Synthesis and structure-activity relationship analysis of 5-HT7 receptor antagonists: Piperazin-1-yl substituted unfused heterobiaryls

Article abstract of DOI:10.3390/molecules21040433

A series of piperazin-1-yl substituted unfused heterobiaryls was synthesized as ligands of the 5-HT₇ receptors. The goal of this project was to elucidate the structural features that affect the 5-HT₇ binding affinity of this class of compounds represented by the model ligand 4-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (2). The SAR studies included systematical structural changes of the pyrimidine core moiety in 2 to quinazoline, pyridine and benzene, changes of the 3-furyl group to other heteroaryl substituents, the presence of various analogs of the 4-methylpiperazin-1-yl group, as well as additional substitutions at positions 5 and 6 of the pyrimidine. Substitution of position 6 of the pyrimidine in the model ligand with an alkyl group results in a substantial increase of the binding affinity (note a change in position numbers due to the nomenclature rules). It was also demonstrated that 4-(3-furyl) moiety is crucial for the 5-HT₇ binding affinity of the substituted pyrimidines, although, the pyrimidine core can be replaced with a pyridine ring without a dramatic loss of the binding affinity. The selected ethylpyrimidine (12) and butylpyrimidine (13) analogs of high 5-HT₇ binding affinity showed antagonistic properties in cAMP functional test and varied selectivity profile-compound 12 can be regarded as a dual 5-HT₇ /5-HT_2A R ligand, and 13 as a multi-receptor (5-HT₇ , 5-HT_2A , 5-HT₆ and D₂ ) agent.

Full text of DOI:10.3390/molecules21040433

molecules
Article
Synthesis and Structure-Activity Relationship
Analysis of 5-HT Receptor Antagonists:
7
Piperazin-1-yl Substituted Unfused Heterobiaryls
1
,
2,
1
1
Lucjan Strekowski *, Jarosław Sa˛czewski *, Elizabeth A. Raux , Nilmi T. Fernando ,
1
1
1
1
1
Jeff Klenc , Shirish Paranjpe , Aldona Raszkiewicz , Ava L. Blake , Adam J. Ehalt ,
Samuel Barnes , Timothy C. Baranowski , Shannon M. Sullivan , Grzegorz Satała
and Andrzej J. Bojarski 3
1
1
1
3
1
Department of Chemistry, Georgia State University, Atlanta, Georgia, GA 30302, USA;
earaux@gmail.com (E.A.R.); fernando.nilmi@gmail.com (N.T.F.); jklenc@merchantgould.com (J.K.);
shirishp007@gmail.com (S.P.); aldona.paluchowska@urpl.gov.pl (A.R.); ablake@bakerlaw.com (A.L.B.);
adamehalt@gmail.com (A.J.E.); sambarnes2@gmail.com (S.B.); tcb@bellsouth.net (T.C.B.);
ssullivan@yahoo.com (S.M.S.)
Department of Chemical Technology of Drugs, Medical University of Gda n´ sk,
Aleja Generała Józefa Hallera 107, Gda n´ sk 80-416, Poland
Department of Medicinal Chemistry, Institute of Pharmacology, Polish Academy of Sciences, Sm e˛ tna 12,
Kraków 31-343, Poland; satala@if-pan.krakow.pl (G.S.); bojarski@if-pan.krakow.pl (A.J.B.)
Correspondence: lucjan@gsu.edu (L.S.); js@gumed.edu.pl (J.S.);
2
3
*
Tel.: +1-770-931-4224 (L.S.); +48-58-349-1648 (J.S.)
Academic Editor: Derek J. McPhee
Received: 27 February 2016; Accepted: 22 March 2016; Published: 31 March 2016
Abstract: A series of piperazin-1-yl substituted unfused heterobiaryls was synthesized as
ligands of the 5-HT receptors. The goal of this project was to elucidate the structural features
7
that affect the 5-HT binding affinity of this class of compounds represented by the model
7
ligand 4-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (2). The SAR studies included systematical
structural changes of the pyrimidine core moiety in
2
to quinazoline, pyridine and benzene,
changes of the 3-furyl group to other heteroaryl substituents, the presence of various analogs of
the 4-methylpiperazin-1-yl group, as well as additional substitutions at positions 5 and 6 of the
pyrimidine. Substitution of position 6 of the pyrimidine in the model ligand with an alkyl group
results in a substantial increase of the binding affinity (note a change in position numbers due to
the nomenclature rules). It was also demonstrated that 4-(3-furyl) moiety is crucial for the 5-HT7
binding affinity of the substituted pyrimidines, although, the pyrimidine core can be replaced with
a pyridine ring without a dramatic loss of the binding affinity. The selected ethylpyrimidine (12
)
and butylpyrimidine (13) analogs of high 5-HT binding affinity showed antagonistic properties
7
in cAMP functional test and varied selectivity profile—compound 12 can be regarded as a dual
5
-HT /5-HT R ligand, and 13 as a multi-receptor (5-HT , 5-HT , 5-HT and D ) agent.
7 2A 7 2A 6 2
Keywords: 5-HT receptor ligands; 5-HT; serotonin; synthesis; structure-affinity relationships (SAR);
7
N-methylpiperazine; 3-furyl
1
. Introduction
For more than 60 years, the studies on the role of serotonin (5-HT) within the central nervous
system (CNS) have constantly provided evidence of the involvement of 5-HT receptors in the
action of various psychiatric drugs. It is now evident that deregulation of serotonergic signaling is
associated with pathogenesis of severe neuropsychological disorders like depression [1,2] Alzheimer’s
Molecules 2016, 21, 433; doi:10.3390/molecules21040433
www.mdpi.com/journal/molecules

Molecules 2016, 21, 433
2 of 22
disease [3,4] or schizophrenia [
5], and not surprisingly the 5-HT signaling system is one of the major
focuses of various CNS drug development strategies [6].
Among the plethora of functions influenced by fourteen 5-HT receptor subtypes, activation of
5
-HT receptors (5-HT R) in the CNS regulates circadian rhythms (facilitates diurnal phase shift
7
7
and REM sleep) [
7,
8], thermoregulation (induces hypothermia) [9,10] and cognitive processes by
facilitation of long and short-term memory consolidation [11
receptors in key cerebral areas as well as high affinity of numerous antidepressants and antipsychotics
for the 5-HT receptors, indicate their potential role in depression and schizophrenia [14 17]. The
results of pre-clinical studies have shown that both genetic inactivation and pharmacological blockade
of the 5-HT receptors leads to antidepressant-like effects in animal models of depression [14 16].
–13]. Moreover, the presence of 5-HT7
–
7
,
7
Similarly, antagonism at 5-HT receptors is one of the key pharmacological features of vortioxetine,
7
an atypical antidepressant that was approved for medication in 2013 by the U.S. FDA [18]. Therefore,
the new individual therapies targeting the blockade of 5-HT receptors in combination with using
7
antidepressants are currently under investigation [19].
In relation to schizophrenic disorders, the involvement of 5-HT receptors in the pro-social action
7
of amisulpride has been recently demonstrated, and 5-HT receptor antagonism was postulated as a
7
possible mechanism for the treatment of social withdrawal—one of the core negative symptoms of
schizophrenia [20]. Other preclinical findings, implicating the role of 5-HT receptors in learning and
7
memory, suggest that blockade of this receptor may be beneficial against cognitive impairments in
schizophrenia [21].
Given the role of 5-HT receptors in the mechanism of action of antidepressants and antipsychotics,
7
various groups of ligands have been developed but attributed mainly with linear arrangement of
basic pharmacophore features that include long chain arylpiperazine structure [22
,23]. Looking for
new leads with different pattern of potential ligand-5-HT receptor interactions, we screened our
7
in-house compound library and high 5-HT R affinity was found for some 4-mono-, or 4,6-di-substutited
7
2
-(4-methylpiperazino)pyrimidine derivatives, developed earlier as 5-HT2A antagonists [24]. Here we
present synthesis and extensive structure-5-HT receptor affinity study on piperazin-1-yl substituted
7
unfused heterobiaryls and related compounds.
2
. Results
2
.1. Chemistry
The commercially available substituted pyrimidines, quinazolines, pyridines and benzenes
were used as starting materials in the synthesis of a novel library of 5-HT receptor ligands.
7
Substituted pyrimidines are the most extensive class of compounds used in this study. The
general synthetic approach developed in our laboratories includes the addition reaction of an
aryl(heteroaryl)lithium reagent to generate a substituted dihydropyrimidine or dihydroquinazoline
intermediate product, which, without isolation, undergoes aromatization upon treatment with
2
,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (Schemes 1 and 2) [24,25].
Many final products were obtained by nucleophilic displacement reaction of a halogen activated
by an appropriately positioned ring nitrogen atom in pyrimidines, quinazolines and pyridines
Schemes 1 and 3, Schemes 4–8). By using this methodology, 4-substituted 2-chloropyrimidine or
(
2
-chloroquinazoline substrates were easily functionalized with amines to give the final products
1, 2,
2
1
,
23 24 26 29 30 32 36 46, and 48. With 2,4-dichloropyrimidines, such nucleophilic displacement
,
,
,
,
,
,
–
is regioselective, giving rise to 4-amino-2-chloropyrimidines, often with yields exceeding 90% [26].
In this work, the regioselective displacement reaction of 2,4-dichloro-6-(3-furyl)pyrimidine with
dimethylamine at ambient temperature yielded a 2-chloro-4-dimethylamino intermediate product, the
subsequent treatment of which with N-methylpiperazine at an elevated temperature furnished the final
product 4-dimethylamino-6-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (19). On the other hand,
two consecutive introductions of heteroaryl groups at positions 4 and 6 of 2-chloropyrimidine followed

Molecules 2016, 21, 433
3 of 22
by nucleophilic displacement of the 2-chloro atom yielded a variety of substituted pyrimidines 11
–
20,
2
2, 25, 27, 28, 31 (Scheme 1).
The classical alkylation and acylation reactions of
1 provided facile entry to compounds 3–
10
(
Scheme 2). A similar strategy afforded a piperazinium derivative 47 (see Experimental). It was also
found that the conjugate addition reaction of dimethylamine with 2-chloro-4-vinylquinazoline [27 28
,
]
proceeds cleanly to furnish 2-chloro-4-(2-dimethylamino)ethyl substituted quinazoline, the subsequent
treatment of which with N-methylpiperazine yielded the desired product 50 (Scheme 5).
The Suzuki mono-coupling reaction of a dihalogeno-substituted pyridine with a heteroarylboronic
acid followed by classical nucleophilic displacement of the second halogen were used to synthesize
pyridine derivatives 51–59 (Schemes 6–8). The reaction sequence depended on type of substrate used.
2
o
1
. R -Li, THF, -78 C
R1
R1
R2
R1
R2
o
o
2
. DDQ, H O, rt
1 or 2 amine
- HCl
N
2
N
N
Cl
N
Cl
N
amino
N
1
, 2, 21, 23, 24, 26
4
o
1
1. R -Li, THF, -78 C
29, 30, 32, 36-46
R = H, n-propyl
2
. DDQ, H O, rt
2
4
2
R , R = alkyl, aryl, heteroaryl
R4
N
R4
HN
N Me
R1
R2
R1
R2
For definition of the substituents,
see Table 1
N
N
- HCl
Cl
N
N
N
Me
1
1-20, 22, 25
2
7, 28, 31
Scheme 1. Synthesis of substituted pyrimidines 1, 2, 11–28, 36–46 (see Table 1).
N
N
RX
N
N
N
N
N
- HX
N
O
O
H
R
1
3-10, 47
Scheme 2. Synthesis of substituted pyrimidines 3–10 and 47 (see Table 1).
o
1
. R-Li, THF, -78 C
MeN
-
NH
2. DDQ, H O, rt
2
N
N
N
HCl
Cl
N
Cl
N
N
N
N
O
O
4
8
Me
Scheme 3. Synthesis of quinazoline 48.
2
MeN
NH
N
N
-
HCl
Cl
N
Cl
N
N
N
N
N
Me
Me
49
Scheme 4. Synthesis of quinazoline 49.

