Phenol Carboxylic Acids and Simple Phenols

In the biochemical transformations so far discussed the C skeleton of the cinnamic acids serving as starting material has been maintained unchanged. The derivation of the two groups of phenols to be discussed in this section from the cinnamic acids involves the partial or total degradation of the side chains of the latter. 

Simple phenols occur in higher plants much less frequently than was earlier supposed. The most important are hydroquinone and its glucoside arbutin (Fig. 104). Both also occur as their methyl ethers. They are known 

The biosynthesis of the phenol carboxylic acids and that of the simple phenols are closely related and should be discussed together for this reason (Fig. 104). Phenol carboxylic acids can be derived from different stages of the shikimic acid pathway (page 119). 

Phenol carboxylic acids can also be supplied by the acetate-malonate pathway, at least in microorganisms. In higher plants, however, they are more likely to be derived from cinnamic acids. Let us consider as an example the conversion of /7-coumaric acid into /7-hydroxybenzoic acid and, subsequently, hydroquinone, a sequence of reactions discovered by Zenk. 

/7-coumaric acid is subjected to a j8-oxidation which reminds us of the j8-oxidation of the fatty acids and it is possible that the mechanism of the two reactions is the same. At any rate an aromatic acid is produced, the side chain of which is 2 C atoms shorter. It is called /7-hydroxybenzoic acid and is an example of a phenol carboxylic acid. In a completely analogous manner protocatechuic acid is obtained by j8-oxidation of caffeic acid, vanillic acid from ferulic acid, etc. 

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Fig. 104. The biosynthesis of phenol carboxylic acids and simple phenols.

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