Design and synthesis of 4-alkynyl pyrazoles as inhibitors of PDE4: A practical access via Pd/C-Cu catalysis

Article abstract of DOI:10.1016/j.bmcl.2012.02.008

The design and synthesis of 4-alkynyl pyrazole derivatives has led to the identification of new class of PDE4 inhibitors. All these compounds were accessed for the first time via a facile Pd/C-CuI-PPh₃ mediated C-C bond forming reaction between

Full text of DOI:10.1016/j.bmcl.2012.02.008

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2480–2487
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design and synthesis of 4-alkynyl pyrazoles as inhibitors of PDE4:
A practical access via Pd/C–Cu catalysis
Dhilli Rao Gorja a,b, K. Shiva Kumar a, Ajit Kandale a, Chandana Lakshmi T. Meda a, Kishore V.L. Parsa a,
K. Mukkanti b, Manojit Pal a,
⇑
a Institute of Life Sciences, University of Hyderabad Campus, Gachibowli, Hyderabad 500 046, India
b Chemistry Division, Institute of Science and Technology, JNT University, Kukatpally, Hyderabad 500 072, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The design and synthesis of 4-alkynyl pyrazole derivatives has led to the identification of new class of
PDE4 inhibitors. All these compounds were accessed for the first time via a facile Pd/C–CuI–PPh3 mediated
C–C bond forming reaction between an appropriate pyrazole iodide and various terminal alkynes. In vitro
PDE4B inhibitory properties and molecular modeling studies of some of the compounds synthesized indi-
cated that 4-alkynyl pyrazole could be a promising template for the discovery of novel PDE4 inhibitors.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 25 November 2011
Revised 18 January 2012
Accepted 3 February 2012
Available online 13 February 2012
Keywords:
Pyrazole
Alkyne
Palladium
PDE4
Pyrazole represents an important class of nitrogen-containing
heterocycles and compounds containing pyrazole framework dis-
play numerous pharmacological properties.1 This is exemplified
by the identification of cyclooxygenase-2 (COX-2) inhibitors2 and
cannabinoid Type 1 receptor antagonists.3 Moreover, O-[(benzyl-
or benzoylpyrazolyl)propynyl]oximes of N-methylpiperidinone, 3-
tropinone, and 3-quinuclidinone (I) (Fig. 1) has been reported as
useful structures for new muscarinic agents of potential application
in Alzheimer’s disease.4 We have a long standing interest in pyra-
zole derivatives5 of potential pharmacological significance. In view
of promising biological activities of alkynes I and as part of our
molecules (C) were derived from a known inhibitor Ibudilast8 (A)
via (B) following a relevant drug design approach (Fig. 1). Herein
we report 4-alkynyl substituted pyrazole as a new template for
the discovery of novel inhibitors of PDE4. The synthesis of library
of small molecules based on 4-alkynyl substituted pyrazole (C)
was carried out using a Pd/C-mediated C–C bond forming reactions.
A number of methods have been reported for the synthesis of
alkynyl substituted pyrazole framework and most of them are based
on Sonogashira coupling of iodo pyrazole with terminal alkynes.4,9
Many of these methods have been developed recently9c–e and
usually involve the use of (PPh3)2PdCl2 as a catalyst. The use of Pd/
C–CuI–PPh3 as a less expensive catalyst system for efficient Sono-
gashira coupling has been explored earlier.10 The catalyst Pd/C is sta-
ble and easy to handle as well as separable from the product.
ongoing program on identification of novel PDE-4/TNF-
a inhibi-
tors6 we became interested in assessing PDE-4 inhibiting properties
of small molecules7 based on 4-alkynyl substituted pyrazole. These
N
R
O
N
N
N
N
H
N
R'
N
N
OEt
R
O
O
O
R
N
I
B
C
A (Ibudilast)
Z
Figure 1. The known bioactive pyrazole (I) and the design of new PDE4 inhibitors (C) based on 4-alkynyl substituted pyrazole.
⇑
Corresponding author. Tel.: +91 40 6657 1500; fax: +91 40 6657 1581.
E-mail address: manojitpal@rediffmail.com (M. Pal).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.02.008

