3-Indolyl sultams as selective CRTh2 antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.04.046

CRTh2 (DP₂) is a prostaglandin D₂ receptor implicated in the recruitment of eosinophils and basophils within the asthmatic lung. Here we report the discovery of a novel series of 3-indolyl sultam antagonists with low nM affinity for

Full text of DOI:10.1016/j.bmcl.2010.04.046

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3287–3290
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
3-Indolyl sultams as selective CRTh2 antagonists
*
L. Nathan Tumey , Michael J. Robarge, Elizabeth Gleason, Jianping Song, Steven M. Murphy, George Ekema,
Chris Doucette, Doug Hanniford, Marc Palmer, Gary Pawlowski, Joel Danzig, Margaret Loftus,
Karen Hunady, Bruce Sherf, Robert W. Mays, Alain Stricker-Krongrad, Kurt R. Brunden, Youssef L. Bennani,
John J. Harrington
Athersys, Inc., 3201 Carnegie Ave., Cleveland, OH 44115, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 February 2010
Revised 8 April 2010
Accepted 12 April 2010
Available online 14 April 2010
CRTh2 (DP2) is a prostaglandin D2 receptor implicated in the recruitment of eosinophils and basophils
within the asthmatic lung. Here we report the discovery of a novel series of 3-indolyl sultam antagonists
with low nM affinity for CRTh2. These compounds proved to be selective over the other primary prosta-
glandin D2 receptor (DP1) as well as the thromboxane A2 receptor (TP).
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
CRTh2
DP
Ramatroban
Antagonist
Prostaglandin D2
Sultam
Indole
Prostaglandin D2 (PGD2) produced by mast cells is a key media-
tor of asthmatic and allergic inflammatory responses.1 There are
two known receptors of PGD2: DP1 and DP2 (also known as
CRTh2).2–4 The latter receptor is expressed on the surface of eosin-
ophils, basophils, and Th2 cells and is responsible for PGD2-induced
chemotaxis in all three cell types.5 Moreover, CRTh2 is involved in
cytokine release from Th2 cells and degranulation of eosinophils.6,7
This has led to widespread interest in antagonists of CRTh2 as po-
tential agents for the treatment of asthma and related allergic dis-
eases.8–13 Recent publication of the effect of CRTh2 antagonists in
various animal models of asthma and allergic rhinitis have spurred
further interest in the exploration of this target.14–16
Ramatroban and its acetic acid analog (1, Fig. 1) have been re-
ported to be potent antagonists of CRTh2.17,18 We previously dis-
closed the SAR of a series of ‘reverse Ramatroban’ analogs with
similar potency to compound 1.10 In our attempts to design addi-
tional novel CRTh2 antagonists, we hypothesized that the satu-
such as Ramatroban naturally adopt. The synthesis of compound
3 and related compounds is illustrated in Scheme 1. The synthesis
begins with AlCl3 promoted electrophilic addition of compound 2
(derived in one step from saccharin)19 to an indole, giving interme-
diate A. The indole nitrogen of A is alkylated with t-butyl bromo-
acetate and the sulfonyl imine is subsequently reduced to give
the benzosultam C. Deprotection of the tert-butyl group is accom-
plished with TFA giving compounds 3–10.
Compound 3 proved to be only weakly active as an antagonist of
CRTh2, with binding affinity (Ki) of approximately 9.2 lM (Table 1).
Incorporation of various substituents at the 5-position (4–6) im-
proved affinity by 2–3-fold. Incorporation of methyl at the 2-posi-
tion (7) also increased binding by threefold. The effect of
substituents at the 2- and 5-positions proved to be additive, as
O
S
O
O
O
O
S
NH
S
F
NH
X
O
NH
rated ring of compound
1
could be excised from the
Y
4
7
tetrahydrocarbazole and instead form a bicyclic sulfonamide, as
shown in Figure 1. One such embodiment of this hypothesis is ben-
zosultam 3. This analog seemed especially fitting given that it
would retain and rigidify the cisoid geometry that sulfonamides
3
5
6
R
2
N
N
N
1
( )n
HO2C
HO2C
HO2C
3
n = 2: Ramatroban
n = 1: 1
* Corresponding author. Tel.: +1 845 602 1876; fax: +1 845 602 5561.
E-mail addresses: tumeyn@wyeth.com, nathantumey@yahoo.com (L.N. Tumey).
Figure 1. Stuctures of Ramatroban and lead molecule 3.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.04.046

