A new chemical access for 3'-acetyl-4'-hydroxychalcones using borontrifluoride-etherate via a regioselective Claisen-Schmidt condensation and its application in the synthesis of chalcone hybrids

Article abstract of DOI:10.1016/j.tetlet.2011.08.120

A new chemical access has been developed for the synthesis of 3'-acetyl-4'-hydroxychalcones from 1-(5-acetyl-2-hydroxy-phenyl)-ethanone and various substituted benzaldehydes via a regioselective Claisen-Schmidt condensation using borontrifluoride-etherate (BF₃·OEt ₂) at room temperature, in good to excellent yields within 12-24 h. Application of this methodology has also been demonstrated in the synthesis of chalcone hybrids.

Full text of DOI:10.1016/j.tetlet.2011.08.120

Tetrahedron Letters 52 (2011) 5794–5798
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Tetrahedron Letters
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A new chemical access for 30-acetyl-40-hydroxychalcones using
borontrifluoride–etherate via a regioselective Claisen-Schmidt condensation
and its application in the synthesis of chalcone hybrids
T. Narender a, , K. Venkateswarlu a, B. Vishnu Nayak b, S. Sarkar a
⇑
a Medicinal and Process Chemistry Division, Central Drug Research Institute (CSIR), Lucknow 226 001, UP, India
b National Institute of Pharmaceutical Education and Research, Raibareli, UP, India
a r t i c l e i n f o
a b s t r a c t
A new chemical access has been developed for the synthesis of 30-acetyl-40-hydroxychalcones from
1-(5-acetyl-2-hydroxy-phenyl)-ethanone and various substituted benzaldehydes via a regioselective
Claisen-Schmidt condensation using borontrifluoride–etherate (BF3ꢀOEt2) at room temperature, in good
to excellent yields within 12–24 h. Application of this methodology has also been demonstrated in the
synthesis of chalcone hybrids.
Article history:
Received 22 July 2011
Revised 19 August 2011
Accepted 21 August 2011
Available online 26 August 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Borontrifluoride–etherate
Chalcone hybrids
Regioselective Claisen-Schmidt
condensation
Chalcones are the main precursors for the biosynthesis of flavo-
noids, which are frequent components of the human diet. Licoch-
alcone A (I) isolated from the roots of Glycyrrhiza inflata (licorice)
has in vitro and in vivo antimalarial1 and antileishmanial activi-
ties,2 3-methoxy-4-hydroxyloncocarpin (II) isolated from the roots
of Lonchocarpus utilis inhibits the NADH:ubiquinone oxidoreduc-
tase activity3 and synthetic chalcones such as 2,4-dimethoxy-40-
allyloxychalcone (III) and 2,4-dimethoxy-40-butoxychalcone (IV)
have been reported as antileishmanial agents4 (Fig. 1). Recent
studies of some of the chalcones on biological evaluation were
found to be anticancer,5 anti-inflammatory,6 antimitotic,7 antitu-
bercular,8 cardiovascular,9 cell differentiation inducing,10 nitric
oxide regulation modulatory11, and antihyperglycemic agents.12
Of the many methods available for the synthesis of chalcones,
the most widely used method is the base catalyzed Claisen–
Schmidt reaction in which the condensation of a ketone with an
aldehyde is carried out in the presence of aq NaOH,13 KOH,14
Ba(OH)2,15 hydrotalcites,16 LiHDMS17, and calcined NaNO3/natural
phosphates.18 The acid catalyzed methodologies include the use of
AlCl3,19 dry HCl,20 Zn(bpy)(OAc)2,21 TiCl4,22 Cp2ZrH2/NiCl2,23 Zeo-
lites16, and RuCl3.24
polyacetoxyacetophenones,27 and synthesis of chromano-chalcones
via regioselective cyclization of prenylated chalcones in high
yields.28 We also reported the antimalarial activity of naturally
occurring prenylated chalcones,29 and antileishmanial activity of
natural chromenodihydrochalcones and synthetic chromenochal-
cones.30 In continuation of our drug discovery program on antipara-
sitic agents, we wanted to synthesize chalcone hybrids, which
contain chalcone core moiety with coumarin (VI), flavone (VII), ben-
zofuran (VIII) skeleton and evaluate their advanced biological activ-
ities (Fig. 2).
