Regioselective O-derivatization of quercetin via ester intermediates. An improved synthesis of rhamnetin and development of a new mitochondriotropic derivative

Article abstract of DOI:10.3390/molecules15074722

The regioselective synthesis of several quercetin (3,3',4',5,7-pentahydroxy flavone) tetraesters bearing a single free OH on 5-C was achieved in good yield by proper choice of reaction conditions using common esterification procedures. Tetracetylated quercetin with the free OH on 7-C was selectively obtained instead via imidazole-promoted deacylation of the corresponding pentaester. Unambiguous structural characterization of the two isomeric tetraacetyl quercetin derivatives was obtained by combined HSQC and HMBC 2D-NMR analysis. These molecules can be used as starting materials for the regioselective synthesis of other derivatives. High yield syntheses of the natural polyphenol rhamnetin (7-O-methylquercetin) and of the new mitochondriotropic compound 7-(4-triphenylphosphoniumbutyl) quercetin iodide are reported as examples.

Full text of DOI:10.3390/molecules15074722

Molecules 2010, 15, 4722-4736; doi:10.3390/molecules15074722
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Regioselective O-Derivatization of Quercetin via Ester
Intermediates. An Improved Synthesis of Rhamnetin and
Development of a New Mitochondriotropic Derivative
Andrea Mattarei 1, Lucia Biasutto 2, Federico Rastrelli 1, Spiridione Garbisa 2, Ester Marotta 1,
Mario Zoratti 2,3 and Cristina Paradisi 1,*
1
Department of Chemical Sciences, Università di Padova, via Marzolo 1, 35131 Padova, Italy
2
Department of Biomedical Sciences, Università di Padova, viale G. Colombo 3, 35121 Padova,
Italy
3
CNR Institute of Neuroscience, viale G. Colombo 3, 35121 Padova, Italy
* Author to whom correspondence should be addressed; E-Mail: cristina.paradisi@unipd.it;
Tel.: +39-049-827-5661; Fax: +39-049-827-5239.
Received: 10 June 2010; in revised form: 22 June 2010 / Accepted: 2 July 2010 /
Published: 6 July 2010
Abstract: The regioselective synthesis of several quercetin (3,3’,4’,5,7-pentahydroxy
flavone) tetraesters bearing a single free OH on 5-C was achieved in good yield by proper
choice of reaction conditions using common esterification procedures. Tetracetylated
quercetin with the free OH on 7-C was selectively obtained instead via imidazole-promoted
deacylation of the corresponding pentaester. Unambiguous structural characterization of
the two isomeric tetraacetyl quercetin derivatives was obtained by combined HSQC and
HMBC 2D-NMR analysis. These molecules can be used as starting materials for the
regioselective synthesis of other derivatives. High yield syntheses of the natural polyphenol
rhamnetin (7-O-methylquercetin) and of the new mitochondriotropic compound 7-(4-
triphenylphosphoniumbutyl) quercetin iodide are reported as examples.
Keywords: regioselective alkylation; mitochondrial targeting; quercetin; rhamnetin;
polyphenols

Molecules 2010, 15
4723
1. Introduction
Due to its widespread diffusion in many foodstuffs and to its complex and rich chemistry, quercetin
[2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one, 1], is an important and highly
representative member of the vast family of natural polyphenols. Such compounds are drawing
increasing interest by the scientific community in view of the potentially beneficial effects
(anti-inflammatory, anti-ageing, cardioprotective, anticancer, etc.) that many of them exhibit in vitro
(for quercetin see, e.g., [1–4]). Despite the fact that several natural polyphenols are being exploited as
additives in nutritional, cosmetic and over-the counter pharmacological formulations, their activity in
vivo is difficult to demonstrate due to their low bioavailability [5–7]. This in turn is largely a
consequence of their high susceptibility to metabolic modifications. Due to the presence of
multiple -OH’s, polyphenols are ready-made substrates for Phase II conjugative metabolism which
rapidly converts them to sulfates, glucuronides and methyl ethers.
One interesting approach to overcome the low bioavailability of polyphenols, so as to test and
hopefully exploit their activity in vivo, relies on chemical modification of the natural compound aimed
at increasing solubility and at slowing down metabolism, while maintaining the capability to
regenerate the original molecule. Efforts are obviously concentrating on the protection of the
polyphenol hydroxyls [8,9]. Another possibility is the modification of the parent molecule by the
introduction of a stable substituent capable of conferring desirable properties. Recently, a charged,
membrane-permeant triphenylphosphonium group has been linked to quercetin (at position 3) and
resveratrol to produce derivatives targeted to mitochondria, where the polyphenol redox properties
either as a pro- or anti-oxidant may be best exploited [10–12]. These products join the growing family
of mitochondriotropic redox-active compounds [13], the best-known of which may be Mito-Q, a
coenzyme Q derivative developed by the pioneering group of Murphy and Smith, currently undergoing
clinical trials [14,15], and the plastoquinone-comprising “SkQs” of Skulachev and coworkers [16].
In such a context it is important to be able to modify selectively the various hydroxyls, which are
far from equivalent from either the chemical or the biofunctional points of view. Thus, in the case of
quercetin, the catechol moiety on the C ring (3’-OH, 4’-OH) is largely responsible for the redox
properties of the molecule [17], the 3-OH is a key group for kinase inhibition [18], the 7-OH is chiefly
responsible for the weak uncoupling activity [19], and the 5-OH is the least acidic and reactive one,
due to intramolecular H-bonding to the carbonyl at 4-C. The literature reports the regioselective
acylation of the 3-OH using enzymatic methods [20] or, after protection of the catecholic hydroxyls,
chemical reactions [21].
We report here the synthesis and characterization of several ester derivatives of quercetin including
pentaesters of a few carboxylic acids with different steric hindrance and selected regioisomers of
tetraesters in which the free OH is either on 5-C or 7-C. The regioselective synthesis of tetraesters with
a free 5-OH was achieved via esterification of quercetin under controlled conditions. Tetraesters with a
free 7-OH can be obtained via selective hydrolysis of pentaester precursors, and we report here the
tetraacetylated derivative. These molecules with a single free OH are meant to serve as entry points for
the production of other compounds. To illustrate their usefulness we report the synthesis of rhamnetin
(7-O-methylquercetin), a natural flavonoid and quercetin metabolite which possesses many of the
activities of quercetin itself (e.g., it is an inhibitor of mitochondrial NADH oxidase [22]), and of 7-O-

