Synthesis, x-ray structure and spectroscopic and electronic properties of two new synthesized flavones

Article abstract of DOI:10.1002/poc.720

Two new flavones (1 and 2) in which one or two di-tert-butylhydroxyphenyl groups replace the B-ring of flavonoids, respectively, were synthesized according to the Baker-Vankataraman method. Their crystal structures were determined by x-ray diffraction met

Full text of DOI:10.1002/poc.720

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2004; 17: 226–235
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.720
Synthesis, x-ray structure and spectroscopic and
electronic properties of two new synthesized flavones
1
2
3
4
4
´
Nicole Cotelle, * Laurence Vrielynck, Guy Nowogrocki, Philippe Cotelle and Herve Vezin
1
´
´
Laboratoire de Chimie Organique et Macromoleculaire, Equipe Polyphenols, UMR CNRS 8009, USTL, 59655 Villeneuve d’Ascq Cedex, France
2Laboratoire de Spectrochimie Infrarouge et Raman, UMR CNRS 8516, USTL, 59655 Villeneuve d’Ascq, Cedex, France
3Laboratoire de Cristallochimie et Physicochimie du solide, UMR CNRS 8012, ENSCL, BP108 59652, Villeneuve d’Ascq Cedex, France
4
´
´
Laboratoire de Chimie Organique et Macromoleculaire, Equipe Molecules Pro et Antioxydantes, UMR, CNRS 8009, USTL, 59655 Villeneuve
d’Ascq Cedex, France
Received 9 November 2002; revised 5 September 2003; accepted 3 October 2003
ABSTRACT: Two new flavones (1 and 2) in which one or two di-tert-butylhydroxyphenyl groups replace the B-ring
of flavonoids, respectively, were synthesized according to the Baker–Vankataraman method. Their crystal structures
were determined by x-ray diffraction methods and compared with those obtained by theoretical calculations using join
Monte Carlo conformational search analysis and geometry optimization ab initio formalism. NMR NOE data, ESR
spectra and electronic properties were obtained in order to understand their implication in the enhancement of low-
density lipoprotein (LDL) resistance to oxidation. The oxidative modification of LDL has been alleged to play an
important role in the development of human atherosclerosis and neurodegenerative diseases. Copyright # 2004 John
Wiley & Sons, Ltd.
KEYWORDS: flavones; crystal structure; electronic properties; low-density lipoprotein
INTRODUCTION
have synthesized two new flavones in which the di-tert-
butylhydroxyphenyl group replaces the catechol moiety.
This motif is present in butylated hydroxytoluene (BHT),
which is well known for its antioxidant properties and
widely used in the USA as a food additive. Nevertheless,
the presence of one or several di-tert-butylhydroxyphenyl
groups may cause drastic constraints in the molecular
structure and consequently could affect the binding mode
to enzymes involved in oxidation systems, e.g. lipoxy-
genases, cyclooxygenases, monooxidase and xanthine
oxidase.
In this work, we first performed a structural analysis by
x-ray diffraction of the two new synthesized flavones
(1 and 2) exhibiting two tert-butyl-substituted groups on
the B ring in the 30 and 50 positions. The structures of both
molecules are shown in Scheme 1(a) and (b), respec-
tively, and 2 differs from 1 by the presence of the D ring
which bears two m-tert-butyl groups and is linked to the
B ring through an ester function. The results of a quantum
chemical conformational analysis, performed at the iso-
lated molecule level with the ab initio 3–21G* formal-
ism, are reported and compared with the crystallographic
data. Subsequently, we focused on many specific spectro-
scopic properties of 1 and 2, especially the electronic
properties, which may be helpful in understanding the
structure–activity relationship for these kinds of com-
pounds. ESR spectroscopic experiments on radical for-
mation analysis were performed in order to understand
the mechanism of radical scavenging. We performed
Flavonoids are a group of low molecular weight natural
molecules widely distributed in the plant kingdom, and
represent a significant part of the average Western daily
diet. Flavonoids are benzo-ꢀ-pyrone derivatives and
consist of a benzene ring (commonly named A ring)
