A straightforward conversion of aurones to 2-benzoylbenzofurans: Transformation of one class of natural products into another

Article abstract of DOI:10.1016/j.tet.2011.08.011

The naturally occurring aurones (2-benzylidene-3(2H)-benzofuran-3-ones) can be easily converted to another class of natural products 2-benzoylbenzo[b] furans, via an effective reduction, acid-mediated rearrangement, and oxidation cascade. This easy conver

Full text of DOI:10.1016/j.tet.2011.08.011

Tetrahedron 67 (2011) 7703e7707
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A straightforward conversion of aurones to 2-benzoylbenzofurans:
transformation of one class of natural products into another
*
ꢀ
Samir Yahiaoui, Marine Peuchmaur, Ahcene Boumendjel
ꢁ
ꢁ
ꢁ
Universite de Grenoble/CNRS, UMR 5063, Departement de Pharmacochimie Moleculaire, BP 53, 38041 Grenoble Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2011
Received in revised form 29 July 2011
Accepted 3 August 2011
Available online 10 August 2011
The naturally occurring aurones (2-benzylidene-3(2H)-benzofuran-3-ones) can be easily converted to
another class of natural products 2-benzoylbenzo[b]furans, via an effective reduction, acid-mediated
rearrangement, and oxidation cascade. This easy conversion was conducted without purification of in-
termediates. This straightforward conversion may be considered as a possible biosynthesis pathway of 2-
benzoylbenzo[b]furans in plants.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Aurones
2-Benzoylbenzo[b]furans
Conversion
Natural products
1. Introduction
reported.5 The therapeutic potential of aurones was frequently
highlighted in various therapeutical domains, such as cancer,
2-Benzoylbenzo[b]furans and aurones (2-benzylidene-3-(2H)-
benzofuran-3-ones) (Fig. 1) are two naturally occurring classes of
compounds, bearing the same carbon skeleton (C6eC3eC6). 2-
Benzoylbenzo[b]furans have been isolated from different tradi-
tional medicine plants1 and they were the subject of various in-
vestigations related to their therapeutic potential.2 In some
circumstances, they were used as intermediates for the synthesis of
biological active compounds, such as aromatase inhibitors.3
Aurones are frequently isolated from flowers of different plant
species, where they are responsible for their gold-yellow color.4
Recently, the occurrence of aurones in marine sponges was
melanoma-related diseases, diabetes, and in cosmetic.6 2-
Benzoylbenzo[b]furans and aurones are regioisomers but so far,
these two classes of compounds have never been isolated from the
same plant source.
The biogenesis of aurones occurs via cyclization of chalcones,
catalyzed by an enzyme, namely aureusidin synthase.4b,7 However,
the biogenesis of 2-benzoylbenzo[b]furans is still unknown.
Given the close structural relation between 2-benzoylbenzo[b]
furans and aurones, it would therefore be very useful to have a re-
liable, mild, and facile method for the conversion of aurones into 2-
benzoylbenzo[b]furans. Herein, we report a convenient method for
the preparation of 2-benzoylbenzo[b]furans via reduction of
aurones, rearrangement of the resulting allylic alcohol, and oxida-
tion of the later.
4'
5'
5'
4'
3'
3'
6'
6'
B
B
7
2. Results and discussion
7
A
4
2'R'
2' R'
O
O
R6
6
A
C
R
C
The conversion pathway is described in Scheme 1. The in-
vestigated aurones were prepared according to previously reported
method8 and obtained as exclusively Z-isomers, corresponding to
the configuration of naturally occurring aurones.
Hβ
5
5
O
3
4
O
2-Benzoylbenzofurans
Aurones
The reduction of aurones was performed with sodium borohy-
dride in methanol at room temperature to give the corresponding
allylic alcohols (2,3-dihydrobenzofuran-3-ols) (I). The obtained al-
cohols require careful handling, due to their sensitivity to excessive
temperature and acidic conditions (see Experimental section). The
isomerization step was carried out at room temperature in
Fig. 1. Structures of aurones (2-benzylidenebenzofuran-3-ones) and 2-benzoylbenzo
[b]furans.
