Tetrahedron Letters 53 (2012) 1300–1303

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Tetrahedron Letters

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A novel reaction mechanism for the formation of deoxyanthocyanidins

André Sousa, Nuno Mateus, Victor de Freitas ⇑Departamento de Química, Faculdade de Ciências, Universidade do Porto, Centro de Investigação em Química, Rua do Campo Alegre 687, 4169-007 Porto, Portugal

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 November 2011Revised 29 December 2011Accepted 4 January 2012Available online 10 January 2012

Keywords:DeoxyanthocyanidinsAldehydesPhloroglucinolFlavonoidsNMRMass spectrometry

0040-4039/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.tetlet.2012.01.006

⇑ Corresponding author. Tel.: +351 226082858; faxE-mail address: vfreitas@fc.up.pt (V.de Freitas).

The synthesis of deoxyanthocyanidins from the reaction between cinnamic aldehydes (coniferaldehydeand sinapaldehyde) and phloroglucinol is reported herein. The resulting structures were characterizedby visible, MS, and NMR spectroscopy.

� 2012 Elsevier Ltd. All rights reserved.

OHO

OH

OCH3

R

H

2

34

A C

B

4a5

6

7

8

8a1'

2'

3'4'

5'

6'

Deoxyanthocyanidins are yellowish pigments found in severalfood plants such as corn, black tea leaves, and sorghum. Sorghumis one of the most important cereal crops in the world and is richin 3-deoxyanthocyanidins, particularly luteolinidin and apigenini-din.1–3 Deoxyanthocyanidins are considered the chemical ances-tors of anthocyanins, the ubiquitous water-soluble pigments thatare found in flowers and fruits and are responsible for their impres-sive blue and purple colors.1

Open chalcone forms of the common anthocyanins are assumedto be crucial in reactions leading to irreversible degradation ofanthocyanins, particularly under weakly acidic to weakly alkalinesolution conditions.4–8 However, the natural yellow deoxyanthocy-anidins are much more stable in slightly acidic solutions thananthocyanins and anthocyanidins, which points to the potentialadvantage of this type of compounds as viable commercial foodcolorants, and justifies the research developed in the chemistryof 3-deoxyanthocyanins and, in particular, the search for new col-orants with significant stability.9–11 In addition, more studies havedemonstrated other potential applications for these compounds,such as their use as hair dyes, laser dyes, sensitizers for solar cells,and molecular-level memory systems.12–14

Despite the interest in these types of compounds, few syntheticapproaches have been made toward deoxyanthocyanidins and theprocedures described in the literature are complex.1,15–19 Only inrecent years attempts have been made to synthesize these com-pounds with simpler methods.20–22

ll rights reserved.

: +351 226082959.

The synthesis of 3-deoxyanthocyanidins 9 (Fig. 1) from thereaction of phloroglucinol 3 with two cinnamic aldehydes 1, conif-eraldehyde and sinapaldehyde, is described herein and their chem-ical structure and mechanism of formation elucidated.

Phloroglucinol 3 (8 mM) was incubated with coniferaldehydeand sinapaldehyde 1 separately under different conditions of pH,percentage of ethanol in water, and molar ratios. These model solu-tions were kept at a temperature of 35 �C and protected from light.The formation of new compounds was followed over time by HPLC–DAD using a reversed phase C-18 (Merck) column (250 � 4.6 mmi.d., particle size 5 lm) at 25 �C. Solvents were (A) water/formic acid(95:5) and (B) acetonitrile. The elution gradient was performedusing L-2130 Merck pump from 10% to 35% B for 55 min at a flowrate of 1.5 mL min�1.

HOH

Figure 1. Structure of the new synthesized 3-deoxyanthocyanidins 9 (R = H,3-deoxypeonidin; R = OCH3, 3-deoxymalvidin).

0

0.01

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350 400 450 500 550 600

A

Wavelength (nm)

0

0.01

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350 400 450 500 550 600

A

Wavelength (nm)

487nm 492nm

A B

Figure 2. Visible spectra of 3-deoxypeonidin (A) and 3-deoxymalvidin (B) determined directly by HPLC–DAD (pH 2).

