Synthesis, SAR and intramolecular hydrogen bonding pattern of 1,3,5-trisubstituted 4,5-dihydropyrazoles as potent cannabinoid CB1 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.01.049

The synthesis, structure-activity relationship (SAR) studies and intramolecular hydrogen bonding pattern of 1,3,5-trisubstituted 4,5-dihydropyrazoles are described. The target compounds 6-18 represent a novel class of potent and selective CB₁ r

Full text of DOI:10.1016/j.bmcl.2010.01.049

Bioorganic & Medicinal Chemistry Letters 20 (2010) 1752–1757
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis, SAR and intramolecular hydrogen bonding pattern
of 1,3,5-trisubstituted 4,5-dihydropyrazoles as potent cannabinoid CB1
receptor antagonists
*
Jos H. M. Lange , Martina A. W. van der Neut, Arnold P. den Hartog, Henri C. Wals, Jan Hoogendoorn,
Herman H. van Stuivenberg, Bernard J. van Vliet, Chris G. Kruse
Solvay Pharmaceuticals, Research Laboratories, Chemical Design & Synthesis Unit, C. J. van Houtenlaan 36, 1381 CP Weesp, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, structure–activity relationship (SAR) studies and intramolecular hydrogen bonding pattern
of 1,3,5-trisubstituted 4,5-dihydropyrazoles are described. The target compounds 6–18 represent a novel
class of potent and selective CB1 receptor antagonists. Based on X-ray diffraction data, the orally active
17 is shown to elicit a different intramolecular H-bonding mode as compared to ibipinabant (3) and
SLV330 (4).
Received 17 November 2009
Revised 5 January 2010
Accepted 6 January 2010
Available online 20 January 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Cannabinoid
CB1 receptor
Antagonist
Inverse agonist
Pharmacophore
Intramolecular hydrogen bonding
Scaffold hopping
X-ray diffraction
Absolute configuration
Fluorination
Crystallography
The endocannabinoid system plays an important role in many
physiological processes.1 In particular, cannabinoid CB1 receptor
antagonists/inverse agonists have shown clinical efficacy in the
treatment of obesity and related cardiovascular and metabolic risk
factors,2,3 and have also been related4,5 to the potential treatment
of addiction, cognitive disorders and peripherally mediated disor-
ders like liver fibrosis, cancer, arthritis and chronic bronchitis.
Although the risk of psychiatric side-effects has terminated6 many
development programs of CB1 receptor blockers for obesity, sug-
gestions have been made to focus on possible therapeutic applica-
tions in peripheral pathologies7 and even for continuing obesity
clinical trials while safeguarding the safety of patients and clinical
trial subjects.8 The majority of the reported9 CB1 receptor antago-
nists and inverse agonists can be described in terms of a general
pharmacophore model.10–14
It was anticipated that a regioisomeric shift of the pyrazoline ring
in the compounds 1–4 would lead to a novel chemotype of CB1
receptor antagonists. As a matter of fact Solvay was the first15 to
patent this principle. More recently, researchers from Esteve spe-
cifically claimed16 the optically active 5—which contains the same
N-piperidinylcarboxamide moiety and aryl-chloro-substitution
pattern as rimonabant—in a patent application and demonstrated
its in vivo activity in an obesity model. Its 5S configuration is
remarkable since—based on the absolute configurations of ibipina-
bant17 3 and 418 and their resulting threedimensional (3D) geome-
tries—the 5R configuration would at forehand be expected for 5.
Srivastava et al. also embarked19 on carboxamides and hydrazides
related to 5 but did not disclose the absolute configurations of their
compounds. These findings prompted us to synthesize and disclose
our results of target compounds20 6–18. The activities of these no-
vel 1,3,5-trisubstituted 4,5-dihydropyrazoles were compared with
their 1,3,4-trisubstituted counterparts 1–4 and a set of novel ana-
logues 19–25. The focus in this study was directed to the stereo-
chemical aspects of the resulting CB1 receptor-ligand interactions
as well as on the potential role of intramolecular H-bonding
interactions in these structures. In addition, the impact of
In considering options for designing novel classes of orally
active and CB1/CB2 subtype selective cannabinoid CB1 receptor
antagonists, the pyrazolines 1–4 were chosen as a starting point.
* Corresponding author. Tel.: +31 (0)294 479731; fax +31 (0)294 477138.
E-mail address: jos.lange@solvay.com (J.H.M. Lange).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.01.049

J. H. M. Lange et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1752–1757
1753
Cl
Cl
Cl
Cl
Cl
Cl
Cl
N
N
N
N
N
N
N
N
N
N
H
H
H
H
N
N
N
N
O
S
O
S
O
S
N
O
S
N
O
NH
N
N
N
N
N
O
O
O
O
Cl
Cl
1
2
3
4
5
fluorination21–23 at the piperidine moiety of 2 was studied since
this recently24 led to retained CB1 antagonism in the structurally
related 1,4,5,6-tetrahydropyridazine series.
