Substituted 3-(2-Benzoxazyl)-benzimidazol-2-(1H)-ones: A new class of GABA(A) brain receptor ligands

Article abstract of DOI:10.1016/S0960-894X(00)00550-3

A novel class of potent benzodiazepine receptor (BZR) ligands has been designed and synthesized aided by molecular modeling of known benzodiazepine ligands such as CGS-8216 and the use of known pharmacophore models. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0960-894X(00)00550-3

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2723±2726
Substituted 3-(2-Benzoxazyl)-benzimidazol-2-(1H)-ones:
A New Class of GABAA Brain Receptor Ligands
Robert W. DeSimone and Charles A. Blum*
Neurogen Corporation, 35 Northeast Industrial Road, Branford, CT 06405, USA
Received 12 June 2000; accepted 25 September 2000
AbstractÐA novel class of potent benzodiazepine receptor (BZR) ligands has been designed and synthesized aided by molecular
modeling of known benzodiazepine ligands such as CGS-8216 and the use of known pharmacophore models. # 2000 Elsevier
Science Ltd. All rights reserved.
Introduction
capable of exhibiting a wide range of intrinsic activities
from full positive or partial positive modulation (agon-
ism) to neutral modulation (antagonism) to partial or
full negative modulation (inverse agonism) of the e€ects
of GABA.4a Along this continuum lie partial agonists
that may have a reduced side e€ect pro®le relative to the
full agonists while maintaining good ecacy as anxio-
lytics.3 More recently, molecular biology studies have
shown that the GABAA receptor complex is even more
heterogenous than once thought with several di€erent
receptor subunits (a, b, g, and d) combining to form the
receptor allowing for di€erent GABAA receptor iso-
forms depending on which subunits are involved. Sev-
eral native receptor subtypes have been identi®ed so far
and as many as 21 recombinant receptor subtypes are
now known.4b Along with this rich heterogeneity has
come the realization that activation of speci®c GABAA
receptor subtypes (by BZR ligands) may mediate spe-
ci®c behavioral responses.4 The promise of discovering
BZ-receptor ligands with partial agonist and/or subtype
selective properties has prompted scientists to continue
their search for anxiolytics, cognition enhancers, and
hypnotics devoid of the side e€ects usually associated
with the classical BZs.
GABA (g-aminobutyric acid) is the main inhibitory
neurotransmitter of the brain and the actions of several
classes of clinically important drugs, including the ben-
zodiazepines (BZs) and barbiturates, are mediated
through the GABAA receptor complex.1 Benzodiaze-
pines, which allosterically modulate the actions of
GABA, have found widespread use as anxiolytics,
sedative/hypnotics, and anticonvulsants.2 The `classical'
1,4-benzodiazepines such as diazepam, however, tend to
be rather full agonists producing not only the desired
anxiolytic, hypnotic, or muscle relaxant e€ects but also
numerous side e€ects including sedation, developement
of tolerance and dependence, rebound symptoms at
withdrawal, amnestic e€ects and potentiation of alcohol
actions.3
In the 1980s several benzodiazepine receptor (BZR)
ligands with distinct chemical structure were identi®ed
that interacted di€erentially with BZ sites in various
brain regions thus supporting the existence of at least
two distinct BZR subtypes associated with the GABAA
complex.3 It was also recognized that BZ-receptor ligands
within a chemical series (e.g., the b-carbolines) are
The goal of the present study was to design a potent and
novel series of BZR ligands that could be evaluated with
regard to ecacy and side e€ect liability.
Design and Synthesis
The design of compounds of formula 1 as potential high
anity ligands for the benzodiazepine (BZ) site of the
GABAA receptor complex, was based on ligands such as
*Corresponding author. Tel.: +1-203-488-8201; fax: +1-203-481-
5290; e-mail: cblum@nrgn.com
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00550-3

2724
R. W. DeSimone, C. A. Blum / Bioorg. Med. Chem. Lett. 10 (2000) 2723±2726
CGS-82165 and CGS-137676 as well as pharmacophore
models developed to understand the binding of these and
other ligands (e.g., the b-carbolines) to the BZR (Fig. 1).7
this carbonyl oxygen with H1 may occur in concert with
the benzoxazole oxygen (or nitrogen) to form a three
centered hydrogen bond. This type of interaction has
been used to explain the high binding anities observed
for other BZR ligands such as certain benzodiazepines
and b-carbolines that are thought to interact with the
same donor site (H1).9 The heteroatom contained
within the benzoxazole moiety, which is oriented away
from H1, is then available to interact with the other H-
bond donor site, H2, present on the receptor (Fig. 1).
