Synthesis of a new bluish pigment from the reaction of a methylpyranoanthocyanin with sinapaldehyde

Article abstract of DOI:10.1016/j.tetlet.2011.02.079

The synthesis of a new pigment with a bluish color was obtained from the reaction of methylpyranomalvidin-3-glucoside with sinapaldehyde and its formation mechanism seems to involve a charge-transfer reaction pathway. The structure of this compound was fully characterized by LC/DAD-MS and NMR. Its equilibrium forms present in water at different pH values and the respective ionization constants were determined by UV-visible spectroscopy. The results revealed the presence of three equilibria involving only deprotonation reactions at the 7-OH and 4′-OH positions of the pyranoanthocyanin moiety and at the 4′″-OH of the syringol moiety (pK_a1 = 3.64 ± 0.01; pK_a2 = 8.02 ± 0.01; pK_a3 = 11.19 ± 0.01).

Full text of DOI:10.1016/j.tetlet.2011.02.079

Tetrahedron Letters 52 (2011) 1996–2000
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of a new bluish pigment from the reaction
of a methylpyranoanthocyanin with sinapaldehyde
⇑
Joana Oliveira, Nuno Mateus, Victor de Freitas
Centro de Investigação em Química, Departamento de Química, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, Porto 4169 007, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a new pigment with a bluish color was obtained from the reaction of methylpyranomal-
vidin-3-glucoside with sinapaldehyde and its formation mechanism seems to involve a charge-transfer
reaction pathway. The structure of this compound was fully characterized by LC/DAD-MS and NMR. Its
equilibrium forms present in water at different pH values and the respective ionization constants were
determined by UV–visible spectroscopy. The results revealed the presence of three equilibria involving
only deprotonation reactions at the 7-OH and 40-OH positions of the pyranoanthocyanin moiety and at
the 4000-OH of the syringol moiety (pKa1 = 3.64 0.01; pKa2 = 8.02 0.01; pKa3 = 11.19 0.01).
Ó 2011 Elsevier Ltd. All rights reserved.
Received 29 December 2010
Revised 16 February 2011
Accepted 18 February 2011
Available online 23 February 2011
Keywords:
Methylpyranoanthocyanins
Sinapaldehyde
Bluish color
Charge-transfer reaction
pKa
UV–visible spectroscopy
Anthocyanins are responsible for the red color observed in sev-
eral flowers and fruits found in nature. Over the last decades differ-
ent research groups have dedicated their work to the identification
and structural characterization of several compounds derived from
anthocyanins that present different colors from yellow to turquoise.
The matrixes used in those studies are usually anthocyanin-rich
foodstuffs like red wines. It is well known that anthocyanins are very
reactive and that they can react with flavanols directly or mediated
by aldehydes.1–5 They can also react with small molecules like acet-
aldehyde,6 acetoacetic acid,7 pyruvic acid,8 hydroxycinnamic
acids,9,10 vinyl–phenol,11 vinyl–guaiacol12 and vinyl–catechol,13
leading to the formation of compounds that belong to the family of
pyranoanthocyanins which present more orange hues compara-
tively to anthocyanins. A few years ago a new family of anthocya-
nin-derived compounds presenting a blue color (portisins) was
identified, and was shown to arise from the reaction of anthocya-
nin–pyruvicacid adducts with vinyl–flavanols or phenolic acids.14,15
Recently, a family of pyranoanthocyanin dimers with a turquoise
color was described from the reaction of an anthocyanin–pyruvic
acid adduct with a methylpyranoanthocyanin.16 More recently,
other newly formed pigments have been identified in red table
wines, namely catechin–vitisin A, catechin–vitisin B and catechin–
pyranoanthocyanin–phenol that can be originated from the reaction
of pyruvic acid, acetaldehyde or vinyl–phenol, respectively with
catechin–anthocyanin adducts.17 The identification of all of these
families of pigments not only demonstrates the high reactivity of
anthocyanins but also the reactivity of some anthocyanin-derived
compounds, such as anthocyanin–pyruvic acid adducts and
methylpyranoanthocyanins.
