3-(2-Aminoethyl)pyridine analogs as α4β2 nicotinic cholinergic receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2005.06.053

An examination of several 3-(2-aminoethyl)pyridine analogs suggests that they likely orient at α4β2 nicotinic cholinergic receptors in a different fashion than their correspondingly substituted nicotine analogs.

Full text of DOI:10.1016/j.bmcl.2005.06.053

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4308–4312
3-(2-Aminoethyl)pyridine analogs as a4b2 nicotinic
cholinergic receptor ligands
Małgorzata Dukat,a Anna Ramunno,a Rita Banzi,a M. Imad Damaj,b
Billy Martinb and Richard A. Glennona,b,
*
aDepartment of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, Richmond, VA 23298-0540, USA
bDepartment of Pharmacology and Toxicology, School of Medicine, Virginia Commonwealth University,
Richmond, VA 23298-0540, USA
Received 15 April 2005; revised 16 June 2005; accepted 16 June 2005
Available online 21 July 2005
Abstract—An examination of several 3-(2-aminoethyl)pyridine analogs suggests that they likely orient at a4b2 nicotinic cholinergic
receptors in a different fashion than their correspondingly substituted nicotine analogs.
Ó 2005 Elsevier Ltd. All rights reserved.
The neurotransmitter acetylcholine produces many of
its effects by interaction with muscarinic and nicotinic
acetylcholinergic receptors. Nicotine (1a) displays selec-
tivity for nicotinic cholinergic (nACh) receptors; how-
ever, these receptors can be classified as belonging to
multiple subtypes depending upon their specific subunit
composition.1 The major population of neuronal nACh
receptors are the a4b2 subtype. S(À)Nicotine (Ki ca.
2 nM) binds with high affinity at a4b2 nACh receptors;
the aminoethylpyridine (i.e., AEP) 2a (Ki = 18 nM),
which was derived from nicotine in a systematic fash-
ion,2,3 retains affinity for this receptor population.
Compound 2a might also be viewed as a partial
structure of the nACh receptor ligand homoazanicotine
(3; Ki = 7.8 nM).4
structural resemblance both to 3 and nicotine (1a), it
was of interest to determine if analogs of 2a also bind
in a similar fashion. Accordingly, we examined several
ring-substituted analogs of 2a for comparison with their
corresponding nicotine analogs 1.
1. Synthesis
Compounds (À)1c and (À)1d, the 4-methoxy and 4-ami-
no analogs of nicotine, respectively, were prepared
following a general approach described by Shibagaky
et al.,5 and Ochiai6 using the nitrocotinine N-oxide 6
as a common intermediate (Scheme 1). Compound 6
was converted to methoxy analog 7 with sodium
methoxide, and the N-oxide function was removed by
treatment with PBr3 to afford 8a. Reduction of the
carbonyl group using diborane afforded 1c. The known
free base5 of 4-aminonicotine (1d) was prepared from
6 according to a literature procedure and converted to
its oxalate salt (Scheme 1; Table 1).
Resolution of ( )5-bromonicotine (1e)7 was accom-
plished using (À)L- and (+)D-di-O,O-p-toluoyltartaric
acid to yield S(À)1e and R(+)1e. The latter two
compounds have been previously characterized as their
dibenzoyl-L-tartrate and di-p-toluoyl-D-tartrate salts,
respectively.8 The enantiomeric purity of the two iso-
mers was monitored by H NMR using the chiral shift
reagent S(+)-2,2,2-trifluoro-1-(9-anthryl)ethanol, and
optical rotations were determined on their maleate salts
On the basis that parallel substituent changes result in
parallel shifts in affinity, we have concluded that analogs
of 3 might bind at nACh receptors in a manner that
mimics nicotine.4 Because compound 2a bears some
1
Keywords: Pharmacophore; Radioligand binding.
*
Corresponding author. Tel.: +1 804 828 8487; fax: +1 804 828
7404; e-mail: glennon@hsc.vcu.edu
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.053

M. Dukat et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4308–4312
4309
Scheme 2. Reagents: (a) i—SOCl2, ii—absolute EtOH; (b) (n-Pr)3Al,
dioxane, Pd(Ph3P)4; (c) N-vinylpyrrolidinone, NaH, toluene; (d)
NaBH4, MeOH; (e) NaCNBH3, HCHO, CH3CN.
