Discovery of MK-7246, a selective CRTH2 antagonist for the treatment of respiratory diseases

Article abstract of DOI:10.1016/j.bmcl.2010.11.015

In this manuscript we wish to report the discovery of MK-7246 (4), a potent and selective CRTH2 (DP2) antagonist. SAR studies leading to MK-7246 along with two synthetic sequences enabling the preparation of this novel class of CRTH2 antagonist are reported. Finally, the pharmacokinetic and metabolic profile of MK-7246 is disclosed.

Full text of DOI:10.1016/j.bmcl.2010.11.015

Bioorganic & Medicinal Chemistry Letters 21 (2011) 288–293
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of MK-7246, a selective CRTH2 antagonist for the treatment
of respiratory diseases
Michel Gallant a, , Christian Beaulieu a, Carl Berthelette a, John Colucci a, Michael A. Crackower b,
⇑
Chad Dalton c, Danielle Denis b, Yves Ducharme a, Richard W. Friesen a, Daniel Guay a, François G. Gervais b,
Martine Hamel b, Robert Houle d, Connie M. Krawczyk b, Birgit Kosjek e, Stephen Lau c, Yves Leblanc a,
Ernest E. Lee c, Jean-François Levesque d, Christophe Mellon a, Carmela Molinaro c, Wayne Mullet c,
Gary P. O’Neill b, Paul O’Shea c, Nicole Sawyer b, Susan Sillaots b, Daniel Simard a, Deborah Slipetz b,
Rino Stocco b, Dan Sørensen a, Vouy Linh Truong a, Elizabeth Wong b, Jin Wu d, Helmi Zaghdane a,
Zhaoyin Wang a
a Medicinal Chemistry, Merck Frosst Centre for Therapeutic Research, Kirkland, Québec, Canada
b Biochemistry and Molecular Biology, Merck Frosst Centre for Therapeutic Research, Québec, Canada
c Basic Pharmaceutical Science, Merck Frosst Centre for Therapeutic Research, Québec, Canada
d DMPK, Merck Frosst Centre for Therapeutic Research, Québec, Canada
e Basic Pharmaceutical Science, Merck Research Laboratories, Rahway, NJ, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
In this manuscript we wish to report the discovery of MK-7246 (4), a potent and selective CRTH2 (DP2)
antagonist. SAR studies leading to MK-7246 along with two synthetic sequences enabling the preparation
of this novel class of CRTH2 antagonist are reported. Finally, the pharmacokinetic and metabolic profile of
MK-7246 is disclosed.
Received 14 October 2010
Revised 27 October 2010
Accepted 1 November 2010
Available online 5 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
MK-7246
CRTH2 antagonist
DP2 antagonist
Asthma
Respiratory diseases
In response to various extracellular stimuli, prostaglandins
(PGE2, PGD2, PGF2, PGI2, and TAX2) are rapidly generated through
the consecutive action of the cyclooxygenase and their respective
synthases from membrane arachidonic acid and exert their action
in close proximity to the site of their synthesis.1 To date, nine pro-
stanoid receptors have been cloned and characterized. These
receptors are members of the growing class of G-protein-coupled
receptors.2 PGE2 binds preferentially on the EP1, EP2, EP3, and
EP4 receptors, PGD2 to the DP1 and CRTH2 (referred occasionally
as DP2) receptors, PGF2 to the FP receptor, PGI2 to the IP receptor
and TAX2 to the TP receptor. Activation of the DP1 receptor by its
endogenous ligand PGD2 results in vasodilatation,3 which was
shown to be reversed pharmacologically by the action of an
antagonist.4 As a result, LaropiprantÒ, a selective and potent DP1
antagonist,5 was developed to reverse the facial flushing induced
by Niacin.5 Alternatively, activation of CRTH26 (chemoattractant
receptor-homologous molecule expressed on T-helper type 2 cells),
the second prostanoid receptor targeted by PGD2, stimulates the
recruitment of leukocytes, the release of Th2 cytokines and the
degranulation of basophils and eosinophils. The combined phar-
macological action of such biological events plays a key role in late
phase allergic inflammation.7 Additionally, high levels of PGD2 are
found in the lungs of asthmatic patients8 and genetic polymor-
phism leading to increased CRTH2 mRNA stability is significantly
associated with asthma in two independent populations.9
Together, these observations provide a sound biological rational
for the development of selective CRTH2 antagonists for the treat-
ment of asthma.10 Consequently numerous pharmaceutical com-
panies have initiated medicinal chemistry programs targeting
selective CRTH2 antagonists for the treatment of respiratory
diseases.11
The affinity and selectivity of compounds described herein for
human CRTH2 and recombinant human prostanoid receptors was
determined by equilibrium competition analysis using the relevant
⇑
Corresponding author. Tel.: +1 514 305–1101.
