13-epi-12-oxo Phytodienoic acid (13-epi-12-oxo PDA) is a lipoxygenase metabolite of α-linolenic acid in the leaves of green plants such as corn. ω-3 and ω-6 polyunsaturated fatty acids in plants are substrates for plant lipoxygenases. 12-oxo PDA is one of the best studied end metabolites of this enzymatic pathway. While the initial enzymatic product and major isomer of 12-oxo PDA contains side chains in the cis position, both being β to the ring, the upper side chain attached at C-13, can and frequently does, isomerize when 12-oxo PDA is extracted, isolated, or stored. 13-epi-12-oxo PDA is the product of this isomerization.