Microwave-assisted synthesis of trazodone and its derivatives as new 5-HT1A ligands: Binding and docking studies

Article abstract of DOI:10.3390/molecules24081609

Trazodone, a well-known antidepressant drug widely used throughout the world, works as a 5-hydroxytryptamine (5-HT₂) and α₁-adrenergic receptor antagonist and a serotonin reuptake inhibitor. Our research aimed to develop a new method for the synthesis of trazodone and its derivatives. In the known methods of the synthesis of trazodone and its derivatives, organic and toxic solvents are used, and the synthesis time varies from several to several dozen hours. Our research shows that trazodone and its derivatives can be successfully obtained in the presence of potassium carbonate as a reaction medium in the microwave field in a few minutes. As a result of the research work, 17 derivatives of trazodone were obtained, including compounds that exhibit the characteristics of 5-HT_1A receptor ligands. Molecular modeling studies were performed to understand the differences in the activity toward 5-HT_1A and 5-HT_2A receptors between ligand 10a (2-(6-(4-(3-chlorophenyl)piperazin-1-yl)hexyl)-[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one) (5-HT_1A K_i = 16 nM) and trazodone. The docking results indicate the lack of the binding of ligand 10a to 5-HT_2AR, which is consistent with the in vitro studies. On the other hand, the docking results for the 5-HT_1A receptor indicate two possible binding modes. Crystallographic studies support the hypothesis of an extended conformation.

Full text of DOI:10.3390/molecules24081609

molecules
Article
Microwave-Assisted Synthesis of Trazodone and Its
Derivatives as New 5-HT1A Ligands: Binding and
Docking Studies
1
1
Jolanta Jas´kowska 1,
*
, Przemysław Zare˛ba , Paweł Sliwa
, Edyta Pindelska 2,
´
Grzegorz Satała 3 and Zbigniew Majka 4
1
Institute of Organic Chemistry and Technology, Faculty of Chemical and Engineering and Technology,
Cracow University of Technology, 24 Warszawska Street, 31-155 Cracow, Poland;
pzareba@chemia.pk.edu.pl (P.Z.); psliwa@chemia.pk.edu.pl (P.S.)
Department of Analytical Chemistry and Biomaterials, Faculty of Pharmacy with the Laboratory Medicine
Division, Medical University of Warsaw, Banacha 1, 02-093 Warsaw, Poland; edyta.pindelska@wum.edu.pl
Department of Medicinal Chemistry, Institute of Pharmacology-Polish Academy of Sciences,
12 Sme˛tna Street, 31-343 Cracow, Poland; satala@if-pan.krakow.pl
TM Labs Sp. z o.o., Al. W. Beliny-Praz˙mowskiego 14, 31-514 Kraków, Poland; zbigniew.majka@tmlabs.pl
Correspondence: jaskowskaj@chemia.pk.edu.pl; Tel.: +48-12-628-27-40
´
2
3
4
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Wei Zhang
Received: 12 March 2019; Accepted: 19 April 2019; Published: 23 April 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Trazodone, a well-known antidepressant drug widely used throughout the world, works
as a 5-hydroxytryptamine (5-HT2) and α1-adrenergic receptor antagonist and a serotonin reuptake
inhibitor. Our research aimed to develop a new method for the synthesis of trazodone and its
derivatives. In the known methods of the synthesis of trazodone and its derivatives, organic
and toxic solvents are used, and the synthesis time varies from several to several dozen hours.
Our research shows that trazodone and its derivatives can be successfully obtained in the presence
of potassium carbonate as a reaction medium in the microwave field in a few minutes. As a result
of the research work, 17 derivatives of trazodone were obtained, including compounds that exhibit
the characteristics of 5-HT1A receptor ligands. Molecular modeling studies were performed to
understand the differences in the activity toward 5-HT1A and 5-HT2A receptors between ligand
10a (2-(6-(4-(3-chlorophenyl)piperazin-1-yl)hexyl)-[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one) (5-HT1A
Ki = 16 nM) and trazodone. The docking results indicate the lack of the binding of ligand 10a to
5-HT2AR, which is consistent with the in vitro studies. On the other hand, the docking results for
the 5-HT1A receptor indicate two possible binding modes. Crystallographic studies support the
hypothesis of an extended conformation.
Keywords: trazodone derivatives; trazodone synthesis; microwave (MW)-assisted synthesis;
serotonin receptor ligands; 5-HT1A ligands; long-chain arylpiperazines
1. Introduction
Trazodone (Figure 1) is a well-known antidepressant drug used since the 1960s, and it is widely
applied throughout the world. The pharmacological effect of trazodone is due to serotonin reuptake
inhibition and 5-hydroxytryptamine (5-HT2) receptor antagonism, whose activation usually leads to
insomnia, anxiety, psychomotor agitation, and sexual dysfunction.
Molecules 2019, 24, 1609; doi:10.3390/molecules24081609
www.mdpi.com/journal/molecules

Molecules 2019, 24, 1609
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N
N
N
Cl
N
N
O
Figure1. Chemical structure of trazodone (2-{3-[4-(3-chlorophenyl)piperazin-1-yl]propyl}[1,2,4]triazolo[4,3-
a]pyridin-3(2H)-one).
In the most commonly described methods of obtaining trazodone, the reaction is carried
out using 2-(3-halopropyl)[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one and 1-(3-chlorophenyl) piperazine
hydrochloride, e.g., in toluene in the presence of triethylamine for about 3 h [
1,2] or in acetonitrile
in the presence of potassium carbonate for 24 h [ ]. There are also known processes for the
3
preparation of trazodone in the reaction of 1-(3-bromopropyl)-4-(3-chlorophenyl)-piperazine and
[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one. The reaction is carried out in two steps in dioxane, where
the total reaction time is about 21 h [
(tetrabutylammonium bromide) and sodium carbonate [
4
], or in isopropyl alcohol, in the presence of TBAB
]. Other known methods involve the
5
use of ester, i.e., 3-(3-oxo-[1,2,4]triazolo[4,3-a]pyridine-2(3H-yl)propyl methanesulfonate as a substrate
in the reaction with 1-(3-chlorophenyl)piperazine, to obtain the expected product within 16 h of heating
in acetonitrile in the presence of potassium carbonate [3].
In summary, the methods of the synthesis of trazodone, which are known so far, require solvents
such as acetonitrile, toluene, dioxane, or isopropyl alcohol, and the synthesis time of the product varies
from several to several dozen hours.
As already mentioned, trazodone is a potent serotonin 5-HT2A and α1-adrenergic
receptor antagonist.
Although trazodone is a well-known antidepressant, its pharmacological activity is not fully
understood, and it is thought to have more than one mechanism of action. There is ample evidence
that suggests that the antidepressant activity of trazodone involves serotonin type 2 (5-HT2) receptor
antagonism and the inhibition of serotonin transporter, which results in agonistic effects against 5-HT1A
.
As a result, 5-HT1A, 5-HT2, and other receptors (5-HT6, 5-HT7, and D2) play an important role in
these diseases of the central nervous system [6–15]. In addition, 5-HT1A receptor ligands are of great
interest due to the fact that new therapeutic targets were identified, i.e., prostate cancer treatment, and
gastrointestinal and cardiopulmonary disorders [16].
We recently reported the synthesis of a number of hexylarylpiperazine derivatives, which showed
high affinity for 5-HT1A receptor [17,18]. As a continuation of this study and in order to test whether
the extension of the trazodone linker will change the activity profile, we present here the design,
synthesis, and biological evaluation of new derivatives of trazodone as potential 5-HT1A ligands. In the
research work, we also attempted to adapt our method of synthesis under microwave radiation [
5,19],
in the synthesis of trazodone itself, as well as of its substrates and its derivatives.
2. Results and Discussion
2.1. Synthesis of Trazodone
Our research shows that trazodone can be successfully obtained in the reaction of
2-(3-halopropyl)[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one (3a/b) and 1-(3-chlorophenyl) piperazine
hydrochloride (4), carried out in the presence of potassium carbonate as a reaction medium, a PTC (phase
transfer catalyst) in the microwave radiation field (Scheme 1, Method I). The same process conditions can
be used successfully to synthesize trazodone with the reaction of 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one
(1
) with chloropropyl-2-chloroarylpiperazine (
5) (Scheme 1, Method II). In addition, these conditions
can be also used in a “one-pot” reaction, without isolating intermediates (Scheme 1, Method III).

Molecules 2019, 24, 1609
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HCl
HN
N
N
NH
Br
X
N
4
N
N
X
Cl
N
Method I
O
2a (X=Cl)
2b (X=Br)
O
3a (X=Cl)
3b (X=Br)
1
N
NH
N
N
N
N
O
HCl
N
HN
Br
Cl
N
Cl
N
O
1
Method II
N
N
2a
Cl
4
5
Cl
Cl
trazodone
N
HCl
HN
Method III
Br
Br
NH
+
+
N
N
O
4
2b
1
Cl
Scheme 1. Methods for obtaining trazodone.
The research started with the synthesis of 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one (
methods (Scheme 2). The first one concerns a two-step process involving the reaction of 2-halopyridine
6a/b) with hydrazine in the first stage (yield 14%) in dimethylformamide (DMF) for 120 s. Bearing in
mind the low efficiency, the reaction was also carried out under conventional conditions by heating
the substrates in ethanol for 25 h [ ], which resulted in a 50% yield. In the case of the synthesis
, the higher yield was obtained by classical synthesis. In the next step, the obtained
was reacted with equimolar amount of urea (yield 49%) or with molar excess (yield
1) using two
(
3
of
7
from 6a
/
b
compound
7
75%) in a solvent-free condition for 50 s. The obtained results, especially in the second stage, are very
interesting because the analogous reaction under conventional conditions described in the literature
takes 2 h and allows the synthesis of compound 1 with a lower yield (57%) [20].
The second method for the synthesis of 1 in a one-step reaction involves the use of semicarbazide in
2-ethoxyethanol (yield 43–48%). All reactions were carried out under microwave radiation, which is a new
way of obtaining 1,2,4-triazolo[4,3-a] pyridin-3(2H)-one (1) and which is yet to be described in the literature.
NH2
NH
NH2
H2N
X
N
O
NH2-NH2, MW, DMF
O
NH
N
N
N
6a (X=Cl)
6b (X=Br)
7
1
NH2CONHNH2, C2H5OCH2CH2OH, H2SO4
Scheme 2. Methodsforobtaining1,2,4-triazolo[4,3-a]pyridin-3(2H)-one(4)undermicrowave(MW)radiation.
2.1.1. Method I
The synthesis of trazodone was started using the method I, which was based on N-alkylation
of the previously obtained 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one (
1) with dihalogenopropane (2a/b)
(Scheme 3).
Br
X
N
O
N
O
NH
N
X
N
N
2a (X = Cl)
2b (X = Br)
3a (X = Cl)
3b (X = Br)
1
Scheme 3. Synthesis 2-(3-halopropyl) [1,2,4] triazolo [4,3-a] pyridin-3 (2H) -one (3a/1b) under
MW radiation.