E-mail address: michel_gallant@videotron.ca (M. Gallant).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.11.015

M. Gallant et al. / Bioorg. Med. Chem. Lett. 21 (2011) 288–293
289
radioligands and cell membranes expressing the various recep-
tors.12 CRTH2 is a Gi-coupled receptor that signals through inhibi-
tion of adenylate cyclase leading to an inhibition of intracellular
cAMP production.13 As a result, a functional assay was developed
to measure intracellular cAMP levels resulting from receptor acti-
vation using recombinant cells overexpressing human CRTH2
(HEK293E/CRTH2). DK-PGD2, a CRTH2-selective metabolite of
PGD2, was used to activate the CRTH2 receptor leading to a de-
crease in intracellular cAMP. The ability of our compounds to block
DK-PGD2-induced inhibition of cAMP formation was measured
and is reported as cAMP IC50. Finally, a physiologically relevant
whole blood assay was developed to assess the potency of com-
pounds on the CRTH2 receptor, endogenously expressed at the sur-
face of human cells. Eosinophil activation triggers a shape change,
as a consequence of cytoskeleton reorganization predisposing the
leukocyte for cell movement and transmigration, which can be
quantified by flow cytometry through changes in forward light
scatter.14 The ability of an antagonist to inhibit the DK-PGD2-in-
Substitution on the indole core was first examined (Table 3). In
general mono- and disubstituted analogs were tolerated but in
some cases, significant loss of selectivity over DP (<20-fold) was
observed as exemplified by compounds 5 and 17. Unexpectedly,
full reversal of selectivity was found for the 4,6-dichloro analog
16, favoring affinity for the DP receptor over CRTH2. Overall, three
compounds demonstrated a superior profile to the lead compound
4. The 6-fluoro (8), 7-fluoro (18), and 6,7-difluoro (15) analogs dis-
played a slight improvement in potency on the CRTH2 receptor,
superior selectivity over DP and improved and whole blood activity
over their unsubstituted counterpart 4.
Next, we studied the substitution of the sulfonamide nitrogen in
order to further improve potency and selectivity (Table 4). As ex-
pected, the unsubstituted analog 20 was equipotent on the CRTH2
and TP receptors. Surprisingly, the addition of one methylene unit
as in the N-ethyl analog 21 lead to substantial lost of selectivity
against DP. A similar observation was made for the trifluoroethyl
compound 25. The N-pentyl derivative 22 exhibited enviable selec-
tivity but was highly shifted (67-fold) in the whole blood eosino-
phil shape change assay. Only the N-(4-fluorobenzyl) analog 24
displayed an improved profile compared to 4 especially in terms
of selectivity over the DP receptor.
Finally, the substitution of the phenyl component of the sulfon-
amide was examined (Table 5). Unfortunately, all attempts to im-
prove potency and selectivity failed. Briefly, repositioning the
fluorine atom or replacing it with larger electron withdrawing
groups led to loss of selectivity over DP. Introduction of a heterocy-
clic group as a phenyl ring replacement was also unsuccessful.
