Conformationally restricted novel pyrazole derivatives: Synthesis of 1,8-disubstituted 5,5-dimethyl-4,5-dihydro-1H-benzo[g]indazoles as a new class of PDE4 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2012.03.029

A number of novel 1,8-disubstituted 5,5-dimethyl-4,5-dihydro-1H-benzo[g] indazoles based on a conformationally restricted pyrazole framework have been designed as potential inhibitors of PDE4. All these compounds were readily prepared by using simple chem

Full text of DOI:10.1016/j.bmcl.2012.03.029

Bioorganic & Medicinal Chemistry Letters 22 (2012) 3248–3255
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Bioorganic & Medicinal Chemistry Letters
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Conformationally restricted novel pyrazole derivatives: Synthesis of
1,8-disubstituted 5,5-dimethyl-4,5-dihydro-1H-benzo[g]indazoles
as a new class of PDE4 inhibitors
T. Shyamsunder Reddy a,e, K. Shiva Kumar b, Chandana L.T. Meda b, Ajit Kandale b, D. Rambabu b,c
,
d
G. Rama Krishna d, C. Hariprasad a, V. Venugopala Rao a, S. Venkataiah a, , C. Malla Reddy ,
⇑
A. Naidu e, P.K. Dubey e, Kishore V.L. Parsa b, Manojit Pal b,
⇑
a CiVentiChem (India) Private Limited, Plot No. 72/A, Part 2 Phase-1, IDA Jeedimetla, Hyderabad 500 055, India
b Institute of Life Sciences, University of Hyderabad Campus, Gachibowli, Hyderabad 500 046, India
c Department of Chemistry, K. L. University, Vaddeswaram, Guntur 522 502, Andhra Pradesh, India
d Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, West Bengal 741 252, India
e 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:
A number of novel 1,8-disubstituted 5,5-dimethyl-4,5-dihydro-1H-benzo[g]indazoles based on a con-
formationally restricted pyrazole framework have been designed as potential inhibitors of PDE4. All these
compounds were readily prepared by using simple chemistry strategy. The in vitro PDE4B inhibitory
properties and molecular modeling studies of some of the compounds synthesized along with the X-
ray single crystal data of a representative compound is presented.
Received 14 January 2012
Revised 24 February 2012
Accepted 7 March 2012
Available online 14 March 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Pyrazole
Benzo[g]indazole
PDE4
X-ray
Docking
The 4,5-dihydro-1H-benzo[g]indazole framework (A, Fig. 1),
found to be integral part of several bioactive agents has attracted
particular interest in the area of medicinal chemistry and new drug
discovery. For example, compounds based on A have been reported
to be effective ligands for cannabinoid receptors1a whereas the lead
molecule PHA-408 (B, Fig. 1) that belongs to this class has been
identified as a highly selective, ATP-competitive and tight-binding
inhibitor of nuclear factor j
-B kinase.1b The 3-(1-piperazinyl)-4,5-
dihydro-1H-benzo[g]indazoles (C,Fig. 1) have been reported as high
affinity ligands for the human dopamine D4 receptor with
improved selectivity over ion channels.1c Being a conformationally
restricted form of 5-phenyl-1H-pyrazole the 4,5-dihydro-1H-
benzo[g]indazole framework A attracted our attention due to our
longstanding interest in pyrazole derivatives.2a–e Moreover,
N
O
N
N
H
CONH2
NR
N
N
N
N
Cl
HN
N
HN
N
F
A
B
C
Figure 1. The 4,5-dihydro-1H-benzo[g]indazole framework (A) and related bioactive molecules B and C.
⇑
Corresponding authors. Tel.: +91 40 23197766; fax: +91 40 2309 1692 (S.V.); tel.: +91 40 6657 1500; fax: +91 40 6657 1581 (M.P.).
