Oxidation of octadecatrienoic acids in the red alga Lithothamnion corallioides: structural and stereochemical studies of conjugated tetraene fatty acids and bis allylic hydroxy acids

Abstract

Enzymatic oxidation of (6Z,9Z,12Z)-octadeca-6,9,12-trienoic acid (γ-linolenic acid; 1a) in a preparation of the red alga Lithothamnion corallioides Crouan led to the formation of (6Z,8E,10E,12Z)-octadeca-6,8,10,12-tetraenoic acid 2a and (11R,6Z,9Z,12Z)-hydroxyoctadeca-6,9,12-trienoic acid 3a as the main products. (9Z,12Z,15Z)-Octadeca-(9,12,15)-trienoic acid (α-linolenic acid; 4a) was oxidized in an analogous way to yield (9Z,11E,13E,15Z)-octadeca-9,11,13,15-tetraenoic acid (α-parinaric acid; 5a), (11S,9Z,12Z,15Z)-hydroxyoctadeca-9,12,15-trienoic acid 6a, and (14R,9Z,12Z,15Z)-hydroxyoctadeca-9,12,15-trienoic acid 7a. Isotope studies demonstrated that enzymatic conversion of the acid 1a into the tetraene 2a was accompanied by stereospecific eliminations of the pro-S and pro-R hydrogens from C-8 and C-11, respectively. The bis allylic hydroxy acid 3a was formed from acid 1a by a reaction involving stereospecific elimination of the pro-S hydrogen from C-11 and incorporation of 1 atom of oxygen from water in the C-11 hydroxy group. Although the bis allylic hydroxy esters 3b, 6b, and 7b were chemically convertible into conjugated tetraenes by rapid acid-catalysed dehydration, enzymatic formation of conjugated tetraenes and hydroxy acids in Lithothamnion occurred by independent pathways.