There is some evidence in the literature to suggest that
compounds such as CGS-8216 and CGS-13767 can
interact with H2 via the imine nitrogen at position 1.6,9
Furthermore, the N±H bond in 1 is situated to allow
favorable interaction with the hydrogen-bond acceptor
site (A1) present on the protein. The orientation of aryl
ring D in 1 allows for interaction with the lipophilic
pocket L1 which is located at the centroid of the D-ring
with compounds in the CGS-8216 series.7a Although the
D-ring of 1 overlaps partially with L1, interaction with
the L2 pocket (normally associated with substituents at
the 4 position of the D-ring in the CGS-8216 series)
appears less likely.
Molecular modeling8 suggested that 1 had all the requi-
site functionality in three-dimensional space to interact
with the BZR in a manner consistent with the CGS ser-
ies of ligands, as shown in Figure 1, and conformed to
the inclusive pharmacophore model described in the lit-
erature.7 Speci®cally, 1 has two heteroatoms capable of
interacting with the hydrogen-bond donor site (repre-
sented by the receptor descriptor H1) present on the
BZR. The importance of the benzimidazolone carbonyl
oxygen is suggested by the SAR observed with CGS-
13767 in which the carbonyl at C-5 was necessary for
binding to the BZR.6 Additionally, the interaction of
Compounds of type 1 (Table 1) were readily prepared
by treating 2(3H)-benzimidazolone 2 with 2.2 equiv-
alents of LDA followed by the addition of either sub-
stituted 2-chlorobenzoxazoles10 3 or substituted 2-
(methylthio)-benzoxazoles 411,12 (Scheme 1). When the
reaction was carried out using 1 equivalent of LDA,
bis-alkylation was the predominant reaction out-
come, possibly due to either the increased acidity or
increased solubility of 1 relative to starting material 2.
Substituted 2-chlorobenzoxazoles (3) and substituted
Figure 1. Overlay of 1 and CGS-8216.
Table 1. Substituted 3-(2-benzoxazyl)-benzimidazol-2-(1H)-ones (1)
Entry
R
X
Y
Z
5
6
7
8
9
Ki (nM)14
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
N
O
O
S
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
C±H
N
C±H
C±H
C±H
C±H
C±H
C±H
H
H
H
F
H
H
H
OMe
H
H
H
H
H
H
H
H
H
H
H
H
H
F
H
F
H
OMe
Me
H
H
H
F
H
H
H
H
H
H
H
H
F
H
H
H
H
H
H
H
H
H
H
H
F
H
H
H
H
H
H
H
H
H
H
H
H
F
54
>10,000
>10,000
162
89
O
O
O
O
O
O
O
O
O
O
O
O
O
O
39
20
91
610
303
44
58
32
25
14
15
723
F
H
H
H
H
H
H
H
F
H
H
H
F
F
F
H
H
F
H
H
CGS-8216
CGS-13767
IC50=0.4 nM15
IC50=4 nM16

R. W. DeSimone, C. A. Blum / Bioorg. Med. Chem. Lett. 10 (2000) 2723±2726
2725
2-(methylthio)-benzoxazoles (4) were prepared from the
corresponding substituted 2-aminophenols by treatment
with the potassium salt of O-ethylxanthic acid in
re¯uxing aqueous ethanol.13 The resulting 2-thiolben-
zoxazoles were then treated with thionyl chloride or
methyl iodide to give the 2-chloro- and 2-(methylthio)-
derivatives, 3 and 4, respectively.10,11
reaction of 5-¯uorobenzimidazolone with 3 followed by
chromatographic separation of the resulting regio-
isomers (1l and 1m).
In similar fashion, 4,5,6,7-tetrahydro-2-benzimidazo-
lone 7 and 3,4,5,6,7,8-hexahydro-2(1H)-cycloheptimi-
dazolone 8 were treated with substituted benzoxazoles
(3 or 4) to give the corresponding A-ring variants 9 and
10, respectively (Scheme 2). Imidazolones 7 and 8 were
prepared from bromoketones 5 and 6 using modi®cations
of literature procedures (Scheme 2).14
Compound 1m (Table 1) was prepared by treating 4-
¯uoro-2-nitroaniline with 2-chlorobenzoxazole followed
by reduction of the nitro group and cyclization with
carbonyldiimidazole. Compound 1l was prepared by
Discussion
The pharmacophore model7 described above was sup-
ported by the structure±activity relationship (SAR) of
the benzimidazolone (1) series as shown in Table 1.