Molecules 2016, 21, 433
4 of 22
1. HN(Me)2
2. MeN
NH
N
N
Cl
N
N
N
-
HCl
N
Me
NMe
2
5
0
Scheme 5. Synthesis of quinazoline 50.
OH
Het-B
OH
MeN
NH
N
N
N
Pd catalyst
Cl
Br
Cl
Het
- HCl
N
Het
Suzuki condensation
N
Me
51-53
Scheme 6. Synthesis of substituted pyridines 51–53 (see Table 4).
OH
Me
Me
Het-B
Me
OH
MeN
-
NH
N
N
N
Pd catalyst
Br
Br
N
Br
N
Het
HCl
Suzuki condensation
N
N
Me
Me
54-56
Scheme 7. Synthesis of substituted pyridines 54–56 (see Table 4).
OH
Het-B
OH
MeN
-
NH
Pd catalyst
Br
N
Br
N
N
Br
N
N
Het
HCl
Suzuki condensation
N
N
Me
Me
57-59
Scheme 8. Synthesis of substituted pyridines 57–59 (see Table 4).
Table 1. 5-HT binding affinities of pyrimidine derivatives 1–41.
7
R4
N
R1
R2
N
R1
R2
R3
R4
Ki ˘ S.D. (nM)
N
N
R3
21a
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
3-furyl
H
H
H
H
H
H
H
H
H
H
H
Me
Et
9 ˘ 2
7.2 ˘ 1.4
10 ˘ 1
Me
3
4
5
6
7
8
9
Et
Pr
Bn
5.3 ˘ 0.9
25 ˘ 3
Bz
(CH2)3Ph
(CH2)2OH
(CH2)2COOEt
(CH2)5COOEt
Me
1405 ˘ 173
1.6 ˘ 0.2
8.5 ˘ 1.1
8.9 ˘ 2.1
1.4 ˘ 0.3
3.9 ˘ 0.7
7.1 ˘ 1.4
1.6 ˘ 0.2
3.3 ˘ 0.5
2.7˘ 0.5
8.3 ˘ 1.7
1
0a
1
1
12
13
14
15
16
Me
n
Me
Bu
n
Me
Hexyl
s
Me
Bu
Me
CF3

Molecules 2016, 21, 433
5 of 22
Table 1. Cont.
R4
R1
R2
N
R1
R2
R3
R4
Ki ˘ S.D. (nM)
18 ˘ 3
N
N
N
R3
17
H
3-furyl
Me
1
8 a
H
H
3-furyl
3-furyl
Me
Me
3-furyl
N(Me)2
22 ˘ 2
19
>10,000
20
H
3-furyl
Me
Me
N
N
>10,000
n
2
1
2
Pr
Pr
3-furyl
3-furyl
Me
Me
H
Bu
1334 ˘ 232
1250 ˘ 116
n
n
2
23
24
25
H
H
H
Me
Me
Me
H
H
1920 ˘ 373
462 ˘ 54
12 ˘ 2
Me
O
Me
O
nhexyl
Me
O
2
6 b
H
H
H
2-furyl
2-furyl
2-furyl
Me
Me
Me
H
Et
Bu
1021 ˘ 96
380 ˘ 44
450 ˘ 63
2
7
n
2
8
2
9
H
Me
H
H
4230 ˘ 680
Me
O
3
0c
H
H
H
H
H
H
3-thienyl
3-thienyl
2-thienyl
2-OH-C6H4
3-OH-C6H4
4-OH-C6H4
Me
Me
Me
Me
Me
Me
83 ˘ 9
15 ˘ 3
n
31
Bu
a,b
d
3
2
H
H
H
H
480 ˘ 65
>10,000
>10,000
>10,000
3
3
3
3
4
5
d
d
3
6
H
H
Me
Me
H
H
7460 ˘ 1230
5280 ˘ 820
O
3
3
7
S
8
H
Me
H
1530 ˘ 125
N
Me
3
9 d
H
H
3-pyridinyl
Me
Me
H
H
780 ˘ 94
N
4
0
>10,000
S
N
N
41
H
Me
H
1980 ˘ 275
a: Ref. [24]; : Ref. [29]; : Ref. [30]; d: Ref. [27].
b
c
In case of benzene derivatives, Buchwald-Hartwig amination of halogeno-substituted benzenes
with N-methylpiperazine in the presence of a palladium catalyst provided piperazine-substituted
products (Scheme 9). The Suzuki condensation of these intermediate products with heteroarylboronic
acids yielded the desired heteroaryl-substituted benzenes 60–71. It should be noted that the majority
of the two-step reactions discussed above were conducted with crude intermediate products. However,
the final products were rigorously characterized in all cases studied.

Molecules 2016, 21, 433
6 of 22
OH
OH
Het-B
Me
N
NH
R
R
R
Pd catalyst
Pd catalyst
Buchwald-Hartwig
amination
Suzuki condensation
Br
Br
N
Br
N
Het
N
N
Me
Me
R = H, Me, nBu
60-71
Scheme 9. Synthesis of substituted benzenes 60–71 (see Table 5).
2
.2. Pharmacology
Novel scaffold endowed with 5-HT R activity was identified by screening of the in-house
7
compound library in a single compound concentration of 1
assay using [ H]5-CT and HEK293 cells overexpressing the human 5-HT7 receptors. Compounds
µ
M in a competition radioligand binding
3
b
showing >60% of inhibition, namely (4-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (
two derivatives 11 and 18 were further tested in full binding experiments, and, as for all the newly
synthesized compounds ( 10 12 17 and 19 71), 5-HT R affinity (K values SD) was determined
in at least two independent experiments run in triplicate [31]. The functional properties of two selected
compounds 12 and 13 active on 5-HT R were further evaluated using their ability to inhibit cAMP
2) and its
1
,
3–
,
–
–
˘
7
i
7
production induced by 5-CT (10 nM) in the same cellular system [32
,33]. In addition, compounds 12
and 13 were evaluated for their affinity at three other serotonergic receptors: 5-HT1A, 5-HT2A, 5-HT6
and dopaminergic D in standard competition binding assays by use of the appropriate radioligand:
2
3
3
3
[
[
H]-8-OH-DPAT (187 Ci/mmol), [ H]-Ketanserin (66.9 Ci/mmol), [ H]-LSD (85.2 Ci/mmol) and
H]-Raclopride (74.4 Ci/mmol), respectively, and cell line with overexpressed receptor (CHO for
-HT2A and HEK293 for all the others) [33,34].
3
5
3
. Discussion
The objective of this study was to determine which combinations of structural features affect
the 5-HT binding affinity of unfused heterobiaryls. The intended SAR studies included synthesis
7
of new analogs of the model compound 2 and encompassed the following points: (1) substitution of
the amino group within the piperazine ring; (2) introduction of alkyl, amino and bulky substituents
into positions 4/6 and/or 5 of the pyrimidine ring; (3) modifications of the heteroaryl group in
position 4/6 of the pyrimidine; (4) replacement of the piperazinyl group by other amino substituents;
and (5) synthesis of a series of pyridines and benzenes allowing comparison of the influence of the
central ring nitrogen atom(s) on the ligand affinity.
First, investigating the influence of substituents at the basic piperazine atom, it was found that
unsubstituted (
1
), simple alkyls (
3
and
4
), arylalkyl (
7
), hydroxyalkyl (
8
) or even esters (9 and 10)
provide a similar level of 5-HT R affinity (K = 1.4–10 nM) as methylated hit compound
2
(K = 7.2 nM).
i
7
i
Slight decrease in 5-HT R affinity is observed for benzyl derivative 5 (K = 25 nM) which is probably
i
7
caused by a negative steric influence of the phenyl group on the key interaction between a protonated
nitrogen and aspartic acid (D3.32) residue in the receptor [35]. Moreover, with the benzoyl group ( ),
6
there is an obvious negative effect on the affinity (K = 1405 nM) that arises from diminished basicity of
i
the piperazine nitrogen atom (Table 1).
In the literature, piperazine has been often a key element of 5-HT R ligands [22], and it is also
7
optimal for heterobiaryl compounds, as its replacement results in decreased affinity (Table 2). Indeed,
introduction of homopiperazine leads to ligand with a moderate affinity for 5-HT R (42, K = 148 nM),
7
i
whereas no affinity is observed for piperidine (43) and morpholine (44) derivatives that lack a basic
nitrogen atom. In the group of compounds incorporating sterically hindered amines (45 47), only
–
the ligand with positively charged quaternary nitrogen atom displayed moderate 5-HT R affinity (47
,
7
K = 299 nM).
i

Molecules 2016, 21, 433
7 of 22
Table 2. Affinities of pyrimidines 42–47 for 5-HT receptor.
7
N
R
K ˘ S.D. (nM)
i
R
N
O
N
4
2
148 ˘ 17
N
Me
4
4
4
3
4
5
N
>10,000
>10,000
O
N
O
2460 ˘ 343
N
NH
NH
4
4
6
7
1280 ˘ 187
299 ˘ 42
NH
N
I
N
N
O
Further modifications of the model compound
of the pyrimidine ring. Alkyl groups (11 16) are very well tolerated in terms of 5-HT receptors
affinity (K values in the range of 1.6 to 8.3 nM), while introduction of bulky cyclohexenylethynyl (17
2 involved introductions of substituents at position
6
–
7
)
i
and 3-furyl (18) groups results only in a minor reduction of the binding affinity with the K values of
i
1
8 and 22 nM, respectively. It can be seen, however, that amino groups (19
substituents at position 5 (21, 22) are highly detrimental for the affinity.
Next, the SAR analysis of the substituents at position 4 of the pyrimidine ring was investigated by
modifications and replacement of the 3-furyl group. Compounds 23 24 with methyl substituents within
, 20) at position 6 and alkyl
,
the 3-furyl ring display reduced affinities (K values of 1,920 and 462 nM, respectively). Interestingly,
i
introduction of an n-hexyl group (25) at position 6 of the pyrimidine in compound 24 restores high
affinity for 5-HT R (K = 12 nM). A similar trend can be recognized for less active ligands 26 (2-furyl,
7
i
K = 1,021 nM) and 30 (3-thienyl, K = 83 nM), where addition of a 6-alkyl substituent results in a 2- or
i
i
3
-fold boost in affinity (ethyl, 27, K = 380 nM; n-butyl, 28, K = 450 nM and n-butyl, 31, K = 15 nM).
i
i
i
As a general rule, 3-furyl (2, 24) and 3-thienyl (30) derivatives are 5 to 140-fold more active than
their 2-furyl (26
p-hydroxyphenyl (33
benzo-fused heterocyclic (36
K > 1000 nM). Thus, it appears that the 3-furyl group is necessary for a high affinity of 5-HT R ligands
,
29) and 2-thienyl (32) counterparts. Replacement of the 3-furyl group for o-, m-,
35), pyridin-3-yl (39), thiazol-2-yl (40) or imidazol-1-yl (41) group as well as
38) moieties does not furnish ligands with noteworthy affinity for 5-HT R
–
–
7
(
i
7
or, alternatively, a 3-thienyl moiety but with an appropriate enhancing substitutent at position 6 of the
pyrimidine ring (Table 1).
Next it was found that quinazoline derivatives (48
–50), which can be treated as 5,6-disubstituted
pyrimidine analogs, were inactive (K > 1910 nM). This result is fully consistent with our finding that
i
substitution at position 5 is highly detrimental for 5-HT R affinity (Table 3).
7

Molecules 2016, 21, 433
8 of 22
Table 3. Affinities of substituted quinazolines 48–50 for 5-HT receptor.
7
N
R
K ˘ SD (nM)
i
N
N
R
N
Me
4
8
9
0
3-furyl
1910 ˘ 235
7540 ˘ 1140
>10,000
4
5
N
N Me
(CH ) N(Me)
2
2
2
Having established reasonable SAR around pyrimidine central ring, we focused our attention on
the role of the pyrimidine core itself, analyzing a series of substituted pyridine- and phenyl-piperazine
analogs. The pyridine derivatives (51–59) were then synthesized to determine whether both nitrogen
atoms within the central heterocycle are needed for the ligand to retain affinity for the binding
pocket of the receptor (Table 4). The biological data show that both isomeric 4-(3-furyl) (51) and
6
-(3-furyl) (57) pyridine ligands still demonstrate high affinities for the 5-HT R (K = 17 nM and 8 nM,
7 i
respectively). However, for the 2-furyl derivatives, position 4 (52 K = 307 nM) is preferred over
i
position 6 (59 K = 1135 nM) of the pyridine as the attachment site. The pyridine ligands incorporating
i
3
-thienyl (58 K = 79 nM) and 2-thienyl (53 K = 396 nM) substituents are nearly equipotent compared
i
i
to their pyrimidine counterparts (30, K = 83 nM and 32, K = 480 nM). Similarly to pyrimidine ligands,
i
i
introduction of an alkyl substituent into position 6 of the pyridine ring results in 2- or 3-fold boost
in affinity (54 K = 8 nM; 55 K = 139 nM and 56 K = 117 nM vs 51, K = 17 nM; 52 K = 307 nM and
i
i
i
i
i
5
3 K = 396 nM, respectively).
i
Table 4. Binding affinities of substituted pyridines 51–59 for 5-HT receptor (see Schemes 6–8).
7
R1
X
R1
R2
X
Y
K ˘ S.D. (nM)
i
N
Y
R2
N
Me
5
1
2
3
4
5
6
7
8
9
H
H
H
Me
Me
Me
H
3-furyl
2-furyl
2-thienyl
3-furyl
2-furyl
2-thienyl
3-furyl
N
N
N
N
N
CH
CH
CH
CH
CH
CH
N
17 ˘ 2
307 ˘ 42
396 ˘ 55
8 ˘ 1
5
5
5
5
5
5
5
5
139 ˘ 22
117 ˘ 12
11 ˘ 2
N
CH
CH
CH
H
H
3-thienyl
2-furyl
N
N
79 ˘ 14
1135 ˘ 154
After finding that the pyrimidine core can be safely replaced with a pyridine ring, our attention
was turned to compounds devoid of any nitrogen atoms within the central moiety of a ligand (Table 5).
-Furyl (60, K = 31 nM) and 3-thienyl (66, K = 342 nM) benzene analogs show significant 5-HT R
3
i
i
7
affinities, although they are 2- and 4-fold less active than their pyrimidine and pyridine equivalents.
-HT R affinities of 2-furyl (63, K = 914 nM) and 2-thienyl (69, K = 542 nM) derivatives remain at
5
7
i
i
moderate levels. Consequently, introduction of methyl groups into position 5 of 1,3-disubstituted
benzene analogs results in 2- to 7-fold increases of the binding properties (64, K = 312 nM; 67
,
i
K = 47 nM and 70, K = 297 nM), except for the 3-furyl ligand (61, K = 37 nM) which stays equipotent
i
i
i
to its unsubstituted counterpart. Surprisingly, the n-butyl substituted benzene derivatives are lower