Molecules 2019, 24, 1609
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The reactions were carried out under microwave radiation in the presence of K2CO3 and PTC
catalysts (TBAB). Table 1 contains the results of the research including the impact of the solvent
(acetonitrile (ACN) and DMF) on the yield and time of obtaining 3a/b.
Table 1. Synthesis 2-(3-halopropyl)[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one (3a/b)—0.01 mol of 1,2,4-
triazolo[4,3-a]pyridin-3-(2H)-one (1), 0.026 mol of 1-bromo-3-chloropropane (2a)/0.026 mol 1,3-
dibromopropane (2b), 0.001 mol of tetrabutylammonium bromide (TBAB), 0.03 mol of K2CO3.
DMF—dimethylformamide; ACN—acetonitrile.
No.
Solvent
Amount of Solvent (cm3)
X
Time (s)
Yield (%)
1
2
3
4
5 *
6
DMF
ACN
ACN
ACN
ACN
ACN
ACN
-
5
6
3
Cl
Cl
Cl
Cl
Br
Br
Cl
Cl
50
50
80
50
120
60
60
80
24
83
92
90
81
73
82
81
0.75
0.075
0.75
0.4
-
7
8
* Reaction was carried out in a MAGNUM II reactor. Molar amounts of the reactants were reduced 10-fold.
The studies showed that, when the reaction is carried out with DMF or ACN under the same
conditions, the obtained efficiency is almost four times better with ACN (Table 1, entries 1 and 2);
therefore, further experiments were carried out with this solvent only. Surprisingly, a subsequent study
showed that, when the amount of the solvent was decreased, or even completely eliminated, this did
not cause significant differences in the performance (Table 1, entries 2, 3, 4, 7, and 8). The replacement
of halogen in the substrate 2a on 2b caused a slight 17% drop in yield (Table 1, entries 4 and 7). In the
case of the reaction carried out in a Magnum II reactor in a closed polytetrafluoroethylene (PTFE)
tube, similar efficiency was obtained, but it was necessary to extend the time of the process (Table 1,
entries 4 and 5).
In the next stage of the synthesis of trazodone with method I, the
obtained
2-(3-halopropyl)[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one
(
3a/b
)
reacted
with
1-(3-chlorophenyl)piperazine hydrochloride (4) (Scheme 4).
N
N
O
HCl
HN
N
X
N
N
N
N
N
+
N
O
3a/b
4
trazodone
Cl
Cl
Scheme 4. Synthesis of trazodone with method I under MW radiation.
The synthesis of trazodone with the method I was carried out with potassium carbonate
and PTC (TBAB—tetrabutylammonium bromide, TEAC—tetraethylammonium chloride, and
DABCO—1,4-diazabicyclo[2.2.2]octane). The reactions were carried out under microwave radiation.
During the experiment, we observed that the addition of a small amount (<10% weight) of DMF or
ACN was beneficial for the duration of the process, but the reaction also took place under completely
solvent-free conditions (Table 2, entries 5, 6, and 11). The obtained results for the synthesis of
trazodone with method I showed that the reaction occurred in less than 5 min, while, in the presence
of DMF or ACN, it occurred within 1–2 min. The highest yield was observed in the reaction with
2-(3-bromopropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (3b) using 10% by weight DMF and TBAB
as a PTC catalyst, which allowed obtaining a product with 98% efficiency in 1 min (Table 2, entry 2).
When the TBAB was exchanged for TEAC or DABCO under the same conditions, the reaction time
was longer by 2 min (Table 2, entries 3 and 4). The exchange of DMF for ACN caused a 13% decrease
in yield (Table 2, entries 1 and 7).

Molecules 2019, 24, 1609
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Table 2. Synthesis of trazodone with method I—10 mmol of 2-(3-chloropropyl)-1,2,4-triazolo[4,3-
a]pyridin-3-(2H)-one (3a) /10 mmol of 2-(3-bromopropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (3b),
10 mmol of 1-(3-chlorophenyl) piperazine hydrochloride ( ), 1 mmol of TBAB, 30 mmol of K2CO3.
PTC— phase transfer catalyst; DABCO—1,4-diazabicyclo[2.2.2]octane.
4
No.
Solvent
Amount of Solvent (cm3)
PTC
X
Time (s)
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14 *
15 *
16 *
DMF
DMF
DMF
DMF
-
4
2
2
2
-
-
3
8
3
1
-
6
50
0.28
0.28
0.28
TBAB
TBAB
TEAC
DABCO
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
60
60
120
120
100
300
60
60
60
80
100
60
60
91
98
90
83
69
82
78
90
86
89
76
50
51
86
86
65
-
ACN
ACN
ACN
ACN
-
DMF
H2O
ACN
ACN
ACN
120 (1 bar)
120 (5 bar)
120 (10 bar)
* Reaction was carried out in a MAGNUM II reactor; reactions were carried out in a closed vessel with a given pressure
1, 5, or 10 bar. Molar quantities of reactants—8 mmol of 2-(3-chloropropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one
(3a), 8 mmol of 1-(3-chlorophenyl) piperazine hydrochloride (4), 0.8 mmol of TBAB, 2.4 mmol of K2CO3.
In the case of the use of 2-(3-chloropropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (3a), the amount
of ACN in the range of 10–40% did not significantly affect the efficiency (Table 2, entries 8–10). For
these reactions, higher yields were observed when ACN was used rather than DMF or H2O. The effect
of pressure on the course of the reaction and the product yield were also evaluated by conducting the
process in a closed PTFE tube in a Magnum II reactor. For pressures of 1 and 5 bar, similar results were
obtained, while the increase of the pressure to 10 bar resulted in a decreased product efficiency by
about 20% (Table 2, entries 14–16).
2.1.2. Method II
In the next stage of our research related to the synthesized trazodone with method II, 1-(3-
chlorophenyl)-4-(3-chloropropyl)piperazine (5) was obtained in the reaction of 3-chloroarylpiperazine
(4) with 1-bromo-3-chloropropane (2a) (Scheme 5) under microwave radiation.
HCl HN
Cl
N
Cl
Br
N
N
Cl
Cl
4
2a
5
Scheme 5. Synthesis 1-(3-chloropropyl)-4-(3-chloropheyl)piperazine (5) under MW radiation.
The best results in the synthesis of 1-(3-chloropropyl)-4-(3-chlorophenyl)piperazine (5) were
obtained by reacting with ACN in a Samsung microwave reactor, where the product was obtained with
a yield of 88% within 40 s. (Table 3) This is a much better result than the analogous method described
in the literature so far under conventional conditions where 1-(3-chloropropyl)-4-(3-chlorophenyl)
piperazine (5) can be obtained with a 60% yield within 7 h [21].

Molecules 2019, 24, 1609
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Table 3. Synthesis 1-(3-chloropropyl)-4-(3-chlorophenyl)piperazine (5)—26 mmol of 1-bromo-3-
chloropropane (2a), 10 mmol of 1-(3-chlorophenyl) piperazine hydrochloride (
4), 10 mmol of TBAB, 30
mmol of K2CO3.
No.
Solvent
Amount of Solvent (cm3)
Time (s)
Yield (%)
1
2
3 *
DMF
ACN
ACN
3
3
0.3
120
40
120
62
88
83
* Reaction was carried out in a MAGNUM II reactor. Molar amounts of the reactants were reduced 10-fold.
To obtain trazodone with method II, the previously prepared 1-(3-chloropropyl)-4-
(3-chlorophenyl)piperazine ( ) was reacted with 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one ( ) (Scheme 6).
5
1
The synthesis was carried out under microwave radiation in the presence of potassium carbonate,
TBAB, and ACN.
N
N
Cl
N
NH
+
N
N
N
N
N
N
O
O
5
1
trazodone
Cl
Cl
Scheme 6. Synthesis of trazodone with method II under MW radiation.
The highest efficiency (92%) in the synthesis of trazodone with method II was obtained in the
reaction for 80 s using about 15 wt.% ACN (Table 4, entry 4).
Table 4.
Synthesis of trazodone with method II—10 mmol of 1-(3-chloropropyl)-4-(3-
chlorophenyl)piperazine hydrochloride (5), (10 mmol) of 1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (1),
10 mmol of TBAB, 30 mmol of K2CO3.
No.
Solvent
Amount of Solvent (cm3)
Time (s)
Yield (%)
1
2
3
4
5 *
ACN
ACN
ACN
ACN
ACN
8
6
6
2
0.2
80
60
80
80
120
92
73
82
92
77
* Reaction was carried out in a MAGNUM II reactor. Molar amounts of the reactants were reduced 10-fold.
2.1.3. Method III—One-Pot Synthesis
Trazodone was also obtained in a “one-pot” variant. The process was carried under microwave
radiation by firstly heating 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one ( ) with 1-bromo-3-chloropropane
2a) for 50 s, followed by the addition of 1-(3-chlorophenyl)piperazine ( ) for another 90 s. The expected
1
(
4
product was obtained with a yield of 31%. The synthesis was also performed in the CEM Discover SP
reactor (100 W), carrying out the process for 30 s in the first stage and for 60 s in the second, which
allowed obtaining trazodone with a yield of 71%.
We also tested the variant in which all of the reactants, 1,2,4-triazolo [4,3-a]pyridin-3(2H)-one (
1-bromo-3-chloropropane (2a), and 1-(3-chlorophenyl)piperazine ( ), were treated with microwave
radiation, but in this case trazodone was not observed in the post-reaction mixture.
1),
4
To summarize the synthesis of trazodone under microwave radiation, it can certainly be assumed
that this method is applicable at every stage of the synthesis. This method, in comparison to
conventional methods, increases the yield of the obtained products, shortens the time of the synthesis,
excludes or reduces the amount of solvents used in the process, and reduces the amount of energy.