Bearing this, the aim of this work was to synthesize and identify
a new pyranoanthocyanin pigment derived from the reaction of an
anthocyanin-derived compound (methylpyranomalvidin-3-gluco-
side) with sinapaldehyde and to study its equilibrium forms by
UV–visible spectroscopy.
The synthesis of a new pyranoanthocyanin-derived compound
presenting a bluish color is presented herein through the reaction
of methylpyranomalvidin-3-glucoside 1 with sinapaldehyde 2. The
structure of the new compound formed was characterized by LC/
DAD-MS and NMR.
Five milligram of methylpyranomalvidin-3-glucoside were left
to react at 30 °C with 21.3 mg of sinapaldehyde (molar ratio of
10:1) in 5 mL of an aqueous solution of 20% (v/v) ethanol at pH
4.0 (adjusted with a NaOH 0.01 M solution). The reaction was mon-
itored by HPLC-DAD-ESI-MS on a 250 ꢀ 4.6 mm i.d. reverse-phase
C18 column as described in the literature.16 A new compound
started to appear after two days of reaction and its maximum of
formation was achieved after 10 days (Fig. 1). The UV–vis spectrum
of this new compound recorded from the HPLC-DAD detector
shows an absorption maximum wavelength of 568 nm indicating
that the compound displays a bluish color. The mass spectrum of
the pigment, obtained by LC-ESI-MS in the positive ion mode
revealed a molecular ion [M]+ at m/z 721 and a fragment ion
[Mꢁ162]+ at m/z 559 corresponding to the loss of the glucose
⇑
Corresponding author. Tel.: +351 220402558; fax: +351 220402658.
E-mail address: vfreitas@fc.up.pt (V.de Freitas).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.079

J. Oliveira et al. / Tetrahedron Letters 52 (2011) 1996–2000
1997
(3), 568 nm
200
(1)
100
0
0
300
400
500
600
700
800
λ (nm)
(3)
568 nm
478 nm
0
5
10
15
20
25
30
35
40
45
Retention time (min)
Figure 1. Chromatograms recorded from the HPLC-DAD at 478 nm and 568 nm of the reaction of methylpyranomalvidin-3-glucoside 1 with sinapaldehyde 2 after 10 days at
30 °C and UV–vis spectrum of the new compound 3 formed, recorded from the HPLC-DAD.
Table 1
moiety. This compound was purified by TSK Toyopearl column
chromatography eluting with an aqueous solution of 50% (v/v)
methanol and isolated by semi-preparative HPLC for further struc-
tural characterization by NMR.
1H and 13C chemical shifts of the new compound 3, determined in DMSO-d6/TFA (9:1)
Position
d1H ppm; J (Hz)
d
13C
HSQC
HMBC
Pyranomalvidin moiety
2C
3C
4C
4aC
5A
6A
7A
8A
H-20, 60
H-9
—
H-6, H-8, H-9
H-6
The identity of the pigment was fully established by 1H
(400.15 MHz) and 13C NMR (100.62 MHz) using 1D and 2D tech-
niques (gCOSY, NOESY, gHMBC and gHSQC)18,19 (Table 1) in DMSO-
d6/TFA (9:1).
—
—
—
—
162.2
136.0
n.a.
109.4
155.1
—
—
—
—
—
—
7.17; d, 1.9
—
7.29; d, 1.9
102.4 H-6
168.8
102.2 H-8
—
H-8
—
The chemical shifts observed for the protons of the pyranoantho-
cyanin moiety are presented in Table 1 and are in agreement with the
ones reported in the literature for other pyranoanthocyanins.20,21
Protons H-2000,6000 from ring E were assigned to a singlet at
7.00 ppm. The protons of the methoxyl groups in the positions
3000,5000 of the ring E were situated at 3.86 ppm. A doublet, a double
doublet and two additional signals were observed, respectively, at
6.78, 7.85, 7.27 and 7.24 ppm. These signals correspond to four
protons present in the structure of the compound and should cor-
respond to a butadienylidene linkage that links the pyranomalvi-
din moiety to the syringol ring. The doublet (J = 15.0 Hz) was
attributed to the proton H-11 and the double doublet (J = 15.0/
10.3 Hz) to the proton H-12. The other two signals were assigned
to protons H-13 and H-14, respectively, through their correlation
with proton H-12 in the 2D NMR spectrum (COSY). The coupling
constants observed for these protons suggest that they are present
in a trans configuration.
