Anthocyanidins

The name proanthocyanidins, previously called leucoanthocyanidins, implies that these are colorless precursors of anthocyanidins. On heating in acidic solution, the C—C bond made during formation is cleaved and terminal flavan units are released from the oligomers as carbocations, which are then oxidized to colored anthocyanidins (cf. 18.1.2.5.3) by atmospheric oxygen (Formula 18.21). Base-catalyzed cleavage via the quinone methode is also possible. 

These red, blue or violet colored benzopyrylium and flavylium salts (Formula 18.22) occur in the form of glycosides, the anthocyanins, in most 

The sugar residues at the 3 or 5 position are easily cleaved in an acid catalyzed reaction with the formation of the corresponding aglycones (anthocyanidins). 

The color of an anthocyanin changes with the pH of the medium (R = sugar moiety cf. Formula 18.24). 

Addition of SO2 bleaches anthocyanins. The flavylium cation reacts to form a carbinol base corresponding to compounds XII or XIII (Formula 18.27). The color is restored by acidification to pH 1 or by addition of a carbonyl compound (e. g. ethanal). Since compounds of type XIV (R = CH3, C2H5) are not affected by SO2, it appears that compound XIII is involved in such bleaching reactions. 

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Hoesch synthesis A variation of the Gattermann synthesis of hydroxy-aldehydes, this reaction has been widely applied to the synthesis of anthocyanidins. It consists of the condensation of polyhydric phenols with nitriles by the action of hydrochloric acid (with or without ZnCl2 as a catalyst). This gives an iminehydrochloride which on hydrolysis with water gives the hydroxy-ketone. 

Phloroglucinol (42) is a colorless and odorless solid which is only spariagly soluble ia cold water (82). It was discovered ia 1855 ia the hydrolysis products of the glucoside phloretia, which was obtained from the bark of fmit trees. Phlorogluciaol occurs ia many other natural products ia the form of derivatives such as flavones, catechins, coumaria derivatives, anthocyanidins, xanthins, and glucosides. 

Glycosides, particularly of phenoHc compounds, are widely distributed in plant tissues (2,10). Glycosides of anthocyanidins, flavones, flavanols, flavanones, flavanonols, stilbenes and saponins, gaUic acid derivatives, and condensed tannins are all common. 

AH anthocyanidins have the 2-phenylben2opyr5lium or flavyhum cation stmcture (86,87) (23), a resonance hybrid of oxonium forms and carbenium... 

Anthocyanidins were first synthesi2ed by reaction of an aryl Grignard reagent with a coumarin (93). 

A moie convenient method for synthesizing anthocyanidins involves the condensation of an o-hydioxybenzaldehyde with an acetophenone (94). 

Because of tfiek pH sensitivity, antiiocyanins have found littie use as industtial dyes. However, a few having the quinoidal form of anthocyanidin (28) were formedy used as dyes. Two of these were carajurin [491 -93-OJ (29) and dracorhodin [643-56-1] (30) ... 

Pink stains occur naturally in some acidic woods, e.g. oak and Douglas fir, but similar anthocyanidin stains can be produced in the wood of sycamore, maple, walnut, agba and sapele by acid-catalysed adhesives. A variety of enzymatic stains can be produced at mildly elevated temperatures in the... 

Fig. 4.20 Compounds 384-388, anthocyanidin derivatives and anthochlors from Gesneriaceae...

The chemical formulae for a variety of plant phenols are given in Fig. 16.2, including examples of simpler phenols, such as cinnamic acid derivative, and of tocopherols, flavonoids, flavonoid glycosides and anthocyanidins. The flavonoids include the following subclasses flavanones (taxifolin), flavones (luteolin), flavonols (quercetin) and flavanols (catechin/epicatechin). The... 

Among the plant phenols, the flavonoids and the anthocyanidins, belonging to the 1,3-diphenylpropans, have been studied in most detail, mainly because of their potential health benefits. With more than 4,000 different flavonoids known, systematic studies of the effects of variation in molecular structure on physico-chemical properties of importance for antioxidative effects have also been possible (Jovanovic et al, 1994 Seeram and Nair, 2002). Flavonoids were originally found not to behave as efficiently as the classic phenolic antioxidants like a-tocopherol and synthetic phenolic antioxidants in donating... 

Anthocyanins and anthocyanidins, compounds present with high structural diversity in fruits and wines, showed a pattern as antioxidants different from that of the tea catechins with respect to the effect of substituents. In a liposomal model system induced peroxidation was inhibited increasingly by anthocyanins/ anthocyanidins with an increasing number of hydroxyl groups in the B-ring (Fig. 16.6), while the opposite was seen for the catechins (Seeram and Nair, 2002). For anthocyanidins, the presence of a 3-hydroxy group is important... 

As we have seen above, anthocyanins comprise an aglycone fraction commonly known as anthocyanidin and a frequently acylated osidic substituent. This characteristic leads to two different approaches for the analysis of these pigments (1) a direct anthocyanin analysis without a hydrolysis stage requiring identification of a number of molecules (several hundreds in the plant kingdom) or (2) an analysis of the anthocyanidin fraction only after hydrolysis of the anthocyanins present in the medium. 

The medium is most often acidihed with hydrochloric acid (0.1% vNY but it should be noted that in light of the work of Revilla et al., the hydrochloric acid concentration should not exceed 0.12 mole/liter to prevent risks of anthocyanidin hydrolysis. Formic acid (2% v/v), with a greater volatility than hydrochloric acid, is preferred because it prevents risks of hydrolysis during the extract concentration stage. ... 

After extraction, an extract purification stage is generally reconunended. This is most often done by liquid-solid exchange using resins such as Sephadex, Amber-lite XAD-7, or Cjg mini-columns. ° All the polar compounds are first trapped on the resin, and then in succession the sugars, acids, and other polar compounds (excluding polyphenolic compounds), polyphenolic compounds (excluding anthocyanidins), and anthocyanidins are respectively eluted with acidified water (HCl 0.01% v/v), ethyl acetate, and acidified methanol (HCl 0.01% v/v). 

Although chromatographic conditions are extremely variable and widely discussed in the literature, broad trends may nonetheless be perceived. A binary solvent such as acidified water/acidified methanol or acidified water/acidified acetonitrile is primarily used because acidification of the solvent converts nearly all anthocyanidins... 

Shimada, S., Inoue, Y., and Sakuta, M., Anthocyanidin synthase in non-anthocyanin-producing Caryophyllales species. Plant J., 44, 950, 2005. 

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