Functionalization at position 3 of the phenyl ring of the potent mGluR5 noncompetitive antagonists MPEP

Article abstract of DOI:10.1016/j.bmcl.2004.12.047

We described the synthesis and biological evaluation of MPEP analogs functionalized at the position 3 of the phenyl ring. The results point out the limitation in the choice of a functional group at this position; the only substituents leading to retention

Full text of DOI:10.1016/j.bmcl.2004.12.047

Bioorganic & Medicinal Chemistry Letters 15 (2005) 945–949
Functionalization at position 3 of the phenyl ring of the potent
mGluR5 noncompetitive antagonists MPEP
David Alagille,a Ronald M. Baldwin,a Bryan L. Roth,b Jarda T. Wroblewski,c
Ewa Grajkowskac and Gilles D. Tamagnana,d,
*
aDepartment of Psychiatry, Yale University and VA Connecticut/116A2, 950 Campbell Avenue, West Haven, CT 06516, USA
bDepartment of Biochemistry and NIMH Psychoactive Drug Screening Program, Case Western Reserve University School
of Medicine, Cleveland, OH 44106, USA
cDepartment of Pharmacology, Georgetown University Medical Center, 3900 Reservoir Road, N.W., Washington, DC 20007, USA
dInstitute for Neurodegenerative Disorders, 60 Temple Street, Suite 8B, New Haven, CT 06510, USA
Received 22 October 2004; revised 15 December 2004; accepted 17 December 2004
Available online 19 January 2005
Abstract—We described the synthesis and biological evaluation of MPEP analogs functionalized at the position 3 of the phenyl ring.
The results point out the limitation in the choice of a functional group at this position; the only substituents leading to retention of
activity are NO2 (IC50 = 13 nM) and CN (IC50 = 8 nM).
Ó 2004 Elsevier Ltd. All rights reserved.
(À)-Cocaine (CAS number [50-36-2]) is a natural alka-
loid produced by Erythroxylum Coca. Early medicinal
application of cocaine was mainly restricted in its use
as local anesthetic for minor surgery and was rapidly re-
placed by simpler, cheaper, more potent synthetic com-
pounds. However, it found new notoriety as being the
most widely used psychostimulant drug worldwide. Co-
caine abuse continues to be a major public health prob-
lem, and despite increased scientific investigation, the
basic biological processes underlying this disorder are
still not well understood, limiting the availability of
pharmacological adjuncts for cocaine treatment. The
role for glutamatergic neurotransmission in the behav-
ioral and reinforcing effects of cocaine is becoming
increasingly evident,1,2 especially for the type 5 metabo-
tropic receptor (mGluR5). Chiamulera et al.3 demon-
strated that genetic deletion of mGluR5 in mice
resulted in reduction of intravenous cocaine self admin-
istration. These investigators also found that cocaine self
administration was reduced following treatment with
the selective mGluR5 antagonist MPEP (1). In our
continuing effort to better understand the biological
process of cocaine reward and craving, we initiated a
program of mGluR5 antagonists discovery and focused
our effort on the systematic exploration of SAR around
the known prototypical noncompetitive mGluR5 antag-
onists MPEP, in which the introduction of the methoxy
moiety as in M-MPEP (2) significantly increased the
activity. We report in this paper the synthesis and bio-
logical evaluation of MPEP and M-MPEP analogs func-
tionalized at the 3 position of the phenyl ring (Fig. 1).
Since previous studies indicated the 3-position to be pre-
ferred, we selected a set of functional groups at this
position that would encompass a broad range of
physicochemical properties in a controlled environment.
The synthesis of intermediate 3-hydroxy-MPEP (3) was
achieved in two steps from 2-bromo-6-methylpyridine
(Scheme 1). The key step of this synthesis was modified
Sonogashira cross coupling between the silyl acetylene
precursor 4 and 3-iodophenol, which led to the desired
compound 3. In contrast, 3-bromophenol under the
same conditions led to degradation of the starting mate-
rial without formation of 3.
Esters 5–10 were prepared by reaction of 3 with the
appropriate acyl chloride in THF using Et3N as scaven-
ger (Scheme 2). Esters 11 and 12 were obtained in good
yield by coupling of 3 with the corresponding carboxylic
acid using DCC as activating agent. Sulfonic esters 13
Keywords: Metabotropic; mGluR5; MPEP; Antagonist.
*
Corresponding author. Tel.: +1 203 401 4309; fax: +1 203 789
2119; e-mail: gtamagnan@indd.org
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.12.047

