Triethanolamine-catalyzed expeditious and greener synthesis of 2-amino-4H-chromenes

Article abstract of DOI:10.1002/jccs.201800203

Triethanolamine (TEOA), an inexpensive and eco-friendly base, was used to efficiently catalyze the three-component condensation reaction of heterocyclic/aromatic aldehyde, (α/β)-naphthol, and malononitrile in water to give the corresponding substituted 2-

Full text of DOI:10.1002/jccs.201800203

Received: 28 May 2018
Revised: 1 October 2018
Accepted: 2 October 2018
DOI: 10.1002/jccs.201800203
C O M M U N I C A T I O N
Triethanolamine-catalyzed expeditious and greener synthesis
of 2-amino-4H-chromenes
Anilkumar S. Patel1 | Satishkumar D. Tala1 | Pankajkumar B. Nariya1 | Kartik D. Ladva1 |
Naval P. Kapuriya2
1Department of Chemistry, Shree M. & N. Virani
Triethanolamine (TEOA), an inexpensive and eco-friendly base, was used to effi-
Science College (Autonomous), Rajkot, Gujarat,
India
ciently catalyze the three-component condensation reaction of heterocyclic/aro-
matic aldehyde, (α/β)-naphthol, and malononitrile in water to give the
corresponding substituted 2-amino-4H-chromene derivatives with excellent yields.
2Department of Chemistry, Bhakta Kavi Narsinh
Mehta University, Junagadh, Gujarat, India
Correspondence
Naval P. Kapuriya, Department of Chemistry,
KEYWORDS
Bhakta Kavi Narsinh Mehta University, Bilkha
Road, Khadia, Junagadh, Gujarat 362263, India.
2-amino-4H-chromene, green synthesis, multicomponent reaction,
triethanolamine (TEOA), water medium
Email: navalkapuriya@gmail.com
Funding information
University Grants Commission (UGC) of India
substituted 2-amino-4H-chromenes exhibit a wide range of
biological properties including, antimicrobial,[14] antiviral,[15]
anticoagulant,[16] potassium channel regulating,[17] anti-
spasmolitic,[18] anti-inflammatory,[19] antiproliferative,[20] and
central nervous system (CNS) activities.[21] There is renewed
interest on 2-amino-4H-chromenes due to their potent antican-
cer activity through vascular disruption[22] and inhibition of
the Bcl-2 protein,[23] which reflects the potential of this class
of derivatives as drug candidates.
A widely used protocol for the synthesis of 2-amino-
4H-chromenes is by refluxing aldehyde, malononitrile, and
activated phenol in the presence of organic bases such as piper-
idine, triethylamine, pyridine, so forth. for several hours.[24–27]
Moreover, other catalysts such as methanesulfonic acid,[28]
imidazole,[29] porous organic polymers (POPs),[30] potassium
phthalimide,[31] L-proline-melamine,[32] sodium malonate,[33]
and Na2CO3[34] have also been utilized for this transformation.
In recent years, several new green methods have been reported
using cetyltrimethylammonium chloride,[35] KF/Al2O3,[36] basic
γ–alumina,[37] MgO,[38] CuSO4Á5H2O,[39] DBU,[40] CTABr/
ultrasound irradiation,[41] nanocatalyst,[42] bael fruit extract
(BFE),[43] water extract of lemon fruit shell ash (WELFSA),[44]
and poly(ethylene glycol) (PEG) in water.[45] Although various
procedures for the synthesis of 2-amino-4H-chromenes are
known, many of these methods have disadvantages such as
expensive and unique catalysts, long reaction times, utilization
1
| INTRODUCTION
In the last two decades, considerable effort has been made for
the development of multicomponent reactions (MCRs) in aque-
ous media.[1] Because of the associated environmental and eco-
nomic advantages, such as the less number of steps, high atom
economy, operational simplicity, and low waste generation,
MCRs complies with the principles of green chemistry.[2] On
the other hand, reactions in aqueous media may circumvent the
problems associated with many of the traditional methods,
which are largely based on the use of toxic or hazardous
organic solvents. In this context, reactions using water as the
medium have emerged as a useful alternative for numerous
organic transformations.[3] Besides being cheap, nontoxic, non-
flammable, and environmentally benign, aqueous solutions
possess the other physicochemical characteristic like high heat
capacity required to be an alternative reaction medium.[4] Con-
sequently, a number of MCRs have been reported in aqueous
media, providing easy access to a variety of bioactive chemical
entities such as pyrido[2,3-d]pyrimidines,[5] dihydropyrano
[2,3-c]pyrazole,[6] 1H-pyrazolo[1,2-b]phthalazine-5,10-dione,[7]
thiopyrano[2,3-b]indole-3-carbonitrile,[8] pyrazolo[3,4-b]quino-
lin-5-one,[9] 3-methyl-isoxazol-5(4H)-one,[10] 1,2-dihydro[1,6]
naphthyridine,[11] and others.[12]
Chromenes are considered medicinally important heterocy-
cles found in many natural products and pigments.[13] Of these,
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2018;1–6.
http://www.jccs.wiley-vch.de
1