—
8aA
9D
10D
10B
—
7.44; s
—
—
7.77; s
—
154.6
105.0
168.3
121.1
—
H-9
—
H-8
—
H-9; H-11
H-20, 60
—
H-20, 60; OCH3
H-20, 60
—
—
20, 60B
30, 50B
40B
110.8 H-20, 60
150.2
144.3
58.7
—
—
OCH3
—
3.93; s
30, 50—OMe
Butadienylidene linkage
11
12
13
14
6.78; d, 15.0
7.85; dd, 15.0/10.3 147.0 H-12
7.27;*
7.24;*
123.3 H-11
—
—
—
—
128.0 H-13
147.4 H-14
Syringol moiety
1000
E
—
7.00; s
—
—
129.1
—
H-2000,6000
2000,6000
E
E
108.5 H-2000,6000
—
3000,5000
4000
150.8
141.1
58.5
—
—
OCH3
H-2000, 6000; OCH3
The anomeric proton H-100 of the glucose moiety was attributed
to a doublet (J = 7.8 Hz) at 4.78 ppm. The coupling constant ob-
served for this proton shows that the molecule of the glucose is
present in its b-configuration. Proton H-200 was assigned to a dou-
ble doublet (J = 8.1/7.8 Hz) at 3.50 ppm. The other protons of the
glucose moiety were situated at 3.11–3.49 ppm.
E
H-2000,6000
—
3000, 5000—OMe 3.86; s
Glucose moiety
100
4.78; d, 7.8
106.5 H-100
—
—
—
—
—
—
—
200
3.50; dd, 8.1/7.8
3.27; t, 8.5
3.17;*
3.11;*
3.49;*
76.4
78.8
72.3
80.2
63.4
63.4
H-200
300
H-300
400
H-400
500
H-500
H-600a, 600b
H-600a, 600b
The assignment of the carbon resonances was possible using 2D
techniques (HSQC and HMBC) as indicated in Table 1.
6a00
6b00
3.33; dd, 11.1/5.5
The structure of this compound is somehow similar to the por-
tisin reported in the literature yielded from the reaction of a carb-
oxypyranoanthocyanin with sinapic acid,15 except that in this new
pigment the binding between the pyranoanthocyanin and the
syringol moieties is not by a vinyl linkage but through two conju-
gated vinyl groups (butadienylidene group). This additional conju-
gated system increases the electronic delocalization on the
structure and is responsible for the bathochromic shift observed
d, doublet, s, singlet, dd, double doublet, t, triplet, *, unresolved, n.a., not assigned.
for the absorption maximum wavelength (ꢂ568 nm) of this new
pigment when compared with the one observed for the similar
portisin (ꢂ540 nm).15

1998
J. Oliveira et al. / Tetrahedron Letters 52 (2011) 1996–2000
Sinapaldehyde
OH
MeO
OMe
OMe
OMe
Methyl-pyranomv-3-glucoside
(2)
OH
OH
OH
MeO
OMe
HO
O
HO
O
O
H
OMe
OMe
OH
OH
Pathway B
HO
HO
O
O
OH
OH
OH
OH
O
O
O
O
(1)
Charge transfer complex
(π stacking and dipolar alignment)
O
-H+
Pathway A
OMe
Ionic/radicalar
OH
OMe
3'
OH
4'
2'
HO
O
OMe
B
8
1'
5'
HO
O
8a
7
OH
OMe
OH
2
6'
HO
O
C
A
OH
OH
O
2''
4''
6''
6
4a
3
3''
HO
O
O
OH
OH
5
4
O
D
1''
5''
+
O
9
10
11
H
O
-H2O
12
(3)
(2)
13
14
1'''
6'''
5'''
2'''
3'''
E
MeO
OMe
MeO
OMe
4'''
Sinapaldehyde
OH
OH
Scheme 1. Hypothetic formation mechanism of the pyranomalvidin–butadienylidene–sinapyl
sinapaldehyde 2.
