trans-3,4-Dimethyl-4-(3-carboxamidophenyl)piperidines: A novel class of μ-Selective opioid antagonists

Article abstract of DOI:10.1016/j.bmcl.2003.09.012

trans-3,4-Dimethyl-4-(3-carboxamidophenyl)piperidines constitute a novel class of μ opioid receptor antagonists. The CONH₂ group was found to be an effective isostere of the phenolic OH moiety. Structure-activity relationships at the piperidine

Full text of DOI:10.1016/j.bmcl.2003.09.012

Bioorganic & Medicinal Chemistry Letters 13 (2003) 4459–4462
trans-3,4-Dimethyl-4-(3-carboxamidophenyl)piperidines:
A Novel Class of ꢀ-Selective Opioid Antagonists
Bertrand Le Bourdonnec,a,* Serge Belanger,b Joel A. Cassel,b
Gabriel J. Stabley,b Robert N. DeHavenb and Roland E. Dollea
aDepartment of Chemistry, Adolor Corporation, 700 Pennsylvania Drive, Exton, PA 19341, USA
bDepartment of Pharmacology, Adolor Corporation, 700 Pennsylvania Drive, Exton, PA 19341, USA
Received 13 June 2003; accepted 4 September 2003
Abstract—trans-3,4-Dimethyl-4-(3-carboxamidophenyl)piperidines constitute a novel class of m opioid receptor antagonists. The
CONH2 group was found to be an effective isostere of the phenolic OH moiety. Structure–activity relationships at the piperidine
nitrogen position led to the identification of several ligands displaying high affinity toward the cloned human m opioid receptors,
good selectivity m/d, m/k, and potent in vitro antagonist activity.
# 2003 Elsevier Ltd. All rights reserved.
In 1978, Zimmerman and collaborators described the
discovery of opioid antagonist activity in a series of
trans - 3,4 - dimethyl - 4 - (3 - hydroxyphenyl)piperidines
(Formula A).1
scientific literature.6 Recently, Wentland and collabora-
tors explored the SAR at the 8-position of cyclazocine
(1), a mixed m/k agonist, in order to identify a potential
bioisostere of the phenolic OH group.7ꢀ9 Results from
these studies showed that the 8-carboxamido analogue
of cyclazocine (2) was found to have high affinity and
agonist activity at the m and k opioid receptors, com-
parable to cyclazocine.
We now wish to report the synthesis, opioid receptor
binding properties and in vitro functional activity of a
series of trans-3,4-dimethyl-4-(3-substituted-phenyl)pi-
peridines. The N-phenethyl trans-3,4-dimethyl-4-(3-
hydroxyphenyl)piperidine (3, Table 1) has been pre-
viously reported to bind opioid receptors with good
affinity [Ki(m)=1.5 nM; Ki(k)=52 nM].2 The in vitro
affinity of 3 measured at cloned human opioid receptors
was in agreement with literature data. On the basis of
the structure of 3, the phenolic OH group was replaced
by various moieties (Table 1) in order to define the SAR
at the phenolic OH position.
These 4-phenyl piperidine antagonists were structurally
unique, since prior to their discovery, opioid antagonists
were generally N-allyl or N-methylcyclopropyl analo-
gues of morphine. The opioid antagonist activity in the
trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines was
a consequence of substitution at the 3-position of the
piperidine ring. Structure–activity relationship (SAR) in
this trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine
series has been focused largely on the substitution of the
piperidine nitrogen.2ꢀ5 Hence, modification of the OH
phenolic position of the trans-3,4-dimethyl-4-(3-hydroxy-
phenyl)piperidines has never been described in the
The target compounds 4–11 were prepared according
to Scheme 1. Condensation of (+)-4(R)-(3-hydroxy-
phenyl)-3(R),4-dimethyl-1-piperidine10 (12) with phe-
nylacetaldehyde under reductive amination conditions
afforded the derivative 3. Palladium catalyzed carbo-
nylation of the triflate derived from 3 provided the
methyl ester 9, which was hydrolyzed under basic con-
ditions to give the carboxylic acid 11. Coupling of 11
*Corresponding author. Tel.: +1-484-595-1061; fax: +1-484-595-
1551; e-mail: blebourdonnec@adolor.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.09.012