2
PATEL ET AL.
TABLE 1 Reaction optimization for triethanolamine (TEOA)-catalyzed
of microwave or ultrasonic irradiation, and affording only mod-
erate yields.[46] Moreover, organic bases (piperidine, triethyla-
mine, pyridine, etc.) used for this MCR as catalysts are toxic
and harmful to the environment.[47] Hence, the development of
inexpensive, mild, and efficient green synthetic methodologies
is of great interest.
synthesis of 2-amino-4H-chromene (4a)a
Reaction
Reaction
time (hr)
Entry
TEOA (Mol%)
temperature (ꢀC)
Yield (%)b
1
2
3
4
5
6
20
RT
50
75
90
90
90
24
8
62
70
83
96
80
15
30
30
4
Triethanolamine (TEOA) has been utilized as surfactant
and an emulsifier in many consumer products such as deter-
gents, dishwashing, hand cleaners, polishes, paints, shaving
creams, and printing inks.[48] Recent reports have demon-
strated the efficiency of TEOA as catalyst in several organic
30
1
15
3
Free
16
a
Reaction conditions: benzaldehyde (3.0 mmol), α-naphthol (3.0 mmol), malo-
nonitrile (3.0 mmol), TEOA in water (10 mL).
Isolated yield after crystallization from MeOH.
transformations[49]
including
phosphate-free
Heck
b
reactions,[50] and Knoevenagel-type cyclocondensation,[51]
and as a reusable reaction medium for the synthesis of aryl
sulfones.[52] From the green chemistry perspective, we
hoped that replacing conventional organic bases (TEA,
piperidine, or pyridine) with TEOA in water can be an effec-
tive strategy for the synthesis of 2-amino-4H-chromenes.
In continuation of our ongoing research on the develop-
ment of simple and efficient synthetic methods for various
heterocycles,[53] here we report on TEOA as an economical,
ingenious, and eco-friendly base catalyst for the synthesis of
2-amino-4H-chromene derivatives in water (Scheme 1).
Note. Bold value represents optimal reaction condition.
2
| RESULT AND DISCUSSION
SCHEME 2 Plausible mechanism for the synthesis of representative
derivative (4a) of 2-amino-4H-chromenes using TEOA
Initially, a mixture of benzaldehyde, malononitrile, and
α-naphthol in water was stirred at room temperature (RT) using
20 mol% of TEOA as catalyst. We found that the reaction took
a long time (24 hr) with only moderate yield of the correspond-
ing 2-amino-4H-chromene derivative 4a (Table 1, entry 1).
When reaction temperature and catalyst loading were raised
from RT to 90ꢀC and 20–30 mol%, respectively, the reaction
gave superior results, providing 4a within 1 hr in excellent
yield (96%). Contrarily, under catalyst-free condition, the yield
of compound 4a was poor (Table 1, entry 6), thus underlining
the crucial role of TEOA for this transformation.
The possible role of TEOA as a base catalyst in the forma-
tion of 2-amino-4H-chromene derivatives is shown in
Scheme 2. Initially, the reaction of benzaldehyde and malono-
nitrile undergoes Knoevenagel condensation in the presence of
TEOA to furnish the intermediate 2-benzylidenemalononitrile
(I). The enolate intermediate (II) generated from the interaction
of α-naphthol with TEOA reacts with I and undergoes hetero-
cyclization via the intermediate (III), resulting in the desired
product.
Several other organic bases were employed, such as
piperidine, pyridine, and triethylamine, under aqueous con-
dition for a comparative study (Table 2). From these results,
it was evident that TEOA showed superior catalytic activity
for the synthesis of 2-amino-4H-chromene derivatives in
aqueous medium over the others.
The structure of the product 4a was confirmed by com-
parison of the melting points and through thin-layer chroma-
tography (TLC) with an authentic sample prepared by
following a reported method[39,51] and also by spectral data
(IR, mass spectrometry, and 1H NMR).
With the optimized protocol, we examined the general-
ity of this method by varying the aldehyde and naphthol
components under same reaction condition (Table 3). It
TABLE 2 Synthesis of 2-amino-4H-chromene (4a) using different organic
catalysts in watera
Entry
Catalyst
Piperidine
Pyridine
Reaction time (hr)
Yield (%)b
1
2
3
4
4
6
4
1
78
62
82
96
Triethylamine
TEOA
a
Reaction conditions: benzaldehyde (3.0 mmol), α-naphthol (3.0 mmol), malo-
nonitrile (3.0 mmol), different base catalysts (30 mol %) in water (10 mL).
Isolated yield after crystallization from MeOH.
b
SCHEME 1 Synthesis of 2-amino-4H-chromenes
Note. Bold value represents optimal reaction condition.