946
D. Alagille et al. / Bioorg. Med. Chem. Lett. 15 (2005) 945–949
N
O
O
S
R
N
O
N
n
Ar
13-14
O
Ar
O
n
O
sulfonylesters
15-24
5-12
aryl or alkylarylesters
alkylarylethers
N
MPEP (1)
N
O
M-MPEP (2)
small functional
groups
bisaryl
pyridine analogs
N
N
Ar or Het
X
N
36-40
27-30
Ar or Het
N
X = CN, NO2, NH2,
CO2Me, COCH3
31-35
Figure 1.
HO
Br
b
HO
I
a
b
N
OH
N
N
Br
SiMe3
4
3
Scheme 1. Reagents and conditions: (a) Me3SiC„CH, CuI, PdCl2(PPh3)2, Et3N, rt, 16 h (71%); (b) TBAF, CuI, PdCl2(PPh3)2, Et3N, DMF, 70 °C,
16 h (67%).
RCOCl
ROH
a
5-10
c
15-19
N
N
OH
O
R
N
O
R
b
d
O
3
RBr
20-24
RCOOH
11-12
RSO2Cl
13-14
a
N
O
O
S
R
O
Scheme 2. Reagents and conditions: (a) Et3N, THF, rt, 3–16 h (70–97%); (b) DCC, DMAP, THF, rt, 16 h (89–93%); (c) PPh3, DEAD, THF, rt, 3 h
(73–91%); (d) NaH, THF or DMF, rt, 16 h (65–97%).
and 14 were synthesized by reaction of 3 with mesyl
chloride or tosyl chloride in THF in the presence of
Et3N. We used two different routes to the ether deriva-
tives of MPEP. Compounds 15–19 were obtained by
Mitsunobu reaction between phenol 3 and the desired
alcohol in THF. Compounds 20–24 were obtained by
nucleophilic substitution of the alkyl halide by the anion
of 3 generated with NaH (Scheme 2).
The synthesis of compounds 27–35 was achieved in
satisfactory yield (46–90%) using a simple Stille reac-
tion between the appropriate aryl or heteroaryltrialkyl

D. Alagille et al. / Bioorg. Med. Chem. Lett. 15 (2005) 945–949
947
a
or
N
N
N
Br
Ar
Br
6
7
SiMe3
N
a
b
b
N
N
N
X
Ar
36-40
27-30
31-35
N
X = CN, NO2, NH2,
CO2Me, COCH3
Scheme 3. Reagents and conditions: (a) aryl or heteroaryl bromide, CuI, PdCl2 (PPh3)2, Et3N, Bu4NF, DMF, 70 °C; (b) aryltrialkylstanane,
Pd(PPh3)4, toluene, reflux.
stannane and the bromo intermediate 6 or 7. Those keys
intermediates as well as compounds 36–40 were easily
obtained (40–69%) using a modified Sonogashira reac-
tion involving the in situ desilylation of 2-methyl-6-tri-
methylsilylethynylpyridine using tetrabutylammonium
fluoride and subsequent palladium cross coupling with
the appropriate aryl or heteroaryl bromide as shown
in Scheme 3. All compounds were tested for antagonism
at mGluR5, a receptors stably expressed in Chinese
hamster ovary (CHO) cells.4 Cultured in 96-well plates,
the receptor-expressing cells were incubated with
0.75 lCi myo-[3H]inositol to label the cell membrane
phosphoinositides. Incubations with test compounds
were carried out for 45 min at 37 °C in LockeÕs buffer
(156 mM NaCl, 5.6 mM KCl, 3.6 mM NaHCO3,
1 mM MgCl2, 1.3 mM CaCl2, 5.6 mM glucose, and
20 mM HEPES, pH 7.4) containing 20 mM LiCl, which
blocks the degradation of inositol phosphates (IPs). The
reaction was terminated by aspiration and addition of
0.1 M HCl. The extracted IPs were separated by anion
exchange chromatography and measured by a liquid
scintillation counting (LKB, Uppsala, Sweden) as previ-
ously described.5 Inhibition of the mGluR5 response by
the test compounds was evaluated by performing dupli-
cate concentration–response curves using seven concen-
trations in presence of 10 lM glutamate. Data from 4 to
6 separate experiments were normalized to the percent
of maximal glutamate response. IC50 values of antago-
nists were determined by fitting the normalized data to
the logistic equation by nonlinear regression using Sig-
maPlot (SPSS Science, Chicago, IL) and are summa-
rized in Tables 1 and 2. For selected compounds,
binding to mGluR5 receptors in vitro was determined
by measuring the displacement by test compounds of
[3H]M-MPEP from rat whole brain as previously de-
scribed6,7 and are summarized in Table 2.
Table 1. mGluR5 IC50 values of selected compounds using IP
hydrolysis assay (triplicate determinations)a
N
R
b
Compound
R
Yield %
96
IC50 SEM (lM)
O
O
O
5
1.50 0.31
O
I
11
7
93
97
87
38
53
1.10 0.13
1.72 0.21
1.01 0.29
0.74 0.23
0.89 0.74
O
O
O
O
8
O
O
O
O
9
O
O
10
O
12
6
89
95
66
1.17 0.37
3.35 0.76
0.18 0.11
O
O
O
Br
O
O
Compounds 5–38 proved to be less active than MPEP or
M-MPEP. Functionalization of the 3-hydroxy moiety of
M-MPEP (carboxylic ester, sulfonic ester, and ether)
did not seem to influence the activity. Similarly, the carb-
oxylic compounds 5, 7, and 11 showed the same range
of activity as the corresponding ester 20–22 and the same
observation can be made for compounds 10, 12 versus 17,
18. The introduction of an aryl moiety spaced by a linker
(9, 10, 12, 15–18, and 20–22) was envisaged to reach a new
13
S
O
O
14
20
84
97
>10
O
O
S
O
1.84 0.33
(continued on next page)