Molecules 2019, 24, 1609
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Bearing in mind that the highest yields of trazodone were obtained with method I, as well as the
fact that this process can be quite easily scaled-up, we decided to synthesize trazodone derivatives
according to this method.
2.2. Synthesis of Trazodone Derivatives
The synthesis of trazodone with method I also works very well in the synthesis of its derivatives
(Scheme 7, Table 5) (10a–r). The reactions were carried out under similar conditions as the previous
ones, i.e., potassium carbonate, TBAB, and a small amount of ACN. All of the syntheses were carried
out in the SAMSUNG device (Table 6). Additionally, for the selected ligands, the synthesis in the
CEM Discover SP reactor was also performed, giving slightly higher yields (10e, yield = 58%; 10o
yield = 61%; 10r yield = 66%). The obtained products were converted into hydrochlorides.
The derivatives of trazodone (10a–k) with a modified aryl substituent were obtained by
the condensation reaction of 2-(3-chloropropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (4a) with
,
appropriate arylpiperazines 7. As already mentioned in the introduction, and bearing in mind the high
activity with 5-HT1A receptors in ligands with the hexyl linker that we previously synthesized [17
–19
,
22],
hexyl derivatives of trazodone were also obtained (10e–k) (Scheme 7).
N
N
( )n
N
Br
HCl
HN
N R
+
( )n
N
N
NR
N
N
O
O
3b, n=1
8, n=4
4, 9a-m
10a-r
n = 4
n = 1
10l
10a
10e
10f
Cl
O
9a
9g
Cl
Cl
Cl
Cl
Cl
Cl
10m
10n
10o
10p
10r
10b
10c
10d
9b
9c
9d
4
9h
9i
O
N
F
O
Cl
F
N
N
Cl
Cl
10g
10h
10i
N
N
O
N
N
O
9j
Cl
O
O
9k
F
Cl
10j
9e
9f
9l
O
Cl
N
Cl
10k
9m
O
Scheme 7. Synthesis of trazodone derivatives with method I.
Table 5. Analytical data of the library 10a–10k. M.P.—melting point.
Entry
No.
10a
n
R
Yield (%)
Purity* (%)
M.P. (◦C)
1
1
25
97
225–230
Cl
Cl
2
3
10b
10c
1
1
30
34
99
99
146–150
235–240
F
N
N

Molecules 2019, 24, 1609
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Table 5. Cont.
Yield (%)
Entry
No.
10d
n
R
Purity* (%)
M.P. (◦C)
4
1
33
44
98
104–111
O
5
10e
4
97
178–183
Cl
6
7
8
10f
10g
10h
4
4
4
45
44
45
92
98
98
145–148
163–167
116–120
Cl
Cl
Cl
Cl
9
10i
10j
4
4
45
32
94
95
183–185
140–141
F
10
O
11
12
14
10k
10l
4
4
4
30
27
31
100
92
137–138
144–146
143–145
O
O
10m
97
O
N
15
10n
4
70
98
175–178
N
N
16
17
10o
10p
4
4
45
55
90
90
146–148
133–137
18
10r
4
61
95
oil
* Percent purity of the purified product was calculated on the peak area integration during HPLC analysis.
The progress of the reaction was monitored with thin-layer chromatography (TLC) and the purity
of the resulting ligands was assessed with UPLC-MS. The structures were confirmed based on the
analysis of data obtained from 1H NMR, 13C NMR, and infrared (IR).
2.3. Biological Evaluation of Trazodone Derivatives
The ligands obtained in the synthesis (10a–r) were tested in vitro for binding to D2, 5-HT1A
,
5-HT2A, 5-HT6, and 5-HT7 receptors on the basis of the screening protocol described previously [17
(Table 6).
]

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Table 6. Activity in Ki ± SD (nM) in vitro tests of trazodone derivatives 10a–10k.
Entry
No.
D2
5-HT1A
5-HT2A
5-HT6
5-HT7
3650 *
116 ± 19
1652 ± 203
122 ± 8
78 *
285 ± 42
1547 ± 314
459 ± 36
593 ± 81
16 ± 2
16*
>10,000 **
1430 ± 294
36,380 ± 5817
3249 ± 401
9459 ± 1138
1945 ± 281
3573 ± 563
2062 ± 173
576 ± 82
1782 **
157 ± 13
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
trazodone
10a
181 ± 22
10b
10c
10d
10e
969 ± 114
178 ± 25
7415 ± 625
174 ± 34
539 ± 68
278 ± 19
2595 ± 359
435 ± 61
196 ± 29
993 ± 214
219 ± 36
1024 ± 184
8297 ± 1351
251 ± 52
1049 ± 91
5569 ± 438
404 ± 67
19 ± 3
152 ± 17
137 ± 11
522 ± 71
362 ± 17
310 ± 42
195 ± 11
57 ± 6
6713 ± 952
342 ± 48
10f
49 ± 4
2600 ± 318
234 ± 46
10g
10h
10i
10j
10k
10l
10m
10n
10o
10p
10r
27 ± 3
19 ± 3
77 ± 14
7 ± 2
417 ± 52
4736 ± 687
11,530 ± 2114
3497 ± 537
5617 ± 743
4816 ± 581
4637 ± 341
7224 ± 827
3328 ± 197
1188 ± 165
4 ± 1
841 ± 105
343 ± 21
21 ± 3
9 ± 2
350 ± 54
1643 ± 219
10 ± 2
826 ± 91
9 ± 2
3903 ± 432
1540 ± 225
1718 ± 193
10,620 ± 1954
391 ± 27
18 ± 3
202 ± 31
1526 ± 116
191 ± 14
104 ± 12
28 ± 4
20 ± 3
328 ± 49
* [23], ** [24].
The in vitro results collected in Table 6 show that replacement of the 3-chloroarylpiperazine system
in trazodone, as well as chain elongation, results in a decreased activity relative to the 5-HT2A receptor.
Interestingly, extending the chain by three carbon atoms, while maintaining the 3-chloroarylpiperazine
system, resulted in a change in the pharmacological profile and binding to the 5-HT1A receptor (10e).
The majority of the synthesized hexyl ligands (Table 6, entries 5–18) provided a high affinity and
selectivity for the 5-HT1A receptor.
Interestingly, adding chlorine to this structure, at position 3 to position 4 (10h) resulted in a
fourfold increase of the activity toward 5-HT2A and 5-HT6. When there was only a substituent in
the 4-position (10g), the activity was similar to that in the case of 10e (Table 6). Among ligands with
chlorine substitution, the compound with the substitution at position 2 (10f) had the least interesting
activity profile. However, when chlorine was replaced with fluorine at this position, the binding
and selectivity to 5-HT1A increased significantly (10i). Among ligands with a hexyl chain and the
arylmethoxy group linked to the piperazine moiety, ligands with substitution at positions 2 (10j) and 3
(10) had a much higher binding to 5-HT1A compared to their 4-substituted analog (10l). Interestingly,
if the methoxy group was replaced by an ethoxy group in position 2 (10m), selectivity increased
significantly. Our research showed that the number of nitrogen atoms in the arylpiperazine system also
strongly affected binding to the 5-HT receptors. When the ligand had two nitrogen atoms at positions
2 and 6, the activity was >100 nM for all receptors (10o). When there was only one nitrogen atom in
the arylpiperazine system (10n), the activity with the 5-HT1A and D2 receptors increased significantly,
after which, when the nitrogen atoms were completely eliminated (10p), the compound was active
only with 5-HT1A. Of all the trazodone derivatives obtained, the characteristics of 5-HT1A receptor
ligands were shown by 10e, 10g, 10h, 10i, 10m, and 10p. It is also worth noting that very interesting
features as a dual 5-HT1A/7 ligand were shown by 10r.
2.4. Molecular Modeling
In order to explain the effect of the alkyl linker elongation in trazodone on the affinity to receptors
5-HT1A and 5-HT2A, we performed the molecular docking of the compound and representative ligand
10e (entry 6 in Table 6). The results showed that 10e, which contained the hexyl chain, did not
bind to any used homology models of the serotonin receptor type 2A. In the case of the 5-HT1AR,
two alternative binding modes were considered (Figure 2). For both, the key salt bridge with D76
and protonated piperazine moiety was observed. In the case of bent conformations, on the left of
Figure 2 the chlorophenyl ring of both compounds formed a CH–π interaction with F321/F322 and