The target compounds 6–18 have been synthesized as depicted
in Scheme 1. Diazotation of the anilines 26 and 27, followed by
reaction25 with ethyl 2-chloro-3-oxobutanoate gave 28 and 29,
respectively. Subsequent ring closure with styrene under basic
conditions at elevated temperature afforded the 3-pyrazoline es-
ters 29 and 30 in high yields. These esters 29 and 30 were hydro-
lyzed under basic conditions into the carboxylic acids 31 and 32,
which were subsequently chlorinated with SOCl2. The in situ
formed acid chlorides were reacted efficiently with aromatic sul-
fonamides under basic conditions to give the intermediates 33–
37. The target compounds 6–10 were obtained in varying yields
by a chlorination reaction with PCl5 at elevated temperature from
33–37, followed by treatment with CH3NH2ꢀHCl in the presence of
Hünig’s base. The key precursor 31 was converted analogously to
crude 38–41 in quantitative yields. The conversion of the obtained
crude 39–41 into the target compounds 12–14 was performed
under milder chlorination conditions (POCl3/DMAP, 40 °C) which
R1
Cl
R1
Cl
c
a, b
NH2
Cl
N
H
N
Cl
N
N
CO2Et
R1
28-29
26-27
CO2Et
29-30
Cl
Cl
Cl
d
N
N
N
N
g, h
e, f
N
N
29-30
R1
R1
R1
O
NHCH3
CO2H
31-32
N
H
N
O
S
O
O
S
O
R2
R2
33-37
6-10
Compound R1
R2
Cl
Cl
6
H
H
H
Cl
4-Cl
4-F
3-Cl
4-Cl
7
N
N
N
N
8
9
l, m
i, j, k
NHCH3
O
31
N
N
H
10
Cl
4-CF3
O
S
N
O
O
S
N
O
Compound R3
R4
H
11
12
H
H
R3
R4
38-41
R3
R4
F
11-14
13
14
H
F
CF3
F
n
o
(5S)-18
13
(5S)-16
(5R)-15
(5R)-17
+
+
14
Scheme 1. Reagents and conditions: (a) NaNO2, HCl, H2O, 0–5 °C, 1 h; (b) ethyl 2-chloro-3-oxobutanoate, NaOAc, EtOH, rt, 16 h (65–73%); (c) styrene, Et3N, benzene, reflux,
1 h (94–97%); (d) NaOH, MeOH, H2O, reflux, 2 h (78–91%); (e) SOCl2, toluene, 80 °C, 1 h; (f) R2C6H4SO2NH2, CH3CN, NaOH, rt, 16 h (78–90%); (g) PCl5, chlorobenzene, 140 °C,
90 min; (h) CH3NH2ꢀHCl, DIPEA, CH2Cl2, rt, 1 h (15–23%); (i) SOCl2, toluene, 80 °C, 1 h; (j) 4,4-R3R4-piperidine-1-sulfamide, NaH, CH3CN, rt, 16 h; (k) 1 N HCl, CH2Cl2, rt, 1 h; (l)
for 11: PCl5, chlorobenzene, 140 °C, 90 min; for 12–14: POCl3, DMAP, CH2Cl2, reflux, 4 h; (m) CH3NH2ꢀHCl, DIPEA, CH2Cl2, 6 °C to rt, 16 h (49–88%); (n) 250 ꢁ 30 mm
CHIRALPAKÒ AD-H 5
lm column, mobile phase: 70/30 CO2/ethanol + 1% Et2NH (supercritical fluid chromatography), flow rate: 120 ml/min; 25 °C, detection: UV 250 nm,
outlet pressure: 130 bars; (o) 250 ꢁ 76 mm CHIRALPAKÒ T-101 column, mobile phase: CH3OH/CH3CN/Et2NH = 84.9/15/0.1 (v/v), flow rate: 250 ml/min; 25 °C, detection: UV
220 nm.

1754
J. H. M. Lange et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1752–1757
Cl
Cl
Cl
Compound R3
R4
F
19
20
21
H
H
F
N
N
H
N
N
N
N
CF3
F
42
a
b,c
NHCH3
O
N
N
H
+
O
O
S
N
O
O
S
N
O
R3
H
N
S
N
O
O
R4
d
O
20
21
(4R)-23
(4R)-25
(4S)-22
(4S)-24
+
+
43-45
R3
R4
46-48
R3
R4
19-21
e
Scheme 2. Reagents and conditions: (a) toluene, reflux, 3 h; (b) POCl3, DMAP, CH2Cl2, reflux, 4 h (70–80%); (c) CH3NH2ꢀHCl, DIPEA, 6 °C to rt, 16 h (85–91%); (d) 250 ꢁ 76 mm
CHIRALPAKÒ AD 20
l
m column, mobile phase: CH3OH/CH3CN = 50/50 (v/v), flow rate: 270 ml/min; 25 °C, detection: UV 250 nm; (e) 250 ꢁ 80 mm CHIRALPAKÒ AD 20
lm
column, mobile phase: CH3OH/CH3CN = 90/10 (v/v), flow rate: 200 ml/min; 25 °C, detection: UV 230 nm.
resulted in significantly higher yields as compared to the conver-
sion of 38 into 11, which was synthesized by applying the conven-
tional reaction conditions (PCl5, elevated temperature). To further
investigate the stereochemical requirements for binding to the
CB1 receptor in this chiral pyrazoline series in more detail, the
key compounds 13 and 14 were separated into their enantiomers
by using chiral preparative HPLC to furnish two sets of compounds
15/16 and 17/18, respectively.