* Corresponding author. Tel.: þ33 4 76 63 53 11; fax: þ33 4 76 63 52 98; e-mail
address: Ahcene.Boumendjel@ujf-grenoble.fr (A. Boumendjel).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.08.011

7704
S. Yahiaoui et al. / Tetrahedron 67 (2011) 7703e7707
O
R'
R'
O
R'
b
O
O
a
R
R
R
R
O
OH
(II)
A1-11
(I)
R'
O
R'
O
c
R'
d
R
R
OH
O
(III)
C1-11
(IV)
Scheme 1. (a) NaBH4, CH3OH, rt; (b) CH3CN/H2O, aqueous HCl, rt; (c) H2O, (d) MnO2, CH2Cl2, molecular sieves, rt.
a mixture of water and acetonitrile and in the presence of catalytic
amount of aqueous HCl. The reaction was monitored by TLC, which
shows that the formed compound (IV) was slightly less polar than
the starting materiel (I) (see Supplementary data). To better handle
the resulting product, water is added and the reaction mixture was
extracted with dichloromethane and subjected to the final step
without evaporating the solvent. The mechanism involves probably
a carbocation formation (II) then rearrangement of the later to the
extracyclic methine carbon (III), which would be stabilized by the
B-aryl group. The organic solution of the rearranged alcohol was
directly used for the oxidation step with 10 equiv of MnO2 in dry
conditions. This three steps conversion was applied to a panel of
aurones (A1-11) (Table 1). The benzoylbenzo[b]furans analogs (C1-
11) were obtained with excellent yields (76e86%). The influence of
the B-ring substitution pattern on the conversion process was in-
vestigated. It appears that, mono-substituted aurones at position 40
with electron-donating groups as well with electro-withdrawing
groups give total conversion (compounds: C1, C2, and C3, Table
1). Additionally, para, ortho, and meta methoxy derivatives were
synthesized in order to determine the impact of the substituents
position on the B ring (compounds: C4, C5, and C6, Table 1). The
results showed that, there is no influence of the position on the
investigation showed that, the conversion is stimulated by the
presence of substituent at the 5 or 7 positions. However, despite the
use of drastic conditions (LiAlH4, LiAlH4/AlCl3 at room temperature
and in refluxed THF) the reduction of 4- and/or 6-substituted
aurones was not fruitful. The reduction was either impossible or
leading to a multitude of degradation compounds, and conse-
quently not convertible to targeted benzoylbenzo[b]furans.
The structures of the resulting compounds were unambiguously
determined by 1H NMR and confirmed by HMBC experiments. In
most cases, the 1H NMR showed a clear difference of the chemical
shifts of protons of aurones and 2-benzoylbenzofurans. The most
important difference concerns the chemical shift of the methine
proton. In aurones, the methine proton (Hb) appears at w6.9 ppm,
whereas in 2-benzoylbenzofurans, the H3-proton was observed
around 7.5 ppm (Table 1).
The HMBC experiment conducted on 2-benzoylbenzo[b]furans
showed a direct correlation between the carbonyl-carbon and the
ortho-protons of the B-phenyl ring. This type of correlation is
missing among aurones (Fig. 2).
It is worth to prove whether this procedure simply isomerizes Z
to E-aurones rather than formation of 2-benzoylbenzofurans
(Scheme 2). To investigate this possibility, we synthesized an E-
aurone (E-isomer of aurone A1 in Table 1) according to
obtained yields. Concerning the
A ring substitution, our
Table 1
Scope and yields of the conversion of aurones to benzoylbenzo[b]furans and the chemical shifts difference of their methine protons
R5
R5
R1
R4
R1
R4
O
R3
O
R3
R2
Hβ
R2
O
O
H3
2-Benzoylbenzo[b]furans (C1-11)
Aurones (A1-11)
Compounds
R1
R2
R3
R4
R5
Yielda (%)
d
H3 (ppm)
d Hb (ppm)
(C1eC11)
(A1eA11)
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
H
H
H
H
H
H
H
H
H
H
OMe
OMe
H
H
H
H
H
H
OMe
H
F
H
H
H
H
H
OMe
H
H
H
H
OMe
CO2Me
F
OMe
H
H
Cl
F
81
79
76
83
82
80
86
81
80
77
79
7.53
7.60
7.56
7.53
7.54
7.31
7.57
7.47
7.47
7.59
7.38
6.89
6.88
6.86
6.90
6.89
7.52
6.85
7.14
6.92
6.95
7.39
OMe
OMe
OMe
OMe
H
H
OMe
OMe
OMe
OMe
OMe
H
OMe
OMe
OMe
H
a
Yield of three steps.

