); 2.13 (m, 4H,
CH2). Anal. (C12H15N3, 1.5 C2H2O4, 0.25 H2O) C, H,
N.
(E)-9d. Mp 133±134 ꢀC. MS (EI) m/z 174 (M+). GC:
1
1
100% (Z). H NMR (400 MHz, DMSO) d 9.1 (bs, 3H,
H+); 8.48 (m, 2H, H-ortho/para); 7.69 (d, 1H, J=9 Hz,
H-ortho); 7.42 (dd, 1H, J=9 Hz, H-meta); 6.60 (s, 1H,
CHˆ); 3.89 (s, 2H, NCH2Cˆ); 3.18 (m, 2H, NCH2);
2.53 (m, 2H, CH2Cˆ); 1.76 (m, 2H, CH2). Anal.
(C11H14N2, 2.25 C2H2O4) C, H, N.
(Z/E)-3-(4-Pyridylmethylene)-1-azabicyclo[2.2.2]octane
8b. Starting from 4-[(trimethylsilanyl)methyl]-pyridine9
and 1-azabicyclo[2.2.2]octan-3-one. The product mix-
ture was crystallized with oxalic acid from acetone.
(Z)-3-Acetonylidene-1-azabicyclo[2.2.2]octane (Z)-10b.6
Dimethyl 2-oxopropylphosphonate (4.9 g, 30 mmol)
was added dropwise to a 5 ꢀC solution of 1-azabicy-
clo[2.2.2]octan-3-one (2.0 g, 16 mmol) and KOH (1.93 g,
30 mmol) in water (7.7 mL). The reaction mixture was
stirred between 0 ꢀC and 5 ꢀC for 90 h. The reaction
mixture was quenched with 1 N HCl (50 mL), rinsed
three times with diethyl ether, made alkaline (K2CO3),
and extracted with dichloromethane (5Â50 mL). The
solvent was removed after drying (MgSO4). The crude
compound was crystallized as the oxalate salt from eth-
anol (100 mL) and then recrystallized from ethanol
(50 mL) to give the title compound in 800 mg (21%)
yield.
(Z/E)-8b. Mp 185±88 ꢀC. MS (EI) m/z 200 (M+). GC:
57% Z, 43% E. Anal. (C13H16N2, 3 C2H2O4) C, H, N.
1
(Z)-8b. H NMR (CDCl3-free base, 300 MHz) d 1.7±2.0
(m, 4H, CH2), 2.55±2.65 (m, 1H, CH), 2.8±3.1 (m, 4H,
NCH2), 3.80±3.90 (m, 2H, NCH2Cˆ), 6.15±6.25 (m,
1H, CHˆ), 7.05±7.15 (m, 2H, H-ortho), 8.45±8.55 (m,
2H, H-meta).
1
(E)-8b. H NMR (CDCl3, 300 MHz) d 1.7±2.0 (m, 4H,
CH2), 2.55±2.65 (m, 1H, CH), 2.8±3.1 (m, 4H, NCH2),
3.80±3.90 (m, 2H, NCH2C=), 6.15±6.25 (m, 1H,
CH=), 7.05±7.15 (m, 2H, H-ortho), 8.45±8.55 (m, 2H,
H-meta).
(Z)-10b. Mp 189±190 ꢀC. MS (EI) m/z 165 (M+). GC:
97% Z. 1H NMR (DMSO-d6) d 1.80 (m, 2H, CH2), 2.00
(m, 2H, CH2), 2.18 (s, 3H, CH3), 2.71 (m, 1H, CH), 3.28
(m, 4H, NCH2), 4.29 (s, 2H, NCH2Cˆ), 6.35 (s, 1H,
CHˆ). Anal. (C10H15NO, HCl, 112 H2O) C, H, N.