CB1 and CB2 receptor, stably expressed into Chinese Hamster Ovary
(CHO) cells,17 utilizing radioligand binding studies (displacement
of the specific binding of [3H]-CP-55,940). CB1 receptor antago-
nism17 was measured using a CP-55,940 induced arachidonic acid
release functional assay, using the same recombinant cell line. In
vivo activities of a set of key compounds after oral administration
was investigated in a CB agonist-induced rat hypotension model.17
Comparison of the CB1 receptor binding data of the novel target
compound 6 and its known counterpart 1 revealed that the regio-
isomeric shift of the pyrazoline ring appeared to have some nega-
tive effect on the CB1 receptor affinity of 6 (Table 1), although it
should be noted that the CB1 affinity SEM value of 6 is relatively
high. However, in the case of their piperidinyl derivatives 11 and
2, respectively, no significant impact on CB1 receptor affinity was
observed. Comparison of the CB1 receptor affinities of the target
compounds 6 and 9 showed a positive effect of the additional 2-
Cl atom in 9. In this arylsulfonyl series 6–10, the CF3-substituted
10 showed the highest CB1 receptor affinity (11.3 nM). However,
Coupling of the dihydropyrazole building block26 42 with the
carbamate esters24 43–45 afforded 46–48 in 70–80% yield, which
was successively chlorinated with POCl3 in the presence of DMAP
and reacted with CH3NH2ꢀHCl in the presence of Hünig’s base to
furnish the target compounds 19–21 in high yields (Scheme 2).
The chiral compounds in this series 20 and 21 were separated into
their enantiomers by using chiral preparative HPLC to furnish two
sets of compounds 22/23 and 24/25, respectively.
The pharmacological results of the compounds 1–4 and 6–25
are given in Table 1. They were evaluated in vitro at the human
Table 1
Pharmacological in vitro and in vivo results of compounds 1–4 and 6–25
Compound
Ki (CB1)a, nM
pA2 (CB1)b
Ki (CB2)c, nM
CB1/CB2 ratio
Hypotension ratd, ID50
1
2
3
4
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
25.2 7.4
152 68
7.8 1.4
58 19
84 56
21.7 10.0
53 10
8.7 0.3
8.7 0.3
9.9 0.6
8.7 0.2
8.4 0.3
8.0 0.3
8.3 0.2
9.0 0.1
8.6 0.1
7.9 0.2
8.4 0.2
9.4 0.3
9.1 0.6
9.5 0.1
8.2 0.1
8.8 0.1
<6.7
9.0 0.2
9.4 0.1
9.2 0.3
9.6 0.1
7.3 0.2
9.0 0.1
6.4 0.2
>1000
>39
8.6
1018
60
>11
>46
6
7
16
8
>18
84
>25
292
>17
>108
>3.5
>133
>68
18
65
>7
>23
>2
15
1321 264
7943 126
3495 968
>1000
n.d.e
5.5
8
>30
30
>1000
330 171
248 96
179 38
1086 86
>1000
606 204
>1000
584 220
>1000
>1000
>1000
>1000
>1000
>30
>30
>30
>30
>30
n.d.e
9
37 23
11.3 2.0
141 58
55 16
7.6 2.0
38.8 14.0
2.0 1.0
58 22
9.2 2.0
280 81
7.5 4.3
14.5 11.9
12.2 5.0
4.7 1.7
140 60
42 24
n.d.e
n.d.e
5
>30
>30
n.d.e
23
2
>30
6
219 91
305 58
>1000
>1000
>1000
463 168
>30
a
b
c
Displacement of specific CP-55,940 binding in CHO cells stably transfected with human CB1 receptor, expressed as Ki SEM (nM).
[3H]-Arachidonic acid release in CHO cells expressed as pA2 SEM values.
Displacement of specific CP-55,940 binding in CHO cells stably transfected with human CB2 receptor, expressed as Ki SEM (nM). The values represent the mean result
based on at least three independent experiments.
d
Antagonism of CB agonist (CP-55,940) induced hypotension (rat), expressed as ID50 (mg/kg, po administration).
n.d. = not determined.
e

J. H. M. Lange et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1752–1757
1755
the presence in 10 of the additional 2-Cl substituent in combina-
tion with the CF3 group led also to a highly lipophilic27 compound
(A log P28 = 5.9). Compound 7 elicited the highest CB1/CB2 receptor
selectivity in this set of compounds.
