Theoretical and experimental exploration of the photochemistry of resveratrol: Beyond the simple double bond isomerization

Article abstract of DOI:10.1039/c2ob26241j

The photochemical isomerization of resveratrol has been the subject of recent studies in which contradictory results were reported. The photoproduct mixture of this reaction needs to be considered more complex than the coexistence of cis and trans isomers

Full text of DOI:10.1039/c2ob26241j

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PAPER
Theoretical and experimental exploration of the photochemistry of
resveratrol: beyond the simple double bond isomerization†
Roi Álvarez Rodríguez, Inmaculada R. Lahoz, Olalla Nieto Faza, María Magdalena Cid* and
Carlos Silva Lopez*
Received 29th June 2012, Accepted 19th September 2012
DOI: 10.1039/c2ob26241j
The photochemical isomerization of resveratrol has been the subject of recent studies in which
contradictory results were reported. The photoproduct mixture of this reaction needs to be considered
more complex than the coexistence of cis and trans isomers. An unidentified third product, at least, has
been detected in various studies although its nature was unknown. In this work, we aim to provide a
thorough description of the photochemical course of this reaction through experimental and
computational approaches working in a synergetic association.
Introduction
Resveratrol (3,4′,5-trihydroxystilbene, see Fig. 1) is a natural
product that is present in a wide variety of natural or processed
foods such as grapes, wine, chocolate, peanuts, tea, soy, etc.1
This biphenolic compound has captivated the attention of the
scientific community due to its biological activity and the associ-
ation of its moderate human consumption with healthy effects.2
Resveratrol is mainly produced in the seeds and pulp of plants
in response to stressor agents like fungi and bacteria, or environ-
mental pressure (excessive radiation or sudden changes in tempe-
rature among other factors). In this sense it can be classified as a
Fig. 1 Resveratrol, cis and trans isomers, together with several other
natural occurring hydroxystilbene derivatives.
phytoalexin.3 The biological activity of resveratrol is twofold: it
is involved in processes related to oxidative stress4 and has also
been reported as a sirtuin activator (vide infra).5
rate than commonly thought (kinh = 2.0 × 105 M−1 s−1 in chloro-
benzene at 303 K).7 The peroxyl radical formally abstracts a H
atom from one of the hydroxyl groups in resveratrol and forms
stable radical species.8 The OH group in the para position has
been suggested as the most active9 due to its lower O–H bond
dissociation energy. This is also supported by the efficient redis-
tribution of the spin density over the entire molecule once the
para OH has lost a hydrogen atom to form a radical species.10
Two main mechanisms have been described for this radical
scavenging process that we have summarized in Fig. 2: one con-
sisting of a hydrogen-atom transfer (HAT)7,8b and the other
being based on a single electron transfer (SET) preceded or fol-
lowed by a proton loss.11
The biphenolic structure of resveratrol furnishes this natural
product with anti-oxidative properties. This activity is however
somehow controversial,6 some studies conclude that resveratrol’s
exceptional properties are Sir2 dependent5c but actually only
moderate capabilities can be found experimentally or computa-
tionally for resveratrol when compared to other compounds.6b
The main mechanism involved in oxidative stress is associated
with very reactive oxygen species formed in the lipid peroxi-
dation chain. Resveratrol is capable of stopping this chain reac-
tion via the scavenging of peroxyl radicals, although at a lower
Departamento de Química Orgánica, Facultade de Química, Campus
Lagoas-Marcosende, 36310 Vigo, Spain. E-mail: carlos.silva@uvigo.es,
mcid@uvigo.es; Tel: +34 986812226
†Electronic supplementary information (ESI) available: Cartesian co-
ordinates, SCF energies and the number of imaginary frequencies for all
stationary points computed in this work. NMR data for compounds 8a,
8b, 9a, 9b and 10; also Fig. 1S showing the instability of 10 and Fig. 2S
depicting the 1H NMR of the oxygen-free photochemical reaction of
EOM-8 after 7 h of irradiation. See DOI: 10.1039/c1ce05336a
On the other hand, resveratrol, as a sirtuin activator,5 has been
the focus of an important number of investigations due to the
relation between sirtuins and lifespan.12 Sirtuins are a family of
proteins present in eukaryotic cells, which use genes to silence
regions of DNA. Resveratrol, as an activator of sirtuins, is
related to several experiments in lifespan,13 cell repair and
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227.2 m/z as the compound responsible for the rather different
fragmentation pattern observed in this case. This interpretation
implies a reaction involving the loss of two hydrogen atoms
from resveratrol. Mark suggested that this unknown compound is
a diphenylacetylene derivative that could be formed through an
oxidative reaction of the exocyclic double bond.19a In another
report, López-Hernández et al. obtained an MS spectrum for this
unknown third compound with the same features as that recorded
by Mark. However, they considered that the mass of the
unknown compound was indeed the same mass of resveratrol.