Following the optimization of our lead compound based on
CRTH2 potency, whole blood activity and selectivity over the
remaining prostanoid receptors, compounds 4, 8, 15, 18, and 24
were further characterized. Their respective Cytochrome P450,
protein covalent binding, off-target activity, physicochemical, and
pharmacokinetic profiles are summarized in Table 6. In general,
compounds in this series of tetrahydropyrido[1,2-a] indoles are
not competitive inhibitors of CYP2D6 and 3A4 nor do they display
affinity for the hERG channel or the pregnane X receptor17 (PXR), a
nuclear receptor involved in the induction of CYP3A4. They were
also found to be orally bioavailable in rats and displayed moderate
to long half-life. Conversely, members of this class of compounds
were found to be moderate CYP2C9 competitive inhibitors. Such
a property could potentially lead to undesired drug–drug interac-
tions if patients are under multiple therapies.18 Of the five com-
pounds selected for further profiling, 4, 18, and 24 maintained
acceptable properties. Time-dependent inhibition of CYP3A4 has
also the potential to perpetrate drug–drug interactions in humans.
Measuring the difference (reported as% lost) between the CYP3A4
duced shape change was measured and is reported as ESC IC50
.
Ramatroban15a (Table 1), a dual TP/CRTH2 antagonist,15b was
reported to exhibit some degree of efficacy in allergic rhinitis and
is currently commercialized in Japan. Shortening the carboxylic
acid chain by one methylene unit, as exemplified by compound
1, reverses the selectivity in favor of the CRTH2 receptor. N-Meth-
ylation of the sulfonamide moiety led the Shionogi team to the dis-
covery of the tetrahydro-carbazole TM30089,16 a potent and
selective CRTH2 antagonist. We prepared the reverse indole of
TM30089 (i.e., compound 2) and found that the potency and selec-
tivity for the CRTH2 receptor was maintained. This discovery
constituted the starting point of SAR studies performed in our lab-
oratories. The results are described in this manuscript.
Chiral resolution of the racemic compound
2 confirmed
previously disclosed data on Ramatroban and TM30089, that the
affinity and selectivity for CRTH2 is superior for the R enantiomer
(i.e., 4) than for the S (i.e., 3) as shown in Table 2. Compound 4 be-
haved as a full antagonist in our functional assay and was barely
shifted (threefold) in the whole blood assay. Additionally, 4 dem-
onstrated low affinity for the six remaining prostanoid receptors
(Ki > 7.5 lM). SAR studies were initiated to further improve the po-
tency and selectivity of 4.
Table 1
Discovery of a new class of CRTH2 ligands
O
S
O
O
O
O
HN
HN
S
mediated degradation of testosterone (250
tosterone, with and without pre-incubation of the potential perpe-
trator (10 M), allowed us to rapidly assess if this class of
lM) to 6-b-hydroxytes-
F
F
F
F
N
N
N
l
1
CO2H
tetrahydropyrido[1,2-a] indoles acted as time-dependent CYP3A4
inhibitors.19 As a result, the 7-fluoro analog 18 was found to be less
susceptible to cause drug–drug interaction by this mechanism. As
part as of our routine characterization of potential development
candidate, the top five analogs were also submitted to various
stress conditions to assess their overall stability. In one of these
experiments, representatives of this class of compounds (2 mM
solution in MeOH) were exposed to full UV light spectrum (1.0
KLux/m2) for 15 min in a Q-Sun Xenon Light Test Chamber. Vari-
able rates of photodegradation were observed. This phenomenon
may represent a developmental challenge as photoinstability of a
molecule that distribute to light exposed tissues (i.e., eye and skin)
has the potential to induce toxicities via the formation of reactive
degradation products.20 The two most photostable analogs found
were those bearing a fluorine atom at the 7 position of the indole
core (i.e., 15 and 18). Photodegradation was observed only in
CO2H
Ramatroban
O
S
Me
O
S
O
Me
N
N
N
CO2H
2
CO2H
TM30089
Compd
Ki (nM)a
CRTH2
DP1
TP
Ramatroban
1
TM30089
2
137
5.5
1.5
6.0
11,020
6140
1115
653
0.58
53
6852
1394
a
Ki determinations are averages of at least two experiments.