Significant differences exist in the SAR between the 1,3,4-tri-
substituted pyrazoline class and the 1,3,5-trisubstituted pyrazoline
class. For example, in the novel 1,3,5-trisubstituted pyrazoline ser-
ies, the 4-F substituted 7 showed a fourfold higher CB1 receptor
affinity than its 4-Cl analogue 6. In contrast, in the earlier re-
ported17 1,3,4-trisubstituted pyrazoline series the 4-Cl substituted
1 showed a 13-fold higher CB1 receptor affinity than its 4-F substi-
tuted counterpart (cf. compounds 67 and 72, respectively, in Ref.
17). Such a marked difference in SAR cannot be rationalized based
on the reported10–14 CB1 inverse agonist pharmacophore model but
is an indicator for distinct structural differences between both clas-
ses of pyrazolines. One of the aims of this investigation was the
identification of relevant factors involved herein.
Figure 1. ORTEP drawing of the X-ray diffraction result of 17. Its intramolecular
H-bond is indicated as a dashed line.
In the piperidinylsulfonyl-derived series 11–14, the CF3-substi-
tuted 13 elicited the highest CB1 receptor affinity and functional
CB1 antagonistic potency. Comparison of the regioisomeric pairs
12/19 and 14/21 showed that the CB1 receptor affinities are higher
in the 1,3,4-trisubstituted pyrazoline series. However, the opposite
was observed for 13 and 20, respectively. In particular, the com-
pounds 13 and 19 share high CB1 receptor affinity values with high
CB1/CB2 subtype selectivities and strong CB1 antagonistic
potencies.
Since 13 and 14 are racemates their enantiomerically pure con-
stituents were also tested. In both cases the dextrorotatory enanti-
omers 15 and 17 had significantly higher CB1 receptor affinities
and CB1 antagonistic potencies than their levorotatory counter-
parts 16 and 18. The highest CB1 receptor affinity (2.6 nM) was
found in the eutomer 15. The distomer 16 showed ꢂ30-fold less
CB1 affinity than the eutomer 15, indicating that these chiral li-
gands bind stereoselectively to the CB1 receptor. A comparable de-
gree of stereoselective binding to the CB1 receptor was observed
for the 1,3,4-pyrazoline counterparts 22 and 23. The distomer 18
showed ꢂ30-fold less CB1 affinity than 17, but in this case the cor-
responding 1,3,4-pyrazoline counterparts 25 and 24 elicited not
more than an 11-fold difference in CB1 receptor affinity.
Interestingly, the impact of variation in stereochemistry on the
observed CB1 receptor affinities in our sulfonylamidine-based ser-
ies of 1,3,5-trisubstituted 4,5-dihydropyrazoles appears to be con-
siderably higher (ꢂ30-fold ratio for the compound sets 15/16 and
17/18, respectively) as compared with the hydrazide-based com-
pounds,19 such as the reported analogue of 5 wherein its piperidi-
nyl moiety was substituted by a 4-morpholinyl group (ꢂthreefold
difference).
The in vivo activity after oral administration was investigated in
a CB1 agonist (CP-55,940) induced hypotension rat model.17 Disap-
pointingly, our initially prepared set of arylsulfonyl-based target
compounds 6–10 were hardly active or inactive in this model,
whereas compound 1 previously had shown significant activity
therein. Therefore, we focused our attention on the piperidine-
based compound 11, but this compound was also found inactive
after oral administration. Based on molecular modeling studies it
became apparent that the CB1 receptor would accommodate an
additional substituent at the 4-position of the piperidine ring of
11. This prompted the design of the compounds 12–14. Gratify-
ingly, the 4,4-difluoro substituted 14 in this series was found orally
active (ID50 = 9 mg/kg). According to expectation, its related
eutomer 17 was approximately twofold more active in this model
after oral administration (ID50 = 5 mg/kg). In our piperidinylsulfo-
nyl-based 1,3,4-trisubstituted pyrazoline series we also identified
some novel compounds with a high oral activity. The 4,4-difluoro
substituted 24 (ID50 = 6 mg/kg) and in particular the 4-CF3 substi-
tuted 22 (ID50 = 1.9 mg/kg) were found very active in our mecha-
nistic in vivo rodent model after oral administration. It is
interesting to note that 22 was found more potent in this model
than ibipinabant (3), as well as rimonabant17 (ID50 = 3.2 mg/kg).
The effects of intramolecular H-bonding on pharmacodynamic
and pharmacokinetic properties of drugs are a known phenome-
non.29–31 Intramolecular H-bonding in general decreases the polar
surface area, thereby enhancing the membrane permeability and
consequently improving oral absorption. It also generally results
in a somewhat higher degree of conformational constraint as com-
pared to structural analogues lacking such an intramolecular bind-
ing feature. The comparison of the X-ray diffraction structure of 17
with the earlier reported X-ray diffraction results of compounds
317 and 418, respectively, showed that intramolecular H-bonding
is present in both structure types. Both the Flack x parameter32 va-
lue (ꢃ0.06 0.04) and the independent analysis of the data in
terms of the Hooft parameter33 (ꢃ0.028 0.016) indicated an
enantiomerically pure crystal of 17 with a high level of confidence.