OH
OMe
OH
O
HO
OMe
OH
O
O
II
I
OMe
OMe
Recently, we utilized BF3ꢀOEt2 as a condensing agent in the syn-
O
O
thesis of chalcones,25 stilbenes,26 regioselective deacetylation of
OMe
OMe
O IV
O
⇑
Corresponding author. Tel.: +91 522 2612411; fax: +91 522 2623405.
III
E-mail
addresses:
tnarender@rediffmail.com,
t_narendra@cdri.res.in
(T. Narender).
Figure 1. Natural and synthetic chalcones of biological importance.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.120

T. Narender et al. / Tetrahedron Letters 52 (2011) 5794–5798
5795
R
HO
O
O
V
R
O
R
R
R
O
O
HO
O
O
O
O
O
O
VIO
VII
O
VIII
Figure 2. Examples for hybrid molecules with chalcone skeleton.
R1
R1
R2
OH
O
HO
R2
R1
R2
HO
O
BF3
.Et2O
O
1,4 Dioxane
OHC
O
O
2a : R1 = NMe2, R2 = H
3a : R1 = NMe2, R2 = H; 80%
3b : R1, R2 = OMe; 71%
3a : R1 =NMe2, R2 = H
3b : R1, R2 = OMe
1
O
2b : R1, R2 = OMe
OH
O
R1
R2
R1
R2
R1
R2
Scheme 2. Synthesis of acetylated chalcones via regioselective Claisen-Schmidt
condensation reaction using BF3ꢀOEt2.
+
KOH/EtOH
rt, 24 h
HO
CHO
O
the acetyl groups to generate the chalcone scaffold and utilize
the second acetyl group to bring other functionalities such as bis-
chalcone (V), coumarin (VI), flavone (VII), and benzofuran (VIII)
(Fig. 2). Initially the diacetylated phenol 1 was subjected to
Claisen-Schmidt condensation with 4-(dimethylamino)benzalde-
hyde (2a) using aqueous KOH in ethanol (Scheme 1), which re-
sulted in the synthesis of mixture of chalcones, 3a, 4a and 5a.32
In our previous studies on the synthesis of chalcones using
BF3ꢀOEt2, we observed that the acetophenones, which have a che-
lated hydroxyl failed to provide the chalcones via Claisen-Schmidt
condensation reaction.25 The acetyl group which is in chelation
with the hydroxyl group might be forming a complex with
BF3ꢀOEt2 to prevent the condensation reaction (Fig. 3). We there-
fore planned to explore the above described property of BF3ꢀOEt2
to perform the regioselective Claisen-Schmidt condensation reac-
tion on 1. On the basis of this background a reaction was attempted
using the same reactants (1 and 2a/2b) as described for KOH with
BF3ꢀOEt2, which provided the desired acetylated chalcone 3a and
1
2a: R1 = NMe2, R2 =H
2b : R1, R2 = OMe
O
O
4a : R1 = NMe2, R2 = H
4b : R1, R2 = OMe
R1
+
HO
R2
O
O
5a : R1 = NMe2, R2 = H
5b : R1, R2 = OMe
Scheme 1. Synthesis of chalcones via Claisen-Schmidt condensation using KOH in
EtOH.
We therefore prepared the key starting material, 1-(5-acetyl-
2-hydroxy-phenyl)-ethanone (1) by carrying out
a
Fries-
rearrangement on O-acetylphenol31 and planned to use one of
F
B
O
F
B
O
F
O
O
R
F
O
O
Sub stituted
BF3.Et2O
HO
Benzaldehydes
H
BF3.Et2O
O
O
O
H
F
O
O
B
O
F
R
R
HO
HO
-H2O
-Boron complexes
O
Figure 3. Possible reaction mechanism in the formation of acetylated chalcones using BF3ꢀOEt2.