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the 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one group interacted with N346. The main differences occurred
in that last region. The elongated structure of 10e caused contacts with G342 and I345, while the
trazodone interacted with T339 and A343. The second arrangement in consideration, which was more
coherent, was additionally supported by the results of crystallographic studies—the crystal structure
of trazodone hydrochloride, CSD-CPTAZP [25] (B on Figure 2). In this case, the triazolopyridine ring
expanded into the cavity between transmembrane domains (TMs) 2 and 7. This binding mode meant
that only trazodone could interact with phenylalanine F322 and W318, which was the main reason for
its greater affinity in comparison to a hexyl derivative. In this case, the ligand with the hexyl chain
additionally had contact with residues from ECL2 (Q57 and Y56) and from TM5 (S159).
Figure 2. Two considered binding modes of trazodone (blue) and 10e (green) in the homology model
of serotonin receptor 5-hydroxytryptamine (5-HT1A). Only key residues of the receptor are marked as
sticks, where those employed in the binding of both are uncolored and those specific in the ligands
are colored.
3. Experimental
3.1. Materials and Methods
The reactions in the microwave radiation field were mainly carried out in an Erlenmeyer flask in
the Samsung M182DN device (300 W) and comparably in a closed PTFE tube in a Magnum II reactor
(600 W) and CEM Discover SP reactor (100 W). All reagents from Sigma Aldrich (Poznan, Poland) and
all organic solvents from POCH were of reagent grade and were used without purification. The progress
of all reactions and purity of the synthesized compounds was confirmed by TLC, performed on Merck
silica gel 60 F254 aluminum sheets (Merck, Darmstadt, Germany). Spots were detected by their
absorption under ultraviolet (UV) light (
Perkin Elmer Series 200 HPLC with an XTerra RP C-18 (3.5
MeOH:H2O 1:1 eluent acidified with 0.1% formic acid as a phase (flow rate of 1 mL
IR spectra were taken on an FTS-165 spectrometer (FTIR Biorad). Melting points were determined on a
Boetius apparatus and are uncorrected. The purification by HPLC was assessed by comparing product
integration to overall integrated spectrum.
λ
= 254 nm). HPLC chromatograms were determined on a
m seed size, 4.6 150 mm) column and
min−1) was used.
µ
×
·
1H-NMR and 13C-NMR spectra were recorded at 300 MHz (Bruker Avance, Cracow, Poland)
using tetramethylsilane (TMS; 0.00 ppm) and chloroform-d1; J values are in Hertz (Hz), and splitting
patterns are designated as follows: s (singlet), d (doublet), t (triplet), m (multiplet).
The three-dimensional structures of the ligands were fully optimized at CAM-B3LYP/6-31G*19
level with the polarizable continuum model (PCM) (solvent = water) using Gaussian 09 software
(Gaussian, Inc., Wallingford, CT, USA). The appropriate ionization states at pH = 7.4
assigned using MarvinSketch 18.29 (ChemAxon Europe, Budapest, Hungary). The AutoDock Tools
±
1.0 were

Molecules 2019, 24, 1609
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was used to assign the bond orders, appropriate amino acid ionization states, and to check for steric
clashes. The receptor grid was generated by centering the grid box with a size of 12 Å on D76 side
chain. Automated flexible docking was performed using AutoDock Vina 1.5.6 [26]. The figures were
prepared using PYMOL.
The homology models of the selected serotonin receptors, namely 5-HT1A and 5-HT2A, were
built on the D3 template (Protein Data Bank (PDB) identifier (ID): 3PBL), using a procedure described
previously [27].
3.2. General Procedure for the Preparation of 2-Hydrazinopyridine (7) under Microwave Conditions
(Samsung M182DN; 300 W)
Firstly, 2.50 g (0.016 mol, 1.5 cm3) of 2-bromopyridine (6b), 6.09 g (0.19 mol, 6 cm3) of hydrazine
anhydrous (98%), and 10 cm3 of DMF were placed in a conical flask. Reactions were carried out under
microwave radiation. The progress of the reaction was monitored by TLC (chloroform–methanol 9:1).
After 2 min, DMF was evaporated in vacuo and 15 cm3 of water was added. The mixture was extracted
2
×
20 cm3 with methylene chloride. The organic extracts were combined and removed in vacuo to
dryness; Rf = 0.41, yield (Y) = 14%.
3.3. General Procedure for the Preparation of 1,2,4-Triazolo[4,3-a]pyridin-3(2H)-one (1)
3.3.1. Preparation of 1,2,4-Triazolo[4,3-a]pyridin-3(2H)-one (1) from 7 under Microwave Conditions
(Samsung M182DN; 300 W)
Firstly, 500 mg (4.58 mmol) g of 2-hydrazinopyridine (7) and 275 mg (4.58 mmol) or 550 mg
(9.16 mmol) of urea were placed in a conical flask. Reactions were carried out under microwave
radiation. The progress of the reaction was monitored by TLC (chloroform–methanol 9:1). After 50 s,
15 cm3 of water was added to the mixture, after which the resulting product was filtered off on a
Büchner funnel. The product was obtained with efficiency 49% in an equimolar reaction and 75%
by reaction with a twofold molar excess of urea; Rf = 0.62, HPLC 97.9% (reaction with molar excess),
tM 1.95 min.
3.3.2. Preparation of 1,2,4-Triazolo [4,3-a] pyridin-3(2H)-one (1) from 6a/b and Semicarbazide under
Microwave Conditions (Samsung M182DN; 300 W)
Firstly, 5.00 g (44 mmol) of 2-chloropyridine (6a), 9.81 g (88 mmol) of semicarbazide hydrochloride,
15 cm3 of 2-ethoxyethanol, and 0.1 cm3 of concentrated sulfuric acid were placed in a conical flask.
Reactions were carried out under microwave radiation. The progress of the reaction was monitored by
TLC (eluent chloroform–methanol 9:1). After 2 min, the reaction mixture was cooled to about 60 ◦C
and 15 cm3 of water was added. The precipitate was filtered on a Büchner funnel and washed with
about 50 cm3 of water. Y = 48%.
3.3.3. Preparation of 1,2,4-Triazolo[4,3-a]pyridin-3(2H)-one (1) from 6b and Semicarbazide under
Microwave Conditions (Magnum II Reactor; 600 W)
Firstly, 350 mg (2.2 mmol) of 2-bromopyridine (6b), 491 mg (4.4 mmol) of semicarbazide
hydrochloride, 0.75 cm3 of 2-ethoxyethanol, and 0.01 cm3 of concentrated sulfuric acid were placed in
a PTFE vessel. Reactions were carried out under microwave radiation. The progress of the reaction
was monitored by TLC (eluent chloroform–methanol 9:1). After 2 min, 0.75 cm3 of water was added to
the reaction mixture. The precipitate was filtered on a Büchner funnel and washed with about 5 cm3
of water.

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3.4. General Procedures for the Preparation of 2-(3-Halopropyl)[1,2,4]triazolo[4,3-a]pyridin-3 (2H)-one (3a/b
)
3.4.1. Synthesis 2-(3-Halopropyl)[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one (3a/b) (Samsung M182DN;
300 W)
Firstly, 1.35 g (0.01 mol) of 1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (1
), 2.50 cm3 (26 mmol, 4.09 g)
of 1-bromo-3-chloropropane (2a)/2.65 cm3 (0.026 mol, 5.25 g) of 1,3-dibromopropane (2b), 320 mg
(0.001 mol) of TBAB, 4.14 g (0.03 mol) of K2CO3, and the appropriate amount of acetonitrile (6, 3,
0.75, or 0.4 cm3)/DMF (5 cm3) were placed in a conical flask, after which the mixture was subjected to
microwave radiation. The progress of the reaction was monitored by TLC (chloroform–methanol 9:1).
The reaction times are summarized in Table 1. After the reaction, about 50 cm3 of water was added
and the resulting product was filtered off on a Büchner funnel. The reaction yields are summarized in
Table 1. The purity of the product obtained with the highest efficiency (Y = 92%) was confirmed using
HPLC 99.3% tM = 3.43 min.
3.4.2. Synthesis 2-(3-Chloropropyl)[1,2,4]triazolo[4,3-a]pyridin-3(2H)-one (3a) (Magnum II Reactor;
600 W)
Firstly, 135 mg (1 mmol) of 1,2,4-triazolo [4,3-a] pyridin-3-(2H)-one (1
), 0.25 cm3 (2.6 mol) of
1-bromo-3-chloropropane (2a), 32 mg (0.1 mmol) of TBAB, and 414 mg (3 mmol) of K2CO3 0.075 cm3 of
acetonitrile were placed in a PTFE vessel, after which the mixture was subjected to microwave radiation.
The progress of the reaction was monitored by TLC (eluent chloroform–methanol 9:1). After 2 min,
5 cm3 of water was added and the resulting product was filtered off on a Büchner funnel. Y = 81%.
3.5. General Procedures for the Preparation of Trazodone According to Method I
3.5.1. Preparation of Trazodone according to Method I under Microwave Conditions
(Samsung M182DN; 300 W)
Firstly, 2.11 g (10 mmol) of 2-(3-chloropropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (3a)/2.56 g
(10 mmol) of 2-(3-bromopropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (3b), 2.33 g (10 mmol) of
1-(3-chlorophenyl) piperazine hydrochloride (4), 320 mg (1 mmol) of TBAB, 4.14 g (30 mmol) of K2CO3,
and the appropriate amount of acetonitrile (8, 3, or 1 cm3)/DMF (6, 4, or 2 cm3)/water (50 cm3), were
placed in a conical flask, after which the mixture was subjected to microwave radiation. The progress
of the reaction was monitored by TLC (eluent chloroform–methanol 9:1). The reaction times are
summarized in Table 2. After the reaction, 50 cm3 water was added and the resulting product was
filtered off on a Büchner funnel. The reaction yields are summarized in Table 2. After drying, the
obtained trazodone was dissolved in acetone and a solution of 2M HCl in dioxane was added until
acidic (universal indicator). The precipitated hydrochloride was filtered off on a Büchner funnel.
3.5.2. Preparation of Trazodone According to Method I under Microwave Conditions
(Magnum II Reactor; 600 W)
Firstly, 168 mg (8 mmol) of 2-(3-chloropropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (3a), 186 mg
(8 mmol) of 1-(3-chlorophenyl)piperazine hydrochloride (4), 25.8 mg (0.8 mmol) of TBAB, 331 mg
(2.4 mmol) of K2CO3, and 0.28 cm3 of ACN were placed in a PTFE vessel, after which the mixture was
subjected to microwave radiation under pressure of 1, 5, or 10 bar. The reaction yields are summarized
in Table 2. After drying, the obtained trazodone was dissolved in acetone and a solution of 2M HCl in
dioxane was added until acidic (universal indicator). The precipitated hydrochloride was filtered off
on a Büchner funnel.