Unexpectedly, a clear difference in intramolecular H-bonding ex-
ists between the two sets of pyrazoline chemotypes. Apparently,
the different sprouting of the three substituents from the common
pyrazoline scaffold in 3 and 4 on the one hand and 17 on the other
hand has a large impact on their preferred intramolecular H-bond-
ing modes. An intramolecular H-bond interaction between the pyr-
azoline N2 atom and its amidine N–H hydrogen atom was present
in both 3 and 4, whereas a strong intramolecular H-bond interac-
tion between one of the oxygen atoms of the sulfonyl group and
its amidine N–H hydrogen atom was observed in 17 (Fig. 1). The
difference in intramolecular H-bond formation between the two
sets of compounds can be rationalized by invoking the different in-
ter-atom distance between the pyrazoline C3 atom and the amidine
C atom in 17 (1.46 Å) as compared to the pyrazoline N1 atom and
the amidine C atom in 3 (1.36 Å), respectively. The shorter distance
in 3 can be explained by a higher degree of conjugation between its
carboxamidine moiety and the pyrazoline ring, thereby resulting in
partial double bond character and a shorter bond length. These
shorter bond lengths in 3 and 4, as well as differences in their over-
all molecular geometry—due to their differently substituted pyraz-
oline ring—as compared to 17, apparently facilitate the specific
intramolecular H-bond as shown in Figure 2. The observed differ-
ences in SAR described hereinabove between some corresponding
members of the two pyrazoline classes can be rationalized by
invoking differences in the resulting 3D shapes, as a result of differ-
ent modes of intramolecular H-bonding. More specific, an analo-
gous orientation of both pyrazoline-bound aromatic moieties and
one of the sulfonyl-oxygen atoms (which is supposed10 to form a
key interaction with the Lys192 residue of the CB1 receptor via
H-bonding) at the CB1 receptor will lead to a different positioning

1756
J. H. M. Lange et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1752–1757
13. Wang, H.; Duffy, R. A.; Boykow, G. C.; Chackalamannil, S.; Madison, V. S. J. Med.
Chem. 2008, 51, 2439.
14. Foloppe, N.; Benwell, K.; Brooks, T. D.; Kennett, G.; Knight, A. R.; Misra, A.;
Monck, N. J. T. Bioorg. Med. Chem. Lett. 2009, 19, 4183.
O
O
S
R
R
S
N
N
1.46
1.33
15. Lange, J. H. M.; Kruse, C. G.; Van Stuivenberg, H. H. WO2005/074920, 2005.
16. Cuberes Altisen, R. EP1743892, 2007.
N 1.36
1.33
1.33
O
O
N
1.33
N
1.97
N
2.30
17. Lange, J. H. M.; Coolen, H. K. A. C.; van Stuivenberg, H. H.; Dijksman, J. A. R.;
Herremans, A. H. J.; Ronken, E.; Keizer, H. G.; Tipker, K.; McCreary, A. C.;
Veerman, W.; Wals, H. C.; Stork, B.; Verveer, P. C.; den Hartog, A. P.; de Jong, N.
M. J.; Adolfs, T. J. P.; Hoogendoorn, J.; Kruse, C. G. J. Med. Chem. 2004, 47, 627.
18. Lange, J. H. M.; van Stuivenberg, H. H.; Veerman, W.; Wals, H. C.; Stork, B.;
Coolen, H. K. A. C.; McCreary, A. C.; Adolfs, T. J. P.; Kruse, C. G. Bioorg. Med. Chem.
Lett. 2005, 15, 4794.
19. Srivastava, B. K.; Joharapurkar, A.; Raval, S.; Patel, J. Z.; Soni, R.; Raval, P.; Gite,
A.; Goswami, A.; Sadhwani, N.; Gandhi, N.; Patel, H.; Mishra, B.; Solanki, M.;
Pandey, B.; Jain, M. R.; Patel, P. R. J. Med. Chem. 2007, 50, 5951.
20. Lange, J. H. M.; den Hartog, A. P.; van Vliet, B. J. WO2009/130234, 2009.
21. Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37,
320.
N
N
0.87 H
CH3
0.79
H3C
H
Cl
Cl
3 R = 4-chlorophenyl
4 R = piperidinyl
17 R = 4,4-difluoropiperidinyl
Figure 2. Schematic representation of the intramolecular H-bonding pattern in the
compounds 3 and 4 versus 17. The depicted inter-atom distances for 3 and 17 from
their X-ray diffraction analyses are expressed in ÅA.
