An efficient, one-pot, green synthesis of tetracyclic imidazo[2,1- b ]thiazoles via electrochemically induced tandem heteroannulation reactions

Article abstract of DOI:10.1002/jhet.959

A series of novel catechol-fused tetracyclic compounds, with an imidazo[2,1-b]thiazole central core, were successfully synthesized through the anodic oxidation of catechols in the presence of 2-mercaptobenzimidazole in aqueous solution. The cyclic voltammetric results indicate that a one-pot four-step sequential reaction occurs between 2-mercaptobenzimidazole and the electrochemically derived o-benzoquinones affording fused polyheterocyclic compounds. The mechanism of this catalyst-free, domino reaction is proved as an ECEC pathway using controlled-potential coulometry. In addition, the electrosyntheses of fused compounds have been successfully performed in ambient conditions in an undivided cell using an environmentally friendly method with high atom economy. The structures of products were characterized by FT-IR, ¹H NMR, ¹³C NMR, and HRMS spectrometric methods.

Full text of DOI:10.1002/jhet.959

January 2013
An Efficient, One‐Pot, Green Synthesis of Tetracyclic
23
Imidazo[2,1‐b]Thiazoles via Electrochemically Induced Tandem
Heteroannulation Reactions
a,b
Mohammad M. Khodaei,a,b Abdolhamid Alizadeh,
*
*
and Tayyebeh Kanjouria,b
aDepartment of Chemistry, Razi University, Kermanshah 67149, Iran
bNanoscience & Nanotechnology Research Centre (NNRC), Razi University,
Kermanshah 67149, Iran
*
E-mail: mmkhoda@razi.ac.ir or ahalizadeh2@hotmail.com
Received November 23, 2010
DOI 10.1002/jhet.959
View this article online at wileyonlinelibrary.com.
A series of novel catechol‐fused tetracyclic compounds, with an imidazo[2,1‐b]thiazole central core, were
successfully synthesized through the anodic oxidation of catechols in the presence of 2‐mercaptobenzimida-
zole in aqueous solution. The cyclic voltammetric results indicate that a one‐pot four‐step sequential reaction
occurs between 2‐mercaptobenzimidazole and the electrochemically derived o‐benzoquinones affording
fused polyheterocyclic compounds. The mechanism of this catalyst‐free, domino reaction is proved as an
ECEC pathway using controlled‐potential coulometry. In addition, the electrosyntheses of fused compounds
have been successfully performed in ambient conditions in an undivided cell using an environmentally
friendly method with high atom economy. The structures of products were characterized by FT‐IR, 1H
NMR, 13C NMR, and HRMS spectrometric methods.
J. Heterocyclic Chem., 50, 23 (2013).
INTRODUCTION
these structures would be of great interest. Catechols are
important species in chemistry and biology, because of their
facility in undergoing electron transfer [16] and involved in
the primary photoelectron donor–acceptor center in bacterial
photosynthesis [17]. The most important property of catechols
is the ease in which they undergo redox transformations; a very
important physiological reaction. Catechol species are electro-
chemically oxidized to the corresponding quinines, and these
can undergo a nucleophilic attack by the nucleophilic species
through a 1,4‐Micheal addition pathway [18,19].
Electric current is one of the cleanest tools in transfor-
mation of organic molecules [20], and accordingly, several
electrochemical techniques have provided a variety of
highly reactive intermediates affording useful and/or novel
organic compounds without the use of any environmen-
tally undesirable catalysts. In particular, direct electro-
chemical oxidation/reduction of substrates utilizes
practically mass‐free electrons as the only reagents. In this
sense, electrochemistry is frequently referred to as one of
the prototypical green procedures for synthesizing various
organic molecules and structures [21].
In recent years, the chemistry and synthesis of polycyclic
compounds possessing an imidazo[2,1‐b]thiazole central core
(Fig. 1, I) has been the focus of great interest [1–5]. This is, in
part, due to the broad spectrum of biological properties of
these compounds. Several imidazo[2,1‐b]thiazole derivatives
have been reported in the literature as antibacterial [6], anti-
fungal [7], antihelminthic [8,9], and antitumor [10–12]
agents. For instance, the imidazo[2,1‐b]thiazole system con-
stitutes the main part of the well‐known antihelminthic and
immunomodulatory agent levamisole (II). A series of imi-
dazo[2,1‐b]thiazole guanyl hydrazones (III) have been
proved to be active against various cancer cell lines [13].
