Flavyliums reduction

The ANR reaction involves a double reduction at the C-2 and C-3 of the anthocyanidin, allowing the inversion of C-3 stereochemistry. Xie et al. postulate four possible reaction mechanisms, proceeding via either flav-3-en-ol or flav-2-en-ol intermediates. The proposed reaction mechanisms are based on anthocyanidins (the flavylium cation forms) as the starting molecules however, as the authors acknowledge, other forms of the anthocyanidin may exist in vivo. In particular, the 3-flaven-2,3-diol pseudobase is thought to be the more likely in vivo product of the ANS. 

The elimination of water (or acetic acid) also occurs during reductive acetylation of flavonols, leading to 3-acetoxyfiav-3-enes (27), which are converted into flavylium salts (28) in the acid medium. This reaction has been the subject of much study because of the complexity of the... 

Benzopyrylium salts are reduced by sodium borohydride to the 4/T-chromene but LAH yields the 2//-isomer. The reduction products of flavylium salts vary with the conditions used even the method of isolating the product has to be carefully controlled since, for example, 4/f-flavenes may be isomerized by contact with acid to 2//-flavenes. Flavylium... 

In view of these and similar reports, it is clear that the reduction products of pyrylium and flavylium salts depend to a considerable extent on the nature of any substituents present, the reducing agent and the precise conditions used. 

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 involvement of radicals 36 and 37 in reductions of 2-arylbenzopy-rylium (flavylium) and 1-arylisobenzopyrylium ions is established. 

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.)... 

Acidification of the solvent is specially important in the case of anthocyanins, since it would determine extraction of the flavylium cation that is stable at acid pH however, care should be taken since too strong acid conditions may cause the hydrolysis of acylated anthocyanins, and thus reduction of the concentration of hydrochloric acid (between 0.1 % and 1 % HCl in methanol) or the use of acetic acid, or weaker acids like tartaric, oxalic, or citric acids have shown good results. ... 

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