E-mail addresses: venkat@civentichem.com (S. Venkataiah), manojitpal@rediffmail.com (M. Pal).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.03.029

T. Shyamsunder Reddy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3248–3255
3249
R'
O
N
N
S
N
S
N
H
N
O
N
a
N
N
O2N
N
N
N
N
N
N
N
Z
R
F
E
Tofimilast (D)
F
F
3d
4
Figure 2. Tofimilast (D) and design of novel PDE4 inhibitors (F).
% yield
61
R
4a methyl
4b phenyl
58
Scheme 2. Reagents and condition: (a) (i) 10% Pd/C, EtOAc, H2, room temp, 12 h; (ii)
R0SO2Cl, pyridine, CH2Cl2, rt, 12 h.
a
b
N
Z
Z
Z
N
O
N
O
R
1
3
2
% yield
82
Z
R
3a NO2 phenyl
81
3b NO2 4-methylphenyl
83
3c NO2 2,3-dimethylphenyl
3d NO2 4-fluorophenyl
3e NO2 4-chlorophenyl
3f NO2 4-bromophenyl
84
84
82
80
3g NO2 4-trifluoromethylphenyl
82
3h Br
3i Br
3j Br
3k Br
3l Br
3m Br
3n Br
3o Br
phenyl
83
4-methylphenyl
4-fluorophenyl
4-chlorophenyl
4-bromophenyl
4-trifluoromethylphenyl
i-propyl
85
82
83
83
82
85
t-butyl
Scheme 1. Reagents and condition: (a) DMF-DMA, 105–110 °C, 2.0 h; (b) RNHNH2 HCl, EtOH, 75–80 °C, 2.0 h.
Figure 3. ORTEP representation of the 3k (Thermal ellipsoids are drawn at 50% probability level).

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T. Shyamsunder Reddy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3248–3255
Table 1
Table 1 (continued)
Inhibition of PDE4B by compound 3 at 30 lM
Entry
Compounds
Average% inhibition
SD
Entry
Compounds
Average% inhibition
SD
Br
O2N
N
N
N
N
N
N
8
68.06
5.76
1
72.13
5.44
N
N
N
N
Cl
3a
3k
Br
O2N
2
68.22
4.93
9
60.34
5.07
Br
3b
3l
O2N
Br
N
N
3
72.74
0.54
10
42.25
5.56
F3C
3c
3m
O2N
Br
11
56.80
3.09
N
N
4
66.81
4.16
N
N
N
N
3n
F
3d
Br
12
71.41
2.36
Br
5
76.24
6.28
N
N
3o
O
3h
S
N
O
H
N
N
13
75.08
3.37
Br
N
N
N
N
6
64.89
5.69
F
4a
3i
O
S
Ph
N
O
H
N
N
14
75.59
3.90
Br
7
72.22
6.43
F
4b
F
SD = standard deviation.
3j

T. Shyamsunder Reddy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3248–3255
3251
Table 2
100
80
60
40
20
0
Glide scores and other parameters of compounds after docking with PDE4B
IC50: 4.57 0.07 µM
Entry
Compounds
Glide scores
(Kcal/mol)
E-1a
(Kcal/mol)
E-2b
(Kcal/mol)
E-3c
(Kcal/mol)
1
2
3
4
3a
3c
3h
3j
ꢀ7.00
ꢀ6.84
ꢀ7.72
ꢀ7.48
ꢀ4.65
ꢀ4.75
ꢀ5.15
ꢀ5.38
ꢀ1.66
ꢀ1.71
ꢀ1.73
ꢀ1.73
ꢀ0.03
ꢀ0.04
ꢀ0.11
ꢀ0.06
a
b
c
E-1 = chemscore lipophilic term and fraction of total Van der Waals energy.
E-2 = hydrophobic reward.
E-3 = electrostatic reward.
0.01
0.1
1
10
100
tofimilast4 (D, Fig. 2) for inhaled administration in asthma and
chronic obstructive pulmonary diseases (COPD). Notably, COPD
and asthma, a major public health burden worldwide, are reported
to be causing the deaths of more than 250,000 people every year
according to the WHO statistics (2007). In our effort2b,f–h towards
the identification of novel inhibitors of PDE4 we became interested
Compound 3h
Figure 4. The IC50 value of compound 3h.
conformationally restricted pyrazole derivatives have been ex-
plored for the identification of phosphodiesterase 4 (PDE4) inhibi-
tors3 as is exemplified by the discovery and development of
Figure 5. Docking of 3a at the active site of PDE4B.