Following this logic, they proposed that the third photoproduct
should be a resveratrol isomer.19b Both studies, therefore,
reported the detection of an unidentified product of resveratrol
under photochemical conditions, but such a compound was
never properly characterized and the structures proposed for it
differ. Very recent results unveiled further complexity to this
chemistry, based on the studies carried out by Ho, who proposed
a reaction cascade pathway for the acid-assisted photoisomeriza-
tion of stilbene,20 Kim, Park and coworkers have assigned the
structure of the fluorescent compound to a naphthalenylbuten-
2-one. This arylbuten-2-one is the result of several isomerization
reactions starting with the pericyclic ring closing of cis-resvera-
trol to the corresponding dihydrophenanthrene.21
Fig. 2 Hydrogen atom transfer and electron transfer mechanisms pro-
posed for the antioxidative activity of resveratrol.
tumoral cell apoptosis.14 Nevertheless the capability of resvera-
trol to activate sirtuins has also been questioned in recent
research.6a,15
Since resveratrol occurs in two isomeric forms (cis/trans, see
Fig. 1), it is important to determine which of them is more bio-
logically active. Of the two stereoisomers of resveratrol, the
trans isomer is the most stable and the most abundant in nature,
but both coexist in different proportions in wine and other foods.
A number of recent studies have therefore been carried out to
measure the absolute and relative amount of the two isomers in
food products.
This situation suggests that the photochemical reactivity of
resveratrol is much more complicated than a simple cis–trans
isomerization. For this reason, and due to our ongoing interest in
the reactivity of food antioxidants and their derivatives,22 we
decided to explore this chemistry and unambiguously identify
the unknown photoproduct isolated by the groups of Mark and
López-Hernández.
trans-Resveratrol, 1trans, undergoes isomerization to the cis
form 1cis when exposed to UV-vis light. The photostationary
equilibrium is highly dependent on the specific spectrum of light
reaching the sample and, at 350 nm, this equilibrium is charac-
terized by the following kinetic parameters: kcis = 0.0472
0.0021 s−1 and t1/2 = 14.7 min.16 The chemical behavior of the
cis isomer depends on the reaction conditions, which sometimes
leads to contradictory results when quantification of resveratrol is
pursued. Many analytical reports base their data on the afore-
mentioned cis–trans equilibrium and do not mention the pres-
ence of a third compound in the reaction mixture. However,
Roggero et al. admitted the possible presence of a phenanthrene
derivative as a photoproduct of trans-resveratrol on the basis of
its UV spectra and in analogy to the photochemical reactivity of
other stilbenes.17 Furthermore, a chromatographic analysis in
wine samples has been reported that takes advantage of the
photoreactivity of cis-resveratrol leading to the corresponding
fluorescent phenanthrene, even if this phenanthrene derivative
was neither isolated nor characterized.18
Experimental studies on resveratrol photoisomerization using
HPLC-MS methods lead to contradictory conclusions. Mark
et al. studied the experimental photoisomerization of resveratrol
at different irradiation times. At short irradiation times they
found two peaks in the UV detector with the characteristic mass
of resveratrol (229.2 m/z), and they were assigned to the trans
and cis isomers, respectively. At longer irradiation times a third
peak appeared showing an MS fragmentation pattern that
departed significantly from that of cis or trans-resveratrol. The
MS spectrum of this unknown third photoproduct also showed a
peak at 229.2 m/z, but Mark et al. pointed at another peak at
Results and discussion
The hydroxyl substitution pattern of resveratrol might affect its
thermal stability. We therefore started our exploration of the iso-
merization reactivity of resveratrol by analyzing computationally
and experimentally its stability under radiationless conditions.