290
M. Gallant et al. / Bioorg. Med. Chem. Lett. 21 (2011) 288–293
Table 2
In vitro characterization of the two enantiomers of compound 2
O
S
O
N S
Me
Me
O
O
N
N
N
F
F
CO2H
Kia (nM)
CO2H
3
4
a
Compd
IC50 (nM)
CRTH2
DP
TP
cAMP
ESC
3
4
144 19(3)
2.5 0.5(8)
2391 439(3)
373 96(6)
>21,660(3)
3804 1290(6)
864 25(3)
3.0 1.3(4)
325 43(3)
2.2 1.0(10)
a
Ki and IC50 are mean standard deviation with n values in parenthesis.
Table 3
Table 4
SAR studies on the indole core substitution
SAR studies on the sulfonamide residue
O
S
O
S
R
Me
O
O
N
N
R2
6
7
4
N
N
R1
F
F
5
CO2H
CO2H
Kia (nM)
IC50 (nM)
a
Compd
R1
R2
Kia (nM)
IC50 (nM)
Compd
R
CRTH2
DP
TP
cAMP
ESC
CRTH2
DP1
TP
cAMP
ESC
2.2
4
20
21
22
23
24
25
Me
H
Et
Pentyl
c-PrCH2
4-F-Bn
CF3CH2
2.7
9.8
1.8
1.6
4.9
1.4
3.7
373
3804
10.9
2142
904
1222
1569
1357
3.0
35.5
8.1
4.9
6.7
2.2
1.6
4
5
H
4-F
H
H
2.5
11
373
216
284
>3235
248
104
395
2093
>3952
1282
121
541
5.6
48
1079
711
3804
888
1872
>6489
727
3949
1518
>7139
>7247
2890
719
3.0
26.5
3.0
>10,000
2.6
3.0
3.6
9.0
38
4.5
3.8
2.5
24
5.7
2.5
9.9
>4069
14.6
1213
130
2292
15.5
6
5-F
H
4.6
1083
1.1
1.1
1.7
8.8
13
2.4
2.5
1.0
26
11.5
107
4.1
7
8
5-SO2Me
6-F
H
H
1.4
11.5
10.8
5.7
6.9
9
6-Cl
6-CF3
6-CN
6-SO2Me
6-Ph
6-F
H
H
H
H
10
11
12
13
14
15
16
17
18
19
a
Ki and IC50 determinations are averages of at least two experiments.
5.0
30.5
H
5-F
7-F
4-Cl
5-F
H
6-F
962
1.1
Table 5
6-Cl
6-Cl
7-F
1330
1840
2152
427
SAR studies on the phenyl residue
3.7
1.6
4.0
O
S
Me
1.8
14.1
O
N
7-F
5-F
R
a
Ki and IC50 determinations are averages of at least two experiments.
N
CO2H
a
Compd
R
Kia (nM)
CRTH2 DP
IC50 (nM)
cAMP
solution under full UV spectrum light (sunlight) but not in ambient
light (laboratory light) nor in the solid form. As a consequence of
these results, two analogs, 4 and 18 were further profiled by per-
forming in vitro covalent binding studies in rat and human micro-
some using radiolabeled material. A significant level of covalent
binding was observed for the photostable analog 18 compared to
the des-fluoro analog 4.
