Structure activity studies of ring e analogues of methyllycaconitine. Part 2: Synthesis of antagonists to the α3β4* nicotinic acetylcholine receptors through modifications to the ester

Article abstract of DOI:10.1016/j.bmcl.2004.05.001

The development of novel agents for the differentiation of neuronal nicotinic acetylcholine receptors (nAChRs) is important for the treatment of a variety of pathological conditions. We have prepared and evaluated a number of simpler analogues of the norditerpeniod alkaloid methyllycaconitine (MLA) in an effort to understand molecular determinants of nAChR?small molecule interactions. We have previously reported the synthesis and evaluation of a series of ring E analogues of MLA. We report here the optimization of the α3β4* functional activity of this series of compounds through modification of the ester.

Full text of DOI:10.1016/j.bmcl.2004.05.001

Bioorganic & Medicinal Chemistry Letters 14 (2004) 3739–3742
Structure activity studies of ring E analogues of methyllycaconitine.
Part 2: Synthesis of antagonists to the a3b4* nicotinic acetylcholine
receptors through modifications to the ester
Stephen C. Bergmeier,a,* Khadiga A. Ismail,b Kristjan M. Arason,a Susan McKay,c
Darrell L. Bryantc and Dennis B. McKayc
aDepartment of Chemistry and Biochemistry, Ohio University, Athens, OH 45701, USA
bDepartment of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Alexandria, Alexandria, Egypt
cDivision of Pharmacology, College of Pharmacy, The Ohio State University, Columbus, OH 43210, USA
Received 23 October 2003; revised 30 April 2004; accepted 3 May 2004
Available online 25 May 2004
Abstract—The development of novel agents for the differentiation of neuronal nicotinic acetylcholine receptors (nAChRs) is
important for the treatment of a variety of pathological conditions. We have prepared and evaluated a number of simpler analogues
of the norditerpeniod alkaloid methyllycaconitine (MLA) in an effort to understand molecular determinants of nAChRꢀsmall
molecule interactions. We have previously reported the synthesis and evaluation of a series of ring E analogues of MLA. We report
here the optimization of the a3b4* functional activity of this series of compounds through modification of the ester.
Ó 2004 Elsevier Ltd. All rights reserved.
The development of agents for the differentiation of
specific subtypes of neuronal nicotinic acetylcholine
receptors (nAChRs) has significant implications for
advancements in the treatment of a variety of patho-
logical conditions.1–4 Neuronal nAChRs are located
throughout the central and peripheral nervous system
and have been implicated in several pathologies2;5;6
including Alzheimer’s disease,7–9 Parkinson’s dis-
ease,10–13 and nicotine addiction.14–17 Currently, several
types of neuronal nAChRs, based on subunit composi-
tion, have been described, including the heteromeric,
a4b2 nAChRs and a3a5b4 nAChRs and the homo-
meric, a7 nAChRs.18 The goal of our research is to
identify and understand molecular determinants of
nAChRꢀsmall molecule interactions.
for a3b4* nAChRs.26 MLA inhibits bovine adrenal
catecholamine secretion (IC50 value, 2.6 lM) mediated
through activation of a3b4* nAChRs.27 The prepara-
tion and evaluation of several bicyclic analogues of
MLA has been reported.28–31 We have prepared and
evaluated a number of simpler analogues of MLA in an
effort to understand the origins of these activities on
multiple nAChRs subtypes27;32;33 (Fig. 1).
A simplification of the MLA core structure provides the
piperidine fragment 2. Further simplification of this
fragment provides lead structure 3. The analogues that
we had initially prepared were simplified derivatives of
MLA that contained only the E ring and the succinim-
idoylanthranilate ester (2). Based upon these early
studies into the structure–activity relationship (SAR) of
compound 3, we determined 3a (R ¼ Ph(CH2)3,
R0 ¼ CH3) to be our lead compound. This compound
has low affinity (IC50 value, 177 lM) for a7 nAChRs and
has no affinity for a3b4* nAChRs; however, 3a appears
to act as a noncompetitive inhibitor (IC50 value, 11 lM)
of a3b4* nAChRs.27 All of these analogues varied sim-
ply through changes in the substitution on the piperidine
nitrogen. One pair of examples, 3b (R ¼ i-Pr,
R0 ¼ CH3) and 3c (R ¼ i-Pr, R0 ¼ H) had the same
Methyllycaconitine (MLA, 1) is a diterpene alkaloid
isolated from plants of the genera Aconitum and Del-
phinium.19–21 MLA is the most potent nonpeptide
nAChR antagonist known, with selectivity for a7
nAChRs,22–25 and moderate affinity (Ki value, 1.3 lM)
* Corresponding author. Tel.: +1-740-517-8462; fax: +1-740-593-0148;
e-mail: bergmeis@ohio.edu
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.05.001

