Cyclization of 2'-hydroxychalcones to flavones using ammonium iodide as an iodine source - An eco-friendly approach

Article abstract of DOI:10.2298/JSC120901119K

Ammonium iodide on exposure to air decomposes to ammonia and iodine. The in situ generated iodine was used for the cyclization of 2'-hydroxychalcones to the corresponding flavones under solvent-free conditions in good to excellent yields. This method could serve as an attractive alternative to the existing methods for synthesis of flavones and the use of toxic molecular iodine is avoided. Copyright

Full text of DOI:10.2298/JSC120901119K

J. Serb. Chem. Soc. 78 (7) 909–916 (2013)
UDC 547.814.5+66.095.25:547.288.1+546.151
JSCS–4468
Original scientific paper
Cyclization of 2′-hydroxychalcones to flavones using ammonium
iodide as an iodine source – an eco-friendly approach
1
1
2
PRAMOD S. KULKARNI , DASHARATH D. KONDHARE , RAVI VARALA *
1
and PUDUKULATHAN K. ZUBAIDHA
1
School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded,
2
Maharashtra – 431 606, India and Department of Chemistry, AP-IIIT Basar, Rajiv Gandhi
University of Knowledge Technologies, Mudhole, Adilabad, Andhra Pradesh - 504 107, India
(
Received 1 September, revised 30 October 2012)
Abstract: Ammonium iodide on exposure to air decomposes to ammonia and
iodine. The in situ generated iodine was used for the cyclization of 2′-hydro-
xychalcones to the corresponding flavones under solvent-free conditions in
good to excellent yields. This method could serve as an attractive alternative to
the existing methods for synthesis of flavones and the use of toxic molecular
iodine is avoided.
Keywords: flavones; 2′-hydroxychalcone; ammonium iodide; solvent-free; in
situ iodine.
INTRODUCTION
There are a number of environmental implications for the use of large volu-
mes of organic solvents since they are utilized in larger quantities than the solutes
they carry and are transferred into the environment through evaporation and
leakage. Due to the increasing concern for the harmful effects of organic solvents
on the environment and human body, organic reactions that are conducted
without conventional organic solvents have aroused the attention of organic che-
mists. Many organic reactions have been reported to proceed efficiently under
1
solvent-free conditions and some showed enhanced selectivity.
Therefore, the synthetic endeavors of more and more chemists are devoted
2
a
toward nature-friendly syntheses and to reduce the drastic prerequisites of
reactions. Thus, a paradigm shift from using solvents toward solvent-free reac-
tions not only simplifies organic syntheses but also improves process conditions
for large-scale syntheses. Therefore, it is now often claimed that “the best solvent
2
b
is no solvent”.
*
Corresponding author. E-mail: ravivarala@gmail.com
doi: 10.2298/JSC120901119K
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KULKARNI et al.
Flavonoids are a group of low molecular weight compounds mainly occur-
ring in the plant kingdom and flavones constitute a major class among the
flavonoids. This class of molecules has been extensively investigated and 4000
3
chemically unique flavonoids have been isolated from plants. They continue to
attract a great deal of attention as they possess biological activities, such as anti-
4
5
6
oxidant effects, antiviral, and leishmanicidal activity, ovipositor stimulant of
7
8
9
10
11
phytoalexins, anti-HIV, vasodilator, bactericidal, DNA cleavage, anti-
1
2
13
14
15–18
inflammatory, antimutagenic, anti-allergic and anticancer.
flavones (2-phenylchromones) exhibit a wide variety of activities.
Especially,
1
9
The main known synthetic methods for obtaining flavones are oxidative
2
0
cyclization of 2′-hydroxychalcones, the cyclodehydration of 1-(2-hydroxyphe-
2
1
22
nyl)-3-phenyl-1,3-propanedione and via an intermolecular Wittig reaction.
Reagents that have been used for the oxidation of 2′-hydroxychalcones and fla-
20
22
23
vanones to obtain flavones are SeO –pentan-1-ol, Pd–C/vacuum, I –DMSO,
SeO –DMSO, 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)–dioxane, NaIO4–
DMSO, nickel peroxide–dioxane, H O –NaOH, Dowex H –2-propa-
2
2
24
25
2
26
27
28
+
–
nol,
–
2 2
23a,29
30
31
SeO –dioxane, SeO –3-methyl-1-butanol (isoamyl alcohol), Br2–
2 2
NaOH,32 Tl(NO ) ·3H O and I –triethylene glycol.
33
34
3 3 2 2
Most of these methods are of limited use as they suffer from low yields and
1
8
often afford a mixture of products containing flavones, flavanones and aurones.
Furthermore, these procedures require prolonged reaction times, use of harsh
organic solvents, high temperatures, expensive catalysts or hazardous reaction
conditions. Hence, there is scope for the development of new methods for the
synthesis of flavones using easily available, inexpensive and eco-friendly reagents.