O

5796
T. Narender et al. / Tetrahedron Letters 52 (2011) 5794–5798
Table 1
Regioselective synthesis of chalcones using BF3ꢀOEt2
Entry
Ketone
Aldehyde
Chalcone
% Yielda
HO
NMe2
NMe2
O
HO
O
OHC
1
80
2a
1
1
O
O
3a
O
HO
O
OMe
OMe
OMe
OMe
HO
OHC
OHC
2
3
71
63
2b
2c
O
3b
3c
O
O
HO
O
HO
O
1
1
O
O
HO
O
HO
4
OHC
OHC
OHC
62
2d
2e
O
3d
O
O
O
HO
O
HO
5
6
7
67
73
71
O
1
1
1
O
O
O
3e
3f
HO
O
HO
O
Cl
Cl
2f
HO
O
Cl
Cl
HO
OHC
OHC
2g
2h
O
3g
3h
O
O
HO
O
HO
OMe
OMe
8
53
72
55
78
O
1
1
O
O
HO
O
Cl
Cl
Cl
Cl
HO
OHC
OHC
9
2i
O
3i
O
O
HO
O
HO
10
11
O
OMe
OMe
1
1
O
O
2j
3j
3k
3l
HO
O
OMe
OMe
Cl
HO
OHC
OHC
2k
O
O
O
HO
O
Cl
HO
12
76
1
O
O
Cl
2l
Cl

T. Narender et al. / Tetrahedron Letters 52 (2011) 5794–5798
5797
Table 1 (continued)
Entry
Ketone
Aldehyde
Chalcone
% Yielda
HO
OH
O
OH
O
HO
O
OHC
13
14
31
65
2m
1
1
O
O
3m
O
HO
O
O
O
O
HO
O
OHC
2n
3n
O
a
Isolated yields.
O
O
6
O
Cl
ii
R
OH
O
O
O
HO
O
R
i
iii
N
7
O
OHC
1
O
O
2a R = NMe2
MeO
3a R = NMe2
2c R = CH3
3c R = CH3
MeO
iv
HO
8
O
O
Scheme 3. Synthesis of hybrid molecules (chalcone-coumarin, chalcone-benzofuran and bischalcone). Reagents and conditions: (i) BF3ꢀOEt2, 1,4-dioxane; (ii) p-
Chlorophenylaceticacid, Ac2O,TEA,16 h, heat; (iii) p-Methoxyphenacyl bromide, K2CO3, acetonitrile, 98 °C;(iv) 4-Methoxybenzaldehyde, KOH, EtOH, rt, 24 h.
3b respectively in good yields (Scheme 2).33 Other Lewis acids such
as AlCl3, ZnCl2, SnCl4, and TiCl4 failed to provide the products
regioselectively.
To explore the generality of the reaction, we carried out a sim-
ilar reaction with various substituted benzaldehydes 2c–2n, which
provided the desired acetylated chalcones 3c–3n in good to excel-
lent yields with the exception of 3m (Scheme 2, Table 1). Starting
materials were recovered in low yield reactions during column
chromatography and we did not observe the formation of their
regioisomers.
vide the regioselective product. We also utilized these intermedi-
ates to synthesize the hybrid molecules such as bischalcones,
coumarin-chalcone, and benzofuran-chalcone to evaluate their
biological potential. This standardized method can be used to gen-
erate a large number of hybrid molecules of biological importance.
Acknowledgments
Authors are thankful to Dr. T. K. Chakraborty, Director CDRI and
Dr. D. K. Dikshit, Project Director, NIPER for their constant encour-
agement for the program on natural products, SAIF for spectral
data and CSIR, New Delhi for financial support. This CDRI commu-
nication No. 8124.
As described in our previous reports,27,28 the reaction mecha-
nism in the regioselective condensation reaction appears to be
BF3ꢀOEt2’s complex formation with chelated hydroxyl and acetyl
groups and second mole of BF3ꢀOEt2 might be participating in the
condensation reaction with second free acetyl group to provide
the desired acetylated chalcones (Fig. 3). Further studies are re-
quired to confirm the exact reaction mechanism.
