Wine Chemistry and Biochemistry

9B Flavanols, Flavonols and Dihydroflavonols 479

OH

R

monophenol

H 2 O

½ O 2

o -quinone

OH

OH

R

o -diphenol

½ O 2 H 2 O

PPO

PPO

O

O

R

OH

OH

R

adducts(o -diphenol)

OH

OH

R

o -diphenol

+

secondary o -quinone

(2)

(1)

(3)

(4)

O.

OH

R

2

(5)

O

OH

R

(6)

2 semiquinones

HO

OH

O

OH

OH

OH

HO

OH

O

OH

OH

OH

HO

OH

O

OH

O

O

O

OH

HO

OH

OH

OH

Fig. 9B.4Enzymatic formation and reactions of quinones

Those involving both flavonol and anthocyanins have been described in Chapter 9A

and will not be detailed here.

9B.3.3.1 Reactions Based on Acid-Catalysed Cleavage of Proanthocyanidins

Precursors

The precursors of these reactions are, on one hand, proanthocyanidins and, on
the other hand, any kind of flavonoid that can act as a nucleophile. The latter

include flavonols, dihydroflavonols, flavanol monomers, proanthocyanidins, and

anthocyanins under their hemiketal form (for anthocyanin reactivity, see

Chapter 9A).

Reaction Mechanism

The reaction starts with acid catalysed cleavage of a proanthocyanidin interfla-

vanic linkage (Figs. 9B.3 and 9B.5(1)). The intermediate carbocation thus gener-

ated then undergoes nucleophilic addition.When the nucleophile is another flavanol

(Fig. 9B.5(2a)), the product is a new proanthocyanidin molecule. As a result of this