attached to a six-membered heterocycle (named C-ring),
which carries at C-2 a phenyl group (named B ring) as a
substituent. The most commonly occurring flavones and
flavonols are those hydroxylated at positions 5 and 7
(A-ring) and 30 and 40 (B-ring). Among this family of
polyphenols, the flavones exhibit a wide variety of
biological properties. Indeed, most of them have been
found to possess anti-ischemic,1 antiplatelet,2 anti-
inflammatory3,4 or antilipoperoxidant5 activities. Fla-
vones are also well known to inhibit a wide range of
enzymes involved in oxidation systems.6–9 These poly-
phenolic compounds can exert their antioxidant activity
by various mechanisms, e.g. by scavenging radicals, by
binding metal ions, but also by inhibiting enzymatic
systems responsible for free radical generation. In con-
trast to the beneficial effects, some flavones with catechol
or pyrogallol moieties have also been reported to be
mutagenic.10,11 In order to prevent these problems, we
*Correspondence to: N. Cotelle, Laboratoire de Chimie Organique et
´
Macromoleculaire, Equipe Polyphenols, UMR CNRS 8009, USTL,
59655 Villeneuve d’Ascq Cedex, France.
´
E-mail: nicole.cotelle@univ-lille1.fr
Copyright # 2004 John Wiley & Sons, Ltd.
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SYNTHESIS, STRUCTURE AND ELECTRONIC PROPERTIES OF FLAVONES
227
Scheme 1. Structures of (a) 1 and (b) 2 with atom numbering
molecular modeling in order to understand the structural
differences induced by the addition of one or two BHT
templates, respectively, at the 20-position on the C ring of
the flavone molecule.
Finally, the antioxidant biological properties of 1 and
2, i.e. protection of low-density lipoprotein (LDL) from
oxidation, were compared with theoretical calculations
particularly with the HOMO energy and its distribution
on the molecule.
DMSO to give the diarylpropane-1,3-dione 4 in 72%
yield. The cyclization of 4 was carried out under reflux in
acetic acid in the presence of sulfuric acid for 1 h, leading
to 2 in 65% yield. Compound 2 was saponified with
sodium hydroxide in dry DMSO for 4 h at 120ꢀC to give 1
in 53% yield.
Crystal structures
The molecular packings of 1 and 2 are shown in Figs 1
and 2, respectively. The experimental crystallographic
parameters of both molecules are summarized in Table 1.
Selected bond lengths and angles are listed in Table 2.
In 1, which crystallizes in the P21/n space group, a
medium–strong intermolecular hydrogen bonding inter-
action was found between the O4 carbonyl oxygen atom
of a molecule and the hydrogen atom of the hydroxyl
group of a neighboring molecule, which are both related
by a glide plane. The intermolecular distances O4ꢁ ꢁ ꢁO40
RESULTS AND DISCUSSION
Chemistry
Compounds 1 and 2 were prepared as previously de-
scribed according to the Baker–Vankataraman proce-
dure12,13 (Scheme 2). The 2-hydroxyacetophenone was
condensed with 3,5-di-tert-butyl-4-hydroxybenzoyl
chloride in the presence of dimethylaminopyridine in
dry pyridine at 60ꢀC for 2 h to give the diester 3 in
34% yield. This result was not surprising since 3,5-
di-tert-butyl-4-hydroxybenzoyl chloride reacted in dry
pyridine at 60ꢀC to give 4-(3,5-di-tert-butyl-4-hydroxy-
benzoyloxy)-3,5-di-tert-butylbenzoic acid in 60% yield.
The diester 3 was treated with sodium hydroxide in dry
0
˚
and O4ꢁ ꢁ ꢁH(O4 ) are 2.658(4) and 1.89(4) A, respec-
tively, and the angle O4ꢁ ꢁ ꢁH (O40) is reported to be 153ꢀ.
These intermolecular hydrogen links entail the formation
of infinite chains roughly parallel to the [1 ꢂ1 0] direc-
˚
tion. All other intermolecular distances are >3.3 A.
The presence of a symmetry center creates pairs of
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228
N. COTELLE ET AL.
Scheme 2. Synthetic pathway
Figure 1. Projection on the (b,c) plane of the 1 structure
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SYNTHESIS, STRUCTURE AND ELECTRONIC PROPERTIES OF FLAVONES
229
Figure 2. Projection on the (a,b) plane of the 2 structure
˚
quasi-parallel molecules which are about 3.5 A apart and
for which the carbon skeletons undergo severe geometric
distortions as the result of the bulky tert-butyl groups on
the phenyl B ring [Fig. 3(a)]. Eventually, the intermole-
cular hydrogen bridges essentially stabilize the three-
dimensional network.
carbonyl oxygen atoms of two molecules which are
˚
related by a symmetry center at 3.11 A, suggesting a
very weak hydrogen bonding interaction. The next short-
˚
est distance (3.46 A) occurs between parallel aromatic
rings where some strong ꢁ–ꢁ interactions are involved
[Fig. 3(b)], thus allowing stabilization of the crystal
lattice.