Generally, the synthetic routes to imidazo[2,1‐b]thiazoles
are as following: (i) alkylations of cyclic thioureas by
appropriate 1,2‐dielectrophiles [14] or (ii) condensations of
2‐aminothiazoles with 1,2‐difunctionalized units, such as
α,β‐unsaturated carbonyl compounds [15]. Although several
synthetic strategies are available, but these suffer from draw-
backs including long reaction periods, toxic solvents, and/or
metal additives, and therefore, the facile and green routes to
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M. M. Khodaei, A. Alizadeh, and T. Kanjouri
Vol 50
Scheme 1
Figure 1. Some typical heterocycles with an imidazo[2,1‐b]thiazole
moiety.
In continuation of our efforts to develop more versatile
and convenient electrochemical and chemical synthesis of
highly functionalized heterocycles [22], here, we report
the electrosynthesis of a series of tetracyclic catechol‐fused
benzimidazo[2,1‐b]thiazoles (Fig. 1, IV) through a one‐pot
electrooxidative‐coupling sequential fashion. The present
protocol provides an efficient procedure via an ECEC electro-
chemical mechanism entry to construct nitrogen‐ and sulfur‐
containing heterocycles of type benzimidazo[2,1‐b]thiazoles,
a relatively rare fused‐ring system. To the best of our knowl-
edge, this is the first report aimed at the synthesizing of tetra-
cyclic adducts bearing imidazo[2,1‐b]thiazole and catechol
moieties utilizing electrochemical techniques.
RESULTS AND DISCUSSION
product at the surface of the electrode, inhibiting to a cer-
tain extent the performance of the electrode process [23].
In addition, similar to the oxidation of 2‐mercaptobenzoxa-
zole, which leads to the formation of bis(benzoxazolyl) disul-
fide [24], 2‐mercaptobenzimidazole (3) can be oxidized to the
corresponding disulfide [Fig. 2(I), curve c], and the anodic
peak (A2) appeared at 0.53 V can be assigned for the oxidation
of 3. In addition, the SH and NH groups of 3 are appropriate
nucleophiles, so it seems that a Michael 1,4‐addition of 3 to 2a
can proceed in a quick and simple way leading to the consid-
erable decrease in the height of cathodic peak (C1).
Initial characterization of the reaction was carried out
using cyclic voltammetry. Cyclic voltammetric responses
of a GC electrode to 1 mM catechol (1a) (Scheme 1) in
water/acetonitrile (90/10) solutions containing 0.2M sodium
acetate in the absence and presence of 2‐mercaptobenzimida-
zole (3) are outlined in Figure 2(I).
Curve a in Figure 2(I) shows a well‐defined oxidative
peak (A1) at 0.5 V (vs. SCE) consistent with the oxidation
of 1a to the o‐benzoquinone species (2a). Upon reversal of
the scan direction, a corresponding reduction peak (C1) is
observed at 0.05 V, which is attributed to back reduction
The cyclic voltammetries of 1a in the presence of 3 in
various sweep rates show that anodic peak current is in-
of 2a to the parent catechol (Scheme 1). A peak current ra-
tio (IpC =Ip ) of nearly unity, especially during the repetitive
A1
creased with the addition of potential sweep rate [Fig. 2
1
C1
(II)]. Also, a plot of peak current ratio (IpA =Ip ) versus log ν
1
recycling of the potential can be observed for 1a in the ab-
sence of 3, and this ratio can be considered as a criterion
for the stability of o‐benzoquinone generated at the surface
of GC electrode under the applied experimental conditions.
In Figure 2(I), curve b shows the cyclic voltammogram
obtained for a 1 mM solution of 1a in the presence of 1
mM of 3. The voltammogram exhibits one anodic and
one decreased cathodic peaks appeared at 0.56 and −0.04
V versus SCE, respectively. The observed slightly shifts
(positive for A1 and negative for C1 peaks) in the presence
of 3 is probably due to the formation of a thin film of
(not shown), for a mixture of 1a and 3 confirmed the reac-
tivity of o‐benzoquinone 2a toward mercaptide anion,
appearing as an increase in the height of the cathodic peak
C1 at higher scan rates. A similar situation was observed
when the concentration of 1a was increased relative to 3
[Fig. 3(I)]. On the other hand, the current function for the
A1 peak, (IpA =υ1=2) decreased on increasing the scan rate.
