Synthesis and SAR of 2-(4-fluorophenyl)-3-pyrimidin-4-ylimidazo[1,2-a]pyridine derivatives as anticoccidial agents

Article abstract of DOI:10.1016/j.bmcl.2006.08.127

Compounds 10a-10d and 10i are very potent inhibitors of Eimeria tenella cGMP-dependent protein kinase (0.081-0.32 nM) and are very efficacious antiparasitic agents in vivo when administered to chickens at 12.5-25 ppm levels in the feed.

Full text of DOI:10.1016/j.bmcl.2006.08.127

Bioorganic & Medicinal Chemistry Letters 16 (2006) 5978–5981
Synthesis and SAR of 2-(4-fluorophenyl)-3-pyrimidin-4-
ylimidazo[1,2-a]pyridine derivatives as anticoccidial agents
Dennis Feng,a,* Michael Fisher,a Gui-Bai Liang,a Xiaoxia Qian,a Chris Brown,b
Anne Gurnett,b Penny Sue Leavitt,b Paul A. Liberator,b John Mathew,b
Andrew Misura,b Samantha Samaras,b Tamas Tamas,b Dennis M. Schmatz,b
Matthew Wyvratta and Tesfaye Biftua
aMerck Research Laboratories, Department of Medicinal Chemistry, Merck and Co., Inc., PO Box 2000, Rahway, NJ 07065, USA
bHuman and Animal Infectious Disease Research, Merck and Co., Inc., PO Box 2000, Rahway, NJ 07065, USA
Received 9 August 2006; accepted 30 August 2006
Available online 26 September 2006
Abstract—Compounds 10a–10d and 10i are very potent inhibitors of Eimeria tenella cGMP-dependent protein kinase (0.081–
0.32 nM) and are very efficacious antiparasitic agents in vivo when administered to chickens at 12.5–25 ppm levels in the feed.
Ó 2006 Elsevier Ltd. All rights reserved.
Coccidiosis is the major cause of morbidity and mortal-
ity in the poultry industry. It is a disease of the avian
intestinal lining due to invasion by protozoan parasites
of the genus Eimeria. Some of the most significant Eime-
ria species in poultry are E. tenella, E. acervulina, E.
necartrix, E. brunetti, and E. maxima. Over 35 billion
chickens are raised annually worldwide and all major
poultry operations use anticoccidial agents prophylacti-
cally. Resistance to current coccidiostats is becoming
widespread1 and new broad spectrum drugs with novel
mechanisms are needed.
The 2-(4-fluorophenyl)-3-pyrimidin-4-ylimidazo[1,2-a]-
pyridine derivatives (10) are potent inhibitors of the E.
tenella cGMP-dependent protein kinase2 (Et-PKG).
Genetic studies in Toxoplasma gondii, a protozoan par-
asite closely related to Eimeria, demonstrate that PKG is
essential for survival and represents a desirable thera-
peutic target.3 In this paper, the synthesis, SAR studies,
and biological data in vitro and in vivo of modified imi-
dazopyridine derivatives which are more potent than
our previously reported pyrrole analogs4 will be
discussed.
Synthesis. The imidazopyridine derivatives in Tables 1
and 25 were prepared as shown in Scheme 1.6 Methyla-
tion of 4-methyl pyrimidine-2-thiol 1 with DMF-DMA
gave 4-methyl-2-(methylthio)pyrimidine 2. The ketone3
was made by coupling the anion generated from 4-meth-
yl-2-(methylthio)pyrimidine 2 and lithium diisopropyla-
mide with 4-fluorobenzoate in tetrahydrofuran. The
bromo ketone 4 was made by brominating ketone 3 with
tetra-n-butylammonium tribromide in methylene chlo-
ride. The imidazopyridine 5 was made by cyclization
of bromo ketones 4 with 2-amino-4-hydroxymethylpyri-
dine 6 by refluxing in ethanol. The primary alcohol 5
was oxidized to the sulfone 7 with oxone and displace-
ment of the sulfone with amines or alcohols gave the
pyrimidine 8. The pyrimidine 8 was easily converted to
the mesylate 9 and displaced with different amines to
R1
N
N
N
N
R3
N
R2
10
F
Keywords: Kinase; Coccidiosis; Parasite; Imidazopyrimidine; PKG
inhibitor.
*
Corresponding author. E-mail: dennis_feng@merck.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.08.127

D. Feng et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5978–5981
5979
Table 1.
R1
N
N
N
N
N
F
Compound
R1
Ten_K IC50(nM)
ppm
Et
Ea
10a
10b
10c
NH–Bn
NH2
NHMe
0.081
0.11
0.17
25
3
3
3
3
3
3
12.5
25
10d
0.32
25
3
3
NH
10e
10f
10g
10h
0.47
3.0
25
25
50
25
3
0
0
0
0
2
0
0
HN
N(Me)–Bn
OBn
0.27
2.6
OMe
Table 2.
NH
N
N
N
N
R3
N
R2
F
Compound
R2
R3
Et
Ten_K IC50 (nM)
ppm
Et
Ea
10i
10j
CH2CH2OH
CH2CH2OH
0.1
25
25
50
N.A.
25
50
25
25
50
25
50
50
3
3
Me
Me
H
0.11
0.13
0.14
0.16
0.16
0.16
0.32
0.81
0.93
0.99
1.4
3
0
10k
10l
Et
Et
3
N.A.
3
N.A.
10m
10n
10o
10d
10p
10q
10r
10s
Me
t-Bu
Et
H
0
3
3
3
3
2
3
3
2
3
0
3
0
0
0
0
H
Et
Me
OH
OMe
COMe
H
Me
Me
Me
H
H
N.A., not available.