Molecules 2010, 15
4724
(4-triphenylphosphoniumbutyl)quercetin iodide. The latter is a novel mitochondria-targeted compound
bearing a free OH at the important 3-position.
2. Results and Discussion
2.1. Synthesis of 3,3’,4’,5,7-pentaacyl quercetins, of 3,3’,4’,7-tetraacyl quercetins and of 3,3’,4’,5-
tetraacetyl quercetin
Acylation of all five hydroxyl groups of quercetin (1) with groups of different steric hindrance was
carried out modifying published procedures [8,23] to obtain derivatives 2a–d in high yield (79–97%)
(Scheme 1). By careful control of the reaction conditions, i.e. temperature (T), type and equivalents
(neq) of acylating agent and reaction time (t), it is possible to stop the acylation at the 3,3’,4’,7-
tetraester stage. Reaction yields did not systematically depend on the acyl group.
Scheme 1. Synthesis of pentaacyl and 3,3’,4’,7-tetraacyl derivatives of quercetin.
O
R
O
3'
R
O
O
O
4'
O
O
O
R
neq1
7
t1, T1
O
3
5
R
O
2 a-d
R
O
O
O
OH
3'
O
OH
4'
R
X
HO
O
Pyridine
O
7
OH
X = Cl, OCOR
3
R
5
OH
O
3'
R
O
O
O
neq2
4'
1
O
O
O
R
7
t2, T2
O
3
5
3 a-d
OH
R
O
a: R=CH3; b: R=CH(CH3)2; c: R=C(CH3)3;
d: R=C6H5.
Thus, a single isomer of tetraacetyl quercetin was obtained in 75% isolated yield from the reaction
with five equiv. of acetic anhydride at room temperature for 3 hours. The product is assigned the
1
structure of 3,3’,4’,7-tetraacetyl quercetin (3a) based on H-NMR analysis. Following literature
protocols [24], the assignment of the single free hydroxyl group utilized the narrow peak shape and
high chemical shift (12 ppm, 20 mM in CDCl3) of the hydroxyl proton [25], and the difference in
chemical shifts between the ring protons of the product and the corresponding ones of
pentaacetylquercetin (2a) (Table 1). The chemical shifts of 2’-H, 5’-H and 6’-H are very similar in 2a
and in 3a, while those of 6-H and 8-H differ significantly.

Molecules 2010, 15
Table 1. Chemical shifts (δ) of the aromatic protons of pentaacetyl quercetin (2a),
4725
3,3’,4’,7-tetraacetyl quercetin (3a) and 3,3’,4’,5-tetraacetyl quercetin (4) measured in
CDCl3. Chemical shift differences (∆δ) relative to 2a are shown in parentheses.
Compound
δ(H-6)
δ(H-8)
δ(H-5’)
δ(H-6’)
δ(H-2’)
2a
3a
4
6.88
7.33
7.35
7.72
7.69
6.60 (-0.28) 6.85 (-0.48) 7.36 (+0.01) 7.75 (+0.03) 7.72 (+0.03)
6.46 (-0.42) 6.71 (-0.62) 7.24 (-0.11) 7.64 (-0.08) 7.58 (-0.11)
HSQC (Heteronuclear Multiple-Quantum Correlation) and HMBC (Heteronuclear Multiple-Bond
Correlation) 2D NMR analysis was then used to unambiguously establish the structure of 3a (see
Section 2.2). Compounds 3b-d were synthesized in good yield (70–85%) and characterized by
analogous procedures. Details about the specific reaction conditions used are given in the
Experimental Section. The good regioselectivity of this reaction can be attributed to the low
nucleophilic reactivity of the 5-OH group. The tetraacetyl isomer of quercetin 4 with a free 7-OH was
instead prepared with high regioselectivity via imidazole-promoted hydrolysis of pentaester 2a
(Scheme 2).
Scheme 2. Synthesis of 3,3’,4’,5-tetraacetyl quercetin (4).
O
O
O
3'
O
3'
O
O
O
O
4'
O
4'
O
O
HO
O
Imidazole (2 Eq.)
CH2Cl2
7
7
3
(87% isolated yield)
3
O
O
5
5
O
O
O
O
O
O
O
O
4
2a
Selective deacylation to free the 7-OH group has been reported by Needs and Williamson [26] and
Shin et al. [27] for daidzen and chrysin, respectively, which have only two hydroxyls. Recently Li et al.
[28] performed the selective deacylation at 7-OH of pentahexanoyl quercetin using imidazole-
catalyzed acyl transfer to an aromatic thiol under basic conditions. In our reaction, without thiols,
using 0.2, 1 or 2 equivalents of imidazole produced compound 4 in 14, 44 and 87% isolated yield,
respectively. Position 7 is preferentially involved presumably because the 7-OH is the most acidic
hydroxyl of quercetin [29]. The assignment of the single free hydroxyl group in 4 is consistent with the
observed differences in the chemical shifts of ring protons (Table 1) and was unambiguously
established by combined HSQC and HMBC spectra (see Section 2.2). Compound 4 is a useful starting
material. Two applications are described below dealing with a new synthesis of rhamnetin (7-O-
methylquercetin) (6) and with the development of a new mitochondria-targeted derivative, 7-O-(4-
triphenylphosphoniumbutyl) quercetin iodide (9).

Molecules 2010, 15
4726
2.2. Spectral assignment of the free hydroxyl position of 3,3’,4’,5-tetraacetylquercetin (4) and
3,3’,4’,7-tetraacetylquercetin (3a)
HSQC and HMBC spectra were obtained in order to confirm the structures of 4 and 3a. The 2D
maps of the relevant spectral regions are shown in Figures 1 and 2 (the 13C-NMR spectra displayed as
a trace in the indirect dimension were obtained with the UDEFT pulse scheme) [30]. The singlet at 8
ppm in the spectrum of 4 clearly belongs to an exchangeable proton, since it has been found
experimentally that trace amounts of water in the solvent mixture change both its lineshape and its
position. From the HMBC spectrum (Figure 1, red) it is clear that this proton bears long-range
correlations with both 6-C and 8-C (whatever their absolute assignment is) as demonstrated by the
superimposed HSQC spectrum (black). This evidence is only compatible with structure 4.
As a counterproof, in the HMBC spectrum of 3a (Figure 2), the OH proton appearing at 12.1 ppm
bears a long-range correlation with 6-H, thus confirming the proposed structure. In this case the
distinction between 6-H and 8-H is unambiguous, since the only proton-bearing carbon seen by the OH
in the HMBC is 6-C, whose corresponding 6-H lies at 6.6 ppm as measured from the HSQC.
Moreover, the cross peak between 5-C and the signal at 6.6 ppm of the 1H spectrum indicates that this
resonance indeed belongs to 6-H. Finally, a cross peak between 8-H and 9-C is observed at 156.2 ppm
for 3a and 158.2 ppm for 4, which further helps to solve the ambiguity in the assignment of 8-H and 6-H.
Figure 1. HSQC (black) and HMBC (red) spectra of 4.