Commercial resveratrol is very stable in the dark, which is in
good agreement with a computed barrier for double bond iso-
merization of 43.6 kcal mol−1. Accordingly, the computed
barrier for stilbene is 43.9 kcal mol−1, which is also in good
agreement with its observed barrier of 42.8 kcal mol−1. This
suggests that the presence of three hydroxyl groups in resveratrol
does not affect its thermal stability.23 Once the thermal stability
of the substrate was confirmed, we decided to also computation-
ally characterize the photochemical isomerization process and
compare it with the well studied photoisomerization of the
parent stilbene.24 A multireference methodology similar to that
employed by Martinez et al. to describe the photochemical iso-
merization of ethylene and stilbene was employed.24a In short,
this implies a state-averaged CASSCF(2,2)/6-31G(d) level of
theory (see the methods section for further details). As expected,
the ground state potential energy surface shows two minima for
the cis and trans isomers and a high energy transition state (cor-
responding to the previously described thermal reaction). In the
first excited state, near the Frank–Condon region, two very
shallow minima can be observed (see Fig. 3). Another relevant
stationary point on this excited state surface is the global
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Fig. 3 Ground and first excited state potential energy surfaces for the
photochemical and thermal isomerization of resveratrol at the CASSCF
(2,2)/6-31G(d) level of theory.25 The two reaction coordinates chosen
correspond to the H–C–C–H and Ar–C–C–Ar dihedrals, which concen-
trate the changes along the reaction path.
Fig. 4 Energy profile for the oxidation of cis-resveratrol, 1cis, to
furnish the derivative 7 via a type-II dyotropic reaction (in blue) and the
dioxetane 7 via singlet oxygen addition to the exocyclic double bond (in
red).
6 is ∼38 kcal mol−1 and the products lie 11 kcal mol−1 lower in
energy than the reactants (see Fig. 4). The alternative oxidation
to yield a dioxetane ring is a formal [π2 + π2] cycloaddition,
which is symmetry forbidden according to the Woodward–
Hoffmann rules.27 Not surprisingly, therefore, the reaction
occurs in a stepwise manner through a diradical intermediate.
Apparently, the energy penalty associated with the formation of
a four membered ring is spread along the multi-step reaction
coordinate, and the rate limiting transition state imposes a barrier
of only 19.2 kcal mol−1. The dioxetane formed is also rather
stable with respect to the reactants (−16.3 kcal mol−1). This
energy profile shows that the formation of dioxetane 7 is more
favorable both kinetically and thermodynamically than the for-
mation of the diphenylacetylene derivative 6. This suggests that,
in the event of a reaction occurring between singlet oxygen and
resveratrol, it is much more likely that this reaction will involve
the addition of oxygen to furnish the dioxetane 7, assuming that
other reactions not examined here do not occur (see Fig. 4).
In summary, computational exploration suggests that: (1) the
chemistry of resveratrol should not depart far away from the
general reactivity of stilbene and (2) in the event of an oxidative
process with photochemically activated oxygen, the expected
course of the reaction is the formal addition to furnish a dioxe-
tane derivative. Under these circumstances we turned to experi-
mental work aiming to either confirm or refute such guidelines.
Following the close similarity found between the energetic
barriers observed for resveratrol and stilbene under both thermal
and photochemical conditions, we decided to find out whether
the oxidative process via oxygen addition could also be experi-
mentally confirmed. To accomplish this, we employed different
irradiation conditions and we observed that the outcome of the
photochemistry of a solution of trans-resveratrol is highly depen-
dent on the specific reaction conditions: energy source, concen-
tration, time of irradiation, etc.28 For instance, the irradiation of a
0.2 mM aqueous ethanolic solution of trans-resveratrol 1trans
with a 200 W lamp placed 5 cm from the reaction flask for
15 min led to a mixture of trans/cis resveratrol 1 : 0.7. Whereas
if a 450 W hanovia lamp was used at the same distance for only
5 min, the ratio was 1 : 4 trans/cis. When it was irradiated for
Scheme 1 Proposed reactions for the dehydrogenation of resveratrol
with singlet oxygen under photochemical conditions.
