Synthesis and structure-activity relationship of imidazo[1,2-a] benzimidazoles as corticotropin-releasing factor 1 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.06.028

8-Aryl-1,3a,8-triaza-cyclopenta[a]indenes represent a novel series of high binding affinity corticotropin-releasing factor 1 receptor antagonists. Here, we report their synthesis, SAR, and pharmacokinetic properties of compound 8e (K_i = 23 nM).

Full text of DOI:10.1016/j.bmcl.2005.06.028

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4029–4032
Synthesis and structure–activity relationship of
imidazo[1,2-a]benzimidazoles as corticotropin-releasing
factor 1 receptor antagonists
Xiaojun Han,* Sokhom S. Pin, Kevin Burris, Lawrence K. Fung,à Stella Huang,
Matthew T. Taber, Jie Zhang§ and Gene M. Dubowchik*
Pharmaceutical Research Institute, Bristol-Myers Squibb Company, 5 Research Parkway,
Wallingford, CT 06492, USA
Received 31 March 2005; revised 24 May 2005; accepted 7 June 2005
Available online 27 June 2005
Abstract—8-Aryl-1,3a,8-triaza-cyclopenta[a]indenes represent a novel series of high binding affinity corticotropin-releasing factor 1
receptor antagonists. Here, we report their synthesis, SAR, and pharmacokinetic properties of compound 8e (Ki = 23 nM).
Ó 2005 Elsevier Ltd. All rights reserved.
Corticotropin-releasing factor (CRF) is a 41-amino-acid
neuropeptide secreted in the hippocampus. It imposes
its physiological effects on depression and other
neuropsychiatric disorders via the hypothalamic–pitui-
tary–adrenal (HPA) axis.1 The CRF receptor, a G-pro-
tein-coupled receptor, has two well-characterized
subtypes (CRF1 and CRF2).2 Compelling clinical evi-
dence supports the hypothesis that overproduction of
CRF may underlie the pathology of depression, anxiety,
and stress-related disorders, and suggests that antago-
nists of CRF1R could be useful for the treatment of
these conditions.1
Figure 1. Small molecule CRF1R antagonists.
reasonable pharmacokinetic properties, and it demon-
strated anxiolytic activity in a mouse canopy model. In
As described in our preceding article,3 we have synthe-
sized 1-aryl-2,3-dihydro-imidazoimidazoles (I) (Fig. 1)
as CRF1R antagonists. An exemplary compound
(X,Y,Z = Me, R1 = Et, R2 = nPr and R3 = cPrCH2)
had relatively good binding affinity (Ki = 42 nM) and
the search for more potent compounds, we elaborated
the core structure I with a phenyl ring as shown in Fig-
ure 1. In this article, we report our efforts on the design,
synthesis, and SAR studies of a novel series of imi-
dazo[1,2-a]benzimidazole CRF1R antagonists.
Keywords: Corticotropin-releasing factor 1; Antagonists; SAR; Phar-
macokinetic properties.
The synthesis of these compounds is outlined in Schemes
1 and 2. The reaction of substituted anilines 1 with o-flu-
oronitrobenzene 2 afforded o-nitroanilines 3.4 Reduc-
tion to the corresponding diamines 4 was effected by
hydrogenation for 2,4,6-trimethyl-substituted anilines,
and with Na2S2O4/NH4OH for chloro- and bromo-
substituted anilines to avoid reduction of the halides.5
2-Aminobenzimidazoles 5 were formed by the reaction
of 4 with cyanogen bromide in ethanol at 150 °C,6 fol-
lowed by alkylation with ethyl bromoacetate in acetone
*
Corresponding authors. Tel.: +1 203 677 7879; fax: +1 203 677
7702; e-mail addresses: xiaojun.han@bms.com; gene.dubowchick@
bms.com
Present address: Palatin Technologies Inc., 4-C Cedar Brook Drive,
Cranbury, NJ 08512, USA.
à Present address: Neurogen Corp., 35 N.E. Industrial Rd., Branford,
CT 06405, USA.
§ Present address: Sanofi Aventis, 1041 Route 202-206, Bridgewater,
NJ 08807, USA.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.028

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X. Han et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4029–4032
Table 1. hCRF1R-binding affinities of amides 7a–e and amines 8a–e
Compound
R1
W
Q
Ki (nM)
7a
7b
7c
7d
7e
8a
8b
8c
8d
8e
Me
Me
Et
O
H
F
360
1180
570
930
3560
20
O
O
H
F
Et
O
CF3
Me
Me
Et
O
H
H
F
H2
H2
H2
H2
H2
Scheme 1. Reagents and conditions: (a) KF, 180 °C, 48–72 h,
20–100%. (b) i—1 atm H2, Pd/C (10%), EtOAc, rt, 4 h, 80–85%; or
ii—Na2S2O4, THF/H2O/concd NH4OH (1:1:1), rt, 16 h, 60–80%. (c)
i—BrCN, EtOH, 150 °C, 40 min; ii—1.2 equiv BrCH2CO2Et, acetone,
65 °C, 16 h. 70–95%. (d) 2.5 equiv R1CO2Na, (R1CO)2O, 150–180 °C,
20 h, 40–76%; (e) R2NHR3, AlMe3, PhMe, 80 °C, 14 h, 60–90%;
(f) Red-Al, PhMe, rt, 24 h, 50–70%.