D. R. Gorja et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2480–2487
2481
Table 1
R
Effect of solvents on Pd/C-mediated reaction of 5-(3-chlorophenyl)-4-iodo-1H-
CO2Et
N
CO2Et
N
pyrazole-3-carboxylate (1a) with phenylaetylenea (2a)
I
10% Pd/C, PPh3, CuI
EtOH, Et3N, 70 °C
O
R
O
Ph
+
OEt
OEt
+
N
N
I
R'
R'
10% Pd/C, PPh3, CuI
Solvent, Et3N, 70 °C
N
1
2
N
Ph
3
N
H
Cl
N
Cl
H
Scheme 1. Pd/C-mediated synthesis of 4-alkynyl substituted pyrazoles.
Cl
Cl
3a
2a
1a
Moreover, the catalyst can be recycled.10 Due to our continuing
interest in Pd/C-mediated alkynylation of aryl and heteroaryl
halides we decided to explore the Pd/C based methodology for the
synthesis of our target compounds that is, 4-alkynyl pyrazoles as
shown in Scheme 1. To the best of our knowledge alkynylation of
pyrazole under Pd/C–Cu catalysis is not common in the literature.
The key starting material that is, 5-(3-chlorophenyl)-4-iodo-
1H-pyrazole-3-carboxylate (1a) required for our study, was pre-
pared (Scheme 2) from ethyl 3-chloroacetophenone (4) which
was converted to ethyl 4-(3-chlorophenyl)-2,4-dioxobutanoate
(5). The compound 5 on treatment with hydrazine provided
5-(3-chlorophenyl)-1H-pyrazole-3-carboxylate11 (6) which was
iodinated12 to give the iodo derivative 1a. The methylation of 1a
provided another starting compound 1b.
Entry
Solvent
Time (h)
% Yieldb
1
2
3
4
5
6
EtOH
EtOH
MeOH
MeCN
1,4-Dioxane
DMF
8
16
8
10
8
8
85
80
80
70
65
65
a
All the reactions were carried out by using 1a (1.0 equiv), 2a (1.5 equiv), 10% Pd/
C (0.026 equiv), PPh3 (0.20 equiv), CuI (0.05 equiv), and Et3N (3.0 equiv) at 70 °C
under a nitrogen atmosphere.
b
Isolated yields.
(0.026 equiv), PPh3 (0.20 equiv), CuI (0.05 equiv), and triethyl-
amine (3.0 equiv) in various solvents. The corresponding results
are summarized in Table 1. The reaction was initially carried out
in ethanol for 8 h and the desired product that is, ethyl 5-(3-chlo-
All the terminal alkynes used are commercially available. Ini-
tially, we chose to examine the coupling reaction of iodo com-
pound 1a with phenylacetylene (2a) in the presence of 10% Pd/C
EtO2C
EtO2C
EtO2C
O
CO2Et
N
N
N
NH
N
NH
(CO2Et)2
Na/EtOH
O
I
O
CH3
Cl
NH2NH2
I2
I
(CH3)2SO4
DMF
CH3COOH
CAN
CH3CN
50 °C
3 h, 55%
rt, 20h
67%
120-125 °C
10h, 79%
KOH
Cl
Cl
Cl
Cl
15 min
63%
4
5
6
1a
1b
Scheme 2. Preparation of iodo pyrazoles (1).
Table 2
Pd/C-mediated synthesis of 4-alkynyl substituted pyrazoles (3)a
Entry
Iodopyrazole (1)
Alkynes (2)
Productsb (3)
% Yieldc
O
O
O
O
I
N
2a
N
N
H
1
85
N
H
Cl
Cl
1a
3a
O
HO
HO
O
2b
N
N
H
2
1a
82
Cl
3b
(continued on next page)

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D. R. Gorja et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2480–2487
Table 2 (continued)
Entry
Iodopyrazole (1)
Alkynes (2)
Productsb (3)
% Yieldc
HO
O
HO
O
2c
N
3
1a
83
N
H
Cl
3c
HO
Cl
HO
O
O
2d
4
1a
83
N
N
H
3d
OH
OH
O
2e
5
6
7
1a
80
82
83
O
N
N
H
Cl
3e
NC
NC
O
O
N
2f
1a
N
H
Cl
3f
O
HO
HO
O
O
O
I
N
N
N
N
2b
2c
2d
Cl
Cl
3g
1b
O
N
O
N
8
1b
80
Cl
3h
HO
O
O
9
1b
83
N
N
Cl
3i