Molecules 2010, 15
4727
Figure 2. HSQC (black) and HMBC (red) spectra of 3a.
2.3. Synthesis of rhamnetin (7-O-methylquercetin)
Methylation of 4 followed by hydrolysis under acidic conditions afforded rhamnetin (6) in 67%
overall yield (Scheme 3).
Scheme 3. Synthesis of rhamnetin (6).
i) CH3I (1.1 eq), K2CO3 (1 eq) in DMF; ii) conc. HCl/CH3CN (1:2), reflux.
The intermediate product 5, isolated by flash chromatography, contained some O-dimethyl
derivative(s) (about 20% by NMR analysis). It was used as such, without further purification, in the
next step since we found that the final product 6 is easily separated by chromatography from any
O-dimethylquercetins. In contrast, it is important to remove any unreacted 4 prior to the hydrolysis
step to avoid the formation of quercetin, which is almost impossible to separate from the major product
6 via silica gel flash chromatography. Our procedure, which converts quercetin to rhamnetin in four
steps with an overall yield of 46%, is a considerable improvement over a previously reported synthesis

Molecules 2010, 15
4728
also starting from quercetin [21]. The previous procedure requires two consecutive protection steps,
the first for the ortho-dihydroxyl groups of the B ring, the second for the 3-OH, followed by
methylation and two deprotection steps to yield the target compound in 11% overall yield.
2.4. Synthesis of 7-O-(4-triphenylphosphoniumbutyl) quercetin iodide (9)
The new mitochondriotropic derivative 9 was synthesized from 4 in three steps as outlined in
Scheme 4.
Scheme 4. Synthesis of 7-O-(4-triphenylphosphoniumbutyl) quercetin iodide (9).
i) Cl(CH2)4Br (1.2 eq), K2CO3 (1 eq), DMF, R.T.; ii) conc. HCl/CH3CN (1:2), reflux; iii) saturated
NaI in acetone, reflux; iv) PPh3 (5 eq), toluene, 95 °C.
The synthesis involved one-pot O-alkylation of 4 to introduce the chlorobutyl group and complete
deacetylation to yield 7-O-(4-butylchloro)-quercetin (7) in 61% yield. The triphenyl phosphonium
(TPP+) cation was then introduced via two consecutive nucleophilic substitution reactions first to
replace chloride by iodide (giving 8 in 90% yield) and then to replace iodide by triphenylphosphine
(giving 9 in 73% yield). This two-step procedure allowed us to avoid the high temperatures required by
+ −
the direct –Cl → –PPh3 I conversion, which were found to lead to some product decomposition.
3. Experimental
3.1. General
Starting materials and reagents were purchased from Aldrich, Fluka, Merck-Novabiochem, Riedel
de Haën, J.T. Baker, Cambridge Isotope Laboratories Inc., Acros Organics, Carlo Erba and Prolabo,
1
and were used as received. H- and 13C-NMR spectra were recorded with a Bruker AC 250F
spectrometer or a Bruker Avance DRX 300 spectrometer. Chemical shifts (δ) are given in ppm relative
to the residual signal of the solvent (for 1H: CDCl3: δ = 7.26 ppm, DMSO-d6:
δ = 2.50 ppm, DMF-d6: δ = 8.03 ppm, CD3CN: δ = 1.94 ppm; for 13C: CDCl3: δ = 77.00 ppm, DMSO-
d6: δ = 39.52 ppm, DMF-d6: δ = 29.76 ppm, CD3CN: δ = 1.32 ppm). Mass spectra were performed
with an Agilent Technologies MSD SL Trap mass spectrometer with ESI source coupled with a 1100
Series HPLC system. TLCs were run on silica gel supported on plastic (Macherey-Nagel

Molecules 2010, 15
4729
Polygram®SIL G/UV254, silica thickness 0.2 mm), or on silica gel supported on glass (Fluka) (silica
thickness 0.25 mm, granulometry 60Å, medium porosity) and visualized by UV detection. Flash
chromatography was performed on silica gel [Macherey-Nagel 60, 230-400 mesh granulometry
(0.063-0.040 mm)] under air pressure. The solvents were analytical or synthetic grade and were used
without further purification. Elemental analyses were performed by the Microanalysis Laboratory of
the Dept. of Chemical Sciences of the University of Padova with a Fison EA1108 CHNS Analyzer.
3.2. HSQC and HMBC spectra
Spectra were obtained using a Bruker Avance DRX 300 spectrometer equipped with a 5-mm BBO
z-gradient inverse probe. The experiments were conducted with 20 mM solutions in 1:5 CD3CN:CDCl3
13
(chosen to insure solubility during analysis). C-NMR spectra were obtained with the UDEFT pulse
scheme [30].
3.3. Synthesis of 3,3’,4’,5,7-pentaacylquercetins and 3,3’,4’,7-tetraacylquercetins
2-(3,4-Diacetoxyphenyl)-4-oxo-4H-chromene-3,5,7-triyl triacetate (2a): Compound 2a was synthesized
from 1 by slightly modifying literature procedures [8,23]. Briefly, quercetin (1, 1.00 g, 3.0 mmol,
1 equiv.), acetic anhydride (6.13 g, 60.0 mmol, 20 equiv.) and pyridine (15 mL) were heated to reflux
and stirred for 5 h. Ice-water (50 g) was added to the warm mixture. The resulting precipitate was
filtered and washed with cold ethyl acetate to afford 2a as a white solid (1.22 g, 79% yield). 1H-NMR
4
(250 MHz, DMSO-d6, 25 °C): δ = 2.32 (s, 3H, CH3), 2.34 (s, 12H, 4 × CH3), 7.16 (d, JH,H = 2.2 Hz,
1 H, 6-H), 7.52 (d, 3JH,H = 8.6 Hz, 1 H, 5’-H), 7.64 (d, 4JH,H = 2.2 Hz, 1 H, 8-H), 7.35-7.65 (m, 2H, 2’-H,
13
6’-H) ppm; C-NMR (62.9 MHz, CDCl3, 25 °C): δ = 170.0, 169.3, 167.9, 167.8, 167.8, 167.8, 156.8,
154.2, 153.7, 150.4, 144.4, 142.2, 134.0, 127.8, 126.4, 123.9, 123.8, 114.7, 113.9, 109.0, 21.2, 21.0,
20.7, 20.5 ppm; MS (ESI-MS): m/z 513, [M+H]+; Elemental analysis: calcd. for C25H20O12: C 58.60, H
3.93; found C 58.62, H 3.97.
4-(3,7-Diacetoxy-5-hydroxy-4-oxo-4H-chromen-2-yl)-1,2-phenylene diacetate (3a): Compound 1
(1.00 g, 3.0 mmol, 1.0 equiv.) was dissolved in CH2Cl2 (20 mL) and pyridine (5 mL). Acetic anhydride
(1.53 g, 15.0 mmol, 5.0 equiv.) was then added dropwise and the mixture was stirred at room
temperature for 3 hours. The obtained solution was diluted in CH2Cl2 (150 mL) and washed with 3 M
aq. HCl (3 × 100 mL). The organic layer was then dried over MgSO4, and filtered. The solvent was
evaporated under reduced pressure and the residue was purified by silica gel flash chromatography
(eluent 9:2:1 CH2Cl2/hexane/ethyl acetate) to afford 3a as a yellow solid (1.06 g, 75% yield). 1H-NMR
(250 MHz, CDCl3, 25 °C): δ = 2.34 (m, 9H, 3 × CH3), 2.37 (s, 3H, CH3), 6.60 (d, 4JH,H = 2.0 Hz, 1 H,
4
3
6-H), 6.85 (d, JH,H = 2.0 Hz, 1 H, 8-H), 7.36 (d, JH,H = 9.0 Hz, 1 H, 5’-H), 7.73-7.77 (m, 2H, 2’-H,
6’-H) ppm; 13C-NMR [75.5 MHz, CDCl3/CD3CN (5:1), 25 °C]: δ = 176.6, 168.5, 168.3, 168.1, 168.0,
161.8, 156.8, 156.3, 155.9, 145.1, 142.7, 132.5, 126.9, 124.5, 124.3, 117.0, 109.0, 105.8, 101.7, 21.3,
20.8, 20.6 ppm; MS (ESI-MS): m/z 471, [M+H]+; Elemental analysis: calcd. for C23H18O11: C 58.73, H
3.86; found C 58.65, H 3.85.

