Esters (RCOOR) are formed by reaction of a carboxylic acid with the hydroxyl group of an alcohol. The hydroxyl group of phenolic compounds can participate in ester formation. Esters of two phenols are not particularly common in nature. The most familiar is the diester of gallic acid (2.22), which is ellagic acid (2.23), along with other gallotannin compounds.
Generally the esters of phenols found naturally are compounds where the phenol contributes the carboxyl group, and another compound contributes the alcoholic hydroxyl group. The hydroxycinnamic acids do not seem to undergo intermolecular condensation, but esters with quinic acid and other acids do occur. For example, chlorogenic acid is an ester of caffeic acid and quinic acid (3-caffeoylquinic acid; 2.24).
o-Hydroxycinnamic acids undergo intramolecular esterification to yield lactones that are called coumarins (see also Chapter 1, Section 3.5). Shown below is the formation of coumarin (2.28) from coumaric acid (2.25; by definition coumaric acid has the hydroxyl group in the ortho-position), which involves glycoslyation (2.26; see also Section 1.7), isomerization from the trans- to the ds-form (2.27), and intramolecular esterification.
Of interest to mycologists, the basidiomycete Polyporus leucomelas produces the phenolic ester protoleucomelone (2.29).
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