Exploring the scope of the Gewald reaction: Expansion to a four-component process

Article abstract of DOI:10.1016/j.tetlet.2017.02.071

An efficient four-component reaction was developed to take advantage of the reactivity of the 2-aminothiophene-3-carbonitrile functionality, which is obtained during the classical three-component Gewald reaction. Various α-methylene bearing ketones were reacted with malononitrile, elemental sulfur, and aryl/heteroarylnitrile derivatives in t-BuOH/NaOH to afford 2-arylthieno[2,3-d]pyrimidin-4-amines in high yields. Preliminary studies revealed the photophysical properties of the products and their potential for use as metal sensors.

Full text of DOI:10.1016/j.tetlet.2017.02.071

Accepted Manuscript
Exploring the scope of the Gewald reaction: expansion to a four-component
process
M. Saeed Abaee, Atefeheh Hadizadeh, Mohammad M. Mojtahedi, Mohammad
R. Halvagar
PII:
S0040-4039(17)30258-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.02.071
TETL 48682
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
6 February 2017
16 February 2017
23 February 2017
Please cite this article as: Abaee, M.S., Hadizadeh, A., Mojtahedi, M.M., Halvagar, M.R., Exploring the scope of
the Gewald reaction: expansion to a four-component process, Tetrahedron Letters (2017), doi: http://dx.doi.org/
10.1016/j.tetlet.2017.02.071
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Graphical Abstract
Exploring the scope of the Gewald reaction:
expansion to a four-component process
Leave this area blank for abstract info.
M. Saeed Abaee, Atefeh Hadizadeh, Mohammad M. Mojtahedi and Mohammad R. Halvagar

Exploring the scope of the Gewald reaction: expansion to a four-component process
M. Saeed Abaee ∗, Atefeheh Hadizadeh, Mohammad M. Mojtahedi and Mohammad R.
Halvagar
Department of Organic Chemistry and Natural Products, Chemistry and Chemical
Engineering Research Center of Iran, Pajohesh Blvrd., 17th km of Tehran-Karaj Highway,
P.O.Box 14335-186, Tehran, Iran
Keywords: Gewald reaction; Four-component reaction; Heterocycles; Aminothiophenes;
Fluorescence
ABSTRACT
An efficient four-component reaction was developed to take advantage of the
reactivity of the 2-aminothiophene-3-carbonitrile functionality, which is obtained during
the classical three-component Gewald reaction. Various α-methylene bearing ketones
were reacted with malononitrile, elemental sulfur, and aryl/heteroarylnitrile derivatives in
t-BuOH/NaOH to afford 2-arylthieno[2,3-d]pyrimidin-4-amines in high yields.
Preliminary studies revealed the photophysical properties of the products and their
potential for use as metal sensors.
The development of multicomponent reactions (MCRs) has gained considerable
popularity among synthetic chemists due to ease of operation, selective preparation of
target compounds, economic aspects, and decreased waste production.1 The development
of new methodologies and improved operational techniques has dramatically expanded
the scope of MCRs from simple three-component reactions2 to more complex processes
engaging a broader range of reactants,3 leading to the synthesis of target molecules4,1a and
libraries of compounds5 with various biological6 and industrial properties.7
One prominent method for further development of a MCR to give complex
products which would otherwise be obtained via multistep processes,8 is to take
advantage of the functionalities already residing in the MCR product, allowing extra
reactants to participate in the process. An appropriate platform to probe the feasibility of
this approach is the Gewald reaction,9 a three-component process utilising α-methylene
bearing ketones, malononitrile, and elemental sulfur to produce 2-aminothiophenes
containing reactive neighboring amino and nitrile functional groups for potential use in
further cyclisation reactions.
The products of the Gewald three-component reaction have been extensively
explored in further synthetic manipulations10 to prepare compounds with desirable
properties11 or access more complex heterocyclic systems.12 However, this has been
mainly limited to step-wise procedures and only a few related one-pot methods involving
a fourth reactant have been reported.13 Some typical structures containing the thieno[2,3-
∗ Corresponding author. Tel.: +98 21 44787749; fax: +098 21 44787785; e-mail: abaee@ccerci.ac.ir