0 2 4 6 80

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A B

Figure 3. Kinetic study of the reaction between phloroglucinol and coniferaldehyde. (A) Influence of molar ratios (phloroglucinol/coniferaldehyde) at pH 1.5, 12% ethanol: (d)1:1; (j) 1:5; (�) 1:10; (N) 5:1; (.) 10:1; (B) Influence of different pH values and percentage of ethanol: (d) pH 1.0/12% ethanol; (j) pH 1.5/12% ethanol; (�) pH 3.5/12%ethanol; (N) pH 1.0/20% ethanol; (.) pH 1.5/30% ethanol (for a molar ratio of 1:10, phloroglucinol/coniferaldehyde).

Table 11H and 13C NMR data and HMBC and HSQC correlations of 3-deoxypeonidin,determined in DMSO/TFA (90:10)

Position d 1H (ppm);J (Hz)

d 13C(ppm)

HMBC HSQC

Ring A5A 158.5 H-6A, H-9C6A 7,03; s 95.4 H-6A7A 170.7 H-6A, H-8A8A 6,76;s 102.3 H-8A4aA 112.5 H-8A8aA 158.9 H-6A, H-8ARing B

10B 120.2 H-60B, H-50B, H-3C, H-4C H-10B

20B 7.91; d, 2.0 112.430B 3.96; s 149.2 H-20B, H-60B, H-50B, OCH3 OCH3

40B 155.8 H-20B, H-60B, H-50B H-40B

50B 7,10; d, 8,6 117.160B 8,06; dd, 2.0 125.6Ring C

2C 170.6 H-4C, H-3C, H-60B, H-20B, H-50B, H-8A3C 8.26; d, 8,9 110.5 H-3C4C 9.05; d, 8,7 148.5 H-3C

s, singlet; d, doublet; dd, double doublet.

A. Sousa et al. / Tetrahedron Letters 53 (2012) 1300–1303 1301

The mass spectra of these compounds obtained by LC–DAD/ESI/MS in the positive ion mode showed a molecular ion [M]+ at m/z285 (3-deoxypeonidin) through the reaction with coniferaldehyde,and [M]+ at m/z 315 (3-deoxymalvidin), from the reaction withsinapaldehyde. In addition, the MS2 spectrum of 3-deoxipeonidinshows a major fragment at m/z 270 (loss of a methyl group,[M�15]+), and further MS3 fragmentation of the ion at m/z 270yielded a fragment at m/z 242 (loss of CO, [M�28]+). The other pig-ment formed in the reaction of phloroglucinol with sinapaldehydefollowed a similar fragmentation scheme.

The two compounds, 3-deoxypeonidin and 3-deoxymalvidin re-vealed maximum absorption in the visible spectrum at 487 nm andat 492 nm (Fig. 2), respectively, conferring them a yellow color.

The kinetics of the formation of 3-deoxypeonidin from the reac-tion between phloroglucinol and coniferaldehyde were studied atdifferent conditions of pH, percentage of ethanol in water, and mo-lar ratios (Fig. 3). It can be seen from Figure 3A that the reactionwas faster and with a better yield (8%) at a molar ratio of 1:10(phloroglucinol/aldehyde) for a constant value of pH 1.5 and 12%ethanol. At this molar ratio (1:10, phloroglucinol/aldehyde), thereaction performed at pH 1.0 and 12% ethanol was faster (the max-imum yield was achieved in 4 days) and yielded larger amounts ofthe new pigment compared with pH 1.5 and higher percentages ofethanol. However the yield obtained by this method (8%) is lower

-H+

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7

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HO

R

CH C

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CH

H3CO

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HCHCHHO

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CH HCHHO

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R OH

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135º

[M+1]+ m/z 287

R= H, coniferaldehydeR= OCH3, sinapaldehyde

-H+

99

[O]

8

[M+1]+ m/z 305

[M]+ m/z 285

+H+

+H+

-H2O

Figure 4. Mechanism proposed for the formation of 3-deoxypeonidin and 3-deoxymalvidin 9 obtained from the reaction between phloroglucinol 3 and cinnamic aldehydes 1.

