Rapid access to novel 2-alkylthiopyrimidine derivatives and attempt of their Tacrine analogs synthesis

Article abstract of DOI:10.1080/00397911.2018.1557687

A variety of novel 2-alkylthiopyrimidines were synthesized through simple condensation of arylidenemalononitriles with different 2-alkylthiouronium halide derivatives catalyzed by anhydrous potassium carbonate (K₂CO₃). The reactions have been carried out under mild conditions in i-PrOH, and the products were obtained in moderate to good yields with a simple work-up method. Subsequently, some examples of these compounds have been converted into Tacrine analogs by applying the Friedl?nder reaction.

Full text of DOI:10.1080/00397911.2018.1557687

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Rapid access to novel 2-alkylthiopyrimidine
derivatives and attempt of their Tacrine analogs
synthesis
Chamseddine Derabli, Raouf Boulcina, Gilbert Kirsch & Abdelmadjid
Debache
To cite this article: Chamseddine Derabli, Raouf Boulcina, Gilbert Kirsch & Abdelmadjid Debache
(2019): Rapid access to novel 2-alkylthiopyrimidine derivatives and attempt of their Tacrine analogs
synthesis, Synthetic Communications, DOI: 10.1080/00397911.2018.1557687
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SYNTHETIC COMMUNICATIONS
https://doi.org/10.1080/00397911.2018.1557687
Rapid access to novel 2-alkylthiopyrimidine derivatives and
attempt of their Tacrine analogs synthesis
a
a,b
c
Chamseddine Derabli
, Raouf Boulcina
, Gilbert Kirsch
, and
a
Abdelmadjid Debache
a
Laboratoire de Synth ꢀe se des Mol ꢁe cules d’Int ꢁe r ^e ts Biologiques, Universit ꢁe des Fr ꢀe res Mentouri-
b
c
Constantine, Constantine, Alg ꢁe rie; Facult ꢁe de Technologie, Universit ꢁe Batna 2, Batna, Alg ꢁe rie; SRSMC,
Universit ꢁe de Lorraine, Metz, France
ABSTRACT
ARTICLE HISTORY
A variety of novel 2-alkylthiopyrimidines were synthesized through
simple condensation of arylidenemalononitriles with different
Received 14 September 2018
2
-alkylthiouronium halide derivatives catalyzed by anhydrous potas-
KEYWORDS
sium carbonate (K CO ). The reactions have been carried out under
2
3
Friedl €a nder reaction;
pyrimidine-based hetero-
cycles; S-substituted
thiopyrimidines;
mild conditions in i-PrOH, and the products were obtained in moder-
ate to good yields with a simple work-up method. Subsequently,
some examples of these compounds have been converted into
Tacrine analogs by applying the Friedl €a nder reaction.
Tacrine analogs
Introduction
Pyrimidines are important structural units found in a wide variety of molecules, includ-
[
1]
[2]
ing natural products, substances of biological and pharmaceutical importance, as
[
3]
[4]
[5]
well in therapeutics such as Voriconazole,
Fig. 1). Especially, pyrimidines have also been utilized as means intermediates in medi-
cinal chemistry to produce new chemical entities with a wide range of pharmacological
Crestor, Gleevec
and Avitriptan
(
[
6]
activities. Also, this exceptional structure has been developed as chemotherapeutic
[
7,8]
agents in the treatment of various types of cancer,
and found to be effective as anti-
agents. Extension of various approaches to pyrimidines
is of great interest and many methods for the synthesis of substituted pyrimidines have
[
9]
[10]
microbial and antioxidant
[
11–19]
been reported.
Recently, the oxidative dehydrogenation of 2-alkylthiodihydropyrimidines was found
to produce 2-alkylthiopyrimidines in the presence of a Cu catalyst with no additional
[
20]
reagents.
This procedure was then applied to generate 3,4-dihydropyrimidin-1H-2-
thiones (DHPMs) and 2-arylthiopyrimidines via Cu-catalyzed C–S cross-coupling and
[
21]
concomitant oxidative dehydrogenation.
However, the development of simple, facile and efficient methodologies to get six-
membered heterocycles is one of the major aspects in organic synthesis. This stimulated
CONTACT Raouf Boulcina
r.boulcina@univ-batna2.dz
Laboratoire de Synth ꢀe se des Mol ꢁe cules d’Int ꢁe r ^e ts
Biologiques, Universit ꢁe des Fr ꢀe res Mentouri-Constantine, Constantine, 25000, Alg ꢁe rie.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
ß 2019 Taylor & Francis Group, LLC