S. Yahiaoui et al. / Tetrahedron 67 (2011) 7703e7707
7705
Fig. 2. The correlation between 20,60 protons with the C]O of 2-benzoylbenzo[b]furans, evidenced by HMBC experiment.
OMe
O
O
O
O
not observed
E-isomer
OMe
OMe
Scheme 2. Possible isomerization of the intracyclic carbocation.
a photochemical approach reported by Venkateswarlu et al.9 and
compared its 1H and 13C NMR with the obtained 2-benzoylbenzo[b]
furans. The NMR data was quite different, thus ruling out the pos-
sibility of simple isomerization.
4. Experimental section
4.1. General information and materials
1H NMR spectra were recorded at 400 MHz and 13C NMR spectra
at 100 MHz at ambient temperature on a Bruker Avance-400 in-
strument. The chemical shifts (d
) for 1H NMR and 13C NMR are
3. Conclusion
reported in parts per million. ESI mass spectra and elemental
analyses were performed at the analysis facilities of the department
of chemistry of the University of Grenoble. Aurones were obtained
following literature methods.8 Sodium borohydride and manga-
nese oxide were purchased from SigmaeAldrich and used as re-
ceived. The other reagents and solvents were obtained from
commercial sources and used as received.
The reported transformation sequence can have multiple ap-
plications and interest. First, the sequence may be exploited for the
preparation of bioactive 2-benzoylbenzo[b]furans starting from
aurones. Second, since the biosynthesis of 2-benzoylbenzo[b]furans
has never been reported, this conversion could be examined as
a potential biogenesis pathway of naturally occurring benzofurans.
The later assessment is supported by the fact that the conversion
process (reduction, rearrangement, and oxidation) could be easily
achieved by plant enzymes. Finally, the reported NMR data makes
clear discrimination between aurones and 2-benzoylbenzo[b]fu-
rans, which is of valuable help for the phytochemists community,
working on the isolation and structure elucidation of aurones and
2-benzoylbenzo[b]furans.
4.2. General experimental procedure
4.2.1. The reduction procedure. To a solution of 100 mg of an aurone
in methanol (15 mL) was added NaBH4 (4 equiv). The mixture was
stirred at room temperature for 10e40 min (see Supplementary

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S. Yahiaoui et al. / Tetrahedron 67 (2011) 7703e7707
data). After reaction completion (as monitored by TLC, cyclohex-
ane/EtOAc 7:3), water was added. After stirring for few seconds,
3.90 (s, 3H). 13C (100 MHz, CDCl3): 183.3, 164.5, 154.1, 147.0, 146.4,
133.1, 130.7, 129.6, 125.5, 116.3, 115.8, 114.8, 110.2, 57.0, 56.5. MS
(ESI) 283 (MþH)þ.
a
suspension was formed. The reduced aurone (2,3-
dihydrobenzofuran-3-ols) was filtered off and washed with water
to obtain a white solid. According to the TLC, the resulting solid was
pure, thus, it was used as obtained for the next step.
4.3.5. 7-Methoxy-2-(m-methoxybenzoyl)benzofuran C5. Known;12
yellow oil. 1H NMR (400 MHz, CDCl3):
d
(ppm) 7.68 (dt, J¼7.6 and
1.2 Hz, 1H), 7.58 (dd, J¼2.5 and 1.6 Hz, 1H), 7.54 (s, 1H), 7.44 (t,
J¼8.0 Hz, 1H), 7.30 (dd, J¼8.0 and 1.2 Hz, 1H), 7.24 (t, J¼7.9 Hz, 1H),
7.18 (ddd, J¼8.4; 2.7 and 0.9 Hz, 1H), 6.97 (dd, J¼7.7 and 1.1 Hz, 1H),
4.03 (s, 3H), 3.89 (s, 3H). 13C (100 MHz, CDCl3): 184.7, 160.6, 153.4,
147.1, 146.7, 139.3, 130.5, 129.5, 125.6, 123.1, 120.4, 117.5, 115.9, 114.8,
110.5, 57.1, 56.4. MS (ESI) 283 (MþH)þ.
