Epoxidation of chromones and flavonoids in ionic liquids

Article abstract of DOI:10.1016/j.tet.2003.11.032

A convenient and efficient procedure for the epoxidation of chromone, isoflavone, and chalcone derivatives using 1-butyl-3-methyl imidazolium tetrafluoroborate [bmim]BF₄ as solvent and alkaline hydrogen peroxide as oxidant is described. All reactions proceed in good yields and faster than in conventional solvents. No evidence of formation of compounds derived from the opening of the epoxide ring was attained.

Full text of DOI:10.1016/j.tet.2003.11.032

Tetrahedron 60 (2004) 967–971
Epoxidation of chromones and flavonoids in ionic liquids
a
a
b
,
Roberta Bernini,a Enrico Mincione, Antonietta Coratti, Giancarlo Fabrizi
,
*
*
and Gianfranco Battistuzzib
a
`
Dipartimento A.B.A.C, Universita degli Studi della Tuscia, Via S. Camillo De Lellis, 01100 Viterbo, Italy
`
b
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Universita degli Studi di Roma “La Sapienza”,
P.le A. Moro 5, 00185 Rome, Italy
Received 22 July 2003; revised 28 October 2003; accepted 13 November 2003
Abstract—A convenient and efficient procedure for the epoxidation of chromone, isoflavone, and chalcone derivatives using 1-butyl-3-
methyl imidazolium tetrafluoroborate [bmim]BF4 as solvent and alkaline hydrogen peroxide as oxidant is described. All reactions proceed in
good yields and faster than in conventional solvents. No evidence of formation of compounds derived from the opening of the epoxide ring
was attained.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
been performed in conventional solvents (acetone, toluene,
tetrahydrofuran) as well as under phase-transfer
conditions.12–14
Natural products having a chromonic and flavonoidic
structure (chalcones and isoflavones) exhibit important
biological properties such as antiviral, cardioprotective,
antioxidant, hepatoprotective, antitumoral and antiflamma-
tory activities.1 Their corresponding epoxides are postulated
as biosynthetic intermediates of some natural compounds.
Chalcone and isoflavone epoxides are useful building blocks
in flavonoid chemistry.2 A nucleophilic oxidant such as
alkaline hydrogen peroxide has been suggested to be the
reagent of choice for the direct preparation of these
compounds (Weitz–Scheffer epoxidation).3 Recently, the
epoxidation of chalcones and isoflavones by using dimethyl-
dioxirane (DMD), an electrophilic oxidant,4 has been
reported. However, long reaction times (up to 140 h) and
a large excess of the reagent (up to 15 equiv.) are required.5
Our interest in the functionalization of natural molecules by
benign oxidative methodologies and in the preparation of
fine-chemicals as well as bioactive compounds15 prompted
us to investigate the development of an epoxidation protocol
for chromenes and flavonoids under environmentally
friendly conditions. Particularly, we focused our attention
on the use of room temperature ionic liquids. They are
considered very promising and attractive substitutes for
volatile organic solvents and are widely used in the Green
Chemistry area.16 In fact, having a negligible vapour
pressure, their loss into the environment by evaporation is
minimal; this property may offer environmental advantages
in industrial processes. Ionic liquids possess several other
attractive properties including a high flash point, thermal
stability, immiscibility with some organic solvents, and
capacity to dissolve a wide range of organic, inorganic and
organometallic compounds. Cations usually present in room
temperature ionic liquids are: tetraalkylammonium, tetra-
alkylphosphonium, trialkylsulfonium, N-alkylpyridinium
and 1,3-dialkylimidazolium. Anions are generally poly-
atomic inorganic species (Fig. 1). They have been employed
in a variety of organic reactions such as hydrogenation,17
olefin dimerization and oligomerization,18 Heck reactions,19
hydroformylation,20 alkoxycarbonylation,21 and allylic
substitution.22 To the best of our knowledge, however,
very little has been done in the oxidation area,23 in particular
in the epoxidation of natural compounds. Recently, the
epoxidation of simple cyclohexenone derivatives by
alkaline hydrogen peroxide in ionic liquids has been
reported.24
Because of the role of chiral epoxides as useful inter-
mediates in the synthesis of natural products and drug
molecules,6 the asymmetric epoxidation of a,b-unsaturated
ketones has also been widely investigated in the last few
years. An asymmetric Weitz–Scheffer epoxidation of
isoflavones mediated by optically active cinchonine
catalysts has been reported,7 as well as the use of the
Jacobsen’s Mn(III)salen catalyst in the presence of DMD as
the source of the oxygen atom.8 Chalcones have been
subjected to asymmetric epoxidation by using a variety of
polymer-bound chiral supports.9–11 All these reactions have
Keywords: Epoxidation; Chromones; Flavonoids; Chalcones; Isoflavones;
Ionic liquids; [bmim]BF4.
