Pyridazines part 41: Synthesis, antiplatelet activity and SAR of 2,4,6-substituted 5-(3-oxo-3-phenylprop-1-en-1-yl)- or 5-(3-phenylprop-2-enoyl) pyridazin-3(2H)-ones

Article abstract of DOI:10.1016/j.bmcl.2005.10.073

As part of the optimization process of the lead compound I a focussed library of diversely substituted pyridazin-3(2H)-ones containing a 3-oxo-3-phenylprop-1-en-1-yl or 3-phenylprop-2-enoyl fragment at position 5 has been obtained and evaluated as antipla

Full text of DOI:10.1016/j.bmcl.2005.10.073

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1080–1083
Pyridazines part 41: Synthesis, antiplatelet activity and SAR
of 2,4,6-substituted 5-(3-oxo-3-phenylprop-1-en-1-yl)- or
5-(3-phenylprop-2-enoyl)pyridazin-3(2H)-onesq
a
b
a
c
c
´
Abel Crespo, Caroline Meyers, Alberto Coelho, Matilde Yanez, Nuria Fraiz,
˜
Eddy Sotelo,a Bert U. W. Maes,b Reyes Laguna,c Ernesto Cano,c
b
a,
*
`
Guy L. F. Lemiere and Enrique Ravina
˜
aDepartamento de Quı´mica Orga´nica, Laboratorio de Quı´mica Farmace´utica, Universidad de Santiago de Compostela,
15782-Santiago de Compostela, Espan˜a, Spain
bOrganic Synthesis, Department of Chemistry, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
cDepartamento de Farmacologı´a, Facultad de Farmacia, Universidad de Santiago de Compostela,
15782-Santiago de Compostela, Espan˜a, Spain
Received 14 September 2005; revised 19 October 2005; accepted 20 October 2005
Available online 14 November 2005
Abstract—As part of the optimization process of the lead compound I a focussed library of diversely substituted pyridazin-3(2H)-
ones containing a 3-oxo-3-phenylprop-1-en-1-yl or 3-phenylprop-2-enoyl fragment at position 5 has been obtained and evaluated as
antiplatelet agents. The structural modification at positions 2, 6 and 4 of the heterocyclic moiety allowed us to obtain preliminary
information on the structure–activity relationship in this family.
Ó 2005 Elsevier Ltd. All rights reserved.
Platelets constitute the primary cellular component of
haemeostasis in mammalian organisms, being their pri-
mary physiologic role to survey the integrity of the
circulatory system and respond rapidly and robustly at
sites of vascular injury.2 Platelet activity must be
exquisitely regulated since inadequate activity leads to
bleeding and excessive activity leads to deleterious
thrombosis3 which causes myocardial and cerebral
infarction, the leading cause of morbidity and mortality
in the industrialised world.
inhibitors. Despite the extensive search and increasing
investment in this important research field, aspirin re-
mains the standard antiaggregatory medication to pre-
vent thrombotic events. Other currently employed
antiplatelet agents are ticlopidine, clopidogrel, sulfinpy-
razone and tirofiban.
As a part of our research programme aiming at the dis-
covery of novel pyridazin-3(2H)-one-based antiplatelet
agents5 we recently reported6 the potent antiaggregatory
effect (IC50 = 25 lM) of the lead compound I which con-
tains a 3-phenyl-3-oxoprop-1-en-1-yl fragment as a key
structural element in the heterocyclic core. Encouraged
by this result and taking into account the well-docu-
mented7 pharmacological properties associated with this
pharmacophoric unit and especially their antiplatelet
activity,8 we present in this communication the synthesis
and preliminary results of the structure–activity rela-
tionship studies obtained during the lead optimization
process performed on I.
In the past decades, major progress has been made in the
knowledge of platelet function. Nonetheless, the number
of antiplatelet agents available as drugs (Fig. 1) is still
insufficient and deleterious side effects are associated
with most of the currently used agents.4 The need to pre-
vent thrombus formation without impairing haemeosta-
sis has spurred large research aimed at the development
of new antithrombotic agents and platelet aggregation
In order to determine the most salient features of the
SAR in this series, a small focussed library having differ-
ent substitution patterns at positions 2, 6 and 4 (2a–e
Keywords: Platelets; Antiplatelet agents; Pyridazinones; Palladium.
q
See Ref. 1.
Corresponding author. Fax: +34 981 594912; e-mail: qofara@usc.es
*
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.10.073

