Pyridazines. Part 36: Synthesis and antiplatelet activity of 5-substituted-6-phenyl-3(2H)-pyridazinones

Article abstract of DOI:10.1016/j.bmcl.2003.11.009

A convenient and efficient palladium-catalysed retro-ene-assisted method has been developed to prepare a series of 5-substituted-6-phenyl-3(2H)- pyridazinones as potential antiplatelet drugs. The most active compounds were those that contain a 3-phenyl-3-

Full text of DOI:10.1016/j.bmcl.2003.11.009

Bioorganic & Medicinal Chemistry Letters 14 (2004) 321–324
Pyridazines. Part 36: Synthesis and antiplatelet activity of
5-substituted-6-phenyl-3(2H)-pyridazinones§
Alberto Coelho,a Eddy Sotelo,a Nuria Fraiz,b Matilde Yanez,b Reyes Laguna,b
Ernesto Canob and Enrique Ravinaa,*
a
´
Departamento de Quımica Organica, Laboratorio de Quımica Farmaceutica, Drug Research & Development Group,
´
´
Universidade de Santiago de Compostela, 15782-Santiago de Compostela, Spain
´
b
´
Departamento de Farmacologıa, Facultad de Farmacia, Universidade de Santiago de Compostela,
15782-Santiago de Compostela, Spain
Received 18 September 2003; revised 24 October 2003; accepted 7 November 2003
Abstract—A convenient and efficient palladium-catalysed retro-ene-assisted method has been developed to prepare a series of
5-substituted-6-phenyl-3(2H)-pyridazinones as potential antiplatelet drugs. The most active compounds were those that contain a
3-phenyl-3-oxo-propenyl fragment or a phenylthio group at position 5 of the heterocyclic ring.
# 2003 Elsevier Ltd. All rights reserved.
Platelet-mediated thrombus formation in the coronary
artery is a primary factor in the development of throm-
botic disorders such as unstable angina, myocardial
infarction, stroke and peripheral artery disease.2,3 The
clinical need for more efficacious antiplatelet agents and
the growing understanding of the role of platelets in
vascular injury have catalysed an extensive evaluation
of novel approaches to control platelet function.
interest not only to be used as drugs but also because
such compounds could be used as pharmacological
tools to provide important information regarding
platelet function.
For a number of years our group has been involved in a
project concerning the synthesis and study of the anti-
platelet activity of 3(2H)-pyridazinones.7ꢀ9 Our initial
target in this project was the general structure I (Fig. 2),
which is structurally related to the two most important
azinone PDE III inhibitors [2(1H)-pyridones (Amrinone
and Milrinone) and 3(2H)-pyridazinones (Zardaver-
ine)]. Pharmacological studies on these compounds
revealed that both the mechanism of action and the
Aspirin, the primary antiplatelet therapy currently in
use, has been shown to reduce the risk of arterial
thrombosis in placebo-controlled clinical trials.4 Despite
the proved efficacy of aspirin, there are reasons to
believe that substantial improvements in antiplatelet
therapy can be made. Other therapeutic-based strategies
to inhibit platelet function include the use of Clopido-
grel, Ticlopidine or Sulfinpyrazone (Fig. 1)—although
some of these drugs have a relatively low activity and up
to one third of patients respond in a limited manner to
treatment. Glycoprotein (GP) IIb/IIIa receptor antago-
nists have recently been identified as a prominent target
for drug development,5 although several complications
involving bleeding have been reported during the use of
these compounds in clinical trials.6 Thus, the search for
new agents with other mechanisms of action is of great
Figure 1.
Keywords: Pyridazinones; Antiplatelet agents.
§See ref 1.
*Corresponding author. Fax: +34-981-594912; e-mail: qofara@usc.es
Figure 2.
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.11.009