*
Corresponding authors. Fax: þ39-0761357230;
e-mail address: berninir@unitus.it
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.11.032

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R. Bernini et al. / Tetrahedron 60 (2004) 967–971
Table 1. Epoxidation of chromones 1, 3, 5, 7, isoflavones 9, 11, 13, and
chalcones 15, 17, 19 with H2O2/NaOHa in conventional solvents
(dichloromethane or acetone) and [bmim]BF4
Entry Substrate
Conditionsa
Epoxide Conversion Yield
(%)b
(%)b
1
2
3
4
5
6
7
8
1
1
3
3
5
5
7
7
CH3COCH3, 0 8C, 2 h
[bmim]BF4, 0 8C, 2 h
CH2Cl2, 0 8C, 2 h
[bmim]BF4, 0 8C, 2 h
CH2Cl2, 0 8C, 2 h
2
2
4
4
6
6
8
8
10
10
12
12
14
14
40
.98
5
62
6
.98
.98
.98
.98
.98
.98
.98c
.98c
.98
.98
.98
.98
.98
.98
.98
.98
.98
.98
.98
.98
[bmim]BF4, 0 8C, 2 h
CH3COCH3, 0 8C, 2 h
[bmim]BF4, 0 8C, 2 h
CH3COCH3, rt, 2 h
[bmim]BF4, rt, 2 h
CH3COCH3, rt, 2 h
[bmim]BF4, rt, 2 h
CH3COCH3, rt, 2 h
[bmim]BF4, rt, 2 h
CH3COCH3, rt, 0.5 h 16
[bmim]BF4, rt, 0.5 h 16
CH3COCH3, rt, 0.5 h 18
[bmim]BF4, rt, 0.5 h 18
CH3COCH3, rt, 0.5 h 20
[bmim]BF4, rt, 0.5 h 20
52
5c
65c
20
98
20
98
12
82
5
9
9
9
10
11
12
13
14
15
16
17
18
19
20
Figure 1. Common cations and anions in room temperature ionic liquids.
11
11
13
13
15
15
17
17
19
19
We report here that satisfactory results in the epoxidation of
the enone moiety of natural compounds such as chromone,
chalcone, and isoflavone derivatives can be obtained by
hydrogen peroxide in 1-butyl-3-methyl imidazolium tetra-
fluoroborate, [bmim]BF4 (Fig. 2).
98
7
98
5
95
a
3 equiv. of H2O2 (35% solution in water) and 2 equiv. of NaOH.
Conversions and yields were determined after chromatographic purifi-
cation of reaction mixtures.
b
c
Conversions and yields were determined by 1H NMR analysis of crude
reaction mixtures.
Figure 2. 1-Butyl-3-methyl imidazolium tetrafluoroborate [bmim]BF4.
by 13C NMR data and GC–MS fragmentation (Mþ¼162).
No evidence of formation of products derived from the
opening of the epoxide ring was attained.
2. Results and discussion
For our initial investigation, the epoxidation of chromone 1
with alkaline hydrogen peroxide was selected as a model
reaction. For comparison, reactions were carried out in
acetone and [bmim]BF4. After 2 h, the epoxide derivative 2
was obtained in only 40% yield in acetone and in .98%
yield in the ionic liquid (Scheme 1, Table 1, entries 1 and 2).
In agreement with the formation of the epoxide ring,3b its 1H
NMR spectrum shows two doublets at 3.70 and 5.72 ppm
(J¼2.9 Hz). This structural assignment is further supported
Most probably, under our conditions, a nucleophilic
mechanism is operating in the oxidation of the
a,b-unsaturated fragment. Therefore, for the sake of
comparison, we also tested the reactivity of methyl-
trioxorhenium/hydrogen peroxide in [bmim]BF4. In fact,
the active peroxodicomplexe dpRe, generated from methyl-
trioxorhenium during the reaction course (Scheme 2), has
been reported to show some nucleophilic character.25
Scheme 1. Epoxidation of chromones 1, 3, 5, 7, isoflavones 9, 11, 13 and chalcones 15, 17, 19.