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T. Shyamsunder Reddy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3248–3255
in evaluating PDE4 inhibitory properties of compound F designed
based on the framework A, compound B and more importantly
tofimilast D (Fig. 2). The potent inhibition of TNF-a and PDE4 shown
by the compound B and D respectively prompted us to explore F.
The structure of compound F was arrived via E by modifying the
core structure of D. The substituents Z and R were introduced to
the basic framework of F for the generation of diversity-based
library of small molecules to be evaluated in the subsequent
in vitro study.
is, 7-substituted-4,4-dimethyl-3,4-dihydronaphthalen-1(2H)-one
(1) required for our synthesis was readily prepared via the reaction
of 4,4-dimethyl-3,4-dihydronaphthalen-1(2H)-one with NBS7 or
HNO3.8 Treating the compound 1 with dimethylformamide di-
methyl acetal (DMF-DMA) afforded the corresponding enaminoke-
tone 2 which on condensation with a variety of hydrazines in
ethanol provided the target compound 3 (or F). A range of 5,5-di-
methyl-1,8-disubstituted-4,5-dihydro-1H-benzo[g]indazole deriv-
atives were prepared by using this methodology. Further, the
compound 3d was converted to the corresponding sulfonamide
derivatives (4) via reduction of the nitro group followed by treating
the resultant amine with R0SO2Cl (Scheme 2). Nevertheless, all the
compounds synthesized were well characterized by spectral (NMR,
MS and IR) data. Additionally, the molecular structure of a repre-
sentative compound 3k was established unambiguously by single
crystal X-ray diffraction (Fig. 3).9
While a number of literature methods are known5 for the
construction of the 4,5-dihydro-1H-benzo[g]indazole ring, the
synthesis of compounds represented by F are, however, not com-
mon. To the best of our knowledge, only one method has been
reported for the preparation of a similar class of compounds in
19596 via a multi-step process that involved preparation and
subsequent rearrangement of 2-benzal-4,4-dimethyl-1-tetralone
oxide to 2-benzoyl-4,4-dimethyl-1-tetralone followed by reaction
with an appropriate hydrazine. We adopted a similar but much
shorter as well as simpler strategy to prepare our target
compounds as shown in Scheme 1. The key starting material, that
Having prepared a range of compounds based on 4,5-dihydro-
1H-benzo[g]indazole scaffold we then tested PDE4 inhibitory prop-
erties of some of these compounds in vitro.11 The isozyme PDE4,
one of the eleven PDE enzyme families10 exists in four different
Figure 6. Docking of 3c at the active site of PDE4B.

T. Shyamsunder Reddy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3248–3255
3253
isoforms, for example, PDE4A, B, C and D and is responsible for the
regulation of intracellular levels of cyclic adenosine monophos-
phate (cAMP). Indeed, PDE4 is specific for the hydrolysis of cAMP
to AMP in mast cells, basophils, eosinophiles, monocytes, and
lymphocytes as well as areas in the brain and airway smooth
muscles. The elevated levels of cAMP is linked with the inhibition
of cellular responses including the production and/or inhibition of
proinflammatory mediators, cytokines and active oxygen species
in inflammatory and immune cells. Thus, in order to suppress the
inflammatory cell function it is therefore necessary to increase
the intracellular concentration of cAMP in the airway and tissue
cells. This can be achieved by inhibiting PDE4 and inhibitors of
PDE4 therefore are beneficial for the treatment of inflammatory
and immunological diseases including asthma and COPD. The
first-generation PDE4 inhibitor rolipram12 however showed
dose-limiting side effects, for example, nausea and vomiting. While
these side effects were reduced by second-generation inhibitors
like cilomilast13 (Ariflo) and roflumilast, their therapeutic index
has delayed the market launch of these drugs. Based on the reports
that PDE4B subtype is linked to inflammatory cell regulation14
while the PDE4D subtype is implied in the emetic response15
efforts have been devoted towards the identification of PDE4B
inhibitors for the potential treatment of asthma and COPD. We ini-
tially evaluated the PDE4B inhibitory properties of the compounds
synthesized in vitro at 30
data generated for active compounds are summarized in Table 1.
l
M using PDE4B enzyme assay11 and the
Figure 7. Docking of 3h at the active site of PDE4B.