Molecules 2010, 15
4730
2-(3,4-bis(Isobutyryloxy)phenyl)-4-oxo-4H-chromene-3,5,7-triyl tris(2-methylpropanoate) (2b): Iso-
butyric anhydride (4.75 g, 30.0 mmol, 10.0 equiv.) was added dropwise to a solution of 1 (1.00 g,
3.0 mmol, 1.0 equiv.) in pyridine (15 mL). The mixture was heated to reflux and stirred for 2 hours.
The obtained solution was diluted in CH2Cl2 (150 mL) and washed with 3 M aq. HCl (6 × 100 mL).
The organic layer was then dried over MgSO4, and filtered. The solvent was evaporated under reduced
pressure. The purification of the resulting residue by silica gel flash chromatography (eluent: 5:5:0.25
CH2Cl2/hexane/ethyl acetate) gave 2b as a pale yellow solid (1.90 g, 97% yield). 1H-NMR (250 MHz,
CD3CN, 25 °C): δ = 1.23-1.34 (m, 30H, 10 × CH3), 2.79-2.94 (m, 5H, 5 × CH), 6.94 (d, 4JH,H = 2.2 Hz,
1 H, 6-H), 7.38-7.41 (m, 2 H, 8-H, 5’-H), 7.73 (d, 4JH,H = 2.0 Hz, 1 H, 2’-H), 7.81 (dd, 3JH,H = 8.8 Hz,
4JH,H = 2.2 Hz, 1 H, 6’-H) ppm; 13C-NMR (62.9 MHz, CD3CN, 25 °C): δ = 175.9, 175.5, 175.3, 175.1,
174.8, 170.7, 157.9, 155.8, 154.5, 151.4, 145.9, 143.6, 134.8, 128.6, 127.6, 125.2, 124.7, 118.2, 115.5,
110.5, 34.9, 34.8, 34.7, 34.6, 34.5, 19.1, 19.0, 19.0, 18.9, 18.9 ppm; MS (ESI-MS): m/z 653, [M+H]+;
Elemental analysis: calcd. for C35H40O12: C 64.41, H 6.18; found C 64.45, H 6.22.
2-(3,4-bis(Isobutyryloxy)phenyl)-5-hydroxy-4-oxo-4H-chromene-3,7-diyl
bis(2-methylpropanoate)
(3b): Isobutyric anhydride (2.00 g, 12.6 mmol, 4.2 equiv.) was added dropwise to a solution of 1
(1.00 g, 3.0 mmol, 1.0 equiv.) in pyridine (15 mL). The mixture was stirred at 70 °C for 1 hour. The
obtained solution was diluted in CH2Cl2 (150 mL) and washed with 3 M aq. HCl (6 × 100 mL). The
organic layer was then dried over MgSO4, and filtered. The solvent was evaporated under reduced
pressure and the resulting residue was purified by silica gel flash chromatography (eluent: 5:5:0.25
1
CH2Cl2/hexane/ethyl acetate) to afford 3b as a yellow solid (1.22 g, 70% yield). H-NMR (250 MHz,
CD3CN, 25 °C): δ = 1.24-1.30 (m, 24H, 8 × CH3), 2.79-2.92 (m, 4H, 4 × CH), 6.61 (d, 4JH,H = 2.0 Hz,
1 H, 6-H), 6.92 (d, 4JH,H = 2.2 Hz, 1 H, 8-H), 7.41 (d, 3JH,H = 8.5 Hz, 1 H, 5’-H), 7.75 (d, 4JH,H = 2.2 Hz,
1 H, 2’-H), 7.82 (dd, 3JH,H = 8.8 Hz, 4JH,H = 2.2 Hz, 1 H, 6’-H) ppm; 13C-NMR (62.9 MHz, DMF-d6, 25
°C): δ = 176.5, 174.7, 174.3, 174.1, 174.0, 161.4, 157.4, 156.4, 156.0, 145.7, 143.1, 132.3, 127.7,
127.3, 124.9, 108.6, 105.9, 102.3, 34.2, 34.0, 33.9, 18.7, 18.6, 18.5 ppm; MS (ESI-MS): m/z 583,
[M+H]+; Elemental analysis: calcd. for C31H34O11: C 63.91, H 5.88; found C 63.99, H 5.92.
2-(3,4-bis(Pivaloyloxy)phenyl)-4-oxo-4H-chromene-3,5,7-triyl tris(2,2-dimethylpropanoate) (2c):
Compound 1 (1.00 g, 3.0 mmol, 1.0 equiv.) was dissolved in pyridine (15 mL). Pivaloyl chloride
(3.62 g, 30.0 mmol, 10.0 equiv.) was then added dropwise and the mixture was heated to reflux and
stirred for 2 hours. The obtained solution was diluted in CH2Cl2 (150 mL) and washed with 3 M aq.
HCl (6 × 100 mL). The organic layer was then dried over MgSO4, and filtered. The solvent was
evaporated under reduced pressure. The purification of the resulting residue by silica gel flash
chromatography (eluent: 9:1:0.5 CH2Cl2/hexane/ethyl acetate) gave 2c as a white solid (1.88 g, 87%
1
4
yield). H-NMR (250 MHz, DMF-d6, 25 °C): δ = 1.35-1.41 (m, 45H, 15 × CH3), 7.27 (d, JH,H = 2.2
Hz, 1 H, 6-H), 7.64 (d, 3JH,H = 8.5 Hz, 1 H, 5’-H), 7.69 (d, 4JH,H = 2.2 Hz, 1 H, 8-H), 7.92 (d, 4JH,H = 2.0
Hz, 1 H, 2’-H), 8.01 (dd, 3JH,H = 8.8 Hz, 4JH,H = 2.2 Hz, 1 H, 6’-H) ppm; 13C-NMR (62.9 MHz, DMF-
d6, 25 °C): δ = 176.3, 176.1, 175.7, 175.6, 175.3, 169.8, 157.2, 155.6, 153.8, 151.1, 145.7, 143.2,
128.0, 127.1, 127.1, 124.9, 124.2, 115.0, 114.9, 110.2, 39.3, 39.3, 39.2, 39.1, 39.0, 27.0, 26.9, 26.9,
26.8, 26.7 ppm; MS (ESI-MS): m/z 723, [M+H]+; Elemental analysis: calcd. for C40H50O12: C 66.47, H
6.97; found C 66.49, H 7.00.