Based on the overall profiles of our top five optimized CRTH2
antagonists and in particular with respect to their relative CYP pro-
files, compound 4 was chosen for further development becoming
MK-7246.21
MK-7246 was further characterized by establishing its overall
metabolism profile and performing additional pharmacokinetic
studies in non-rodent preclinical species (Table 7). The rank order
of metabolic stability of MK-7246 in hepatocytes was Sprague–
Dawley rat > beagle dog > cynomolgus monkey > rhesus mon-
key ꢀ human. In all species, the acyl glucuronide of the parent
(40) was the major metabolite observed (Fig. 1). Minor metabolites
detected included a product of oxidative decarboxylation (41) and
its putative acyl glucuronide (42). Low turnover of MK-7246 was
TP
4
26
27
28
29
30
31
32
33
34
35
36
37
38
39
4-F-Ph
2-F-Ph
2-Tol
3-F-Ph
3-Tol
H
4-Cl
4-CF3-Ph
4-MeO-Ph
4-Et-Ph
4-F-Bn
5-Cl-thiophene-2-yl
1-Me-pyrazole-4-yl
2.7
17
373
3804 3.0
892 >21,160 22
7.6
21
51
1075
250
537
47
123
26
16
>5591 27
>7054 33
>5591 13
>7054 12
1039 8.5
2118 214
>7096 5.5
>5591 15
>5745 25
2840 13
8.7
4.3
5.5
40.8
4.4
20
20
6.3
124
799
283
>3901
322
>3901
>7181 63
>7054 250
>7181 308
3,5-Di Me-isoxazole-4-yl 296
2-Pyridinyl 117
a
Ki and IC50 determinations are averages of at least two experiments.
observed in rat, dog, monkey and human liver microsomes and
the major metabolites detected were 41, and two mono-
hydroxylated products 43 and 44 of undetermined structure. All

M. Gallant et al. / Bioorg. Med. Chem. Lett. 21 (2011) 288–293
291
Table 6
Comparative profile of optimized CRTH2 antagonists
a
Compd
CYP inhibition IC50
M)
3A4 TDI
PXR
EC50
Photosensitivity
hERG
In vitro CB (pmol equiv/
mg protein)
Rat PKb
(
l
a
2D6
2C9
3A4
% Lost @ 10
lM
(l
M) % Degradation in solution Kia
(
l
M) Rat
Human
F (%) Cl (mL/min/kg) T1/2 (h)
4
8
15
18
24
>100
>50
>50
>50
>50
9.4
13.6
6.4
4.2
21.0
34
>50
>50
>50
>50
20
17
26
12
49
>30
>30
>30
>30
>30
45
89
1
4
26
>29
35
154 27
455 30
205 23
107
100
72
2.2
1.8
0.4
0.4
5.6
4.3
13
>60
>60
568 42
62
16
a
Ki, IC50, EC50, % lost and % degradation are averages of at least two experiments
Rat PK studies were conducted in Sprague–Dawley rats at 10 mg/kg po in 0.5% methocel (n = 2) and 5 mg/kg iv in PEG200 60% (n = 2).
b
Table 7
Pharmacokinetic profile of MK-7246 in preclinical species
Species
F (%)
Cl (mL/min/kg)
Vdss (L/kg)
T1/2 (h)
AUC40/AUCMK-7246
SD Rat
107
67
10
2.2
15
4.8
6.9
0.98
2.3
0.5
5.6
8.4
11
0.02
0.06
0.5
Beagle dog
Cynomolgus
Rhesus
57
2.6
8.1
1.7
Rat PK studies were conducted in Sprague–Dawley rats at 10 mg/kg po in 0.5% methocel (n = 2) and 5 mg/kg iv in PEG 200 60% (n = 2). Dog and monkey PK studies were
conducted using the same vehicle at 1 mg/kg po and 0.5 mg/kg iv.
pharmacokinetic studies. To this end, plasma samples were
quenched with CH3CN containing 0.5% formic acid to stabilize
the glucuronide 40 and allow its quantification. The results are re-
ported in Table 7 as a ratio to the parent compound MK-7246. In all
species, MK-7246 exhibited a low to moderate plasma clearance
(2.2–15 mL/min/kg), a moderate volume of distribution (0.5–
4.8 L/kg), and a moderate plasma terminal half-life, ranging from
5.6 to 11 h. The oral bioavailability was greater than 57% in all spe-
cies with the exception of cynomolgus monkey where it was only
10%. The acyl glucuronide 40 was found circulating in all species,
however, its relative plasma exposure versus that of MK-7246 re-
vealed marked species differences and was highest in non-human
primates (0.5–1.5) compared to dogs (0.06) and SD rats (0.02).