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3.6. General Procedures for the Preparation of 1-(3-Chloropropyl)-4-(3-chlorophenyl)piperazine (5)
3.6.1. Preparation of 1-(3-Chloropropyl)-4-(3-chlorophenyl)piperazine (
(Samsung M182DN; 300 W)
5) under Microwave Radiation
Firstly, 2.50 cm3 (26 mmol, 4.09 g) of 1-bromo-3-chloropropane (2a), 2.33 g (10 mmol) of
1-(3-chlorophenyl)piperazine hydrochloride (4), 320 mg (10 mmol) of TBAB, 4.14 g (30 mmol) of
K2CO3, and 3 cm3 of acetonitrile/DMF were placed in a conical flask, after which the mixture was
subjected to microwave radiation. The progress of the reaction was monitored by TLC (eluent
chloroform–methanol 9:1). After the reaction, 50 cm3 of water was added and the resulting product
was filtered off. After drying, obtained 5 was dissolved in acetone and a solution of 2M HCl in dioxane
was added until acidic (universal indicator). The precipitated hydrochloride was filtered off on a
Büchner funnel; Rf = 0.83. The reaction yields are summarized in Table 3.
3.6.2. Preparation of 1-(3-Chloropropyl)-4-(3-chlorophenyl)piperazine (
(Magnum II Reactor; 600 W)
5) under Microwave Radiation
Firstly, 0.25 cm3 (2.6 mmol, 409 mg) of 1-bromo-3-chloropropane (2a) 233 mg (1 mmol) of
1-(3-chlorophenyl) piperazine hydrochloride (4), 32 mg (1 mmol) of TBAB, 414 mg of K2CO3 (3 mmol),
and 0.3 cm3 of acetonitrile were added in a PTFE vessel, after which the mixture was subjected to
microwave radiation. The progress of the reaction was monitored by TLC (eluent chloroform–methanol
9:1). After drying, obtained 5 was dissolved in acetone and a solution of 2M HCl in dioxane was added
until acidic (universal indicator). The precipitated hydrochloride was filtered off on a Büchner funnel.
3.7. General Procedures for the Preparation of Trazodone According to Method II
3.7.1. Preparation of Trazodone According to Method II under Microwave Conditions
(Samsung M182DN; 300 W)
Firstly, 2.76 g (10 mmol) of 1-(3-chloropropyl)-4-(3-chlorophenyl)piperazine hydrochloride (
5),
1.35 g (10 mmol) of 1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one ( ), 322 mg (1 mmol) of TBAB, 4.14 g
1
(30 mmol) of K2CO3, and a corresponding amount of acetonitrile (2, 6, or 8 cm3) were placed in a
conical flask, after which the mixture was subjected to microwave radiation. The progress of the
reaction was monitored by TLC (eluent chloroform–methanol 9:1). The reaction times are summarized
in Table 4. After the reaction, about 50 cm3 of water was added and the resulting product was filtered
off on a Büchner funnel. The reaction yields are summarized in Table 4; Rf = 0.75.
3.7.2. Preparation of Trazodone According to Method II under Microwave Conditions
(Magnum II Reactor; 600 W)
Firstly, 276 mg (1 mmol) of 1-(3-chloropropyl)-4-(3-chlorophenyl)piperazine hydrochloride (
5),
135 mg (1 mmol) of 1,2,4-triazolo [4,3-a] pyridin-3-(2H)-one (
1), 32 mg (0.1 mmol) of TBAB, 414 mg
(3 mmol) of K2CO3, and 0.2 cm3 of acetonitrile were placed in a PTFE vessel, after which the mixture
was subjected to microwave radiation. The progress of the reaction was monitored by TLC (eluent
chloroform–methanol 9:1). After the reaction, water was added and the resulting product was filtered
off. After drying to solution 1-(3-chloropropyl)-4-(3-chloropheyl)piperazine (5) in acetone, a solution
of HCl in dioxane was added until acidic. The precipitated hydrochloride was filtered off on a
Büchner funnel.
3.8. General Procedure for the Preparation of Trazodone in One-Pot Synthesis
Firstly, 1.35 g (10 mmol) of 1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (1
), 2.5 cm3 (26 mmol) of
1-bromo-3-chloropropane (2a), 322 mg (1 mmol) of TBAB, 8.28 g (60 mmol) of K2CO3, and 5 cm3 of
acetonitrile were placed in a conical flask. The reactions were carried out under microwave radiation
(Samsung M182DN; 300 W) for 50 s. After this time, 2.3 g (10 mmol) of 1-(3-chlorophenyl)piperazine

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hydrochloride (4
) and 5 cm3 of acetonitrile were added to the reaction mixture. Reactions were carried
out in the presence of microwave radiation for another 90 s. The progress of the reaction was monitored
by TLC (eluent chloroform–methanol 9:1). After the reaction, 50 cm3 of water was added and the
resulting product was filtered off on a Büchner funnel. Yield (Samsung M182DN; 300 W) 31%, yield
(CEM Discover SP reactor; 100 W) 71%.
3.9. General Procedure for the Preparation of 2-(3-Bromohexyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (8)
Firstly, 1.35 g (10 mmol) of 1,2,4-triazolo [4,3-a] pyridin-3-(2H)-one (1
), 4.6 cm3 (30 mmol) of
1,6-dibromohexane, 0.322 g (1 mmol) of TBAB, 4.14 g (30 mmol) of K2CO3, and 2.5 cm3 of acetonitrile
were placed in a conical flask, after which the mixture was subjected to microwave radiation for 60 s.
The progress of the reaction was monitored by TLC (eluent chloroform–methanol 9:1). After the
reaction, 15 cm3 of water was added to the mixture and extracted with methylene chloride. The product
was purified by column chromatography (eluent chloroform–methanol 9:1).
2-(3-bromohexyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (8 δ 7.78 (ddd,
): 1H NMR (400 MHz, CDCl3)
J = 5.5, 3.3, 2.2 Hz, 1H, ArH), 7.15–7.07 (m, 2H, ArH), 6.51 (ddd, J = 7.2, 4.4, 3.0 Hz, 1H, ArH), 4.02
(t, J = 7.1 Hz, 2H, CHNCO), 3.42 (dd, J = 8.0, 5.6 Hz, 2H, CHBr), 1.89 (ddd, J = 10.3, 7.3, 3.4 Hz, 4H,
CHAliph), 1.55–1.48 (m, 2H, CHAliph), 1.46–1.39 (m, 2H, CHAliph). 13C NMR (101 MHz, CDCl3)
δ 148.57
(Ar), 141.47 (Ar), 129.75 (Ar), 123.77 (Ar), 115.38 (Ar), 110.48 (Ar), 45.77 (CAliph), 33.69 (CAliph), 32.56
(CAliph), 28.62 (CAliph), 27.71 (CAliph), 25.75 (CAliph). HPLC 91% (tR = 7.31), Rf = 0,96, yield = 79%, oil.
3.10. General Procedure for the Preparation Trazodone Derivatives
Firstly, 529 mg (2.5 mmol) of 2-(3-chloropropyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (3a)/745 mg
(2.5 mmol) of 2-(3-bromohexyl)-1,2,4-triazolo[4,3-a]pyridin-3-(2H)-one (8), 2.5 mmol of the
corresponding arylpiperazine hydrochloride (9a–m), 80 g (0.25 mmol) of TBAB, 1.04 g (7.5 mmol) of
K2CO3, and 0.75 cm3 of acetonitrile were placed in a conical flask, after which the mixture was subjected
to microwave radiation (Samsung M182DN; 300 W). The progress of the reaction was monitored by
TLC (eluent chloroform–methanol 9:1). After the reaction, water was added and the resulting product
was filtered off on a Büchner funnel. After drying, the obtained 10a–k were dissolved in acetone
and a solution of 2M HCl in dioxane was added until acidic (universal indicator). The precipitated
hydrochloride was filtered off on a Büchner funnel.
1
2-[3-[4-(2,3-dichlorophenyl)piperazin-1-yl]propyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10a): H
NMR (300 MHz, CDCl3) δ 7.76 (d, J = 7.1 Hz, 1H, ArH), 7.17 (ddd, J = 17.0, 8.8, 3.0 Hz, 4H, 1H, ArH),
7.02 (d, J = 8.0 Hz, 1H, 1H, ArH), 6.54 (t, J = 6.5 Hz, 1H, 1H, ArH), 4.18 (t, J = 6.1 Hz, 2H, CHNCO), 3.67
(dd, J = 22.9, 11.2 Hz, 4H, CHpip), 3.37 (d, J = 13.5 Hz, 2H, CHpip), 3.17 (s, 2H, CHpip), 3.06 (d, J = 11.5
Hz, 2H, CHaliph), 2.60 (s, 2H, CHaliph).13C NMR (75 MHz, CDCl3)
δ 149.07 (Ar), 148.89 (Ar), 142.14 (Ar),
134.44 (Ar), 130.61 (Ar), 128.09 (Ar), 127.82 (Ar), 126.45 (Ar), 123.92 (Ar), 119.52 (Ar), 115.58 (Ar), 111.10
(Ar), 55.54 (Ctriaz), 52.66 (Cpip, Cpip), 48.15 (Cpip, Cpip), 43.45 (Cpip), 23.55 (Caliph). Fourier-transform
(FT)-IR 3000 (C–H Ar, Str), 2954, 2850 (C-HAliph, Str), 1704 (C=O, Str), 1650 (C=N, Str), 1500, 1450
(C=C, Str), 1350 (C–N, Str), 750 (C–Cl, Str). HPLC 97% (tR = 4.33), m/z = 406,19, Rf = 0.87, yield = 25%,
melting point (mp) = 225–230 ◦C.
2-[3-[4-(2-fluorophenyl)piperazin-1-yl]propyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10b): 1H
NMR (300 MHz, CDCl3) δ 7.76 (d, J = 6.9 Hz, 1H, ArH), 7.26 (s, 2H, ArH), 7.12 (d, J = 10.0 Hz, 4H, ArH),
6.54 (s, 1H, ArH), 4.18 (s, 2H, CHNCO), 4.03–3.91 (m, 2H, CHpip), 3.62 (s, 2H, CHpip), 3.58–3.49 (m,
2H, CHpip), 3.46–3.30 (m, 2H, CHpip), 3.26–3.16 (m, 2H, CHaliph), 2.66–2.53 (m, 2H, CHaliph). 13C NMR
(75 MHz, dimethyl sulfoxide (DMSO)) δ 155.88 (Ar), 152.63 (Ar), 147.58 (Ar), 140.62 (Ar), 137.71(Ar)
130.04 (Ar), 122.79 (Ar), 119.02 (Ar), 115.72 (Ar), 115.45 (Ar), 114.47 (Ar), 110.31 (Ar), 52.42 (Ctriaz),
50.35 (Cpip, Cpip), 46.41 (Cpip, Cpip), 42.01 (Cpip), 22.38 (Caliph). FT-IR 3000 (C–H Ar, Str), 2946, 2852