Molecules 2016, 21, 433
9 of 22
in 5-HT R affinity (62, K = 106 nM; 65, K = 743 nM; 68, K = 263 nM and 71, K = 734 nM). The
7
i
i
i
i
observed phenomenon may arise from higher lipophilicity of the ligands, which increases non-specific
interactions with the tested 5-HT receptor.
7
Table 5. Binding affinities of substituted benzenes 60–71 for 5-HT receptor.
7
R1
R1
R2
K ˘ S.D. (nM)
i
N
R2
N
Me
6
6
6
6
6
6
6
6
6
6
7
7
0
H
Me
Bu
H
Me
Bu
H
Me
Bu
H
Me
3-furyl
3-furyl
3-furyl
2-furyl
2-furyl
31 ˘ 4
37 ˘ 3
1
2
3
4
5
6
7
8
9
0
1
n
106 ˘ 9
914 ˘ 121
312 ˘ 25
743 ˘ 66
342 ˘ 51
47 ˘ 6
n
2-furyl
3-thienyl
3-thienyl
3-thienyl
2-thienyl
2-thienyl
2-thienyl
n
263 ˘ 19
542 ˘ 66
297 ˘ 42
734 ˘ 59
n
Bu
Two selected compounds 12 and 13 with high 5-HT R affinity were next evaluated in in vitro
7
functional cAMP assay (Figure 1) and their selectivity for other serotonin (5-HT1A, 5-HT2A, 5-HT6)
and dopamine D receptors was investigated (Table 6). It was found that both 12 and 13, similarly to
2
(
2R)-1-((3-hydroxyphenyl)sulfonyl)-2-(2-(4-methyl-1-piperidinyl)ethyl)pyrrolidine (SB-269970) [32]—a
selective 5-HT7R antagonist used as a control, dose-dependently decrease 5-CT-induced cAMP
accumulation in transfected HEK cells expressing 5-HT R (Figure 1). It has to be stressed, however,
7
that 5-HT R antagonistic properties of 12 and 13 are weaker (K = 12 and 60 nM, respectively) than
7
B
that of SB-269970 (K = 1.9 nM) and that, unexpectedly, n-butyl derivative 13, despite showing higher
B
affinity, is less potent 5-HT R antagonist than ethyl analog 12. Alkyl substituent at position 4 also
7
influences selectivity profile—both compounds show almost the same, high affinity for 5-HT2A
R
(
K = 10 and 12 nM for 12 and 13, respectively) but n-butyl derivative is 10- and 13-fold more active
i
than 12 at 5-HT and D receptors. Both analogs are weak 5-HT1AR binders.
6
2
Figure 1. The functional properties of compounds 12 and 13 and a reference antagonist SB-269970 on
-HT R evaluated using their ability to inhibit cAMP production induced by 5-CT (10 nM) in HEK-293
5
7
cells expressing 5-HT R.
7

Molecules 2016, 21, 433
10 of 22
Table 6. Extended results for compounds 12 and 13.
K ˘ S.D. (nM)
i
D2
5-HT1A
5-HT
5-HT6
5-HT7
2A
1
1
2
3
466 ˘ 39
37 ˘ 4
1501 ˘ 164
637 ˘ 53
12 ˘ 2
10 ˘ 1
121 ˘ 14
12 ˘ 2
7.1 ˘ 1.4
1.6 ˘ 0.2
4
. Materials and Methods
4
.1. General Methods
All air-sensitive reactions were conducted under a nitrogen atmosphere. Tetrahydrofuran (THF)
was distilled from sodium benzophenone ketyl immediately before use. Heteroaryllithium reagents
were generated by direct lithiation of a heterocyclic substrate or by a bromine-lithium exchange reaction
as previously reported [28] Final products were purified on a chromatotron using silica gel-coated
rotors. Oily products were transformed into hydrobromide salts by using a general procedure [30
]
and the salts were crystallized from 95% ethanol. Melting points (Pyrex capillary) are not corrected.
1
13
The H-NMR (400 MHz) and C-NMR (100 MHz) spectra of free bases and hydrobromide salts
were obtained in CDCl and DMSO-d , respectively. Mass spectra were recorded using electrospray
3
6
ionization in a positive ion mode.
4
.2. Synthesis of Substituted Pyrimidines 1, 2, 21, 23, 24, 26, 29, 30, 32, 36–46 and Quinazoline 48
A solution of the corresponding lithium reagent (10 mmol) in THF (10 mL) was stirred
˝
at ´50 C and treated dropwise with a solution of 2-chloropyrimidine, 2-chloro-5-propylpyrimidine or
˝
2
-chloroquinazoline (12 mmol) in THF (15 mL). The mixture was allowed to reach 0 C within 2 h, then
quenched with a solution of water in THF (1:5, 6 mL), stirred at 0 C and treated with a solution of DDQ
˝
˝
(
2.3 g, 10 mmol) in THF (5 mL). After stirring for an additional 10 min at 0 C, the mixture was treated
with a cold solution of sodium hydroxide (4 M, 5 mL, 20 mmol), stirred and extracted immediately
with ether/hexanes (1:1, 3 10 mL). The combined extracts were dried with anhydrous sodium sulfate,
decolorized by filtration through a pad of silica gel (5 g) and concentrated on a rotary evaporator. The
resultant crude 4-substituted 2-chloropyrimidine or 2-chloroquinazoline was treated with a primary
or secondary amine (30 mmol) in toluene (20 mL) in the presence of anhydrous potassium carbonate
ˆ
˝
and the mixture was heated at 75 C for 5–10 h, after which time a TLC analysis on silica gel eluting
with ether/triethylamine (9:1) showed the absence of the substrate. Preparative chromatography was
conducted eluting with ether/triethylamine/hexanes (9:5:5) to give product 1, 2, 21, 23, 24, 26, 29, 30,
3
2, 36–46 (Scheme 1) and 48 (Scheme 2).
4
.3. Synthesis of Polysubstituted Pyrimidines 11–20, 22, 25, 27, 28, 31
Alternatively, the crude 4-substituted 2-chloropyrimidine, obtained as described above,
was treated with a lithium reagent followed by aromatization with DDQ of the intermediate
dihydropyrimidine under the conditions described above and the resultant crude 4,6-disubstituted
2
4
-chloropyrimidine was allowed to react with N-methylpiperazine, as described above, to yield
,6-disubstituted 2-(N-methylpiperazine-1-yl)pyrimidine 11–20, 22, 25, 27, 28, 31 (Scheme 1).
4
.4. Synthesis of [N-Substituted 2-(Piperazin-1-yl)]pyrimidines 3–10 and 47
In the preparation of compounds 3–10, a mixture of 4-(3-furyl)-2-(piperazin-1-yl)pyrimidine
(
2
1, 200 mg, 0.87 mmol), synthesized as described above, and anhydrous potassium carbonate (300 mg,
˝
.2 mmol) in anhydrous acetonitrile (10 mL) was stirred at
´10 C and treated with ethyl bromide,
propyl bromide, benzyl chloride, benzoyl chloride, 3-phenylpropyl bromide, 2-bromoethanol, ethyl
3
˝
-bromopropionate or ethyl 6-bromohexanoate (2.0 mmol) (Scheme 2). The mixture was stirred at 23 C
for 2 h and then quenched with water (2 mL) and extracted with ether (3 20 mL). The extract was
ˆ