Thus, when 1a was methylated, thereby removing pos-
sible H-bonding interactions with A1, the resulting
compound (1b) had no anity for the BZR. Further-
more, when one of the hydrogen bond acceptors present
in 1a (the oxygen atom of the benzoxazole group) is
replaced by sulfur, the resulting compound (1c) exhib-
ited a marked loss of anity for the BZR.
Scheme 1. (a) 2.2 equiv LDA, THF, rt, 15 min, then 3 or 4 in THF,
0.5±2 h.
Placing ¯uorine substituents on the benzoxazole group
(D-ring) did not signi®cantly change the anity of this
series although the di¯uoro compound (1g) displayed a
modest improvement in potency relative to the parent
compound (1a). In an attempt to maximize interactions
with the lipophilic cavity (L2) of the receptor, larger
Scheme 2. (a) 1 equiv urea, H2O, HOAc, NH4OAc, re¯ux 4 h; (b)
2.2 equiv LDA, THF, rt, 15 min; (c) 1 equiv 3 or 4 in THF, 0.5±2 h.
Table 2. Substituted imidazolones (9 and 10)
Entry
n
Z
5
6
7
Ki (nM)14
9a
9b
9c
9d
9e
9f
9g
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
C±H
C±H
C±H
C±H
C±H
C±H
C±H
N
C±H
C±H
C±H
C±H
C±H
C±H
N
C±H
C±H
C±H
C±H
C±H
C±F
C±H
H
F
H
H
H
OMe
H
H
H
F
H
H
OMe
H
H
Me
H
F
F
H
H
H
F
H
F
H
OMe
H
H
H
F
H
H
OMe
H
H
Me
F
H
F
H
H
H
F
56
26
47
52
21
81
F
H
H
H
H
H
H
F
H
H
H
H
H
H
F
239
>1000
8
9h
10a
10b
10c
10d
10e
10f
10g
10h
10i
10j
10k
10l
10m
10n
5
5
5
25
254
137
15
158
87
7
F
H
H
3
65
140
H
Cl
F
H

2726
R. W. DeSimone, C. A. Blum / Bioorg. Med. Chem. Lett. 10 (2000) 2723±2726
substituents were placed at positions 5 and 6 of 1a (Fig.
1). However, substitution of a methoxy group at the 5
position (1h) produced a deleterious e€ect on anity
while substitution with methyl or methoxy at the 6
position (1i and 1j) had an even greater negative impact
on receptor binding. Close examination of Figure 1
indicates that there is no position on 1 that is ideally
suited to project a substituent into the area of space
corresponding to the para position of CGS-8216 (where
interactions with L2 are maximized). It is plausible that
substituents at positions 5 or 6 of 1 encounter steric repul-
sions from regions (S1 or S2) present on the receptor.7
References and Notes
1. (a) Sieghart, W. Pharmacol. Rev. 1995, 47, 181. (b) Teuber,
L.; Watjen, F.; Jensen, L. H. Curr. Pharmaceut. Des. 1999, 5,
317.
2. (a) Bowery, N. G. In The GABA Receptors; Enna, S. J.,
Bowery, N. G., Eds.; Humana: NJ, 1997; pp 209±236. (b)
Gupta, S. P. In Progress in Drug Research; Jucker, E., Ed.;
Birkhauser Verlag: Basel, 1995; Vol. 45, pp 67±106.
3. Costa, E.; Guidotti, A. Trends Pharmacol. Sci. 1996, 17,
192, and references therein.
4. (a) Rudolph, U.; Crestani, F.; Benke, D.; Brunig, I.; Ben-
son, J.; Fritschy, J.-M.; Martin, J. R.; Bluethmann, H.; Moh-
ler, H. Nature 1999, 401, 796. (b) Mehta, A. K.; Ticku, M. K.
Brain Res. Rev., 1999, 29, 196.
5. Yokoyama, N.; Ritter, B.; Neubert, A. D. J. Med. Chem.
1982, 25, 337.
6. (a) Francis, J. E.; Cash, W. D.; Barbaz, B. S.; Bernard, P.
S.; Lovell, R. A.; Mazzenga, G. C.; Friedmann, R. C.; Hyun,
J. L.; Braunwalder, A. F.; Loo, P. S.; Bennett, D. A. J. Med.
Chem. 1991, 34, 281. (b) Francis, J. E.; Bennett, D. A.;
Hyun, J. L.; Rovinski, S. L.; Amrick, C. L.; Loo, P. S.;
Murphy, D.; Neale, R. F.; Wilson, D. E. J. Med. Chem.
1991, 34, 2899.
7. (a) Liu, R.; Hu, R. J.; Zhang, P.; Skolnick, P.; Cook, J. M.