1
It is well known that such behaviors are indicative of an
ECEC mechanism [25]. We also studied the effect of
solution’s pH on the electrochemical oxidation of 1a in
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An Efficient, One-Pot, Green Synthesis of Tetracyclic Imidazo[2,1-b]Thiazoles via
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Electrochemically Induced Tandem Heteroannulation Reactions
Figure 3. (I) Cyclic voltammograms obtained for the various ratio of the
concentration of 1a to 3. (II) Cyclic voltammograms of 1a in the presence
of 3 obtained at various pHs: (a) 2, (b) 4, (c) 7, and (d) 9, at glassy carbon
electrode versus SCE in water/acetonitrile (90/10) solution containing
0.2M sodium acetate. t = 25 1°C. (e) CV of 1a in the absence of 3 at
pH = 7 is given for comparison.
Figure 2. (I) Cyclic voltammograms of 1.0 mM 1a in the absence (a) and
presence (b) of 1.0 mM of 3 and (c) 1.0 mM of 3 in the absence of 1a at
glassy carbon electrode versus SCE in water/acetonitrile (90/10) solution
containing 0.2M sodium acetate. Scan rate: 100 mV s−1; t = 25 1°C.
(II) Scan rates for (a)–(d) are: 50, 100, 200, and 500 mV s−1
.
the presence of 3. Although, similar decreasing in the cur-
rent density for the cathodic peak (indicating the reactivity
of 2a toward 3) was observed in all pHs [Fig. 1(II)], how-
ever, it was found that the easier oxidation (less potential)
of 1a to 2a can be occurred at pH = 7.
Considering the closeness of oxidation potential peaks of
1a and 3 [A1 and A2 in Fig. 2(I), curves a and c], to minimize
the oxidation of 3 and hence, achieving higher selectivity and
efficiency, we applied 0.25 V potential versus SCE in both
coulometry and preparative synthesis processes.
4e− per molecule of 1a. These results confirm the proposed
ECEC mechanism for this electrochemically induced reac-
tion and allow us to suggest the pathways shown in
Scheme 1 for the electrooxidation of 1a in the presence
of 3. According to our results, it seems that anodic oxida-
tion process of 1a to 2a is followed by an intermolecular
Michael‐type reaction of 3 with 2a, and this event seems
to occur much faster than other side reactions, leading to
the formation of intermediate 4a.
The over‐oxidation of 4a is easier than the oxidation of
the parent‐starting molecule by virtue of the presence of
an electron‐donating group and hence, following the oxida-
tion of 4a to 5a, an intramolecular Michael‐type heteroan-
nulation of 5a occurs to give 6a. Although the final product
can be oxidized at a lower potential than 1a, however,
over‐oxidation of 6a was circumvented during the prepara-
tive reaction because of the insolubility of the products in
To determine electrochemical efficiency, controlled‐po-
tential coulometry of 1a in the presence of 3 was per-
formed at 0.25 V versus SCE. On the basis of obtained
results, the electrochemical efficiency is >88%. The mon-
itoring of electrolysis progress was carried out using cyclic
voltammetry. It was shown that, proportional to the prog-
ress of coulometry, the anodic peak (A1) decrease and
disappears when the charge consumption becomes about
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M. M. Khodaei, A. Alizadeh, and T. Kanjouri
Vol 50
Scheme 2
the water/acetonitrile medium. As it can be seen, there is an
electrooxidative/intermolecular Michael‐type addition/
electrooxidative/intramolecular heteroannulation sequence
(ECEC) in this one‐pot four‐step coupling reaction of 1a
with 3 leading to the formation of a novel catechol‐fused
tetracyclic compound (6a), with an imidazo[2,1‐b]thiazole
central core in excellent yield (92%).
In examining the scope and generality of the developed
protocol as well as the influence of structural variation of
catechol ring on the reactivity of o‐benzoquinones toward
3, we studied the electrochemically induced reaction of
some other catchols bearing electron‐withdrawing or elec-
tron‐donating groups at the C‐3 or C‐4 positions (1b–f).