(Z)-3-(3-Pyridylmethylene)-piperidine (Z)-9d. (Z)-1-Ben-
zyl-3(3-pyridylmethylene)-piperidine ((Z)-5d) (650 mg,
2.46 mmol) was dissolved in toluene (50 mL) and con-
centrated in vacuo. The now anhydrous (Z)-5d was then
redissolved in toluene (50 mL) under N2 and cooled to
0 ꢀC on an ice bath. Addition of 1-chloroethyl chloro-
formate (400 mL, 3.70 mmol) was followed by removal
of the ice bath, upon which the reaction mixture was
re¯uxed for 4 h. The reaction mixture was then cooled
to 20 ꢀC, concentrated in vacuo, and the residue dis-
solved in methanol (25 mL). Upon stirring at re¯ux
temperature for 2 h, the mixture was evaporated and the
resulting oil dissolved in 1 N HCl (75 mL). The water
phase was extracted with diethyl ether (3Â50 mL), made
alkaline (K2CO3), and extracted with ethyl acetate
(3Â75 mL). The combined ethyl acetate extracts were
washed with brine (75 mL), dried (MgSO4), and evapo-
rated. The crude product (310 mg) was dissolved in ethyl
acetate (15 mL) and crystallized with 2 M HCl in ethyl
acetate to give 55 mg (11%) of (Z)-3-(3-pyridyl-
methylene)-piperidine ((Z)-9d) hydrochloride.
Compounds 10a and 12b were prepared as described for
compound 10b with the variations stated.
(R/S)-(Z)-3-Acetylmethylene-1-azabicyclo[2.2.1]heptane
(Z)-10a. Starting from 1-azabicyclo[2.2.1]heptan-3-one
andꢀdimethyl 2-oxoethylphosphonate. (Z)-10a. Mp 176±
1
177 C. MS (EI) m/z 151 (M+). GC: 96% Z. H NMR
(DMSO-d6) d 1.55 (m, 1H, CH2), 2.15 (s, 3H, CH3),
2.20 (m, 1H, CH2), 3.15 (s, 2H, NCH2), 3.20 (m, 1H,
NCH2), 3.35 (m, 1H, NCH2), 3.50 (d, 1H, CH), 4.17 (s,
2H, NCH2Cˆ), 6.50 (s, 1H, CHˆ). Anal. (C13 H16N2,
C2H2O4, 1 H2O) C, H, N.
4
(R/S)-3-Propionylmethylene-1-azabicyclo[2.2.1]heptane
(Z)-11a.6 GC: 95% Z.
3-Propionylmethylene-1-azabicyclo[2.2.2]octane (Z)-11b.6
GC: 100% Z.
(Z)-3-Butyrylmethylene-1-azabicyclo[2.2.2]octane (Z)-12b.
Starting from 1-azabicyclo[2.2.2]octan-3-one and di-
methyl 2-oxopentylphosphonate.15 Mp 135±137 ꢀC. MS
(Z)-9d. Mp 218±221 ꢀC. MS (EI) m/z 174 (M+). GC:
1
1
97% (Z). H NMR (300 MHz, CDCl3) d 8.42 (m, 2H,
(EI) m/z 193 (M+). GC: 86% Z. H NMR (300 MHz,
H-ortho/para); 7.45 (d, 1H, J=9 Hz, H-ortho); 7.23 (dd,
1H, J=9 Hz, H-meta); 6.22 (s, 1H, CHˆ); 3.55 (s, 2H,
NCH2Cˆ); 2.95 (t, 2H, J=5 Hz, NCH2); 2.51±2.32 (m,
3H, NH+CH2Cˆ); 1.76 (quintet, 2H, J=5 Hz, CH2).
Anal. (C11H14N2, 2 HCl, 1.25 H2O) C, H, N.
MeOH) d 6.41 (m, 1H, CHˆ); 4.50 (s, 2H, NCH2Cˆ);
3.52±3.29 (m, 4H, NCH2); 2.82 (m, 1H, CH); 2.53 (t,
2H, J=7 Hz, CH2CO); 2.26±2.12 (m, 2H, CH2); 2.18±
1.93 (m, 2H, CH2); 1.61 (sextet, 2H, J=7 Hz, CH2); 0.94
(t, 3H, J=7 Hz, CH3). Anal. (C12H19NO, C2H2O4, 0.25
H2O) C, H, N.
