3,4,5-Trihydroxycinnamic acid

Figure 3. Proposed biosynthetic pathways to gallic acid (5). (1) Dehy-droshikimic acid (2) caffeic acid (3) 3,4,5-trihydroxycinnamic acid (4) protocatechuic acid.

The biosynthesis of gallic acid (3.47) has been under investigation for more than 50 years. Different biosynthetic routes have been proposed, as depicted in Figure 3-6 (/) direct biosynthesis from an intermediate of the shikimate pathway, (2) biosynthesis via phenylalanine (3.27), cinnamic acid (3.29), />coumaric acid (3.30), caffeic acid (3.32), and 3,4, 5-trihydroxycinnamic acid (3.44), or (3) biosynthesis via caffeic acid (3.32) and protocatechuic acid (3.45). The possibility that different pathways co-existed in different species or even within one species was also considered. 

Parenthetically it should perhaps be noted that the absence of 3,4,5-trihydroxycinnamic acid (6) from plant tissues is now, in contrast to the early 1960s, more readily explicable. Thus if one accepts that the occurrence of the hydroxycin-namic acids and their various conjugates in plants derive from their formation... 

Zenk222 formulated a conventional pathway Figure 5.13, b) from L-phenylalanine (42) to 3,4,5-trihydroxycinnamic acid (119) fol-... 

Analogous schemes, with some variations, may be used to accommodate the biosynthesis of esters and glycosides of caffeic acid, quercetin and hence the procyanidins (1, R = H). However several serious uncertainties arise when the origin of flavonoids such as myricetin and the prodelphinidins (1, R = OH) is considered. 3,4,5-Trihydroxycinnamic acid (5) is not an intermediate in lignin synthesis and, except as its various o-methyl ethers, it has not been encountered in nature. Its role as a putative precursor of the vicinal trihydroxyaryl (pyrogallol) group in... 

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