4.2.2. The rearrangement procedure. In a dry flask, the reduced
aurone was dissolved in acetonitrile (10 mL), then a solution of
aqueous HCl (10 mL H2Oþ0.1 mL of HCl 10%) was added. The re-
action mixture was allowed to stir for 10 min to 4 h (see
Supplementary data) at room temperature. The reaction was
monitored by TLC (cyclohexane/EtOAc 7:3), most of cases, the
product of isomerization was slightly less polar than the reduced
aurone. TLC with several migrations was needed if the resulting
product possesses nearly the same retention factor (Rf) as the re-
duced aurone. By the end of the reaction, water was added and the
mixture was extracted with dicholoromethane (three times), then,
the organic layer was dried over MgSO4 and filtered off. The
obtained dichloromethane solution was directly used for the oxi-
dation step.
4.3.6. 7-Methoxy-2-(o-methoxybenzoyl)benzofuran C6. Unknown;
yellow oil. 1H NMR (400 MHz, CDCl3):
d
(ppm) 7.51 (d, J¼7.5 Hz, 1H),
7.49 (td, J¼7.2 and 1.8 Hz,1H), 7.31 (s,1H), 7.24 (dd, J¼7.9 and 1.5 Hz,
1H), 7.21 (t, J¼7.9 Hz,1H), 7.07e7.05 (m,1H), 7.03e7.01 (m,1H), 6.94
(dd, J¼7.5 and 1.5 Hz, 1H), 4.01 (s, 3H), 3.80 (s, 3H). 13C (100 MHz,
CDCl3): 185.5, 158.7, 154.2, 147.1, 146.7, 133.5, 130.7, 129.7, 128.8,
125.4, 121.3, 117.2, 116.0, 112.7, 110.7, 57.1, 56.8. MS (ESI) 283
(MþH)þ.
4.2.3. The oxidation procedure. The previous dichloromethane so-
lution was treated with molecular sieves (powder) and the mixture
was stirred at room temperature for 5 min, then, 10 equiv of MnO2
were added and stirring was continued at room temperature for
25 mine3 h 45 min. After reaction completion (monitored by TLC,
cyclohexane/EtOAc 7:3), the resulting mixture was filtered off
through Celite, then the filtrate was washed with water and the
dichloromethane solution was dried over MgSO4 and filtered off.
The solvent was removed under reduced pressure to give pure 2-
aroylbenzofuran without purification.
4.3.7. 7-Methoxy-2-(p-chlorobenzoyl)benzofuran
yellow oil. 1H NMR (400 MHz, CDCl3):
C7. Known;13
(ppm) 8.07 (d, J¼8.7 Hz,
d
2H), 7.57 (s, 1H), 7.52 (d, J¼8.7 Hz, 2H), 7.31 (dd, J¼7.9 and 1.2 Hz,
1H), 7.25 (t, J¼7.8 Hz, 1H), 6.97 (dd, J¼7.7 and 1.2 Hz, 1H), 4.03 (s,
3H). 13C (100 MHz, CDCl3): 183.5, 153.4, 147.0, 146.7, 140.4, 136.2,
132.0, 129.8, 129.4, 125.7, 117.3, 115.9, 110.6, 57.0. MS (ESI) 287
(MþH)þ. Anal. Calcd for C16H11O3Cl: C 67.03%; H 3.87%. Found: C
66.67%; H 3.96%.