The electrooxidation of 3‐methylcatechol (1b) and 3‐meth-
oxycatechol (1c) in the presence of 3 proceed in a similar
fashion to that of 1a. Also notably, the existence of a
methyl or methoxy group at the C‐3 position of 1b or 1c
may have subtle effects on the reactivity of their relevant
o‐bezoquinones and would probably cause these Michael
acceptors (2b and 2c, Scheme 1) to be attacked by 3 from
the C‐5 or C‐4 positions. Because the methyl and methoxy
groups are both electron‐donating substituents, we think
that o‐benzoquinones 2b and 2c are more electropositive
at C‐5 position and therefore, can be selectively attacked
in all probability only at the C‐5 position by 3 leading to
the formation of intermediates 4b and 4c (Scheme 1), re-
spectively. As depicted in Scheme 1, similar to that of
4a, further oxidation of these intermediates is followed
by an intramolecular Michael‐type heteroannulation which
leads to the final products. These one‐pot four‐step oxidative
coupling reactions (ECEC) of 1b and 1c with 3 lead to the
formation of novel tetracyclic benzimidazo[2,1‐b]thiazole
compounds in high yields (Scheme 1: 6b: 82%; 6c: 88%).
On the other hand, anodic oxidation of 3,4‐dihydroxy-
benzoic acid (1d) with an electron‐withdrawing group, in
the presence of 3 proceeds in a nearly different manner
than that of 1a–c. Oxidation of 1d to 2d (Scheme 2), and
subsequent intermolecular Michael addition reaction of 3
with 2d (at the more electropositive C‐5 position) leads
to adduct 4d which is capable of undergoing further
oxidation to afford intermediate 5d. An intramolecular
1,4‐addition (Michael) reaction in 5d followed by an electro‐
decarboxylation process [26, 27] can lead to the final product.
As it can be seen, this unique sequential heteroannulation
reaction of 1d with 3 leads to the formation of a novel
catechol‐fused tetracyclic benzimidazo[2,1‐b]thiazole
compound (6d) in excellent yield (90%).
As mentioned earlier, any decrease observed in the cur-
rent density for the cathodic peak in cyclic voltammogram
of catechol relates to the reactivity of electrochemically de-
rived o‐benzoquinone toward nucleophile. Compared with
that of 1a, cyclic voltammograms of 1e and 1f in the pres-
ence of 3 show less decrease in the current density for the
cathodic parts. These results prove that 2e and 2f are
weaker Michael‐acceptor than 2a and therefore, have less
reactivity in the Michael‐addition reaction toward mercap-
tide anion of 3, appearing as an increase in peak current ra-
A1
tio (IpC =Ip ). More interestingly, contrast to the cases of
1
4a–d, no further oxidations and subsequent heteroannula-
tion reaction were observed in the cases of 4e and 4f, and
these adducts (catechol‐benzimidazole thioetheres) were
obtained as the final products in moderate yields (4e:
68%; 4f:54%). In addition to insolubility of 4e and 4f in
water/acetonitrile media, one can assume that the presence
of a methyl or tert‐butyl group at the C‐4 position of these
adducts would probably cause a streic inhibition of the
accessibility of their C‐4 position to nucleophilic nitrogen
atom of adjacent benzimidazole moiety and therefore, there
is no possibility for the subsequent heteroannulation reaction.
CONCLUSIONS
The effect of an electron‐donating group located at the
reactive site of o‐benzoquinone (C‐4 or β‐position to car-
bonyl group) on its reactivity was also investigated in some
detail. The electrochemical oxidations of 4‐methylcatechol
(1e) and 4‐tert‐butylcatechol (1f; Scheme 1) were studied
in the presence of 3 in water/acetonitrile (90/10) solution
containing 0.20M sodium acetate.
