Synthesis and biological activity of anticoccidial agents: 5,6-Diarylimidazo[2,1-b][1,3]thiazoles

Article abstract of DOI:10.1016/j.bmcl.2008.08.063

Novel 5,6-diarylimidazo[2,1-b][1,3]thiazoles bearing an amine substituent at the imidazothiazole 2-position have been synthesized and evaluated as anticoccidial agents in both in vitro and in vivo assays. Both subnanomolar in vitro activity and broad spec

Full text of DOI:10.1016/j.bmcl.2008.08.063

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5263–5267
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological activity of anticoccidial agents:
5,6-Diarylimidazo[2,1-b][1,3]thiazoles
a
b
b
c
c
Andrew Scribner a, , Susan Meitz , Michael Fisher , Matthew Wyvratt , Penny Leavitt , Paul Liberator ,
*
Anne Gurnett c, Chris Brown c, John Mathew c, Donald Thompson c, Dennis Schmatz c, Tesfaye Biftu b
a SCYNEXIS, Inc., Discovery Chemistry, PO Box 12878, Research Triangle Park, NC 27709-2878, USA
b Merck Research Laboratories, Department of Medicinal Chemistry, Merck & Co., Inc., Rahway, NJ 07065, USA
c Merck Research Laboratories, Department of Human and Animal Infectious Disease Research, Merck & Co., Inc., Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel 5,6-diarylimidazo[2,1-b][1,3]thiazoles bearing an amine substituent at the imidazothiazole 2-posi-
tion have been synthesized and evaluated as anticoccidial agents in both in vitro and in vivo assays. Both
subnanomolar in vitro activity and broad spectrum in vivo potency were detected for several compounds,
particularly compound 10.
Received 2 June 2008
Revised 18 August 2008
Accepted 18 August 2008
Available online 22 August 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Anticoccidial
Antiprotozoan
Protein kinase G
PKG inhibitor
Imidazothiazole
Coccidiosis is a parasitic disease that is the major cause of mor-
bidity and mortality in the poultry industry. It is a disease of the
avian intestinal lining due to invasion by Apicomplexan protozoan
parasites of the genus Eimeria.1 The most significant Eimeria spe-
cies in poultry include E. tenella, E. acervulina, E. mitis, and E. max-
ima. Over 35 billion chickens are raised annually worldwide, and
all major poultry operations use anticoccidial agents as prophylac-
tics, such as polyether ionophores. Nevertheless, resistance to cur-
rent coccidiostats has become widespread,2 creating the need for
new broad spectrum drugs with novel mechanisms of action.
Recently, we have reported on novel anticoccidial agents with
potent in vitro and in vivo activity against Eimeria parasites. Reduc-
tion of parasite growth by these compounds was found to be due to
the inhibition of parasite-specific cGMP-dependent protein kinase
(PKG), a serine and/or threonine protein kinase.3,4 In particular, we
have found that various 2,3-diarylpyrroles5–8 and 2,3-diarylimi-
dazopyridines9–13 show exceptional potency as anticoccidial
agents.
3-position, and an amine-bearing side chain at the pyrrole 5-posi-
tion or imidazopyridine 7-position. We therefore used this func-
tionality to explore the imidazo[2,1-b][1,3]thiazole template.
Scheme 1 depicts the synthesis of imidazothiazoles bearing a 2-
aminopyrimidin-4-yl or pyrimidin-4-yl ring at the imidazothiazole
5-position. Treatment of 4-fluorophenacyl bromide with methyl 2-
aminothiazole-5-carboxylate yielded imidazothiazole 1. Subse-
quent treatment of 1 with DIBAL reduced the ester completely to
the corresponding alcohol 2, which was then refluxed in acetic
anhydride with catalytic sulfuric acid to afford diacylated product
3. DMFDMA then selectively reacted with the ketone and not the
ester of 3 to give N,N-dimethylaminoenone 4. Treatment of 4 with
either guanidine–HCl or formamidine–HCl under basic conditions
yielded 5-(2-aminopyrimidin-4-yl)imidazothiazole 5 and 5-(pyr-
imidin-4-yl)imidazothiazole 6, respectively. Oxidation with man-
ganese(IV) oxide then gave aldehydes
7 and 8. Reductive
amination of the 5-(2-aminopyrimidin-4-yl) substrate with either
dimethylamine or 1-methylpiperazine gave the corresponding
amine products 9 and 10, while reductive amination of the pyrim-
idin-4-yl substrate was carried out with 1-methylpiperazine only
to give benzylic piperazine 11.
Synthesis of the corresponding 5-(pyridin-4-yl)imidazothiaz-
oles is shown in Scheme 2. Introduction of the amine side chain
in this case worked best when taking place prior to installation
of the 5-(pyridin-4-yl) ring. An alternative approach for converting
a benzylic alcohol to an amine was implemented here, entailing
mesylation of alcohol 2 to mesylate 12, followed by nucleophilic
In this paper, we present the synthesis and biological activity of
a series of 5,6-diarylimidazo[2,1-b][1,3]thiazoles to see how such
compounds compared with the pyrroles and imidazopyridines.
Optimal substituents from these two series included a 4-fluoro-
phenyl ring at the core heterocycle 2-position, a pyrimidin-4-yl,
2-aminopyrimidin-4-yl, or pyridin-4-yl ring at the core heterocycle
* Corresponding author. Tel.: +1 919 544 8610; fax: +1 919 544 8697.
E-mail address: andrew.scribner@scynexis.com (A. Scribner).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.08.063