3740
S. C. Bergmeier et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3739–3742
R
H
OMe
OMe
O
O
O
N
R'
Et
N
OH
OH
MeO
O
OMe
O
O
O
N
NEt
N
O
O
4
Ph
O
O
O
O
3
O
O
O
N
N
R
H3C
O
N
R
H3C
N
2
1
O
5
Ph
Figure 1. Methyllycaconitine (MLA), 1, ring E analogue 3 and new analogues 4 and 5.
substituent on the nitrogen (R ¼ i-Pr) and either the
presence or absence of a methyl group on the succini-
mide ring. The differences in a3 nAChR functional
activity between these two compounds (ꢁ20 lM) sug-
gested that modification to this part of the molecule
could be a fertile area for enhancement of the affinity at
R
N
O
O
O
O
R
O
b
a
8
O
N
N
the a3b4* nAChR. We have prepared two series of
molecules in which this general area of the molecule has
been modified. The goals for this work are to optimize
Ph
5
4
Ph
Scheme 2. Reagents and conditions: (a) 10, DCC, 4a, R ¼ 3,3-di-
methyl, 85%; 4b, R ¼ 3,4-dimethyl, 83%; 4c, R ¼ Et, 95%; 4d, R ¼ n-Bu,
97%; 4e, R ¼ octyl, 76%; 4f, R ¼ i-Pr, 98%; 4g, R ¼ cyclopentyl, 75%;
4h, R ¼ allyl, 94%; 4i, R ¼ Bn, 85%. (b) R–PhCO2H, DCC, 5a,
R ¼ 2-Cl, 92%; 5b, R ¼ 2-OMe, 80%; 5c, R ¼ 2-CF3, 86%; 5d, R ¼ 2-
F, 49%; 5e, R ¼ 2-acetamide, 45%; 5f, R ¼ 2-phthalimide, 58%; 5g,
R ¼ 2-phenyl, 80%; 5h, R ¼ 4-phenyl, 81%.
the a3b4* functional activity of this series of compounds
through modification of the ester. In the first series we
have replaced the methyl group on the imide ring with a
variety of alkyl chains, 4. In the second series we have
replaced the entire succinimide ring with other substi-
tuents, 5. For both series of analogues we have retained
the phenylpropyl chain on the piperidine nitrogen.
These succinic anhydrides were coupled to anthranilic
acid to provide acid 10.35 As shown in Scheme 2, these
acids were then coupled to 8 using DCC to provide a
series of ring E analogues (4). The second series of
analogues (5) was prepared using a variety of substituted
benzoic acids which were coupled to 8 using DCC.
The synthesis of the requisite piperidine methanol 8 is
shown in Scheme 1. Alkylation of ethyl nipecotate with
1-bromo-3-phenylpropane followed by reduction of the
ester with LiAlH4 provides 8 in excellent yield.
For the first series of compounds we needed to prepare a
series of substituted succinimido–anthranilic acids. As
shown in Scheme 1, this synthesis starts with either
commercially available or known succinic anhydrides.34
These analogues possess significant inhibitory activity in
functional assays involving a3b4* nAChRs (Table 1).
However, the analogues showed little or no inhibitory
effects on binding to a7, a4b2, or a3b4* nAChRs at
concentrations up to 10 lM (data not shown) support-
ing noncompetitive interactions. Very few competitive
antagonists of a3b4* nAChRs exist. Most a3b4*
nAChR antagonists are noncompetitive and their
potencies are in the micromolar range.36–38 For example,
hexamethoniun and decamethonium have IC50 values of
ꢁ17 lM; mecamylamine, one of the most potent non-
competitive inhibitors, has an IC50 value of 0.1 lM.
Tubocurarine, a competitive antagonist, has an IC50
value of 2 lM. Our novel analogues have comparable
IC50 values of 1–11 lM.
CO2Et
CO2Et
b
a
OH
Ph
N
N
H
N
Ph
6
8
7
R
N
R
O
O
c
O
HO2C
O
O
9
10
Given that the presence or absence of a methyl group at
the 3-position of the imide ring is significant, we wished
to study the consequences of di-substitution at this po-
sition. Compound 4a shows a small but significant
improvement in potency relative to 3a.39
Scheme 1. Reagents and conditions: (a) Ph(CH2)3Br, K2CO3, 85%. (b)
LiAlH4, 95%. (c) Anthranilic acid, 145–160 °C, 0.1 mmHg, 10a,
R ¼ 3,3-dimethyl, 34%; 10b, R ¼ 3,4-dimethyl, 66%; 10c, R ¼ Et,
63%; 10d, R ¼ n-Bu, 65%; 10e, R ¼ octyl, 71%; 10f, R ¼ i-Pr, 69%;
10g, R ¼ cyclopentyl, 65%; 10h, R ¼ allyl, 63%; 10i, R ¼ Bn, 70%.