The structures of 1 and 2 are essentially controlled by
the value of specific dihedral angles such as ꢂ (C3—
C2—C10 —C60) for 1 and 2 and ꢃ (C50 —C40 —O40 —
Cc) and ꢄ (Oc—Cc—C100 —C200) for 2, which describe
the principal degrees of freedom in these two molecules.
The small ꢂ torsion angle value in 1, measured as 1.7ꢀ,
appears to favor the ꢁ system delocalization between the
B ring and the ꢀ-pyrone part, although in 2 the conjuga-
tion is strongly restricted owing to an angle value of
almost 6ꢀ. In 2, the reported bond lengths for C2—C10
In 2, which crystallizes in the P-1 space group, the
shortest intermolecular distance occurs between the O400
Table 1. Crystal data and details of data collections
Compound
1
2
Formula
Formula weight
C23H26O3
350.44
C38H46O5
582.75
(g molꢂ1
Space group
)
P21/n
P-1
Crystal size (mm3)
0.35 ꢃ 0.30 ꢃ 0.25 0.30 ꢃ 0.15 ꢃ 0.10
˚
˚
˚
a (A)
10.164 (4)
16.135 (7)
12.090 (5)
90.0
14.317 (3)
13.338 (3)
9.434 (2)
104.22 (3)
97.86 (3)
100.86 (3)
1683.2 (6)
2
(1.474 A) and C3—C4 (1.444 A) show that these bonds
exhibit single-bond character, even though the C2—C3
˚
b (A)
˚
c (A)
˚
bond, which is very short (1.333 A), presents an obvious
ꢂ (ꢀ)
ꢃ (ꢀ)
ꢀ (ꢀ)
double bond order. The non-coplanarity of the phenyl
ring and the benzopyrane moiety supports a decrease in
the conjugation between the C2—C10 bond and the ꢁ
electrons of the ꢀ-pyrone ring. In 2, the value of the
dihedral angle C50 —C40 —O40 —Cc that is reported to
be close to 100ꢀ and can be explained by the strong steric
hindrance of the tert-butyl motifs on the B ring. The
99.909 (9)
90.0
1953 (1)
4
0.077
3
˚
V (A )
Z
Linear absorptionꢂ1
0.075
coefficient (m mm
)
Dc (g cmꢂ3
ꢄ limits
hkl limits
)
1.192
2–31
1.150
3.2–25.0
˚
ꢂ14, 14; ꢂ23, 23; ꢂ17, 17; ꢂ15,
carbonyl bond Cc—Oc is short (1.200 A) and does not
appear to be conjugated with this phenyl D ring as
suggested by the single-bond character of the Cc—C100
ꢂ17, 16
16505
5281
15; ꢂ11; 11
11719
5847
No. of data collected
No. of intensities
˚
bond (1.472 A). The low conjugation of the carbonyl
No. of intensities I >2ꢅ(I) 2537
2578
R
Rw
0.0439
0.1055
0.851
313
0.0514
0.1098
1.049
group is also confirmed by a dihedral angle ꢄ value of
ꢂ168.3ꢀ measured from x-ray data indicating the out-of-
plane position of the CO group. Moreover, ab initio
calculations also show that for all the theoretical
Goodness of fit
No. of variables
527
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230
N. COTELLE ET AL.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 226–235

Plate 1. Structures of the most stable conformers of 2: 21 (left) corresponding to the x-ray structure and 22 (right) to the folded
structure. Structures were obtained by Monte Carlo analysis and then fully optimized at the Hartree–Fock level with the 3–21G*
basis set
Plate 2. Three-dimensional contour plot for the molecular orbital HOMO (left) and LUMO (right) for 1 (top) and 21 conformer
molecule (bottom)
Plate 3. Three-dimensional contour plot for the molecular orbital HOMO (left) and LUMO (right) for the 22 conformer
Copyright # 2004 John Wiley & Sons, Ltd.
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SYNTHESIS, STRUCTURE AND ELECTRONIC PROPERTIES OF FLAVONES
231
Figure 3. Stacking of molecules to show the quasi-parallelism of benzopyran groups for (a) 1 and (b) 2
conformers these values are far from that for the planar
structure of this CO bond (Table 3).
solution, the NMR results show the equivalence of
hydrogens (and carbons) on symmetry-related sites of
the phenyl B and D rings and also for those of the 30, 50-
and 300, 500-t-Bu groups. Thus rapid exchange occurs on
the NMR time-scale between all these pairs of sites
through rotation around the single bonds C2—C10,
C40—O40, Cc—C100. The mean distances between H20,
H60 and H3 do not differ significantly in the two con-
formers envisaged, 21 and 22, hence NOE does not allow
are to make a choice between the conformations.