322
A. Coelho et al. / Bioorg. Med. Chem. Lett. 14 (2004) 321–324
Scheme 1.
antiplatelet activity of this series are highly dependent
on the nature of the chemical group present at the 5-
position of the heterocyclic ring.10
The potent antiplatelet activity shown by 6-phenyl-
3(2H)-pyridazinones that have alkylidene groups in the
5-position II11 (Fig. 2) prompted us to prepare other
pyridazinones with different carbon-based functions at
this position in the heterocycle. The synthetic pathway
selected to prepare these compounds involves the use of
palladium-catalysed reactions (PCR) (Stille, Heck,
Sonogashira and Suzuki) on precursor 1 (Scheme 1).
During the course of these studies we observed that the
NH group of the lactam function completely inhibits
palladium-catalysed reactions.12ꢀ14 Indeed, prior to
submitting the 3(2H)-pyridazinone to the catalytic cycle
it is essential to protect the NH group in 1. The protec-
tion was initially achieved using the methoxymethyl
(MOM) group,12ꢀ14 but attending to the substituent at
the 5-position several unexpected results were obtained
during the cleavage.15
Scheme 2. Reagents: (a) HCꢁC-R/Pd(PPh3)2Cl2/CuI/Et3N/DMF/
55 ꢂC; (b) HCꢁC–CH(OH)Ph/Pd(PPh3)2Cl2/CuI/Et3N/DMF/55 ꢂC;
(c) H2C¼CH-X/PdCl2[P(o-tolyl)3]2/DMF/110 ꢂC/24 h; (d) ArB(OH)2/
Pd(PPh3)4/DME/H2O (3:1)/90 ꢂC.
compound represents a valuable advantage, although
the most noteworthy aspect in terms of the synthetic
applicability of this compound as a 2-blocked pyr-
idazinone in cross-coupling reactions is derived from its
chemical and thermal stability. Compound 2 is a 1-O,
3-N, 5-O ene-adduct,18 which easily loses formaldehyde
through a retro-ene reaction promoted by base and/or
by heat once the PCR has occurred. This process yields
the corresponding 3(2H)-pyridazinones 3–6 in a very
efficient and short pathway (Scheme 1).
In a very recent paper, we briefly reported the use of the
hydroxymethyl group as a protecting group for the NH
group of pyridazinones16 and described some of the
advantages of its use in cross-coupling reactions
(Scheme 1). We now describe the extensive exploitation
of this new methodology to prepare 5-substituted-6-
phenyl-3(2H)-pyridazinones and present the preliminary
results of the antiplatelet activity and structure–activity
relationships in this series.
In order to rapidly achieve structural diversification at
position 5 in this system, we proceeded to study several
palladium-catalysed cross coupling reactions on 2
(Scheme 2).16 As can be observed, these transformations
give different 5-alkynyl, 5-alkenyl and 5-aryl-6-phenyl-
3(2H)-pyridazinones in satisfactory to excellent yields
(Table 1).16
The reaction sequences are outlined in Schemes 1 and 2.
The readily obtainable 5-bromo-6-phenyl-3(2H)-pyr-
idazinone 117 was converted into 5-bromo-2-hydroxy-
methyl-6-phenyl-3-pyridazinone 2 (89%) by hydroxy-
methylation using a 35% solution of formaldehyde
(Scheme 1).16 The straightforward preparation of this
The most significant result obtained during PCR on 2
was found with the Sonogashira coupling with 1-phenyl-
2-propyn-1-ol. The expected 5-alkynylpyridazinone was
Table 1. 5-Substituted-6-phenyl-3(2H)-pyridazinones 3–6
Compd
Substituent at
position 5
Mpa (ꢂC)
References
Yield (%)
Compd
Substituent at
position 5
Mpa (ꢂC)
References
Yield (%)
3a
3b
3c
3d
3e
4
5a
5b
5c
CꢁC–Hb
202–203
200–201
167–169
199–201
133–135
231–232
203–204
189–190
21
13
21
21
—
21
—
—
58c
87
78
82
63
65
52
50
5d
5e
6a
6b
6c
6d
6e
7
CH¼CH–CN
CH¼CH–Ph
Ph
231–232
251–252
178–180
222–223
198–200
172–173
235–236
162–164
—
—
12
12
12
12
12
10
68
75
72
82
74
72
88
90
CꢁC–CH2OH
CꢁC–(CH2)2OH
CꢁC–(CH2)3OH
CꢁC–CH(OEt)2
CH¼CHCOPh
CH¼CH–COOMe
CH¼CH–COOEt
CH¼CH–COO
(CH2)2OMe
4-Cl–Ph
4-CH3–Ph
4-CHO–Ph
2-Furyl
S–Ph
169–170
—
56
8
OPh
202–204
10
88
a All compounds were recrystallyzed from i-PrOH.
b Obtained by desilylation of 6-phenyl-5-trimethylsilylethynyl-3(2H)-pyridazinone employing KOH/MeOH.
c Overall yield from 2 (cross-coupling+desilylation).

