Biosynthesis of flavonoids

A rapid and substantial progress to a broad understanding of flavonoid biosynthesis marked the previous reviews of this series of books (Hahlbrock and Grisebach, 1975; Ebel and Hahlbrock, 1982; Heller and Forkmann, 1988), and several other reviews have also appeared (Harborne, 1988; Zaprometov, 1989; Stafford, 1990). Advances achieved in the past few years are restricted to specific steps, filling some of the remaining gaps in this metabolic pathway. Among these reactions are the formation of 3-deoxyanthocyanins and 5-deoxyflavonoids, and the synthesis of the structurally derived pterocarpans, which represent the most prominent group of flavonoid phytoalexins. Among the flavonoid modifications, enzymatic formation of flavonoid sulfate esters and aromatic acylation of anthocyanins with hydroxycinnamic acid glucose esters as donors can be mentioned. Yet the key reaction to one of the main flavonoid classes, the anthocyanins, as well as the formation of epicatechin, the proanthocyanidins and some minor flavonoid-related compounds, such as aurones and dihydrochalcones, still remains a matter of debate.