3288
L. N. Tumey et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3287–3290
Table 2
O
S
N
O
S
N
SAR of sulfonamide substitution
O
S
O
O
O
N
a
b
O
S
N
O
R1
R2
R1
R2
CO2tBu
R1
R2
Cl
R1
N
H
2
N
N
H
B
A
R2
CO2H
Kib
N
O
S
O
S
NH
O
NH
O
Compda
R1
R2
(lM)
c
d
R1
R2
3
11
12
13
14
H
Me
H
H
H
H
H
9.2
8.1
2.3
1.1
R1
R2
N
N
C
CH2CH2CH(CH3)2
CH2Ph
CH2CH2Ph
CO2t Bu
CO2H
3-10
f
,
d
e,d
O
S
N
0.63
O
S
N
O
O
N
O
R3
15
H
0.018
Ph
R1
R2
CO2H
7
16
17
18
H
Me
Me
Me
Me
2.8
N
N
CH2CH2CH(CH3)2
CH(CH3)2
Ph
0.56
0.92
0.48
11-17, 19-26
CO2H
18
Scheme 1. Regents and conditions: (a) AlCl3, DCE, 65 °C; (b) t-butyl bromoacetate,
K2CO3, DMF, 80 °C; (c) NaBH4, MeOH, 30 min; (d) TFA, DCM; (e) CuI (1 equiv),
K2CO3, MeNHCH2CH2NHMe (2 equiv), PhBr, toluene, 110 °C, 3 days; (f) alkyl halide,
DMF, K2CO3, 80 °C, 1 h.
19
20
21
Me
Me
Me
X = Me, 0.012
X = Ph, 0.013
X = Me, 0.008 (5-Cl indole core)
N
O
X
22
23
24
25
26
CH2CH2Ph
Me
Me
Me
Me
Me
0.22
0.029
0.035
1.4
CH2CH2OPh
CH2CH2O-(4-Cl-Ph)
CH2CH2CH2OPh
CH2CH2OH
Table 1
SAR of unsubstituted sulfonamides
1.4
O
a
S
NH
O
All compounds were purified by preparative HPLC and were evaluated for
proper identity and purity by analytical HPLC-MS and by 1H NMR.
b
Values shown are the means of at least triplicate samples from radioligand
R2
R1
binding inhibition assays utilizing cells expressing human CRTh2.
N
CO2H
see 12/16 and 14/22). Interestingly, however, the addition of a 2-
methyl group to compound 15 had little effect on the potency
(19). Rather, the presence of the isoxazole ring itself seems to drive
the exceptional potency. Substitution of the isoxazole (20) and
substitution of the indole core (21) had only minor effects on po-
tency. Interestingly, the incorporation of a hydrogen bond acceptor
into compound 22 resulted in compounds (23/24) with similar po-
tency to the isoxazoles. Movement of the hydrogen bond acceptor
further from the core (25) or replacement of the acceptor with a
donor (26) resulted in compounds with significantly decreased po-
tency. Taken together, these results clearly indicate that optimal
potency can be obtained by incorporation of a strategically placed
hydrogen bond acceptor within a few bond lengths of the sultam
nitrogen.
Compda
R1
H
5-Me
5-Cl
5-CN
H
5-Me
5-Cl
5-F
R2
Kib
(lM)
3
4
5
6
7
8
9
10
H
H
H
H
Me
Me
Me
Me
9.2
2.9
2.9
2.0
2.8
1.5
0.69
0.43
a
All compounds were purified by preparative HPLC and were evaluated for
proper identity and purity by analytical HPLC-MS and by 1H NMR.
b
Values shown are the means of at least triplicate samples from radioligand
binding inhibition assays utilizing cells expressing human CRTh2.
As previously stated, the most unexpected finding from this
series of compounds was the identification of a dimethyl isoxazole
side chain that gave dramatically improved binding over the par-
ent compounds. Holding this piece of the molecule constant, a
series of analogs were made to further understand the SAR of
the core sultam ring. Scheme 2 shows the synthesis of a ring-de-
leted analog of 19. 2-Methyl-indole-3-carbaldehyde (27) was
alkylated and treated with (E)-2-phenylethenesulfonamide to give
imine 28. AlCl3 mediated addition of vinyl Grignard gave com-
pound 29. The key step of this sequence is the ring-closing
metathesis of 29 to form 30, which occurs in good yield in reflux-
ing DCM. Hydrogenation of 30 resulted in C–N bond cleavage giv-
ing the undesired ring-opened analog 31. However, conjugate
reduction via sodium borohydride gave the desired key intermedi-
ate 32, which was alkylated and deprotected to give the desired
product, 33. Interestingly, acid promoted deprotection to give 32
gave a complex mixture of products and therefore a one-pot
transesterification/saponification was used to remove the tert-bu-
tyl ester.
can be seen by comparing compounds 3–5 with compounds 7–9.
Combining a methyl at the 2-position with a halogen at the 5-posi-
tion gave our first sub-lM compounds in this series, 9 and 10.
We next set about to explore substitution on the sultam nitro-
gen. The nitrogen of intermediate C (Scheme 1) was alkylated by
standard conditions ultimately giving compounds 11–17 and 19–
26 after deprotection of the carboxylic acid. Alternatively, an Ull-
mann-like coupling20 gave direct aryl substitution on nitrogen
(18). We began our explorations with various alkylations on the
unsubstituted indole scaffold (R2 = H, Table 2). Small alkyl groups,
such as methyl (11) and neopentyl (12) gave little improvement
over the parent compound (3). However, larger groups such as 2-
phenylethyl (14) gave more than a 10-fold improvement in affin-
ity. To our astonishment, a dimethyl isoxazole (15) gave nearly a
500-fold boost in receptor-binding. This compound was greater
than 10-fold more potent than any other compound in the series.
Combining a 2-methyl indole core with N-alkyl substituents
generally improved the affinity of compounds by 2–3-fold (e.g.,