A. Crespo et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1080–1083
1081
R
O
HN
CO2H
Ph
Ph
O
N
N
N
O
S
OCOMe
Ph
CO H
2
S
Cl
O
HN
R = H Ticlopidine
R = CO2Me Clopidogrel
Aspirin
Tirofiban
Sulfinpyrazone
SO
2
Figure 1. Structure of the most prominent antiplatelet agents currently employed.
O
O
O
and 6a–e) of the scaffold was prepared. In addition, 3-
phenylprop-2-enoyl positional isomers of the 3-oxo-3-
phenylprop-1-en-1-yl system (4a–f) were synthesised
(Fig. 2). Further work will be devoted to evaluate the ef-
fect of substitution on the phenyl group of the enone
system.
O
Cl
Me
Cl
Me
Cl
Me
a
N
N
N
N
N
N
Ph
+
Ph
OTf
O
5
6a
6b
d, e or f
O
4
Me
R4
6c) R = OEt
4
Schemes 1 and 2 illustrate the synthetic methods devel-
oped to access the target compounds 2, 4 and 6. Sono-
gashira coupling of N-2-blocked 5-halopyridazinones 1
with 1-phenylprop-2-yn-1-ol, under previously reported
conditions,1,9 afforded the trans-configured enones 2
(Scheme 1). The transformation involves alkynylation
followed by a base-promoted isomerisation of the ini-
tially formed 5-(3-hydroxy-3-phenylprop-1-yn-1-yl)pyri-
dazin-3(2H)-one. The isomeric enones 4 were prepared
by a Claisen–Schmidt condensation of the easily accessi-
ble methyl ketones 31,10 with benzaldehyde dimethyl
acetal in the presence of anhydrous aluminium chloride
as a catalyst (Scheme 1).11
N
N
6d) R = N(Me)
2
Ph
4
6e) R = Ph
O
6c-e
Scheme 2. Synthesis of 4-substituted 2-methyl-5-(3-oxo-3-phenylprop-
1-en-1-yl)pyridazin-3(2H)-ones 6.11 Reagents and conditions: (a)
CH„CH(OH)Ph, PdCl2(PPh3)2, CuI, Et3N, Bu4NI, DMF, rt; (d)
EtOH, K2CO3, reflux; (e) HN(Me)2, EtOH, rt; (f) PhB(OH)2,
Pd(PPh3)4, Na2CO3, toluene, H2O, reflux.
group at C-5, based on chemoselective oxidative addi-
tion, afforded a 48/52 mixture of the E/Z enones 6a
and b. The E/Z mixture could be isomerised to a 92/8 ra-
tio using triethylamine in dioxane at reflux for 24 h.
Introduction of diversity at position 4 of 6a (Scheme
2) was easily performed by exploiting the reactivity of
the chlorine atom in nucleophilic substitution reactions
(6c,d) or palladium-catalysed reactions (Suzuki–Miya-
ura coupling) (6e).
An effective and divergent approach based on the differ-
ent reactivities of the triflate group and chlorine atom in
the previously reported precursor 512 was developed to
access the targeted 4-substituted enones 6 (Scheme 2).
Regioselective Sonogashira coupling on the triflate
O
Table 1 shows the antiplatelet activity of compounds 2,
4, 6 and two reference compounds evaluated by the tur-
bidimetric method of Born14 employing thrombin as
platelet aggregation inductor. Although the limited
number of compounds precludes a detailed structure–
activity relationship study, a few general features can
be deduced and will be taken into account for further
work.
2
N
N
4
H
Ph
6
O
I
Figure 2. Structure of the lead compound I and points modified during
lead optimization.
O
O
O
O
R2
R2
R2
R2
a
b
c
N
N
N
N
N
N
N
N
Ph
Ph
X
R6
O
R6
R6
O
R6
O
2a-g
3
4a-f
1
2
6
2
6
2a) R = H, R = Ph
4a) R = H, R = Ph
2
6
2
6
2b) R = Me, R = Ph
4b) R = Me, R = Ph
2
6
2
6
2c) R = MOM, R = Ph
4c) R = MOM, R = Ph
2
6
2
6
2d) R = Bn, R = Ph
4d) R = H, R = H
2
6
2
6
2e) R = H, R = H
4e) R = Me, R = H
2
6
2
6
2f) R = Me, R = H
4f) R = Bn, R = H
2
6
2g) R = Bn, R = H
2
6
2h) R = Ph, R = H
Scheme 1. Synthesis of isomeric 5-(3-oxo-3-phenylprop-1-en-1-yl)pyridazin-3(2H)-ones 2 and 5-(3-phenylprop-2-enoyl)pyridazin-3(2H)-ones 4.11
Reagents and conditions: (a) CH„CH(OH)–Ph, PdCl2(PPh3)2, Et3N, DMF, rt for X = I, 60 °C for X = Br; (b) (1) CH2@C(OEt)Sn(Bu)3,
PdCl2(PPh3)2, DMF, Et3N, reflux; (2) N HCl, reflux; (c) PhCH(OMe)2, AlCl3, dioxane, reflux.