To lessen the main metabolic pathway by which MK-7246 is
O
S
Me
O
O
S
N
Me
O
N
O
N
N
O
F
F
40
41
CO2H
O
S
Me
O
N
O
HOOC
HO
OH
N
O
S
Me
O
F
OH
Me
N
O
S
CO2H
MK-7246
N
O
OH
N
F
42
N
O
eliminated, three additional analogs were prepared bearing a-sub-
stituent to the carboxylic acid, with the anticipation that steric hin-
O
F
OH
43, 44
O
drance would reduce the rate of glucuronidation. To this end, the
CO2H
HOOC
OH
cyclopropyl 4524 and the two
a-methyl diastereoisomers 46 and
OH
4725 were prepared (Fig. 2). Unfortunately, these modifications re-
sulted in loss of affinity for the CRTH2 receptor (CRTH2 Ki = 318,
341, and 316 nM for 45, 46, and 47, respectively).
Figure 1. Structure of metabolites of MK-7246.
The syntheses of the tetrahydropyrido[1,2-a] indoles described
in this manuscript were prepared according to two distinct syn-
thetic sequences. MK-7246 and analogs bearing various substitu-
ents on the nitrogen of the sulfonamide moiety (Table 4) or the
phenylsulfonyl group (Table 5) were prepared as described in
Scheme 1. Briefly, condensation of phenyl hydrazine hydrochloride
with diethyl 4-oxopimelate in refluxing toluene, followed by treat-
ment of the resulting tetrahydropyridazine 48 with methanesulf-
onic acid in refluxing n-propanol afforded the diester indole 49.
This procedure is a slight modification to the classic Fisher indole
metabolites identified in human liver preparations were also de-
tected in the corresponding incubations from preclinical species.
The structure of the acyl glucuronide metabolite 40 was confirmed
with a synthetic standard22 and was found to have low affinity for
the CRTH2 receptor (Ki > 207 nM)23 and for the remaining eight
prostanoid receptors (Ki > 10 lM). The decarboxylated metabolite
41 was isolated from a large-scale microsomal incubation. It was
also found to display low affinity for the CRTH2 receptor
(Ki > 7.2
(Ki > 2
from permanently bile-cannulated male rats dosed iv with 3H-
MK-7246 (1 mg/kg, 300 Ci/kg) showed that the major component
lM) as well as for the others prostanoid receptors
l
M). Analysis of bile, feces and urine recovered for 48 h
l
O
S
O
N S
Me
Me
O
O
excreted in bile was the acyl glucuronide 40 (90%), the parent MK-
7246 (2.7%), the decarboxylated metabolite 41 (2.3%) and various
oxidative metabolites (5%) accounting for 77% of total radioactiv-
ity. Remaining radioactive material was found in feces (6%) and ur-
ine (1%) for a cumulative recovery of 84%.
N
N
N
F
F
45
46, 47
CO2H
CO2H
To evaluate the level of acyl glucuronidation of MK-7246 in pre-
clinical species, the compound was monitored in all subsequent
Figure 2. Structure of a-substituted analogs of MK-7246.

