Flavylium ions with

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). 

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. 

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... 

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 benzopyrylium ion structure is widespread in nature. It is frequently found substituted with a 2-phenyl group, and such structures (3.59) are known as flavylium ions. Various hydroxylated forms, the anthocyanidins, are usually responsible for the beautiful colors of flowers and fruits. An example of such a structure is the reddish-brown pelargonidin chloride (3.60). 

Hydrophobic effects are thought to position the anthocyanin chromophore and the copigment to form a ti-ti complex [244], and by this way, the most efficient overlap would occur with planar flavonols when compared with hydroxyciimamoyl, galloyl esters, or with the nonplanar flavan-3-ols [245]. Usually, it is thought that the flavylium ion is the major colored species that contributes to the copigmentatirm phenomenon [244, 246]. However, some authors have suggested that the neutral quinoidal base is the main species involved [247, 248]. 

Benzopyrylium salts (chromylium salts) are colored and possess long-wave UV/VIS maxima at 385 nm. A bathochromic shift is observed in the presence of a 2-phenyl substituent, that is, for 2-phenyl-l-benzopyrylium ions (flavylium ions). H NMR data (cf pyrylium ion, p. 298) show the influence of the positively charged oxygen, which reduces the electron density on C-2 and C-4 5 = 9.75 (2-H), 8.40 (3-H), and 8.75 (4-H) (CF3COOD). This effect is even more pronounced with two benzannulations thus, the dibenzo[b,e]pyrylium ion (xanthyHum ion) shows shifts at 5 = 10.18 (9-H) and 165.1 (C-9) (CF3COOD). 

In the context of mechanistic studies, the electrochemical behavior and reactions with nucleophiles of 4-chloro-2,6-diphenylpyrylium and 4-chloro(bromo)flavylium have been studied <1999CHE653>. The proposed mechanism for nucleophilic substitution in halogen-substituted pyrylium and flavylium salts passes through formation of a charge-transfer complex that is converted into an ion-radical pair by simple electron transfer. Heterocyclic cleavage of the C-halogen bond occurs at the stage of the radical or the adduct from the reaction of the pyrylium salt and the nucleophile. In this study, an amine nucleophile was used however, the data are likely relevant for other types of nucleophiles as well (Scheme 5). 

The natural anthocyanin pigments form adducts with bisulfite ion. The bisulfite ion forms a bond to position C-2 or C-4 of the flavylium nucleus, which decolorises the pigment and simultaneously stabilises the glycosidic bond at position C-3 (9-84). The adducts are stable at pH about 3, but acidification to pH < 1 and heating leads to reversal of the reaction with quantitative recovery of the pigment. Such adducts are therefore likely to be likewise unstable under gastric conditions. 

---