Molecules 2016, 21, 433
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dried with anhydrous sodium sulfate and concentrated. Purification by chromatography eluting with
dichloromethane/methanol (10:1) gave product 3–10.
In the preparation of 4-[4-(3-furyl)pyrimidin-2-yl]piperazine-1-spiro-1-morpholinium iodide (47),
a mixture of compound 1 (1 mmol), 1-iodo-2-(2-iodoethoxy)ethane (1 mmol) and anhydrous potassium
carbonate (140 mg, 1 mmol) in anhydrous acetonitrile (10 mL) was heated under reflux for 1 h and
then quenched with water (5 mL). Product 47 was extracted with ethyl acetate and purified on a
chromatotron eluting with dichloromethane/methanol (8:2).
1
4
-(3-Furyl)-2-(piperazin-1-yl)pyrimidine (
1
). Yield 70% (free base); an oil; H-NMR:
δ
8.33 (d, J = 5.1 Hz,
1
H), 8.10 (s, 1H), 7.51 (m, 1H), 6.89 (m, 1H), 6.67 (d, J = 5.1 Hz, 1H), 3.88 (m, 4H), 2.98 (m, 4H).
˝
A hydrobromide salt: m.p. 241–242 C. Anal. Calcd for C H N O2‚HBr
‚
0.5H O: C, 45.01; H, 5.03;
12
14
4
2
N, 17.50. Found: C, 44.82; H, 4.85; N, 17.39.
4
-(3-Furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (2) [24].
-(4-Ethylpiperazin-1-yl)-4-(3-furyl)pyrimidine (
1
2
3
). Yield 61% (free base); an oil; H-NMR:
δ
8.29 (d,
J = 5.1 Hz, 1H), 8.07 (m, 1H), 7.48 (m, 1H), 6.86 (m, 1H), 6.63 (d, J = 5.1 Hz, 1H), 3.91 (m, 4H), 2.53 (m,
4
H), 2.49 (q, J = 7.2 Hz, 2H), 1.14 (t, J = 7.2 Hz, 3H). A hydrobromide salt. Anal. Calcd for C H N O 2
‚
14
18
4
HBr‚0.5 H O: C, 39.18; H, 4.93; N, 13.06. Found: C, 39.29; H, 4.36; N, 12.81.
2
1
4-(3-Furyl)-2-(4-propylpiperazin-1-yl)pyrimidine (
4). Yield 39% (free base); an oil; H-NMR: δ 8.29 (d,
J = 5.1 Hz, 1H), 8.06 (s, 1H), 7.48 (s, 1H), 6.86 (s, 1H), 6.63 (d, J = 5.1 Hz, 1H), 3.90 (t, 4H), 2.52 (m, 4H),
˝
.36 (t, J = 7.4 Hz, 2H), 1.57 (m, 2H), 0.94 (t, J = 7.4 Hz, 3H). A hydrobromide salt: m.p. 274–277 C.
2
Anal. Calcd for C H N O
‚
2 HBr
‚
H O: C, 39.84; H, 5.35; N, 12.39. Found: C, 40.39; H, 4.84; N, 12.45.
15
20
4
2
1
2-(4-Benzylpiperazin-1-yl)-4-(3-furyl)pyrimidine (
5
). Yield 73% (free base); an oil; H-NMR:
δ
8.28 (d,
J = 5.1 Hz, 1H), 8.05 (m, 1H), 7.47 (m, 1H), 7.31 (m, 5H), 6.85 (s, 1H), 6.62 (d, J = 5.1 Hz, 1H), 3.89 (m, 4H),
˝
3
.57 (s, 2H), 2.53 (m, 4H). A hydrobromide salt: m.p. >280 C. Anal. Calcd for C H N O
‚
2HBr H O:
‚
19
20
4
2
C, 45.62; H, 4.84; N, 11.20. Found: C, 45.93; H, 4.67; N, 11.11.
1
2-(4-Benzoylpiperazin-1-yl)-4-(3-furyl)pyrimidine (
6). Yield 33% (free base); an oil; H-NMR: δ 8.31 (d,
J = 5.1 Hz, 1H), 8.06 (m, 1H), 7.48 (m, 1H), 7.44 (m, 5H), 6.85 (m, 1H), 6.69 (d, J = 5.1 Hz, 1H), 4.27
(
1
broad m, 8H); 13C-NMR:
06.3, 44.0, 42.3. A hydrobromide salt: m.p. 221 C (dec.). Anal. Calcd for C H N O2‚HBr
δ
170.6, 161.7, 159.2, 158.13, 144.0, 143.1, 135.8, 129.8, 128.6, 127.1, 126.0, 108.5,
˝
‚
0.5 H O:
19
18
4
2
C, 53.79; H, 4.75; N, 13.20. Found: C, 53.97; H, 4.32; N, 13.15.
-(3-Furyl)-2-[(4-(3-phenylpropyl)piperazin-1-yl]pyrimidine ( ). Yield 44% (free base); an oil; H-NMR:
δ 8.27 (d, J = 5 Hz, 1H), 8.05 (s, 1H), 7.46 (t, J = 2 Hz, 1H), 7.27 (dd, J = 8 Hz and 15 Hz, 2H), 7.18 (m, 3H),
1
4
7
6
4
.85 (t, J = 2 Hz, 1H), 6.61 (d, J = 5 Hz, 1H), 3.88 ( t, J = 5 Hz, 4H), 2.67 (t, J = 8 Hz, 2 H), 2.50 (t, J = 5 Hz,
H), 2.41 (t, J = 7 Hz, 2H); 13C-NMR:
δ
161,8, 159.0, 158.0, 143.8, 143.0, 142.1, 128.4, 128.3, 126.2, 125.7,
+
108.5, 105.6, 58.1, 53.1, 43.7, 33.7, 28.5. HR-MS (ESI). Calcd for C H N O (M + 1): m/z 349.2028.
21
25
4
Found: m/z 349.2022.
1
2
-[4-(2-Hydroxyethyl)piperazin-1-yl]-4-(3-furyl)pyrimidine (
δ 8.34 (t, J = 4.8 Hz, 1H), 8.27 (d, J = 5.2 Hz, 1H), 7.47 (d, J = 1.2 Hz, 1H), 6.85 (s, 1H), 6.62 (d, J = 5.2 Hz,
H), 3.89 (t, J = 4.8 Hz, 4H), 3.68 (t, J = 5.2 Hz, 2H), 3.20 (broad s, exchangeable with D O, 1H), 2.58
8). Yield 58% (free base); an oil; H-NMR:
1
(
(
2
m, 6H); 13C-NMR:
δ
161.9, 159.2, 158.2, 143.3, 143.1, 126.3, 108.7, 105.9, 59.8, 58.0, 53.0, 43.8. HR-MS
ESI). Calcd for C H N O (M + 1): m/z 275.1508. Found: m/z 275.1508. A hydrobromide salt:
+
14
19
4
2
˝
m.p. >280 C. Anal. Calcd for C H N O2‚2HBr
‚
H O: C, 37.02; H, 4.88; N, 12.34. Found: C, 37.12;
14
19
4
2
H, 4.51; N, 12.32.
1
Ethyl 3-{4-{[4-(3-furyl)pyrimidin-2-yl]piperazin-1-yl}}propanoate (
9
). Yield 50% (free base); an oil; H-NMR:
δ
8.27 (d, J = 5 Hz, 1H), 8.06 (s, 1H), 7.47 (t, J = 2 Hz, 1H), 6.85 (m, 1H), 6.62 (d, J = 5 Hz, 1H), 4.15 (q,
J = 7 Hz, 2H), 3.87 (t, J = 5 Hz, 4H), 2.74 (m, 2H), 2.54 (m, 6H), 1.26 (t, J = 7 Hz, 3H); 13C-NMR:
δ
172.6,

Molecules 2016, 21, 433
12 of 22
1
61.9, 159.1, 158.1, 144.0, 143.1, 126.3, 108.6, 105.8, 60.6, 53.8, 53.0, 43.8, 32.5, 14.4. HR-MS (ESI). Calcd
+
for C H N O (M + 1): m/z 331.1770. Found: m/z 331.1756.
17
23
4
3
1
Ethyl 3-{4-{[4-(3-furyl)pyrimidin-2-yl]piperazin-1-yl}}hexanoate (10). Yield 85% (free base); an oil; H-NMR:
8.27 (d, J = 5 Hz, 1H), 8.06 (s, 1H), 7.48 (t, J = 2 Hz, 1H), 6.85 (m, 1H), 6.62 (d, J = 5 Hz, 1H), 4.12
q, J = 7 Hz, 2H), 3.88 (t, J = 5 Hz, 4H), 2.50 (m, 4H), 2.30 (t, J = 8 Hz, 2H), 2.31 (t, J = 8 Hz, 2H), 1.66
δ
(
(
1
13
m, 2H), 1.56 (m, 2H), 1.37 (m, 2H), 1.25 (t, J = 7 Hz, 3H); C-NMR:
43.0, 108.5, 105.6, 60.2, 58.6, 53.2, 43.6, 34.2, 27.1, 26.5, 24.9, 14.2. HR-MS (ESI). Calcd for C H N O
3
δ 173.7, 161.7, 158.9, 158.0, 143.8,
20
29
4
+
(M + 1): m/z 373.2240. Found: m/z 373.2240.
4
-(3-Furyl)-6-methyl-2-(4-methylpiperazin-1-yl)pyrimidine (11) [24].
-Ethyl-6-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (12). Yield 67% (free base); an oil; H-NMR:
1
4
δ 8.06 (m, 1H), 7.50 (m, 1H), 6.88 (m, 1H), 6.56 (s, 1H), 3.94 (m, 4H), 2.65 (d, J = 7.5 Hz, 2H), 2.51 (m,
˝
4
H), 2.38 (s, 3H), 1.33 (t, J = 7.5 Hz, 3H). A hydrobromide salt: m.p. 287-288 C. Anal. Calcd for
C H N O‚2HBr‚0.5H O: C, 40.47; H, 5.66; N, 12.58. Found: C, 40.41; H, 5.32; N, 12.55.
15
20
4
2
1
4-(n-Butyl)-6-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (13). Yield 30% (free base); an oil; H-NMR:
δ 0.91 (m, 3H), 1.35 (m, 2H), 1.73 (m, 2H), 2.35 (s, 3H), 2.47 (t, J = 5 Hz, 4H), 2.58 (t, J = 7 Hz, 2H),
3
.90 (t, J = 5 Hz, 4H), 6.50 (s, 1H), 6.85 (m, 1H), 7.46 (m, 1H), 8.06 (m, 1H). A hydrobromide salt:
˝
m.p. 179–180 C. Anal. Calcd for C H N O
C, 43.95; H, 6.30; N, 11.96.
‚
2HBr
‚
1.5H O: C, 43.89; H, 6.28; N, 12.04. Found:
17
24
4
2
1
4
-(3-Furyl)-6-n-hexyl-2-(4-methylpiperazin-1-yl)pyrimidine (14). Yield 51% (free base); an oil; H-NMR:
δ 0.90 (m, 3H), 1.32 (m, 6H), 1.70 (m, 2H), 2.34 (s, 3H), 2.47 (t, J = 5 Hz, 4H), 2.57 (t, J = 7.5 Hz, 2H),
3
3
2
.91 (t, J = 5 Hz, 4H), 6.51 (s, 1H), 6.85 (m, 1H), 7.46 (m, 1H), 8.06 (m, 1H); 1 C-NMR:
δ
14.1, 22.6, 28.5,
9.1, 31.7, 38.1, 43.6, 46.3, 55.1, 104.6, 108.6, 126.4, 142.7, 143.7, 158.6, 162.0, 171.8. A hydrobromide
˝
salt: m.p. 230–233 C. Anal. Calcd for C H N O
C, 54.34; H, 7.06; N, 13.29.
‚HBr
‚
0.5H O: C, 54.54; H, 7.23; N, 13.39. Found:
19
28
4
2
1
4
-sec-Butyl-6-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (15). Yield 37% (free base); an oil; H-NMR:
δ
0.87 (t, J = 7.5 Hz, 3H), 1.25 (m, 3H), 1.57 (m, 1H), 1.76 (m, 1H), 2.34 (s, 3H), 2.47 (t, J = 5 Hz), 2.57
(
m, 1H), 2.90 (t, J = 5 Hz, 4H), 6.49 (s, 1H), 6.87 (m, 1H), 7.47 (m, 1H), 8.06 (m, 1H); 13C-NMR:
δ 12.1,
1
9.6, 29.1, 43.4, 43.7, 46.3, 55.1, 103.7, 108.6, 126.5, 142.7, 143.6, 158.6, 162.0, 175.8. A dihydrobromide
˝
salt: m.p. 165–170 C. Anal. Calcd for C H N O
C, 43.05; H, 5.64; N, 11.88.
‚
2HBr 0.5H O: C, 43.33; H, 5.77; N, 11.89. Found:
‚
17
24
4
2
4
-(3-Furyl)-2-(4-methylpiperazin-1-yl)-6-(trifluoromethyl)pyrimidine (16). Yield 81% (free base); an oil.
˝
1
A hydrochloride salt: m.p. >300 C; H-NMR:
δ
8.77 (s, 1H), 7.88 (m, 1H), 7.56 (m, 1H), 3.98 (m, 4H),
2
.55 (m, 4H), 2.39 (s, 3H). Anal. Calcd for C H N F O HCl 0.25H O: C, 47.00; H, 4.71; N, 15.86.
‚
‚
14
15
4
3
2
Found: C, 47.40; H, 4.61; N, 15.72.
4
-(Cyclohexenylethynyl)-6-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (17). Yield 8%; a colorless oil (free
1
base); H-NMR: 1.68 (m, 4H, CH ), 2.18 (m, 2H), 2.27 (m, 2H), 2.34 (s, 3H, CH ), 2.48 (t, J = 5 Hz, 4H),
3
2
3
13
.90 (t, J = 5 Hz, 4H), 6.39 (m, 1H), 6.74 (s, 1H), 6.86 (m, 1H), 7.49 (m, 1H), 8.07 (m, 1H); C-NMR:
δ 21.3, 22.2, 25.9, 28.7, 43.6, 46.2, 55.0, 85.7, 93.2, 108.3, 108.5, 119.9, 125.9, 138.5, 143.1, 143.9, 151,8, 159.1,
˝
1
61.8. A dihydrobromide salt: m.p. 188–190 C. Anal. Calcd for C H N O
‚
2HBr 1.75H O: C, 46.55;
‚
21
24
4
2
H, 5.49; N, 10.34. Found: C, 46.63; H, 5.09; N, 10.00.
4
,6-Bis(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (18) [24].
4
.5. Synthesis of 4-Dimethylamino-6-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (19)
This compound was obtained by: (1) addition reaction of 3-furyllithium with
,4-dichloropyrimidine; (2) aromatization of the intermediate dihydropyrimidine with DDQ
2
by using the general procedure described above; (3) treatment of the crude product [2,4-dichloro-6-