2
C. DERABLI ET AL.
Figure 1. Different drugs containing pyrimidine nucleus.
our interest for the synthesis of new fused pyrimidines from simple condensation of
arylidenemalononitrile with different 2-alkylthiouronium halide derivatives.
In our continuous interest for the development of new efficient catalytic synthesis of
[
22]
biologically important heterocycles,
as only few general and highly methods for the
synthesis of S-substituted thiopyrimidines have been reported, in this paper we further
report a simple and effective method for synthesis of 2-alkylthiol-substituted pyrimi-
dines in good yields, employing i-PrOH as solvent and using easily available anhydrous
potassium carbonate, subsequently some examples of these products will be converted
to Tacrine analogs applying the Friedl €a nder reaction.
Results and discussion
Herein, we are interested in the synthesis of new 4-amino-2-(alkylthio)-6-arylpyrimi-
dine-5-carbonitriles 3a–u. In fact, the reactivity of 2-arylidenemalononitriles with differ-
ent 2-alkylthiouronium halides has been studied, relying on the utility of the catalyst in
the reaction. For this purpose, we first synthesized 2-alkylthiouronium halides 1a–d
from the condensation between thiourea and different alkyl halides using the method
[
23]
described by Masquelin et al.
(Scheme 1)
Furthermore, the condensation between various substituted aromatic or heterocyclic
aldehydes and malononitrile in the presence of piperidine as catalyst in ethanol at room
temperature, give rise to the formation of variety of 2-arylidenemalononitriles 2
(Scheme 2).
The optimization of the reaction conditions was carried out on the condensation of
S-methylthiouronium iodide 1a with 2-benzylidenemalononitrile 2a (Scheme 3).
In an initial experiment, a reaction has been performed in ethanol at room tempera-
ture for 24 h. After simple workup, the corresponding product was isolated in very low

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SYNTHETIC COMMUNICATIONS
3
Scheme 1. Synthetic layout for the synthesis of 4-amino-2-(alkylthio)-6-arylpyrimidine-5-carbonitrile
derivatives 3a-u and some of their Tacrine analogs 5a-d.
Scheme 2. Synthesis of 2-alkylthiouronium halides 1a-d.
Scheme 3. Synthesis of arylidenemalonitriles 2a-i.
yield (Table 1, entry 1). Running the same reaction at room temperature in other sol-
vents such as i-PrOH (Entry 2), MeOH (Entry 3), and CH CN (Entry 4) did not
3
improve the reaction yields.
From these initial results, we further optimized the reaction conditions by employing
various catalysts. The results are listed in Table 1. When the same reaction was

