Occurence and reactions of long-chain epoxy acids

Abstract

The possibility that long-chain epoxy acids may be key intermediates in the biosynthesis of acids with conjugated unsaturation has been investigated. By isomerisation of several epoxy acids with acidic and basic catalysts it was hoped to provide a chemical model to support this postulate. Acid-catalysed rearrangement of methyl vernolate, although giving no support for the above hypothesis, yielded a novel oxo-cyclopropane ester, identified by degradative and synthetic techniques as methyl 12-oxo-9,10-methyleneheptadecanoate. This reaction was studied in two solvents under different conditions, and a mechanism has been proposed for the formation of this unusual ester. Under optimum conditions the ester was obtained in yield. Base-catalysed rearrangement of suitably activated epoxy esters provided useful support for the biosynthetic postulate. By this means partial syntheses were effected of methyl coriolate from vernolate, racemio methyl L-dimorphecolate from 9,10-epoxy-octadec-12-enoate, and racemic methyl helenynolate from methyl 9,10-epoxyoctadec-12-ynoate. A synthesis of methyl parinarate from methyl linolenate via an epoxy intermediate was also attempted. Re-examination of three seed oils for unknown epoxy acids which would be biosynthetic intermediates, led to the discovery of a new epoxy acid, cis-9,10-epoxyoctadec-12-ynoic acid, in Helichrysum bracteatum seed oil. Finally, the inter- and intraglyceride distribution of vernolic acid in six seed oils was examined. It was shown that vernolic acid, like oleic and linoleic acids, competes effectively for the 2-position in the triglycerides.