292
M. Gallant et al. / Bioorg. Med. Chem. Lett. 21 (2011) 288–293
O
NH2
HCl
CO2Et
HN
H
N
N
CO2Pr
49
CO2Pr
CO2Et
a
b
N
+
O
EtO2C
48
c
O
O
H
N
H
N
CO2H
N
N
e
d
CHN2
O
50
51
CO2Pr
52
CO2Pr
CO2Pr
O
O
O
f
S
S
NH2
N
HN
g
h
N
N
F
F
F
54
55
53
CO2Pr
CO2Pr
CO2Pr
i
j
O
S
N
O
O
S
O
NH
N
N
N
g, i
R1
N
MK-7246
CO2H
56
CO2Pr
I
CO2H
O
S
R2
N
O
O
S
R2
O
N
N
N
i
k
54
F
F
II
III
CO2H
CO2Pr
Scheme 1. Synthetic sequence A. Reagents and conditions: (a) toluene, reflux; (b) CH3SO3H, n-propanol, 80 °C; (c) KOH, n-propanol, 50 °C, 4 h; (d) (i) EtOCOCl, THF, 0 °C then
dropwise addition of NMM; (ii) CH2N2, 0 °C; (e) Rh2(Oct)4 (10%), CH2Cl2, rt; (f) ATA enzyme, alanine, FDH system, 99% ee; (g) 4-F-phenylsulfonyl chloride, NEt3, DMAP, CH2Cl2,
rt; (h) NaH, DMF, 0 °C then MeI; (i) THF, iso-propanol, aqueous LiOH (1 M); (j) Mg, MeOH; (k) appropriately substituted aryl sulfonyl chloride, NEt3, DMAP, CH2Cl2, rt.
O
HO
O
MeO
O
O
O
OH
O
S
a
OH
OMe
b
S
NH
58
NH
HO
O
NH2
O
57
D-Aspartic acid
F
c
F
O
S
O
S
d
O
O
F
F
60
59
N
N
OTBS
O
OH
O
O
O
S
S
N
N
H
R
N
e
R
f
R
N
N
CHO
F
F
OTBS
CO2Me
IV
VI
V
CO2Me
O
CO2Me
g
O
S
O
O
S
N
N
R
R
N
N
h, i
F
F
VII
VIII
CO2Me
CO2H
Scheme 2. Synthetic sequence B. Reagents and conditions: (a) (i) SOCl2, MeOH; (ii) NEt3, THF; (b) NaBH4, EtOH; (c) 1,10-(azodicarbonyl)dipiperidine, PBu3, THF; (d) TBSCl,
imidazole, THF; (e) (i) NaH, DMF; (ii) 59; (iii) MeI; (f) (i) TBAF, THF; (ii) DMSO, (COCl)2, cat. DMF; (g) PPTS, toluene; (h) H2, MeOH, cat. Pd/C 10%; (i) aqueous LiOH (1 M), THF,
MeOH.
synthesis described by Sapi26 on related compounds. On larger
scale, we found that using methanesulfonic acid in n-propanol in-
stead of the usual HCl gas/MeOH gave higher and more reproduc-
ible yields. Selective saponification26 of 49 and subsequent
formation of the diazomethylketone 51 enabled the rhodium
catalyzed carbene insertion yielding the tricyclic ketone 52 as

M. Gallant et al. / Bioorg. Med. Chem. Lett. 21 (2011) 288–293
293
described by Capretta.27 The conversion of 52 to the corresponding
chiral amine 53 was the key step in this sequence. Reductive
amination via a transaminase/dehydrogenase catalytic system
yielded the desired chiral amine with a 99% ee. Details on this piv-
otal reaction will be disclosed in a separated manuscript. Coupling
of the chiral amine 53 with 4-fluorobenzenesulfonyl chloride, fol-
lowed by N-methylation of the resulting sulfonamide 54 and
hydrolysis of the propyl ester afforded the desired enantiomeri-
cally pure MK-7246. Removal of the 4-fluoro phenylsulfonyl group
under mild conditions (Mg/MeOH) afforded the desired N-methyl
analog 56. Coupling of various arylsulfonyl chlorides with 56 and
subsequent saponification afforded the desired analogs described
in Table 4. Finally, alkylation of the sulfonamide 54 with different
alkyl and benzyl bromides followed by hydrolysis yielded the com-
pounds described in Table 3.