1302 A. Sousa et al. / Tetrahedron Letters 53 (2012) 1300–1303

A. Sousa et al. / Tetrahedron Letters 53 (2012) 1300–1303 1303

than the yield obtained with the methods described in the litera-ture for the preparation of other deoxyanthocyanidins.1,15–22

3-Deoxypeonidin was the only deoxyanthocyanidin synthe-sized in large amounts and its structure was fully elucidated by1H and 13C NMR spectroscopy using 2D techniques (COSY, HSQC,HMBC) (Table 1).

Concerning the pyrylium ring C, the new vinylic protons 3C and4C revealed a clear correlation in the COSY spectrum and wereattributed to the two doublets located at d 8.26 (J = 8.9 Hz) and9.05 ppm (J = 8.7 Hz), respectively. Carbons 3C and 4C wereassigned at d 110.5 and 148.5 ppm through HSQC correlation withthe respective protons. Carbon 2C was assigned at d 170.6 from itslong distance correlations observed in the HMBC spectrum.

The protons 20B, 50B were assigned to the doublets at d 7.91(J = 2.0 Hz) and 7.10 ppm (J = 8.6 Hz), respectively, and the proton60B to the double doublet at d 8.06 ppm. The methoxyl group fromring B was attributed to the singlet at d 3.96 ppm.

Protons 6A and 8A revealed a clear correlation in the COSY spec-trum and were attributed to the singlets at d 7.03 and 6.76 ppm,respectively. Carbons 6A and 8A were assigned at d 95.4 and102.3 ppm, respectively, through HSQC correlation with therespective protons. Carbons 5A and 7A were assigned at d 158.5and 170.7 ppm from their long distance correlations with protons6A, 4C and 8A, 6A, respectively, observed in the HMBC spectrum.

The carbons 4aA and 8aA were assigned at d 112.5 and158.9 ppm from their long distance correlations with protons 8A,3C and 8A, 4C, 6A, respectively, observed in the HMBC spectrum.

These correlations identify unambiguously the position of thearomatic ring A linked to the pyrylium ring C by carbons 8aA and4aA.

The chemical shifts of the remaining protons and carbons iden-tified in Table 1 were easily established by HSQC and HMBCtechniques.

The hypothetic mechanism of the formation of these pigmentsfrom the reaction between phloroglucinol and cinnamic aldehydesis represented in Figure 4. The reaction starts with the protonationof the cinnamic aldehyde 1 in acidic medium, forming a carbocationin the carbonyl carbon 2, followed by a nucleophilic attack of phlor-oglucinol 3, leading to structure 4. The putative presence of com-pound 4 was evidenced by ESI-MS direct analysis of the reactionsolution showing a protonated molecular ion [M+1]+ at m/z 305,with a fragmentation scheme in MS2 ([M+1�18]+, [M+1�126]+,[M+1�124]+) and MS3 ([M+1�124�28]+, [M+1�124�18]+) spectra.The dehydration of the resulting protonated adduct 5 yields a newcarbocation 6, which undergoes a rearrangement leading to carbo-cation 7. This carbocation suffers an intra-molecular nucleophilic

attack by the hydroxyl group at the adjacent carbon of phloroglu-cinol, leading to the structure 8. The putative presence of compound8 was also confirmed in the ESI-MS spectra which showed a proton-ated molecular ion [M+1]+ at m/z 287, with a fragmentation schemein MS2 ([M+1�32]+, [M+1�126]+) spectra. The final oxidation yieldsthe structure 9 which has the new pyrylium ring C associated withthe aromatic ring A and constitutes a chromophore group. Theresulting structure is a 3-deoxyanthocyanidin and the extendedconjugation of the p electrons is at the origin of their maximumabsorption around 490 nm, which represents a hypsochromic shiftfrom their anthocyanidin relatives.23

Acknowledgment

The authors thank FCT (Fundação para a Ciência e Tecnologia)for a PhD grant (ref. SFRH/BD/68736/2010). This research was alsosupported by a research project grant (PTDC/QUI-QUI/117996/2010) funded by FCT (Fundação para a Ciência e Tecnologia).

References and notes

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