The average of inter-proton distance in conformer 22 is
related to a modification of the ꢂ dihedral angle value.
The main geometric features resulting from the energy
global minimum search (for 1 and 2) are reported in
Table 2. The presence of the tert-butyl groups in the 30
and 50 positions on the B ring of 1 and 2 leads to a torsion
angle (O1C2C1,C2,) of the B ring with the rest of the
molecule (A and C rings) close to 1 ꢄ 0.3 and 4.1 ꢄ 04ꢀ
(Table 2). This means that 1 and 2 are almost planar. It
has been stated in general that the flavone B ring is
slightly (ꢄ20ꢀ) twisted relative to the plane of the A and
C rings. For example, apigenin (5,7,40-trihydroxyflavone)
presents a torsion angle of 16.48ꢀ, unlike flavonols, which
are planar, e.g. quercetin (3,5,7,3040-pentahydroxyfla-
vone) presents a torsion angle of ꢂ0.29ꢀ. The cause of
the planarity of the flavonols appears to be a hydrogen
bond-like interaction between the 3-OH and 20- or 60-
proton.14
Computational chemistry
The Monte Carlo conformational analysis of 2 revealed
nine series of stable conformers resulting from the energy
global minimum search, which are all characterized by
different values of ꢄ, ꢂ and ꢃ angles. Each conformer was
then fully optimized at the Hartree–Fock level using a
3–21G* basis set. Two groups of structures with a heat of
formation of ꢂ162.36 kcal molꢂ1 (1 kcal ¼ 4.184 kJ)
(two structures) and ꢂ164.0 kcal molꢂ1 (two structures)
respectively for 2 (Plate 1) were obtained from these
calculations. Only one structure with a heat of formation
of ꢂ84.33 kcal molꢂ1 was found for 1. The first 2 con-
former of ꢂ162.36 kcal molꢂ1 corresponds to the crystal-
lographic structure whereas that of ꢂ164.0 kcal molꢂ1 is
related to a folded structure, folding between the D and B
rings. The main differences found in this spatial structure
arise from drastic changes in the ꢄ, ꢂ and ꢃ angle values
and are displayed in Table 3. At this stage we hypothe-
sized that this folded structure may reflect the possible
conformation in solution where the ꢁ–ꢁ interaction found
in the crystal structure is vanishes. NMR steady-state
NOE experiments tend to confirm the reality of these
structures. Irradiation of the signal of hydrogen H3 of 2
produced a strong NOE (27%) of the signal of hydrogens
H20 and H60. Conversely, a strong NOE of approximately
same value (34%) was observed for the signal of hydro-
gen H3 when H20/H60 were irradiated. However, in
For conformers 1 and 21, good agreement is observed
between the calculated geometric parameters and the
crystallographic data, notably for distances and valence
angles.
Table 3. Energy formation and dihedral angle values for the
two sets of conformer for 2: 21 related to crystallographic
structure and 22 to folded conformationa
UV–visible spectroscopy
From the three-dimensional contour plotting of the mo-
lecular orbital (Plate 2), the nature of the chromophore
mainly involved in the HOMO–LUMO electronic transi-
tion on the UV–visible spectra of 1 and 2 may be
predicted.
For 1 and 21, the HOMO ! LUMO transition does not
appear as specific to a peculiar part of the molecule and
induces a drastic change in the electronic distribution on
the whole flavone moiety, notably on the ꢀ-pyrone ring.
Heat of formation
)
ꢂ (ꢀ)
ꢃ (ꢀ)
ꢄ (ꢀ)
(kcal molꢂ1
21
22
ꢂ162.36
ꢂ162.35
ꢂ164.09
ꢂ163.9
4.1
ꢂ4.3
101.5
ꢂ101.6
83.5
177.5
147.0
ꢂ105.8
60.5
ꢂ130.9
83.7
ꢂ128.7
a
The energy of formation was computed at the RHF/PM3 level and the
dihedral angle values were calculated at the RHF/PM3/3–21G* ab initio
level.
Copyright # 2004 John Wiley & Sons, Ltd.
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232
N. COTELLE ET AL.
Table 4. ESR hyperfine splitting constants (G) and proton
assignment (number, in parentheses)
Compound
A1
A2
BHT
1
2
1.56 (2)
2.15 (2)
2.13 (2)
11.5 (3)
ESR spectroscopy
ESR spectroscopic experiments using the cerium(IV)
oxidation system showed the ability of 1 and 2 to form
a phenoxy radical. These results were compared with
those for the BHT reference for which the hyperfine
splitting constants are close to those found in the litera-
ture.16 The hyperfine splitting constants are given in
Table 4 and the related spectra (experimental and simu-
lated) are displayed in Fig. 5 for BHT and 2 respectively.