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A. Scribner et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5263–5267
O
O
OH
Br
N
N
a
b
S
N
N
S
O
F
F
F
2
1
c
R
O
O
O
O
N
N
N
N
N
N
OH
e or f
d
N
F
S
S
N
N
O
O
S
F
F
R = NH2 (5)
R = H (6)
4
3
g
R
R
H
N
N
R'
N
N
O
h or i
N
N
N
S
S
N
F
F
R = NH2 (7)
R = H (8)
R = NH2; R' = CH2NMe2 (9)
R = NH2; R' = CH2-piperazin-4-yl (10)
R = H; R' = CH2-piperazin-4-yl (11)
Scheme 1. Reagents and conditions: (a) methyl 2-aminothiazole-5-carboxylate, EtOH, 60 °C; (b) DIBAL, CH2Cl2, 0 °C to rt; (c) Ac2O, H2SO4, reflux; (d) DMFDMA, reflux; (e)
guanidine–HCl, NaOMe, 1-propanol, reflux; (f) formamidine–HCl, NaOMe, 1-propanol, reflux; (g) MnO2, CH2Cl2; (h) HNMe2, NaB(OAc)3H, CH2Cl2; (i) 1-Me-piperazine,
NaB(OAc)3H, CH2Cl2.
Scheme 3. Treatment of mesylate 12 with tetra-n-butylammonium
cyanide gave nitrile 19, which was reduced with DIBAL to yield
aldehyde 20. Subsequent reductive amination with either dimeth-
ylamine or 1-methylpiperazine afforded amine products 21 and 22,
respectively. Subsequent bromination and Suzuki coupling as de-
scribed before gave bromides 23 and 24, and then 5-(pyridin-4-
yl)imidazothiazoles 17 and 18.
R'
OMs
N
N
N
N
a
b or c
S
S
2
F
F
R' = CH2NMe2 (13)
12
R' = CH2-piperazin-4-yl (14)
Schemes 4 and 5 show the synthesis of analogs of compound 18
bearing modification to the imidazothiazole 2- and 3-positions,
respectively. Synthesis of the benzamide analog of 18 is shown
in Scheme 4. Hydrolysis of ester 1 under basic conditions yielded
carboxylic acid 27, which was then coupled with 1-methylpipera-
zine to afford amide 28. Subsequent bromination and Suzuki cou-
pling as described before gave bromide 29, and then 5-(pyridin-4-
yl)imidazothiazole 30.
d
R'
N
R'
Br
N
e
N
N
S
S
N
Synthesis of the 3-methylimidazothiazole analog of 18 is shown
in Scheme 5. Treatment of 4-fluorophenacyl bromide with ethyl 2-
amino-4-methylthiazole-5-carboxylate yielded imidazothiazole
31. DIBAL reduction afforded alcohol 32, which was then oxidized
with manganese(IV) oxide to give aldehyde 33. Reductive amina-
tion with 1-methylpiperazine gave amine target 34, which was
treated with N-bromosuccinimide to give bromide 35. Suzuki cou-
pling with pyridine-4-boronic acid ultimately gave 5-(pyridin-4-
yl)imidazothiazole 36.
Table 1 presents both in vitro and in vivo biological data of
each compound tested.14 In vitro activity was assessed by mea-
suring compound inhibition of native E. tenella (Et) PKG enzyme
activity, and is reported as an IC50. In vivo activity was deter-
mined by administering each compound orally in feed, and then
ranking each for anticoccidial activity using a 7 day efficacy
model. A quantitative measure of E. tenella (Et), E. acervulina
F
F
R' = CH2NMe2 (17)
R' = CH2-piperazin-4-yl (18)
R' = CH2NMe2 (15)
R' = CH2-piperazin-4-yl (16)
Scheme 2. Reagents and conditions: (a) MsCl, Et3N, THF; (b) HNMe2, THF; (c) 1-Me-
piperazine, THF; (d) NBS, CH2Cl2; (e) pyridine-4-boronic acid, Pd(PPh3)4, XANT-
PHOS, Na2CO3, 2:1 1,4-dioxane:water, 90 °C.
displacement by either dimethylamine or 1-methylpiperazine to
give amine products 13 and 14, respectively. Regioselective bro-
mination at the imidazothiazole 5-position proceeded with N-bro-
mosuccinimide to give bromides 15 and 16, which were ultimately
subjected to Suzuki coupling conditions with pyridine-4-boronic
acid to give 5-(pyridin-4-yl)imidazothiazoles 17 and 18.
The 5-(pyridin-4-yl) series was expanded to include amine side
chains homologated relative to compounds 17 and 18, shown in