3254
T. Shyamsunder Reddy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3248–3255
Rolipram12 was used as a reference compound in this assay. Most
of the compounds except 3m showed significant inhibition of
In order to understand the nature of interactions of these
molecules with PDE4B docking studies were carried out using
compounds 3a, 3c, 3h and 3j. The xp (extra precision) docking
was performed for all the molecules using glide module of
Schrödinger 2011. The glide scores and other parameters obtained
after docking of these molecules into the PDE4B protein are sum-
marized in Table 2. The data shown in Table 2 suggests that these
molecules bind well with PDE4B. The individual interaction of
compound 3a and 3h with the PDE4B protein (Figs. 5 and 7) was
PDE4B when tested at 30
compound 3h showed dose dependent inhibition of PDE4B with
an IC50 value of 4.57 0.07 M (Fig. 4). To assess the PDE4D inhib-
lM (Table 1). In a dose–response study
l
itory potential of 3 and 4 few selected compounds were tested
against this enzyme when compound 3a, 3c, 3h, 3j and 3o showed
55.8%, 54.9%, 50.0%, 65.4% and 41.7% inhibition, respectively. A re-
cent study have demonstrated that inhibition of PDE4D by alloste-
ric inhibitors (maximum inhibition, Imax 80–90%) did not cause
emetic side effects raising a possibility that PDE4B inhibitors with
partial but not complete inhibition of PDE4D (Imax of ꢁ60–80%)
could be beneficial for the treatment of COPD and asthma without
causing emetic side effects.16 Thus, the present class of compounds
that showed a general tendency towards selective inhibition of
PDE4B over PDE4D may have medicinal value.
mainly contributed by
p–p stacking interaction between (i) the
benzene ring of benzo[g]indazole of 3a and phenylalanine (Phe
414 and 446), and (ii) the pyrazole moiety and tyrosine (Tyr
233). Whereas
p–p stacking interaction of two benzene rings of
1-phenyl benzo[g]indazole moiety with phenylalanine (Phe 446)
of PDE4B protein was observed in case of 3c (Fig. 6). Similarly,
p–p stacking interaction of the benzene ring of benzo[g]indazole
Figure 8. Docking of 3j at the active site of PDE4B.

T. Shyamsunder Reddy et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3248–3255
3255
Joseph, S. C.; Reddy, K. S.; Reddy, L. S.; Kumar, P. M.; Krishna, G. R.; Reddy, C. M.;
Rambabu, D.; Kapavarapu, R.; Lakshmi, C.; Meda, T.; Priya, K. K.; Parsa, K. V. L.;
Pal, M. Chem. Commun. 2011, 47, 7779; (g) Reddy, T. R.; Reddy, G. R.; Reddy, L.
S.; Jammula, S.; Lingappa, Y.; Kapavarapu, R.; Meda, C. L. T.; Parsa, K. V. L.; Pal,
M. Eur. J. Med. Chem. 2012, 48, 266; (h) Pal, S.; Durgadas, S.; Nallapati, S. B.;
Mukkanti, K.; Kapavarapu, R.; Lakshmi, C.; Parsa, K. V. L.; Pal, M. Bioorg. Med.
Chem. Lett. 2011, 21, 6573.
moiety and phenylalanine (Phe 414 and 446) was observed in case
of 3j ( Fig. 8). Overall, all four molecules showed nearly same
binding orientations with more lipophilic vdw energy for 3h and
3j compared to 3a and 3c whereas more electrostatic reward was
observed for 3h over 3j. Nevertheless, 5,5-dimethyl-1,8-disubsti-
tuted-4,5-dihydro-1H-benzo[g]indazoles designed based on a con-
formationally restricted pyrazole framework showed promising
PDE4B inhibitory properties in vitro and good interactions with
PDE4B protein in silico.
