BIOSYNTHESIS OF THE COUMARINS: VI. FURTHER STUDIES ON HERNIARIN FORMATION IN LAVENDER

Abstract

Analyses of lavender plants, with careful precautions to avoid enzymic hydrolysis, have shown that herniarin exists in the plant at least 99% in a bound form, presumably cis-2-glucosyloxy-4-methoxycinnamic acid (cis-GMC). Traces of a glycoside of 2-hydroxy-4-methoxycinnamic acid (HMC), which appears identical with trans-GMC, have also been identified. Several pieces of evidence, principally the failure to demonstrate conversion of p-coumaric acid to p-methoxycinnamic acid by a trapping technique, now indicate that the latter compound, although an efficient precursor of herniarin when administered, may not be a natural intermediate in herniarin formation. O-Methylation, therefore, may occur at a later stage. Feedings of C14-labelled HMC have shown that it is converted to herniarin about one-fifth as efficiently as trans-GMC, the best herniarin precursor yet found. Trapping experiments with C14-labelled p-coumaric or p-methoxycinnamic acid and nonradioactive HMC demonstrated conversion of both labelled compounds to free HMC and to a bound form hydrolyzable with emulsin, presumably trans-GMC. The consistently higher specific activity of the bound HMC in these experiments suggests that free HMC, contrary to earlier assumptions, may not be an intermediate in the formation of bound HMC. Syntheses of herniarin-1-C14 and HMC-carboxy-C14 are described.