Scheme 1. Reagents: (a) H2O2 (30%), CH3COOH; (b) HNO3/H2SO4;
(c) NaOMe, anhydrous MeOH; (d) H2/Raney-Ni, MeOH; (e) PBr3,
CHCl3; (f) BH3/THF, dry THF.
Several different procedures were used in the synthesis of
the aminoethylpyridines 2. Amines 2b, 2e, and 2l were
obtained as shown in Scheme 3 from commercially
available pyridine acetic acid or 5-bromopyridine acetic
acid; 6-bromopyridine acetic acid12 was prepared in situ
from (6-bromopyrid-3-yl)acetonitrile13 according to a
literature method. Acids 12 were converted to amides
13 using 1-[3-(dimethylamino)propyl]-3-ethylcarbodii-
mide hydrochloride (EDCI), and reduced with diborane
to the target amines. Treatment of 2e with (n-Pr)3Al, as
described above for 1i, gave 5-propyl analog 2i.
(Table 1). Compound 1h was prepared according to the
procedure of Jacob et al.9
The 5-propyl analog 1i (Scheme 2) was obtained by
reaction of ethyl 5-bromonicotinate with (n-Pr)3Al in
the presence of Pd(Ph3P)4 using a procedure adapted
from Bracher and Papke,10 followed by formation
of 5-n-propylnornicotine which was subsequently
reductively methylated using the Borch and Hassid
procedure.11
Table 1. Physicochemical properties of substituted nicotine analogs 1 and aminoethylpyridine analogs 2
X
R
Recrystallization solvent
Melting point (°C)
Empirical formulaa
b
c
(À)1c
(À)1d
(À)1e
(+)1e
1h
4-OCH3
d
Me
Me
Me
Me
Me
Me
H
—
78–80
C11H16N2 0.3H2O
C10H15N2 2C2H2O4
4-NH2
EtOH/Et2O
2-PrOH/EtOH
2-PrOH/EtOH
MeOH/Et2O
2-PrOH
197–198
100–102
100–102
129–130
208–210
79–83
5-Bre
5-Bre
5-CH3
C10H13BrN2 C4H4O4
C10H13BrN2 C4H4O4
C11H16N2
1i
2b
5-CH2CH2CH3
H
4-OCH3
C13H20N2 2HCl 0.5 H2O
C9H14N2 2C4H4O4
EtOH/Et2O
EtOH/Et2O
MeOH
2c
2d
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
139–140
203–205
124–125
125–127
102–104
97–98
C11H18N2O 1.5C4H4O4
C10H17N3 2.5C2H2O4 0.25H2O
C10H15BrN2 C2H2O4
C10H15ClN2 C2H2O4 0.25H2O
C11H18N2O 2C4H4O4
C11H16N2O 2C2H2O4
C13H22N2 2C2H2O4 0.25H2O
C11H18N2 2C4H4O4
4-NH2
5-Br
2e
EtOH/Et2O
2-PrOH/EtOAc
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
2f
2g
5-Cl
5-OCH3
5-CH3
2h
2i
5-CH2CH2CH3
6-CH3
6-Br
119–121
71–74
74–75
2j
2l
2n
C10H15BrN2 C4H4O4
C18H23ClN2 2C2H2O4 0.25H2O
C19H26N2O 2C2H2O4 H2O
6-CH2CH2Ph(4-Cl)
6-CH2CH2Ph(4-OMe)
137–141
138–140
2o
a Compounds analyzed within 0.4% of theory for C, H, and N. C4H4O4 = maleate salt; C2H2O4 = oxalate salt. All compounds were homogeneous as
determined by thin-layer chromatographic analysis, and assigned structures are consistent with 1H NMR spectra.
b Compound (À)1c: [a]D = À140° (c = 1%, MeOH, 27 °C; free base).
c Purified by silica gel column chromatography (eluent: CH2Cl2/MeOH, 8.5:1.5).
d Compound (À)1d: [a]D = À87° (c = 0.7%, MeOH, 24 °C; free base).
e Compounds (À)- and (+)1e: [a]D = À100° (c = 1.45%, EtOH, 24 °C); [a]D = 97° (c = 1.45%, EtOH, 25 °C).