In conclusion, a series of novel compounds based on a conform-
ationally restricted pyrazole framework have been designed as
potential inhibitors of PDE4. All these compounds were readily
prepared by using simple chemistry strategy and several of them
showed promising PDE4B inhibitory properties in vitro. Some of
them showed good interactions with PDE4B protein in silico. The
benzo[g]indazole framework presented here therefore has poten-
tial for the discovery and development of novel PDE4 inhibitors.
3. For a review, see: Kodimuthali, A.; Jabaris, S.; Pal, M. J. Med. Chem 2008, 51,
5471.
4. Duplantier, A. J.; Bachert, E. L.; Cheng, J. B.; Cohan, V. L.; Jenkinson, T. H.; Kraus,
K. G.; McKechney, M. W.; Pillar, J. D.; Watson, J. W. J. Med. Chem. 2007, 50, 344.
5. (a) Peruncheralathan, S.; Khan, T. A.; Ila, H.; Junjappa, H. J. Org. Chem. 2005, 70,
10030; (b) Cho, C. S.; Patel, D. B. Tetrahedron 2006, 62, 6388; (c) Katritzky, A. R.;
Denisenko, A.; Denisenko, S. N.; Arend, M. J. Heterocycl. Chem. 2000, 37, 1309;
(d) Despotopoulou, C.; Knochel, P.; Gignoux, C.; McConnell, D. Synthesis 2009,
21, 3661.
6. Cromwell, N. H.; Bambury, R. E.; Barkley, R. P. J. Am. Chem. Soc. 1959, 81, 4294.
7. Johnson, A. T.; Wang, L.; Gillett, S. J.; Chandraratna, R. A. S. Bioorg. Med. Chem.
Lett. 1999, 9, 573.
8. Mathur, N. C.; Snow, M. S.; Young, K. M.; Pincock, J. A. Tetrahedron Lett. 1985, 41,
1509.
9. Crystal
data
of
3k:
Molecular
formula = C19H18BrClN2,
Formula
weight = 388.71, Crystal system = Triclinic, space group = P-1, a = 6.317 (9) Å,
b = 12.439 (18) Å, c = 12.950 (18) Å, V = 880.5 (2) Å3, T = 296(2) K, Z = 2,
Dc = 1.466 Mg mꢀ3 (Mo-K ) = 2.49 mmꢀ1, 15572 reflections measured, 3822
, l
a
Acknowledgment
independent reflections, 2558 observed reflections [I > 2.0
r
(I)],
R1_obs = 0.042, Goodness of fit = 1.01. Crystallographic data (excluding
structure factors) for 3k have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication numbers CCDC
861584.
The authors (TSR, VVR, SV and PKD) thank authorities of Civenti
Chem (India) Pvt Ltd of Hyderabad for laboratory facilities and JNT
University of Hyderabad for encouragement.
10. (a) Houslay, M. D.; Schafer, P.; Zhang, K. Y. J. Drug Disc. Today 2005, 10, 1503;
(b) Bender, A. T.; Beavo, J. A. Pharmacol. Rev. 2006, 58, 488.
Supplementary data
11. Wang, P.; Myers, J. G.; Wu, P.; Cheewatrakoolpong, B.; Egan, R. W.; Billah, M. M.
Biochem. Biophys. Res. Commun. 1997, 19, 320. (b) PDE4 enzymatic assay (for
further details see ESI): The inhibition of PDE4 enzyme was measured using
PDElight HTS cAMP phosphodiesterase assay kit (Lonza) according to
manufacturer’s recommendations. Briefly, 10 ng of in house purified PDE4B
or 0.5 ng commercially procured PDE4D enzyme was pre-incubated either
with DMSO (vehicle control) or compound for 15 min before incubation with
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.03.029.