d]pyrimidin-4-amine fragment which were obtained from 2-aminothiophene precursors
via step-wise routes are shown in Figure 1. These compounds exhibit various biological14
and medicinal15 features and thus, their one-pot synthesis taking advantage of the Gewald
products reactivity could be of interest and lead to new approaches using more efficient
four-component processes.
Figure 1. Examples of important structures obtained via step-wise synthetic manipulation
of Gewald products.
As part of our interest in the development of new multicomponent methods,16 we
elected to expand the scope of the Gewald reaction by engaging the two vicinal amino
and nitrile functional groups in further reactions. Herein, we report the four-component
reaction of various α-methylene bearing ketones, malononitrile, elemental sulfur, and
aryl/heteroarylnitrile derivatives to efficiently obtain thienopyrimidine compounds via a
single synthetic operation.
Scheme 1. Step-wise vs multicomponent approaches for the synthesis of 2-
phenylthieno[2,3-d]pyrimidin-4-amine 4a.
Initially, the two steps were optimized independently. Thus, the model reaction
(Scheme 1) between cyclohexanone 1a, 2, and S8 gave compound 5 in 90% yield after 75
min when conducted in t-BuOH/Et3N at rt (Table 1, entry 1). A similar outcome was
observed with shorter times when Et3N was replaced with either NaOH or K2CO3
(Entries 2–3), while t-BuOK was also appropriate when used in substoichiometric
amounts, due to the suppression of undesired side-reactions (Entries 4–5). Other protic

(Entries 6–8) or non-protic solvents (Entries 9–10) demonstrated that t-BuOH was the
optimum solvent.
Table 1. Optimization for the synthesis of 5 from 1, 2, and S8.
Entry Conditionsa Base (equiv) Time (h) Yield 5 (%)b
1
2
3
4
5
6
7
8
9
10
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
n-BuOH
i-PrOH
EtOH
Et3N (1.0)
1.25
0.5
0.75
0.5
0.5
1.0
0.75
3.0
24
90
94
91
63
92
90
88
86
32
0
NaOH (1.0)
K2CO3 (1.0)
t-BuOK (1.0)
t-BuOK (0.5)
NaOH (1.0)
NaOH (1.0)
NaOH (1.0)
NaOH (1.0)
NaOH (1.0)
DMSO
PhMe
24
a
Reagents and conditions: 1a (2.0 mmol), 2 (2.0 mmol), S8 (2.0 mmol), solvent (4 mL),
rt.
b GC yield.
Similar optimization reactions were explored for the conversion of 5 to 4a using t-
BuOH as the solvent (Table 2). Although t-BuOK still acted as an appropriate base
(Entries 1–2), Et3N (Entries 3–4) or K2CO3 (Entries 5–6) were either not active or less
effective for the cyclisation of 5 with benzonitrile 3a. The use of NaOH gave 4a in 95%
yield, but with extended reaction times at rt (Entry 7). Gratifyingly, quantitative
formation of 4a was observed within 3 h at 50 °C, making NaOH the base of choice
(Entry 8).
Table 2. Optimization for the conversion of 5 to 4a.
Entry Base (1.0 equiv)a T (oC) Time (h) Yield 4a (%)b
1
2
3
4
5
6
7
8
t-BuOK
t-BuOK
Et3N
25
50
25
50
25
50
25
24
40
92
0
1.75
24
Et3N
12
0
K2CO3
K2CO3
NaOH
NaOH
24
20
54
95
24
24
50
3
99
a Reagents and conditions: 5 (2.0 mmol), 3a (4.0 mmol), t-BuOH (4 mL).
b Isolated yield.
With these results in hand, we sought to further improve the process by using all
the reactants simultaneously in a four-component reaction, as opposed to a tandem

sequence. Thus, implementation of this approach gave 4a in 95% yield (Table 3, entry 1).
Consequently, the generality of this one-pot process was evaluated by conducting the
reactions of analogues of 1. Cyclohexanone homologues 1b–c reacted equally well to
produce the respective products 4b–c in high yields (Entries 2–3). Similarly, heterocyclic
analogues were amenable to the reaction conditions, producing 4d–f in 82-85% yield
(Entries 4–6).
Table 3. Synthesis of compounds 4a-f.
Entry Ketone X
Product Time (h) Yield 4 (%)a,b
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
CH2
4a
3.5
4.0
4.5
3.5
3.5
4.0
93
86
78
84
82
85
(CH2)2 4b
(CH2)3 4c
S
4d
4e
O
NMe 4f
a
Reagents and conditions: 1 (2.0 mmol), 2 (2.0 mmol), S8 (2.0 mmol), 3a (4.0 mmol),
NaOH (4.0 mmol), t-BuOH (4 mL).
b Isolated yield.
To further evaluate the generality of the procedure, additional ketones were
subjected to the reaction conditions (Fig. 2). The use of 1,4-dioxaspiro[4.5]decan-8-one
or dimedone led to the selective formation of compounds 4g–h in good yields. Similarly,
the reactions of cyclopentanone, aryl methyl ketone derivatives, or diethyl ketone
proceeded equally well to give compounds 4i–l. Additionally, when the unsymmetrical
reactant methyl isobutyl ketone was subjected to the reaction, a 2.5:1 mixture of 4m and
4m' in an overall yield of 80% was obtained.17 The structures of the products were
determined by spectroscopic methods, and in the cases of 4e and 4m, were confirmed by
single crystal X-ray analysis (Fig. 3).