In conclusion, we have described a one‐pot, environ-
mentally friendly and reagent‐less protocol for the prepara-
tion of a series of polyfunctional tetracyclic benzimidazo
[2,1‐b]thiazoles through a domino reaction of commercially
available starting materials. Besides the high efficiency and
atom economy as a domino process, this reaction has the fol-
lowing advantages: (1) reaction proceeds smoothly under
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An Efficient, One-Pot, Green Synthesis of Tetracyclic Imidazo[2,1-b]Thiazoles via
27
Electrochemically Induced Tandem Heteroannulation Reactions
1
1529, 1446, 1257, 1196, 739; H NMR (200 MHz, DMSO‐d6): δ
= 4.27 (broad, OH), 6.94 (s, 1H, Har), 7.11 (s, 1H, Har) 7.17–7.55
(m, 4H, Har); 13C NMR (50 MHz DMSO‐d6): δ = 109.8, 113.8,
121.5, 122.6, 123.6, 128.1, 128.7, 143.0, 144.2, 149.4, 156.1,
157.1, 157.6; ms (EI) m/z (relative intensity): 256 [M]+ (30), 224
(33), 208 (25), 150 (100), 134 (37), 110 (75), 39 (80); HRMS (EI):
m/z calcd for C13H8N2O2S: 256.0306; found: 256.0324.
very mild conditions without introducing any acid, base or
metal catalyst and (2) it is an environmentally benign transfor-
mation because of the fact that only electrons are used as
reagent instead of oxidative ones. We think that this proce-
dure with its advantages of complementary reactivity, espe-
cially dramatically technical feasibility and because of the
diversity of this method, can be adopted in organic heterocy-
clic chemistry to synthesize and screen libraries of related bi-
ologically important imidazo[2,1‐b]thiazoles.
Compound 6b. Yield: 82%, amorphous, beige solid, mp
240–244°C; IR (KBr): ν (cm−1) = 3424, 3056, 2991, 1625, 1529,
1
1488, 1449, 1382, 1267, 1209, 739; H NMR (200 MHz, DMSO‐
d6): δ = 2.31 (s, 3H, CH3), 4.46 (broad, OH), 7.19 (s, 1H, Har),
7.45–7.75 (m, 4H, Har); 13C NMR (50 MHz DMSO‐d6): δ = 9.01,
98.6, 110.8, 114.4, 118.2, 119.0, 121.9, 122.0, 123.3, 127.0, 131.8,
141.5, 145.6, 148.0; ms (EI) m/z (relative intensity): 270 [M]+ (32),
269 (10), 155 (22), 150 (30), 110 (100), 77 (40), 43 (80); HRMS
(EI): m/z calcd for C14H10N2O2S: 270.0463; found: 270.0471.
Compound 6c. Yield: 88%, amorphous brown solid, mp
210–213°C; IR (KBr): ν (cm−1) = 3408, 3042, 2928, 1628,
1532, 1449, 1270, 1216, 1100, 742; 1H NMR (200 MHz,
DMSO‐d6): δ = 3.66 (s, 3H, OCH3), 4.04 (broad, OH), 6.94–
7.55 (m, 5H, Har); 13C NMR (50 MHz DMSO‐d6): δ = 56.4,
109.8, 114.7, 120.7, 122.2, 122.5, 136.1, 139.7, 140.1, 140.8,
146.8, 149.1, 150.1, 151.1; ms (EI) m/z (relative intensity): 286
[M]+ (15), 178 (25), 164 (100), 150 (80), 140 (25), 131 (70), 39
(30); HRMS (EI) calcd. for C14H10N2O3S: 286.0412; found:
286.0401.
EXPERIMENTAL
Apparatus and reagents.
The working electrode used in
voltammetry experiments was a glassy carbon disk (1.8‐mm
diameter), and platinum wire was used as the counter electrode.
The working electrode used in controlled‐potential coulometry
and bulk electrolysis (using an electronic potentiostat) was an
assembly of four rods, 6‐mm diameter, and ∼ 10‐cm length, and
a large platinum gauze constituted the counter electrode. The
working electrode potentials were measured versus SCE. 1H and
13C NMR spectra were recorded on a spectrometer operating at
200 MHz for proton and carbon nuclei. Chemical shifts were
recorded as δ values in parts per million (ppm). Spectra were
1
acquired in DMSO‐d6 at 25°C. For H NMR spectra, the peak
Compound 6d. Yield: 90%, amorphous, pale‐yellow solid,
mp: 251–253°C; IR (KBr): ν (cm−1) = 3376, 3056, 2993, 1622,
due to residual DMSO (δ 2.54) was used as the internal
reference. H NMR spectra are reported as follows: chemical shift
1
1
1529, 1446, 1257, 1196, 739; H NMR (200 MHz, DMSO‐d6):
(δ) [multiplicity (where multiplicity is defined as: br = broad; s =
singlet; d = doublet; t = triplet; q = quartet; m = multiplet),
coupling constant(s) J (Hz), relative integral, and assignment]. 13C
NMR spectra were conducted using the central peak (δ 39.5) of the
DMSO multiplet as the internal reference. Mass spectra and exact
masses were recorded on a high resolution mass spectrometer; the
latter used a mass of 12.0000 for carbon, and the data are listed as
follows: mass‐to‐charge ratio (m/z). Infrared spectra were recorded
using a drop‐casting technique on NaCl plates and are reported in
wavenumbers (cm−1). All chemicals (catechols 1a–f and 2‐
mercaptobenzimidazole) were reagent‐grade materials, and sodium
acetate and other solvents and reagents were of proanalysis grade.