4.3.8. 5-Methoxy-2-(2,4-difluorobenzoyl)benzofuran C8. Unknown;
yellow powder; mp 130 ꢀC. 1H NMR (400 MHz, CDCl3):
d (ppm) 7.77
4.3. Characterization
(m, 1H), 7.54 (d, J¼9.0 Hz, 1H), 7.47 (br s, 1H), 7.18 (dd, J¼9.0 and
2.6 Hz, 1H), 7.14 (d, J¼2.5 Hz, 1H), 7.10e7.06 (m, 1H), 7.04e6.98 (m,
1H), 3.90 (s, 3H). 13C (100 MHz, CDCl3): 180.4, 165.2 (dd, JCeF¼255.5
and 12.0 Hz), 161.1 (dd, JCeF¼257.3 and 12.4 Hz), 156.8, 152.8, 151.5,
132.3 (dd, JCeF¼10.4 and 4.1 Hz), 127.5, 122.8 (dd, JCeF¼14.1 and
3.7 Hz), 119.3, 116.7 (d, JCeF¼2.2 Hz), 113.3, 111.9 (dd, JCeF¼21.6 and
3.7 Hz), 105.0 (t, JCeF¼25.5 Hz), 103.9, 55.9. MS (ESI) 289 (MþH)þ.
Anal. Calcd for C16H10O3F2: C 66.67%; H 3.50%. Found: C 66.82%; H
3.87%.
4.3.1. 2-(p-Methoxybenzoyl)benzofuran C1. Known;10 yellow pow-
der; mp 96 ꢀC. 1H NMR (400 MHz, CDCl3):
d
(ppm) 8.12 (d, J¼8.9 Hz,
2H), 7.74 (br d, J¼7.8 Hz, 1H), 7.63 (dd, J¼8.5 and 0.8 Hz, 1H), 7.53 (d,
J¼0.9 Hz, 1H), 7.50 (td, J¼7.8 and 1.2 Hz, 1H), 7.34 (td, J¼7.6 and
0.9 Hz,1H), 7.03 (d, J¼8.9 Hz, 2H), 3.92 (s, 3H). 13C (100 MHz, CDCl3):
183.8, 164.5, 156.8, 153.6, 132.9, 130.8, 129.0, 128.0, 124.8, 124.1,
116.5, 114.8, 113.4, 56.5. MS (ESI) 253 (MþH)þ.
4.3.2. 2-(p-Methyesterbenzoyl)benzofuran C2. Unknown; white
4.3.9. 5-Methoxy-2-(p-methoxybenzoyl)benzofuran C9. Unknown;
powder; mp 171 ꢀC. 1H NMR (400 MHz, CDCl3):
d
(ppm) 8.25 (d,
yellow powder; mp 88 ꢀC. 1H NMR (400 MHz, CDCl3):
d (ppm) 8.11
J¼8.5 Hz, 2H), 8.14 (d, J¼8.5 Hz, 2H), 7.79 (br d, J¼7.9 Hz, 1H), 7.70
(br d, J¼8.5 Hz, 1H), 7.60 (d, J¼0.8 Hz, 1H), 7.57 (td, J¼7.8 and 1.2 Hz,
1H), 7.40 (td, J¼7.5 and 0.8 Hz, 1H), 4.03 (s, 3H). 13C (100 MHz,
CDCl3): 183.7, 166.3, 156.2, 152.0, 140.8, 133.7, 129.8, 129.4, 128.8,
126.9, 124.2, 123.5, 117.1, 112.7, 52.6. MS (ESI) 303 (MþNa)þ. Anal.
Calcd for C17H12O4: C 72.85%; H 4.32%. Found: C 72.51%; H 4.42%.
(d, J¼8.9 Hz, 2H), 7.52 (br d, J¼9.7 Hz, 1H), 7.47 (d, J¼1.0 Hz, 1H), 7.11
(s, 1H), 7.10 (dd, J¼7.7 and 2.7 Hz, 1H), 7.02 (d, J¼8.9 Hz, 2H), 3.91 (s,
3H), 3.86 (s, 3H). 13C (100 MHz, CDCl3): 183.6, 164.5, 157.5, 154.4,
151.9, 132.9, 130.8, 128.5, 119.0, 116.5, 114.8, 114.1, 104.8, 56.8, 56.5.
MS (ESI) 283 (MþH)þ. Anal. Calcd for C17H14O4: C 72.33%; H 5.00%.
Found: C 72.08%; H 5.17%.