minimum just above the thermal transition state. Both minima at
the Frank–Condon region are kinetically very unstable since a
very low barrier separates them from the global minimum. In
analogy to stilbene, and also in good agreement with the experi-
mental data, the vertical excitation energy of cis-resveratrol is
slightly larger than in the trans isomer. trans-Resveratrol, there-
fore, absorbs UV light at 308 nm and reaches the shallow
minima at the first excited state surface. Surmounting a barrier of
less than 0.5 kcal mol−1 the excited trans-resveratrol reaches the
global minimum associated with a 90° distortion of the exocyclic
double bond dihedral angle. Vibrational relaxation to the ground
state makes the isomerization transition state accessible and,
from there, both isomers can be formed completing either a glob-
ally unproductive reaction or a double bond isomerization
process (see Fig. 3).
At this stage we decided to explore the possibility of oxidative
photocyclization to yield a compound weighting 2 Da less than
resveratrol. Oxidation under photochemical conditions calls for
the intervention of singlet oxygen. Two different oxidation
routes were considered for the reaction of resveratrol with this
highly reactive oxidant: (1) the route proposed by Mark, in
which the resveratrol is oxidized through the loss of the two
hydrogen atoms attached to the exocyclic double bond yielding
diphenylacetylene, and (2) the [π2 + π2] cycloaddition of singlet
oxygen to the exocyclic double bond to furnish a dioxetane
derivative (see Scheme 1).
The dehydrogenation of the exocyclic double bond of cis-
resveratrol to form a diphenylacetylene derivative occurs via a
concerted transition state. Both hydrogen atoms shift synchro-
nously from the double bond to the singlet oxygen molecule in a
type-II dyotropic reaction, a sort of chemical transformation that
has gathered significant attention recently.26 The energy barrier
associated with the formation of the diphenylacetylene derivative
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15 min more a trans/cis mixture was also obtained, but along
fractions whereas the mass of the third compound was 227.2 Da.
These results suggest that the controversy arising from the nature
of the third compound in the photochemistry of resveratrol was
likely due to traces of cis-resveratrol present in the HPLC frac-
tion of the unknown compound. Under these circumstances two
peaks with m/z ratios of 229.2 and 227.2 appear in the MS spec-
trum. Mark interpreted that the molecular peak of the unknown
compound was 227.2 whereas López-Hernández opted for not
interpreting the observations and assumed that the unknown
compound was an isomer of resveratrol. Our results clearly
demonstrate that the third compound found by these authors is
formed through the loss of two hydrogen atoms from resveratrol.
All the efforts aimed to isolate this compound in amounts
large enough for complete characterization were unsuccessful.
Therefore, we decided to synthesize this phenanthrene via an
indirect route to confirm this hypothesis. Thus, trimethyl-
protected resveratrol 8a,33 prepared in good yields by nucleophi-
lic substitution of 1 with methyl iodide, was irradiated with a
450 W lamp to furnish the phenanthrene derivative 9a in 37%
yield.34 The methyl derivative 9a was demethylated with TMSI
in dichloromethane at 50 °C or MeOH/HCl 10% at 25 °C to give
trihydroxyphenanthrene 1035 that proved to be highly unstable
under these reaction conditions. The same results were obtained
when the methoxymethoxy derivatives 8b and 9b were used
(Scheme 2).36
1
with other compounds.29 The H-NMR spectrum of these reac-
tion mixtures showed a proton signal resonating at 9.1 ppm; the
presence of which prompted us to assume that a phenanthrene
derivative could be involved, since the chemical shift of H5 in
this structural motif usually appears at such displacement.30
HPLC31 of the mixture was then performed. Two main signals
at 6.98 and 13.75 min were observed with UV detection, which
were assigned to trans- and cis-resveratrol, as in previous
studies,19 along with signals at higher retention times. Further-
more, with fluorescence detection (λexc = 260 nm, λem
=
364 nm) a peak at 14.95 min appeared as the most intense one
suggesting an increased aromatic character of this unknown com-
pound with respect to resveratrol (Fig. 5, top panel).