These chemicals were used without further purification.
δ = 4.27 (broad, OH), 6.94 (s, 1H, Har), 7.11 (s, 1H, Har)
7.17–7.55 (m, 4H, Har); 13C NMR (50 MHz DMSO‐d6): δ =
109.8, 113.8, 121.5, 122.6, 123.6, 128.1, 128.7, 143.0, 144.2, 149.4,
156.1, 157.1, 157.6; ms (EI) m/z (relative intensity): 256 [M]+ (30),
224 (33), 208 (25), 150 (100), 134 (37), 110 (75), 39 (80); HRMS
(EI): m/z calcd for C13H8N2O2S: 256.0306; found: 256.0322.
Compound 4e. Yield: 68%, amorphous brown solid, mp 161–
1
165°C; H NMR (200 MHz, DMSO‐d6): δ = 2.22 (s, 3H, CH3),
3.63 (broad, OH), 6.99–7.45 (m, 6H, Har), 12.58 (broad, NH); 13C
NMR (50 MHz DMSO‐d6): δ = 20.0, 114.4, 115.9, 118.4, 121.1,
122.8, 124.7, 133.0, 139.6, 144.3, 147.7; ms (EI) m/z (relative
intensity): 272 [M]+ (10), 255 (15), 150 (100), 134 (70), 119 (45), 93
(70); HRMS (EI) calcd. for C14H12N2O2S: 272.0619; found: 272.0607.
Compound 4f. Yield: 54%, amorphous beige solid, mp 201–
204°C; 1H NMR (200 MHz, DMSO‐d6): δ = 1.25 (s, 9H, 3‐
CH3), 3.91 (broad, OH), 7.11–7.38 (m, 4H, Har), 7.60 (s, 1H,
Har), 7.85 (s, 1H, Har), 12.48 (broad, NH); 13C NMR (50 MHz
DMSO‐d6): δ = 29.8, 34.6, 109.8, 114.8, 121.3, 122.6, 126.1,
132.5, 139.3, 141.1, 144.1, 144.3; ms (EI) m/z (relative
intensity): 314 [M]+ (10), 152 (20), 150 (100), 122 (35). HRMS
(EI) calcd. for C17H18N2O2S: 314.1089; found: 314.1028.
General procedure for the synthesis of tetracyclic imidazo
[2,1‐b]thiazols.
In a typical procedure, an aqueous solution
(ca. 100 mL) of water/acetonitrile (90/10), containing 0.2M
sodium acetate, 1.0 mmol of catechols (1a–f), and 1.0 mmol of
2‐mercaptobenzimidazole (3), was electrolyzed in an undivided
cell equipped with a carbon anode (an assembly of four rods, 6‐
mm diameter, and ∼ 10‐cm length) and a large platinum gauze
cathode at 0.20 V versus SCE, at 25°C. The electrolysis was
terminated, when the decay of the current became more than
95%. The process was interrupted during the electrolysis, and
the graphite anode was washed in acetone to reactivate it. At the
end of electrolysis, a few drops of acetic acid were added to the
solution, and the cell was placed in a refrigerator overnight. The
precipitated solid was collected by filtration, washed copiously
Acknowledgments. Financial support of this work by Razi
University is appreciated. We also thank Professor M.S.
Workentin (Department of Chemistry, UWO, Canada) for the
use of the HRM S spectrometer.
1
with distilled water, and characterized by: FT‐IR, H NMR, 13C
NMR, and HRMS. The final products (6a–d and 4e–f) were
obtained purely, and no extra purification was needed. Selected
characterization data for the products are given below.
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M. M. Khodaei, A. Alizadeh, and T. Kanjouri
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet