Highly efficient and green synthesis of 2,4-diphenyl substituted thiazoles

Article abstract of DOI:10.1080/00397911.2020.1718711

2,4-Diphenyl thiazole is an important organic intermediate and its derivatives contain multiple biological properties. In the present study, we reported a new protocol to synthesize 2,4-diphenyl thiazole analogs, which involved the bromination of ethyl benzoylacetates with NBS in the presence of 2-hydroxypropyl-β-cyclodextrin, followed by a direct cyclization with thiobenzamides in water. Compared with the reported method, the current protocol contains less reaction steps, milder reaction conditions, and simpler workup procedure.

Full text of DOI:10.1080/00397911.2020.1718711

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
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Highly efficient and green synthesis of 2,4-
diphenyl substituted thiazoles
Jungan Zhang, Peipei Li, Hongyun Zeng, Yu Huang & Wei Hong
To cite this article: Jungan Zhang, Peipei Li, Hongyun Zeng, Yu Huang & Wei Hong (2020): Highly
efficient and green synthesis of 2,4-diphenyl substituted thiazoles, Synthetic Communications, DOI:
10.1080/00397911.2020.1718711
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https://doi.org/10.1080/00397911.2020.1718711
Highly efficient and green synthesis of 2,4-diphenyl
substituted thiazoles
c
Jungan Zhanga,b , Peipei Li , Hongyun Zenga,b, Yu Huangc, and Wei Honga,b
ꢀ
ꢀ
aSchool of Chemistry and Chemical Engineering, North Minzu University, Yinchuan, P. R. China; Key
b
Laboratory of Chemical Engineering and Technology, State Ethnic Affairs Commission, North Minzu
c
University, Yinchuan, P. R. China; School of Pharmacy, Ningxia Medical University, Yinchuan, P.
R. China
ABSTRACT
ARTICLE HISTORY
Received 16 December 2019
2,4-Diphenyl thiazole is an important organic intermediate and its
derivatives contain multiple biological properties. In the present
study, we reported a new protocol to synthesize 2,4-diphenyl thia-
zole analogs, which involved the bromination of ethyl benzoylace-
tates with NBS in the presence of 2-hydroxypropyl-b-cyclodextrin,
followed by a direct cyclization with thiobenzamides in water.
Compared with the reported method, the current protocol contains
less reaction steps, milder reaction conditions, and simpler workup
procedure.
KEYWORDS
Cyclization; cyclodextrin; 24-
diphenyl thiazoles;
green synthesis
GRAPHICAL ABSTRACT
Introduction
Thiazole ring is an important five-membered aromatic heterocyclic ring containing
nitrogen and sulfur heteroatoms and it is a common feature in many drugs. When
binding with biological targets (proteins or nucleic acids), it can form multiple kinds of
non-covalent interactions, such as hydrogen bonds, electrostatic interactions, and hydro-
phobic interactions. Therefore, thiazole compounds have wide applications in the field
of medicine. At present, many thiazole compounds are first-line drugs for varies disease
treatments, such as the antibiotic drug Cefixime,[1] the anticancer drug Dasatinib,[2,3]
the antiparasitic drug Nitazoxanide,[4,5] the anti-inflammatory drug Meloxicam,[6,7] the
antiviral drug Ritonavir,[8,9] and so on. Moreover, 2,4-diphenyl thiazole derivatives are
CONTACT Wei Hong
University, Yinchuan, P. R. China; Yu Huang
hongwei136@hotmail.com
School of Chemistry and Chemical Engineering, North Minzu
huangyunxmu@163.com School of Pharmacy, Ningxia Medical
University, Yinchuan, P. R. China.
ꢀ
Jungan Zhang and Peipei Li have contributed equally to this work.
Supplemental data for this article is available online at https://doi.org/10.1080/00397911.2020.1718711
ß 2020 Taylor & Francis Group, LLC