4
C. DERABLI ET AL.
Table 1. Optimization of the reaction conditions.
ꢀ
a
Entry
Solvent
Time (h)
Catalyst
Amount of catalyst (mol %)
T ( C)
Yield (%)
1
2
3
4
5
6
7
8
9
EtOH
i-PrOH
MeOH
24
24
24
24
24
24
24
5
5
2
5
2
–
–
–
–
HCl
HCl
–
–
–
25
25
25
25
Traces
Traces
Traces
–
3
CH CN
–
2–3 drops
2–3 drops
10
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
i-PrOH
25
–
–
–
51
54
55
23
42
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
2
ZnCl .H
2
O
DABCO
DMAP
10
10
1
1
1
1
1
a
0
1
2
3
4
Et
3
N
5
2–3 drops
20
2–3 drops
P
2
O
NaOH (1N)
2
1
K
K
2
CO
CO
3
100
100
50
68
2
3
Yields of pure isolated product.
performed at room temperature in EtOH in the presence of a few drops of HCl as cata-
lyst, only a trace amount of the corresponding product 7a was observed, even when the
reaction was carried out under reflux (Entry 6). Similarly, the use of ZnCl .H O was
2
2
not able to afford the desired product (Entry 7). Other catalysts such as DABCO,
DMAP, Et N, P O and NaOH (1N) were also screened. However, all of them gave the
3
2 5
final product in low to moderate yields (entries 8–12).
The use of K CO as catalyst in refluxed ethanol facilitated the reaction to some
2
3
extent but did not exceed the yield of 50% (Entry 13). However, this reaction was most
efficient when using K CO (1 equiv) in i-PrOH under reflux (Entry 14).
2
3
These results show the advantage of using K CO as catalyst with notable improve-
2
3
ment in the reaction giving rise to 4-amino-2-(methylthio)-6-phenylpyrimidine-5-car-
bonitrile 3a. This method was subsequently generalized to various other
2-arylidenemalononitriles and different 2-alkylthiouronium halides, thus allowing the
synthesis of a series of products 3a–u bearing electron donating or withdrawing groups
with moderate to good yields varying between 56 and 68% within 1–2 h in refluxed
i-PrOH (Table 2, entries 1–21). The reactions were remarkably cleans, and no chroma-
tographic separation were required. The use of aromatic or heterocyclic 2-arylidenema-
lononitriles has no significant impact on the conversions. Similarly, various other
2-alkylthiouronium halides have been used with success.
The proposed mechanistic path for this reaction can be suggested first by a condensa-
tion of 2-alkylthiouronium halides with 2-arylidenemalononitriles according to a
Michael type 1,4-addition. Then, intermediate A cyclizes to give rise to a second inter-
mediate B obtained in situ from the reaction, finally complete aromatization in the pres-
ence of K CO provide the desired products (Scheme 4).
2
3
On the other hand, we attempted to extend the process we have taken four randomly
selected derivatives of the previously synthesized 3-amino-2-cyanopyrimidines for use in
the Friedl €a nder reaction. We therefore performed the reaction by applying standard
[
24c]
conditions described in our previous work.
The structures of all the products were
confirmed on the basis of their spectroscopic data, particularly nuclear magnetic reson-
1
13
ance ( H and C NMR) spectroscopy, high-resolution mass spectroscopy (HRMS) and
elemental analysis (Table 3).

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5
Table 2. Synthesis of 4-amino-6-aryl-2-methylthio pyrimidine-5-carbonitrile in ethanol catalyzed
with K CO .
2
3
1
2
a
ꢀ
ꢀ
Lit. Mp ( C)
Entry
Product
R
R
Yield (%)
m.p. ( C)
[
23a]
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
C
6
H
5
H
CH
CH
CH
CH
3
3
3
3
68
65
63
53
61
62
59
54
56
54
61
52
51
70
68
66
65
60
69
62
64
202–204
215–217
207–209
250
210
[23a]
4-ClC
4-(MeO)C
2-(MeO)-1-Naphthyl
6
4
225.5[
211.5
23a]
6 4
H
– [
23b]
23b]
C
6
H
5
H
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
2
2
2
2
4
4
4
4
4
8
8
8
8
8
8
8
8
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
5
177–179
175–177
166–168
218–220
132–134
159–161
151–153
106–108
195–197
105–107
120–122
107–109
116–118
147–149
190–192
146–148
172–174
170–171[
4-ClC
4-(MeO)C
2-(MeO)-1-Naphthyl
6
4
5
191–192
6
H
4
5
–
5
– [
23b]
23b]
C
6
H
5
H
9
128–129[
1
1
1
1
1
1
1
1
1
1
2
2
a
0
1
2
3
4
5
6
7
8
9
0
1
4-ClC
4-(MeO)C
3-Pyridin-C
2-(MeO)-1-Naphthyl
6
4
9
158–159
6
H
4
9
–
–
–
–
–
–
–
–
–
–
–
6
H
4
9
9
C
6
H
5
17
17
17
17
17
17
17
17
4-ClC
4-(MeO)C
4-(MeS)C
4-(NO )C
2-(Cl)-5-(NO
-C
2-(MeO)-1-Naphthyl
6 4
H
6 4
H
6
H
4
2
6
H
)C H
2 6 3
4
3s
3t
3u
C
6
H
5
6 4
H
Isolated yield.
Scheme 4. Proposed mechanism for the S-substituted thiopyrimidines 3.
Table 3. Synthesis of pyrimidine analogs of Tacrine.
1
2
ꢀ
Product
R
R
Yield (%)
m.p. ( C)
5
5
5
5
a
a
b
c
4-ClC
4-ClC
4-(MeO)C
4-(MeO)C
6
H
H
4
4
C H
2 5
C H
2 5
C H
4 9
C H
4 9
47
51
43
40
219–220
232–234
190–192
224–226
6
6
H
H
4
d
6
4
Isolated yield.