(E)-3-(3-Pyridylmethylene)-piperidine (E)-9d. The title
compound was prepared as described for (Z)-9d, start-
ing from (E)-1-benzyl-3(3-pyridylmethylene)-piperidine
((E)-5d). The crude product was crystallized as the oxa-
late salt in 15% overall yield.
(Z)-3-(3-Methyl-[1,2,4]-oxadiazol-5-yl)methylene-1-aza-
bicyclo[2.2.2]octane (Z)-14b. Sodium (150 mg, 6.0 mmol)
was dissolved in dry ethanol (15 mL) under N2, and (Z)-
3-(ethoxycarbonyl-methylene)-1-azabicyclo[2.2.2]octane

1450
P. H. Olesen et al. / Bioorg. Med. Chem. 8 (2000) 1443±1450
((Z)-13b) (600 mg, 3.0 mmol) and N-hydroxy-acet-
amidine (450 mg, 6.0 mmol) were added dissolved in dry
ethanol (8 mL). Upon this the reaction mixture was
stirred for 90 min at 80 ꢀC, then at 20 ꢀC overnight. The
reaction mixture was ®ltered, the residue washed
thoroughly with methanol, and the combined ®ltrates
concentrated in vacuo. The resulting oil was dissolved
in ethyl acetate (50 mL) and the organic phase was
washed with brine (2Â50 mL). Drying (MgSO4) and
evaporation gave 790 mg (128%) of a yellow oil. Col-
umn chromatography (eluent: ethyl acetate:methanol:
ammonium hydroxide, 25% in water: 80:20:0.5%) gave
570 mg (93%) of (Z)-14b.
0.001. As the compounds were protonated, the dielectric
constant was set to 4 while keeping the cut-o€ value
at 8 A. The resulting low-energy structures were then
minimized, but this time without any constraints.
Molecular dynamics. The remaining compounds were
investigated using the molecular dynamics option in
Sybyl. In the simulation, the compounds were heated
to 1000 K for 100 ps. The step size was 10 fs, and every
500 fs a structure was selected giving a total of 201
structures. The maximum number of iterations was set
to 10,000. Due to the amount of heat applied, some of
the compounds racemized and/or Z/E interchanged.
The 200 structures resulting from a dynamics run were
then minimized.16 The structures were superpositioned
using the fit atoms command.
Crystallization with oxalic acid from acetone gave
700 mg of (Z)-14b oxalate as white crystals.
Mp 146±147 ꢀC. MS (EI) m/z 205 (M+). GC: 100% Z.
1H NMR (400 MHz, DMSO): d 6.68 (s, 1H, CHˆ),
4.41 (s, 2H, NCH2Cˆ), 3.40±3.22 (m, 4H, NCH2),
2.96 (s, 1H, CH), 2.37 (s, 3H, CH3), 2.08 (m, 2H, CH2),
1.86 (m, 2H, CH2). Anal. (C11H15N3O, C2H2O4, 14 H2O)
C, H, N.
References and Notes
1. Garvey, D. S.; Wasicak, J. T.; Decker, M. W.; Brioni, J. D.;
Buckley, M. J.; Sullivan, J. P.; Carrera, G. M.; Holladay, M.
W.; Arneric, S. P.; Williams, M. J. Med. Chem. 1994, 37, 1055.
2. Arneric, S. P.; Sullivan, J. P.; Briggs, C. A.; Donnelly-
Roberts, D.; Anderson, D. J.; Raszkiewicz, J. L.; Hughes, M.
L.; Cadman, E. D.; Adams, P.; Garvey, D. S.; Wasicak, J. T.;
Williams, M. J. Pharmacol. Exp. Ther. 1994, 270, 310.
3. Wonnacott, S.; Jackman, S.; Swanson, K. L.; Rapoport, H.;
Albuquerque, E. X. J. Pharmacol. Exp. Ther. 1991, 259, 387.
4. Wright, E.; Gallagher, T.; Sharples, C. G. V.; Wonnacott, S.
Bioorg. Med. Chem. Lett. 1997, 7, 2867.