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B. Le Bourdonnecet al. / Bioorg. Med. Chem. Lett. 13 (2003) 4459–4462
Table 1. Opioid receptor (m, k and d) binding data and in vitro antagonist activity (m) of N-phenethyl-trans-3,4-dimethyl-4-(3-substituted-phenyl)
piperidines
Compd
R
Ki(m) (nM)a or % inh.@10 mMc IC50(m) (nM)b Ki(k) (nM)a or % inh.@10 mMc Ki(d) (nM)a or % inh.@10 mMc
Naloxone
3
4
5
6
7
8
9
10
11
3.7
1.9
4.7
11
68
71
150
850
1200
25%
7.3
2.0
1.9
2.1
16
24
ndd
nd
nd
nd
9.2
17
95
840
1100
1300
1400
44%
23%
9%
33
33
72
270
51%
1200
50%
18%
17%
8%
OH
CONH2
CH2OH
NHCOCH3
CONHCH3
NH2
CO2CH3
CON(CH3)2
CO2H
aThe potencies of the compounds were determined by testing the ability of a range of concentrations of each compound to inhibit the binding of the
non-selective opioid antagonist, [3H]diprenorphine, to cloned human m, k, and d opioid receptors, expressed in separate cell lines.11 Ki values are
geometric means computed from at least three separate determinations.
bThe potencies of the antagonists were assessed by their abilities to inhibit agonist (loperamide) stimulated [35S]GTPgS binding to membranes con-
taining the cloned human m opioid receptor.
c% inhibition of [3H]diprenorphine binding to the cloned human m, k, and d opioid receptors using a concentration of the competitor of 10 mM.
dNot determined.
Scheme 2. Reagents and conditions: (a) Boc2O, Et3N, THF, 25 ꢁC,
100%; (b) C6H5N(SO2CF3)2, Et3N, CH2Cl2, 25 ꢁC, 74%; (c) CO(g),
Pd(OAc)2, DPPF, Et3N, DMSO/MeOH, 65 ꢁC, 75%; (d) HCl/Et2O,
MeOH, reflux, 98%; (e) 2-(trimethylsilyl)ethyl-p-nitrophenylcarbo-
nate, (iPr)2EtN, CH3CN, 25 ꢁC, 92%; (f) LiOH, H2O/THF, 25 ꢁC,
97%; (g) Fmoc-deprotected Rink amide resin, HATU, HOBt,
(iPr)2EtN, CH2Cl2/DMF, 25 ꢁC; (h) TBAF, THF, 25 ꢁC; (i) aldehyde
or ketone, NMP/EtOH, BH3/C5H5N, 25 ꢁC; (j) TFA/CH2Cl2.
of the resulting Boc protected aniline derivative afforded
the derivative 8. Acylation of 8 with acetyl chloride in
the presence of triethylamine provided the amide 6.
Opioid receptor binding data are found in Table 1.11
Compounds 3–11 were tested for their affinities toward
Scheme 1. Reagents and conditions: (a) C6H5CH2CHO, BH3/C5H5N,
EtOH, 25 ꢁC, 71%; (b) C6H5N(SO2CF3)2, Et3N, CH2Cl2, 25 ꢁC, 67%;
(c) CO(g), Pd(OAc)2, DPPF, Et3N, DMSO/MeOH, 65 ꢁC, 92%; (d) aq
the cloned human m, d and k opioid receptors as mea-
sured by their abilities to displace [3H]-diprenorphine
from its specific binding sites.11 The antagonist poten-
cies of selected compounds (Ki <100 nM) were assessed
by their abilities to inhibit agonist (loperamide)-stim-
ulated guanosine 50 - O - (3 - [35S]thio)triphosphate
([35S]GTPgS) binding to membranes containing m
opioid receptors.11 The antagonist potencies of these
selected compounds were expressed as IC50 values. No
agonist activity was detectable for compounds 3–11 at
concentrations up to 10 mM. The primary carboxamido
analogue 4 derived from 3 displayed high affinity for the
cloned human m opioid receptor, and good k and d
.
NaOH, THF, reflux, 95%; (e) CH3NH2 HCl, (CH3)2NH or NH4Cl,
EDCI, HOBt, Et3N, DMF, 25 ꢁC, 63% (7), 63% (10), 50% (4);
(f) LiAlH4, THF, 25 ꢁC, 63%; (g) DPPA, Et3N, molecular sieves,
t-BuOH, reflux, 53%; (h) HCl/diethyl ether, MeOH, reflux, 96%;
(i) CH3COCl, Et3N, CH2Cl2, 25 ꢁC, 87%.
with methylamine, dimethylamine or ammonium chlor-
ide afforded the secondary, tertiary or primary amides
7, 10 and 4, respectively. The primary alcohol 5 was
prepared by reduction of the methyl ester 9 using
lithium aluminum hydride. Curtius rearrangement of
the carboxylic acid 11 followed by acidic deprotection