J. Med. Chem. 1996, 39, 1928. (b) Zhang, W.; Koehler, K. F.;
Zhang, P.; Cook, J. M. Drug Des. Discov. 1995, 12, 193. (c)
Cox, E. D.; Diaz-Arauzo, H.; Huang, Q.; Reddy, M. S.; Ma,
C.; Harris, B.; McKernan, R.; Skolnick, P.; Cook, J. M. J.
Med. Chem. 1998, 41, 2537.
As with the D-ring, substitution of ¯uorine at positions
8 or 9 (1l and 1m) of the A-ring produced only modest
e€ects on receptor anity. It has been reported that
electron withdrawing substituents (e.g., Cl) at the 5-
position (position 8 of compound 1) of indolic BZR
ligands can enhance binding anity.16 However, place-
ment of a ¯uorine at position 8 (compound 1l) did not
improve receptor anity. Combining the best ¯uorine
substituents from the A-ring and the D-ring yielded 1o,
which showed about a 4-fold gain in potency relative to
1a. Incorporation of a nitrogen into the A-ring of 1a
yielded a compound (1q) with a more than 10-fold
decrease in potency at the BZR.
The SAR of the corresponding tetrahydro-
benzimidazolone series (9) closely paralleled that of the
aromatic series (1) with the former having a slightly
greater anity for the BZR (Table 2). Previous obser-
vations suggest that a more three-dimensional A-ring or
one bearing substituents capable of forming lipophilic
interactions in this area of space (e.g., placement of Cl
at position 9 of CGS-13767)6a can have a positive in¯u-
ence on binding anity. Indeed, this is the case when
moving to the more lipophilic seven-membered ring
homologues of the present series (10a±10n) with several
compounds having Kis of less than 10 nM at the BZR
(Table 2). Compounds 10a±n exhibited as much as a 30-
fold increase in anity relative to the analogous benz-
imidazolones (1) re¯ecting a more favorable interaction
with the receptor. In the case of compounds such as
CGS-13767, which are thought to bind in a similar
fashion to those of the present series, an increased
interaction with this lipophilic pocket is often accom-
panied by an increase in agonist function.
8. Structures shown in Figure 1 were minimized on a Silicon
Graphics Indigo 2 Personal workstation using SYBYL (Tripos
Associates, St. Louis, MO).
9. Allen, M. S.; Tan, Y.-C.; Trudell, M. L.; Narayanan, K.;
Schindler, L. R.; Martin, M. J.; Schultz, C.; Hagen, T. J.;
Koehler, K. F.; Codding, P. W.; Skolnick, P.; Cook, J. M. J.
Med. Chem. 1990, 33, 2343.
10. Forster, H.; Boehm, S.; Marhold, A.; Santel, H.; Luers-
sen, K.; Schmidt, R. R. Eur. Patent 0 572 893, 1993; Chem.
Abstr. 1993, 120, 164158.
11. (a) Yamato, M.; Takeuchi, Y.; Hashigaki, K.; Hirota, T.
Chem. Pharm. Bull. 1983, 31, 733. (b) Chu-Moyer, M. Y.;
Berger, R. J. Org. Chem. 1995, 60, 5721.
12. Compounds gave correct mass spectral analysis and had
NMR spectroscopic data consistent with the structures given.
13. Van Allan, J. A.; Deacon, B. D. Organic Syntheses; Wiley:
New York, 1963; Collect. Vol. IV, pp 569±570.
14. Zav'yalov, S. I.; Sitkareva, I. V.; Ezhova, G. I.; Dor-
ofeeva, O. V.; Zavozin, A. G. Khim. Geterotsikl. Soedin. 1990,
6, 847; Chem. Abstr. 1991, 114, 23873.
3
15. Compounds were assayed for their ability to displace H-
RO15-1788 (3H-Flumazenil) from rat cortical tissue. For con-
ditions see: (a) Thomas, J. W.; Tallman, J. F. J. Neurosci.
1983, 3, 433. (b) Thomas, J.; Tallman, J. J. Biol. Chem. 1981,
256, 9838. (c) DeSimone, R. W.; Blum, C. A. US Patents 5 637
724, 1997; 5 637 725, 1997; 5 936 095, 1999.
The compounds of the present series are structurally
unique and display good to moderate anity for the
BZR. The pharmacology of this series and its potential
to produce anxiolytic or sedative/hypnotic e€ects
devoid of the adverse side-e€ects frequently associated
with other BZ ligands, will be reported in due course.
16. Primo®ore, G.; Marini, A. M.; DaSettimo, F.; Martini,
C.; Bardellini, A.; Giannacchini, G.; Lucacchini, A. J. Med.
Chem. 1989, 32, 2514.