Biosynthesis of the coumarins—V.

14C-labelled p-coumaric and o-coumaric acids, when administered to cut shoots of Hydrangea macrophylla Ser., were both converted with good efficiency to umbelliferone. No randomization of 14C could be detected when the umbelliferone formed from o-coumaric acid was chemically degraded. Trapping experiments showed that both coumaric acids can be converted to umbelliferone via umbellic (2,4-dihydroxycinnamic) acid, which is itself an extremely efficient umbelliferone precursor when fed in labelled form. When cinnamic acid-β-14C was administered to H. macrophylla with non-radioactive o-coumaric acid, no 14C could be detected in the recovered o-coumaric acid, although p-coumaric acid, umbelliferone, and hydrangetin (7-hydroxy-8-methoxycoumarin) were radioactive. The results establish the existence of a pathway: cinnamic acid → p-coumaric acid → umbellic acid → umbelliferone in Hydrangea, but the normal participation of o-coumaric acid in umbelliferone biosynthesis cannot be considered proved. The results of one experiment indicate that cinnamic and p-coumaric acids are also precursors of hydrangetin.