Figure 2. Synthesis of 4g-m′. Reagents and conditions: ketone (2.0 mmol), 2 (2.0 mmol),
S8 (2.0 mmol), 3a (4.0 mmol), NaOH (4.0 mmol), t-BuOH (4 mL), 50 °C.
Figure 3. ORTEP representation of 4e (top) and 4m (bottom). Thermal ellipsoids set at
the 40% probability level.18
The process was further developed using nitrile derivatives possessing different
electronic natures. As shown in Table 4, the reaction was amenable to the use of 3b-f,
producing products 4n–t in 75-85% yield.
Table 4. Synthesis of 4 using benzonitriles 3b-f.

Entry Ketone [X] ArCN (Y, Z)
Product Time (h)
Yield 4 (%)a,b
1
2
3
4
5
6
7
1b [(CH2)2] 3b (N, H)
1c [(CH2)3] 3b (N, H )
4n
4o
4p
4
5
4
7
5
7
75
78
78
82
83
85
1a [CH2]
1a [CH2]
3b (N, H )
3c (CH, 4-MeO) 4q
1b [(CH2)2] 3d (CH, 3-Cl)
4r
4s
4t
1a [CH2]
1a [CH2]
3e (CH, 4-Me)
3f (CH, 4-Br)
8
80
a
Reagents and conditions: ketone (2.0 mmol), 2 (2.0 mmol), S8 (2.0 mmol), 3 (4.0
mmol), NaOH (4.0 mmol), t-BuOH (4 mL).
b Isolated yield.
There are limited reports on the fluorescence properties of products containing the
thieno[2,3-d]pyrimidine structure.19 For preliminary evaluation of this aspect, 4e was
selected due to the pyran ring in its structure which is important from biological20 and
natural product points of view.21 Compound 4e exhibited two significant absorption
bands at 231 and 325 nm in the UV−visible spectrum (Fig. 4a). When excited at 325 nm,
a relatively strong red-shifted emission (20 nm) at ~390−410 nm was observed. Similar
analyses in other solvents showed maximum emission for the MeCN solution (Fig. 4b).
Additionally, the fluorescence intensity was shown to increase using lower
concentrations of 4e, showing maximum emission at 10 µM and demonstrating a self-
quenching fluorescence effect without any extra band on the red side (Fig. 4c).
An area of intense studies in recent years has involved the design of fluorescence
sensors for various ions and molecules.22 Regarding our products, preliminary results
were obtained for the titration of product 4n with various Pd2+ concentrations (Fig. 4d).
While a MeCN solution of 4n showed three main absorption bands at 221, 244, and 332
nm, the addition of Pd2+ (in 9:1 MeCN/H2O) caused the band intensities to gradually
decrease. Meanwhile, a new absorption band at 378 nm with an isosbestic point at 362
nm formed which at higher concentrations of Pd2+ was accompanied by a color change
from colorless to yellow that was easily detectable by the naked eye.

Figure 4. (a) UV-vis absorption (100 µM) in CH2Cl2; (b) emission spectra of 4e in
various solvents (10 µM) at 298 K; (c) emission spectra of 4e in CH2Cl2 (1000 to 10-4
µM) at 298 K; (d) absorption spectra of 4n in MeCN, in the presence of various Pd2+
concentrations.
Conclusion
In summary, we report a four-component Gewald reaction which incorporates an
extra reactant in the process in order to take advantage of the reactivity of the two
neighboring functional groups found in the typical three-component products. This
reaction is currently being developed using more diverse reactants. Detailed study of the
photophysical behavior of the products and their possible use as metal sensors are also
underway.
Acknowledgments
The Analytical Department is acknowledged for conducting some of the analyses.
Supplementary Material
Supplementary data (spectra and characterization of all products) associated with this
the
article
http://dx.doi.org/10.1016/j.tetlet.2016.XXXX.
can
be
found,
in
online
version,
at

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Highlights
• A four-component approach was developed for the Gewald reaction employing an
extra reactant.
• Various ketones were reacted with malononitrile, S8 and ArCN derivatives.
• Selected products were studied for their photophysical properties.