In plants coumarins and hydroxycoumarins are believed to be synthesized from trans-cinnamic acid (3.29) and trans-p-coumaric acid (3.30), respectively, but the exact mechanism for its synthesis is still unknown. One possible biosynthetic route toward coumarin is via o-hydroxylation of 3.29 to give coumaric acid (3.94), followed by glycosylation to result in trans-coumaric acid-2-O-glucoside (3.95) (Figure 3-14).
Figure 3-14. Possible biosynthetic route towards coumarin. (a) 2-hydroxylase, (b) glucosyl transferase, (c) P-glucosidase, (d) dimethylallyl transferase and/or UV light. The enzymes have not yet been identified.
Either the aglycone resulting from the action of a P-glucosidase or the glucoside, or possibly both, undergo cis-trans isomerization under influence of UV-light or possibly mediated by a dimethylallyl transferase. The last step of the biosynthetic pathway is an intramolecular esterification reaction, which can occur spontaneously, to yield coumarin (3.96). The enzymes that involved in these reactions have not been purified.
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