0
22. Hagmann, W. K. J. Med. Chem. 2008, 51, 4359.
23. Filler, R.; Saha, R. Future Med. Chem. 2009, 1, 777.
24. Lange, J. H. M.; den Hartog, A. P.; van der Neut, M. A. W.; Kruse, C. G. Bioorg.
Med. Chem. Lett. 2009, 19, 5675.
of both their amidine N–CH3 substituent and the hydrophobic
piperidine region therein. It should be noted that the presence of
an alternative hydrophobic pocket in the CB1 receptor has been
suggested earlier.34
25. Shawali, A. S.; Elsheikh, S.; Parkanyi, C. J. Heterocycl. Chem. 2003, 40, 207.
26. Grosscurt, A. C.; Van Hes, R.; Wellinga, K. J. Agric. Food. Chem. 1979, 27, 406.
27. Leeson, P. D.; Springthorpe, B. Nat. Rev. Drug Disc. 2007, 6, 881.
28. Ghose, A. K.; Viswanadhan, V. N.; Wendoloski, J. J. J. Phys. Chem. A 1998, 102,
3762.
Surprisingly, the key compound 17 showed the opposite abso-
lute configuration as compared to Esteve’s 5. Both compounds
would be expected to bind analogously to the CB1 receptor, based
on the published10–14 CB1 inverse agonist pharmacophore model,
since their pyrazoline moieties are almost identical and an
H-acceptor moiety is present in their lipophilic tail region. It
should be borne in mind that both the size and nature of their tails
(N-piperidinylcarboxamide in 5 vs 4,4-difluoropiperidinyl-sulfo-
nylcarboxamidine in 17) is different which can lead to different
preferential binding modes14 at the CB1 receptor. Additional
sophisticated molecular modeling investigations will be manda-
tory to provide a clear explanation for these observations.
Novel 1,3,5-trisubstituted 4,5-dihydropyrazoles are described.
The target compounds35 6–18 represent a novel class of potent
and selective CB1 receptor antagonists. Although the key com-
pound 17 showed the opposite absolute configuration—according
to the Cahn-Ingold-Prelog rules—as compared to 3 and 4, the ste-
reochemical orientation of the relevant side chains for CB1 receptor
binding in 17 matched the orientation of the corresponding sub-
stituents in 3 and 4. The orally active 17 was shown to elicit a dif-
ferent intramolecular H-bonding pattern as compared to 3 and 4.
The observed clear differences in SAR described hereinabove be-
tween some corresponding members of the two pyrazoline classes
was rationalized by invoking differences in 3D shape, as a result of
different modes of intramolecular H-bonding as well as the differ-
ent attachment points of the three substituent at their pyrazoline
ring.
29. Sasaki, S.; Cho, N.; Nara, Y.; Harada, M.; Endo, S.; Suzuki, N.; Furuya, S.; Fujino,
M. J. Med. Chem. 2003, 46, 113.
30. Mire, D. E.; Silfani, T. N.; Pugsley, M. K. J. Cardiovasc. Pharmacol. 2005, 46, 585.
31. Kasagami, T.; Kim, I.-H.; Tsai, H.-J.; Nishi, K.; Hammock, B. D.; Morisseau, C.
Bioorg. Med. Chem. Lett. 2009, 19, 1784.
32. Flack, H. D. Acta Crystallogr., Sect. A 1983, 39, 876.
33. Hooft, R. W. W.; Straver, L. H.; Spek, A. L. J. Appl. Crystallogr. 2008, 41, 96.
34. Carpino, P. A.; Griffith, D. A.; Sakya, S.; Dow, R. L.; Black, S. C.; Hadcock, J. R.;
Iredale, P. A.; Scott, D. O.; Fichtner, M. W.; Rose, C. R.; Day, R.; Dibrino, J.; Butler,
M.; DeBartolo, D. B.; Dutcher, D.; Gautreau, D.; Lizano, J. S.; O’Connor, R. E.;
Sands, M. A.; Kelly-Sullivan, D.; Ward, K. M. Bioorg. Med. Chem. Lett. 2006, 16,
731.
