Synthesis and nicotinic receptor activity of a hydroxylated tropane

Article abstract of DOI:10.1016/j.bmcl.2003.09.081

(±)-3α-Hydroxy homoepibatidine 4 has been synthesized from the alkaloid scopolamine 5 and its properties as a nicotinic agonist assessed. While still binding strongly, the compound showed reduced agonist potency for the α₄β₂ nAChR compared with the parent compound epibatidine 1. Compound 4 also displayed generally similar binding and selectivity profiles at α₄β₂, α ₂β₄, α₃β₄, and α₄β₄ nAChR subtypes to those for nicotine.

Full text of DOI:10.1016/j.bmcl.2003.09.081

Bioorganic & Medicinal Chemistry Letters 14 (2004) 271–273
Synthesis and nicotinic receptor activity of a hydroxylated tropane
John B. Bremner,a,* Colette A. Godfrey,a Anders A. Jensenb and Reginald J. Smithc
aDepartment of Chemistry and Institute for Biomolecular Science, University of Wollongong, Wollongong, NSW 2522, Australia
bDepartment of Medicinal Chemistry, The Danish University of Pharmaceutical Sciences, Copenhagen, DK-2100, Denmark
cPhytex Australia Pty Ltd, Glenhaven, NSW 2156, Australia
Received 12 May 2003; accepted 5 September 2003
Abstract—(ꢀ)-3a-Hydroxy homoepibatidine 4 has been synthesized from the alkaloid scopolamine 5 and its properties as a nico-
tinic agonist assessed. While still binding strongly, the compound showed reduced agonist potency for the a4b2 nAChR compared
with the parent compound epibatidine 1. Compound 4 also displayed generally similar binding and selectivity profiles at a4b2, a2b4,
a3b4, and a4b4 nAChR subtypes to those for nicotine.
# 2003 Elsevier Ltd. All rights reserved.
Nicotinic acetylcholine receptors (nAChRs) are widely
distributed throughout the PNS and CNS. Efforts are
now being made to synthesize ligands that exhibit
selectivity for different nAChR subtypes and that also
display selectivity for central over peripheral nAChRs.
As the a4b2 nAChR is one of the most abundant in the
CNS, this has been a major target.1 The powerful analge-
sic epibatidine 1 [(+)-enantiomer shown] has a high affi-
nity for the a4b2 nAChR and has therefore acted as a
structural template for many nicotinic ligands produced in
this field. Epibatidine is also a full agonist at other nAChR
subtypes, resulting in high toxicity as a result of deleter-
ious actions on CNS responses and respiratory, gastro-
intestinal and cardiovascular function.2
Epibatidine analogues have so far failed to yield a
highly selective agonist,3 and the search is complicated
by the complex pharmacology of the nAChR subtypes
and lack of a reliable pharmacophore.4 One of the most
promising analogues to date is
a 3,8-diazabicy-
clo[3.2.1]octane 2. It showed no life-threatening side
effects in vivo (mouse) at antinociceptive doses, evi-
dently due to selective binding to the a4b2 receptor sub-
type, while lacking strong binding to neuromuscular
junction nAChRs.5ꢁ7 Another seven-membered ring ana-
logue of epibatidine, homoepibatidine 3, retains analgesic
potency but also shows little selectivity (Fig. 1).8,9 Most
bicyclic analogues experiment with positioning of the
pyridyl substituent, retaining an aliphatic region, thus
Figure 1. Chemical structure of epibatidine and analogues.
* Corresponding author. Tel.: +61-2-4221-4255; fax: +61-2-4221-5431; e-mail: john_bremner@uow.edu.au
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.09.081