Molecules 2016, 21, 433
13 of 22
˝
3-furyl)pyrimidine] with dimethylamine in ethanol at 23 C, which caused a selective displacement
(
of the 4-chloro atom to yield 2-chloro-4-dimethylamino-6-(3-furyl)pyrimidine; and (4) the treatment
of this product with 4-methylpiperazine at elevated temperature using the general procedure
described above.
1
Yield 30%; a colorless oil (free base); H-NMR:
3
δ
8.17 (m, 1H), 7.90 (m, 1H), 7.48 (m, 1H), 6.95 (s, 1H),
1
.86 (m, 4H), 2.48 (m, 4H), 2.35 (s, 3H), 2.20 (s, 6H); H-NMR NOE experiment: Irradiation at
δ
2.20
(
NMe ) gave a strong NOE signal at
δ
6.95 for C5-H of the pyrimidine, which is fully consistent with
2
the given structure. HR-MS (ESI). Calcd for C H N O: m/z 287.1745. Found: m/z 2871748.
15
21
5
4
-(3-Furyl)-2,4-bis(4-methylpiperazin-1-yl)pyrimidine (20)
This compound was obtained by treatment of 2,4-dichloro-6-(furan-3-yl)pyrimidine described
˝
above with 4-methylpiperazine at 75 C by using the general procedure described above.
1
Yield 37% (free base); an oil; H-NMR:
δ
7.47 (m, 1H), 7.05 (m, 1H), 6.53 (m, 1H), 6.31 (s, 1H), 3.85 (t,
163.4, 161.7,
55.4, 153.8, 143.2, 111.8, 109.7, 86.9, 55.2, 54.8, 46.3, 46.2, 43.9, 43.8. Anal. Calcd for C H N O 3HBr:
J = 4.8 Hz, 4H), 3.67 (t, J = 4.8 Hz, 4H), 2.47 (t, J = 4.8 Hz, 8H), 2.34 (s, 6H); 13C-NMR:
δ
1
‚
18
26
6
C, 36.94; H, 5.00; N, 14.36. Found: C, 37.14; H, 5.29; N, 14.62.
1
4
-(3-Furyl)-2-(4-methylpiperazino)-5-propylpyrimidine (21). Yield 44%; an oil; H-NMR (free base):
δ 8.17
(
(
m, 1H), 7.91 (m, 1H), 7.48 (m, 1H), 6.95 (s, 1H), 3.85 (m, 4H), 2.59 (t, J = 7.6 Hz, 2H), 2.49 (m, 4H), 2.35
s, 3H), 1.59 (sxt, J = 7.6 Hz, 2H), 0.99 (t, J = 7.6 Hz, 3H). A hydrobromide salt: m.p. 206–208 C. Anal.
˝
Calcd for C H N O‚HBr: C, 52.32; H, 6.31; N, 15.25. Found: C, 52.34; H, 6.11; N, 15.12.
16
22
4
4
-n-Butyl-6-(3-furyl)-2-(4-methylpiperazino)-5-propylpyrimidine (22). Yield 73% (free base); an oil;
1
H-NMR:
δ 7.80 (m, 1H), 7.47 (m, 1H), 6.86 (m, 1H), 3.84 (m, 4H), 2.66 (m, 2H), 2.59 (m, 2H), 2.47
(
m, 4H), 2.34 (s, 3H), 1.69 (m, 2H), 1.47 (m, 4H), 1.01 (t, J = 7.6 Hz, 3H), 0.96 (t, J = 7.6 Hz, 3H).
˝
A dihydrobromide salt: m.p. 203 C (dec.). Anal. Calcd for C H N O 2HBr: C, 47.63; H, 6.40; N,
1
‚
20
30
4
1.11. Found: C, 47.34; H, 6.65; N, 11.08.
4
-(2-Methyl-3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (23).
Yield 39% (free base); an oil.
δ 2.66 (s, 3H, CH ), 2.85 (s, 3H), 3.11 (m, 2H),
˝
1
A dihydrobromide salt: m.p. 263–265 C; H-NMR:
.40 (m, 2H, CH ), 3.55 (m, 2H), 4.73 (m, 2H), 6.96 (m, 1H), 6.98 (m, 1H), 7.59 (m, 1H), 8.41 (m, 1H),
3
3
2
1
0.00 (bs, 1H, exchangeable with D O). Anal. Calcd for C H N O
‚
2HBr 0.5 H O: C, 39.18; H, 4.93;
‚
2
14 18
4
2
N, 13.06. Found: C, 39.18; H, 4.77; N, 13.32.
4
-(5-Methyl-3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (24).
Yield 58% (free base); an oil.
δ 2.30 (s, 3H, CH ), 2.84 (m, 3H, CH ), 3.10
˝
1
A dihydrobromide salt: m.p. 275 C (dec.); H-NMR:
m, 2H), 3.31 (m, 2H), 3.53 (m, 2H), 4.79 (m, 2H), 6.72 (m, 1H), 7.04 (m, 1H), 8.41 (m, 2H), 9.98 (bs, 1H,
3
3
(
exangeable with D O). Anal. Calcd for C H N O
C, 39.01; H, 4.70; N, 13.02.
‚
2HBr 0.5 H O: C, 39.18; H, 4.93; N, 13.06. Found:
‚
2
14 18
4
2
4
-n-Hexyl-6-(5-methyl-3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (25). Yield 22% (free base); an oil.
˝
1
A dihydrobromide salt: m.p. 242 C (dec.); H-NMR:
2
2
δ
0.88 (m, 3H), 1.34 (m, 6H), 1.69 (m, 2H),
.31 (s, 3H), 2.57 (t, J = 4.5 Hz , 2H), 2.86 (m, 3H), 3.10 (m, 2H), 3.26 (m, 2H), 3.50 (m, 2H), 4.81 (m,
H), 6.67 (s, 1H), 6.91 (s, 1H), 8.28 (s, 1H), 9.37 (bs, 1H, exchangeable with D O). Anal. Calcd for
2
C H N O‚2HBr‚H O: C, 45.99; H, 6.56; N, 10.73. Found: C, 45.81; H, 6.44; N, 10.54.
20
30
4
2
4
-(2-Furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (26) [29].
1
4
-Ethyl-6-(2-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (27). Yield 61% (free base); an oil; H-NMR: δ 7.52
(
2
1
m, 1H), 7.12 (d, J = 3.2 Hz, 1H), 6.77 (s, 1H), 6.51 (m, 1H), 3.91 (t, J = 4.8 Hz, 4H), 2.64 (q, J = 7.4 Hz, 2H),
.48 (t, J = 4.8 Hz, 4H), 2.34 (s, 3H), 1.27 (t, J = 7.4 Hz, 3H); 13C-NMR:
δ
173.2, 162.0, 155.8, 153.3, 144.1,
+
12.1, 110.8, 102.4, 55.3, 46.5, 43.8, 31.4, 12.7. HR-MS (ESI). Calcd for C H N O (M + 1): m/z 273.1725.
1
5
21
4

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˝
Found: m/z 273.1715. A hydrobromide salt: m.p. >250 C. Anal. Calcd for C H N O
H, 5.11; N, 12.70. Found: C, 41.79; H, 4.87; N, 12.38.
‚2HBr: C, 41.50;
15
20
4
1
4
-n-Butyl-6-(2-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (28). Yield 78% (free base); an oil; H-NMR:
δ 7.52 (m, 1H), 7.11 (d, J = 3.2 Hz, 1H), 6.75 (s, 1H), 6.51 (m, 1H), 3.90 (t, J = 4.6 Hz, 4H), 2.60 (t,
J = 7.4 Hz, 2H), 2.48 (t, J = 4.6 Hz, 4H), 2.34 (s, 3H), 1.70 (m, 2H), 1.40 (m, 2H), 0.94 (t, J = 7.4 Hz, 3H);
1
3
C-NMR:
δ
172.4, 162.1, 155.8, 153.4, 144.1, 112.2, 110.8, 103.1, 55.3, 46.5, 43.9, 38.1, 30.8, 22.7, 14.2.
+
HR-MS (ESI). Calcd for C H N O (M + 1): m/z 301.2026. Found: m/z 301.2028. Anal. Calcd for
17
25
4
C H N O‚HBr: C, 53.55; H, 6.61; N, 14.10. Found: C, 53.34; H, 6.60; N, 14.49.
17
24
4
4
-(5-Methyl-2-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (29).
Yield 48% (free base); an oil.
˝
1
A dihydrobromide salt: m.p. 263–265 C; H-NMR:
δ
2.39 (s, 3H), 2.85 (m, 3H), 3.11 (m, 2H), 3.28
(m), 3.54 (m, 2H), 4.79 (m, 2H), 6.36 (m, 1H), 6.99 (m, 1H), 7.28 (m, 1H), 8.45 (m, 1H), 9.78 (bs, 1H,
exchangeable with D O). Anal. Calcd for C H N O
‚
2HBr: C, 40.02; H, 4.80; N, 13.34. Found: C,
2
14 18
4
3
2
4
2
4
4
4
4
9.70; H, 4.85; N, 13.25.
-(Methylpiperazin-1-yl)-4-(3-thienyl)pyrimidine (30) [25].
-n-Butyl-2-(methylpiperazin-1-yl)-6-(3-thienyl)pyrimidine (31) [30].
-(Methylpiperazin-1-yl)-4-(2-thienyl)pyrimidine (32) [24,26].
-(2-Hydroxyphenyl)-2-(4-methylpiperazin-1-yl)pyrimidine (33) [27].
-(3-Hydroxyphenyl)-2-(4-methylpiperazin-1-yl)pyrimidine (34) [27].
-(4-Hydroxyphenyl)-2-(4-methylpiperazin-1-yl)pyrimidine (35) [27].
1
-(Benzofuran-2-yl)-2-(4-methylpiperazin-1-yl)pyrimidine (36). Yield 71% (free base); an oil; H-NMR:
δ 8.45 (m, 1H), 7.69 (m, 1H), 7.58 (m, 1H), 7.42 (m, 1H), 7.30 (m, 2H), 7.09 (m, 1H), 3.98 (m, 4H), 2.55
˝
(
m, 4H, 2.39 (s, 3H). A hydrobromide salt: m.p. >300 C. Anal. Calcd for C H N O
‚2HBr: C, 44.76;
17
18
4
H, 4.42; N, 12.28. Found: C, 44.60; H, 4.48; N, 12.15.
1
4
-(Benzothiophen-2-yl)-2-(4-methylpiperazin-1-yl)pyrimidine (37). Yield 78% (free base); an oil; H-NMR:
δ 8.39 (d, J = 5.1 Hz, 1H), 7.96 (s, 1H), 7.88 (m, 2H), 7.41 (m, 2H), 6.99 (d, J = 5.1 Hz, 1H), 3.98 (m, 4H),
˝
2
.55 (m, 4H), 2.40 (s, 3H). A hydrobromide salt: m.p. 299–300 C. Anal. Calcd for C H N S
‚2HBr:
17
18
4
C, 43.24; H, 4.27; N, 11.86. Found: C, 43.20; H, 4.33; N, 11.80.
4
2
2
-(1-Methylindol-2-yl)-2-(4-methylpiperazin-1-yl)pyrimidine (38) [27].
-(4-Methylpiperazin-1-yl)-4-(pyridin-3-yl)pyrimidine (39) [27].
1
-(4-Methylpiperazin-1-yl)-4-(thiazol-2-yl)pyrimidine (40). Yield 77% (free base); an oil; H-NMR: δ 8.44 (d,
J = 5.0 Hz, 1H), 7.95 (d, J = 3.2 Hz, 1H), 7.48 (d, J = 3.2 Hz, 1H), 7.29 (d, J = 5.0 Hz, 1H), 3.92 (t, J = 5.0 Hz,
1
3
4
4
4
H), 2.50 (t, J = 5.0 Hz, 4H), 2.36 (s, 3H); C-NMR: δ 168.4, 161.4, 159.0, 157.7, 144.3, 122.1, 104.3, 54.8,
˝
6.1, 43.5. A hydrobromide salt: m.p. 273 C (decomp.). Anal. Calcd for C H N S
1.03; H, 4.88; N, 19.94. Found: C, 41.32; H, 4.64; N, 19.93.
‚
HBr 0.5H O: C,
‚
12
15
5
2
1
(
1H-Imidazol-1-yl)-2-(4-methylpiperazin-1-yl)pyrimidine (41). Yield 87% (free base); an oil; H-NMR: δ 8.36
(
2
4
s, 1H), 8.36 (d, J = 5.4 Hz, 1H), 7.60 (s, 1H), 7.17 (s, 1H), 6.50 (d, J = 5.4 Hz, 1H), 3.90 (t, J = 5.1 Hz, 4H),
13
.49 (t, J = 5.1 Hz, 4H), 2.35 (s, 3H); C-NMR:
δ
161.4, 160.1, 155.3, 135.0, 130.8, 115.6, 96.5, 54.8, 46.1,
˝
3.7. A hydrobromide salt: m.p. 229–231 C. Anal. Calcd for C H16‚2HBr
‚
H O: C, 33.98; H, 4.75;
12
2
N, 19.81. Found: C, 33.93; H, 4.55; N, 19.60.
1
4
-(3-Furyl)-2-(4-methylhomopiperazin-1-yl)pyrimidine (42). Yield 69% (free base); an oil; H-NMR:
δ
8,32
(
2
d, J = 5.1 Hz, 1H), 8.08 (s, 1H), 7.50 (m, 1H), 6.88 (m, 1H), 6.64 (d, J = 5.1 Hz, 1H), 4.01 (m, 2H), 3.89 (m,
˝
H), 2.74 (m, 2H), 2.60 (m, 2H), 2.41 (s, 3H), 2.04 (m, 2H). A hydrobromide salt: m.p. 176–177 C. Anal.
Calcd for C H N O‚2HBr‚1.25H 0: C, 37.99; H, 5.12; N, 12.66. Found: C, 37.96; H, 5.12; N, 12.51.
14
18
4
2