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J. ZHANG ET AL.
important in organic synthesis, which can be used as building blocks in the drug design
because of their potential activities in the anti-anoxic,[10] antiprion,[11] antimalarial,[12]
and antimycobacterial,[13] etc.
Normally, the preparation of 2,4-diphenyl thiazole (3a) rested on the following meth-
ods: (1) two-step reactions: conversion of ethyl benzoylacetate to a-bromo-b-keto
ester,[12,14] a-p-toluenesulfonyloxy-b-keto ester[15,16] or a-diazo-b-keto ester,[17,18] and
then cyclization with thiobenzamide; (2) three-step reactions: cyclization of ethyl ben-
zoylacetate with thiourea, followed by diazo reaction and bromination, and then fin-
ished by Suzuki reaction.[19] All these reactions involved organic solvents and tedious
workup. In order to overcome these shortcomings, it was necessary to develop a mild
and highly efficient synthetic methodology.
Since the outer edge of the cyclodextrin is hydrophilic and the cavity is hydrophobic,
it can encapsulate organic molecules in the inner cavity and utilize the hydrophilic abil-
ity of the outer edge to increase the water solubility of organic molecules.[20–23]
Therefore, cyclodextrin is helpful to catalyze the organic reactions in the water.
Recently, Kakulapati, etc.[24] reported the water phase synthesis of 2-amino-thiazole in
the presence of b-cyclodextrin. However, the green synthetic methods of 2,4-diphenyl
substituted thiazoles are still needed to be explored.
Herein, an efficient and green approach for the synthesis of 2,4-diphenyl substituted
thiazoles was reported by using cyclodextrin from ethyl benzoylacetate and thiobenza-
mide in water. After the formation of hydrogen bonds with cyclodextrin, ethyl benzoy-
lacetate is able to dissolve in water and converted to a-bromo-b-keto ester in the
presence of NBS. Moreover, thiobenzamide cannot dissolve in water, but the acetone
solution of thiobenzamide can cyclize with a-bromo-b-keto ester in water to form 2,4-
diphenyl thiazoles.
Result and discussion
The current methodology allows the preparation of 2,4-diphenyl substituted thiazole
derivatives with high efficiency. The great efforts have been made to investigate the
effects of catalyst, reaction temperature, and solvent toward the formation of thiazoles.
Initially, four different cyclodextrins (a-cyclodextrin, b-cyclodextrin, c-cyclodextrin, and
2-hydroxypropyl-b-cyclodextrin) were chosen to prepare 3a in H2O/acetone at 50 ꢁC.
As shown in Table 1, except the c-cyclodextrin which cannot catalyze the formation of
3a, the target compound was obtained by the rest three catalysts with different yields
varying from 56 to 63% and 2-hydroxypropyl-b-cyclodextrin was regarded as the best
catalyst (Entry 4). Subsequently, the most suitable reaction temperature was studied
with a mixture solvent of H2O/acetone under different temperatures from 25 to 80 ꢁC
(Table 1, Entry 4–6). Interestingly, when the reaction temperature elevated from 25 to
50 ꢁC, the yield of 3a was dramatically increased from 41 to 63%. However, the yield
was no longer increased when the reaction temperature was elevated to 80 ꢁC.
Therefore, the 50 ꢁC was treated as the most suitable reaction temperature. In order to
find the most suitable solvent for the thiazole formation, two different solvents (H2O
and H2O/THF) were evaluated in the preparation of 3a with 2-hydroxypropyl-b-cyclo-
dextrin as the catalyst at 50 ꢁC. Compared to Entries 7 and 8 with 4 (Table 1), the