35. Yields refer to isolated pure products unless otherwise noted and were not
maximized. Selected data for compounds 14, 15, 17, 22 and 24. Synthesis of
compound 14: To a stirred solution of 26 (15.68 g, 0.123 mol) in ice (30 ml) and
concentrated HCl (30 ml) was slowly added
a solution of NaNO2 (9.0 g,
0.13 mol) in H2O (16 ml) and the resulting solution was stirred for 1 h at 0–5 °C
and subsequently added to a cold mixture of NaOAc (32 gram, 0.39 mol), EtOH
(520 ml) and ethyl 2-chloro-3-oxobutanoate (16.6 ml, 0.12 mol). After stirring
the resulting mixture for 1 h the formed precipitate was collected by filtration,
washed with EtOH and dried in vacuo to give 28 (22.99 g, 73% yield). Mp
147.5–149.5 °C. 1H NMR (200 MHz, CDCl3) d 1.40 (t, J = 7 Hz, 3H), 4.39 (q,
J = 7 Hz, 2H), 7.16 (br d, J = 8 Hz, 2H), 7.30 (br d, J = 8 Hz, 2H), 8.31 (br s, 1H). To
a stirred boiling solution of 28 (22.95 g, 0.088 mol) and styrene (30.3 ml,
0.264 mol) in benzene (140 ml) was added Et3N (34.3 ml, 0.247 mol) and the
resulting solution was heated at reflux temperature for 1 h. The resulting
solution was cooled to rt and the formed precipitate was removed by filtration
and washed with toluene. The filtrate was concentrated in vacuo and purified
by flash chromatography (silica gel, CH2Cl2) to give 29 (27.2 g, 94% yield) as a
syrup, which slowly solidified on standing. 1H NMR (200 MHz, CDCl3) d 1.38 (t,
J = 7 Hz, 3H), 3.06 (dd, J = 18 and 7 Hz, 1H), 3.73 (dd, J = 18 and 13 Hz, 1H), 4.33
(q, J = 7 Hz, 2H), 5.38 (dd, J = 13 and 7 Hz, 1H), 7.02 (br d, J = 8 Hz, 2H), 7.08–
7.40 (m, 7H). To a stirred suspension of 29 (23.0 g, 0.07 mol) in CH3OH (200 ml)
was added H2O (15 ml) and concentrated NaOH (10 ml) and the resulting
solution was heated at reflux temperature for 2 h. The CH3OH was partly
removed by evaporation and the residue was dissolved in a mixture of H2O and
EtOAc. Ice, concd HCl (20 ml) and EtOAc were successively added, the EtOAc
layer collected, dried over MgSO4, filtered and concentrated in vacuo. The
resulting residue was washed with Et2O (100 ml) and diisopropyl ether,
respectively, to give 31 as a solid, mp 177–179 °C. To 31 (18.77 g, 62.4 mmol)
in toluene (200 ml) was added SOCl2 (18.0 ml, 246.8 mmol). The reaction
mixture was stirred at 80 °C for 1 h. Volatiles were thoroughly removed in
vacuo. The residue was dissolved in CH3CN (250 ml): solution A. To a solution
of 4,4-difluoropiperidine-1-sulfonamide (12.5 g, 62.4 mmol) in CH3CN
(500 ml) was added aqueous NaOH (8.25 ml, 157.8 mmol). After 10 min,
solution A was slowly added. The reaction mixture was stirred overnight at rt.
Volatiles were removed in vacuo to give crude 41 (39.01 g). This crude residue
was extracted with CH2Cl2/1 N HCl. Layers were separated. The CH2Cl2 layer
was dried over Na2SO4, filtered and evaporated to give 41 (30.41 g, quantitative
yield). 1H NMR (400 MHz, DMSO-d6) d 1.93–2.10 (m, 4H), 2.75 (dd, J = 18 and
6 Hz, 1H), 3.12–3.21 (m, 4H), 3.36 (br s, probably NH and H2O), 3.62 (dd, J = 18
and 13 Hz, 1H), 5.42 (dd, J = 13 and 6 Hz, 1H), 6.93 (br d, J = 8, 2H), 7.14–7.36
(m, 7H). Compound 41 (30.14 g, 62.4 mmol) was dissolved in CH2Cl2 (500 ml).
DMAP (33.80 g, 276.7 mmol) was added. POCl3 (7.35 ml, 80.3 mmol) in CH2Cl2
(50 ml) was added dropwise. The reaction mixture was heated at reflux
temperature for 4 h. After cooling down to 6 °C CH3NH2ꢀHCl (19.0 g,
281.4 mmol) was added, followed by dropwise addition of DIPEA (72.0 ml,
420.6 mmol). The reaction mixture was stirred overnight at rt. Water (100 ml)
Acknowledgments
Jan Jeronimus and Hugo Morren are gratefully acknowledged
for supply of the analytical data.
References and notes
1. Lambert, D. M.; Fowler, C. J. J. Med. Chem. 2005, 48, 5059.
2. van Gaal, L. F.; Rissanen, A. M.; Scheen, A. J.; Ziegler, O.; Roessner, S. Lancet
2005, 365, 1389.
3. Antel, J.; Gregory, P. C.; Nordheim, U. J. Med. Chem. 2006, 49, 4008.
4. Lange, J. H. M.; Kruse, C. G. Chem. Rec. 2008, 8, 156.
5. Di Marzo, V. Nat. Rev. Drug Disc. 2008, 7, 438.
6. Jones, D. Nat. Rev. Drug Disc. 2008, 7, 961.
7. Bifulco, M.; Pisanti, S. Nat. Rev. Drug Disc. 2009, 8, 594.
8. Le Foll, B.; Gorelick, D. A.; Goldberg, S. R. Psychopharmacology 2009, 205, 171.
9. Högenauer, E. K. Exp. Opin. Ther. Patents 2007, 17, 1457.
10. Reggio, P. H. Curr. Pharm. Des. 2003, 9, 1607.
11. Lange, J. H. M.; Kruse, C. G. Curr. Opin. Drug Discovery Dev. 2004, 7, 498.
12. Lange, J. H. M.; Kruse, C. G. Drug Discovery Today 2005, 10, 693.

J. H. M. Lange et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1752–1757
1757
was added, followed by acidification with 1 N HCl. Layers were separated. The
CH2Cl2 layer was dried over Na2SO4, filtered and concentrated in vacuo.