For BHT the spectrum gives four packets of three lines
due to the electron coupling with the two hydrogen atoms
of the ring and that of the methyl group. For 1 and 2, the
spectra consist of three lines due to two hydrogen atoms
coupled with the electron centered on the oxygen atom.
We have recently reported results related to the biolo-
gical activities of 1 and 2 on the inhibition of copper ion
or AAPH (2,20-azobis-(2-amidinopropane) dihydrochlor-
ide)-induced LDL oxidation.17 In this paper, we have
shown that 2 was 10 times more active than BHTwhereas
1 showed the same activity as BHT. These results can be
correlated with the theoretical HOMO and LUMO en-
Figure
4. Electronic
absorption
spectra
of
2
(4 ꢃ 10ꢂ5 mol lꢂ1) dissolved in DMSO (dashed line) and
methanol (full line)
As reported previously for flavone,15 the inter-ring bond
that exhibits a ꢁ antibonding character in the HOMO
adopts a bonding character in the LUMO. As far as the 22
conformer is concerned, the electronic density in the
HOMO appears localized on the terminal D ring and
the HOMO ! LUMO transition is expected to be char-
acteristic of an intramolecular charge-transfer band from
the D ring towards the entire flavonoic moiety (Plate 3). If
the HOMO–LUMO energy gap is quasi-equal for 21 and
22, the electronic nature of the transition is largely
controlled by the molecular conformation.
As in solution many conformational structures may co-
exist (depending also on the solvent properties), the band
arising at higher wavelengths in the UV–visible spectrum is
expected to be complex. The electronic absorption spectra
of 2 (4 ꢃ 10ꢂ5 mol lꢂ1) in methanol and in dimethyl sulf-
oxide (DMSO) are presented in Fig. 4. A large and strong
absorption pattern centered experimentally at 297 nm ap-
pearing in both solvents surely corresponds to the HOMO–
LUMO transition of ꢁ–ꢁ* character. Nevertheless, in
DMSO, an additional band at 352 nm clearly arises, in-
dicating that solvation of the solute particularly influences
the electronic spectra of this kind of flexible molecule.
ergies calculated and band gap energy (EHOMO–ELUMO
)
shown in Table 5. Effectively, we can see that the lowest
band gap and the highest activity were observed with 0.9
and 1.7 eV differences, respectively, comparing BHT
with 2 and 1 with 2. The major differences in band gap
energy are principally supported by the LUMO energy
values, which are 1.7 and 2.7 eV higher for BHT and 1,
respectively, compared with the 2 active one. In another
part the HOMO energy value is 0.5 eV lower for 2.
Figure 5. Experimental and simulated ESR spectra of BHT (left) and 2 (right) radical formed by oxidation with the Ce(IV) system.
Experimental settings were amplitude modulation 1 G, microwave power 1 mW and receiver level gain 2 ꢃ 104
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 226–235

SYNTHESIS, STRUCTURE AND ELECTRONIC PROPERTIES OF FLAVONES
Table 5. HOMO and LUMO energy values
233
order (HPLC retention times 73.31 and 79.22 min, re-
spectively). Oxidation of 1 and 2 by cerium(IV) gives the
same radical species, which are stable in the course of
time. Nevertheless, the role of the D ring in the electronic
behavior indicates that it seems to be responsible for the
antioxidant properties. The major difference that explains
this reactivity is the observed band gap energy value
principally supported by the LUMO energy value. These
results open the route towards the rational design of new
antioxidant leads.
EHOMO (eV)
ELUMO (eV)
Band gap (eV)
BHT
1
21
ꢂ8.50
ꢂ8.27
ꢂ9.26
ꢂ9.4
0.95
1.96
ꢂ9.45
ꢂ10.23
ꢂ8.57
ꢂ8.67
ꢂ0.70
22
ꢂ0.73
Previous studies with other flavonoid compounds such
as quercetin and taxifolin have shown that the formation
of a phenoxy radical occurs with maximum spin density
(84%) localized on the oxygen when a hydrogen radical
is removed. Calculation of spin densities with the 1 and
unfolded 21 phenoxy species revealed a distribution of
spin densities on the D ring with a higher value on the
oxygen (34%). In this case the SOMO (singly occupied
molecular orbital) is localized at the spin density site. By
contrast, for the folded 22, the spin densities are also
distributed on the D ring whereas the SOMO is mainly
localized on the A ring on carbon 3. The SOMO energy is
close to the LUMO energy, indicating the presence of a
reactive site for the scavenging mechanism. We have
previously shown that carbon at the 3-position is the
preferential site of hydroxylation in basic aqueous solu-
tion.13 In order to confirm our hypothesis, we performed
mass spectrometric analysis of 2 after initiation of phe-
noxy radical with cerium(IV) in the presence of a small
amount of NaOH. We can observe mainly two peaks at
m/z 583 (M þ H)þ and m/z 604 (M þ Na)þ, which are
related to the molecular ion peaks of the molecule. An
additional peak at m/z 598 is also observed, which can be
attributed to an oxygen addition resulting from hydro-
xylation of the molecule.