Molecules 2019, 24, 1609
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(C–HAliph, Str), 1711 (C=O, Str), 1639 (C=N, Str), 1574, 1451 (C=C, Str), 1355 (C–N, Str), 1108 (C–F, Str).
HPLC 99% (tR = 3.38), m/z = 356.21; Rf = 0.70, yield = 30%, mp = 146–150 ◦C.
1
2-[3-(4-pyrimidin-2-ylpiperazin-1-yl)propyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10c): H NMR
(300 MHz, CDCl3)
δ 7.76 (d, J = 7.1 Hz, 1H, ArH), 7.26–7.07 (m, 4H, ArH), 7.02 (dd, J = 7.8, 1.7 Hz, 1H,
ArH), 6.57–6.50 (m, 1H, ArH), 4.18 (t, J = 6.1 Hz, 2H, CHNCO), 3.67 (dd, J = 22.2, 11.3 Hz, 4H, CHpip),
3.37 (d, J = 12.9 Hz, 2H, CHpip), 3.25–3.15 (m, 2H, CHpip), 3.06 (d, J = 11.3 Hz, 2H, CHaliph), 2.59 (t, J =
14.4 Hz, 2H, CHaliph). 13C NMR (75 MHz, CDCl3)
δ 148.85 (Ar), 148.67 (Ar), 141.93 (Ar), 130.41 (Ar),
127.89 (Ar, Ar) 126.23 (Ar), 119.32 (Ar), 115.37 (Ar), 110.90 (Ar), 55.32(Ctriaz), 52.46 (Cpip, Cpip), 47.94
(Cpip, Cpip), 43.24 (Cpip), 23.34 (Caliph). FT-IR 2990 (C-H Ar, Str), 2946, 2850 (C–HAliph, Str), 1706 (C=O,
Str), 1636 (C=N, Str), 1601, 1459 (C=C, Str), 1355 (C–N, Str). HPLC 99% (tR = 4.32), m/z = 340.46; Rf =
0.68, yield = 34%, mp = 235–240 ◦C.
1
2-[3-[4-(2-ethoxyphenyl)piperazin-1-yl]propyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10d): H
NMR (300 MHz, CDCl3)
δ 7.76 (d, J = 7.1 Hz, 1H, ArH), 7.18–6.95 (m, 4H, ArH), 6.89 (dd, J = 17.3
7.4 Hz, 2H, ArH), 6.54 (t, J = 6.5 Hz, 1H, ArH), 4.17 (t, J = 6.1 Hz, 2H, CHNCO), 4.08 (q, J = 6.9 Hz, 2H,
OCH), 3.65–3.53 (m, 4H, CHpip), 3.16 (s, 4H, CHpip), 2.60 (s, 2H, CHaliph), 1.64 (s, 2H, CHaliph), 1.59 (s,
2H, CHaliph). 13C NMR (75 MHz, CDCl3)
δ
151.66 (Ar), 148.87 (Ar), 142.17 (Ar, Ar), 130.62 (Ar), 123.92
(Ar, Ar), 121.47 (Ar), 115.59 (Ar, Ar), 113.05 (Ar), 111.11 (Ar), 64.37 (Coxy), 55.39 (Ctriaz), 51.37 (Cpip
,
Cpip), 48.08 (Cpip, Cpip), 43.41 (Cpip), 23.53 (Caliph), FT-IR 3001 (C–H Ar, Str), 2941, 2858 (C–HAliph, Str),
1708 (C=O, Str), 1611 (C=N, Str), 1531, 1458 (C=C, Str), 1349 (C–N, Str), 1260, 1021 (C–O, Str). HPLC
98% (tR = 3.73), m/z = 382.27; Rf = 0.66, yield = 33%, mp = 104–111 ◦C.
1
2-[6-[4-(3-chlorophenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10e): H NMR
(300 MHz, CDCl3)
δ 7.76 (d, J = 7.1 Hz, 1H, ArH), 7.61 (s, 1H, ArH), 7.49 (d, J = 8.2 Hz, 1H, ArH), 7.39
(d, J = 8.2 Hz, 1H, ArH), 7.31 (d, J = 8.3 Hz, 1H, ArH), 7.15–7.05 (m, 2H, ArH), 6.51 (dt, J = 7.3, 3.7
Hz, 1H, ArH), 4.45 (d, J = 11.4 Hz, 2H, CHNCO), 4.02 (t, J = 6.7 Hz, 2H, CHpip), 3.83 (s, 2H, CHpip),
3.73–3.63 (m, 4H, CHpip), 3.11 (s, 2H, CHaliph), 2.00–1.81 (m, 4H, CHaliph), 1.46 (s, 4H, CHaliph). 13C
NMR (75 MHz, CDCl3)
δ 148.62(Ar), 146.74 (Ar), 141.56 (Ar), 135.80 (Ar), 131.06 (Ar), 129.92 (Ar),
125.69 (Ar), 123.72 (Ar), 119.31 (Ar), 116.98 (Ar), 115.37 (Ar), 110.65 (Ar), 57.28 (Ctriaz), 50.21 (Cpip,
Cpip), 48.35 (Cpip, Cpip), 45.33 (Cpip), 28.32 (Caliph), 26.06 (Caliph), 25.67 (Caliph), 23.39 (Caliph). FT-IR
2981 (C–H Ar, Str), 2935, 2851 (C–HAliph, Str), 1703 (C=O, Str), 1639 (C=N, Str), 1593, 1451 (C=C, Str),
1354 (C–N, Str), 734 (C–Cl, Str). HPLC 97% (tR = 4.23), m/z = 414.30, Rf = 0.66, yield = 47% (Samsung),
yield = 58 (CEM Discover SP reactor), mp = 178–183 ◦C.
1
2-[6-[4-(2-chlorophenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10f): H NMR
(300 MHz, DMSO)
δ 8.44 (d, J = 4.8 Hz 1H, ArH), 8.33–8.17 (m, 2H, ArH), 7.93–7.82 (m, 1H, ArH), 7.63
(dd, J = 11.7, 6.5 Hz, 2H, ArH), 7.08 (dd, J = 13.6, 6.4 Hz, 1H, ArH), 6.75 (t, J = 4.7 Hz, 1H, ArH), 3.84
(t, J = 7.3 Hz, 2H, CHNCO), 3.58 (s, 8H, CHpip), 3.04 (m, 2H, CHaliph), 1.84 (m, 4H, CHaliph), 1.52 (m,
4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ 161.06 (Ar), 158.58 (Ar), 136.22 (Ar), 132.01 (Ar), 130.58
(Ar), 130.23 (C Ar), 129.15 (Ar), 120.70 (Ar), 118.62 (Ar), 118.54(Ar), 111.66 (Ar), 104.35 (Ar), 55.87
(Ctriaz), 50.74 (Cpip, Cpip), 42.71 (Cpip, Cpip), 41.81 (Cpip), 27.98 (Caliph), 27.82 (Caliph), 27.61 (Caliph),
26.22 (Caliph). FT-IR 3008 (C–H Ar, Str), 2942, 2856 (C–HAliph, Str), 1738 (C=O, Str), 1616 (C=N, Str),
1538, 1456 (C=C, Str), 1349 (C–N, Str), 738 (C–Cl, Str). HPLC 92% (tR = 4.86), m/z = 452.19; Rf = 0.67,
yield = 45%, mp = 145–148 ◦C.
1
2-[6-[4-(4-chlorophenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10g): H NMR
(300 MHz, CDCl3)
δ 7.85 (d, J = 8.8 Hz, 2H, ArH), 7.77 (d, J = 7.1 Hz, 1H, ArH), 7.51 (d, J = 8.9 Hz,
2H, ArH), 7.13–7.09 (m, 2H, ArH), 6.52 (dd, J = 7.2, 3.6 Hz, 1H, ArH), 4.78 (m, 2H, CHNCO), 4.21 (m,
2H, CHpip), 4.04 (t, J = 6.8 Hz, 2H, CHpip), 3.75–3.56 (m, 4H, CHpip), 3.15 (m, 2H, CHaliph), 2.03–1.81
(m, 4H, CHaliph), 1.45 (m, 4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ 148.89 (Ar), 148.39 (Ar), 141.35
(Ar), 130.85 (Ar), 129.27 (Ar, Ar), 124.30 (Ar) 124.00 (Ar), 117.93 (Ar, Ar), 115.44 (Ar), 111.30 (Ar), 55.69
(Ctriaz), 50.79 (Cpip, Cpip), 45.64 (Cpip, Cpip), 45.26 (Cpip), 28.44 (Caliph), 25.96 (Caliph), 25.87 (Caliph),