5. Olesen, P. H.; Swedberg, M. D. B.; Rimvall, K.; Eskesen,
K.; Judge, M. E.; Egebjerg, J.; Hansen, J. B.; Tùnder, J. E.;
Rasmussen, T.; Sheardown, M. J. Society for Neuroscience
Abstracts 1996, 22, 1262.
6. Tùnder, J. E.; Hansen, J. B.; Begtrup, M.; Pettersson, I.;
Rimvall, K.; Christensen, B.; Ehrbar, U.; Olesen, P. H. J.
Med. Chem. 1999, 42, 4970.
(Z)-3-(3-Ethyl-[1,2,4]oxadiazol-5-yl)methylene-1-azabi-
cyclo[2.2.2]octane (Z)-15b. The title compound was pre-
pared in 46% yield as described for (Z)-14b starting
from (Z)-3-(ethoxycarbonyl-methylene)-1-azabicyclo-
[2.2.2]octan ((Z)-13b) and N-hydroxy-propionamidine.
The reaction mixture was stirred for 5 h at 80 ꢀC, before
stirring at 20 ꢀC overnight. Mp 125±127 ꢀC. MS (EI)
m/z 219 (M+). GC: 100% Z. 1H NMR (400 MHz,
DMSO): d 6.69 (s, 1H, CHˆ), 4.44 (s, 2 H, NCH2Cˆ),
3.32 (m, 4H, NCH2), 2.98 (s, 1H, CH), 2.74 (quartet,
2H, J=10 Hz, Ar-CH2), 2.08 (m, 2H, CH2), 1.88 (m,
2H, CH2), 1.25 (t, 3H, J=10 Hz, CH3). Anal. (C12H17
N3O, 1.5 C2H2O4) C, H, N.
7. Arneric, S. P.; Holladay, M. W.; Sullivan, J. P. Exp. Opin.
Invest. Drugs 1996, 5, 79.
8. Newhouse, P. A.; Potter, A.; Levin, E. D. Drugs Aging
1997, 11, 206.
Molecular modelling
9. Vohra, R.; Maclean, D. B. Can. J. Chem. 1994, 72, 1660.
10. Wadsworth, H. J., Hadley, M. S., Wyman, P. A.; Jenkins,
S. M. European Patent 363085A2 1989; Chem. Abstr. 1990, 28,
783.
11. Olesen, P. H.; Swedberg, M. D. B.; Eskesen, K.; Judge,
M. E.; Egebjerg, J.; Tùnder, J. E.; Rasmussen, T.; Sheardown,
M. J.; Rimvall, K. Bioorg. Med. Chem. Lett. 1997, 7, 1963.
12. Swedberg, M. D. B.; Jacobsen, F.; Honore, T. J. Pharma-
col. Exp. Ther. 1995, 274, 1113.
13. Tripos Inc., 1699 South Hanley Road, St Louis, Missouri,
63144, USA.
14. Langlois, Y.; Konopski, L.; Bac, N. V.; Chiaroni, A.;
Riche, C. Tetrahedron Lett. 1990, 31, 1865.
The calculations were performed using Sybyl 6.5.13 The
structures were built by combining and altering stan-
dard fragments from Sybyl. Using Gasteiger±Huckel
charges the structures were minimized until a local
minimum was reached.
Grid search. The conformational search employing
rotation around the single bond connecting the bicycle
and the heteroaromatic ring or the ketone was done
using the grid search command, which contains an
energy minimization method that keeps the torsion
angle under examination constant. The torsional scans
were performed from 0 to 360ꢀ in 5ꢀ intervals, and the
inherent minimization used the TRIPOS force ®eld and
Gasteiger±Huckel charges. The maximum number of
iterations was set to 1000 and the termination gradient to
15. Khatri, N. A.; Schmitthenner, H. F.; Shringarpure, J.;
Weinreb, S. M. J. Am. Chem. Soc. 1981, 103, 6387.
16. The macro for molecular dynamics data procession was
kindly supplied by Inge Thùger Christensen, Novo Nordisk
A/S, Novo Nordisk Park, 2760 Malùv, Denmark.