EXPERIMENTAL
Synthesis
TLC analyses were performed on a 3 ꢃ 10 cm plastic
sheet precoated with silica gel 60F254 (Merck) with the
solvent system ethyl acetate–hexane (1:4). SiO2 (200–
400 mesh) (Merck) was used for column chromatogra-
phy. Melting-points were obtained on a Reichert Ther-
mopan melting-point apparatus, equipped with
a
microscope and are uncorrected. Infrared spectra were
obtained on a Perkin-Elmer 881 spectrometer on KBr
pellets. NMR spectra were obtained at 25ꢀC on a Bruker
AC 200 spectrometer, for H at 200 MHz and for 13C at
50 MHz. Chemical shifts are indirectly referenced to
TMS via the solvent signal (chloroform-d1 7.26 and
77.0 ppm; DMSO-d6 2.50 and 39.5 ppm). J values are
given in Hz. Mass spectra were recorded on a Finnigan
MAT Vision 2000 spectrometer [for matrix-assisted laser
desorption/ionization (MALDI)]. Elemental analyses
were performed at CNRS Laboratories (Vernaison) and
were within 0.4% of the theoretical value.
1
Diester 3. A 1.7 g (12.5 mmol) amount of 2-hydroxyace-
tophenone, 5.12 g (19 mmol) of 3,5-di-tert-butyl-4-
hydroxybenzoyl chloride and 0.122 g of 4-dimethylami-
nopyridine were dissolved in 50 ml of dry pyridine and
the stirred mixture was heated at 60ꢀC for 2 h under an
atmosphere of argon. The solution was cooled and 200 ml
of water were added. The solid was filtered and washed
with water until the pH was 7. The solid was dried and
crystallized from ethanol to give 1.94 g of the diester 3
(34% yield). Elemental analysis for C38H48O6, calculated
C 75.97, H 8.05, O 15.98, found C 76.14, H 8.21, O
CONCLUSION
The modified analogues of the natural flavones 1 and 2
have been fully characterized in the solid state and in
solution. The crystallographic structure of 1 displays
particular hydrogen bonding networks and that of 2
presents some strong ꢁ–ꢁ interactions, allowing the
stabilization of the crystal lattice. Theoretical calcula-
tions have shown that 2 can adopt a folded conformation
in solution. Interestingly, 2 is 12 and three time more
active than 1 and quercetin (a well-known antioxidant
flavonoid), respectively, in the LDL lipid peroxidation
test.17 Antioxidant activity and, in particular, inhibition of
lipid peroxidation is a multi-factor event. The ability of
radical formation, stabilization of the radicals, capability
of metal chelation and lipophilicity remain important
factors for the inhibitory activity. In this study we focused
on the electronic behavior of these two compounds and
the radical stabilization properties since 1 and 2 do not
complex copper and their lipophilicitity is of the same
15.51%; m.p. ¼ 227ꢀC; infrared (ꢆ/cmꢂ1), 3580 (ꢆ phe-
—
nolic OH), 2880 (ꢆ C–H), 1725 (ꢆ C O ester), 1700
—
(ꢆ C O ketone), 1600 (ꢆ C C aromatic); H NMR
1
—
—
—
—
(CDCl3), ꢇ 1.38 [18H, s, 2 ꢃ C (CH3)3], 1.49 [18H, s,
2 ꢃ C (CH3)3], 2.59 (3H, s, COCH3), 5.91 (1H, bs, OH),
7.25 (1H, dd, 3J ¼ 8.3 Hz, 4J ¼ 1.6 Hz, H3), 7.35 (1H, td,
4
3
3J ¼ 8.3 Hz, J ¼ 1.6 Hz, H5), 7.58 (1H, td, J ¼ 8.3 Hz,
4J ¼ 1.6 Hz, H4), 7.86 (1H, dd, J ¼ 8.3 Hz, J ¼ 1.6 Hz,
H6), 8.08 (2H, s, ring B or D), 8.22 (2H, s, ring B or D);
MALDI þ MS, m/z 623.2 (M þ Na), 639.2 (M þ K).