To demonstrate the wide application of this methodology, the
acetylated chalcone, 3c was reacted with p-chlorophenylacetic acid
in the presence of Ac2O and triethylamine (TEA), which gave the
coumarin-chalcone hybrid 6.34 Similarly 3c upon reaction with
p-methoxyphenacyl bromide provided us the bezofuran-chalcone
hybrid 7.35 Claisen-Schmidt condensation of 3a with p-methoxy-
benzaldehyde using KOH resulted in the synthesis of bischalcone 8
(Scheme 3).32
Supplementary data
Supplementary data (compounds characterization data and
spectra (NMR and Mass) of all new compounds) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.08.120.
References and notes
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In conclusion, a new chemical access has been developed for the
synthesis of 30-acetyl-40-hydroxychalcones via a regioselective
Claisen-Schmidt condensation using BF3ꢀOEt2 for the first time.
Other Lewis acids such as AlCl3, ZnCl2, SnCl4, and TiCl4 failed to pro-
3. Fang, N.; Casida, J. E. J. Nat. Prod. 1999, 62, 205–210.

5798
T. Narender et al. / Tetrahedron Letters 52 (2011) 5794–5798
4. Chen, M.; Zhai, L.; Christensen, S. B.; Theander, T. G.; Kharazmi, A. Antimicrob.
Agents Chemother. 2001, 45, 2023–2029.
32. Representative procedure for the synthesis of, chalcones 3a, 5a and bischalcone 4a.
To solution of 1-(5-acetyl-2-hydroxy-phenyl)-ethanone (1) (200 mg,
a
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(150 mg, 43%), 5a (50 mg, 14%), and bischalcone 4a (80 mg, 16%).
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Compound 3a:1H NMR (300 MHz, CDCl3) d 13.89 (s, 1H), 8.65 (d, J = 2.1 Hz, 1H),
8.08 (dd, J = 1.4 Hz, 8.7 Hz, 1H), 7.97 (d, J = 15.0 Hz, 1H), 7.66 (d, J = 7.8 Hz, 2H),
7.56 (d, J = 15.0 Hz, 1H), 7.07 (d, J = 8.7 Hz, H), 6.75 (d, J = 7.8 Hz, 2H), 3.10 (s,
6H), 2.40 (s, 3H); MS (ESI) m/z 310 (M+H)+.
Compound 4a:1H NMR (300 MHz, CDCl3) d 13.79 (s, 1H), 8.71 (d, J = 2.7 Hz, 1H),
8.16 (dd, J = J = 1.0 Hz, 8.7 Hz, 1H), 7.94 (d, J = 16.7 Hz, 1H), 7.88 (d, J = 15.1 Hz,
2H), 7.64 (m, 5H), 7.55 (d, J = 15.1 Hz, 1H), 7.09 (d, J = 13 Hz, 2H), 6.73 (d,
J = 13 Hz, 4H), 3.08 (s, 6H), 3.05 (s, 6H); MS (ESI) m/z 441 (M+H)+.
´
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Compound 5a:1H NMR (300 MHz, CDCl3) d 12.66 (s, 1H), 8.53 (d, J = 2.7 Hz, 1H),
8.20 (dd, J = 2.0 Hz, 8.8 Hz, 1H); 7.87 (d, J = 15.3 Hz, 1H), 7.60 (m, 2H), 7.35 (d,
J = 15.3 Hz, 1H), 7.09 (d, J = 8.7 Hz, 1H), 6.74 (d, J = 8.8 Hz, 2H), 3.07 (s, 6H), 2.76
(s, 3H); MS (ESI) m/z 310 (M+H)+.
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33. Representative procedure for the synthesis of acylatedchalcone 3a. To a stirred
solution of 1 (100 mg, 0.56 mmol) and 4-(dimethylamino)benzaldehyde (2a)
(83 mg, 0.56 mmol) in dry 1,4 dioxane was added gradually BF3ꢀOEt2
(0.142 mL, 1.12 mmol) at room temperature. The whole reaction mixture
was stirred for 24 h. The reaction mixture was then diluted with ethyl acetate
(100 mL) and washed with water (3 x 25 mL) to decompose the BF3ꢀOEt2
complex. The organic solution obtained after extraction was dried over anhyd.