B. Le Bourdonnecet al. / Bioorg. Med. Chem. Lett. 13 (2003) 4459–4462
4461
selectivity. Furthermore, compound 4 displayed potent
in vitro m antagonist activity (IC50=1.9 nM) that was
comparable to that of its phenolic counterpart 3
(IC50=2.0 nM). When the carboxamido nitrogen of 4
was substituted by a single methyl group as in 7, the
affinity for the m opioid receptor decreased 15-fold. The
affinity for m further decreased 17-fold when the carbox-
amido nitrogen of 4 was replaced by two methyl groups
(10). These data indicated that both H-atoms of the car-
boxamide are important for m binding. The primary
alcohol derivative 5, which also has the potential for
H-bond donation, displayed high affinity for the m
Table 2. Opioid receptor (m, k and d) binding data of N-substituted-trans-3,4-dimethyl-4-(3-carboxamido)phenyl piperidines
Compd
Rc
Ki(m)
(nM)a or % inh.@10 mMb or % inh.@10 mMb
Ki(k) (nM)a
Ki(d) (nM)a
Compd
Rc
Ki(m)
(nM)a or % inh.@10 mMb or % inh.@10 mMb
Ki(k) (nM)a
Ki(d) (nM)a
15
0.5 (0.7d)
5.4 (8.1d)
1
19d)
4 ( 24
8.3
210
62%
16
17
1.7
120
110
25
26
8.8
9.7
67%
190
59%
57%
3.1e (1.8d) 140 (160d)
73 (53d)
18
3.2
120
70%
27
13
71%
74%
19
20
21
22
23
5.0
100
460
310
28
29
15
16
29
53
69
58%
210
32%
42%
59%
58%
24%
5.3
70%
5.5 (11d)
6.2 (12d)
8.0 (13d)
220 (380d)
300 (480d)
600 (750d)
71
76%d) % ( 30
140
72% (400d)
31
32
140
220 (420d)
64%
aSee Table 1, footnote a.
bSee Table 1, footnote c.
cUV purity (library compounds) >90%; UV detector; Waters 996 PDA. 200–300 nm. Total scan; compound identity was established by MS analysis.
dBiological data of purified library compound expressed as the geometric mean of at least three separate determinations.
eUV purity of library compound: 73%.