3
4
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 226–235

234
N. COTELLE ET AL.
Diarylpropane-1,3-dione 4. A 2.25 g (37.5 mmol) amount
of diester 3 and 1.5 g (37.5 mmol) of sodium hydroxide
(granulometry: 20–40 mesh) were dissolved in 60 ml of
dry DMSO and the mixture was stirred at room tempera-
ture for 2 h. The solution was poured into 200 ml of ice–
water. Acetic acid was added until the pH was 7. The
solid was filtered and washed with water. The solid was
dried and crystallized from ethanol to give 1.62 g of
diarylpropane-1,3-dione 4 (72% yield). Elemental ana-
lyses for C38H48O6, calculated C 75.97, H 8.05, O 15.98;
found C 75.58, H 8.32, O 15.63%; m.p. ¼ 190ꢀC; infrared
(ꢆ/cmꢂ1), 3620 (ꢆ phenolic OH), 2880 (ꢆ C–H), 1740 (ꢆ
C 78.48, H 7.43, O 13.81%; m.p. ¼ 240ꢀC; infrared
(ꢆ/cmꢂ1), 3450 (ꢆ phenolic OH), 2980 (ꢆ C–H), 1620
1
(ꢆ C O ketone), 1590 and 1570 (ꢆ C C aromatic); H
—
—
—
—
NMR (CDCl3), ꢇ 1.55 [18H, s, 2 ꢃ C (CH3)3], 5.70 (1H,
3
bs, OH), 6.80 (1H, s, H3), 7.45 (1H, td, J ¼ 7.5 Hz,
3
4
4J ¼ 1.2 Hz, H6), 7.61 (1H, dd, J ¼ 8.8 Hz, J ¼ 1.2 Hz,
H8), 7.73 (1H, td, 3J ¼ 7.7 Hz, 4J ¼ 1.2 Hz, H7), 7.80 (2H,
s, ring B), 8.26 (1H, dd, 3J ¼ 8.0 Hz, 4J ¼ 1.6 Hz, H5); 13
C
NMR (CDCl3), ꢇ 30.2 [C (CH3)3], 34.6 [C (CH3)3], 106.2
(C3), 118.1 (C8), 122.8 (C10), 123.7 (C4a), 124.0 (C20),
125.1 (C5), 125.7 (C6), 133.5 (C7), 136.7 (C30), 156.3
(C8a), 157.3 (C40), 164.8 (C2), 178.6 (C4); MALDI-
þ MS, m/z 351.3 (M þ H), 373.3 (M þ Na), 389.3
(M þ K).
—
—
—
—
C
C
O ester), 1690 (ꢆ C O ketone), 1600 and 1560 (ꢆ
—
—
1
C aromatic); H NMR (CDCl ), ꢇ 1.38 [18H, s, 2 ꢃ C
3
(CH3)3], 1.51 [18H, s, 2 ꢃ C (CH3)3], 5.82 (1H, bs,
phenolic OH), 6.80 (1H, bs, ethylenic H), 6.97 (2H, m,
3
4
H6 and H4), 7.48 (1H, td, J ¼ 8.4 Hz, J ¼ 1.5 Hz, H5),
NMR NOE measurements
3
4
7.76 (1H, dd, J ¼ 8.4 Hz, J ¼ 1.5 Hz, H3), 7.93 (2H, s,
ring B or D), 8.09 (2H, s, ring B or D), 12.05 (1H, bs,
phenolic OH), 14.5 (1H, bs, enolic OH); MALDI þ MS,
m/z 623.1 (M þ Na), 639.0 (M þ K).