3
from the reaction of methylpyranomalvidin-3-glucoside 1 with
Methylpyranoanthocyanins were first thought to react as nucle-
ophiles through their methyl group at carbon C-10 in their methy-
lene deprotonated form (Scheme 1, pathway A). However, a recent
study dealing with the methylpyranomalvidin-3-glucoside equilib-
rium forms in aqueous solutions at different pH values showed
that the deprotonation at the methyl group of a methylpyrano-
anthocyanin to form a methylene group occurs only at very high
pH (>11.5).22 Therefore, these results confirm that the formation
of the new pigment 3 should not occur by this pathway (Scheme
1, pathway A). Alternatively, the condensation reaction of methyl-
pH 1.71
pH 2.34
pH 3.40
pH 4.35
pH 5.74
pH 7.34
pH 8.40
pH 9.32
pH 10.66
pH 11.07
pH 11.66
pH 12.52
pH 1.94
pH 2.97
pH 3.92
pH 4.98
pH 6.83
pH 7.97
pH 8.83
pH 10.25
pH 10.91
pH 11.33
pH 12.29
1.4
1.2
1
0.8
0.6
0.4
0.2
0
250
350
450
550
650
750
850
λ (nm)
Figure 2. Absorption spectra of aqueous solutions of compound 3 as a function of pH (1.76–12.04).

J. Oliveira et al. / Tetrahedron Letters 52 (2011) 1996–2000
1999
OMe
OH
OMe
A
OH
HO
O
O
O
Ka1
OMe
OH
OMe
H+
OH
HO
HO
O
O
OH
OH
OH
OH
O
O
O
O
+
SinapH3
SinapH2
MeO
OMe
MeO
OMe
OH
OH
OMe
OMe
B
OH
O
O
O
O
O
H+
OMe
OMe
OH
Ka2
OH
HO
HO
O
O
OH
OH
OH
OH
O
O
O
O
SinapH-
SinapH2
MeO
OMe
MeO
OMe
OH
OH
OMe
OMe
C
O
O
O
O
O
O
OMe
OMe
OH
Ka3
H+
OH
HO
HO
O
O
O
OH
OH
OH
OH
O
O
O
SinapH-
Sinap2-
MeO
OMe
MeO
OMe
OH
O
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
4
6
8
10
12
pH
Figure 3. Proton transfer equilibria of compound 3 in aqueous solutions at pH between 2 and 12 and respective molar fraction distribution diagram as a function of the pH.
pyranomalvidin-3-glucoside 1 with sinapaldehyde 2 that leads to
the formation of compound 3 can be initiated by a charge-transfer
reaction between the two reactants (Scheme 1, pathway B), as
previously suggested by Chassaing et al. for the reaction of a

2000
J. Oliveira et al. / Tetrahedron Letters 52 (2011) 1996–2000
4-methyl-flavylium with several aldehydes.23 The progression of
the charge-transfer complex may include ionic or radicalar reac-
tions, but these pathways remain to be clarified.
anionic quinoidal base form of this kind of compounds reducing its
ability to lose the third proton.
In this work, a new pyranoanthocyanin-derived pigment with
The equilibrium forms of the new compound 3 in water were
studied at different pH values and its pKa values determined by
UV–visible spectroscopy.
extensive p delocalization was synthesised for the first time from
the reaction of a methylpyranoanthocyanin and a cinnamic alde-
hyde (sinapaldehyde) and the structure fully characterized through
LC–MS and NMR. The structure of the new compound includes a
double vinyl linkage between the pyranoanthocyanin moiety and
the syringol one. These results reinforces the importance of methyl-
pyranoanthocyanins as precursors for the formation of new pig-
ments in food matrixes such as red wines, as already described in
the literature for pyranoanthocyanin dimers.16
A solution of pigment 3 (0.24 mM) was prepared in 30% (v/v)
ethanol/water 0.1 M HCl. One milliliter of the universal buffer of
Theorell and Stenhagen24 at pH 1.24 was added to a 10 ꢀ 10 mm
quartz cell, with 1 mL of 0.1 M NaOH solution and 1 mL of the pig-
ment solution. The final concentration of the pigment was
0.08 mM in 10% (v/v) ethanol/water. The prepared solution was
then titrated using concentrated aqueous solutions of NaOH 10
or 1 M. The pH values of all samples were measured with a pH me-
ter WTW pH 320 (Weilheim, Germany) with a CRISON 5209 com-
bined glass electrode of 3 mm diameter (Barcelona, Spain). The
estimated uncertainty in pH measurements is 0.01 units. The
UV–visible absorption spectra were recorded from 250 to 900 nm
in a Thermo Scientific Evolution Array UV–visible spectrophotom-
eter. Determination of pKa values were made with the software
pHab 2006 developed by Gans et al. (1999) which refines the
Acknowledgements
The authors thank Dr. Zélia Azevedo for the LC/DAD-MS analy-
sis and Dr. Mariana Andrade for the NMR analysis. This research
was supported by a post-doc grant from FCT (Fundação para a
Ciência e a Tecnologia—Praxis BPD/65400/2009) and a grant from
FCT (Fundação para a Ciência e a Tecnologia—REDE/1517/RMN/
2005, PTDC/AGR-ALI/65503/2006 and PTDC/QUI/67681/2006) all
from Portugal and by FEDER funding.