R. Bernini et al. / Tetrahedron 60 (2004) 967–971
969
ACS reagent grade solvents, were redistilled and dried
according to standard procedures. Preparation of
[bmim]BF4 was carried out according to Ref. 27. Thin
layer chromatography was carried out using Merck platen
Kieselgel 60 F254. Reaction products were purified by flash
chromatography by using Merck silica gel 60, 230–400
(eluents: hexane/ethyl acetate, 9:1 or 8:2). IR spectra were
recorded on a Perkin–Elmer 298 spectrophotometer using
NaCl paltes. NMR spectra were recorded on a Bruker
(200 MHz) spectrometer and are reported in d values. Mass
spectra were recorded on a VG 70/250S spectrometer with
an electron beam of 70 eV. Elementary analyses were
performed by a Carlo Erba 1106 Analyser.
Scheme 2. Epoxidation of alkenes by the H2O2/CH3ReO3 catalytic system.
4.1. General procedure for the oxidation of chromone
and flavonoid derivatives with H2O2/NaOH in
[bmim]BF4
Nevertheless, though the desired epoxide derivative was
formed as almost the only product, a very low conversion of
the substrate was observed (10–15%), even charging a large
excess of the oxidation system (10% of methyltrioxo-
rhenium and up to 12 equiv. of hydrogen peroxide).
The substrate (1.0 mmol) was solubilized in [bmim]BF4
(1 mL). Then, hydrogen peroxide (35% aqueous solution,
3.0 mmol) and NaOH power (2.0 mmol) were added.
Reactions were monitored by thin layer chromatography
and by gas-chromatography. Products were extracted
with diethyl ether, the organic layer was evaporated
under vacuum and the mixture was purified by flash-
chromatography.
Our reaction conditions (H2O2/NaOH in [bmim]BF4 at 0 8C
or rt) were next extended to other chromone derivatives
such as 6-chlorochromone 3, 6-methylchromone 5,
2-methylchromone 7 and to some natural flavonoids such
as isoflavone 9, 6-methoxyisoflavone 11, 7-methoxyiso-
flavone 13, chalcone 15, 40-methoxychalcone 17, and
4-methoxychalcone 19 (Scheme 2). In every case, the use
of [bmim]BF4 resulted to be superior to that of molecular
solvents: the corresponding epoxides were isolated in
significantly higher yields in all the comparisons (Table 1,
compare entries 4, 6, 8, 10, 12, 14, 16, 18, and 20 with
entries 3, 5, 7, 9, 11, 13, 15, 17, and 19). In addition, faster
reaction times were always observed. Chromone epoxides 4,
6, and 8 were obtained in satisfactory yields in 2 h. The new
4.1.1. Chromone epoxide (1). Yellow solid, mp 64–66 8C
(lit.3b 65–66 8C). Found: C, 66.65; H, 3.73; O, 29.62.
C9H6O3 requires C, 66.67; H, 3.73; O, 29.60%; nmax (KBr):
3052, 1682; dH (CDCl3, 200 MHz): 7.87 (1H, dd,
J1¼7.9 Hz, J2¼1.8 Hz, Ph), 7.59–7.50 (1H, m, Ph), 7.18–
7.02 (2H, m, Ph), 5.72 (1H, d, J¼2.9 Hz, –COCHOCH–),
3.70 (1H, d, J¼2.9 Hz, –COCHOCH–); dC (CDCl3,
50 MHz): 188.1, 155.4, 136.3, 127.1, 123.4, 119.8, 118.0,
77.2, 55.3; m/z (EI) 162 (Mþ, 51.1%).
1
cromone products 4 and 6 were fully characterized by H
NMR, 13C NMR, IR spectra and GC–MS while 8 was found
to be unstable.3b After usual work-up with diethyl ether, the
reaction mixture could not be purified by chromatography
4.1.2. 6-Chloro chromone epoxide (4). Yellow solid, mp
92–94 8C. Found: C, 54.98; H, 2.57; O, 24.40; Cl, 18.05.