Molecules 2019, 24, 1609
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23.29 (Caliph). FT-IR 3034 (C–H Ar, Str), 2934, 2860 (C–HAliph, Str), 1708 (C=O, Str), 1637 (C=N, Str),
1539, 1482 (C=C, Str), 1354 (C–N, Str), 755 (C–Cl, Str). HPLC 98% (tR = 1.63); Rf = 0.50, yield = 44%,
mp = 163–167 ◦C.
1
2-[6-[4-(3,4-dichlorophenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10h): H
NMR (300 MHz, DMSO)
δ 7.84 (d, J = 7.1 Hz, 1H, ArH), 7.45 (d, J = 8.9 Hz, 2H, ArH), 7.23 (dd, J = 5.9,
3.4 Hz, 2H, ArH), 7.00 (dd, J = 9.1, 2.8 Hz, 1H, ArH), 6.66–6.58 (m, 1H, ArH), 4.67 (s, 2H, CHNCO), 3.89
(dd, J = 12.3, 5.2 Hz, 4H, CHpip), 3.60–3.47 (m, 4H, CHpip), 3.16 (m, 2H, CHaliph), 1.75 (m, 4H, CHaliph),
1.32 (m, 4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ 149.77 (Ar), 148.38 (Ar), 141.35 (Ar), 132.11 (Ar),
131.10 (Ar), 130.84 (Ar), 124.29 (Ar), 121.22 (Ar), 117.44 (Ar), 116.29 (Ar), 115.44 (Ar), 111.30 (Ar), 55.73
(Ctriaz), 50.54 (Cpip), 45.71–44.67 (Cpip, Cpip), 42.66 (Cpip, Cpip), 32.66 (Cpip), 28.44 (Caliph), 25.68 (Caliph),
23.34 (Caliph), 22.55(Caliph). FT-IR 3016 (C–H Ar, Str), 2949, 2860 (C–HAliph, Str), 1738 (C=O, Str), 1642
(C=N, Str), 1542 (C=C, Str), 1365 (C–N, Str), 757 (C–Cl, Str). HPLC 95% (tR = 4.79), m/z = 448.20; Rf =
0.54, yield = 45%, mp = 116–120 ◦C.
1
2-[6-[4-(2-fluorophenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10i): H NMR
(400 MHz, DMSO) δ 7.85 (dt, J = 7.1, 1.1 Hz, 1H, ArH), 7.20–7.03 (m, 5H, ArH), 6.62 (ddd, J = 7.2, 5.0,
2.4 Hz, 1H, ArH), 3.90 (t, J = 6.9 Hz, 2H, CHNCO), 3.55 (t, J = 9.5 Hz, 2H, CHpip), 3.46 (d, J = 11.1 Hz,
2H, CHpip), 3.24–3.15 (m, 4H, CHpip), 3.08 (dd, J = 10.9, 5.5 Hz, 2H, CHaliph), 1.78–1.69 (m, 4H, CHaliph),
1.35–1.29 (m, 4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ 154.21–153.60 (Ar), 148.39 (Ar), 141.35 (Ar),
138.74 (Ar), 130.84 (Ar), 125.46 (Ar), 124.31 (Ar), 120.01 (Ar), 116.71 (Ar), 116.51 (v), 115.45 (Ar), 111.30
(Ar), 55.79 (Ctriaz), 51.22 (Cpip, Cpip), 47.41 (Cpip, Cpip), 45.26 (Cpip), 28.44 (Caliph), 26.05 (Caliph), 25.89
(Caliph), 23.29 (Caliph). FT-IR 3021 (C–H Ar, Str), 2936, 2850 (C–HAliph, Str), 1713 (C=O, Str), 1635 (C=N,
Str), 1541, 1500 (C=C, Str), 1355 (C–N, Str), 1141 (C–F, Str). HPLC 94% (tR = 1.46), Rf = 0.51, yield =
45%, mp = 183–187 ◦C.
1
2-[6-[4-(2-methoxyphenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10j): H
NMR (300 MHz, CDCl3)
δ 8.26 (d, J = 6.4 Hz, 1H, ArH), 7.77 (d, J = 7.0 Hz, 1H, ArH), 7.55–7.42 (m,
1H, ArH), 7.08 (dd, J = 14.9, 6.3 Hz, 4H, ArH), 6.50 (m, 1H, ArH), 5.16 (m, 3H, OCH), 4.46 (m, 2H,
CHNCO), 4.13–3.95 (m, 4H, CHpip), 3.66–3.52 (m, 4H, CHpip), 3.14 (s, 2H, CHAliph), 2.02–1.76 (m, 4H,
CHaliph), 1.45 (m, 4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ 152.27 (Ar), 148.39 (Ar), 141.36 (Ar),
139.75 (Ar), 130.85 (Ar), 124.30 (Ar), 124.00 (Ar), 121.30 (Ar), 118.72 (Ar), 115.45 (Ar), 112.41 (Ar), 111.31
(Ar), 55.84 (Coxy), 51.45 (Ctriaz), 47.32 (Cpip), 45.75 (Cpip), 45.27 (Cpip, Cpip), 32.18 (Cpip), 28.44 (Caliph),
26.56 (Caliph), 26.16 (Caliph), 25.79 (Caliph), 23.30 (Caliph). FT-IR 3016 (C–H Ar, Str), 2942, 2860 (C–HAliph
,
Str), 1708 (C=O, Str), 1640 (C=N, Str), 1540, 1448 (C=C, Str), 1366 (C–N, Str), 1261, 1020 (C–O, Str).
HPLC 95% (tR = 3.66), Rf = 0.45, yield = 32%, mp = 140–141 ◦C.
1
2-[6-[4-(3-methoxyphenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10k): H
NMR (300 MHz, CDCl3)
δ 7.77 (d, J = 7.1 Hz, 1H, ArH), 7.49 (s, 1H, ArH), 7.42 (d, J = 7.5 Hz, 2H, ArH),
7.11 (d, J = 4.1 Hz, 2H, ArH), 7.01 (d, J = 7.4 Hz, 1H, ArH), 6.51 (dd, J = 7.2, 3.8 Hz, 1H, ArH), 4.78 (s,
3H, OCH), 4.29 (s, 2H, CHNCO), 4.03 (t, J = 6.8 Hz, 2H, CHpip), 3.86 (s, 2H, CHpip), 3.66 (d, J = 10.9
Hz, 4H, CHpip), 3.15 (s, 2H, CHaliph), 1.91 (dd, J = 14.3, 7.3 Hz, 4H, CHaliph), 1.48 (d, J = 24.1 Hz, 4H,
CHaliph). 13C NMR (101 MHz, DMSO)
δ 150.21 (Ar), 140.44 (Ar, Ar), 139.99 (Ar), 129.15 (Ar), 120.10
(Ar, Ar), 121.16 (Ar), 119.89 (Ar), 114.42 (Ar), 112.11 (Ar), 111.11 (Ar), 55.44 (Coxy), 50.42 (Ctriaz), 47.39
(Cpip), 45.85 (Cpip, Cpip), 32.21 (Cpip), 28.41 (Caliph), 27.14 (Caliph), 28.12 (Caliph), 25.67 (Caliph), 23.31
(Caliph). FT-IR 2982 (C–H Ar, Str), 2938, 2861 (C–HAliph, Str), 1640 (C=O, Str), 1615 (C=N, Str), 1541,
1491 (C=C, Str), 1356 (C–N, Str), 1271, 1027 (C–O, Str). HPLC 100% (tR = 3.88), Rf = 0.54, yield = 30%,
mp = 137–138 ◦C.
2-[6-[4-(4-methoxyphenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride: (10l) 1H
NMR (300 MHz, CDCl3) δ 7.86 (d, J = 9.2 Hz, 2H, ArH), 7.76 (m, J = 7.0 Hz, 1H, ArH), 7.10 (d, J = 2.9
Hz, 2H, ArH), 7.00 (d, J = 9.1 Hz, 2H, ArH), 6.51 (m, 1H, ArH), 4.80 (m, 3H, OCH), 4.45–4.26 (m, 2H,
CHNCO), 4.04 (t, J = 6.8 Hz, 2H, CHpip), 3.84 (m, 2H, CHpip), 3.62 (m, 4H, CHpip), 3.13 (s, 2H, CHaliph),