59
21
H
F
Et
CF3
150
23
H
In the related dihydroimidazoimidazole series,3 amides
displayed better potency than corresponding amines.
However, in this series, amines showed a >10-fold
potency advantage (8a–e vs. 7a–e). This is similar to
what was seen with a previous series of arylaminothiaz-
oles.13 For amines (W = H2), there was a little differ-
ence when R1 was Me, Et, or CF3 (8a, 8c, and 8e).
However, long-term hydrolytic stability was only en-
sured when at least one electron withdrawing group
(R1 = CF3) was present on the core.13 Therefore, it
was hoped that, instead of the rather lipophilic CF3
at R1, a single fluorine on the C ring would permit
methyl or ethyl substitution at R1. Although 8b and
8d were sufficiently stable (data not shown), fluorina-
tion resulted in a 3- to 8-fold loss of potency for
amines and amides (8b,d vs. 8a,c).
Scheme 2. Reagents and conditions: (a) 5 equiv i-Bu2AlH, PhMe,
0 °C, 1 h, 80–100%. (b) 2 equiv SOCl2, CH2Cl2, 0 °C, 1 h; (c) 5 equiv
R2NHR3.HCl, 7 equiv i-Pr2NEt (or 5 equiv R2NHR3, 2 equiv
i-Pr2NEt, MeCN, rt, 1 h, then chlorides 9, rt, 24 h, 50–70% for two
steps.
at reflux. Condensation with acetic, propionic, or triflu-
oroacetic anhydride along with the respective sodium
salt at 160 °C afforded esters 6.7 Because of the volatility
of trifluoroacetic anhydride (bp = 40 °C), esters 6 where
R1 = CF3 were prepared in a sealed bomb. Weinreb ami-
dation8 of esters 6 formed amides 7, and the Red-Al
reduction of amides 7 afforded amines 8.9
A series of aminomethyltrifluoromethylimidazoles using
a small set of amine substituents was prepared next to
probe the requirement for 2,4- versus 2,4,6-aryl substitu-
tion on the pendant aryl ring. In some fused bicyclic aro-
matic CRF antagonist chemotypes, 2,4,-disubstitution
resulted in high affinity binding, while in others activity
was greatly diminished.14 This appeared to depend on
the presence of a substituent on the B ring that projected
into the Ôortho-spaceÕ of the pendant aryl ring, presum-
ably helping to enforce an orthogonal conformation.
As given in Table 2, 2,4,6-trisubstitution (8e–g) is clearly
preferred for this core structure, with the bulkier substit-
uents on 8h–j provided some advantage over the smaller
chlorines in 8k–m.
Amines 8 could also be efficiently synthesized in a paral-
lel fashion (Scheme 2) using a variety of secondary (and
primary) amines. This was especially convenient, since
the CRF1R is tolerant of diversity in this area. The
DIBAL-H reduction of esters 6 cleanly afforded the
corresponding alcohol with minimal workup.10 These
alcohols were treated with SOCl2 briefly to form chlo-
rides 9 that, after concentration in vacuo, were treated
with a variety of secondary amines in acetonitrile to give
amines 8 in good yields.
Lastly, we explored some SAR of the aminomethyl side
chains. Additional polar atoms were avoided since their
presence abolished the activity in related series.3,13 The
results are summarized in Table 3. Interestingly, while
Ôbenzyl-likeÕ cyclopropylmethyl-containing compounds
showed better binding potency than cyclopropylethyl
compounds (8f vs. 8aa, 8o vs. 8cc, and 8n vs. 8ff), phen-
ylethyl derivatives were clearly superior to benzyl-con-
taining compounds (8g vs. 8ll and 8x vs. 8jj). Clearly,
Initial SAR studies focused on amide (Fig. 1, II,
W = O) versus amine (Fig. 1, II, W = H2) functionality,
preferences for the small alkyl group on the A ring
(R1), and fluoro substitution on the C ring. CRF1R-
binding affinities were determined as described previ-
ously,11 and the results are summarized in Table 1.