Molecules 2010, 15
4731
2-(3,4-bis(Pivaloyloxy)phenyl)-5-hydroxy-4-oxo-4H-chromene-3,7-diyl bis(2,2-dimethylpropanoate)
(3c): Pivalic anhydride (4.47 g, 24.0 mmol, 8.0 equiv.) was added dropwise to a solution of 1 (1.00 g,
3.0 mmol, 1.0 equiv.) in pyridine (15 mL). The mixture was heated to reflux and stirred for 45
minutes. The obtained solution was diluted in CH2Cl2 (150 mL) and washed with 3 M aq. HCl (6 ×
100 mL). The organic layer was then dried over MgSO4, and filtered. The solvent was evaporated
under reduced pressure and the resulting residue was purified by silica gel flash chromatography
(eluent: 9:1:0.5 hexane/CH2Cl2/ethyl acetate) to afford 3c as a pale green solid (1.62 g, 85% yield). 1H-
NMR
(250 MHz, DMF-d6, 25 °C): δ = 1.36-1.38 (m, 36H, 12 × CH3), 6.79 (d, 4JH,H = 2.0 Hz, 1 H, 6-H), 7.18
(d, 4JH,H = 2.2 Hz, 1 H, 8-H), 7.66 (d, 3JH,H = 8.5 Hz, 1 H, 5’-H), 7.94 (d, 4JH,H = 2.0 Hz, 1 H, 2’-H), 8.0
(dd, 3JH,H = 8.8 Hz, 4JH,H = 2.2 Hz, 1 H, 6’-H) ppm; 13C-NMR (62.9 MHz, DMF-d6, 25 °C): δ = 176.6,
176.1, 175.7, 175.6, 175.4, 161.5, 157.7, 156.4, 156.0, 146.0, 143.2, 132.5, 127.7, 127.4, 125.0, 124.4,
108.6, 105.9, 102.3, 39.3, 39.3, 39.2, 39.1, 27.0, 26.9, 26.8, 26.7 ppm; MS (ESI-MS): m/z 639,
[M+H]+; Elemental analysis: calcd. for C35H42O11: C 65.82, H 6.63; found C 65.76, H 6.66.
2-(3,4-bis(Benzoyloxy)phenyl)-4-oxo-4H-chromene-3,5,7-triyl tribenzoate (2d): Compound 1 (1.00 g,
3.0 mmol, 1.0 equiv.) was dissolved in CH2Cl2 (20 mL) and pyridine (5 mL). Benzoyl chloride (4.22 g,
30.0 mmol, 10.0 equiv.) was then added dropwise and the mixture was stirred at room temperature for
3 hours. The obtained solution was diluted in CH2Cl2 (150 mL) and washed with 3 M aq. HCl
(3 × 100 mL). The organic layer was then dried over MgSO4, and filtered. The solvent was evaporated
under reduced pressure and the resulting residue was purified by silica gel flash chromatography
(eluent: 5:5:0.5 CH2Cl2/hexane/ethyl acetate) to afford 2d as a white solid (2.4 g, 97% yield). 1H-NMR
(250 MHz, DMF-d6, 25 °C): δ = 7.49-7.83 (m, 15 H, Ar-H), 7.87 (d, 3JH,H = 8.5 Hz, 1 H, 5’-H), 8.01-
4
13
8.08 (m, 5 H, Ar-H), 8.19-8.29 (m, 8 H, Ar-H), 8.43 (d, JH,H = 2.0 Hz, 1 H, 2’-H) ppm; C-NMR
(62.9 MHz, DMF-d6, 25 °C): δ = 168.8, 164.9, 164.4, 164.1, 163.9, 163.6, 157.4, 155.5, 154.5, 150.7,
145.6, 143.4, 134.8, 134.7, 134.6, 134.5, 134.2, 133.1, 130.6, 130.5, 130.4, 130.2, 130.2, 129.8, 129.7,
129.4, 129.4, 129.3, 129.1, 128.9, 128.9, 128.6, 128.5, 128.4, 128.4, 127.7, 125.1, 124.6, 115.8, 115.2,
111.0 ppm; MS (ESI-MS): m/z 823, [M+H]+; Elemental analysis: calcd. for C50H30O12: C 72.99, H
3.67; found C 72.98, H 3.63.
4-(3,7-bis(Benzoyloxy)-5-hydroxy-4-oxo-4H-chromen-2-yl)-1,2-phenylene dibenzoate (3d): Compound
1 (1.00 g, 3.0 mmol, 1.0 equiv.) was dissolved in CH2Cl2 (20 mL) and pyridine (5 mL). Benzoyl
chloride (1.77 g, 12.6 mmol, 4.2 equiv.) was then added dropwise and the mixture was stirred at room
temperature for 2 hours. The obtained solution was diluted in CH2Cl2 (150 mL) and washed with 3 M
aq. HCl (3 × 100 mL). The organic layer was then dried over MgSO4, and filtered. The solvent was
evaporated under reduced pressure. The purification of the resulting residue by silica gel flash
chromatography (eluent: 5:5:0.5 CH2Cl2/hexane/ethyl acetate) gave 3d as a pale yellow solid (1.55 g,
72% yield). 1H-NMR (250 MHz, DMF-d6, 25 °C): δ = 7.08 (d, 4JH,H = 2.0 Hz, 1 H, 6-H), 7.49-7.57 (m,
3
5 H, Ar-H), 7.65-7.75 (m, 6 H, Ar-H), 7.80-7.82 (m, 2 H, Ar-H), 7.88 (d, JH,H = 8.5 Hz, 1 H, 5’-H),
8.02-8.06 (m, 4 H, Ar-H), 8.22-8.33 (m, 5 H, Ar-H), 8.44 (d, 4JH,H = 2.0 Hz, 1 H, 2’-H) ppm; 13C-NMR
(62.9 MHz, DMF-d6, 25 °C): δ = 176.6, 164.4, 164.0, 163.9, 163.8, 161.6, 157.5, 156.6, 156.4, 151.1,
145.9, 143.4, 135.0, 134.7, 134.6, 132.7, 130.7, 130.4, 130.2, 130.2, 129.5, 129.4, 129.3, 129.1, 128.6,