Molecules 2019, 24, 1609
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1.91 (m, 4H, CHaliph), 1.73 (m, 4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ
154.37 (Ar), 148.39 (Ar),
143.82 (Ar), 141.36 (Ar), 130.85 (Ar), 124.30 (Ar), 118.65 (Ar), 115.45 (Ar, Ar) 114.88 (Ar, Ar), 111.31 (Ar),
56.98 (Coxy), 55.70 (Ctriaz), 51.03 (Cpip, Cpip), 47.31 (Cpip, Cpip), 45.27 (Cpip), 28.44 (Caliph), 26.50–25.58
(Caliph), 23.31 (Caliph), 13.96 (Caliph). FT-IR 3002 (C–H Ar, Str), 2939, 2840 (C–HAliph, Str), 1708 (C=O,
Str), 1640 (C=N, Str), 1541, 1509 (C=C, Str), 1374 (C–N, Str), 1260, 1025 (C–O, Str). HPLC 92% (tR =
1.40), Rf = 0.65, yield = 27%, mp = 144–146 ◦C.
2-[6-[4-(2-ethoxyphenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10m): 1H
NMR (300 MHz, CDCl3)
δ 8.20–8.15 (s, 1H, ArH), 7.77 (d, J = 7.1 Hz, 1H, ArH), 7.43 (m, 1H, ArH),
7.08 (dd, J = 15.7, 4.7 Hz, 4H, ArH), 6.51 (s, 1H, ArH), 4.96 (m, 2H, OCH), 4.45 (m, 2H, CHNCO), 4.33
(t, J = 7.3 Hz, 2H, CHpip), 4.02 (t, J = 6.9 Hz, 2H, CHpip), 3.61 (t, J = 13.0 Hz, 4H, CHpip), 3.13 (s, 2H,
CHaliph), 1.89 (m, 4H, CHaliph), 1.64 (t, J = 7.0 Hz, 3H, CHaliph), 1.44 (m, 4H, CHaliph). 13C NMR (101
MHz, DMSO)
δ 155.67 (Ar), 147.23 (Ar), 144.81 (Ar), 139.45 (Ar), 137.84 (Ar), 120.12 (Ar), 120.01 (Ar),
116.42 (Ar, Ar), 112.98 (Ar, Ar), 110.31 (Ar), 58.91 (Coxy), 55.74 (Ctriaz), 50.04 (Cpip, Cpip), 48.21 (Cpip
,
Cpip), 44.18 (Cpip), 29.60 (Caliph), 26.12 (Caliph), 21.13 (Caliph), 13.90 (Caliph), 13.40 (Caliph). FT-IR 2990
(C–H Ar, Str), 2936; 2860 (C–HAliph, Str), 1701 (C=O, Str), 1636 (C=N, Str), 1541, 1491 (C=C, Str), 1356
(C–N, Str), 1251, 1040 (C–O, Str). HPLC 97% (tR = 1.63), Rf = 0.46, yield = 31%, mp = 143–145 ◦C.
1
2-[6-[4-(2-pyridyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10n): H NMR
(300 MHz, DMSO)
δ 8.14 (d, J = 4.1 Hz, 1H, ArH), 7.83 (t, J = 7.0 Hz, 2H, ArH), 7.22 (d, J = 3.2 Hz,
2H, ArH), 7.15 (d, J = 8.9 Hz, 1H, ArH), 6.88 (s, 1H, ArH), 6.67–6.57 (m, 1H, ArH), 4.40 (d, J = 13.8 Hz,
2H, CHNCO), 3.88 (dd, J = 16.3, 9.5 Hz, 2H, CHpip), 3.58 (d, J = 11.6 Hz, 2H, CHpip), 3.37 (d, J = 14.1
Hz, 2H, CHpip), 3.22 (s, 2H, CHpip), 3.08 (s, 2H, CHaliph), 1.75 (d, J = 6.8 Hz, 4H, CHaliph), 1.28 (d, J =
27.2 Hz, 4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ 148.38 (Ar), 141.35 (Ar), 130.85 (Ar, Ar), 124.30
(Ar, Ar), 115.44 (Ar), 114.27 (Ar, Ar), 114.16 (Ar), 111.31 (Ar), 55.80 (Ctriaz), 50.29 (Cpip), 45.27 (Cpip),
43.34 (Cpip), 42.34 (Cpip), 28.44 (Caliph), 26.02 (Caliph), 25.88 (Caliph), 23.28 (Caliph). FT-IR 2970 (C–H Ar,
Str), 2936, 2861 (C–HAliph, Str), 1701 (C=O, Str), 1633 (C=N, Str), 1536, 1495 (C=C, Str), 1365 (C–N, Str).
HPLC 98% (tR = 2.32), m/z = 381.27, Rf = 0.48, yield = 70%, mp = 175–178 ◦C.
1
2-[6-(4-pyrimidin-2-ylpiperazin-1-yl)hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10o): H NMR
(300 MHz, CDCl3) δ 8.61 (d, J = 5.2 Hz, 1H, ArH), 8.29 (s, 1H, ArH), 7.76 (s, 1H, ArH), 7.10 (s, 2H, ArH),
6.99 (s, 1H, ArH), 6.54–6.50 (m, 1H, ArH), 4.28–4.21 (m, 2H, CHNCO), 4.01 (s, 4H, CHpip), 3.70 (s, 4H,
CHpip), 3.07–3.00 (m, 2H, CHaliph), 1.95–1.81 (m, 4H, CHaliph), 1.45–1.38 (m, 4H, CHAliph). 13C NMR
(75 MHz, CDCl3) δ 157.67 (Ar), 148.81 (Ar), 141.73 (Ar), 130.08 (Ar), 123.96 (Ar), 115.59 (Ar), 111.38 (Ar),
110.82 (Ar), 109.87 (Ar), 57.58 (Ctriaz), 51.54 (Cpip, Cpip), 45.57 (Cpip, Cpip), 41.78 (Cpip), 29.56 (Caliph),
28.11 (Caliph), 26.16 (Caliph), 23.54 (Caliph). FT-IR 3023 (C–H Ar, Str), 2937, 2857 (C–HAliph, Str), 1700
(C=O, Str), 1616 (C=N, Str), 1540 (C=C, Str), 1350 (C–N, Str). HPLC 90%, (tR = 1.42), Rf = 0.35, yield =
45% (Samsung), yield = 61% (CEM Discover SP reactor), mp = 146–148 ◦C.
2-[6-(4-phenylpiperazin-1-yl)hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10p): 1H NMR
(300 MHz, DMSO)
δ 7.84 (d, J = 7.0 Hz, 1H, ArH), 7.25 (dd, J = 14.6, 5.8 Hz, 4H, ArH), 7.00 (d,
J = 8.1 Hz, 2H, ArH), 6.86 (t, J = 7.2 Hz, 1H, ArH), 6.61 (dd, J = 8.4, 5.8 Hz, 1H, ArH), 3.77 (t, J = 12.0 Hz,
2H, CHNCO), 3.64–3.35 (m, 4H, CHpip), 3.16 (dd, J = 22.3, 9.6 Hz, 4H, CHpip), 3.06 (m, 2H, CHaliph),
1.74 (m, 4H, CHaliph), 1.32 (m, 4H, CHaliph). FT-IR 2987 (C–H Ar, Str), 2937, 2861 (C–HAliph, Str), 1693
(C=O, Str), 1639 (C=N, Str), 1541, 1491 (C=C, Str), 1375 (C–N, Str). HPLC 90% (tR = 3.69), m/z = 380.27,
Rf = 0.58, yield = 55%, mp = 133–137 ◦C.
1
2-[6-[4-(2-phenylphenyl)piperazin-1-yl]hexyl]-[1,2,4]triazolo[4,3-a]pyridin-3-one hydrochloride (10r): H NMR
(300 MHz, CDCl3)
δ 7.76 (d, J = 7.2 Hz, CH-, 1H, ArH), 7.52 (d, J = 7.2 Hz, 2H, ArH), 7.42 (t, J = 7.5 Hz,
2H, ArH), 7.37–7.29 (m, 2H, ArH), 7.24–7.22 (m, 1H, ArH), 7.21–7.13 (m, 2H, ArH), 7.09 (d, J = 3.2 Hz,
2H, ArH), 6.54–6.46 (m, 1H, ArH), 3.99 (t, J = 7.0 Hz, 2H, CHNCO), 3.53 (t, J = 12.0 Hz, 2H, CHpip), 3.35
(d, J = 10.9 Hz, 2H, CHpip), 3.10 (d, J = 13.4 Hz, 2H, CHpip), 2.86 (m, 2H, CHpip), 2.69 (m, 2H, CHaliph),
1.81 (m, 4H, CHaliph), 1.39 (m, 4H, CHaliph). 13C NMR (101 MHz, DMSO)
δ 148.71 (Ar), 148.37 (Ar),

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141.34 (Ar), 140.59 (Ar), 134.47 (Ar), 131.74 (Ar), 130.84 (Ar), 129.03 (Ar, Ar), 128.73 (Ar, Ar), 127.55 (Ar),
124.29 (Ar), 123.96 (Ar), 119.01 (Ar, Ar), 115.44 (Ar), 111.30 (Ar), 55.75 (Ctriaz), 51.25 (Cpip), 47.76 (Cpip,
Cpip), 45.25 (Cpip, Cpip), 28.42 (Caliph), 26.01 (Caliph), 25.84 (Caliph), 23.25 (Caliph). FT-IR 3023 (C–H Ar,
Str), 2937, 2858 (C–HAliph, Str), 1702 (C=O, Str), 1637 (C=N, Str), 1541, 1500 (C=C, Str), 1351 (C–N, Str).
HPLC 95% (tR = 4.99), m/z = 456.94, Rf = 0.65, yield = 61% (Samsung), yield = 66% (CEM Discover SP
reactor), oil.
4. Conclusions
As part of the research, 17 new trazodone derivatives were obtained, which were tested for their
activity toward the 5-HT1A receptor, which is an important hold of drugs used in the diseases of the
central nervous system. All ligands, as well as substrates for them, were obtained by means of the
new synthesis method in the presence of potassium carbonate as a reaction medium under microwave
radiation. The in vitro studies showed that, in the obtained group of ligands, there are selective ligands
for the 5-H1A receptor (10e, 10g, 10h, 10i, 10m, and 10p). Among the ligands obtained, those having a
hexyl chain, as compared with a propyl chain, have a higher incremental activity. It is very interesting
that the elongation in the trazodone of the alkyl chain by three carbon atoms changes the activity
profile from the 5-HT2A ligand (trazodone, 5-HT1A Ki = 78, 5-HT2A Ki = 16) to 5-HT1A (10e, 5-HT1A Ki
= 16, 5-HT2A Ki = 342). This change in the activity was evaluated in the molecular modeling procedure
and additionally supported with crystallographic studies. A very interesting pharmacological profile
is shown by the compound 10r, which has the properties of a dual 5-HT1A/7 ligand (5-HT1A Ki = 20,
5-HT7 Ki = 19). Bearing in mind all of the above, we are continuing the research on the activity of these
ligands (10e, 10r). Currently, we are conducting in vivo studies, the aim of which is to confirm their
antidepressant activity.
5. Patents
Some of the results concerning the synthesis of trazodone are presented in “Method for the
preparation trazodone” PCT/PL2018/000024, 6 March 2018.
Author Contributions: J.J. was responsible for conceptualization, methodology, writing—original draft
preparation, supervision, compound design, and synthesis. P.Z. was responsible for synthesis, compound
´
design, and analysis of physico-chemical data. P.S. was responsible for molecular modeling. G.S. was responsible
for in vitro studies. E.P. was responsible for calculations of geometry optimization. Z.M. was responsible for
conceptualization and methodology.
Funding: This research was funded by the National Center for Research and Development, LIDER VI project
(LIDER/015/L-6/14/NCBR/2015).
Conflicts of Interest: The authors declare no conflicts of interest.
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