Aqua mediated synthesis of substituted 2-amino-4H-chromenes and in vitro study as antibacterial agents

Article abstract of DOI:10.1016/j.bmcl.2005.06.041

A simple, clean, environmentally benign route to the synthesis of 2-amino-chromenes is described using K₂CO₃ as a green catalyst in water under microwave irradiation. This implies a convenient route avoiding the usage of hazardous or

Full text of DOI:10.1016/j.bmcl.2005.06.041

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4295–4298
Aqua mediated synthesis of substituted 2-amino-4H-chromenes
and in vitro study as antibacterial agents
Mazaahir Kidwai,a,* Shilpi Saxena,a M. Khalilur Rahman Khanb
and Sharanjit S. Thukralb
aDepartment of Chemistry, University of Delhi, Delhi 110007, India
bDepartment of Microbiology, Vallabhbhai Patel Chest Institute, University of Delhi, Delhi 110007, India
Received 13 April 2005; accepted 18 June 2005
Abstract—A simple, clean, environmentally benign route to the synthesis of 2-amino-chromenes is described using K2CO3 as a green
catalyst in water under microwave irradiation. This implies a convenient route avoiding the usage of hazardous organic solvents and
organic bases. This technique requires only water in both the reaction step and workup, thus rendering the whole procedure into a
truly ecofriendly green protocol. All the synthesized compounds were shown to possess antibacterial activity as tested in vitro
against standard strains of Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
metics, pigments,5 and potential biodegradable agro-
chemicals.6 Fused chromenes are biologically active
compounds with a wide spectrum of activities viz. anti-
microbial,7 antiviral,8,10 mutagenicity,9 antiprolifera-
tive,11 sex pheromone,12 antitumor,13 and central
nervous system activity.14 Thus, in view of the diverse
therapeutic activity of chromenes and in continuation
to our ongoing endeavour15–17 aimed at developing
new selective and environmentally benign methodolo-
gies using MWI, we report herein the synthesis of substi-
tuted 2-amino-4H-chromene and benzo[e]chromene
derivatives and their in vitro antibacterial profile against
gram negative and gram positive standard strains of
bacteria using Broth Microdilution MIC method.18
The environment calls on the entire research edifice to
define long-term strategic goals for clean chemistry and
to reduce the amount of pollutants produced including
organic solvents whose recovery is mandated by ever-
more strict laws. To reduce the dependence on ecolog-
ically unsafe chemicals, it is most advantageous to
carry out reactions in aqueous media. Water is the
cheapest abundantly available solvent. Indeed, water
is recognized as an attractive medium for many organic
reactions. Reactions in aqueous media are generally
environmentally safe, devoid of any carcinogenic ef-
fects, simple to handle, comparatively cheaper to oper-
ate, and especially important in industry.1,2 Further,
coupling of this solvent-free synthesis with microwave
irradiation (MWI) has associated benefits of shorter
reaction times, uniform heating, higher yields, en-
2. Results and discussion
hanced
selectivity,
and
associated
ease
of
2-Amino-chromenes are generally prepared by refluxing
malononitrile, aldehyde, and activated phenol in the
presence of hazardous organic bases like piperdine in
organic solvents like ethanol and acetonitrile for several
hours.19 A literature survey revealed that several modi-
fied procedures using CTACI,20 TEBA,21 and c alumi-
na22 as catalyst have been recently reported but all
these methods require long refluxing hours. To study
the role of base used and to make the classical method
a clean, efficient, economical, and green method utilizing
MWI under solvent-free conditions, the reaction was
manipulation.3,4
2-Amino-chromenes represent an important class of
compounds being the main components of many natural
occurring products and are widely employed as cos-
Keywords: Microwave irradiation; Antibacterial agents; Solvent-free;
4H-chromenes.
*
Corresponding author. Tel./fax: +91 1127666235.; e-mail: kidwai_
chemistry@yahoo.co.uk
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.041