X. Han et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4029–4032
Table 2. The effects of aryl substitutions on hCRF1R-binding affinities
4031
Compound
X
Y
Z
R2
R3
Ki (nM)
8f
Me
Me
Me
Br
Br
Br
Cl
Me
Me
Me
i-Pr
i-Pr
i-Pr
Cl
Me
Me
Me
H
cPrCH2
cPrCH2
cPrCH2
nPr
nPr
11
23
8e
8g
8h
8i
PhCH2CH2
cPrCH2
cPrCH2
250
54
cPrCH2
Pr
nPr
H
150
410
240
370
1600
8j
H
PhCH2CH2
cPrCH2
cPrCH2
8k
8l
H
cPrCH2
nPr
nPr
Cl
Cl
Cl
Cl
H
8m
H
PhCH2CH2
Table 3. The effects of amine substitutions on hCRF1R-binding
affinities
Table 4. Rat PK parameters for 8e (10 mg/kg, p.o.; 2 mg/kg, i.v.)a
Cl
Vd
5.1 mL/min/kg
0.6 L/kg
4.1 h
t1/2
Fp.o.
35%
AUC (plasma, p.o.)
Cmax
B/P (2 h)
11438 ng h/mL
2380 ng/mL
0.03
a Dosing vehicle was 10/10/80 Cremphor/DMSO/water. Dosing vol-
umes were 1 and 3 mL/kg for i.v. and p.o., respectively. Brain-to-
plasma concentration ratio (B/P) was determined after i.v.
administration.
Compound
R2
R3
Ki (nM)
8n
8o
cPrCH2
cPrCH2
CF3CH2
CF3CH2CH2
nPr
7.1
13
8p
8q
cBuCH2
cPrCH2
26
28
Et
8r
cPrCH2
cBuCH2
CF3CF2CH2
CF3CH2
nPr
34
53
Compound 8e was chosen for in vivo pharmacokinetic
profiling (Table 4). In rats, 8e had a low plasma clear-
ance, low volume of distribution, moderate terminal
half-life, acceptable oral bioavailability, but essentially
no brain penetration.
8s
8t
8u
CF3CH2CH2
CF3CF2CH2
CF3CH2
68
75
nPr
nPr
8v
75
8w
8x
Allyl
Allyl
Et
138
249
291
333
362
428
457
573
618
653
704
744
1412
2774
3388
4049
7708
PhCH2CH2
CF3CH2CH2
PhCH2CH2
cPrCH2CH2
PhCH2
In summary, imidazo[1,2-a]benzimidazoles represent a
new series of CRF1R antagonists with good receptor-
binding affinities. Efforts to improve the physiochemical
properties of this series to improve brain penetration
will be reported shortly.
8y
8z
CF3CH2
PhCH2CH2
nPr
8aa
8bb
8cc
8dd
8ee
8ff
CF3CH2CH2
CF3CH2CH2
Me
cPrCH2CH2
PhCH2
PhCH2CH2
cPrCH2CH2
Et
CF3CH2CH2
CF3CH2
nBu
Acknowledgments
8gg
8hh
8ii
PhCH2CH2
cPrCH2CH2
PhCH2
CF3CH2
Et
Et
We thank Dr. John Macor and Dr. Joanne Bronson for
critical reading of the manuscript.
8jj
8kk
8ll
PhCH2CH2
PhCH2
PhCH2
Et
nPr
References and notes
8mm
nBu
1. (a) Contoreggi, C.; Ayala, A.; Grant, S.; Eckelman, W.;
Webster, E.; Rice, K. C. Drug of Future 2002, 27, 1093; (b)
and as seen in our other studies,3,13 the binding pocket
available for these residues is comparatively small and
favors aromatic character. Trifluorination of the other
side chain (R3) was preferred to a greater (8n vs. 8q)
or lesser extent (8e vs. 8o).
´
Paez-Pereda, M.; Arzt, E.; Stalla, G. K. Expert Opin.
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Psychopharmacol. Clin. Exp. 2001, 16, 81.

4032
X. Han et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4029–4032
2. Hauger, R. L.; Grigoriadis, D. E.; Dallman, M. F.;
Plotsky, P. M.; Vale, W. W.; Dautzenberg, F. M.
Pharmacol. Rev. 2003, 55, 21.
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A.; Fung, L.; Fiedler, T.; Browman, K. E.; Taber, M. T.;
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4. Kulagowski, J. J.; Rees, C. W. Synthesis 1980, 215.