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1
4
-(3-Furyl)-2-(piperidin-1-yl)pyrimidine (43). Yield 68% (free base); an oil; H-NMR:
δ
8.32 (d, J = 5.1 Hz,
1
H), 8.10 (m, 1H), 7.51(m, 1H), 6.90 (m, 1H), 6.62 (d, J = 5.1 Hz, 1H), 3.89 (m, 4H), 1.67 (m, 6H).
˝
A hydrobromide salt: m.p. 190–191 C. Anal. Calcd for C H N O
N, 13.35. Found: C, 49.60; H, 5.11, N, 13.37.
‚
HBr 0.25H O: C, 49.66; H, 5.28;
‚
13
15
3
2
˝
4
-(4-(3-Furyl)-2-(morpholin-1-yl)pyrimidine (44). Yield 53% (free base); a white solid: m.p. 58–59 C;
1
HNMR:
8
δ
3.79 (m, 4H), 3.87 (m, 4H), 6.68 (d, J = 4.8 Hz, 1H), 6.88 (m, 1H), 7.49 (m, 1H), 8.08 (m, 1H),
˝
.31 (d, J = 4.8 Hz, 1H). A hydrobromide salt: m.p. 215-216 C; Anal. Calcd for C H N O2‚HBr
‚
0.25
12
13
3
H O: C, 45.51; H, 4.62; N, 13.27. Found: C, 45.38; H, 4.52; N, 13.37.
2
1
4
-(3-Furyl)-N-[2-(morpholin-4-yl)ethyl]pyrimidin-2-amine (45). Yield 69% (free base); an oil; H-NMR:
δ 8.25 (d, J = 5.2 Hz, 1H), 8.07 (s, 1H), 7.48 (d, J = 1.8 Hz, 1H), 6.85 (m, 1H), 6.66 (d, J = 5.2 Hz, 1H),
5
.78 (s, 1H), 3.71 (t, J = 4.4 Hz, 4H), 3.55 (q, J = 6 Hz, 2H),2.59 (t, J = 6 Hz, 2H), 2.49 (t, J = 4.4 Hz, 4H).
˝
A hydrobromide salt: m.p. 143–146 C. Anal. Calcd for C H N O2‚2HBr
‚
0.5H 0: C, 37.77; H, 4.75;
14
18
4
2
N, 12.59. Found: C, 37.78; H, 4.76; N, 12.20.
1
4
-(3-Furyl)-N-[3-(1H-imidazol-2-yl)propyl]pyrimidin-2-amine (46). Yield 62% (free base); an oil; H-NMR:
δ
8.24 (d, J = 5.2 Hz, 1H), 8.06 (m, 1H), 7.49 (m, 2H), 7.06 (s, 1H), 6.94 (s, 1H), 6.84 (m, 1H), 6.70 (d,
J = 5.2 Hz, 1H), 4.04 (t, J = 6.8 Hz, 2H), 3.46 (t, J = 6.8 Hz, 2H), 2.09 (m, 2H). A hydrobromide salt: m.p.
˝
2
34–236 C. Anal. Calcd for C H N O
‚
2HBr 0.5H O: C, 38.30; H, 4.12; N, 15.91. Found: C, 38.23;
‚
14
15
5
2
H, 3.93; N, 15.78.
4
.6. Synthesis of 4-[4-(3-Furyl)pyrimidin-2-yl]piperazine-1-spiro-1-morpholinium iodide (47)
A mixture of compound (1 mmol), 1-iodo-2-(2-iodoethoxy)ethane (1 mmol) and anhydrous
potassium carbonate (140 mg, 1 mmol) in anhydrous acetonitrile (10 mL) was heated under reflux for
and then quenched with water (5 mL). Product 47 was extracted with ethyl acetate and purified on
a chromatotron eluting with dichloromethane/methanol (8:2).
1
1
h
˝
1
Yield 66%; m.p. >200 C (decomp.); H-NMR (DMSO-d ):
δ
8.55 (m, 1H), 8.46 (d, J = 5.2 Hz, 1H), 7.83
6
(
m, 1H), 7.10 (m, 2H), 4.13 (m, 4H), 3.98 (m, 4H), 3.74 (m, 4H), 3.67 (m, 4H). HR-MS (ESI). Calcd for
+
C H N O (M ): m/z 301.1665. Found: m/z 301.1650. Anal. Calcd for C H N IO : C, 44.87; H, 4.94;
16
21
4
2
16 21
4
2
N, 13.08. Found: C, 44.97; H, 5.09; N, 12.69.
4
-(3-Furyl)-2-(methylpiperazin-1-yl)quinazoline (48) [27].
4
.7. Synthesis of 2,4-bis(4-Methylpiperazin-1-yl)quinazoline (49)
This compound was obtained by treatment of 2,4-dichloroquinazoline with 4-methylpiperazine
by using the general necleophilic displacement procedure described above.
1
Yield 46% (free base); an oil; H-NMR:
4
δ 7.67 (d, J = 8.4 Hz, 1H), 7.51 (m, 2H), 7.05 (m, 1H), 3.93 (m,
H), 3.70 (t, J = 4.6 Hz, 4H), 2.61 (t, J = 4.6 Hz, 4H), 2.51 (t, J = 4.6 Hz, 4H), 2.37 2s, 6H); 13C-NMR:
δ 165.8, 158.4, 154.6, 132.6, 126.4, 125.3, 120.8, 112.2, 55.3, 55.1, 49.9, 46.4, 46.3, 44.1. HR-MS (ESI). Calcd
for C H N (M + 1): m/z 327.2282. Found: m/z 327.2297. A hydrobromide salt: m.p. 108–112 C.
+
˝
18
27
6
Anal. Calcd for C H N ‚2HBr: C, 44.28; H 5.78; N, 17.21. Found: C, 44.60; H, 5.73; N, 17.42.
18
26
6
4
.8. Synthesis of 4-[2-(Dimethylamino)ethyl]-2-[2-(4-methylpiperazin-1-yl)quinazoline (50)
The starting material, 2-chloro-4-vinylquinazoline, was obtained by the reaction
of 2-chloroquinazoline with vinyllithium followed by aromatization of the resultant
-vinyldihydroquinazoline adduct by treatment with potassium ferricyanide(III) according to
4
the described procedure [36].
The conjugate addition reaction of dimethylamine with 2-chloro-4-vinylquinazoline to give
2-chloro-4-(2-dimethylaminoethyl)quinazoline was conducted as described for the preparation of

Molecules 2016, 21, 433
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analogous compounds [27]. Treatment of this compound with 4-methylpiperazine to give the final
product 50 was conducted by using a general procedure described above.
1
Yield 27% (free base), an oil; H-NMR:
δ
7.86 (d, J = 8.0 Hz, 1H), 7.59 (m, 2H), 7.18 (t, J = 7.2 Hz, 1H),
3
2
3
.99 (t, J = 5.0 Hz, 4H), 3.29 (t, J = 7.2 Hz, 2H), 2.85 (t, J = 7.2 Hz, 2H), 2.51 (t, J = 5.0 Hz, 4H), 2.36 (s, 6H),
.35 (s, 3H); 13C-NMR:
δ
170.1, 158.4, 152.3, 133.3, 126.4, 124.6, 122.1, 118.6, 57.6, 55.1, 46.3, 45.5, 43.8,
+
2.4. HR-MS (ESI). Calcd for C H N (M + 1): m/z 300.2175. Found: m/z 300.2188. A hydrobromide
17
26
5
˝
salt: m.p. 100–106 C. Anal. Calcd for C H N5‚3.5HBr
‚
H O: C, 34.00; H, 5.12; N, 11.66. Found: C,
1
7
25
2
3
3.74; H, 5.40; N, 11.26.
4
.9. Synthesis of 2-(4-Methylpiperazin-1-yl)pyridines 51–53
A mixture of furan-3-boronic acid, furan-2-boronic acid or thiophene-2-boronic acid (3.4 mmol),
-bromo-2-chloropyrimidine (0.25 mL, 2.3 mmol), potassium carbonate (1.0 g, 7 mmol) and
4
tetrakis(triphenylphosphine)palladium [Pd(PPh3)4, 0.2 g, 0.2 mmol] in anhydrous dioxane (6 mL) was
stirred under a nitrogen atmosphere and heated under reflux for 4 days. After cooling the mixture was
filtered through a 0.5-cm pad of Celite and concentrated under reduced pressure. A mixture of the
crude product (2-chloro-4-heteroarylpyridine, 0.5 mmol) and 1-methylpiperazine (1 mL) in a sealed
˝
tube was heated at 200 C for 12 h. The resultant brown oil was subjected to chromatography eluting
with dichloromethane/methanol (95:5) to yield analytically pure product 51–53.
1
4
-(3-Furyl)-2-(4-methylpiperazin-1-yl)pyridine (51). Yield 20% (free base); an oil; H-NMR:
δ
8.16 (m, 1H),
7
.81 (s, 1H), 7.49 (m, 1H), 6.72 (m, 3H), 3.60 (t, J = 5.0 Hz, 4H), 2.53 (t, J = 5.0 Hz, 4H), 2.34 (s, 3H);
13
C-NMR:
δ
160,1, 148.4, 144.0, 141.4, 140.0, 125.2, 110.9, 108.5, 103.5, 54.9, 46.1, 45.2. A hydrobromide
˝
salt: m.p. >250 C. Anal. Calcd for C H N 0
H, 4.39; N, 11.07.
‚2HBr: C, 41.51; H, 4.73; N, 10.37. Found: C, 41.14;
14
17
3
1
4
-(2-Furyl)-2-(4-methylpiperazin-1-yl)pyridine (52). Yield 55% (free base); an oil; H-NMR:
δ
8.17 (m, 1H),
7
.49 (m, 1H), 6.39 (s, 1H), 6.86 (m, 1H), 6.78 (m, 1H), 6.49(m, 1H), 3.62 (t, J = 5.0 Hz, 4H), 2.54 (t,
13
J = 5.0 Hz, 4H), 2.35 (s, 3H); C-NMR:
δ
160.1, 152.3, 148.3, 143.1, 138.8, 111.9, 108.4, 107.9, 100.9,
+
54.9, 46.2, 45.2. HR-MS (ESI). Calcd for C H N O (M + 1): m/z 244.1450. Found: m/z 244.1459.
14
18
3
˝
A hydrobromide salt: m.p. >250 C. Anal. Calcd for C H N O
Found: C, 41.24; H, 4.78; N, 10.51.
‚2HBr: C, 41.51; H, 4.73; N, 10.37.
14
17
3
1
2
-(4-Methylpiperazin-1-yl)-4-(2-thienyl)pyridine (53). Yield 34% (free base); an oil; H-NMR:
δ
8.18 (d,
J = 5.2 Hz, 1H), 7.44 (m, 1H), 7.36 (d, J = 4.5 Hz, 1H), 7.11 (m, 1H), 6.86 (m, 1H), 6.83 (s, 1H), 3.62
t, J = 5.0 Hz, 4H), 2.55 (t, J = 5.0 Hz, 4H), 2.36 (s, 3H). 13C-NMR:
160.1, 148.5, 142.8, 142.5, 128.1,
26.3, 124.8, 110.7, 103.2, 54.9, 46.2, 45.3. A hydrobromide salt: m.p. >250 C. Anal. Calcd for
C H N S‚2HBr‚0.5H 0: C, 39.09; H, 4.69; N, 9.77. Found: C, 39.46; H, 4.80; N, 9.44.
(
1
δ
˝
14
17
3
2
4
.10. Synthesis of Pyridines 54–59
A mixture of 4-bromo-6-methyl-2-(methylpiperazin-1-yl)pyridine or 4-bromo-2-(methylpiperazin-
1
(
0
-yl)pyridine (0.4 mmol), furan-3-boronic acid, furan-2-boronic acid or thiophene-2-boronic acid
0.5 mmol), tetrakis(triphenylphosphine)palladium (0.03 g, 0.03 mmol) and potassium carbonate
.17 g, 1.1 mmol) in DMF (4.0 mL) containing water (0.3 mL) in a sealed tube under a nitrogen
˝
atmosphere was stirred at 90 C for 12 h. After cooling, the mixture was filtered through Celite and
dissolved in ethyl acetate (20 mL). The solution was washed with water (3 25 mL), brine (20 mL)
and concentrated under reduced pressure. The residue was subjected to chromatography eluting with
hexanes/ether (4:1, 200 mL; 1:1, 150 mL) and finally with hexanes/ether/methanol (10:10:1, 200 mL)
to give product 54–59.
ˆ
1
4
-(3-Furyl)-2-methyl-6-(4-methylpiperazin-1-yl)pyridine (54). Yield 31% (free base); an oil; H-NMR: δ 7.79
(
4
s, 1H), 7.48 (s, 1H), 6.69 (s, 1H), 6.62 (s, 1H), 6.61 (s, 1H), 3.60 (t, J = 4.8 Hz, 4H), 2.55 (t, J = 4.8 Hz,
H),2.42 (s, 3H), 2.36 (s, 3H); 13C-NMR:
δ
157.5, 144.0, 141.8, 140.0, 128.7, 125.6, 110.5, 108.8, 100.6,