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Table 1. Synthesis of 2,4-diphenyl substituted thiazole 3a in different conditions.
Entry
Cyclodextrin
a-cyclodextrin
b-cyclodextrin
Solvent
Temperature (ꢁC)
Yield (%)
1
2
3
4
5
6
7
8
H2O/acetone (20/1)
H2O/acetone (20/1)
H2O/acetone (20/1)
H2O/acetone (20/1)
H2O/acetone (20/1)
H2O/acetone (20/1)
H2O
50
50
50
50
25
80
50
50
60
56
0
63
41
63
45
50
c- cyclodextrin
2-hydroxypropyl-b-cyclodextrin
2-hydroxypropyl-b-cyclodextrin
2-hydroxypropyl-b-cyclodextrin
2-hydroxypropyl-b-cyclodextrin
2-hydroxypropyl-b-cyclodextrin
H2O/THF (20/1)
suitable reaction solvent was determined as H2O/acetone. Finally, the reaction condition
was regarded as in situ formation of the 2-hydroxypropyl-b-cyclodextrin complex of the
b-keto ester 1a and NBS in water at 50 ꢁC, followed by the addition of acetone solution
of thiobenzamide 2a; stirring at the same temperature gave the corresponding 2,4-
diphenyl substituted thiazole 3a. The treatment of ethyl benzoylacetate with NBS may
afford a-bromo-b-keto esters as intermediates, which then undergo cyclization with thi-
obenzamide to give the corresponding compound.
With the determined reaction procedure, we introduced different substituents at the R1
position of ethyl benzoylacetate 1 (Table 2), such as methyl, methoxy, isopropyloxy, and
halogen groups, which were reacted with thiobenzamide 2a to generate thiazole derivatives
3b–g. Reaction with compound 1b afforded 4-methyl phenyl substituted thiazole 3b in 61%
isolated yield. Methoxy and isopropyloxy derivatives 3c and 3d were isolated in similar
yields (66 and 60%, Entries 3 and 4, respectively). Surprisingly, reactions with halogen (F,
Cl, and Br) substitution on compounds 1e–g afforded moderate yields (45, 41, and 52%,
Entries 5–7, respectively). This result pointed out that the current methodology required the
presence of electron-donor substituents in the aromatic ring of ethyl benzoylacetate 1.
In order to investigate the influences of different substituents at the R2 position of
thiobenzamides, ethyl benzoylacetate 1a was reacted with substituted thiobenzamides
2b–f to generate thiazole derivatives 3h–l. As shown in Table 2, the reaction with com-
pound 2b (R2 ¼ methyl group) and 2c (R2 ¼ methoxy group) afforded methyl or
methoxy derivatives 3h and 3j, respectively in moderate yields (45 and 51%, Entries 1
and 2, respectively). Moreover, reactions with halogen (F, Cl, and Br) substitutions on
compounds 2d–f afforded moderate yields (41, 56 and 42%, Entries 3–5, respectively).
Simultaneously, it can be observed that the electron-donor substituents in thiobenza-
mides contain little effects on the yield.
Finally, the scopes with different substitutes on ethyl benzoylacetates and thiobenzamides
were presented in Table 2. The reaction of electron-donor substituent (methoxy) on com-
pound 1c with compound 2c bearing the same substitute group generated the thiazole
derivative 3m in good yield (70%, Entry 1). However, reaction with compound 2d bearing
the electron-withdrawing group (fluoro) afforded the thiazole derivative 3n in a moderate

4
J. ZHANG ET AL.
Table 2. Preparation of substituted 2,4-diphenyl thiazoles 3a–p.
Reactant
Entry
R1
R2
Product
Yield (%)
1
2
3
4
5
6
7
8
H (1a)
CH3 (1b)
OCH3 (1c)
CH(CH3)2 (1d)
F (1e)
Cl (1f)
Br (1g)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
OCH3 (1c)
OCH3 (1c)
F (1e)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
CH3 (2b)
OCH3 (2c)
F (2d)
Cl (2e)
Br (2f)
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
63
61
66
60
45
41
52
45
51
41
56
42
70
56
59
52
9
10
11
12
13
14
15
16
OCH3 (2c)
F (2d)
OCH3 (2c)
F (2d)
F (1e)
yield (56%, Entry 2). Subsequently, the reactions of electron-withdrawing group (fluoro) on
compound 1e with compound 2c and 2d afforded moderate yields (59 and 52%, Entries 3
and 4, respectively). The result pointed out that if the electron-donor substituents were pre-
sented on the aromatic rings of both ethyl benzoylacetate and thiobenzamide, good yields
for the generations of thiazole derivatives could be expected. Chemical structures of com-
1
1
pounds 3a–p were characterized with H NMR, 13C NMR, and HRMS. For example, H
NMR spectrum of 3m assigned two singlet signal at 3.90 and 3.89ppm assignable for two
–OCH3 protons, respectively, two doublet signal at 8.00 and 7.86ppm with J-value 8.8 Hz
for 2-phenyl protons and two doublet signal at 7.00 and 6.98ppm with J value 3.8 Hz for 4-
phenyl protons. 13C NMR spectrum of 3m showed the characteristic signals at 169.10ppm
assigned for carbonyl carbon. The molecular weight of 3m for [Mþ H]þ is 370.1113, and
the signal peak at 370.1108 was found on the HRMS spectrum. The characterization of other
compounds can be found in Supporing Information.
The plausible mechanism for the formation of 2,4-diphenyl substituted thiazole 3a
was explained in Scheme 1. The outer edge of the 2-hydroxypropyl-b-cyclodextrin is
hydrophilic and the cavity is hydrophobic, so it can encapsulate ethyl benzoylacetate 1a
in the inner cavity and increase the water solubility of compound 1a. Therefore, in the
water, compound 1a was reacted with NBS to generate a-bromo-b-keto ester 4.
Thiobenzamide 2a was tautomerized to intermediate 5, which could attack a-bromo
position on 4 with thiol group by nucleophilic substitution to give intermediate 6. By
tautomerization, the amino group on intermediate 7 could attack b-keto group and
then the intramolecular dehydration could generate intermediate 9. Finally, the