Molecules 2010, 15
4732
128.5, 128.3, 128.2, 127.9, 125.2, 124.8, 108.9, 106.3, 102.8 ppm; MS (ESI-MS): m/z 719, [M+H]+;
Elemental analysis: calcd. for C43H26O11: C 71.86, H 3.65; found C 71.79, H 3.61.
Synthesis of 4-(3,5-diacetoxy-7-hydroxy-4-oxo-4H-chromen-2-yl)-1,2-phenylene diacetate (4): A
solution of imidazole (0.05 g, 0.78 mmol, 2.00 equiv.) in CH2Cl2 (5 mL) was added dropwise to a
solution of 2a (0.20 g, 0.39 mmol, 1.00 equiv.) in CH2Cl2 (10 mL) at –15 °C in an ice/acetone bath.
The resulting solution was allowed to warm to room temperature and stirred for 2 hours. The reaction
mixture was diluted in CH2Cl2 (50 mL) and washed with 3 M aq. HCl (3 × 50 mL). The organic layer
was then dried over MgSO4, and filtered. The solvent was evaporated under reduced pressure. The
purification of the resulting residue by silica gel flash chromatography (eluent: CHCl3/methanol, 97:3)
1
gave 4 as a white solid (0.16 g, 87% yield). H-NMR (300 MHz, CDCl3/CD3CN 5:1, 25 °C):
4
δ = 2.23-2.30 (m, 9H, 3 × CH3), 2.34 (s, 3H, CH3), 6.48 (d, JH,H = 2.2 Hz, 1 H, 6-H), 6.74
4
3
(d, JH,H = 2.2 Hz, 1 H, 8-H), 7.26 (d, JH,H = 9.0 Hz, 1 H, 5’-H), 7.60-7.67 (m, 2H, 2’-H, 6’-H) ppm;
13C-NMR [75.5 MHz, CDCl3/CD3CN (5:1), 25 °C]: δ = 170.31, 169.67, 168.32, 168.2, 162.3, 158.3,
153.3, 151.1, 144.6, 142.6, 133.9, 128.4, 126.7, 124.3, 124.0, 117.0, 110.6, 109.6, 101.6, 21.3, 20.9,
20.7 ppm; MS (ESI-MS): m/z 471, [M+H]+; Elemental analysis: calcd. for C23H18O11: C 58.73, H 3.86;
found C 58.68, H 3.83.
Synthesis of 2-(3,4-dihydroxyphenyl)-3,5-dihydroxy-7-methoxy-4H-chromen-4-one (6): A solution of
methyl iodide (0.13 g, 0.95 mmol, 1.1 equiv.) in DMF (10 mL) was added dropwise to a solution of 4
(0.4 g, 0.85 mmol, 1 equiv.) and potassium carbonate (0.12 g, 0.85 mmol, 1.0 equiv.) in DMF (20 mL).
The mixture was stirred at –78 °C in a bath of dry ice/acetone for 10 minutes and then allowed to
warm and stirred for 12 hours at room temperature. The reaction mixture was diluted in CH2Cl2
(100 mL) and washed with 3 M aq. HCl (6 × 100 mL). The organic layer was then dried over MgSO4,
and filtered. The solvent was evaporated under reduced pressure. The purification of the resulting
residue by silica gel flash chromatography (eluent: 7:3 CHCl3/ethyl acetate) gave a white solid (0.38 g)
1
composed of 5 and of a dimethylated product in 8:2 ratio, as determined by NMR analysis. H-NMR
(250 MHz, DMSO-d6, 25 °C, signals of 5): δ = 2.27-2.34 (m, CH3), 3.93 (s, OCH3), 6.87 (d,
4
3
4JH,H = 2.2 Hz, 6-H), 6.30 (d, JH,H = 2.5 Hz, 8-H), 7.53 (d, JH,H = 9.2 Hz, 5’-H), 7.83-7.90 (m, 2’-H,
6’-H) ppm; MS (ESI-MS): m/z 485, [M+H]+, m/z 457, [M’+H]+. Without further purification, crude 5
(0.30 g) was added to a solution of acetonitrile (20 mL) and 6 M aq. HCl (10 mL). The resulting
solution was stirred and refluxed for 1.5 hours. Then ethyl acetate (100 mL) and water (100 mL) were
added. The organic layer was washed with 3 M aq. HCl (3 × 100 mL), dried over MgSO4, and filtered.
The solvent was evaporated under reduced pressure and the purification of the resulting residue by
silica gel flash chromatography (eluent: 9:1 CHCl3/methanol) gave 6 as a bright yellow solid (0.14 g,
67% yield with respect to 4). 1H-NMR (250 MHz, DMSO-d6, 25 °C): δ = 3.86 (s, 3H, OCH3), 6.35 (d,
4
3
4JH,H = 2.0 Hz, 1 H, 6-H), 6.70 (d, JH,H = 2.0 Hz, 1 H, 8-H), 6.89 (d, JH,H = 8.5 Hz, 1 H, 5’-H), 7.58
(dd, 3JH,H = 8.5 Hz, 4JH,H = 2.0 Hz, 1 H, 6’-H), 7.73 (d, 4JH,H = 2.0 Hz, 1 H, 2’-H) ppm; 13C-NMR (62.9
MHz, DMSO-d6, 25 °C): δ = 175.9, 164.8, 160.3, 156.0, 147.8, 147.2, 145.0, 136.0, 121.8, 119.9,
115.5, 115.1, 103.9, 97.4, 91.8, 55.9 ppm; MS (ESI-MS): m/z 317, [M+H]+; Elemental analysis: calcd.
for C16H12O7: C 60.76, H 3.82; found C 60.72, H 3.80.