A. Scribner et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5263–5267
5265
O
CN
N
N
N
a
b
H
S
S
12
N
F
F
19
20
c or d
N
R'
R'
R'
Br
N
N
N
N
N
N
f
e
S
S
S
F
F
F
R' = CH2CH2NMe2 (25)
R' =
R' = CH2CH2NMe2 (23)
R' =
R' = CH2CH2NMe2 (21)
R' =
CH2CH2-1-Me-piperazin-4-yl (22)
CH2CH2-1-Me-piperazin-4-yl (26)
CH2CH2-1-Me-piperazin-4-yl (24)
Scheme 3. Reagents and conditions: (a) n-Bu4NCN, CH2Cl2; (b) DIBAL, CH2Cl2; (c) HNMe2, NaB(OAc)3H, CH2Cl2; (d) 1-Me-piperazine, NaB(OAc)3H, CH2Cl2; (e) NBS, CH2Cl2; (f)
pyridine-4-boronic acid, Pd(PPh3)4, XANTPHOS, Na2CO3, 2:1 1,4-dioxane:water, 90 °C.
O
O
OH
N
a
b
N
N
N
N
N
S
S
1
F
F
27
28
c
O
O
N
N
N
d
Br
N
N
N
N
N
N
S
S
F
F
30
29
Scheme 4. Reagents and conditions: (a) NaOH, CH3OH, 60 °C; (b) 1-Me-piperazine, EDC–HCl, Et3N, CH2Cl2; (c) NBS, CH2Cl2; (d) pyridine-4-boronic acid, Pd(PPh3)4,
XANTPHOS, Na2CO3, 2:1 1,4-dioxane:water, 90 °C.
(Ea), E. mitis (Emi), and E. maxima (Ema) oocyst shedding from in-
fected birds provided an assessment of antiparasitic activity.
Treatments resulting in reduction of oocyte burden by 100%
are scored a ‘4’, those with 99% reduction are scored a ‘3+’, those
with 80–98% reduction are scored a ‘3’, those with 50–79%
reduction are scored a ‘2’, and those with <50% reduction are
scored a ‘0’.
Regarding in vitro activity, it is seen that subnanomolar
activity was achieved with the 5-(2-aminopyrimidin-4-yl) com-
pounds bearing either a benzylic dimethylamine or 1-methylpi-
perazine side chain (9 and 10, respectively), as well as the
5-(pyridin-4-yl) compound bearing a benzylic 1-methylpipera-
zine (18). Within the 5-(2-aminopyrimidin-4-yl) and 5-(pyri-
din-4-yl) families, the most tightly binding compounds possess
a piperazine ring in the side chain. Compounds bearing an
amide at the benzylic carbon (30), or a methyl group at the
3-position (36), either of which can alter the most stable con-
formation of the key pharmacophore sites of the molecule, were
considerably less potent.
ring, and that the optimal distance between this amine nitrogen
and the imidazothiazole 2-position is five atoms. The limited
entropic freedom of this nitrogen, enforced by the piperazine ring,
may also be important for in vivo activity. The most potent com-
pounds include benzylic piperazines 10 and 11, which were
strongly active against all four species of Eimeria tested, as well
as benzylic piperazine 18, which was strongly active against three
out of four species of Eimeria tested.
In conclusion, we have prepared several novel 5,6-diaryl-2-
substituted imidazo[2,1-b][1,3]thiazoles, three of which show
subnanomolar potency in vitro against E. tenella cGMP-dependent
protein kinase (PKG), and three of which show strong in vivo po-
tency against at least three of the four species of Eimeria tested.
Of these, 5-(2-aminopyrimidin-4-yl)-2-(CH2-1-methylpiperidin-4-
yl)imidazo[2,1-b][1,3]thiazole 10 also showed significant activity
versus E. tenella, E. acervulina, E. maxima, and E. mitis, respectively,
when tested at 25 ppm (4,4,4,4), 12.5 ppm (4,4,4,0), and
6.25 ppm (4,4,4,0). This level of potency compares with that of
our most potent imidazopyridine (IC50 = 0.044 nM, with 6.0 ppm
scores of 4,3,3,4).13 Hence, the 5,6-diarylimidazo[2,1-b][1,3]thia-
zole family holds significant promise for the treatment of
coccidiosis.
In vivo activity was optimal in compounds bearing a benzylic
piperazine ring, suggesting that the most external nitrogen is likely
the more important of the two basic nitrogens on the piperazine