A. Coelho et al. / Bioorg. Med. Chem. Lett. 14 (2004) 321–324
323
not isolated but the isomeric E-chalcone 419 was
obtained in good yield (85%) (Scheme 2). The unusual
formation of chalcone 4 is not dissimilar to results
described in previous papers20 and can be explained as
a consequence of base-catalysed isomerisation of the
5-(3-hydroxy-3-phenylprop-1-ynyl)-2-methoxymethyl-6-
phenyl-3-pyridazinone intermediate—a process that is
facilitated by the electron-deficient nature of the pyr-
idazinone system.21
A general view of the antiplatelet activities listed in
Table 2 reveals the important effect of the substituent at
position 5 on the antiplatelet activity. In the series of
carbon-based 5-substituted-3(2H)-pyridazinones, 5-alky-
nyl 3 and 5-alkenyl 5 derivatives showed only a weak
activity, but these derivatives were slightly more potent
than their analogues without the multiple bonds (gen-
eral structure I). Unfortunately, most of the 5-aryl deri-
vatives 6 were either not sufficiently soluble to be tested
or precipitated during the study. However, the data
obtained for two compounds (6c and 6e) are particu-
larly interesting for further SAR studies.
In order to study the reactivity of 2 in other transfor-
mations, and taking into account that several authors
have previously described how the introduction of an
alkyl or hydroxymethyl group at position 2 in the pyr-
idazinone ring facilitates nucleophilic halogen substitu-
tion reactions,22 we decided to study the reaction of 2
with mercaptobenzene and phenol (Scheme 3). These
experiments confirmed that 2 is a more reactive pre-
cursor than 1 in this transformation10 and gave, in a
more efficient way, the previously obtained10 phenylthio
and phenyloxy derivatives 7 and 8.
Comparison of antiplatelet activities of the lead com-
pounds II10 and the acrylates 5a and 5b shows that the
elimination of one of the ester groups in II causes a
dramatic drop in activity (Fig. 3). Interestingly, the
most sterically hindered derivative of the 5-alkenyl ana-
logues (compound 4) shows the highest efficacy as pla-
telet aggregation inhibitor within these series and has an
IC50 value in the micromolar range (25 mM). This fact
demonstrates the importance of the steric effect.
The platelet aggregation inhibitory activity of the
3(2H)-pyridazinone derivatives described above was
examined on washed human platelets using thrombin as
inducer of platelet aggregation. The turbidimetric method
described by Born was used in this study.23 The results
of these experiments are summarised in Table 2. Most
of the compounds studied inhibit platelet aggregation in
a dose-dependent manner, with their antiplatelet effects
being similar or superior to Sulfinpyrazone (IC50=500
mM).
Interestingly, the 5-styryl derivative 5d, which can be
considered a bioisoster of the previously described10
potent antiplatelet agent 6-phenyl-5-phenylthio-3(2H)-
pyridazinone 7, proved to be inactive while the 5-(3-
oxo-3-phenylpropenyl) derivative 4 had an antiplatelet
activity comparable to 7. These results, along with oth-
ers from our group,24 suggest that the 3-oxo-3-phenyl-
propenyl fragment may be a more effective bioisosteric
group for the phenylthio radical than the styryl group in
compounds where the activity is particularly influenced
by both steric and electronic factors (Fig. 4).
In conclusion, we have applied a short, efficient and
versatile palladium-assisted synthetic approach to achieve
structural diversification at the 5-position of 6-phenyl-
3(2H)-pyridazinone system. The most significative fea-
tures emerging from these preliminary studies have been
taken into account for the design of more potent anti-
platelet agents. Further studies are in progress in our
laboratory and will be reported in detail in a series of
forthcoming papers.
Scheme 3.
Table 2. Antiplatelet activity (IC50) of compounds 3–6a
Compd
Substituent
at 5
IC50 mM Compd Substituent IC50 mM
at 5
3a
3b
3c
3d
3e
4
5a
5b
5c
CꢁC–H
500
>1000
5d
5e
6a
6b
6c
6d
6e
7
CH¼CH–CN >1000
CꢁC–CH2OH
CH¼CH–Ph
Ph
—
b
CꢁC–(CH2)2OH >2000
CꢁC–(CH2)4OH >1000
CꢁC–CH(OEt)2 >1000
b
4-Cl–Ph
4-CH3–Ph
4-CHO–Ph
2-Furyl
S-Ph
152
b
CH¼CHCOPh
25
CH¼CH–COOMe >1000
635
15
CH¼CH–COOEt
CH¼CH–COO
(CH2)2OMe
—
Figure 3.
c
>1000
8
OPh
a The antiplatelet activities of the standards and the synthesised com-
pounds were tested by the turbidimetric method in washed human
platelets using thrombin as agonist. Methods for these assays have
been published elsewhere.10 Results shown are means of at least three
experiments. The highest mean standard error of IC50 values was
10%. —Inactive.
b Precipitates under the conditions of the study.
c Promotes platelet aggregation.
Figure 4.

324
A. Coelho et al. / Bioorg. Med. Chem. Lett. 14 (2004) 321–324
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This work was supported in part by Xunta de Galicia
PGYDT (TQ1PX20309PR). E. Sotelo thanks the Uni-
versity of Santiago de Compostela for his appointment
as Post-doctoral fellow.
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constant of 15.5 Hz, which confirms a trans stereo-
chemistry for the double bond. The observation of signals
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,
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