Molecules 2010, 15
4733
Synthesis of 7-(4-chlorobutoxy)-2-(3,4-dihydroxyphenyl)-3,5-dihydroxy-4H-chromen-4-one (7):
1-Bromo-4-chlorobutane (0.62 g, 3.59 mmol, 1.20 equiv) and potassium carbonate (0.413 g, 2.97
mmol, 1.00 equiv.) were added to a solution of 4 (1.39 g, 2.97 mmol, 1.00 equiv.) in DMF (20 mL)
under nitrogen and stirred overnight at R.T. The reaction mixture was diluted in CH2Cl2 (100 mL) and
washed with 3 M aq. HCl (3 × 100 mL). The organic layer was then dried over MgSO4 and filtered.
The solvent was evaporated under reduced pressure. Without further purification, the crude product
was added to a mixture of acetonitrile (60 mL) and 3 M aq. HCl (30 mL). The resulting solution was
stirred and refluxed for 1 hour, and then ethyl acetate (100 mL) and water (100 mL) were added. The
organic layer was washed with 3 M aq. HCl (3 × 100 mL), dried over MgSO4, and filtered. The solvent
was evaporated under reduced pressure and the purification of the resulting residue by silica gel flash
chromatography (eluent: 8:2 toluene/methanol) gave 7 as a bright yellow solid (0.71 g, 61% yield).
1H- NMR (250 MHz, DMSO-d6, 25 °C): δ = 1.80-2.01 (m, 4H, 2 × CH2), 3.66-3.80 (m, 2H, CH2),
4.00-4.22 (m, 2H, CH2), 6.34 (d, 4JH,H = 2.2 Hz, 1 H, 6-H), 6.70 (d, 4JH,H = 2.0 Hz, 1 H, 8-H), 6.89 (d,
3JH,H = 8.5 Hz, 1 H, 5’-H), 7.57 (dd, 3JH,H = 8.5 Hz, 4JH,H = 2.0 Hz, 1 H, 6’-H), 7.73 (d, 4JH,H = 2.0 Hz,
1 H, 2’-H) ppm; 13C-NMR (62.9 MHz, DMSO-d6, 25 °C): δ = 175.5, 163.7, 159.9, 155.6, 147.4, 146.8,
144.6, 135.6, 121.5, 119.6, 115.1, 114.8, 103.6, 97.4, 91.9, 67.3, 44.7, 28.3, 25.5 ppm; MS (ESI-MS):
m/z 393, [M+H]+; Elemental analysis: calcd. for C19H17ClO7: C 58.10, H 4.36; found C 58.18, H 4.41.
Synthesis
of
7-(4-iodobutoxy)-2-(3,4-dihydroxyphenyl)-3,5-dihydroxy-4H-chromen-4-one
(8):
Compound 7 (0.50 g, 1.27 mmol, 1 equiv.) was added to a saturated solution of NaI in dry acetone
(20 mL) and heated at reflux for 20 h. After cooling, the resulting mixture was diluted in EtOAc
(100 mL), filtered and washed with water (3 × 30 mL). The organic layer was dried over MgSO4 and
filtered. The solvent was evaporated under reduced pressure to afford the product in 90% yield after
flash chromatography using 7:3 CHCl3/acetone. 1H-NMR (250 MHz, DMSO-d6, 25 °C): δ = 1.71-1.98
(m, 4H, 2 × CH2), 3.24-3.43 (m, 2H, CH2), 4.06-4.23 (m, 2H, CH2), 6.32 (d, 4JH,H = 2.2 Hz, 1 H, 6-H),
4
3
3
6.70 (d, JH,H = 2.0 Hz, 1 H, 8-H), 6.89 (d, JH,H = 8.5 Hz, 1 H, 5’-H), 7.57 (dd, JH,H = 8.5 Hz,
4JH,H = 2.0 Hz, 1 H, 6’-H), 7.73 (d, JH,H = 2.0 Hz, 1 H, 2’-H) ppm; C-NMR (62.9 MHz, DMSO-d6,
25 °C): δ = 175.8, 164.0, 160.3, 155.9, 147.7, 147.1, 145.0, 141.5, 135.9, 121.8, 119.9, 115.5, 115.2,
103.9, 92.3, 67.3, 29.5, 29.3, 8.3 ppm; MS (ESI-MS): m/z 485, [M+H]+; Elemental analysis: calcd. for
C19H17IO7: C 47.13, H 3.54; found C 47.16, H 3.59.
4
13
Synthesis of 7-(4-triphenylphosphoniumbutoxy)-2-(3,4-dihydroxyphenyl)-3,5-dihydroxy-4H-chromen-
4-one iodide (9): A mixture of 8 (300 mg, 0.63 mmol, 1 equiv.) and triphenylphosphine (0.825 g,
3.15 mmol, 5 eq.) in toluene (15 mL) was heated at 95°C under argon. After 3 h, the solvent was
eliminated at reduced pressure and the resulting yellow solid was dissolved in the minimum volume of
dichloromethane (1 mL) and precipitated with diethyl ether (5 × 50 mL). The solvents were decanted
after each precipitation. Residual solvent was then removed under reduced pressure to afford
1
compound 6 in 73% yield. H-NMR (250 MHz, DMSO-d6, 25 °C): δ = 1.64-1.80 (m, 2H, CH2),
1.88-2.00 (m, 2H, CH2), 3.60-3.73 (m, 2H, CH2), 4.13-4.20 (m, 2H, CH2), 6.28 (d, 4JH,H = 2.0 Hz, 1 H,
6-H), 6.66 (d, 4JH,H = 2.0 Hz, 1 H, 8-H), 6.89 (d, 3JH,H = 8.5 Hz, 1 H, 5’-H), 7.55 (dd, 3JH,H = 8.5 Hz, 4JH,H
=
2.0 Hz, 1 H, 6’-H), 7.72-7.93 (m, 16 H: 15 H = PPh3, 1 H = 2’-H) ppm; 13C-NMR (62.9 MHz, DMSO-d6,
4
25 °C): δ = 175.7, 163.7, 160.1, 155.8, 147.6, 147.1, 144.8, 135.8, 134.9 [Ph, J(13C/31P) = 2.8 Hz],

Molecules 2010, 15
4734
3
2
133.4 [Ph, J(13C/31P) = 10.1 Hz], 130.0 [Ph, J(13C/31P) = 12.3 Hz], 121.6, 119.7, 118.1
[Ph, 1J(13C/31P) = 86.1 Hz], 115.3, 115.1, 103.8, 97.5, 92.2, 74.1, 28.5, 19.1, 18.2 ppm; MS (ESI-MS):
m/z 619, [M]+; Elemental analysis: calcd. for C37H32IO7P: C 59.53, H 4.32; found C 59.60, H 4.30.
4. Conclusions
Basic procedures and inexpensive and common reagents for esterification and ester hydrolysis were
adapted to achieve the regioselective synthesis of quercetin tetraesters bearing the single free OH
group either on 5-C or on 7-C. Selective hydrolysis of pentaacetyl quercetin to 3,3’,4’,5-tetraacetyl
quercetin (4) formed the basis for a new, relatively easy synthesis of rhamnetin (6) and for the
development of a new quercetin derivative of biomedical interest, the mitochondria-targeted 7-O-(4-
triphenylphosphoniumbutyl) quercetin iodide (9). The cation of 9 is expected to accumulate into cells
and mitochondria under the influence of the negative- inside transmembrane potential difference
maintained across the cellular and inner mitochondrial membrane. Compound 9 is an isomer of already
available 3-O-(4-triphenylphosphoniumbutyl) quercetin iodide [10], with which it may be compared in
terms of redox reactivity and biological effects. The properties and effects of the two isomers differ
indeed significantly (Mattarei, A., et al., in preparation), and the comparison will allow an informed
choice of the most suitable compound to be tested in experimental models of pathophysiological
relevance. The approaches used for the production of these quercetin derivatives may be easily
extended to the regioselective modification of other flavonoids.
Acknowledgements
This work was partially supported by grants from the University of Padova (Progetti d’Ateneo) and
from the Fondazione Cassa di Risparmio di Padova e Rovigo (Progetti d’Eccellenza). L.B.
acknowledges support by a post-doctoral fellowship by the University of Padova (Assegno di Ricerca).
References and Notes
1. Bischoff, S.C. Quercetin: Potentials in the prevention and therapy of disease. Curr. Opin. Clin.
Nutr. Metab. Care 2008, 11, 733-740.
2. Murakami, A.; Ashida, H.; Terao, J. Multitargeted cancer prevention by quercetin. Cancer Lett.
2008, 269, 315-325.
3. Boots, A.W.; Haenen, G.R.; Bast, A. Health effects of quercetin: From antioxidant to
nutraceutical. Eur. J. Pharmacol. 2008, 585, 325-337.
4. Hirpara, K.V.; Aggarwal, P.; Mukherjee, J.; Joshi, N.; Burman, A.C. Quercetin and its
derivatives: Synthesis, pharmacological uses with special emphasis on anti-tumor properties and
prodrug with enhanced bio-availability. Anticancer Agents Med. Chem. 2009, 9, 138-161.
5. Manach, C.; Williamson, G.; Morand, C.; Scalbert, A.; Rémésy, C. Bioavailability and
bioefficacy of polyphenols in humans. I. Review of 97 bioavailability studies. Am. J. Clin. Nutr.
2005, 81 (Suppl. 1), 230S-242S.
6. Williamson, G.; Manach, C. Bioavailability and bioefficacy of polyphenols in humans. II. Review
of 93 intervention studies. Am. J. Clin. Nutr. 2005, 81 (Suppl. 1), 243S-255S.

