Flavylium

Flavor transport Flavylium cation Flaws Flax... 

The reaction of flavylium salts (403a) with hydroxylamine in pyridine gave 2,5-dihy-droisoxazoles (404) in an analogous manner (75T2884). Pyrimidines have also been converted into isoxazoles, and the reaction of the pyrimidines (405) with hydroxylamine hydrochloride gave the isoxazoles (338). 

Jurd reported the isolation of a 2//-3-isoxazoline by the reaction of the flavylium salt (488) with hydroxylamine. Gentle heating of the material caused isomerization to the more stable 2-isoxazoline. Treatment with base generated an a,/3-unsaturated oxime which on photolysis regenerated the starting flavylium salt (Scheme 129) (70CI(L)624). 

A convenient method leading to pyrans (38) consists in the nucleophilic addition of R anions to 2,6-disubstituted pyrjdium salts, in which the y-position (secondary carbonium ion) is more reactive than the a-positions (tertiary carbonium ions), in opposition to the reactivity of 2,4,6-trisubstituted pyrylium salts.Krohnke and Dickore as well as Dimroth and WolH showed that 2,6-diphenyl-pyrylium salts add the anions R of nitromethane, 1,3-diketones, malonodinitrile, ethyl cyanoacetate, and benzoylacetonitrile. Similar reactions are known in the flavylium series. -Nonactivated R ... 

Photochrome flavylium compounds as multistate/multifunction molecular-level systems 99CC107. 

Aus Flavylium-Salzen werden mit Lithiumalanat hauptsachlich 4H-Flavene gebildet1. Natriumboranat ergibt in niedrigeren Alkoholen Dimerisierungsprodukte, in Acetonitril oder tert.-Butanol liefert es ebenfalls die 4H-Flavene z. B.2 ... 

Auch Tropylium-3, Benzopyrylium-4, Flavylium-3 und Acridinium-Salze3 unterliegen dieser Dimerisierung. 

As frequently mentioned in the literature, anthocyanins co-exist in equilibrium in four different forms. The pH conditions shift this equilibrium toward a variety of structural forms, with the direct consequences of color changes of these pigments. As a rule, at pH above 4, yellow compounds (chalcone form), blue compounds (quinoid base), or colorless compounds (methanol form) are produced. Anthocyanins have the highest stabilities at a pH between 1 and 2 since the flavylium cation is the most stable predominant form. 

The overall anthocyanin analysis is generally conducted using the Giusti and Wrolstad method based on the differences in absorbance of anthocyanins at pH 1 and pH 4.5. Then the pigment content is determined using the coefficient of molar extinction of the predominant anthocyanin. It should be noted that this technique only allows dosing of anthocyanins with a color difference between the two pH values (due to transition to the flavylium cation form). A more global analysis of total anthocyanin content may be conducted by direct spectrophotometry of the... 

Garcia-Viguera, C. and Bridle, R, Influence of structure on colour stability of anthocyanins and flavylium salts with ascorbic acid, Food Chem., 64, 21, 1999. 

Stabilities and colors of anthocyanins are dependent on the nature and number of sugars attached to the flavylium ion and the nature and number of acids linked to the glycosylic moiety. Tint and hue, however, are related to the numbers and positions of hydroxyl and methoxyl substituents in the flavylium ion. In addition, 3-deoxy-anthocyanins that are yellow due to dehydroxylation of the carbon at C-3 have higher stabilities than the corresponding 3-hydroxy anthocyanins that in turn are red but lose much of their stability. 

The stability of cyanidin 3-glncosyl-arabinoside was investigated in different solvents, water, and dimethyl snlfoxide (DMSO, an aprotic solvent) under the same conditions. The valnes nnexpectedly showed that the anthocyanin was more stable in water solntion. DMSO was chosen as a solvent so that it would make the addition of DMSO to C-2 difficnlt and thus the anthocyanin would be more stable in DMSO than in a water solntion. However, since the experiment took place in acidic solntion (pK > pKj,), the preferential addition at C-2 or C-4 of the flavylium ion probably took place by a protonated molecule of DMSO. ... 

The best cofactors are typically flavonoid derivatives that contain many hydroxyl groups, the most favorable at position 3 of the flavones. The strongest cofactors have electron-rich systems that associate with electron-poor compounds such as the flavylium cation. 

In 1967, Timberlake and Bridle proposed that copigmentation complex formation reactions between cyanidin and quercetin in aqueous buffered solutions took place between the colored forms of the flavylium cation (AH+) cyanidin at pH 3.0 and the quinoidal base (A) at pH 5.0. 

Houbiers, C. et al.. Color stabilization of malvidin 3-glucoside self-aggregation on the of flavylium cation and copigmentation with the Z-chalcone form, J. Phys. Chem. B, 102, 3578, 1998. 

Jurd, L., Anthocyanidins and related compounds. XL Catechin-flavylium salt condensation reactions. Tetrahedron, 23, 1057, 1967. 

Pina, R, Caffeine interaction with synthetic flavylium salts. A flash photolysis study for the adduct involving 4,7-dihydroxyflavylium, J. Photochem. Photobiol. A Chem., 117, 51, 1998. 

Timberlake, C.F. and Bridle, P., Flavylium salts anthocyanidins and anthocyanins. Structural transformations in acid solutions, J. Sci. Food Agric., 18, 473, 1967. 

Figure 9.2 Chemical structure of unsubstituted flavylium cation.

Anthocyanins usually give a purple red colour. Anthocyanins are water soluble and amphoteric. There are four major pH dependent forms, the most important being the red flavylium cation and the blue quinodial base. At pHs up to 3.8 commercial anthocyanin colours are ruby red as the pH becomes less acid the colour shifts to blue. The colour also becomes less intense and the anthocyanin becomes less stable. The usual recommendation is that anthocyanins should only be used where the pH of the product is below 4.2. As these colours would be considered for use in fruit flavoured confectionery this is not too much of a problem. Anthocyanins are sufficiently heat resistant that they do not have a problem in confectionery. Colour loss and browning would only be a problem if the product was held at elevated temperatures for a long while. Sulfur dioxide can bleach anthocyanins - the monomeric anthocyanins the most susceptible. Anthocyanins that are polymeric or condensed with other flavonoids are more resistant. The reaction with sulfur dioxide is reversible. 

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