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M. Kidwai et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4295–4298
attempted in K2CO3. As a test case, a saturated solution
of K2CO3 in water was added to the reaction mixture
containing equimolar amount of aldehyde 1a–d, mal-
ononitrile, and resorcinol (Scheme 1), and was subjected
to MWI at low power (560 W). On completion of reac-
tion as monitored by TLC, the reaction mixture was
cooled and the solid obtained was triturated with few
milliliters of water to give the required product 2a–d in
excellent yields within just few minutes of MWI. To ex-
tend the scope of this reaction and its utility as a new
synthetic approach to substituted 2-amino-4H-chro-
mene, we also studied the synthesis of substituted 2-ami-
no-benzo[e]chromene derivatives (Scheme 2) by the
condensation reaction of aldehyde 1a–d, malononitrile,
and b-naphthol in aqueous K2CO3. Excellent yield of
products were obtained within few minutes of MWI
(Table 1). Furthermore, the use of several heterocyclic
aldehydes led to the synthesis of novel 4H-chromene
derivatives. The structure of the products was confirmed
on the basis of spectroscopic and analytical data (Table
2). To assess the efficiency of MWI in inducing these
reactions, the reaction of activated phenols with alde-
hyde and malononitrile with classical heating under
comparable reaction conditions (time and temperature)
was also studied. However, in this case, the reaction
gave impure product even after long hours of heating
and in certain cases led to charring.
All the synthesized compounds 2a–d and 3a–d were dis-
solved in dimethylformamide (DMF) and tested for
their antibacterial activity at concentrations of 0.5, 1.0,
2.0, 4.0, 8.0, 16.0, 32.0, 64.0, and 128.0 lg/ml using
Broth Microdilution MIC method. All the compounds
showed antibacterial activity against standard reference
strains of Escherichia coli (ATCC 25922), Pseudomonas
aeruginosa (ATCC 27853), and Staphylococcus aureus
(ATCC 25923), and their MICs ranged between 64
and 128 lg/ml. Ampicillin was used as the standard drug
(Table 3).
3. Conclusion
Thus, an easier, practically convenient, and ecofriendly
green synthesis of substituted 2-amino-4H-chromene
and benzo[e]chromene derivatives has been developed
that avoids the use of harsh and highly environment
contaminating conditions, viz., classical heating, use of
expensive and toxic reagents, solvents, and catalyst.
Moreover, it offers benefits like elimination of solvent,
Scheme 1. Synthesis of substituted 2-amino-4H-chromene derivatives in aqueous medium.
Scheme 2. Synthesis of substituted 2-amino-4H-benzo[e]chromene derivatives in aqueous medium.
Table 1. Reaction times and yields for compounds 2a–d and 3a–d
Compound
R
Melting point, mp (ꢁC)
Time (min)a
Yield (%)
2a
2b
2c
2d
3a
3b
3c
3d
Phenyl
Piperonyl
Indolyl
232–234b
164–165b
207–209
185–187
278–279b
188–190b
218–220
296–298
2.8
2.5
3.5
1.9
3.2
2.9
3.3
2.0
91
92
90
88
90
93
89
87
2-Chloro-3-quinolyl
Phenyl
Piperonyl
Indolyl
2-Chloro-3-quinolyl
a Microwave heating (560 W, 2450 MHz, 95–105 ꢁC, 30 s).
b See Ref. 24.