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7.19 (m, 2H), 7.02 (s, 2H), 6.89–6.86 (m, 1H), 3.69 (t,
J = 7.0 Hz, 2H), 3.49 (d, J = 6.9 Hz, 2H), 2.39 (s, 3H), 2.35
(s, 3H), 1.98 (s, 6H), 1.74–1.65 (m, 2H), 1.16–1.06 (m, 1H),
0.90 (t, J = 7.2 Hz, 3H), 0.59–0.53 (m, 2H), 0.23–0.20 (m,
2H); 13C NMR (CDCl3, 75 MHz) d 163.7, 139.2, 137.0,
136.8, 135.2, 129.8, 129.6, 128.8, 124.8, 123.4, 121.0, 114.2,
113.1, 110.2, 51.1, 48.5, 21.0, 17.7, 15.4, 14.1, 11.2, 10.1,
3.7; Mass spec.: 428.32 (MH)+. For 8f: 1H NMR (CDCl3,
300 MHz) d 8.09–8.06 (m, 1H), 7.37–7.33 (m, 2H), 7.04 (s,
2H), 6.97–6.94 (m, 1H), 4.95 (s, 2H), 3.27–3.16 (m, 4H),
2.22 (s, 3H), 1.96 (s, 6H), 1.87–1.79 (m, 2H), 1.23–1.20 (m,
1H), 0.97 (t, J = 7.2 Hz, 3H), 0.84–0.80 (m, 2H), 0.46–0.43
(m, 2H); 13C NMR (CDCl3, 75 MHz) d 161.3 (q,
J = 36.8 Hz), 148.4, 140.0, 137.0, 135.8, 129.8, 128.2,
128.0, 125.6, 123.9, 122.8 (q, J = 270 Hz), 122.0, 111.1,
110.3, 57.4, 53.1, 45.9, 21.1, 17.6, 17.3, 11.0, 5.5, 4.9; Mass
spec.: 469.30 (MH)+.
6. Scherz, M. W.; Fialeix, M.; Fischer, J. B.; Reddy, N. L.;
Server, A. C.; Sonders, M. S.; Tester, B. C.; Webber, E.;
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1
The structure of bromide 5a was confirmed by H NMR
studies. First, the CH2 in –CH2CO2Et is a singlet, and there
is no coupling between this CH2 and NH; secondly, there
are positive nOe observed between –CH2CO2Et and –NH2
and also H1.
10. For a description of the workup procedures, see Han, X.;
Corey, E. J. Org. Lett. 2000, 2, 2543.
11. Membranes were prepared from IMR-32 cells as previ-
ously described12 and incubated with [125I]Tyr-o-CRF
(100 pM) and increasing concentrations of test com-
pound for 100 min at 25 °C (assay bufffer: 50 mM Tris
(pH 7.2), 10 mM MgCl2, 0.5% BSA, 0.005% Triton X-
100, 0.01 mg/mL aprotinin, and 0.01 mg/mL leupeptin).
Assays were stopped by the addition of ice-cold buffer.
Nonspecific binding was defined with 0.01 mM o-CRF.
These compounds are full antagonists of the CRF1R, as
determined by their ability to inhibit CRF-stimulated
cAMP production in IMR-32 cells.12 Compound 8e was
also tested against the CRF2R and found to have
IC50 > 10 lM.15
12. Dieterich, K. D.; DeSouza, E. B. Brain Res. 1996, 733, 113.
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9. All new compounds gave satisfactory analytical data. For
5a (Ar = 2,4,6-trimethylphenyl, Q = H): 1H NMR
(CDCl3, 500 MHz) d 8.28 (br s, 2H), 7.40 (t, J = 7.6 Hz,
1H), 7.32 (d, J = 7.4 Hz, 1H), 7.29 (d, J = 8.0 Hz, 1H),
7.14 (s, 2H), 6.87 (d, J = 7.8 Hz, 1H), 5.73 (s, 2H), 4.30 (q,
J = 7.1 Hz, 2H); 2.41 (s, 3H), 2.01 (s, 6H), 1.34 (t,
J = 7.2 Hz, 3H). For 6a (Ar = 2,4,6-trimethylphenyl,
Q = H, R = CF3): 1H NMR (CDCl3, 300 MHz) d 8.76–
8.73 (m, 1H), 7.43–7.34 (m, 2H), 7.08 (s, 2H), 7.01–6.98
(m, 1H), 4.54 (q, J = 7.1 Hz, 2H), 2.39 (s, 3H), 1.99 (s,
6H), 1.50 (t, J = 7.2 Hz, 3H); 13C NMR (CDCl3,
125 MHz) d 159.1, 147.0, 139.9, 139.1 (q, J = 39 Hz),
136.9, 135.7, 129.7, 129.5, 128.0, 125.4, 125.0, 121.9, 120.9
(q, J = 268 Hz), 116.4, 110.6, 61.4, 21.0, 17.6, 14.0. For 7a:
1H NMR (CDCl3, 300 MHz) d 7.86–7.82 (m, 1H), 7.23–