Flavylium ion

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 flavylium ions of direct flavanol-anthocyanin adducts (i.e., A" -F and F-A" ") and anthocyanin dimers (A+-AOH) have the same UV-visible spectra as anthocyanins. Species in which the anthocyanin is substituted in the 4-position (A" -F, A" -AOH) are expected to be resistant to sulfite bleaching and hydration whereas F-A is as susceptible to water addition as their anthocyanin precursor. ... 

Escribano-Bailon, M., Dangles, O., and Brouillard, R., Coupling reactions between flavylium ions and catechin. Phytochemistry 41, 1583, 1996. 

Methoxide ion attacks pyrylium salts to give methoxypyrans, e.g. (217), (218). Flavylium ion (220) gives (221) (with NaOAc-EtOH). 

Pyrylium salts with a free a- or -position react in a similar way without ring fission, e.g. flavylium ions (220) add dimethylaniline and the product aromatizes to give (297) xanthylium ions (210) form adducts at the 9-position with (3-diketones, 3-keto esters and malonic esters e.g. 298). 

Pyridinium and pyrylium cations, pyridones and pyrones are all readily hydrogenated e.g. flavylium ion (220) and coumarin yield (387) and (388), respectively. 

Turning to experimental measurements, the majority of equilibrium constants measured for carbocation formation refer to ionization of alcohols or alkenes in acidic aqueous solution, and correspond to pAR or pAa. Considering the instability of most carbocations it is hardly surprising that only unusually stable ions such as the tropylium ion l49 or derivatives of the flavylium ion 250,51 are susceptible to pA measurements in the pH range. 

Flash photolysis is a powerful technique for investigating the kinetics of conversion of the various forms of flavylium ions. 47cI Even with a simple flash-photolysis apparatus, with a time resolution of approximately 0.2 s, it is possible to obtain kinetic data that can complement and/or replace those obtainable by the pH-jump technique. 

An intuitive way to describe the interconversion processes in flavylium-type compounds is their description by a hydraulics analogy.1411 By such an analogy, the behavior of aqueous solution of flavylium ions upon a pH-jump from 1.0 to 4.2 can be schematically represented as in Figure 14. In the case of 4 -hydroxyflavylium, Cc... 

Fig. 20 Write-lock-read-unlock-erase cycles for the unsubstituted flavylium ion. 43 For more detail, see text.

The flavonoid hydroxylases (F3 H, F3 ,5 H) and the phenylpropanoid enzyme cinnamate 4-hydroxylase (C4H) are cytochrome P-450s that are anchored to the cytoplasmic face of the ER.14 F3 H and F3 ,5 H add hydroxyl groups to the 3 and/or 5 of the B ring bringing about chemical and spectral diversity to the flavonoids. Dihydroflavonol 4-reductase (DFR) is the committed step to the production of the precursor of the colored compounds - the leucoanthocyanidins (Fig. 3.2). It is upon the reduction of the 4 keto of the C -ring by DFR (Fig. 3.1) and its further reduction by leucoanthocyanidin dioxygenase/anthocyanidin synthase (LDOX/AS) that the de-localization of the electrons necessary for the formation of the planar flavylium ion is permitted.2... 

The copigmentation phenomenon consists of hydrophobic interactions (-rr-Tr stacking) between the planar polarizable nuclei of the colored forms of the anthocyanins (i.e., flavylium ion and quinonoidal forms) with other organic molecules... 

The only pyrylium benzologue for which mass spectral data are available is the flavylium ion (12) (69AK(30)393>. On electron impact, (12) forms a highly stable molecular ion as the base peak which decomposes by two pathways (Scheme 27). Pathway A generates the [M-H]+ ion (12a). There is some evidence to suggest thaft loss of CO from (12a) gives the phenanthrene radical ion (12b) (m/e 178). Subsequent ejection of an acetylene molecule generates the biphenylene ion (12c). 

In general, the radicals dimerize rapidly unless inhibited by pyranyl ring substitution 2-(4-nitrophenyl)benzopyranyl is reported not to dimerize, however.The kinetics of the formation of a series of 36 by reduction of the precursor flavylium ions by Cr(II) have been investigated a Hammett plot of the second-order rate constants versus has p = 1.03. Most work has been concerned with the polarography of the parent cations there appear to be no ESR results for benzopyranyl radicals. The mediation of benzopyranyl radicals in the oxidation of 2-phenyl-2//-benzopyrans by potassium permanganate has been suggested.(See Section II,B,5,b for radicals from anthocyanidins.)... 

The polarography of anthocyanidins was studied several years ago by Zuman. A more recent investigation of pelargonidin (62), cyanidin, and various model flavylium ions has been published by Harper and... 

Escribano-Bailon T., Dangles O., Brouillard R. 1996. Coupling reaction between flavylium ions and catechin. PtQftochemistry. 41,1583-1592. 

Coumarin 13 (27/ -l-benzopyran-2-one), the 1-benzopyrylium ion 14 and the flavylium ion 15 (2-phenyl-l-benzopyrylium ion) derived from flavane (2-phenyl 2i/-chromene) are important derivatives of 277-chromene. They are discussed in chapters 6.7 and 6.8. 

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