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B. Le Bourdonnecet al. / Bioorg. Med. Chem. Lett. 13 (2003) 4459–4462
opioid receptor, good receptor selectivity and potent m
in vitro antagonist activity. The methyl ester derivative
9 displayed only weak affinity (Ki=850 nM) toward the
m opioid receptor, while the carboxylic acid analogue
(11) was devoid of appreciable opioid receptor binding.
In the previously reported agonist series, the 8-amino
analogue of cyclazocine had significant affinity for the m
and k opioid receptors.12 In our antagonist series, the
aniline derivative 8 displayed only modest affinity
(Ki=150 nM) for the m opioid receptor. Interestingly,
acetylation of 8 was accompanied by a 2-fold increase in
the affinity toward the m opioid receptor (6: Ki=68 nM).
Of the substituents examined, CONH2 appeared to be
an effective bioisostere of OH in this series of trans-3,4-
dimethyl-4-(3-substituted-phenyl)piperidines.
lipophilic entity (phenyl ring or cyclohexyl) separated
from the piperidine nitrogen by three atoms.2 Similarly,
in the new carboxamide series, the 3-phenylpropyl deri-
vative 17 bound with a higher affinity (Ki=1.8 nM) to m
opioid receptors than its phenylethyl (4) (Ki=4.7 nM) or
benzyl (29) analogues (Ki=16 nM). With regard to
N-alkyl substitution, increasing the alkyl length to 5, 6
and 7 carbons was previously shown in the phenolic series
to increase opioid antagonist potencies.2 Similar results
were found in the carboxamide series. In particular, the
most active compound, the (3,5,5)-trimethylhexyl deriva-
tive 15 (diastereomeric mixture), displayed high affinity
toward the m opioid receptor (Ki=0.7 nM) and potent in
vitro antagonist activity (IC50=2.2 nM).
In summary, this study showed that the new series of
trans-3,4-dimethyl-4-(3-carboxamidophenyl) piperidines
provides ligands with good affinity toward the m opioid
receptor and potent in vitro antagonist activity. The car-
boxamide moiety was found to be an effective bioisostere
of the phenolic OH. These results extend the earlier work
of Wentland on m/k agonists to selective m antagonists. In
drug substances, phenolic groups are generally prone to
sulfonation and glucuronidation, both of which facilitate
rapid excretion of the active compounds, resulting in
short half lives. In this regard, future in vivo evaluation of
the trans-3,4-dimethyl-4-(3-carboxamidophenyl) piper-
idines in vivo will be of particular interest.
With the identification of 4 as a novel m antagonist, the
SAR at the piperidine nitrogen was investigated. Thus, an
80 member library of tertiary amines, that is N-substituted
trans-3,4-dimethyl-4-(3-carboxamidophenyl)piperidines
(general formula B), was synthesized according to
Scheme 2. The key intermediate 13, used as starting
material for the solid phase synthesis of compounds of
Formula B, was prepared in six steps from (+)-4(R)-(3-
hydroxyphenyl)-3(R),4-dimethyl-1-piperidine 12. Cou-
pling of the acid 13 with the Fmoc-deprotected Rink
amide resin, followed by removal of the Teoc protecting
group using TBAF, provided the resin 14. Condensa-
tion of 14 with a selected range of aldehydes and
ketones under reductive amination conditions in
N-methylpyrrolidinone/ethanol (3:1) using borane/pyri-
dine as reducing agent afforded the corresponding resin-
bound tertiary amines. Compound cleavage was carried
out using a mixture of trifluoroacetic acid/dichloro-
methane 1:1, yielding the desired carboxamide deriva-
tives (Formula B). The purity of the cleaved products
was generally >90% as determined by LC/MS. The
library compounds were tested for their affinities toward
cloned human m, d and k opioid receptors. The biologi-
cal data obtained for selected compounds 15–32 are
summarized in Table 2. Representative compounds (15,
17 and 21–23) were resynthesized according to Scheme 2
and further purified by preparative HPLC in order to
confirm the binding data obtained for the library com-
pounds. The Ki values obtained for the purified pro-
ducts were generally within 2–3-fold of the Ki values
obtained for the library compounds. The various sub-
stituted benzyl derivatives 16, 18, 20, 21, 25, 26, 28–30
(Table 2) displayed nanomolar range affinity (Ki
<30 nM) toward the human cloned m opioid receptor,
suggesting that the m opioid receptor seems to accom-
modate various lipophilic sustituents at this position.
Similar results have been reported by Zimmerman and
collaborators for the phenolic compounds.2 Replace-
ment of the phenyl ring of 29 by various heterocycles
(compounds 22–24, 27) provided ligands with good
affinity and selectivity for the m opioid receptor. For
example, the quinoline derivative 23 bound to the m
opioid receptor with high affinity (Ki=13 nM) and was
a potent m antagonist in vitro (IC50 (m)=16 nM). It has
been previously determined in the phenolic series that
maximum potency and selectivity for the m opioid recep-
tor was achieved when the N-substituent incorporated a
References and Notes
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C. H. J. Med. Chem. 1993, 36, 2833.
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Med. Chem. 1993, 36, 2842.
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