6
C. DERABLI ET AL.
Conclusions
We reported in this paper the synthesis of 4-amino-6-aryl-2-alkylthiopyrimidine-5-car-
bonitrile derivatives from the condensation of 2-benzylidenemalononitriles with differ-
ent 2-alkylthiouronium halides. Because of the easy work-up procedure, low cost, and,
especially, good product yields, this method is a useful and attractive procedure for syn-
thesis of S-substituted thiopyrimidines compounds. An attempt to convert some 2-
amino-6-aryl-2-alkylthiopyrimidine-5-carbonitriles to the corresponding Tacrine analogs
applying the Friedl €a nder reaction has been accomplished with success. The continuation
of this project as well as the biological evaluation of these compounds will be the sub-
ject of another paper.
Experimental section
Instruments and materials
All the chemicals were purchased from the Sigma-Aldrich and used without further purifi-
cation. All solvents used for spectroscopic and synthesis studies were reagent grade and
were further purified by literature methods. Melting points were determined on an
1
13
Electrothermal capillary fine control apparatus. H and C NMR spectra were recorded on
a Br u€ ker Avance 400 instrument at 400 and 100 MHz, respectively, and Br u€ ker advance
1
13
DPX 250 (250 MHz for the H, 62.5 for the C), in CDCl or DMSO-d . Chemical shifts
3
6
(d) are given in part per million downfield from TMS as an internal standard and J values
in Hz. High-Resolution Mass spectra were recorded with a MicroTof-Q 98. Elemental anal-
yses were carried out on a Microanalyzer Flash EA1112 CHNS/O Thermo Electron.
[
22]
General procedure for the preparation of 2-alkylthiouronium halides 1a-d
A mixture of thiourea (1 mmol) and alkyl halides (1.2 mmol) in EtOH (5 mL) was
stirred overnight under reflux, the mixture was cooled to room temperature and evapo-
rated. The residue was suspended in Et O and filtered. The final product was used in
2
the next step without any additional purification.
2
-Octylthiouronium bromide (1d)
ꢀ
1
Yield: >95%; white solid; m.p.: 98–100 C; H NMR (250 MHz, CDCl ) d (ppm) 8.48 (s,
3
2
1
3
H, NH ), 3.29 (t, J ¼ 7.2 Hz, 2H, S–CH ), 1.71 (qt, J ¼ 7.4 Hz, 2H, CH ), 1.46–1.28 (m,
2
2
2
1
3
0 H), 0.89 (t, J ¼ 6.8 Hz, 3H). C NMR (62.5 MHz, CDCl ) d (ppm) 172.2, 119.9, 32.3,
3
1.9, 29.2, 28.7, 28.4, 22.8, 14.3.
General procedure for the preparation of 4-amino-6-aryl-2-methylthio pyrimidine-
-carbonitrile 3a–u
5
In a 100 mL flask was introduced 2-alkylthiouronium halides 1 (1 mmol) and 2-benzyli-
denemalononitriles derivatives 2 (1 mmol) with 1 mmol of K CO in refluxed i-PrOH
2
3
(10 mL). The mixture was cooled to room temperature (the progress of the reaction was