4310
M. Dukat et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4308–4312
Scheme 3. Reagents and conditions: (a) EDCI, HOBT, CH2Cl2, rt
and either H2NC2H5 or CH3-NH-C2H5; (b) BH3/THF, D, dry THF;
(c) (n-Pr)3Al, dioxane, Pd(Ph3P)4.
Scheme 5. Reagents: (a) NaHDMS; NaOCl, THF; (b) H2, 10% Pd/C,
MeOH; (c) i—SOCl2, CH2Cl2; ii—H2, 10% Pd/C, MeOH; (d) BH3/
THF, dry THF.
Modification of a procedure described by Dormoy and
Heymes14 was employed for the preparation of 2c and
2d (Scheme 4). The reported N,N-dimethyl-[2-(4-nitro-
1-oxypyridin-3-yl)vinyl]amine14 (14) was converted to
its homologous amine 15; using the procedure of Ochiai6
the intermediate was treated with sodium methoxide to
form 16, and deprotection to 2c was achieved using
H2/Raney-Ni. Compound 2d was obtained directly from
15 by reduction with H2/Raney-Ni.
CH3
CH3
CH3
CH3
O
O
N
N
a
O
O
N
H3C
N
22 R = OCH3
23 R = NO2
21
R
b,c
CH3
CH3
N
d
For the synthesis of 2g, 2h, and 2j, the appropriate 5-
or 6-substituted methyl nicotinate was treated with
N-methyl-N-ethylcyanamide15 (18) to yield the corre-
sponding glyoxylamides 19 (Scheme 5). Two-step reduc-
tion of 19 afforded the desired products. Intermediates
20 were occasionally isolated, but were not character-
ized. Methyl 5-methoxynicotinate16 used in the prepara-
tion of 2g was prepared from commercially available
methyl 5-hydroxynicotinate by treatment with
diazomethane.
2o
O
N
(R = OCH3)
R
24 R = OCH3
25 R = NH2
(R = NH2)
e
CH3
CH3
N
d
2n
O
N
26
Cl
Scheme 6. Reagents: (a) (4-R)PhCHO, Ac2O; (b) H2, 10% Pd/C,
MeOH; (c) i—SOCl2, CH2Cl2; ii—H2, 10% Pd/C, MeOH; (d) BH3/THF,
dry THF; (e) NaNO2, HCl, CuCl2.
The 6-phenylethyl derivatives 2o and 2n were prepared
from compound 21—an intermediate in the synthesis
of 2j (Scheme 6). Condensation of 21 with 4-methoxy-
or 4-nitrobenzaldehyde afforded intermediates 22 and
23, respectively. The remainder of the synthetic route
was similar to that shown in Scheme 5 except that amine
25 was converted via a Sandmeyer-type reaction to its
corresponding chloro analog 26 prior to reduction with
diborane.
2. Pharmacology
The results of this investigation can be viewed from
three perspectives. First, the binding data (Table 2) ex-
tend what is currently known about the structure–affin-
ity relationships for the binding of nicotine analogs at
a4b2 receptors and, in particular, for 4-substituted nico-
tine analogs—a position that has not previously received
much attention. Second, they provide a structure–affin-
ity assessment for the binding of aminoethylpyridine-
type nicotinic ligands. And third, they allow
comparisons to be made between the nicotine and
aminoethylpyridine series. In studies aimed at pharma-
cophore elucidation, there is evidence that certain
Scheme 4. Reagents: (a) N-Methyl-N-ethylamine, DMF, 100 °C, 4 h;
(b) H2, Raney-Ni, MeOH; (c) NaOMe, anhydrous MeOH.