5980
D. Feng et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5978–5981
S
SH
i-Pr2NEt
.
HCl
N
O
+
N
N
N
N
O
1
2
S
S
N
N
4
N
N
3
n-Bu4NBr3
Br
O
LDA
O
O
F
O
F
F
OH
EtOH, 60oC
5
N
NH2
SO2
N
S
N
N
N
Oxone
N
OH
N
N
OH
N
F
F
7
6
ROH or RNH2
R1
R1
N
N
N
N
MsCl, Et3N
N
N
OMs
OH
N
N
8
R1 = RNH or RO
R1 = RNH or RO
F
F
9
HNR2R3,
i-Pr2NEt
R1
N
N
N
R3
N
R2
N
F
10 R1 = RNH or RO
Scheme 1.
yield 10. Compounds in Tables 1 and 2 were made in a
similar manner.
anhydride. Oxidation of the 12 by potassium permanga-
nate gave the acid 13. Bubbling of hydrochloric gas into
ethanolic solution of the 13 provided the ester 14 and
reduction of the 14 by lithium aluminum hydride yielded
the alcohol 5.
Preparation of intermediate 5 is outlined in Scheme 2.7
2-Aminopicoline 11 was converted to 12 with acetic

D. Feng et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5978–5981
5981
both host and parasites) that could be affected by the
basicity and lipophilicity of the compounds. In general,
for compounds listed in Tables 1 and 2, the in vivo activ-
ity tracks with the PKG activity fairly well. Potent PKG
inhibitors (10a–10d, 10i,10k, and 10o) are active in in vi-
vo assays when administered to birds in feed. The most
potent analogs are 10a–10d and 10i. These compounds
are active in feed at a level of 12.5–25 ppm. Commercial
coccidiostats such as salinomycin and amprol are active
at 66 and 130 ppm, respectively.
KMnO4
O
acetic anhydride
N
NH2
N
N
H
11
12
OH
O
OH
O
O
HCl
LAH
O
EtOH
Conclusion. Novel imidazopyridine analogs were found
to be potent inhibitors of parasite PKG activity. The
most potent compounds are the tertiary amines 10a–
10d and 10i (PKG activity 0.081–0.32 nM). These com-
pounds were also fully active in in vivo assays as anti-
coccidial agents at 12.5–25 ppm level in feed.
N
5
NH2
N
N
N
NH2
H
14
13
Scheme 2.
Biological assays. To evaluate these compounds as anti-
coccidial agents, an enzyme inhibition assay against E.
tenella cGMP-dependent protein kinase (Et-PKG) was
used as the initial in vitro screening assay; oocyst reduc-
tion against two major Eimeria species in poultry (E.
tenella (E.t.) and E. acervulina (E.a.)) was used for in vi-
vo assay. Treatments which provide at least 80% reduc-
tion in oocyst production are rated 3, those with 50–79%
are rated 2, and those with <50% are rated 0. Details of
these procedures and rules of scoring were published in
the preceding paper.4,6
Acknowledgments
The authors thank Dr. Phil Eskola, Dr. Gerard Kieczy-
kowshi, Ms. Amanda Makarewicz, Mr. Bob Frankshun,
and Mr. Glen Reynolds for scale-up of key intermedi-
ates, Dr. Ann E. Weber for review of the manuscript,
and Merck Research Laboratory Basic Chemistry
Analytical Support Group for open access service on
LC/MS.
References and notes
Results and discussion. The data shown in Table 1 sum-
marize the effect of substituents in the 2-position of the
pyrimidine ring on PKG activity and in vivo efficacy. In
general, primary amine (10b) and secondary amines
(10a, 10c, and 10d) in this position have excellent
PKG activity (0.081–0.32 nM). However, a tertiary
amine (10f) is at least 14-fold less potent. Steric interac-
tion may not be very important in this position, since the
1-phenylethaneamine (10d) retains activity. Orientation
of methyl group plays a very important role in the
PKG activity since the S isomer (10d) is more potent
than the R isomer (10e). Strong hydrogen bonding
reduces the PKG activity as shown for the methoxy
(10h) and benzyloxy (10g) analogs which are less potent
than the methylamine (10c) and benzylamine (10a),
respectively. Anticoccidial activity of 10a–10d in vivo
is better than that of 10e–10h, and is consistent with
PKG activity in vitro.
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As shown in Table 2, basicity in the 7-position of imi-
dazopyridine is very important for PKG activity. Sub-
stituents such as hydroxyl and methoxy amine (10p
and 10q), amide (10r), and primary amine (10s) are
much less potent than the secondary and tertiary amines
10i–10o. For active PKG inhibitors, in vivo efficacy is
determined by many factors (including absorption in
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