272
J. B. Bremner et al. / Bioorg. Med. Chem. Lett. 14 (2004) 271–273
incorporation of polar substituents in 3, capable of both
H-bond donation and acceptance, was of interest to
explore the potential effects on receptor binding and
selectivity.10 To this end, the hydroxylated tropane
derivative (ꢀ)-3a-hydroxy homoepibatidine 4 was syn-
thesized from the tropane alkaloid scopolamine 5, and
receptor binding studies conducted. The results are dis-
cussed in this paper.
recombinant nAChRs expressed in HEK293 or tsA
cells.15 Furthermore, the compound was characterized
functionally at the a3b4 nAChR in a FLIPR1 Mem-
brane Potential Assay using a NOVOstarTM. The bind-
ing assays and functional assay were performed
essentially as previously described.15 Affinity and com-
parison data are shown in Table 1. At native nAChRs
compound 4 had a lower binding affinity8 than either
enantiomer of homoepibatidine 3. However, at these
native receptors, analogue 4 showed a binding affinity
similar to that of (S)-nicotine but 100-fold lower bind-
ing affinity than epibatidine. This was also true in the
The synthesis of (ꢀ)-3a-hydroxy homoepibatidine 4 is
summarized in Scheme 1. Scopolamine 5 was used as the
starting material, as supplied by Phytex Australia. Zinc–
copper reduction of 5 in ethanol gave 6,7-dehy-
drohyoscyamine 6 (96%),11 followed by base hydrolysis
to provide tropenol 7 (90%).12 Protection of the hydroxy
group as a TBDMS ether to give 8 (90%), followed by
demethylation and protection of the nitrogen with ethyl
chloroformate gave the carbamate 9 (58%). The reduc-
tive Heck coupling between the commercially available
2-chloro-5-iodopyridine and 9 was carried out in a solu-
tion of DMF containing tetrakis(triphenylphos-
phine)palladium(0), piperidine and formic acid.9,10 The
coupled product 10 was obtained in fair yield (49%).
Finally, cleavage of both the carbamate and TBDMS
ether with trimethylsilyl iodide afforded the target
molecule 4 (45%).13 The exo stereochemistry for the 2-
chloropyridyl substituent was confirmed from the 1H
NMR spectrum with the coupling constants for the
doublet of doublets signal being 9.3 and 5.4 Hz (H6
endo–H7 endo and H6 endo–H7 exo, respectively, with
no coupling to H5); similar coupling constants were
observed14 in homoepibatidine 3.
Table 1. Binding affinities (Ki, nM) for 4 and standard nicotinic
agonists in a [3H]-epibatidine binding assay to native and recombinant
nAChRsa
Compd
Rat brain tissue a4b2
a2b4
a3b4
a4b4
Epibatidine
(S)-Nicotine
4
0.034
27
18
0.046
9.1
7.4
0.091
103
31
0.457
320
88
0.072
129
38
a Values are determined using the equation Ki ¼ IC50=ð1 þ ½LŠ=KDÞ,
where [L] is the radioligand concentration, n the Hill coefficient, and
KD the dissociation constant and are the means of 3–4 independent
assays.
Table 2. Agonistic potencies of 4 and standard nicotinic agonists at
the a3b4 nAChR in a FLIPR1 membrane potential assay
Compd
EC50 (mM)a
Epibatidine
(S)-Nicotine
4
0.019
4.3
2.1
The pharmacological activity of 4 was evaluated in a
[3H]-epibatidine binding assay at native nAChRs and at
a Means of three independent assays.
Scheme 1. Synthesis of 3a-hydroxy homoepibatidine. Reagents and conditions: (a) Zn–Cu, EtOH; (b) NaOH, MeOH; (c) TBDMSTf, 2,6-lutidine,
DCM; (d) ClCO2Et, K2CO3, CHCl3; (e) 2-chloro-5-iodopyridine, Pd(PPh3)4, HCO2H, piperidine, DMF; (f) Me3SiI, CHCl3.

J. B. Bremner et al. / Bioorg. Med. Chem. Lett. 14 (2004) 271–273
273
recombinant a4b2 binding assay and the functional
assay at the a3b4 nAChR. Compound 4 displayed low
nanomolar binding affinities at all recombinant nAChR
subtypes studied and is an agonist at the a3b4 nAChR
(Table 2).
Anderson, D. J.; Kuntzweiler, T. A.; Colombo, D.; Toma,
L. J. Med. Chem. 1998, 41, 674.
6. For further ring analogues, see: Toma, L.; Quadrelli, P.;
Bunnelle, W. H.; Anderson, D. J.; Meyer, M. D.; Cignar-
ella, G.; Gelain, A.; Barlocco, D. J. Med. Chem. 2002, 45,
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zon, P.; Donnelly-Roberts, D.; Puttfarcken, P. S.; Bitner,
R. S.; Diaz, A.; Dickenson, A. H.; Porsolt, R. D.; Wil-
liams, M.; Arneric, S. P. Science 1998, 279, 77.
8. Malpass, J. R.; Hemmings, D. A.; Wallis, A. L.; Fletcher,
S. R.; Patel, S. J. Chem. Soc., Perkin Trans. 1 2001, 1044.
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Chem. Lett. 1996, 6, 279.
Acknowledgements
We are grateful to the Australian Research Council and
Phytex Australia for an APAI scholarship and support
to CAG. Drs. Ken Kellar, Yingxian Xiao and Joe
Henry Steinbach are thanked for generously providing
us with the a3b4 and a4b2 nAChR cell lines. A.A.J. was
supported by the Lundbeck Foundation.
10. For recently published hydroxylated epibatidine analo-
gues see: Wei, Z.-L.; George, C.; Kozikowski, A. P.
Tetrahedron Lett. 2003, 44, 3847.
11. Bremner, J. B.; Smith, R. J.; Tarrant, G. J. Tetrahedron
Lett. 1996, 37, 97.
12. Aberle, N. S.; Ganesan, A.; Lambert, J. N.; Sauber, S.;
Smith, R. Tetrahedron Lett. 2001, 42, 1975.
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J=2.7 Hz, H20), 7.79 (1H, dd, J=8.7 Hz, J=2.7 Hz, H40),
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