Molecules 2016, 21, 433
17 of 22
+
5
5.1, 46.4, 45.5, 24.8. HR-MS (ESI). Calcd for C H N O (M + 1): m/z 258.1606. Found: m/z 258.1598.
15 20 2
˝
A hydrobromide salt: m.p. 150–152 C. Anal. Calcd for C H N O
‚HBr
‚
0.5H 0: C, 51.88; H, 6.10;
1
5
19
3
2
N, 12.10. Found: C, 51.90; H, 6.50; N, 12.06.
1
4
-(2-Furyl)-2-methyl-6-(4-methylpiperazin-1-yl)pyridine (55). Yield 10% (free base); an oil; H-NMR:
δ
7.47
(
3
s, 1H), 6.75 (m, 2H), 6.72 (s, 1H), 6.47 (m, 1H), 3.62 (t, J = 4.8 Hz, 4H), 2.54 (t, J = 4.8 Hz, 4H), 2.41 (s,
H), 2.35 (s, 3H); 13C-NMR: 159.9, 157.5, 152.8, 143.0, 139.4, 111.9, 108.0, 107.7, 98.2, 55.0, 46.2, 45.3, 24.8.
+
HR-MS (ESI). Calcd for C H N O (M + 1): m/z 258.1606. Found: m/z 258.1618. A hydrobromide salt:
m.p. 148–150 C. Anal. Calcd for C H N O
H, 6.42; N, 11.39.
15
20
3
˝
‚HBr
‚
H O: C, 50.57; H, 6.22; N, 11.79. Found: C, 50.41;
15
19
3
2
1
2
-Methyl-6-(4-methylpiperazin-1-yl)-4-(2-thienyl)pyridine (56). Yield 6% (free base); an oil; H-NMR: δ 7.41
(
2
d, J = 3.2 Hz, 1H), 7.34 (d, J = 4.8 Hz, 1H), 7.09 (m, 1H), 6.74 (s, 1H), 6.63 (s, 1H), 3.61 (t, J = 4.8 Hz, 4H),
˝
.55 (t, J = 4.8 Hz, 4H), 2.43 (s, 3H), 2.36 (s, 3H). A hydrobromide salt: m.p. 132–134 C. Anal. Calcd for
C H N S‚HBr: C, 50.85; H, 5.68; N, 11.86. Found: C, 51.10; H, 5.69; N, 11.53.
15
19
3
1
6-(3-Furyl)-2-(4-methylpiperazin-1-yl)pyridine (57). Yield 80% (free base); an oil; H-NMR:
δ 7.99 (s,
1
4
H), 7.49 (m, 1H), 7.30 (m, 2H), 6.85 (m, 1H), 6.54 (m, 1H), 3.64 (t, J = 5.0 Hz, 4H), 2.57 (t, J = 5.0 Hz,
H), 2.37 (s, 3H); 13C-NMR:
δ
159.1, 149.8, 143.4, 141.1, 138.0, 127.7, 109.5, 108.8, 105.0, 54.9, 46.3, 45.5.
+
HR-MS (ESI). Calcd for C H N O (M + 1): m/z 244.1461. Found: m/z 244.1450. A hydrobromide salt:
m.p. >250 C. Anal. Calcd for C H N O
H, 5.33; N, 9.72.
14
18
3
˝
‚
2HBr
‚
2H O: C, 38.12; H, 5.25; N, 9.52. Found: C, 38.23;
14
17
3
2
1
2
-(4-Methylpiperazin-1-yl)-6-(3-thienyl)pyridine (58). Yield 42% (free base); an oil; H-NMR:
δ
7.86 (m,
1
H), 7.63 (m, 1H), 7.49 (t, J = 4.0 Hz, 1H), 7.33 (m, 1H), 6.97 (d, J = 4.0 Hz, 1H), 6.54 (d, J = 4.0 Hz,
13
1
H), 3.64 (t, J = 5.0 Hz, 4H), 2.53 (t, J = 5.0 Hz, 4H), 2.35 (s, 3H); C-NMR:
δ 159.0, 154.4, 143.0, 138.2,
˝
126.4, 125.7, 123.0, 109.7, 105.2, 55.0, 46.3, 45.1. A hydrobromide salt: m. p. >250 C. Anal. Calcd for
C H N S‚2HBr‚H O: C, 39.09; H, 4.69; N, 9.77. Found: C, 39.11; H, 4.79; N, 9.90.
14
17
3
2
1
2
-(2-Furyl)-6-(4-methylpiperazin-1-yl)pyridine (59). Yield 38% (free base); an oil; H-NMR:
J = 5.2 Hz, 1H), 7.48 (d, J = 1.6 Hz, 1H), 6.92 (m, 1H), 6.85 (d, J = 5.2 Hz, 1H), 6.77 (d, J = 3.6 Hz, 1H),
.57 (d, J = 3.6 Hz, 1H), 3.61 (t, J = 5.2 Hz, 4H), 2.52 (t, J = 5.2 Hz, 4H), 2.34 )s, 3H); 13C-NMR:
160.1,
52.3, 148.3, 143.1, 138.8, 111.9, 108.4, 107.9, 100.9, 54.9, 46.2, 45.2. A hydrobromide salt: m.p. >250 C.
Anal. Calcd for C H N O‚2HBr‚2H O: C, 38.12; H, 5.25; N, 9.52. Found: C, 37.94; H, 5.02; N, 9.54.
δ 8.17 (d,
6
1
δ
˝
14
17
3
2
4
.11. Synthesis of Substituted Benzenes 60–71
A mixture of 1,3-dibromobenzene, 1,3-dibromo-5-methylbenzene or 1,3-dibromo-5-n-butylbenzene
(
21 mmol) and N-methylpiperazine (0.8 mL), 7.0 mmol) in toluene (20 mL) was stirred under a nitrogen
1
1
atmosphere and treated quickly with 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP, 0.13 g,
.21 mmol) and tris (dibenzylideneacetone)dipalladium(0) (Pd dba , 0.05 g, 0.06 mmol). The flask was
0
2
3
refilled with nitrogen, capped with a rubber septum, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU,
˝
.6 mL, 17.5 mmol) was added via a syringe. The mixture was warmed to 60 C before treatment
with sodium tert-butoxide (0.01 g, 0.05 mmol) in one portion. The mixture was heated at 110 C
for 12 h, then cooled and partitioned between ethyl acetate (EtOAc, 40 mL) and water (40 mL). The
aqueous layer was extracted with EtOAc (40 mL). The organic layers were combined and washed
2
˝
with hydrochloric acid (1.6 M, 2
basic with NaOH solution (1M, 50 mL) and the mixture was extracted with EtOAc (2
extract was washed with brine (25 mL), dried over magnesium sulfate, filtered and concentrated under
reduced pressure to provide an analytically pure product.
ˆ
25 mL). The aqueous acidic layer containing product was made
40 mL). The
ˆ
1
-(3-Bromophenyl)-4-methylpiperazine; yield 30%; an oil; 13C-NMR:
δ
152.6, 130.5, 123.4, 122.3, 118.8,
+
114.5, 55.1, 48.8.46.3. HR-MS (ESI). Calcd for C H BrN (M + 1): m/z 255.0497. Found: m/z 255.0493.
11
15
2

Molecules 2016, 21, 433
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1
1
-(3-Bromo-5-methylphenyl)-4-methylpiperazine; yield 54%; an oil; 13C-NMR:
δ
152.5, 140.6, 131.1, 123.2,
+
16.1, 115.4, 55.2, 48.9, 46.3, 21.7. HR-MS (ESI). Calcd for C H BrN (M + 1): m/z 269.0653.
12
17
2
Found: 269.0653.
1
1
-(3-Bromo-5-n-butylphenyl)-4-methylpiperazine; yield 32%; an oil; 13C-NMR: δ 152.3, 145.5, 122.9, 122.4,
+
16.0, 114.7, 55.0, 48.7, 46.1, 35.8, 33.4, 22.3, 13.9. HR-MS (ESI). Calcd for C H BrN (M + 1):
15 24 2
m/z 311.1123. Found: m/z 311.1133.
A bromophenylpiperazine derivative (1.2 mmol), obtained as described above, was
allowed to react with furan-3-boronic acid, furan-2-boronic acid, thiophene-3-boronic acid or
thiophene-2-boronic acid (1.8 mmol) in DMF (5 mL) and water (0.5 mL) in the presence of
tetrakis(triphenylphosphine)palladium(0) (0.09 mmol) and potassium carbonate (0.5 g, 3.6 mmol)
˝
at 90 C under a nitrogen atmosphere in a sealed tube for 12 h. After cooling, the mixture was
filtered through Celite and the adsorbent was washed with EtOAc (20 mL). Concentration followed by
chromatography eluting with hexanes/ether (4:1) gave the oily product 60–71 that was additionally
purified by crystallization of a hydrobromide salt from ethanol.
1
1
-[3-(3-Furyl)phenyl]-4-methylpiperazine (60). Yield 34%; H-NMR:
δ
7.70 (s, 1H), 7.46 (s, 1H), 7.26 (t,
J = 5.6 Hz, 1H), 7.03 (s, 1H), 6.99 (d, J = 7.6 Hz, 1H), 6.85 (m, 1H), 6.68 (s, 1H), 3.27 (t, J = 4.8 Hz, 4H),
2
.62 (t, J = 4.8 Hz, 4H), 2.38 (s, 3H); 13C-NMR:
δ
151.9, 143.6, 138.7, 133.5, 129.7, 127.1, 117.8, 115.0, 113.9,
+
109.2, 55.3, 49.3, 46.3. HR-MS (ESI). Calcd for C H N O (M + 1): m/z 243.1497. Found: m/z 243.1492.
15
19
2
˝
A hydrobromide salt: m.p. 215–217 C. Anal. Calcd for C H N O 2HBr: C, 44.58; H, 4.99; N, 6.93.
‚
15
18
2
Found: C, 44.31; H, 4.99; N, 6.62.
-[3-(3-Furyl)-5-methylphenyl]-4-methylpiperazine (61). Yield 25%; H-NMR:
.85 (s, 1H), 6.82 (s, 1H), 6.67 (s, 2H), 3.23 (t, J = 4.8 Hz, 4H), 2.59 (t, J = 4.8 Hz, 4H), 2.34 (2s, 6H);
1
1
δ 7.68 (s, 1H), 7.44 (s, 1H),
6
13
C-NMR:
δ
151.7, 143.4, 139.2, 138.5, 133.1, 126.9, 118.6, 115.7, 111.1, 109.1, 55.1, 49.2, 46.1, 21.7. HR-MS
+
(ESI). Calcd for C H N O (M + 1): m/z 257.1654. Found: m/z 257.1657. A hydrobromide salt:
16
21
2
˝
m.p. 230–232 C. Anal. Calcd for C H N O
N, 6.34.
‚2HBr: C, 45.96; H, 5.30; N, 6.70. Found: C, 46.30; H, 5.28;
16
20
2
1
1
-[3-n-Butyl-5-(3-furyl)phenyl]-4-methylpiperazine (62). Yield 70%; H-NMR:
δ
7.69 (s, 1H), 7.43 (t,
J = 1.6 Hz, 1H), 6.85 (s, 1H), 6.82 (s, 1H), 6.67 (m, 2H), 3.23 (t, J = 5.2 Hz, 4H), 2.57 (m, 6H), 1.61 (m,
2
H), 1.37 (m, 2H), 0.93 (t, J = 7.2 Hz, 3H); 13C-NMR:
δ
151.8, 144.4, 143.4, 138.5, 133.1, 127.1, 118.1,
+
115.4, 111.3, 109.2, 55.3, 49.4, 46.2, 36.2, 33.8, 22.6, 14.1. HR-MS (ESI). Calcd for C H N O (M + 1):
19
27
2
˝
m/z 299.2123. Found: m/z 299.2136. A hydrobromide salt: m.p. 215–219 C. Anal. Calcd for
C H N O‚2HBr‚0.5H O: C, 48.63; H, 6.23; N, 5.97. Found: C, 48.55; H, 6.14; N, 6.20.
19
26
2
2
1
1-[3-(2-Furyl)phenyl]-4-methylpiperazine (63). Yield 24%; H-NMR:
δ 7.44 (s, 1H), 7.27 (m, 2H), 7.16
(
(
1
d, J = 7.6 Hz, 1H), 6.84 (d, J = 7.6 Hz, 1H), 6.62 (m, 1H), 6.45 (m, 1H), 3.27 (t, J = 4.6 Hz, 4H), 2.60
t, J = 4.6 Hz, 4H), 2.36 (s, 3H); 13C-NMR:
δ
154.5, 151.7, 142.0, 131.8, 129.6, 115.5, 115.4, 111.7, 111.5,
+
05.1, 55.3, 49.2, 46.3. HR-MS (ESI): Calcd for C H N O (M + 1): m/z 243.1497. Found: m/z 243.1487.
15
19
2
˝
A hydrobromide salt: m.p. 240–242 C. Anal. Calcd for C H N O
‚
2HBr 0.5H O: C, 43.61; H, 5.12;
‚
15
18
2
2
N, 6.78. Found: C, 43.24; H, 5.12; N, 7.15.
-[3-(2-Furyl)-5-methylphenyl]-4-methylpiperazine (64). Yield 15%; H-NMR:
.03 (s, 1H), 6.69 (m, 1H), 6.63 (m, 1H), 6.47 (m, 1H), 3.31 (t, J = 5.0 Hz, 4H), 2.67 (t, J = 5.0 Hz, 4H),
1
1
δ 7.46 (s, 1H), 7.09 (s, 1H),
7
2
5
.42 (s, 3H), 2.36 (s, 3H); 13C-NMR:
4.9, 48.9, 45.8, 21.8. HR-MS (ESI). Calcd for C H N O (M + 1): m/z 257.1654. Found: m/z 257.1651.
δ
154.4, 151.4, 141.7, 139.1, 131.5, 116.6, 116.2, 111.5, 108.9, 104.8,
+
16
21
2
˝
A hydrobromide salt: m.p. 245–249 C. Anal. Calcd for C H N O 2HBr: C, 45.97; H, 5.30; N, 6.70.
Found: C, 45.64; H, 5.26; N, 6.81.
‚
16
20
2
1
1
-[3-n-Butyl-5-(2-furyl)phenyl]-4-methylpiperazine (65). Yield 76%; H-NMR:
δ
7.44 (s, 1H), 7.07 (s, 1H),
7
2
.01 (s, 1H), 6.68 (m, 1H), 6.61 (d, J = 3.2 Hz, 1H), 6.45 q, J = 3.2 Hz, 1H), 3.26 (t, J = 4.8 Hz, 4H),
.59 (t, J = 5.6 Hz, 6H), 2.36 (s, 3H), 1.62 (m, 2H), 1.37 (m, 2H), 0.93 (t, J = 7.2 Hz, 3H); 1 C-NMR:
3