HPLC-APCI+-MS performed under the conditions reported by
Galeano-Díaz and coworkers18,32 provided a chromatogram
showing three peaks in close analogy to those obtained both by
Mark and López-Hernández.19 Unfortunately, this means that the
fraction of the third unknown compound overlaps with cis-
resveratrol and, therefore, the resulting MS spectrum is unreli-
able. By slightly modifying the elution conditions (reducing the
flux from 0.8 ml min−1 to 0.3 ml min−1) total separation of the
three peaks was achieved with retention times of 4.22, 7.06 and
7.74 min (see Fig. 5, bottom panel). The mass spectra of these
fractions reported molecular peaks of 229.2 Da for the first two
A comparative 1H NMR study is shown in Fig. 6. Spectrum 1
corresponds to a mixture of 1trans and 1cis obtained after
irradiation with a 450 W lamp placed 25 cm from the sample for
5 min; spectrum 2 corresponds to the reaction mixture when irra-
diated at 5 cm for 20 min. and 3 corresponds to the isolated
phenanthrene. With these results, we can conclude that the signal
at 9.1 ppm corresponds to phenanthrene 10. Moreover, when a
sample of 10 was injected into the HPLC column under the con-
ditions of this study, we could unequivocally assign compound
10 to the aforementioned third compound. Interestingly, we
1
could identify the same H-NMR signals of 10 in a spectrum of
the reaction mixture that Prof. Lopez-Hernández kindly sent us.
In summary, the computational and experimental evidence
gathered clearly suggests that the photochemistry of resveratrol is
similar to that found in the parent system stilbene. Under photo-
chemical activation, resveratrol undergoes rapid cis–trans iso-
merization and, under longer irradiation times, a six electron
electrocyclic reaction followed by oxidation furnishes 2,4,6-tri-
hydroxyphenanthrene. The formation of the trihydroxyphenan-
threne derivative from resveratrol requires intense irradiation and
therefore this compound is not expected to participate in the
Fig. 5 HPLC chromatograms corresponding to the reaction mixture
after irradiating commercial trans-resveratrol for 20 min at 5 cm with a
450 W lamp (top) and an APCI+-MS chromatogram of the same reaction
mixture (bottom).
Scheme 2 Synthetic route followed in the preparation of trihydroxy-
phenanthrene 10.
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Fig. 6 1H-NMR spectra: (1) reaction of resveratrol at 25 cm for 5 min;
(2) reaction at 5 cm for 20 min; (3) isolated phenanthrene (c: cis-resvera-
trol; t: trans-resveratrol; p: 2,4,6-trihydroxyphenanthrene).
Fig. 7 1H-NMR spectra of the reaction mixture for the photoactivation
of trans-resveratrol 1trans (top) and trimethyl-protected trans-resveratrol
8a (bottom).
phytochemistry of the plant. However, due to its high fluor-
escence it could be valuable when developing accurate detection
protocols for resveratrol from natural resources.
The preliminary computational work performed to explore the
possibility of the formation of diphenylacetylene derivative 6
suggested that, under photochemical conditions, singlet oxygen
could readily react with resveratrol to give a dioxetane derivative.
The energy barrier associated with this reaction was only
19.2 kcal mol−1, which renders this process very facile. Guided
1
by these results we decided to carefully analyze the H-NMR
spectrum of the reaction mixture to look for evidence of this
reaction pathway being operative. Dioxetane species are very
unstable and they spontaneously decompose yielding two carbo-
nyl species.37 To further confirm this reactivity a photoactivated
reaction was performed while bubbling oxygen through the
mixture. Such conditions favoured the formation of the transient
dioxetane intermediate and we identified 4-hydroxy-benzal-
dehyde and 3,5-dihydroxy-benzaldehyde or 4-methoxy-benzal-
dehyde and 3,5-dimethoxy-benzaldehyde as products of the
reaction when resveratrol 1 or the trimethoxy derivative 9a were
irradiated, respectively (Fig. 7).38
The complete picture of the photochemistry of resveratrol is
therefore considerably more complicated than a simple cis–trans
interconversion. Not only oxidative processes are operative and
competitive under regular conditions, furnishing phenanthrene
derivatives, but also fragmentation of fleeting dioxetane inter-
mediates has been observed (see Fig. 8).
Fig. 8 Summary of the observed photochemistry of trans-resveratrol
1trans
.
exploration uncovered the feasibility of singlet oxygen addition
to cis or trans-resveratrol to furnish a dioxetane intermediate.
Such prediction was confirmed experimentally through the
detection of the products of dioxetane by spontaneous cyclo-
reversion (benzaldehydes 11 and 12), thus reinforcing the idea of
the synergetic relationship between theoretical and experimental
work.