1082
A. Crespo et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1080–1083
Table 1. Structure11 and antiplatelet activity13 of pyridazin-3(2H)-ones 2, 4, 6 and reference compounds
Compound
R2
R6
R4
R5
Mp (°C)
Yield (%)
IC50 (lM)
2a
2b
H
Ph
Ph
Ph
Ph
H
H
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CO–CH@CH–Ph
CO–CH@CH–Ph
CO–CH@CH–Ph
CO–CH@CH–Ph
CO–CH@CH–Ph
CO–CH@CH–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
CH@CH–CO–Ph
230–231
209–210
177–178
180–182
193–194
192–194
174–175
156–157
175–177
152–153
134–136
188–190
212–214
73–75
69a
70
80
75
60
89
70
77
60
70
67
64a
62
76
25.9 1.7
b
Me
MOM
Bn
H
b
c
2c
2d
H
H
2e
H
H
19.7 0.8
15.7 0.7
4.2 0.3
3.3 0.3
35.5 2.0
38.5 1.6
2f
2g
Me
Bn
H
H
H
H
2h
4a
Ph
H
H
H
Ph
Ph
Ph
H
H
4b
4c
MOM
Me
H
H
H
43.0 8.2
c
4d
4e
H
c
Me
Bn
H
H
d
4f
6a
H
H
Me
Me
Me
Me
Me
H
Cl
Cl
OEt
N(Me)2
Ph
167–168
109–111
120–121
96–97
30
e
14.4 1.8
31.3 4.9
48.5 5.6
221.3 2.2
18.9 0.7
509.1 49.0
4.7 0.5
6b
6c
H
H
43
91
55
6d
6e
H
H
159–160
Sulfinpyrazone
Milrinone
a Overall yield of coupling and deprotection.
b IC50 > 150 lM (precipitate at higher concentrations).
c Precipitate under test conditions.
d Stimulates platelet aggregation above 100 lM.
e Attempts to separate the 48/52 (E/Z) mixture yielded only a small fraction of pure Z.
O
O
Analysis of the data obtained for the 5-(3-oxo-3-phenyl-
Ph
N
N
Ph
N
N
prop-1-en-1-yl)pyridazin-3(2H)-ones (2a–h) suggests
that removal of the phenyl group at C-6 increases the
antiplatelet activity (Table 1, compare 2a and 2e, 2b
and 2f). Furthermore, in this series the introduction of
substituents at N-2 produces a different effect depending
on the presence or not of the phenyl ring at C-6. Thus,
for those derivatives containing a 6-phenyl group, alkyl-
ation of the heterocyclic lactam led to a decreased activ-
ity (Table 1, compare 2a with 2b and 2c). On the
contrary for compounds 2e–h, the activity is increased
by substitution at N-2. Interestingly, a substantial in-
crease in potency (4-fold) is associated with the incorpo-
ration of bulky groups at N-2 (benzyl (2g) or phenyl
(2h)) independently of the rigidity of their structure.
Ph
Ph
O
O
4f
2g
Potent Antiplatelet Agent
Platelet Aggregation Stimulant
IC50 = 4.2 µM
Figure 3. Different pharmacological profile of the isomeric derivatives
2f and 4f.
antiplatelet agent (IC50 = 4.2 lM), while its positional
isomer 4f stimulates platelet aggregation at relatively
high concentrations (100 lM).15
One interesting but not unexpected result coming from
the evaluation of compounds 6 is the slight difference
in activity (2-fold) observed for the geometric isomers
6a and 6b. Although at this stage of the research pro-
gramme there are only a limited number of compounds
available, a preliminary comparison of the activity of
enones 6a–e with their parent compound 2e reveals that
replacement of the 4-H atom by a substitutent had only
a modest effect on the antiplatelet activity, with the dim-
ethylamino group (6d) as the only exception. These
observations suggest a pharmacophoric tolerance that
could be judiciously employed to improve the pharma-
co-kinetic profile in further pyridazin-3(2H)-one series.
Although derivatives 6 did not retain the excellent activ-
ity observed for some compounds of the series (2g and
2h), it should be noted that they are 2-methylpyrida-
zin-3(2H)-ones. It can be expected that introduction of
a benzyl or a phenyl group at this position will increase
their activity.
Regarding the positional isomeric pyridazin-3(2H)-ones
4 in the 6-phenyl series it is evident that the group at N-2
(H, Me and MOM) does not substantially affect the
antiplatelet activity (Table 1, see 4a–c). Unfortunately,
the low solubility of compounds 4d and 4e did not allow
us to obtain information about the effect of the removal
of the C-6-phenyl ring. Comparison of the available
data for the 6-phenyl derivatives suggests that the 5-(3-
phenylprop-2-enoyl)pyridazin-3(2H)-ones 4a–c are gen-
erally slightly more potent than their corresponding iso-
mers 2a–c (Table 1). These results reveal that isomerism
of the oxopropenyl system does not produce significant
differences in the antiplatelet activity of the prepared ser-
ies. Importantly, analysis of the pharmacological data of
the isomeric derivatives 2g and 4f (Fig. 3) clearly indi-
cates that such a modification could produce severe con-
sequences for the activity either at the quantitative but
also at the qualitative level since the former is a potent