948
D. Alagille et al. / Bioorg. Med. Chem. Lett. 15 (2005) 945–949
Table 2. mGluR5 IC50 values of selected compounds using IP
Table 1 (continued)
hydrolysis assay (mean of n = 3 determinations) and in vitro binding
affinity (n = 2)a
b
Compound
R
Yield %
IC50 SEM (lM)
Compound Structure
Ki (nM) IC50 (nM)
O
22
81
1.30 0.22
O
27
993
629
493
N
O
O
15
16
21
17
91
76
65
73
1.59 2.31
2.05 1.97
1.98 0.36
1.19 0.60
O
28
2360
N
F
I
O
O
S
N
N
29
30
36
371
656
O
O
147
18
23
85
72
1.16 0.68
2.40 1.51
N
31
32
33
34
35
1.7
20
282
205
271
257
O
N
O
O
O
OMe
N
N
74
24
19
60
80
0.29 0.03
0.42 0.02
N
N
O
S
O
6.5
36
37
NH2
40
69
58
47
44
—
—
0.188 0.01
0.208 0.05
0.146 0.11
0.013 0.002
0.005 0.001
0.0368
CO2Me
COMe
NO2
CN
38
39
N
N
155
13
40
MPEP
M-MPEP
N
H
OMe
N
0.008
a IP hydrolysis assay. All values represent means SEM.
b Isolated yield.
N
a IP hydrolysis assay and displacement by test compounds of [3H]M-
MPEP bound to rat whole brain.6,7
interaction with the close trans-membrane domain TMI
and or TMII9 of the receptor, but this modification led
to inactive compounds. The only functional group leading
to activity comparable to the reference compound were
the nitro (39, IC50 = 13 nM) and cyano (40, IC50 = 8 nM)
derivatives. Recently, Roppe et al.10 reported a series with
good to low binding affinity for various 3-aryl substituted
of analogs of MTEP. As with the thiazole moiety of
MTEP, we found similar results with the pyridine ring
of MPEP. Compounds 27–30 presented a weak affinity
for mGluR5 receptors. In contrast, introduction of a pyr-
idine moiety led to remarkably increased binding, espe-
cially for compound 31 (Ki = 1.7 nM), which is 584-fold
more active than the carbocyclic parent 27.
MPEP with various functionalities at the 3 position of
the phenyl ring. These modification point out the limita-
tion in the choice of a functional group at this position;
the only substituents leading to retention of activity are
NO2 (39) and CN (40). In the bis-aryl series, only the pyr-
idine analogs (31–35) showed good binding for mGluR5,
despite a lack of activity in the functional receptor assay.
Acknowledgments
This work was supported by the National Institutes of
Health (DA16180-01), from Department of Veterans
Affairs, National Center for PTSD Alcohol Research
Center and NARSAD to GDT and by the National
Institute of Mental Health through the Psychoactive
Drug Screening Program (N01MH80005) and
KO2MH01366 to BLR.
The results in Table 2 also indicate that the rank order of
potency (IC50) shown by the particular compounds in the
PI hydrolysis assay does not correlate with their binding
affinities (Ki). This suggests that while some of the com-
pounds may bind with high affinity to the mGluR5 recep-
tors they may not be able to induce the conformational
change necessary to inhibit receptor signal transduction.
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