5. Sturino, C. F.; O’Neill, G.; Lachance, N.; Boyd, M.; Berthelette, C.; Labelle, M.; Li,
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H.; Lévesque, J. F.; Seto, C.; Silva, J. H.; Trimble, L. A.; Carriere, MC.; Denis, D.;
Greig, G.; Kargman, S.; Lamontagne, S.; Mathieu, MC.; Sawyer, N.; Slipetz, D.;
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quence B described in Scheme 2. Briefly, esterification of D-aspartic
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Lamontagne, S.; Rochette, C.; Sawyer, N.; Tremblay, N. M.; Belley, M.; Gallant,
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and Kostenis (Ki = 4.3 nM, Ref. 16a).
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097598.
acid followed by treatment with 4-fluorophenylsulfonyl chloride
and subsequent reduction afforded the diol 58. The aziridine 60
was prepared via a Mitsunobu variation of the Wenker reaction28
and subsequent silylation of the alcohol moiety. Opening of the
aziridine 60 by the sodium salt of appropriately substituted indole
ester (IV)29 followed by direct methylation of the resulting sulfon-
amide sodium salt afforded the silyl ether V. Removal of the silyl
ether protecting group and subsequent Swern oxidation yielded
to key aldehyde precursor VI. Acid promoted cyclization of alde-
hyde VI in refluxing toluene afforded the alkene indole VII. Hydro-
genation and saponification of the unsaturated ester completed the
synthetic sequence. This convergent approach enabled a rapid ac-
cess to compounds described in Table 5.
In summary, the results presented in this manuscript recapit-
ulate our effort towards the identification of a potent and selec-
tive CRTH2 antagonist. MK-7246 is the end result of this
endeavor and after successfully completing preclinical safety
studies in rats and rhesus monkeys, MK-7246 entered Phase I
clinical trials. The results of those studies will be disclosed in sep-
arate manuscripts.
17. Tompkins, L. M.; Wallace, A. D. J. Biochem. Mol. Toxicol. 2007, 21, 176.
18. Zhang, Z. Y.; Wong, Y. N. Curr. Drug Metab. 2005, 6, 241.
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Houle, R.; Lacombe, P.; Laliberté, S.; Liu, S.; MacDonald, D.; Mackay, B.; Martin,
D.; McKay, D.; Powell, D.; Lévesque, J.-F. Bioorg. Med. Chem. Lett. 2010, 20, 5074.
20. (a) Moore, D. E. Drug Saf. 2002, 25, 345; (b) Lankerani, L.; Baron, E. D. J. Cutan.
Med. Surg. 2004, 424.
21. (a) Wang, Z., Indole derivatives as CRTH2 receptor antagonists. WO07019675
and WO10031182.; (b) Gervais, F. G.; Sawyer, N.; Stocco, R.; Hamel, M.;
Krawczyk, C.; Sillaots, S.; Denis, D.; Wong, E.; Wang, Z.; Gallant, M.; Abraham,
W. M.; Slipetz, D.; Crackower, M. A.; O’Neill, G. P. Mol. Phamacol. 2011, 79, 1.
22. Juteau, H.; Gareau, Y.; Labelle, M. Tetrahedron Lett. 1997, 38, 1481.
23. To determine the affinity and selectivity of the glucuronide 40 for the human
CRTH2 and recombinant human prostanoid receptors, the binding assays
conditions were slightly modified. Because of the instability of 40 at
physiologically relevant pH (7) the assays were run at pH 5. MK-7246 was
tested as a reference in these conditions and we obtained similar Ki (within
twofold). Despite these modifications in the assay, 0.4% of degradation of 40
was found in the new assay condition. The potency found for 40 on the CRTH2
receptor was 207 nM but can be accounted for the most part by the presence of
MK-7246 (0.4% with Ki = 2.5 nM). Consequently, the CRTH2 Ki of 40 should be
>207 nM.
Acknowledgments
The authors would like to thank Dr. Yongxin Han, Dr. Scott Pet-
erson and Nicolas Lachance for proofreading this manuscript.
References and notes
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25. Compounds 46 and 47 were prepared from the propyl ester of MK-7246 (i.e.
55) by the action of t-BuOK and MeI followed by saponification and chiral
resolution.
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27. Salim, M.; Capretta, A. Tetrahedron 2000, 56, 8063.
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