PATEL ET AL.
3
TABLE 3 Synthesis of substituted 2-amino-4H-chromenes (4a–i or 5a–i) catalyzed by TEOA in watera
M.p. (ꢀC)
Found
Entry
R
Naphthol
2a
Time (min)
Productb
4a
Yield (%)c
Reportedref
207–208[54]
1
60
96
205–206
2
2a
45
4b
90
204–205
205–206[55]
3
4
2a
2a
60
60
4c
93
90
202–203
138–140
203–205[56]
136–138[55]
4d
5
2a
90
4e
91
203–204
—
6
7
2a
2a
60
45
4f
92
96
230–231
232–233
231–232[55]
230–231[55]
4g
8
9
2a
2a
45
90
4h
4i
92
88
167–168
198–199
169–171[55]
199–200[35]
10
11
2b
2b
60
90
5a
5b
90
87
278–279
254–255
280[28]
256–257[54]
12
2b
60
5c
91
240–242
242–243[39]
13
14
2b
2b
90
90
5d
5e
90
86
141–143
263–264
148–150[39]
262–263[57]

4
PATEL ET AL.
TABLE 3 (Continued)
M.p. (ꢀC)
Found
Entry
R
Naphthol
2b
Time (min)
Productb
5f
Yield (%)c
Reportedref
210–212[20]
15
60
89
210–212
16
2b
60
5g
90
183–185
186[28]
17
18
2b
2b
60
90
5h
5i
94
89
222–224
231–232
225–226[57]
229–231[20]
a
b
c
Reaction condition: aldehyde (3.0 mmol), α- or β-naphthol (3.0 mmol), malononitrile (3.0 mmol), triethanolamine (30 mol%) in water (10 mL) at 90ꢀC.
All products were characterized by their m.p. and IR, mass, and 1H NMR spectra and compared with reported data.
Isolated yields after crystallization from MeOH.
was revealed that the reaction is also applicable to different
aromatic aldehydes and weakly reactive β-naphthols as
well, giving the desired 2-aminochromene derivatives (4a–i
or 5a–i) in very high yields. Moreover, the yields were
independent of the electronic effect of substituents present
on the aromatic aldehyde (Table 3). It is noteworthy that
the product isolation and purification procedure were
extremely simple in this protocol. Moreover, easy recovery
of the catalyst from filtrate underlines the convenience of
this method.
using TLC on aluminum silica gel 60 F254 (Merck, Ger-
many) and detected by UV light (254 nm).
4.2 | General procedure for the synthesis of substituted
2-amino-4H-chromenes (4a–i or 5a–i) catalyzed by
TEOA in water
In a Teflon septa-capped reaction vessel, a mixture of the
appropriate aldehyde (1, 3.0 mmol), α or β-naphthol (2a or
2b, 3.0 mmol), malononitrile (3, 3.0 mmol), and TEOA
(0.9 mmol) in water (10 mL), was heated and stirred at 90ꢀC
for an appropriate time ( as mentioned in Table 3). The pro-
gress of the reaction was monitored by TLC (ethyl acetate/
hexane 3:1). After the starting material was completely con-
sumed, the reaction mixture was cooled to RT, and the sepa-
rated solid product was collected by Buchner filtration and
crystallized from methanol to give the corresponding
2-amino-4H-chromenes (4a–i, 5a–i).
3
| CONCLUSIONS