sum of weighted, squared residuals,
r, minimized by adjusting
the refinable stability constants.25
Figure 2 shows the UV–visible spectrophotometric titration of
compound 3 from pH 1.72 to 12.04. The decrease of the color
intensity of solution with the increasing of the pH for values be-
tween 1.72 and 4.84 occurs concomitantly with a hypsochromic
shift of the maximum absorption wavelength, which indicates that
this correspond to the equilibrium between the pyranoflavylium
cation and the respective neutral quinoidal base forms (Fig. 3A).
In fact, it is also possible to observe an isobestic point at 490 nm
corresponding to that equilibrium. Thus, the decrease of the color
intensity does not correspond to the hydration reaction as de-
scribed in the literature for flavyliums.26–28 As already reported
in the literature pyranoanthocyanin compounds are not likely to
undergo hydration reactions but only acid–base equilibria.22,29,30
With the increase of the pH values from 4.84 to 9.50 it is possi-
ble to observe a slight increase in the absorbance and a bathochro-
mic shift in the maximum absorption wavelength of the solutions.
These features correspond to the equilibrium between the neutral
and the anionic quinoidal base forms (Fig. 3B). For higher pH values
(pH >9.50) it was possible to observe the decrease in the absor-
bance of the solutions at ꢂ535 nm and an increase at ꢂ635 nm.
At this pH range the deprotonation at the hydroxyl group of the
syringol group may occur and the consequent formation of a di-an-
ionic specie (Fig. 3C) that absorbs at kmax ꢂ635 nm.
References and notes
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Based on the data discussed previously (Figs. 2 and 3) and using
specific software (pHab 2006)25 it was possible to determine the
three equilibrium constants for the sinapyl-derived compound 3
(pKa1 = 3.64 0.01; pKa2 = 8.02 0.01; pKa3 = 11.19 0.01).
18. Bax, A.; Subramanian, S. J. Magn. Reson. 1986, 67, 565–569.
19. Bax, A.; Summers, M. F. J. Am. Chem. Soc. 1986, 108, 2093–2094.
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The pKa1 and pKa2 determined for the sinapyl-derived compound
concur with the ones reported in the literature for a vitisin B
(pKa1 = 4.43 0.02; pKa2 = 7.34 0.03),30 for methylpyranomv-3-
glc (pKa1 = 4.57 0.07; pKa2 = 8.23 0.04)22 and for other pyranoan-
thocyanins.29 The main difference concerns the first ionization
constant that in this case is slightly smaller than the ones observed
for the other pyranoanthocyanins described in the literature.22,29,30
This means that butadienylidene linkage decreases the stability of
the pyranoflavylium cation form of this compound leading to a dis-
placement towards the formation of the neutral quinoidal base form
of the sinapyl-derived compound. The pKa3 value (11.19 0.01) is
higher than the ones reported in the literature for catechol
(pKa3 = 10.28 0.06)31 or for a pyranoanthocyanin–catechol pig-
ment.29 In fact, the presence of additional electron-donating groups
of the syringol comparing to the catechol seems to stabilize the
31. Timberlake, C. F. J. Chem. Soc. 1957, 4987–4993.