Purification by flash chromatography (silica gel, Et2O/pa (40/60) = 1/1 (v/v))
afforded 14 (27.1 g; 88% yield.) 1H NMR (400 MHz, DMSO-d6) d 2.00–2.15 (m,
4H); 2.85 (br d, J ꢂ5 Hz, 3H), 3.09–3.21 (m, 5H), 3.94 (dd, J = 18 and 13 Hz, 1H),
5.61 (dd, J = 13 and 6 Hz, 1H), 7.04 (br d, J = 8, 2H), 7.20–7.38 (m, 7H), 8.80–
8.85 m, 1H); ESI+-MS exact mass calcd for C22H25ClF2N5O2S m/z, 496.1386
([MH+]), found: 496.1411. Compound 15: 1H NMR (400 MHz, DMSO-d6) d 1.43–
1.60 (m, 2H), 1.83–1.92 (m, 2H), 2.39–2.48 (m, 1H), 2.57–2.69 (m, 2H), 2.88 (br
s, 3H), 3.14 (dd, J = 18 and 7 Hz, 1H), 3.60 (t, J = 9 Hz, 2H), 3.94 (dd, J = 18 and
13 Hz, 1H), 5.60 (dd, J = 13 and 7 Hz, 1H), 7.03 (d, J = 9 Hz, 2H), 7.23 (d, J = 9 Hz,
2H), 7.26–7.39 (m, 5H), 8.86 (br s, 1H); ESI+-MS exact mass calcd for
under the supervision of Professor Dr. A. L. Spek (Bijvoet Centre for
Biomolecular Research, Utrecht University, The Netherlands) with a Nonius
KappaCCD diffractometer on
a rotating anode using MoKa radiation,
temperature: 150 K, crystal size: 0.21 ꢁ 0.30 ꢁ 0.66 mm, crystal system:
monoclinic, space group: P21, unit cell dimensions: a = 6.0743; b = 10.4341;
c = 17.6350 Å, Calculated density: 1.475 g cmꢃ3. The CIF has been deposited at
the Cambridge Crystallographic Data Centre (CCDC), deposition number:
760300. Compound 22: mp 164.5–165 °C. 1H NMR (400 MHz, CDCl3) d 1.67–
1.80 (m, 2H), 1.88–1.95 (m, 2H), 1.99–2.13 (m, 1H), 2.51–2.63 (m, 2H), 3.25 (d,
J = 7 Hz, 3H), 3.77–3.86 (m, 2H), 4.10–4.17 (m, 1H), 4.57 (t, J = 11 Hz, 1H), 4.67
(dd, J = 11 and 5.5 Hz, 1H), 6.81 (br s, 1H), 7.15 (d, J = 8 Hz, 2H), 7.23–7.36 (m,
5H), 7.52 (br d, J = 8 Hz, 2H); ESI+-MS exact mass calcd for C23H26ClF3N5O2S m/
C23H26ClF3N5O2S m/z, 528.1448 ([MH+]), found: 528.1475; ½a 2D5
ꢄ
= 167 (c 1 g/
z, 528.1448 ([MH+]), found: 528.1472; ½a 2D5
= ꢃ130 (c 1 g/100 ml, CH3OH).
ꢄ
100 ml, CH3OH). Compound 17: mp 158 °C (EtOH). 1H NMR (400 MHz, DMSO-
d6) d 2.00–2.15 (m, 4H), 2.85 (br d, J ꢂ5 Hz, 3H), 3.09–3.21 (m, 5H), 3.94 (dd,
J = 18 and 13 Hz, 1H), 5.61 (dd, J = 13 and 6 Hz, 1H), 7.04 (br d, J = 8 Hz, 2H),
7.20–7.38 (m, 7H), 8.80–8.85 m, 1H); ESI+-MS exact mass calcd for
Compound 24: mp 185.5–186 °C. 1H NMR (400 MHz, CDCl3) d 2.03–2.16 (m,
4H), 3.24 (d, J = 7 Hz, 3 H), 3.26–3.34 (m, 4H), 4.14 (dd, J = 12 and 5.5 Hz, 1H),
4.57 (t, J = 12 Hz, 1H), 4.67 (dd, J = 12 and 5.5 Hz, 1H), 6.79 (br s, 1H), 7.15 (d,
J = 8 Hz, 2H), 7.23–7.35 (m, 5H), 7.53 (d, J = 8 Hz, 2H); ESI+-MS exact mass calcd
C22H25ClF2N5O2S m/z, 496.1386 ([MH+]), found: 496.1403; ½a 2D5
ꢄ
= 165 (c 1 g/
for C22H25ClF2N5O2S m/z, 496.1386 ([MH+]), found: 496.1399; ½a 2D5
= ꢃ148 (c
ꢄ
100 ml, CH3OH). Selected crystallographic data for 17: X-ray data were collected
1 g/100 ml, CH3OH).