Molecules 2010, 15
4735
7. Silberberg, M.; Morand, C.; Mathevon, T.; Besson, C.; Manach, C.; Scalbert, A., Rémésy, C. The
bioavailability of polyphenols is highly governed by the capacity of the intestine and of the liver
to secrete conjugated metabolites. Eur. J. Nutr. 2006, 45, 88-96.
8. Biasutto, L.; Marotta, E.; De Marchi, U.; Zoratti, M.; Paradisi, C. Ester-based precursors to
increase the bioavailability of quercetin. J. Med. Chem. 2007, 50, 241-253.
9. Biasutto, L.; Marotta, E.; Mattarei, A.; Beltramello, S.; Caliceti, P.; Salmaso, S.; Bernkop-
Schnürch, A.; Garbisa, S.; Zoratti, M.; Paradisi, C. Absorption and metabolism of resveratrol
carboxyesters and methanesulfonate by explanted rat intestinal segments. Cell. Physiol. Biochem.
2009, 24, 557-566.
10. Mattarei, A.; Biasutto, L.; Marotta, E.; De Marchi, U.; Sassi, N.; Garbisa, S.; Zoratti, M.; Paradisi,
C. A mitochondriotropic derivative of quercetin: A strategy to increase the effectiveness of
polyphenols. ChemBioChem 2008, 16, 2633-2642.
11. Biasutto, L.; Mattarei, A.; Marotta, E.; Bradaschia, A.; Sassi, N.; Garbisa, S.; Zoratti, M.;
Paradisi, C. Development of mitochondria-targeted derivatives of resveratrol. Bioorg. Med. Chem.
Lett. 2008, 18, 5594-5597.
12. Biasutto, L.; Sassi, N.; Mattarei, A.; Marotta, E.; Cattelan, P.; Toninello, A.; Garbisa, S.; Zoratti,
M.; Paradisi, C. Impact of mitochondriotropic quercetin derivatives on mitochondria. Biochim.
Biophys. Acta 2010, 1797, 189-196.
13. Hoye, A.T.; Davoren, J.E.; Wipf, P.; Fink, M.P.; Kagan, V.E. Targeting mitochondria. Acc. Chem.
Res. 2008, 41, 87-97.
14. Murphy, M.P.; Smith, R.A. Targeting antioxidants to mitochondria by conjugation to lipophilic
cations. Annu. Rev. Pharmacol. Toxicol. 2007, 47, 629-656.
15. Murphy, M.P. Targeting lipophilic cations to mitochondria. Biochim. Biophys. Acta 2008, 1777,
1028-1031.
16. Skulachev, V.P.; Anisimov, V.N.; Antonenko, Y.N.; Bakeeva, L.E.; Chernyak, B.V.; Erichev,
V.P.; Filenko, O.F.; Kalinina, N.I.; Kapelko, V.I.; Kolosova, N.G.; Kopnin, B.P.; Korshunova,
G.A.; Lichinitser, M.R.; Obukhova, L.A.; Pasyukova, E.G.; Pisarenko, O.I.; Roginsky, V.A.;
Ruuge, E.K.; Senin, I.I.; Severina, I.I.; Skulachev, M.V.; Spivak, I.M.; Tashlitsky, V.N.; Tkachuk,
V.A.; Vyssokikh, M.Y.; Yaguzhinsky, L.S.; Zorov, D.B. An attempt to prevent senescence: A
mitochondrial approach. Biochim. Biophys. Acta 2009, 1787, 437-461.
17. Metodiewa, D.; Jaiswal, A.K.; Cenas, N.; Dickancaité, E. Quercetin may act as a cytotoxic
prooxidant afyer its metabolic activation to semiquinone and quinoidal product. Free Radic. Biol.
Med. 1999, 26, 107-116.
18. Sarno, S.; Moro, S.; Meggio, F.; Zagotto, G.; Dal Ben, D.; Ghibellini, P.; Battistutta, R.; Zanotti,
G.; Pinna, L.A. Toward the rational design of protein kinase casein kinase-2 inhibitors.
Pharmacol. Ther. 2002, 93, 159-168.
19. van Dijk, C.; Driessen, A.; Recourt, K. The uncoupling efficiency and affinity of flavonoids for
vesicles. Biochem. Pharmacol. 2000, 60, 1593–1600.
20. Montenegro, L.; Carbone, C.; Maniscalco, C.; Lambusta, D.; Nicolosi, G.; Ventura, C.A.; Puglisi,
G. In vitro evaluation of quercetin-3-O-acyl esters as topical drugs. Int. J. Pharm. 2007, 336,
257-262.

Molecules 2010, 15
4736
21. Bouktaib, M.; Lebrun, S.; Atmani, A.; Rolando, C. Hemisynthesis of all the O-monomethylated
analogues of quercetin including the major metabolites, through selective protection of phenolic
functions. Tetrahedron 2002, 58, 10001-10009.
22. Hodnick, W.F.; Duval, D.L.; Pardini, R.S. Inhibition of mitochondrial respiration and cyanide-
stimulated generation of reactive oxygen species by selected flavonoids. Biochem. Pharmacol.
1994, 47, 573-580.
23. Picq, M.; Prigent, A.F.; Némoz, G.; André, A.C.; Pacheco, H. Pentasubstituted quercetin
analogues as selective inhibitors of particulate 3’:5’-cyclic-AMP phosphodiesterase from rat brain.
J. Med. Chem. 1982, 25, 1192-1198.
24. Op de Beck, P.; Dijoux, M.G.; Cartier, G.; Mariotte, A.-M. Quercitrin-3’-sulphate from leaves of
Leea guinensis. Phytochemistry 1998, 47, 1171-1173.
25. Rietjens, I.M.; Awad, H.M.; Boersma, M.G.; van Iersel, M.L.; Vervoort, J.; Van Bladeren, P.J.
Structure activity relationship for the chemical behaviour and toxicity of electrophilic
quinones/quinone methides. Adv. Exp. Med. Biol. 2001, 500, 11-21.
26. Needs, P.W.; Williamson, G. Syntheses of daidzein-7-yl beta-D-glucopyranosiduronic acid and
daidzein-4’,7-yl di-beta-D-glucopyranosiduronic acid. Carbohydr. Res. 2001, 330, 511-515.
27. Shin, J.S.; Kim, K.S.; Kim, M.B.; Jeong, J.H.; Kim, B.K. Synthesis and hypoglycemic effect of
chrysin derivatives. Bioorg. Med. Chem. Lett. 1999, 9, 869-874.
28. Li, M.; Han, X.; Yu, B. Facile synthesis of flavonoid 7-O-glycosides. J. Org. Chem. 2003, 17,
6842-6845.
29. Musialik, M.; Kuzmicz, R.; Pawlowski, T.S.; Litwinienko, G. Acidity of hydroxyl groups: an
overlooked influence on antiradical properties of flavonoids. J. Org. Chem. 2009, 74, 2699-2709.
30. Piotto, M.; Bourdonneau, M.; Elbayed, K.; Wieruzeski, J.M.; Lippens, G. New DEFT sequences
for the acquisition of one-dimensional carbon NMR spectra of small unlabelled molecules. Magn.
Reson. Chem. 2006, 44, 943-947.
Sample Availability: Not available.
© 2010 by the authors; licensee MDPI, Basel, Switzerland. This article is an Open Access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).