M. Dukat et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4308–4312
Table 2. Binding of agents at nACh receptorsa
4311
X
R
nACh receptor affinity (Ki, nM SEM)b
( 0.4) 2a
H
Me
H
(À)1a
1b
1.8
30
18
H
2b
2c
2d
2e
1780
1840
>10,000
360
( 100)
( 460)
4-OCH3
4-NH2
5-Br
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
(À)1c
(À)1d
1e
2015
3150
( 720)
( 1420)
6.9
5.4
270
( 90)
(À)1e
(+)1e
( 0.5)
( 50)
—
—
5-Cl
5-OCH3
5-CH3
—
2f
310
19
( 30)
( 4)
1g
1h
1i
14
1.8
2g
2h
2i
( 0.2)
( 0.4)
370
52
( 40)
( 3)
( 30)
5-CH2CH2CH3
6-CH3
6-Cl
2.1
1.8
0.6
0.5
1j
2j
240
70
1k
1l
2k
2l
6-Br
83
( 4)
6-CH2CH2Ph
6-CH2CH2Ph(4-Cl)
6-CH2CH2Ph(4-OMe)
1m
1n
1o
15
2m
2n
2o
>10,000
>10,000
>10,000
40
10
a Radioligand binding assays performed as previously described.24
b Ki values previously reported for certain compounds and are included here for comparison: 1b,1 1e,18 1g,18 1j–1l,19 1m–1o,25 2a,2 2k,2 and 2m.25
nicotinoids bind in a manner similar to that of nicotine,
whereas others do not.17 A comparison of aminoethyl-
pyridine analogs with their similarly substituted nicotine
analogs can provide information on how these two ser-
ies bind relative to one another.
(Table 2); although certain analogs, such as 5-methoxy
AEP (2g; Ki = 19 nM compared with 1g; Ki = 14 nM),
bind with an affinity comparable to their nicotine coun-
terparts, most AEP analogs bind with from about 50- to
>200-fold lower affinity. In particular, the 6-phenylethyl
substituted compound 2o (Ki > 10,000 nM) binds with
>1000-fold reduced affinity relative to 1o (Ki = 10 nM).
With regard to nicotine, it was found that introduction
of polar substituents at the ring 4-position results in a
>1000-fold reduction in affinity; i.e., the 4-methoxy
and 4-amino analogs (À)1c and (À)1d (Ki = 2015 and
3150 nM, respectively) bind with substantially lower
affinity than (À)nicotine (Ki = 1.8 nM). In contrast, the
5-position seems to tolerate a variety of substitutents
including a methyl (1h; Ki = 1.8 nM) and an n-propyl
(1i; Ki = 2.1 nM) group. We have previously examined
the effect of a 5-bromo and 5-methoxy group on the
affinity of nicotine,18 as well as the influence of substitu-
ents at the 6-position.19
In general, it would seem that the nicotine (i.e., 1) and
AEP (i.e., 2) series are affected differently by substituent
modifications. In fact, a plot of pKi values for ten 2-se-
ries compounds (2a–c, 2e, 2g–l) against the pKi values of
their corresponding nicotine analogs indicated only a
very modest correlation (r = 0.604). It would appear
that the AEP analogs 2 and nicotine analogs 1 might
not orient similarly upon binding at a4b2 nACh recep-
tors. This is perhaps most pronounced with the 6-phen-
ylethyl-substituted compounds (2m–o—which could not
be included in the correlation due to their indeterminate
receptor affinities).
We have described in detail the pharmacology of ( )5-
bromonicotine (1e).18 Because (À)nicotine binds with
at least 20-fold higher affinity than its (+)-enantiomer,
and because the same stereochemical effect was observed
with several other 6-substituted compounds,1 it was felt
important to make at least one enantiomeric potency
comparison with a 5-substituted analog. Consequently,
we resolved 1e. S(À)5-Bromonicotine (Ki = 5.4 nM)
was found to bind with 50-fold higher affinity than its
(+)isomer (+)1e (Ki = 270 nM).20
It is unknown how these ligands bind relative to one
another. The AEP analogs 2 possess a longer internitro-
gen distance, and greater conformational flexibility,
than the corresponding nicotine analogs.21 Nevertheless,
according to recent vector pharmacophore models for
a4b2 binding that seemingly account for conformation-
ally flexible agents with various internitrogen distanc-
es,22,23 the binding of AEP analogs should not be
excluded.17,21 Indeed, many of the AEP analogs 2 bind
at a4b2 receptors and probably share some receptor
binding features with nicotine because binding is
competitive. But, it is unlikely that binding occurs in a
Presented for the first time is the binding of variously
substituted aminoethylpyridine (AEP; 2) analogs. AEP
2a binds with 10-fold lower affinity than nicotine

4312
M. Dukat et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4308–4312
8. Bleicher, L. S.; Cosford, N. D. P.; Herbaut, A.; McCal-
lum, J. S.; McDonald, I. A. J. Org. Chem. 1998, 63, 1109.
9. Jacob, P.; Benowitz, N. L.; Yu, L.; Shulgin, A. Anal.
Chem. 1986, 58, 2218.