D. R. Gorja et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2480–2487
2483
Table 2 (continued)
Entry
Iodopyrazole (1)
Alkynes (2)
Productsb (3)
NC
% Yieldc
O
N
O
N
10
1b
2f
83
Cl
3j
a
All the reactions were carried out by using 1 (1.0 equiv), 2 (1.5 equiv), 10% Pd/C (0.026 equiv), PPh3 (0.20 equiv), CuI (0.05 equiv), and Et3N (3 equiv) in EtOH at 70 °C for
8–9 h.
b
c
Identified by 1H NMR, IR, and MS.
Isolated yields.
I
O
N
Pd
Pd
OEt
Pd(0)-PPh3
complex in
solution
PPh3
1
leaching
Pd in
Pd/C
solution
N
R'
X
CuI
Et3N
Cu
R
2
Precipitation of
R
Pd on C at the end
of catalytic cycle
Pd(0)
+
CO2Et
N
3
Pd
N
R'
Scheme 3. Plausible mechanism of Pd/C-mediated alkynylation of 5-(3-chlorophenyl)-4-iodo-1H-pyrazole-3-carboxylate (1).
Table 3
80
Inhibition of PDE4B by compound 3 at 30 lM
Entry
Compounds
Average % inhibition
SD
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3f
3g
3h
3i
70.0
68.1
65.1
70.0
21.1
71.1
69.1
59.2
67.3
73.4
1.0
3.1
2.5
2.8
4.5
2.2
1.9
2.3
1.3
3.6
60
40
20
0
10
3j
SD = standard deviation.
0.01
0.1
1
10
100
rophenyl)-4-(phenylethynyl)-1H-pyrazole-3-carboxylate (3a) was
isolated in 85% yield (Table 1, entry 1). The reaction proceeded well
in EtOH via a C–C bond forming reaction to give the alkynylated
product. The increase of reaction time did not improve the product
yield of 3a further (Table 1, entry 2). The use of other solvents such
as MeOH, MeCN, 1,4-dioxane and DMF was examined and found to
be effective in terms of product yields (Table 1, entries 3–6). Since
the best result was achieved by using EtOH as a solvent hence all
other studies were carried out using EtOH.
Having established the optimum reaction conditions for the
preparation of 3a we then used this methodology for the prepara-
tion of a library of compounds related to 3a. Thus, a variety of com-
mercially available terminal alkynes were employed under the
Figure 2. Dose dependent inhibition of PDE4B by 3j.
reaction conditions presented in entry 1 of Table 1 and the results
are summarized in Table 2.
As evident from Table 2 that the reaction proceeded well with
both aliphatic (Table 2, entries 2–10) and aromatic alkynes
(Table 2, entry 1). All the reactions were generally completed
within 8–9 h irrespective of the nature of substituents present
in the terminal alkynes (2) affording the desired products (3a–j)
in good to excellent yields.
Mechanistically, the alkynylation of iodopyrazole derivative 1
proceeds via generation of an active Pd(0) species in situ that

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D. R. Gorja et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2480–2487
Table 4
elimination of Pd(0) to afford the 4-alkynylpyrazole derivatives (3).
The iodide displacement step was perhaps aided by an intramolec-
ular coordination of the neighboring carbonyl oxygen to the
palladium center of X.
Docking scores and binding energy of compounds after docking with PDE4B
Compound
Docking score
Binding energy (kcal/mol)
3a
3b
3d
À8.70
À8.12
À8.14
À2345.97
À2338.44
À2346.08
All the compounds synthesized were tested for their PDE4B
inhibitory potential in vitro.13 In inflammatory and immune cells,
the inhibition of cellular responses, including the production
and/or release of proinflammatory mediators, cytokines, and active
oxygen species, is associated with elevated levels of cAMP. Since
PDE4 (that exists in four different isoforms e.g., PDE4A, B, C and
D) plays a key role in the hydrolysis of cAMP to AMP7 hence inhi-
bition of PDE4 should result in elevated levels of cAMP in the air-
way tissues and cells thereby suppressing inflammatory cell
function. This is supported by the fact that PDE4 inhibitors have
been found to be beneficial for the treatment of inflammatory
and immunological diseases including asthma and chronic
obstructive pulmonary disease (COPD). Notably, rolipram14 the
first-generation PDE4 inhibitor showed adverse effects such as
nausea and vomiting. More recently, cardiovascular effects of
undergoes oxidative addition with 1 to give the organo-Pd(II)
species X (Scheme 3). The active Pd(0) species is generated from
the minor portion of the bound palladium (Pd/C) via a Pd leaching
process into the solution.10 The leached Pd then becomes an active
species by interacting with phosphine ligands that is, a dissolved
Pd(0)–PPh3 complex actually catalyzes the C–C bond forming
reaction in solution. At the end of the reaction re-precipitation of
Pd occurs on the surface of the charcoal. Once generated, the org-
ano-Pd(II) species X then facilitates the stepwise formation of C–C
bond via (i) trans organometallation with copper acetylide gener-
ated in situ from CuI and terminal alkyne followed by (ii) reductive
Figure 3. Docking of 3a at the active site of PDE4B.