M. Kidwai et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4295–4298
4297
simplification of the work up, procedures, facility of
scale up, and savings in energy consumption, and in
addition giving higher yields of products and enhance-
ment in reaction rate. Further, the use of water as a
green solvent combined with the exploitation of the
multicomponent strategy open to this process, suggests
good prospects for its industrial applicability. All the
compounds synthesized were shown to have antibacteri-
al activity against standard strains of gram negative and
gram positive bacteria.
4. Experimental
Melting points were determined on a Thomas Hoover
melting point apparatus and are uncorrected. IR spectra
were recorded on a Perkin-Elmer FTIR-1710 spectro-
1
photometer using KBr pellets. H NMR spectra were
recorded on
a Bruker Avance 300 Spectrospin
(300 MHz) instrument using TMS as an internal stan-
dard and CDCl3/DMSO as a solvent. Microwave irradi-
ation was carried out in a Kenstar Microwave Oven,
Model No. OM 9925E (2450 MHz, 800 W). The reac-
tion was monitored by TLC, using silica gel coated Al
plates (Merck). The reaction temperature was measured
through AZ, Mini Gun Type Non-Contact IR Ther-
mometer (Model No. 8868). 2-Chloro-3-quinoline alde-
hyde was prepared according to literature method.23
4.1. General procedure for the synthesis of 2-amino-3-
cyano-7-hydroxy-4-substituted-4H-chromene derivatives
(2a–d)
Equimolar amounts of neat reactants, aldehyde (1a–d),
malononitrile, and resorcinol were taken in an Erlen-
meyer flask, and 10 ml saturated solution of K2CO3 in
water was added to it. The reaction mixture was subject-
ed to MWI for a specific time (Table 1) at low power
(560 W). The progress of the reaction was monitored
by TLC examination at an interval of every 30 s. On
completion of reaction, the reaction mixture was cooled
and was triturated with 2–3 ml of ice cold water to get
the solid product 2a–d, leaving behind K2CO3 dissolved
in water. The product obtained was filtered, washed
with cold water, dried, and recrystallized from ethanol.
4.2. General procedure for the synthesis of 2-amino-3-
cyano-4-substituted-4H-benzo[e]chromene derivatives
(3a–d)
An equimolar mixture of aldehyde (1a–d), malononitrile,
and b-naphthol in 10 ml saturated solution of 10 ml
K2CO3 was taken in an Erlenmeyer flask and subjected
to MWI. On completion of reaction as monitored by
TLC examination at an interval of 30 s, the reaction mix-
ture was cooled and triturated with few milliliters of
water and the product was recrystallized from ethanol.
4.3. Antibacterial activity
Screening of all the synthesized compounds for their
antibacterial activity was performed by employing Broth
Microdilution MIC method. Using sterile microtiter

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M. Kidwai et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4295–4298
Table 3. Antibacterial activity of different compounds (2a–d and 3a–d) and ampicillin by Broth Microdilution MIC method
Concentration used (lg/ml)
Antibacterial activity against standard strains—Compounds
2a
2b
2c
2d
3a
3b
3c
3d
Ampicillin
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
0.5
1.0
+
+
+
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+
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+
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+
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+
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+
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2.0
4.0
8.0
–
16.0
32.0
64.0
128.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1, E. coli (ATCC 25922); 2, P. aeruginosa (ATCC 27853); 3, S. aureus (ATCC 25923); +, resistant; À, susceptible.
6. Hafez, E. A.; Elnagdi, M. H.; Elagemey, A. G. A.;
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plates, 0.1 ml of Mueller Hinton Broth was added to
each of the 96 wells. Doubling dilutions of each com-
pound were made in the wells, thus a plate contained
0.5–128 lg/ml dilutions of eight different compounds
and of ampicillin. In each plate, one well was kept as po-
sitive control (broth + inoculum) and another one as neg-
ative control (broth only). The inoculum was adjusted to
a turbidity equivalent to McFarland 0.5 turbidity stan-
dard. The inoculum was suitably diluted so as to get a
final concentration of approximately 5 · 105 cfu/ml of
bacteria in each well. Each well was inoculated with
0.01 ml of the prepared inoculum using a multichannel
micropipette and the plates were incubated overnight at
37 ꢁC. The MICs of these compounds and ampicillin were
determined by using the standard protocol of NCCLS
Broth Microdilution MIC method.18
14. Eiden, F.; Denk, F. Arch. Pharm. Weinhein Ger (Arch.
Pharm.) 1991, 324, 353.
15. Kidwai, M.; Saxena, S.; Khan, M.K.R.; Thukral, S.S.
European J. Med. Chem. 2005 (in press).
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Med. Chem. Lett. 2005, 15, 915.
Acknowledgment
17. Kidwai, M.; Venkataramanan, R.; Dave, B. Green Chem.
2001, 3, 278.
18. NCCLS, Performance Standards for Antimicrobial Sus-
ceptibility Testing: Twelfth Information Supplement M,
In National Committee for Clinical Laboratory Stan-
dards, Wayne, PA, 2002, pp 100–512.
The authors M. Kidwai and S. Saxena are thankful to
University Grants Commission, New Delhi, for financial
assistance.
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