Synthesis of flavones from 2′-hydroxychalcone using iodine in dimethyl sul-
2
3
foxide is reported in the literature. However, molecular iodine is highly cor-
rosive, toxic and expensive, making its use somewhat unattractive. In order to
overcome the problems associated with molecular iodine, herein, for the first
time, the oxidative cyclization of 2′-hydroxychalcones to flavones by in situ
generated iodine from ammonium iodide in the presence of air under solvent-free
conditions is reported. However, the use of ammonium iodide in organic syn-
thesis is very rare; it is used in the iodination of aromatic compounds with ozone
3
5
as the oxidizing agent. Recently there was a report on the use of ammonium
iodide and H O in organic synthesis for iodination of ketones and aromatic
2
2
3
6
compounds, and dethioacetalization. In continuation of ongoing interest in the
development of novel synthetic methodologies, particularly of carbon–carbon
and carbon–heteroatom bond formation of biologically relevant heterocycles,37
brief findings are reported herein on the use of ammonium iodide in the oxidative
cyclization of 2′-hydroxychalcone to flavones.
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SYNTHESIS OF FLAVONES
911
EXPERIMENTAL
2
′-Hydroxychalcones (1a–q) were prepared by base-catalyzed condensation between
38
2
-hydroxyacetophenone and the appropriate benzaldehyde using a literature procedure.
2-Hydroxyacetophenone, substituted benzaldehydes, ammonium iodide and solvents were
purchased from Loba Chemicals, Merck and Sigma–Aldrich. Progress of the reaction was
monitored by TLC. All the yields were calculated after purification of the products by column
chromatography using EtOAc:petroleum ether (1:4, boiling range 40–60 °C) on silica gel. The
melting points of the compounds were determined in open capillary tubes and are uncorrected.
1
The IR spectra were recorded on a Perkin-Elmer FTIR-1710 spectrophotometer. The H-NMR
and 13C-NMR spectra were recorded at room temperature on Bruker AC-250 spectrometers
using TMS as an internal standard.
General procedure for the preparation of flavones (2a–q)
A mixture of 1.0 mmol of 1 (2′-hydroxychalcone) and ammonium iodide (0.1 mmol) was
heated in air under neat condition at 120 °C for 1 h. After completion of the reaction, the
reaction mixture was cooled to room temperature and poured into 20 mL water. The formed
precipitate was filtered, washed with 10 % sodium thiosulfate (3×10 mL) and then with 5 mL
ice-cold ethanol. The crude product obtained was purified by column chromatography (silica
gel, ethyl acetate–petroleum ether (1:4)) to give pure flavone (2).
RESULTS AND DISCUSSION
Ammonium iodide gradually turns yellow on standing in moist air, owing to
3
9
decomposition with liberation of iodine and this was utilized for the oxidative
cyclization of 2′-hydroxychalcones to their corresponding flavones (Scheme 1).
Scheme 1. Synthesis of flavones using NH4I.
For this purpose, 2′-hydroxychalcones (1a–q) were synthesized using easily
accessible starting materials, substituted 2-hydroxyacetophenones and electroni-
cally divergent benzaldehydes in good to excellent yields in presence of base.38 It
was observed that 1-(2-hydroxyphenyl)-3-phenylprop-2-en-1-one (1 mmol, 1a)
when heated with a solution of ammonium iodide (10 mol %) in DMSO at 120
°
C becomes transformed to the corresponding flavone (2a) in 1 h in good yield
(
88 %). The reaction was monitored by TLC and the structure of product was
1
confirmed by spectroscopic data. The H-NMR spectra of 2a showed a singlet at
6
.86 due to 1H of 3H, i.e., pyrone ring, which is the characteristic singlet for flavo-
1
nes. Such observed H-NMR data and the complete absence of a peak near 12.73
ppm due to an ortho-hydroxy group is in agreement with the oxidation of the
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KULKARNI et al.
chalcone into the corresponding flavone. Among the solvents screened for opti-
mum conditions for the transformation, DMSO was found to be the most suit-
able, as is obvious from the data presented in Table I. Of the tested solvents,
ethanol (reflux) and diethylene glycol (120 °C) gave poor or no yields. The same
reaction was explored under neat conditions and, to our utmost satisfaction, pro-
duct 2a was obtained in a good isolated yield (92 %).
TABLE I. Cyclization of 2′-hydroxychalcone in different solvents using ammonium iodide;
reaction conditions: 2′-hydroxychalcone (1 mmol), NH I (0.1 mmol)
4
a
Entry
Solvent
Time, h
Yield , %
Product
1
2
3
4
5
DMSO
DMF
EtOH
1
4
12
12
1
88
72
10
0
Flavone (2a)
Unidentified compound
Flavone (2a)
Diethylene glycol
Solvent free
No reaction
Flavone (2a)
92
a
Isolated yield
In the absence of NH I, only the starting materials were isolated from the
4
reaction mixture, even after 12 h. This indicates that the reagent exhibits high
catalytic activity in this transformation. In order to evaluate the most appropriate
reagent loading, the above test reaction was performed using 5, 10 and 20 mol %
of NH I under solvent-free conditions. It was found that 10 mol % of the reagent
4
showed maximum yield in the minimum time at 120 °C. Further increasing of the
reagent loading did not affect the yield.