In summary, we have developed a greener and expeditious
methodology for the synthesis of 2-amino-4H-chromenes
via a three-component condensation reaction catalyzed by
TEOA. Several advantages of this method, such as the use
of an inexpensive catalyst, short reaction time, use of water
as solvent, and easy purification technique, imply that this
method may have industrial applicability.
4.2.1
| Catalyst separation and purification procedure
The filtrate (containing TEOA) collected after Buckner fil-
tration, as describe above, was extracted sequentially with
diethyl ether (5 mL) and hexane (5 mL) in order to remove
the absorbed organic substrates. The remaining aqueous
medium was evaporated under reduced pressure to obtain
sufficiently pure TEOA for further use. The recovery of
TEOA was about 95%.
4
| EXPERIMENTAL SECTION
4.1 | General information
The melting points of all compounds were recorded in open
capillary tubes and are uncorrected. IR spectra were recorded
on an alpha FT-IR (Bruker) spectrophotometer using the
1
All the synthesized compounds 4a–i and 5a–i were char-
KBr pallet technique. H NMR spectra were recorded on a
1
acterized by IR, mass spectrometry, and H NMR spectros-
Bruker-400 MHz FT-NMR spectrometer in DMSO-d6 as
solvent, and the chemical shift values are reported in units of
δ (ppm) relative to tetramethylsilane as the internal standard.
The mass spectra were recorded on an Agilent GC-MS-
5977A spectrometer. Reaction monitoring was carried out
copy. Also, the melting points determined were compared
with those in the literature and found to be matching
(Table 3). The spectral data for selected compounds are
listed below.

PATEL ET AL.
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4.4 | 2-Amino-4-(3-bromo-4-hydroxy-
5-methoxyphenyl)-4H-benzo[h]chromene-
3-carbonitrile (4e)
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1
1278, 1198, 794, 768; H NMR (400 MHz, DMSO-d6) δ
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120.36, 123.50, 124.04, 125.56, 126.13, 126.24, 126.65,
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1
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also thank Central Instrumental facility (CIF), Shree M. &
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Naval P. Kapuriya
http://orcid.org/0000-0002-2178-5156
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How to cite this article: Patel AS, Tala SD,
Nariya PB, Ladva KD, Kapuriya NP. Triethanola-
mine-catalyzed expeditious and greener synthesis of
2-amino-4H-chromenes. J Chin Chem Soc. 2018;1–6.
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