Computational methods
Density functional theory39 employed throughout this work was
based on the Kohn–Sham formulation40 and all calculations
were performed with the Gaussian 09 suite.41 The three-para-
meter hybrid B3LYP42 functional was used in all DFT calcu-
lations along with the double-ζ basis set 6-31G(d,p). Due to
significant high spin contamination occurring in singlet oxygen
and compounds related to its chemistry the projection scheme of
Yamaguchi43 has been employed for all the structures illustrated
in Fig. 4. This methodology has been successfully employed to
study singlet oxygen reactivity very similar to that described in
the current work.44 Harmonic analysis has been applied to the
second derivatives of the energy with respect to the nuclear
motion for all structures. Through this analysis the nature
Conclusions
Under photochemical activation trans-resveratrol undergoes not
only a double bond isomerization. The third compound observed
in previous experimental studies has been characterized as 2,4,6-
trihydroxyphenanthrene, formed via a 6-electron electrocyclic
ring closure followed by dehydrogenation. The dyotropic reac-
tion of singlet oxygen to afford a diphenylacetylene derivative
has an associated activation energy of 37.6 kcal mol−1 and can
be discarded as a competitive route. However, computational
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(minimum or transition structure) of the stationary points was
identified and the zero point vibrational energies and the contri-
butions to the free energy were obtained.
([M+], 66), 167 (79), 149 (100). HRMS (EI+) m/z calc. for
C20H24O6 360.1573; found 360.1584.
Due to the potential diradical character of some of the struc-
tures considered in this work, the internal and external stability
of the wavefunctions was computed via the Hermitian stability
matrices A and B in all cases.45 For all the structures exhibiting
unstable restricted wavefunctions, the spin-symmetry constraint
of the wavefunction was released (i.e. expanding the SCF calcu-
lation to an unrestricted space, UB3LYP) leading to stable
unrestricted wavefunctions.
Complete active space SCF (CASSCF) calculations were per-
formed with the 6-31G(d) basis set to accurately describe the di-
radical character of several key intermediates. These calculations
were performed with the GAMESS package.46 The active space
in the CASSCF calculations presented in this work includes the
π and π* orbitals of the aliphatic double bond in resveratrol,
resulting in two electrons in two orbitals active space, usually
noted CASSCF(2,2).
General procedure for the oxidative photocyclization:
2,4,6-trimethoxyphenanthrene (9a)
A 4 mM solution of the (E)-1,3-dimethoxy-5-(4-methoxystyryl)-
benzene (27 mg, 0.1 mmol) in ethanol, under an air atmosphere,
was irradiated using a 450 W medium pressure mercury lamp in
a Pyrex immersion well. RMN was used to follow the irradiation
progress at regular intervals. When the reaction was complete
(6 h), the reaction mixture was concentrated in vacuo in the dark
and purification by flash column chromatography on silica gel
(hexane–DCM 70 : 30) afforded 10 mg (37%) of 2,4,6-
trimethoxyphenanthrene.34
2,4,6-Tris(methoxymethoxy)phenanthrene (9b)
A 4 mM solution of 8b (59 mg, 0.22 mmol) afforded 13 mg
1
(16%) of 9b. Data: H NMR (400 MHz, CDCl3) δ 9.28 (d, J =
Experimental methods
2.4, 1H), 7.75 (d, J = 8.7, 1H), 7.62 (d, J = 8.7, 1H), 7.48 (d, J
= 8.7, 1H), 7.26 (dd, J = 2.4, 8.7, 1H), 7.14 (d, J = 2.4, 1H),
7.06 (d, J = 2.4, 1H), 5.48 (s, 2H), 5.33 (s, 2H), 5.29 (s, 2H),
3.59 (s, 3H), 3.54 (s, 3H), 3.53 (s, 3H). 13C NMR (100 MHz,
CDCl3) δ 157.4, 156.1, 155.8, 135.9, 131.5, 129.6, 128.1, 127.7,
125.3, 116.5, 116.3, 113.3, 106.3, 103.4, 95.2, 95.1, 94.8, 56.7,
56.4, 56.2. EM (EI+) m/z (%) 359 (17), 358 ([M+], 88), 327
(17), 326 (100), 280 (50), 265 (48), 250 (50), 223 (23), 196
(17), 195 (93), 165 (32), 151 (30). HRMS (EI+) m/z calc. for
C20H22O6 358.1416; found 358.1420.