1H NMR spectra of 1 and 2, dissolved in DMSO-d6
solution (2.5 mg mlꢂ1), were recorded on a Bruker AC
200 spectrometer equipped with an Aspect 3000 compu-
ter operating in the Fourier transform mode with quad-
rature detection at 200 MHz using tetramethylsilane
(TMS) as the internal reference. One-dimensional NOE
values were obtained in the difference mode by subtract-
ing two types of spectra, one in which the desired signal
was saturated at low power for 30 s and the other in which
the off irradiation was out of the spectrum. The steady
state was obtained with two dummy scans. Homonuclear
Overhauser effect experiments on 1 and 2 gave the
following results: irradiation of the hydrogen H3 at
6.80 ppm (1) or 6.85 ppm (2) produced NOEs of 32%
and 27%, respectively, of aromatic hydrogens of the B ring
and irradiation of the aromatic hydrogens H2’ and H6’ at
7.80ppm (1) and 7.91 ppm (2) induced NOEs of 37% and
34%, respectively, of the hydrogen H3. Most commonly,
Flavone 2. A 1 g (1.7 mmol) amount of diarylpropane-
1,3-dione 4 was dissolved in 200 ml of acetic acid and
8 ml of sulfuric acid and the stirred mixture was refluxed
for 1 h. The solution was cooled and 200 ml of water were
added. The solid was filtered and washed with water until
pH ¼ 7. The solid was dried and crystallized from ethanol
to give 0.62 g of 2 (65% yield). Elemental analyses for
C38H46O5, calculated C 78.32, H 7.96, O 13.72; found C
78.45, H 8.04, O 13.52%; m.p. ¼ 240ꢀC; infrared
(ꢆ/cmꢂ1), 3620 (ꢆ phenolic OH), 2880 (ꢆ C–H), 1740
—
—
—
—
(ꢆ C O ester), 1690 (ꢆ C O ketone), 1600 and 1560
—
—
(ꢆ C C aromatic); H NMR (CDCl ), ꢇ1.40 [18H, s,
1
3
2 ꢃ C ꢃ (CH3)3], 1.49 [18H, s, 2 ꢃ C (CH3)3], 5.81 (1H,
3
bs, OH), 6.85 (1H, s, H3), 7.43 (1H, td, J ¼ 7.5 Hz,
˚
intense NOEs correspond to short distances (1.8–2.7A).
3
4
4J ¼ 1.2 Hz, H6), 7.61 (1H, dd, J ¼ 8.8 Hz, J ¼ 1.2 Hz,
H8), 7.71 (1H, td, 3J ¼ 7.7 Hz, 4J ¼ 1.2 Hz, H7), 7.91 (2H,
s, ring B), 8.08 (2H, s, ring D) 8.26 (1H, dd, 3J ¼ 8.0 Hz,
4J ¼ 1.6 Hz, H5); 13C NMR (CDCl3), ꢇ 30.2 [C (CH3)3],
31.4 (C (CH3)3], 34.4 [C (CH3)3], 35.8 [C (CH3)3], 107.5
(C3), 118.2 (C8), 121.1 (C100), 124.0 (C4a), 124.5 (C20),
125.2 (C5), 125.7 (C6), 128.0 (C200), 128.4 (C10), 133.6
(C7), 136.2 (C300), 144.2 (C30), 151.7 (C40), 156.3 (C8a),
158.7 (C400), 164.1 (C2), 168.6 (Cc), 178.6 (C4);
MALDI þ MS, m/z 583.4 (M þ H), 605.3 (M þ Na).
ESR spectroscopy
ESR spectroscopy was performed on a Bruker ELEXYS
580E spectrometer operating at 9.7 GHz and 100 kHz
frequency modulation. Amplitude modulation and micro-
wave power were set at 0.8 G and 1 mW, respectively.
Radical generation was carried out by using the ceriu-
m(IV) oxidant procedure.18 Spectrum simulation was
performed with Bruker Simfonia software.
Flavone 1. A 1.0 g (1.7 mmol) amount of 2 and 1.0 g
(25 mmol) of sodium hydroxide were dissolved in 40 ml
of dry DMSO and the stirred mixture was heated at 60ꢀC
for 2 h under an atmosphere of argon. The solution was
cooled and 200 ml of water were added. The solid was
filtered and washed with water until the pH was 7. The
solid was dried and crystallized from ethanol to give
0.32 g of 1 (53% yield). Elemental analyses for
C23H26O3, calculated C 78.83, H 7.48, O 13.69, found
X-ray diffraction
X-ray diffraction measurements were performed on a
Bruker AXS three-circle diffractometer equipped with a
charge-coupled device (CCD) two-dimensional detector
(ꢈ Mo Kꢂ ¼ 0.71069 A, graphite monochromator,
J. Phys. Org. Chem. 2004; 17: 226–235
˚
Copyright # 2004 John Wiley & Sons, Ltd.

SYNTHESIS, STRUCTURE AND ELECTRONIC PROPERTIES OF FLAVONES
235
T ¼ 294 K). An empirical absorption correction was ap-
Electronic absorption spectroscopy
plied by using the SADABS program. Structure solution
was achieved by the direct method (SHELXS-86) and
refinement by using the full-matrix least-squares techni-
ques (SHELXTL program). All hydrogen atoms were
found on a Fourier difference map and their positions
refined, their thermal parameter being fixed at 1.2, the
value of the equivalent thermal parameter of the atom to
which they are bound. The experimental parameters for 1
and 2 are summarized in Table 1.
The UV–visible absorption spectra of 1 and 2 dissolved in
DMSO were recorded on a Varian Cary 1 double-beam
spectrophotometer in the 200–500 nm range with 2 nm
spectral resolution.
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Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 226–235