References and notes
the substrate cAMP (5 lM) for 1 h. The reaction was halted with stop solution
and reaction mix was incubated with detection reagent for 10 min in dark.
Dose–response studies were performed at 13 different concentrations ranging
from 200 lM to 0.001 lM. Luminescence values (RLUs) were measured by a
1. (a) Murineddu, G.; Ruiu, S.; Mussinu, J.-M.; Loriga, G.; Grella, G. E.; Carai, M. A.
M.; Lazzari, P.; Pani, L.; Pinna, G. A. Bioorg. Med. Chem. 2005, 13, 3309; (b)
Mbalaviele, G.; Sommers, C. D.; Bonar, S. L.; Mathialagan, S.; Schindler, J. F.;
Guzova, J. A.; Shaffer, A. F.; Melton, M. A.; Christine, L. J.; Tripp, C. S.; Chiang, P.
C.; Thompson, D. C.; Hu, Y.; Kishore, N. J. Pharmacol. Exp. Ther. 2009, 329, 14; (c)
Collins, I.; Rowley, M.; Davey, W. B.; Emms, F.; Marwood, R.; Patel, S.; Patel, S.;
Fletcher, A.; Ragan, I. C.; Leeson, P. D.; Scott, A. L.; Broten, T. Bioorg. Med. Chem.
1998, 6, 743.
Multilabel plate reader (Perklin–Elmer 1420 Multilabel counter).The
percentage of inhibition was calculated using the following formula:%
inhibition = [(RLU of vehicle control ꢀ RLU of inhibitor)/(RLU of vehicle
control)] ꢂ 100.
12. Demnitz, J.; LaVecchia, L.; Bacher, E.; Keller, T. H.; Muller, T.; Schurch, F.;
Weber, H. P.; Pombo-Villar, E. Molecules 1998, 3, 107.
2. (a) Kumar, P. M.; Kumar, K. S.; Mohakhud, P. K.; Mukkanti, K.; Kapavarapu, R.;
Parsa, K. V. L.; Pal, M. Chem. Commun. 2012, 48, 431; (b) Kodimuthali, A.; Gupta,
R.; Parsa, K. V. L.; Prasunamba, P. L.; Pal, M. Lett. Drug Des. Disc. 2010, 7, 402; (c)
Kodimuthali, A.; Nishad, T. C.; Prasunamba, P. L.; Pal, M. Tetrahedron Lett. 2009,
50, 354; (d) Pal, M.; Veeramaneni, V. R.; Kumar, S.; Vangoori, A.; Mullangi, R.;
Misra, P.; Rajjak, S. A.; Lohray, V. B.; Casturi, S. R.; Yeleswarapu, K. R. Lett. Drug
Des. Disc. 2005, 2, 329; (e) Pal, M.; Madan, M.; Padakanti, S.; Pattabiraman, V.
R.; Kalleda, S.; Vanguri, A.; Mullangi, R.; Mamidi, N. V. S. R.; Casturi, S. R.;
Yeleswarapu, K. R. J. Med. Chem. 2003, 46, 3975; (f) Reddy, G. R.; Reddy, T. R.;
13. Dastidar, S. G.; Rajagopal, D.; Ray, A. Curr. Opin. Invest. Drugs 2007, 85, 364.
14. Jin, S.-L. C.; Lan, L.; Zoudilova, M.; Conti, M. J. Immunology 2005, 175, 1523.
15. Robichaud, A.; Stamatiou, P. B.; Jin, S.-L. C.; Lachance, N.; MacDonald, D.;
Laliberte, F.; Liu, S.; Huang, Z.; Conti, M.; Chan, C.-C. J. Clin. Invest. 2002, 110,
1045.
16. Burgin, A. B.; Magnusson, O. T.; Singh, J.; Witte, P.; Staker, B. L.; Bjornsson, J. M.;
Thorsteinsdottir, M.; Hrafnsdottir, S.; Hagen, T.; Kiselyov, A. S.; Stewart, L. J.;
Gurney, M. E. Nat. Biotech. 2010, 28, 63.