Chalcones tautomerism

Anthocyanins are usually represented as the red flavylium cations (Figure 5.1, left). However, this form is predominant only in very acidic solvents (pH < 2) such as those used for HPLC analysis. In mildly acidic media, the flavylium cations undergo proton transfer and hydration reactions, respectively, generating the quinonoidal base and the hemiketal syn carbinol) form (Figure 5.1, right) that can tautomerize to the chalcone. Thus, at wine pH, malvidin 3-glucoside occurs mostly as the colorless hemiketal (75%), the red flavylium cation, yellow chalcone, and blue quinonoidal base being only minor species. 

The opening of the heterocycle and rearrangement as chalcone corresponds to a tautomerism. This balance is slow and takes a few minutes, or even a few hours according to the temperature ... 

Candenatone 4a represents the first example of a new type of fused isoflavan. A biogenesis of the compound is outlined in Scheme 1 in which a radical coupling of the chalcone 6 and the preformed isoflavan vestitol (7) could lead to the radical species 8. Elimination of a H radical and tautomerization would give the intermediate 9. Candenatone would be obtained through cyclization to 10 and subsequent elimination of water. 

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