(E)-1,3-Dimethoxy-5-(4-methoxystyryl)benzene (8a)
Sodium hydride (134 mg, 3.5 mmol, 60% in mineral oil) was
suspended in DMF (12 mL) at 0 °C in a round-bottomed flask
under an argon atmosphere. Then a solution of trans-resveratrol
(250 mg, 1.095 mmol) in DMF (10 mL) was added slowly, and
the ice bath was removed. A solution of iodomethane (1.140 g,
8 mmol) in DMF (20 mL) was added slowly into the mixture
from a dropping funnel with a pressure equalizing arm. After 5 h
at 25 °C, the reaction mixture was plunged into crushed ice con-
taining 5% HCl, and the reaction product was extracted with
ethyl acetate. The combined organic layers were washed with
brine and water, dried over Na2SO4, filtered and concentrated
in vacuo. Purification by flash column chromatography on silica
gel (hexane–ethyl acetate 70 : 30) afforded 27 mg (46%) of
(E)-1,3-dimethoxy-5-(4-methoxystyryl) benzene.33
Phenanthrene-2,4,6-triol (10)
From 9a: Trimethylsilyl iodide (281 mg, 1.4 mmol) was added
to a solution of 9a (13 mg, 0.05 mol) in DCM (2.5 mL). After
24 h at 50 °C, under an argon atmosphere, an additional amount
of Me3SiI (141 mg, 0.7 mmol) was added. The reaction was
complete after 23 h more. Then, MeOH (1 mL) was added and
the solvent was removed to yield the trihydroxyphenanthrene
10.35
(E)-1,3-Bis(methoxymethoxy)-5-(4-methoxymethoxy)styryl
benzene (8b)
Sodium hydride (302 mg, 2.9 mmol, 60% in mineral oil) was
suspended in THF (9 mL) at 0 °C in a round-bottomed flask
under an argon atmosphere. Then a solution of trans-resveratrol
(200 mg, 0.88 mmol) in THF (9 mL) was added slowly, and the
ice bath was removed. After 30 min, a solution of chloromethyl
methyl ether (922 mg, 11.45 mmol) was added via a syringe.
After 15 h at 25 °C, the reaction mixture was partitioned
between water and ethyl acetate. The aqueous phase was
extracted with ethyl acetate, the combined organic layers were
dried over Na2SO4, filtered and concentrated in vacuo. Purifi-
cation by flash column chromatography on silica gel (hexane–
ethyl acetate 70 : 30) afforded 73 mg (23%) of 8b. Data: 1H
NMR (400 MHz, CDCl3) δ 7.46 (d, J = 8.7, 2H), 7.05 (m, 3H),
6.93 (d, J = 16.3, 1H), 6.87 (d, J = 2.2, 2H), 6.66 (t, J = 2.2,
1H), 5.22 (s, 6H), 3.52 (s, 9H). 13C NMR (100 MHz, CDCl3) δ
158.7, 157.2, 140.0, 131.2, 129.0, 128.0, 126.9, 116.6, 107.8,
104.26, 94.7, 94.6, 56.3, 56.2. EM (EI+) m/z (%) 361 (8), 360
From 9b: HCl(c) (30 μL) was added to a solution of 9b
(3 mg, 0.01 mmol) in MeOH (0.3 mL) and the resulting reaction
mixture was stirred under a drying tube at 25 °C for 24 h. Elim-
ination of the solvent under reduced pressure rendered com-
pound 10.
Acknowledgements
C.S.L. and M.C. are grateful to the Ministerio de Ciencia e Inno-
vación (CTQ2009-13703) and the Xunta de Galicia (INCITE08
PXIB383129PR and INCITE09 314 294 PR) for financial
support. The authors wish to thank Mr Berto Acuña and Mr
Esteban Guitián for their assistance on HPLC runs and mass
spectroscopy. The Centro de Supercomputación de Galicia
(CESGA) is also acknowledged for generous allocation of com-
puting time.
9180 | Org. Biomol. Chem., 2012, 10, 9175–9182
This journal is © The Royal Society of Chemistry 2012

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