L. N. Tumey et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3287–3290
3289
O
S
O
S
NH
O
O
N
O
Ph
Ph
a,b
c
d
N
N
H
N
29
28
CO2tBu
O
CO2tBu
27
O
O
S
N
S
NH
O
S
NH
O
O
N
O
g,h
e
f
N
N
N
30
32
CO2tBu
CO2tBu
CO2H
33
O
S
O
NH2
N
31
CO2tBu
Scheme 2. Reagents and conditions: (a) t-butyl bromoacetate, K2CO3, DMF, 80 °C; (b) (E)-2-phenylethenesulfonamide, PPTS, toluene, reflux; (c) AlMe3, vinyl MgBr 10 min, rt,
toluene; (d) Grubb’s catalyst, DCM, reflux; (e) NaBH4, iPrOH, 80 C; (f) H2, Pd, EtOH (g) 4-(chloromethyl)-3,5-dimethylisoxazole, K2CO3, DMF, 80 °C; (h) NaOH, EtOH, 80 °C, 1 h.
O
S
NH
Table 3
O
S
N
O
S
N
O
Selectivity of the most potent compounds against the DP and TP receptors
O
O
N
O
H3C
DP1a,b
TPa,b
H3C
a
Compd
CRTh2 Ki (lM)
b,c
19
21
23
24
0.012
0.008
0.029
0.035
0% at 10
15% at 10
0% at 10
28% at 10
l
l
l
l
M
M
M
M
0% at 10
0% at 10
l
l
M
M
M
M
N
N
N
35
34
36
CO2t Bu
CO2t Bu
Ki = 7.8
l
CO2H
Ki = 3.4 l
Scheme 3. Reagents and conditions: (a) AlMe3, MeMgBr, 10 min, 0 °C; (b) 4-
(chloromethyl)-3,5-dimethylisoxazole, K2CO3, DMF, 80 °C; (c) TFA, DCM.
a
Values shown are % inhibition of specific radioligand binding unless otherwise
noted.
b
Values shown are the means of at least triplicate samples from radioligand
binding inhibition assays utilizing cells expressing human CRTh2.
Exploration of the stereogenic carbon21 was explored via AlCl3
mediated addition of a methyl Grignard to compound 34 as out-
lined in Scheme 3. Addition of the dimethylisoxazole tail followed
by standard deprotection of the tert-butyl ester gave the desired
compound 36.
As illustrated in Figure 2, addition of a methyl group to the ste-
reogenic carbon (36) results in only a threefold loss of potency.
Complete removal of the ‘northern’ aromatic ring (33) results in
only a modest loss of affinity. This taken together with the results
in Table 2, suggests that the primary driver of potency for these
molecules is the combination of the ‘southern’ 2-methyl indole
core and the strategically placed isoxazole ring. The southern car-
boxylate/indole moiety presumably anchors the molecule in such a
way that allows the isoxazole to make one or more key hydrogen
bonds with the receptor.
screened for selectivity against the TXA2 receptor (TP). The results
are shown in Table 3. As can be seen, these compounds are clearly
very selective for CRTh2 over the DP1 and TP receptors. The assays
described above are receptor-binding assays which do not directly
measure the function (antagonism/agonism) of the compounds. To
this end, compound 19 was evaluated in a FLIPR-based functional
assay in order to verify that one of the most potent compounds
of this series was, in fact, an antagonist of the CRTh2 receptor in
a cellular context. Gratifyingly, compound 19 was shown to block
the activation of the receptor by PGD2, with a Ki of 28 nM.
In conclusion, we have identified and explored the SAR of a ser-
ies of novel 3-indolyl substituted sultams that are low nanomolar
antagonists of the CRTh2 receptor. The SAR indicates the impor-
tance of a hydrophobic pocket encompassing the 2-position of
the indole ring along with key H-bond acceptors in the vicinity
of the sulfonyl and the sultam N-substituent. The most potent
compounds show essentially no activity against the other prosta-
glandin D2 receptor, DP1. Studies are ongoing to explore the utility
of these compounds in inflammation disease models.
A few of the most active compounds were screened for selectiv-
ity against the other prostaglandin D2 receptor, DP1. Since the
compounds described herein are ultimately relatives of Ramatro-
ban, a Thromboxane-A2 (TXA2) receptor antagonist22, we also
O
S
N
O
S
N
O
S
N
References and notes
N
O
O
N
O
N
O
O
O
Me
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