Na2SO4, filtered and the solvent was evaporated under reduced pressure. The
crude product was purified by silica gel column chromatography using hexane-
ethyl acetate (93:7) as a mobile phase to afford 3a. (140 mg, 80%), 1H NMR
(300 MHz, CDCl3) d 13.89 (s, 1H), 8.65 (d, J = 2.1 Hz, 1H), 8.08 (dd, J = 1.4 Hz,
8.7 Hz, 1H), 7.97 (d, J = 15.0 Hz, 1H), 7.66 (d, J = 7.8 Hz, 2H), 7.56 (d, J = 15.0 Hz,
1H), 7.07 (d, J = 8.7 Hz, 1H), 6.75 (d, J = 7.8 Hz, 2H), 3.10 (s, 6H), 2.40 (s, 3H); MS
(ESI) m/z 310 (M+H)+.
34. General procedure for the synthesis of coumarin-chalcone hybrid 6. A mixture of
acetylated chalcone 3c (100 mg, 0.35 mmol), p-chlorophenylacetic acid
(182 mg, 0.35 mmol), acetic anhydride (3 mL), and TEA (1.2 mL) was refluxed
for 16 h, then cooled to room temperature and poured on to ice-water. The
separated solid was filtered, washed successively with aq. sodium bicarbonate,
water and crystallized from methanol to obtain the desired compound 6
(95 mg, 65%). 1H NMR (300 MHz, CDCl3) d 8.39 (s, 1H), 8.23 (d, J = 8.4 Hz, 1H),
7.90 (d, J = 15.6 Hz, 1H), 7.60 (d, J = 15.6 Hz, 1H), 7.56 (m, 2H), 7.47 (m, 3H),
7.25 (m, 4H), 2.43 (s, 6H);); 13C (50Mz, CDCl3) d 188.3, 160.3, 155.5, 148.2,
146.2, 141.9, 134.8, 132.6, 132.1, 131.7 (4C), 130.9 (2C), 129.1 (3C), 128.9 (2C),
126.5, 120.8, 120.4, 117.3, 21.8, 17.0; MS (ESI) m/z 415 (M+H)+.
35. General procedure for the synthesis of benzofuran-chalcone hybrid 7. A mixture of
p-methoxyphenacyl bromide (81 mg, 0.35 mmol), acetylated chalcone 3c
(100 mg, 0.35 mmol) and K2CO3 (49 mg, 0.35 mmol) was taken up in 5 mL of
acetonitrile (CH3CN) and stirred continuously. The heterogeneous mixture was
heated at reflux for 3 h under atmosphere of N2 until the transformation was
complete (TLC). After the heat source was removed, 20 mL of water was added,
and the reaction mixture was allowed to cool to ambient temperature. The
mixture was filtered, washed with H2O/CH3CN; 2:1, CH3CN and then dried to
give desired compound 7 in (105 mg, 72%), 1H NMR (300 MHz, CDCl3) d 8.41 (s,
1H), 8.23 (m, 3H), 7.90 (d, J = 15.6 Hz, 1H), 7.64 (d, J = 15.6 Hz, 1H), 7.62 (m, 3H),
7.25 (d, J = 6.9 Hz, 2H), 7.06 (d, J = 8.8 Hz, 2H), 3.93 (s, 3H), 2.74 (s, 3H), 2.43 (s,
3H); 13C NMR (75 MHz, CDCl3) d 189.4, 184.3, 163.8, 156.6, 150.1, 145.4, 141.5,
134.5, 132.6 (2C), 132.4, 130.5, 129.7 (2C), 128.8, 128.7 (2C), 128.4, 126.7, 123.0
121.2, 114.0 (2C), 112.6, 55.8, 21.8, 10.3; MS (ESI) m/z 411 (M+H)+.
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