Anthocyanins biosynthetic routes

FIGURE 2.4.1 Biosynthetic routes leading to betalains and anthocyanins. ... 

The PAs, or condensed tannins, are polymers synthesized from flavan-3-ol monomer units. The phlobaphenes are 3-deoxy-PAs formed from flavan-4-ol monomers. The biosynthesis of both types of PAs follows the biosynthetic route of anthocyanins from chalcones through to the branch points to flavan-3-ol and flavan-4-ol formation. In this section, the specific enzymes forming the monomers are discussed, along with a discussion on the polymerization process. Although the chemistry of tannins is described in detail elsewhere in this book, it is useful to briefly mention the nature of the monomer subunit types and the polymer forms. 

Aromatic Amino Acid Biosynthesis. The shikimate pathway is the biosynthetic route to the aromatic amino acids tryptophan, tyrosine and phenylalanine as well as a large number of secondary metabolites such as flavonoids, anthocyanins, auxins and alkaloids. One enzyme in this pathway is 5-enolpyruvyl shikimate-3-phosphate synthase (EPSP synthase) (Figure 2.9). 

Scheme 10. Possible biosynthetic routes leading from the shikimic acid pathway to betalains and the coexisting flavonoids (excluding anthocyanins) in betalain-bearing members of the Caryophyl-lales.

Since the early contributions of Willstatter and Robinson, several alternative approaches following mainly two routes have been considered for synthesis of anthocyanins.One of the routes includes condensation reactions of 2-hydroxybenzaldehydes with acetophenones, while the other uses transformations of anthocyanidin-related compounds like flavonols, flavanones, and dihydroflavonols to yield flavylium salts. The urge for plausible sequences of biosynthetic significance has sometimes motivated this latter approach. In the period of this review, new synthetically approaches in the field have also predominantly been following the same general routes however, some new features have been shown in synthesis of pyranoanthocyanidins. 

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