4.3.3. 2-(p-Fluorobenzoyl)benzofuran C3. Known;11 yellow-pale
4.3.10. 7-Methoxy-2-(3,4-dimethoxybenzoyl)benzofuran
crystal; mp 133 ꢀC. 1H NMR (400 MHz, CDCl3):
d
(ppm) 8.14 (dd,
C10. Known;14 beige powder; mp 111 ꢀC. 1H NMR (400 MHz,
J¼8.9 and 5.5 Hz, 2H), 7.75 (d, J¼7.9 Hz,1H), 7.65 (br d, J¼7.5 Hz,1H),
7.56 (br s, 1H), 7.52 (td, J¼7.4 and 1.2 Hz, 1H), 7.35 (br td, J¼7.5 and
0.7 Hz, 1H), 7.23 (t, J¼8.6 Hz, 2H). 13C (100 MHz, CDCl3): 183.7, 166.6
(JCeF¼255.1 Hz), 156.9, 153.1, 134.3 (JCeF¼2.4 Hz), 133.1
(JCeF¼9.2 Hz), 129.4, 127.9, 125.0, 124.3, 117.3, 116.7 (JCeF¼21.8 Hz),
113.5. MS (ESI) 263 (MþNa)þ. Anal. Calcd for C15H9O2F: C 75.00%; H
3.78%. Found: C 75.36%; H 4.13%.
CDCl3):
d
(ppm) 7.94 (dd, J¼8.4 and 2.1 Hz, 1H), 7.73 (d, J¼2.0 Hz,
1H), 7.59 (s, 1H), 7.34 (dd, J¼7.9 and 1.0 Hz, 1H), 7.28 (t, J¼7.9 Hz,
1H), 7.02 (d, J¼8.4 Hz, 1H), 6.99 (dd, J¼7.8 and 0.9 Hz, 1H), 4.07 (s,
3H), 4.02 (s, 3H). 13C (100 MHz, CDCl3): 182.3, 153.4, 153.2, 149.1,
146.1,145.5, 129.8, 128.7, 124.8,124.6, 115.4,114.9,112.0,110.2,109.3,
56.2, 56.1, 56.0. MS (ESI) 313 (MþH)þ.
4.3.11. 7-Methoxy-2-(2,3,4-trimethoxybenzoyl)benzofuran
4.3.4. 7-Methoxy-2-(p-methoxybenzoyl)benzofuran C4. Known;12
C11. Unknown; yellow oil. 1H NMR (400 MHz, CDCl3):
d (ppm) 7.38
yellow oil. 1H NMR (400 MHz, CDCl3):
d
(ppm) 8.16 (d, J¼8.9 Hz,
(s, 1H), 7.31 (d, J¼8.6 Hz, 1H), 7.26 (dd, J¼8.1 and 1.3 Hz, 1H), 7.21 (t,
J¼7.9 Hz, 1H), 6.94 (dd, J¼7.7 and 1.2 Hz, 1H), 6.75 (d, J¼8.6 Hz, 1H),
4.00 (s, 3H), 3.94 (s, 3H), 3.91 (s, 3H), 3.88 (s, 3H). 13C (100 MHz,
2H), 7.53 (s,1H), 7.29 (dd, J¼7.9 and 1.1 Hz,1H), 7.23 (t, J¼7.8 Hz,1H),
7.02(d, J¼8.9 Hz, 2H), 6.95 (dd, J¼7.8 and 1.0 Hz, 1H), 4.03 (s, 3H),

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5. Atta-ur-Rahman; Choudhary, M. I.; Hayat, S.; Khan, A. M.; Ahmed, A. Chem.
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CDCl3): 184.4, 157.6, 154.3, 154.0, 147.0, 146.6, 143.1, 129.7, 126.5,
126.2, 125.4, 116.6, 116.0, 110.4, 107.6, 63.1, 61.9, 57.1, 57.0. MS (ESI)
343 (MþH)þ.
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Supplementary data
The table of reactions times, the Rf of intermediates, and the
NMR copies (1H and 13C) of all compounds (aurones and 2-ben-
zoylbenzo[b]furans). Supplementary data associated with this ar-
ticle can be found, in the online version, at doi:10.1016/
j.tet.2011.08.011.
References and notes
€
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