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A. Scribner et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5263–5267
O
H3C
H3C
Br
OH
O
b
N
N
a
N
S
S
O
N
F
F
F
31
32
c
H
H3C
H3C
H3C
N
N
O
d
Br
e
N
N
N
N
N
N
N
N
S
S
S
F
F
F
35
34
33
f
H3C
N
N
N
N
N
S
F
36
Scheme 5. Reagents and conditions: (a) ethyl 2-amino-4-methylthiazole-5-carboxylate, EtOH, 60 °C; (b) DIBAL, CH2Cl2, 0 °C to rt; (c) MnO2, CH2Cl2; (d) 1-Me-piperazine,
NaB(OAc)3H, CH2Cl2; (e) NBS, CH2Cl2; (f) pyridine-4-boronic acid, Pd(PPh3)4, XANTPHOS, Na2CO3, 2:1 1,4-dioxane:water, 90 °C.
Table 1
Et PKG inhibition and in vivo anticoccidial activity of 5,6-diaryl-2-substituted imidazo[2,1-b][1,3]thiazoles
R
N
A
R'
N
N
S
F
Compound
R
A
R0
Et PKG IC50 (nM)
Anticoccidial activity at 50 ppm
Et
Ea
Emi
Ema
9
NH2
NH2
H
H
H
H
H
H
H
N
N
N
CH
CH
CH
CH
CH
CH
CH2NMe2
0.9
3+
4
4
3
3+
2
4
4
3+
2
4
2
4
4
0
3
3+
0
0
4
4
0
0
0
2
10
11
17
18
25
26
30
36
CH2-1-Me-piperazin-4-yl
CH2-1-Me-piperazin-4-yl
CH2NMe2
CH2-1-Me-piperazin-4-yl
CH2CH2NMe2
CH2CH2-1-Me-piperazin-4-yl
C(@O)-1-Me-piperazin-4-yl
CH2-1-Me-piperazin-4-yl*
0.08
1.04
22
0.7
10.1
2.5
2
3
3+
100
>100
Not tested
Not tested
*
Imidazothiazole core bears a methyl substituent at the 3-position.
6. Liang, G.-B.; Qian, X.; Biftu, T.; Feng, D.; Fisher, M.; Crumley, T.; Darkin-Rattray,
S. J.; Dulski, P. M.; Gurnett, A.; Leavitt, P. S.; Liberator, P. A.; Misura, A. S.;
Samaras, S.; Tamas, T.; Schmatz, D. M.; Wyvratt, M. Bioorg. Med. Chem. Lett.
2005, 15, 4570.
7. Qian, X.; Liang, G.-B.; Feng, D.; Fisher, M.; Crumley, T.; Rattray, S.; Dulski, P.
M.; Gurnett, A.; Leavitt, P. S.; Liberator, P. A.; Misura, A. S.; Samaras, S.;
Tamas, T.; Schmatz, D. M.; Wyvratt, M.; Biftu, T. Bioorg. Med. Chem. Lett.
2006, 16, 2817.
8. Liang, G.-B.; Qian, X.; Feng, D.; Fisher, M.; Crumley, T.; Darkin-Rattray, S. J.;
Dulski, P. M.; Gurnett, A.; Leavitt, P. S.; Liberator, P. A.; Misura, A. S.; Samaras,
S.; Tamas, T.; Schmatz, D. M.; Wyvratt, M.; Biftu, T. Bioorg. Med. Chem. Lett.
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