Having optimized the reaction parameters (10 mol % NH I, heating at 120
4
°
C in open air under neat conditions), this methodology was extended to the other
2
′-hydroxychalcones and the results are presented in Table II. The substituents on
B ring of 2′-hydroxychalcones were varied from electron donating to electron
withdrawing and in all the cases, the studied the transformation went smoothly to
4
0
yield the corresponding flavones in good yield. Contrary to previously reported
methods, this procedure tolerates a wide range of substituents, such as methyl,
chloro, methoxy, bromo, hydroxyl, N,N-dimethylamino and nitro. Generally, it
was observed that oxidation of substrates with unprotected hydroxyl groups on
the aromatic rings gave poor yields. However, it was found that this new reagent
is equally suitable and efficient for the oxidation of such derivatives (entries 15
and 16).
The mechanism of this reaction is still not clear. However, the reaction path-
way shown in Scheme 2 is tentatively proposed. Initially, electrophilic addition
of iodine to the enone to form 3, followed by elimination of HI with oxidative
cyclization yielding 5. A β-elimination of HI from 5 gave flavone 2. In support of
this mechanism, the model reaction on a relatively larger scale (10 mmol) was
performed and during the course of reaction, the pH of aqueous layer was
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SYNTHESIS OF FLAVONES
913
measured. The pH was found to be acidic, confirming thereby the elimination
product to be HI.
TABLE II. Cyclization of 2′-hydroxychalcone to flavones using NH I under solvent free con-
4
ditions; reaction conditions: 2′-hydroxychalcone (1 mmol), NH I (0.1 mmol) under neat
4
condition
a
Entry
Chalcone
R1
H
H
H
H
H
H
H
H
H
H
OMe
H
H
R2
H
H
H
H
H
H
H
Cl
Cl
H
H
OMe
H
H
H
H
H
R3
H
H
R4
H
OMe
R5
H
H
Flavone Yield , % M.p., °C
974
0a
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
92
89
82
94
79
82
78
74
81
75
79
84
91
93
71
67
72
4
0a
0a
0b
0c
0d
0e
157
174
4
OMe OMe OMe
H
H
OMe OMe
OMe
H
H
H
H
H
H
Br
H
H
H
4
4
Cl
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
189
110
4
154
132
118
172
4
H
H
Cl
NO2
OMe
OMe
Br
H
H
OMe
NMe2
4
4
4
0f
0i
0j
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
242
4
0k
143
95
4
0k
4
0l
177
4
0m
H
115
240
264
192
4
0g
0h
0n
OH
OH
H
4
4
1
a
Isolated yield after purification
Scheme 2. Possible mechanism of flavone synthesis using NH I via oxidative cyclization.
4
CONCLUSIONS
In conclusion, an efficient general method for the conversion of 2′-hydroxy-
chalcones to the corresponding flavones using in situ generated iodine is
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KULKARNI et al.
reported. The products were obtained in a shorter time, in high yields, and
employed a less hazardous and inexpensive reagent than molecular iodine. Other
attributes include its applicability to substrates bearing electron donating and
electron withdrawing as well as a free hydroxyl group on the B ring of chalcones.
Thus, this methodology would serve as attractive alternative to the use of toxic
molecular iodine.
Acknowledgement. One of the authors D. D. Kondhare would like to thank DST-New
Delhi for the financial assistance provided by an Inspire Fellowship. Dr. Ravi Varala thanks
Prof. Rajendra Sahu (Director, IIIT Basar) and Prof. R. V. Raja Kumar (Vice Chancellor,
RGUKT) for their encouragement. We thank the referees for their healthy suggestions.
И З В О Д
ЦИКЛИЗАЦИЈА 2′-ХИДРОКСИХАЛКОНА ДО ФЛАВОНА УПОТРЕБОМ
АМОНИЈУМ-ЈОДИДА КАО ИЗВОРА ЕЛЕМЕНТАРНОГ ЈОДА –
ЕКОЛОШКИ ПРИХВАТЉИВ ПРИСТУП
2
2
1
2
PRAMOD S. KULKARNI , DASHARATH D. KONDHARE , RAVI VARALA и PUDUKULATHAN K. ZUBAIDHA
1
School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded, Maharashtra
2
-
431 606, India и Department of Chemistry, AP-IIIT Basar, Rajiv Gandhi University of Knowledge
Technologies, Mudhole, Adilabad, Andhra Pradesh – 504 107, India
Амонијум-јодид се у присуству ваздуха разлаже на амонијак и елементарни јод.
Генерисан in situ у реакционој смеши, јод омогућава циклизацију 2′-хидроксихалкона до
одговарајућих флавона, у одсуству раставарача, у одличном приносу. Поступак се може
користити као добра алтернатива постојећим методама синтезе флавона.
(
Примљено 1. септембра, ревидирано 30. октобра 2012)
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