C9H5O3Cl requires C, 54.99; H, 2.56; O, 24.42; Cl, 18.03%;
1
without decomposition of the epoxide. However, H NMR
analysis of the crude reaction mixture (singlet at 3.64 ppm)
confirmed the presence of the epoxide ring3b as well as also
the GC–MS fragmentation. Isoflavone epoxides 10, 12, 14
and chalcone epoxides 16, 18 and 20 were obtained in
satisfactory yields. All products were fully characterized by
NMR, IR, and GC–MS (Section 4).
n
max (KBr): 3049, 1691; dH (CDCl3, 200 MHz): 7.85 (1H, d,
J¼2.6 Hz, Ph), 7.53–7.46 (1H, m, Ph), 7.03 (1H, d,
J¼8.8 Hz, Ph), 5.68 (1H, d, J¼2.8 Hz, –COCHOCH–),
3.71 (1H, d, J¼2.4 Hz, –COCHOCH–), dC (CDCl3,
50 MHz): 186.9, 153.8, 136.1, 129.0, 125.6, 120.2, 119.7,
77.4, 55.0; m/z (EI) 198 (Mþþ2, 17.0%).
4.1.3. 6-Methyl chromone epoxide (6). Yellow solid, mp
80–82 8C. Found: C, 68.21; H, 4.56; O, 27.23. C10H8O3
requires C, 68.19; H, 4.57, O, 27.24%; nmax (KBr): 3055,
1679; dH (CDCl3, 200 MHz): 7.67 (1H, d, J¼1.6 Hz, Ph),
7.38–7.33 (1H, m, Ph), 6.95 (1H, d, J¼8.4 Hz, Ph), 5.64
(1H, d, J¼2.4 Hz, –COCHOCH–), 3.67 (1H, d, J¼2.5 Hz,
–COCHOCH–), 2.31 (3H, s, Me); dC (CDCl3, 50 MHz):
188.3, 153.5, 138.1, 133.0, 126.7, 119.4, 117.7, 77.2, 55.3,
20.4; m/z (EI) 177 (Mþ, 6.7%).
3. Conclusions
We have developed a simple and efficient procedure for the
epoxidation of natural compounds containing the a,b-enone
moiety in [bmim]BF4. The method merits attention due to
the simplicity of the experimental procedure, the reduced
waste production and the high yields in short reaction time.
4. Experimental
4.1.4. Isoflavone (9). White solid, mp 129–131 8C (lit.26
131–132 8C). Found: C, 81.10; H, 4.53; O, 14.37. C15H10O2
requires C, 81.07; H, 4.53; O, 14.40%; nmax (KBr):
1675, 1638, 1567, 1465; dH (CDCl3, 200 MHz): 8.30
(1H, dd, J1¼7.9 Hz, J2¼1.6 Hz, Ph); 7.99 (1H, s, vCH),
7.70–7.36 (8H, m, Ph); dC (CDCl3, 50 MHz): 176.1, 156.1,
Melting points were determined with a Bu¨chi apparatus and
are uncorrected. Chromones 1, 3, 5, 7 and chalcones 15, 17,
19 are commercially available (Aldrich) and were used as
purchased. Isoflavones 9, 11 and 13 were synthesized
according to literature.26 Acetone and dichloromethane,

970
R. Bernini et al. / Tetrahedron 60 (2004) 967–971
153.0, 136.0, 133.7, 133.5, 131.7, 128.8, 128.4, 128.1,
127.7, 127.1, 126.3, 125.1, 117.9; m/z (EI) 222 (Mþ,
49.8%).
Acknowledgements
We are sincerely grateful to Consorzio I.N.C.A and Italian
MURST 5% “Progetto Reflui Oleari” for their financial
support.
4.1.5. Isoflavone epoxide (10). Colourless oil. Found: C,
75.60; H, 4.24; O, 20.16. C15H10O3 requires C, 75.62; H,
4.23; O, 20.15%; nmax (KBr): 3043, 1683; dH (CDCl3,
200 MHz): 8.01 (1H, dd, J1¼7.9 Hz, J2¼1.7 Hz, Ph),
7.64–7.38 (6H, m, Ph), 7.23–7.08 (2H, m, Ph), 5.51 (1H,
s, –OCHO); dC (CDCl3, 50 MHz): 187.4, 155.0, 136.1,
130.4, 129.0, 128.4, 128.3, 127.7, 127.1, 127.0, 123.3,
120.0, 117.8, 82.9, 63.0; m/z (EI) 238 (Mþ, 1.0%).