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Scheme 1. Plausible mechanism for the synthesis of 2,4-diphenyl substituted thiazole 3a.
compound 3a was formed by tautomerization again. Thus, the 2-hydroxypropyl-b-cyclo-
dextrin is a good catalyst to promote the reaction in water.
Experimental
All reagents and solvents were purchased from the suppliers and used directly in the
experiment. Melting point was measured on a SGW X-4 micro melting point instru-
1
ment, uncorrected. H and 13C NMR spectra were recorded on a Bruker Avance III 400
spectrometer. The solvent used for the NMR spectrum was chloroform-d or DMSO-d6,
and TMS was used as the internal standard. HRMS was recorded on a Thermo Fisher
LTQ Orbitrap XL LC/MS. TLC was performed on silica gel (silica gel 60 pre-coated alu-
minum plates from Macherey-Nagel).
General procedure for the synthesis of 3a–p
2-Hydroxypropyl-b-cyclodextrin (1 mmol, 1 eq) was dissolved in H2O (20 mL) and then
substituted ethyl benzoylacetate (1 mmol, 1 eq) and NBS (1.2 mmol,1.2 eq) were added.
After the mixture was stirred at 50 ꢁC for 2 h, a solution of substituted thiobenzamide
(1.2 mmol, 1.2 eq) in acetone (1 mL) was added dropwise. The reaction mixture was
stirred at 50 ꢁC for 4 h and extracted with EtOAc (10 mL ꢂ 3). The combined organic
layers were washed with water, dried over Na2SO4, filtered, and concentrated. The resi-
due was purified by column chromatography on silica gel using EtOAc/petroleum as
eluent to give 3a–p.
Ethyl 2,4-diphenylthiazole-5-carboxylate (3a)
1
Colorless solid, yield 63%; MP 101–103 ꢁC (lit.[25] MP 101–102 ꢁC); H NMR (400 MHz,
Chloroform-d) d 8.11–8.03 (m, 2H), 7.89–7.82 (m, 2H), 7.54–7.44 (m, 6H), 4.33 (q,
J ¼ 7.1 Hz, 2H), 1.33 (t, J ¼ 7.1 Hz, 3H); 13C NMR (101 MHz, DMSO-d6) d 169.43,
161.18, 160.14, 134.19, 132.52, 132.08, 130.31, 129.89, 129.67, 128.19, 127.18, 122.70,

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J. ZHANG ET AL.
61.91, 14.38; HRMS (ESI) m/z [M þ H]þ Calcd for C18H15NO2S: 310.0896,
Found: 310.0896.
1
Full experimental detail, H and 13C NMR spectra. This material can be found via
the “Supplementary Information”.
Funding
This study was supported by the National Natural Science Foundation of China [Grant No.
81660588, 81960623]; and the Key R&D Program of Ningxia [Grant No. 2018BFG02004].
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