A. Crespo et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1080–1083
1083
´
5. Sotelo, E.; Fraiz, N.; Yan˜ez, M.; Terrades, V.; Laguna, R.;
Cano, E.; Ravin˜a, E. Bioorg. Med. Chem. 2002, 10,
In summary, we prepared a focussed library of diversely
substituted pyridazin-3(2H)-ones containing the 3-oxo-
3-phenylprop-1-en-1-yl or 3-phenylprop-2-enoyl frag-
ments as key pharmacophoric elements. The biological
evaluation of these compounds allowed the identification
of several new, potent antiplatelet agents but also the
establishment of relevant features of the SAR in this ser-
ies. Further studies are in progress in our laboratories to
exploit these preliminary results for the synthesis of a larg-
er library incorporating substitution on/and bioisosteric
replacement of the phenyl group in the enone system.
´
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E.; Brea, J.; Ravin˜a, E. Bioorg. Med. Chem. Lett.
´
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11. Complete details of the synthesis and spectral data
References and notes
1. For the previous paper in this series, see: Coelho, A.;
Novoa, H.; Peeters, H.; Blaton, O. N.; Sotelo, E.
Tetrahedron 2005, 61, 4785.
2. Stormoken, H.. In Platelets Responses and Metabolism;
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3. Davies, M. J.; Thomas, A. C. Br. Heart J. 1985, 53, 363;
Fuster, V.; Badimon, L.; Badimon, J. J.; Chesebro, J. H.
N. Engl. J. Med. 1992, 326, 242; Blockmans, D.; Deck-
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pp 3–22; Resnekov, L.; Chediak, J.; Hirsch, J.; Lewis, H.
D. Chest 1989, 95, 52S.
will be published elsewhere in
a full paper. The
structures of described compounds have been unam-
biguously established on the basis of the spectroscopic
methods (NMR, FTIR and mass spectroscopy). All
compounds gave satisfactory microanalytical data (C,
H, N 0.4).
`
12. RÕkyek, O.; Maes, B. U. W.; Jonckers, T. H. M.; Lemiere,
G. L. F.; Dommisse, R. A. Tetrahedron 2001, 57, 10009.
13. Antiplatelet activity is expressed as the mean SEM of at
least five experiments.
14. Born, G. V. R. Nature 1962, 194, 927.
15. Compound 4f stimulates platelet aggregation above
100 lM, at lower concentrations it did not inhibit platelet
aggregation.
4. Majerus, P. W.; Tollefson, D. M. In Goodman s The
Pharmacological Basis of Therapeutics; Hardman, J. G.,
Limbird, L. E., Eds., 10th ed.; McGraw-Hill: New York,
2001; p 1534.