References and notes
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4.1.6. 6-Methoxyisoflavone (11). Yellow solid, mp 137–
139 8C. Found: C, 76.21; H, 4.78; O, 19.01. C16H12O3
requires C, 76.18; H, 4.79; O, 19.03%; nmax (KBr): 1670,
1640, 1440; dH (CDCl3, 200 MHz): 7.99 (1H, s, vCH), 7.65
(1H, d, J¼3.0 Hz, Ph), 7.57 (2H, dd, J¼8.2 Hz, Ph), 7.47–
7.39 (4H, m, Ph), 7.25 (1H, dd, J1¼9.1 Hz, J2¼3.2 Hz, Ph),
3.89 (3H, s, OMe); dC (CDCl3, 50 MHz): 175.9, 157.0,
152.9, 151.0, 132.0, 128.9, 128.7, 128.6, 128.4, 128.1,
125.2, 124.5, 123.6, 119.4, 105.4, 55.7; m/z (EI) 252 (Mþ,
80.7%).
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4.1.7. 6-Methoxyisoflavone epoxide (12). Colourless oil.
Found: C, 71.60; H, 4.50; O, 23.90. C16H12O4 requires C,
71.63; H, 4.51; O, 23.86%; nmax (KBr) 3020, 1670;?dH
(CDCl3, 200 MHz): 7.44–7.38 (7H, m, Ph), 7.03 (1H, d,
J¼9.1 Hz, Ph), 5.48 (1H, s, –OCHO), 3.81 (3H, s, OMe); dC
(CDCl3, 50 MHz): 187.4, 155.4, 149.4, 130.6, 128.9, 128.7,
128.3, 127.2, 126.1, 125.0, 120.6, 119.1, 108.2, 82.9, 62.8,
55.8; m/z (EI) 268 (Mþ, 1.7%).
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4.1.8. 7-Methoxyisoflavone (13). White solid, mp 140–
142 8C. Found: C, 76.15; H, 4.79; O, 19.06. C16H12O3
requires C, 76.18; H, 4.79; O, 19.03%; nmax (KBr): 1680,
1634, 1441; dH (CDCl3, 200 MHz): 8.20 (1H, d, J¼8.9 Hz,
Ph), 7.92 (1H, s, vCH); 7.56–7.52 (2H, m, Ph), 7.46–7.35
(3H, m, Ph), 6.98 (1H, dd, J1¼8.9 Hz, J2¼2.4 Hz, Ph), 6.84
(1H, d, J¼2.36 Hz), 3.89 (3H, s, OMe); dC (CDCl3,
50 MHz): 190.5, 175.5, 164.0, 157.9, 152.5, 131.9, 128.9,
128.7, 128.4, 128.0, 127.8, 125.2, 118.4, 114.5, 100.1, 55.8;
m/z (EI) 252 (Mþ, 69.8%).
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4.1.9. 7-Methoxyisoflavone epoxide (14). White solid, mp
121–123 8C (lit.3b 123–124 8C). Found: C, 71.62; H, 4.50;
O, 23.88. C16H12O4 requires C, 71.63; H, 4.51; O, 23.86%;
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max (KBr) 3005, 1689; dH (CDCl3, 200 MHz): 7.94 (1H, d,
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J¼8.8 Hz, Ph), 7.48–7.37 (5H, m, Ph), 6.74 (1H, dd,
J¼11.2 Hz, Ph), 6.53 (1H, d, J¼2.3 Hz, Ph), 5.47 (1H, s,
–OCHO), 3.86 (3H, s, OMe); dC (CDCl3, 50 MHz): 186.0,
166.1, 157.1, 130.7, 129.4, 128.9, 128.7, 128.4, 128.3,
127.1, 113.5, 111.6, 101.0, 83.2, 62.3, 55.7; m/z (EI) 268
(Mþ, 2.5%).
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4.1.10. Chalcone epoxide (16). White solid, mp 88–90 8C
(lit.5a 88–89 8C).
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4.1.11. 40-Methoxychalcone epoxide (18). White solid, mp
74–76 8C (lit.5a 75–76 8C).
4.1.12. 4-Methoxychalcone epoxide (20). White solid, mp
82–84 8C (lit.5a 82–83 8C).

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