10. Bracher, F.; Papke, T. Monatsh. Chem. 1995, 126, 805.
11. Borch, R. F.; Hassid, A. T. J. Org. Chem. 1972, 37,
1673.
12. Hendrix, M.; Kleymann, G.; Betz, U.; Baumeister, J.;
Bender, W.; Eckenberg, P.; Fischer, R.; Handke, G.;
Henninger, K.; Jensen, A.; Keldenich, J.; Schneider, U.;
Weber, O. German Patent DE 10044358 A1, 2002.
13. Schaetzer, J.; Wenger, J.; Berteina-Raboin, S.; Stoller, A.;
Nebel, K.; Hall, R. G. WO Patent 03/050087, 2003.
14. Dormoy, J. R.; Heymes, A. Tetrahedron 1993, 49, 2885.
15. Chiba, T.; Takata, Y. J. Org. Chem. 1977, 42, 2973.
16. Urban, R.; Schnider, O. Helv. Chim. Acta 1964, 47, 363.
17. Glennon, R. A.; Dukat, M. Bioorg. Med. Chem. Lett.
2004, 14, 1841.
manner where the pyridine nuclei of 1 and 2 are strictly
superimposed. Consequently, parallel substituent chang-
es might not necessarily result in parallel affinity shifts;
this is supported by the present results. Compounds 2
also possess structural similarity to homoazanicotine 3
and, in fact, might be viewed as abridged, ring-open
analogs of 3. Yet, we have found that the homoazanic-
otine analogs seem to bind in a manner consistent with
that of their corresponding nicotine counterparts.4 One
possibility we already suggested4 to account for the
binding of homoazanicotine analogs is that, upon pro-
tonation, the charge is dispersed over the entire amidine
moiety; obviously, this is something that cannot occur
with the AEP analogs because they lack this
functionality.
In addition to providing new structure–affinity informa-
tion on the binding of nicotine analogs 1 and 3-(2-ami-
noethyl)pyridine analogs 2 at a4b2 nACh receptors,
the present results suggest that despite their structural
similarity the two series likely orient differently upon
interaction with the receptor.
18. Dukat, M.; Damaj, I. M.; Young, R.; Vann, R.; Collins,
A. C.; Marks, M. J.; Martin, B. R.; Glennon, R. A. Eur. J.
Pharmacol. 2002, 435, 171.
19. Dukat, M.; Dowd, M.; Damaj, I.; Martin, B.; El-Zahabi,
M. A.; Glennon, R. A. Eur. J. Med. Chem. 1999, 34, 31.
20. The two optical isomers of 5-bromonicotine were submit-
ted to the NIMH Psychoactive Drug Screening Program
to obtain a preliminary binding profile at several nACh
receptor subtypes. Standard binding protocols were
employed; for methods, see: http://kidb.cwru.edu/nimh/
binding.php. S(À)5-Bromonicotine was found to bind
with slightly greater selectivity than (À)nicotine at the
different nACh receptor types (receptor type followed in
parenthesis by Ki values for (À)1e and (À)1a): a2b2
(55 nM; 13 nM), a2b4 (740 nM; 70 nM), a3b2 (135 nM;
45 nM), a3b4 (5000 nM; 390 nM), a4b4 (650 nM; 50 nM).
The R(+)isomer of 5-bromonicotine displayed lower
affinity (by at least 10-fold) than (À)1e for each receptor
type examined.
Acknowledgments
This work was supported in part by DA 05274. R.B. was
supported by a Research Fellowship from the Depart-
ment of Pharmaceutical Science, Universita degli Studi
di Bologna-Sede di Bologna, Italy. The NIMH PDSP
is also acknowledged for providing the non-a4b2 bind-
ing data on the isomers of 5-bromonicotine.
21. Glennon, R. A.; Dukat, M.; Liao, L. Curr. Top. Med.
Chem. 2004, 4, 631.
References and notes
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3. Glennon, R. A.; Dukat, M. In Neuronal Nicotinic Recep-
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reported18 using rat brain (minus cerebellum) homoge-
nates and [3H]S(À)nicotine. IC50 values were determined
from a plot of the log concentration versus percent
displacement and converted to Ki values (at least in
triplicate).
25. Ramunno, A.; Dukat, M.; Lee, M.; Young, R.; El-Zahabi,
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