S. C. Bergmeier et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3739–3742
3741
Table 1. Functional effects of analogues on a3b4* nAChR-stimulated
adrenal catecholamine secretion
of activity. However, replacement of the imide ring (3a)
with a phthalimide ring (5f) significantly increased the
potency (ꢁ10-fold). We next chose to carry out an
isosteric replacement of the imide ring (3a) with a simple
phenyl ring (5g). This again provided a significant level
of enhancement. Movement of the phenyl ring from the
2-position (5g), as in the original molecule, to the 4-
position (5h) resulted in no significant loss of activity.
Compound #
R
Catecholamine secretiona
(IC50 value, lM)
MLA
3a
4a
4b
4c
––
––
2.6 (2.3–3.0)b
11.4 (10.9–11.9)b
7.5 (6.8–7.6)
6.0 (5.5–6.6)
4.5 (4.4–4.5)
1.5 (1.5–1.5)
1.3 (0.7–2.4)
2.5 (2.3–2.6)
1.2 (1.2–1.3)
1.7 (1.7–1.7)
1.3 (1.1–1.4)
3.4 (3.2–3.5)
3.2 (3.1–3.4)
3.2 (3.1–3.2)
5.2 (4.8–5.6)
8.4 (7.8–9.1)
1.3 (1.2–1.6)
1.7 (1.6–1.8)
3.0 (2.3–3.9)
3,3-Dimethyl
trans-3,4-Dimethyl
Et
4d
4e
n-Bu
Octyl
i-Pr
In summary the imide ring can be substituted with 3–7
carbon chains and provide enhanced potency at a3b4*
nAChRs. Perhaps more intriguing is the total replace-
ment of the imide ring with a phthalimide or phenyl ring
to also provide compounds with greatly enhanced po-
tency at a3b4* nAChRs. These studies are beginning to
define a novel noncompetitive binding site on neuronal
nAChRs.
4f
4g
4h
4i
Cyclopentyl
Allyl
Bn
5a
5b
5c
2-Cl
2-OCH3
2-CF3
2-F
5d
5e
2-Acetamide
2-Phthalimide
2-Phenyl
4-Phenyl
It is significant that none of these new compounds (4a–i,
5a–h) showed any affinity for the agonist binding sites of
either a7, a4b2, or a3b4* nAChRs. In a comparison of
several of these compounds (4d,e,g,h,i, 5f, and 5g) with
other known a3b4* nAChR antagonists37 we find that
these ring E analogues with imide substitutions are po-
tent nAChR antagonists. Studies on further modifica-
tions to substitution on the aryl ring as well as the
piperidine ring are underway and will be reported in due
course.
5f
5g
5h
a Secretion studies were performed as previously described.27 Values
represent geometric mean (95% confidence limits) of 5–6 experiments.
b Data from Bryant et al.27
The 3,4-dimethyl imide analogue (4b) was prepared in
an effort to address the issue of rotational isomers
around the aryl–imide bond. Computational results
indicate that the barrier to this rotation is quite low
(ꢁ1 kcal/mol) thus an analogue with a trans disubsti-
tuted dimethyl might show enhanced potency as both
rotational isomers are present at all times. This com-
pound shows a small, but significant increase in potency
relative to 3a. While the enhancements in potency of 4a
and 4b relative to 3a are small (less than twofold), they
do suggest that the stereochemistry of substitution on
the imide ring is an important consideration in the
design of future analogues.
Acknowledgements
We would like to thank the National Institute on Drug
Abuse at the National Institutes of Health for support
of this research (RO3 DA12706 and RO1 DA13939).
K.A.I. would like to thank the Egyptian Cultural and
Educational Bureau for a fellowship.
We next chose to examine the effects of increasing chain
length on the 3-substituent on the imide ring. The ethyl
compound (4c) showed significant improvements (two-
fold) in potency relative to 3a. Increasing the chain
length to an n-Bu or octyl (4d,e) provided significant
enhancement (ꢁ10-fold) in potency relative to 3a. Sub-
stitution of the ethyl chain (4c) with a methyl to provide
an isopropyl group (4f) again provided significant
improvements in potency compared to 3a. Constraining
the methyls of the isopropyl group into a cyclopentyl
group (4g) provided a significant improvement in po-
tency. Inclusion of a p-bond with an allyl (4h) or phenyl
(4i) also provided significantly improved activity.
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