V
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SYNTHETIC COMMUNICATIONS
7
monitored by TLC). Then the mixture was poured onto 100 mL of water. The obtained
precipitate was filtered on B u€ chner and the resulting solid was recrystallized
from ethanol.
4
-Amino-2-(butylthio)-6-(4-methoxyphenyl)pyrimidine-5-carbonitrile (3k)
ꢀ
1
Yield: 61%; white solid; m.p.: 151–153 C; H NMR (400 MHz, CDCl ) d (ppm) 7.98
3
(
3
dd, J ¼ 8.8, 2.0 Hz, 2H), 6.94 (dd, J ¼ 8.8, 2.0 Hz, 2H), 5.54 (br s, 2H, NH ), 3.81 (s,
2
H, O–CH ), 3.10 (t, J ¼ 7.6 Hz, 2 H, S–CH ), 1.65 (qt, J ¼ 7.6 Hz, 2H, CH ), 1.42 (qt,
3
2
2
13
J ¼ 7.6 Hz, 2H, CH ), 0.89 (t, J ¼ 7.2 Hz, 3H, CH ). C NMR (100 MHz, CDCl ) d
2
3
3
(ppm) 175.1, 166.2, 163.6, 162.4, 130.5, 128.1, 117.0, 114,0, 81.9, 55.5, 31.3, 30.8, 22.0,
þ
þ.
1
3.7. HRMS (ESI ) m/z 337.1101 [M þ Na] , Calcd for C H N ONaS: 337.1099.
16
18 4
General procedure for the preparation of compounds 5a-d
In a 100 mL round bottom flask, aluminum chloride (2 mmol) were added into adequate
volume of dry 1,2-dichloroethane (DCE) and stirred at reflux for few minutes. Then, a
mixture of 4-amino-6-aryl-2-methylthio pyrimidine-5-carbonitrile 3 (1 mmol), and
cyclohexanone 4 (2 mmol) were added into the reaction. The mixture was continuously
stirred for 24 h (monitored by TLC) at constant temperature. After completion, solvent
was evaporated and water was added and the mixture was basified with 10% sodium
hydroxide solution to pH = 8–9. After stirring for 30 min, the precipitate was filtered
and washed with water. Purification by column chromatography on silica gel (ethyl ace-
tate:cyclohexane = 3:1, v/v) afforded the title compounds 5a–d.
2
-(Butylthio)-4-(4-chlorophenyl)-6,7,8,9-tetrahydropyrimido[4,5-b]quinolin-5-
amine (5b)
ꢀ
1
Yield: 51%; yellow solid; m.p.: 232–234 C; H NMR (400 MHz, CDCl ) d (ppm) 7.44
3
(
m, 4H), 4.56 (br s, 2H, NH ), 3.28 (t, J ¼ 7.2 Hz, 2H),2.98 (t, J ¼ 6.0 Hz, 2H), 2.33 (t,
2
J ¼ 6.0 Hz, 2H), 1.84–1.81 (m, 4H),1.68 (qt, J ¼ 6.0 Hz, 2H), 1.41 (qt, J ¼ 6.0 Hz, 2H),
1
3
0
1
.86 (t, J ¼ 7.2 Hz, 3H). C NMR (100 MHz, CDCl ) d (ppm) 170.0, 165.9, 165.6, 158.4,
3
49.2, 137.5, 136.7, 130.1, 129.5, 111.9, 104.0, 33.8, 30.9, 23.5, 22.3, 22.2, 22.0, 13.8.
þ
þ.
HRMS (ESI ) m/z 399.1437 [M þ H] , calcd for C H ClN S: 399.1410.
21
24
4
Funding
We thank “le Minist ꢀe re de l’Enseignement Sup ꢁe rieur et de la Recherche Scientifique” (Algeria)
for the financial supports.
ORCID
Chamseddine Derabli
http://orcid.org/0000-0002-5283-2058
Raouf Boulcina
Gilbert Kirsch
http://orcid.org/0000-0001-9102-3854
http://orcid.org/0000-0001-8500-1755
Abdelmadjid Debache
http://orcid.org/0000-0002-0152-993X

8
C. DERABLI ET AL.
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