Molecules 2016, 21, 433
19 of 22
δ 154.7, 151.7, 144.4, 141.9, 131.7, 116.1, 116.0, 111.7, 109.2, 105.0, 55.3, 49.3, 46.2, 36.3, 33.8, 22.6, 14.2.
+
HR-MS (ESI). Calcd for C H N O (M + 1): m/z 299.2123. Found: m/z 299.2124. A hydrobromide salt:
19
27
2
˝
m.p. 180–182 C. Anal. Calcd for C H N O
‚HBr: C, 60.16; H, 7.17; N, 7.38. Found: C, 60.37; H, 7.15;
19
26
2
N, 7.54.
1
1
-Methyl-4-[3-(3-thienyl)phenyl]piperazine (66). Yield 25%; H-NMR:
δ
7.42 (t, J = 2.0 Hz, 1H), 7.36 (d,
J = 2.0 Hz, 2H), 7.29 (t, J = 8 Hz, 1H), 7.14 (s, 1H), 7.09 (d, J = 8.0 Hz, 1H), 6.87 (m, 1H), 3.27 (t, J = 4.8 Hz,
4
H), 2.62 (t, J = 4.8 Hz, 4H), 2.38 (s, 3H); 13C-NMR:
δ
151.8, 143.1, 137.0, 129.7, 126.7, 126.2, 120.5,
+
1
18.5, 115.2, 114.6, 55.3, 49.3, 46.3. HR-MS (ESI). Calcd for C H N S (M + 1): m/z 259.1269. Found:
15
19
2
˝
m/z 259.1269. A hydrobromide salt: m.p. 200–205 C. Anal. Calcd for C H N S
H, 4.22; N, 5.66. Found: C, 35.97; H, 4.19; N, 5.66.
‚3HBr: C, 35.95;
15
18
2
1
1
2
6
-Methyl-4-[3-methyl-5-(3-thienyl)phenyl]piperazine (67). Yield 25%; H-NMR:
δ
7.40 (s, 1H), 7.35 (s,
H), 6.95 (s, 1H), 6.92 (s, 1H), 6.70 (s, 1H), 3.25 (t, J = 4.8 Hz, 4H), 2.61 (t, J = 4.8 Hz, 4H), 2.35 (2s,
151.9, 143.1, 139.3, 136.9, 126.8, 126.0, 120.3, 119.4, 116.0, 111.9, 55.3, 49.3, 46.2, 22.0.
HR-MS (ESI). Calcd for C H N S (M + 1): m/z 273.1425. Found: m/z 273.1432. A hydrobromide salt:
1
3
H); C-NMR:
δ
+
16
21
2
˝
m.p. 245–247 C. Anal. Calcd for C H N S
H, 5.23; N, 6.42.
‚
1.5HBr
‚
2H O: C, 44.89; H, 4.85; N, 6.03. Found: C, 44.71;
16
20
2
2
1
1
-[3-n-Butyl-5-(3-thienyl)phenyl]-4-methylpiperazine (68). Yield 23%; H-NMR:
δ
7.40 (t, J = 2.4 Hz,
1
(
H), 7.35 (m, 2H), 6.96 (s, 1H), 6.92 (s, 1H), 6.71 (s, 1H), 3.26 (t, J = 4.8 Hz, 4H), 2.60 (m, 6H), 2.36
s, 3H), 1.62 (m, 2H), 1.39 (m, 2H), 0.93 (t, J = 7.6 Hz, 3H); 1 C-NMR:
3
δ
151.9, 144.5, 143.3, 136.8,
1
26.9, 126.0, 120.3, 118.9, 115.6, 112.1, 55.4, 49.5, 46.3, 36.3, 33.9, 22.7, 14.2. HR-MS (ESI). Calcd for
+
˝
C H N S (M + 1): m/z 315.1895. Found: m/z 315.1910. A hydrobromide salt: m.p. 215–217 C. Anal.
19
27
2
Calcd for C H N S‚2HBr: C, 47.91; H, 5.93; N, 5.93. Found: C, 47.60; H, 6.19; N, 6.20.
19
26
2
1
1
-Methyl-4-[3-(2-thienyl)phenyl]piperazine (69). Yield 23%; H-NMR:
δ 7.27 (m, 3H), 7.15 (s, 1H), 7.10 (d,
J = 8.4 Hz, 1H), 7.06 (m, 1H), 6.86 (m, 1H), 3.26 (t, J = 4.8 Hz, 4H), 2.59 (t, J = 4.8 Hz, 4H), 2.36 (s, 3H);
1
3
C-NMR:
δ
151.7, 144.9, 135.2, 129.6, 127.8, 124.6, 123.0, 117.7, 115.2, 113.8, 55.1, 49.0, 46.1. HR-MS
+
(ESI). Calcd for C H N S (M + 1): m/z 259.1269. Found: m/z 259.1279. A hydrobromide salt: m.p.
15
19
2
˝
>
250 C. Anal. Calcd for C H N S
‚
2HBr: C, 42.87; H, 4.80; N, 6.67. Found: C, 42.55; H, 4.78; N, 6.65.
15
18
2
1
1
-Methyl-4-[3-methyl-5-(2-thienyl)phenyl]piperazine (70). Yield 27%; H-NMR:
δ
7.26 (m, 2H), 7.05 (t,
J = 3.6 Hz, 1H), 6.97 (m, 2H), 6.69 (s, 1H), 3.26 (t, J = 4.8 Hz, 4H), 2.62 (t, J = 4.8 Hz, 4H), 2.34 (s, 3H), 2.36
(
s, 3H); 13C-NMR:
δ
151.7, 145.0, 139.3, 135.1, 127.8, 124.4, 123.0, 118.7, 116.2, 111.1, 55.1, 49.1, 46.1, 21.7.
+
HR-MS (ESI). Calcd for C H N S (M + 1): m/z 273.1425. Found: m/z 273.1434. A hydrobromide salt:
m.p. >250 C. Anal. Calcd for C H N S
H, 5.15; N, 6.61.
16
21
2
˝
‚2HBr
‚
0.5H O: C, 43.36; H, 5.23; N, 6.23. Found: C, 43.47;
16
20
2
2
1
1
-[3-n-Butyl-5-(2-thienyl)phenyl]-4-methylpiperazine (71). Yield 26%; H-NMR:
δ
7.25 (m, 2H), 7.05 (m,
1
3
1
H), 6.97 (t, J = 2.0 Hz, 1H), 6.94, (s, 1H), 6.70 (s, 1H), 3.25 (t, J = 5.2 Hz, 4H), 2.59 (m, 6H), 2.60 (s,
H), 1.59 9M, 2H), 1.37 (m, 2H), 0.93 (t, J = 7.2 Hz, 3H); 13C-NMR:
151.9, 145.4, 144.6, 135.2,127.9,
24.6, 123.1, 118.3, 115.9, 111.5, 55.3, 49.4, 46.3, 36.2, 33.8, 22.7, 14.2. HR-MS (ESI). Calcd for C H N O
δ
19
27
2
+
˝
(M + 1): m/z 315.1895. Found: m/z 315.1885. A hydrobromide salt: m.p. 234–236 C. Anal. Calcd for
C H N S‚1.5HBr: C, 52.36; H, 6.36; N, 6.34. Found: C, 52.01; H, 6.40; N, 6.50.
19
26
2
5
. Conclusions
Piperazin-1-yl substituted unfused heterobiaryls incorporating 4-(3-furyl) moiety demonstrate
high affinities for 5-HT7 receptor. The SAR studies on the model ligand 4-(3-furyl)-2-(4-
methylpiperazin-1-yl)pyrimidine ( ) (K = 7.2 nM) revealed that introduction of an alkyl group to
2
i
position 6 of the pyrimidine leads to increase in the binding affinity, however, a substituent at position 5
is highly detrimental. It was also shown that the pyrimidine core can be replaced with a pyridine
ring without a significant loss of the binding affinity, although piperazine moiety is optimal, as its

Molecules 2016, 21, 433
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replacement results in decreased 5-HT affinity. The selected 6-ethylpyrimidine 12 (K = 7.1 nM) and
7
i
6
-butylpyrimidine 13 (K = 1.6 nM) ligands demonstrated 5-HT antagonistic properties in cAMP
i
7
functional test (K = 12 and 60 nM, respectively). The extended selectivity studies with use of other
B
types of serotonin (5-HT1A, 5-HT2A, 5-HT ) and dopamine D receptors showed that compound 12
6
2
displays also pronounced affinity for 5-HT2A receptors (K = 12 nM), which is consistent with our
i
previous studies of piperazin-1-yl substituted heterobiaryl derivatives. 6-Butylpyrimidine analog
13 lacks selectivity towards 5-HT2A, 5-HT and dopamine D receptors. In summary, piperazin-1-yl
6 2
substituted heterobiaryls constitute a potent group of serotonergic agents that could be further studied
in animal models of CNS disorders.
Acknowledgments: This work was supported by the Program in Brains and Behavior at Georgia State University
(L.S.) and, in part, by the Polish Ministry of Science and Higher Education, Grant No. 2-P05F-019-30 (A.J.B.)
Author Contributions: The structural design of compounds, the design of synthetic routes to compounds, and
analysis of data were conducted by L.S. at Georgia State University. Compounds were synthesized and purified
by his graduate students E.A.R., N.T.F., J.K., S.P., A.L.B., A.J.E., S.B., T.C.B., S.M.S. and his postdoctoral research
associate A.R. A substantial amount of work indicated above was provided by J.S. a visiting professor to the L.S.
research group and subsequently an independent researcher at the Medical University of Gda n´ sk. Compounds
were assayed at the National Institutes of Health, Bethesda, USA and by G.S. in the A.J.B. laboratories using the
same procedures. A.J.B. helped analyze the affinity results. L.S., J.S. and A.J.B. wrote the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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