D. R. Gorja et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2480–2487
2485
PDE4 inhibitors have been reported.15 While these dose-limiting
side effects were reduced by second-generation inhibitors like cilo-
milast16 (Ariflo) and roflumilast, their therapeutic index has de-
layed market launch so far. It is therefore necessary to devote a
continuing effort in exploring new class of compounds for their
PDE4 inhibitory potential. Moreover, identification of novel small
molecules possessing balanced inhibitory properties could be use-
ful in minimizing the adverse side effects. Accordingly, our com-
pounds were evaluated against PDE4B by using PDE4B enzyme
isolated from Sf9 cells.13 Rolipram, a well known inhibitor of
PDE4 was used as a reference compound. Most the compounds ex-
cept 3e showed significant inhibition of PDE4B in compared to
of interactions of present 4-alkynyl substituted pyrazole deriva-
tives (3) with PDE4B docking studies were carried out using few
representative compounds for example, 3a, 3b and 3d. The docking
score and binding energy obtained after docking of these mole-
cules with PDE4B protein is summarized in Table 4. The data
shown in Table 4 clearly suggests that these molecules bind well
with PDE4B. The interaction of compound 3a with the PDE4B pro-
tein (Fig. 3) was mainly contributed by a H-bonding between the
pyrazole nitrogen and OH of tyrosine (Tyr233), p–p stacking inter-
action between pyrazole of 3a and histidine (His234), coordinate
interaction between Mg ion and the ester carbonyl group and coor-
dinate interaction between Zn ion and pyrazole nitrogen.
other molecules when tested at 30
l
M (Table 3). A dose response
Similarly, the interaction of compound 3b with the PDE4B pro-
tein (Fig. 4) was contributed by H-bonding between pyrazole nitro-
gen and OH of tyrosine (Tyr233), p–p stacking interaction between
pyrazole and histidine (His234) and coordinate interaction be-
tween Mg ion and the ester carbonyl group of 3b. The interaction
of compound 3d with the PDE4B protein (Fig. 5) was mainly con-
study was carried out using a representative compound 3j which
showed dose dependent inhibition of PDE4B (Fig. 2). Since inhibi-
tion of PDE4D subtype is linked to the emetic response17 hence
some of the compounds were evaluated for their PDE4B selectivity
over PDE4D. Accordingly compounds 3a, 3d, 3f and 3j that showed
PDE4B inhibition >70% (Table 3) were tested against PDE4D using
commercially procured PDE4D enzyme. All these compounds
tributed by a p–p stacking interaction between pyrazole and histi-
dine (His234) and the coordinate interaction between Mg ion and
the ester carbonyl group of 3d. Overall, the present 4-alkynyl pyr-
azoles18 showed good interactions with PDE4B protein where the
showed 20–30% inhibition at 30
l
M indicating their ꢀtwofold
selectivity towards PDE4B. Nevertheless, to understand the nature
Figure 4. Docking of 3b at the active site of PDE4B.

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D. R. Gorja et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2480–2487
Figure 5. Docking of 3d at the active site of PDE4B.
central pyrazole ring and its ester moiety played a key role while
the alkyne side chain attached to the pyrazole ring was extended
into a hydrophobic pocket. All these compounds also showed a
coordinate interaction with both metals Zn and Mg. Finally, all li-
gands occupied enough space with lot of hydrophobic interaction
contributing to their activity.
Acknowledgments
The authors thank management of ILS for encouragement and
support. K.S.K. thank UGC for Kothari fellowship (No.F.4-2/
2006(BSR)/13-324/2010(BSR). M.P. thanks DST, New Delhi, India
for financial support (Grant No. SR/S1/OC-53/2009).
In conclusion, new 4-alkynyl substituted pyrazoles have been
designed and accessed via a facile and inexpensive methodology
based on Pd/C-mediated coupling of an appropriate iodide with
terminal alkynes. All the compounds synthesized were tested for
PDE4B inhibitory properties in vitro and many of them showed sig-
nificant inhibition. The docking results suggested that these mole-
cules interact well with PDE4B protein where the central pyrazole
ring and its ester moiety played a key role while the alkyne side
chain was extended into a hydrophobic pocket. Overall, 4-alkynyl
pyrazole has been identified as a new template for the identifica-
tion of small molecule based novel inhibitors of PDE4.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2012.02.008.
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