Novel polyunsaturatedn-alkenes in the marine diatomRhizosolenia setigera

Four previously unknown n ‐C 25 and n ‐C 27 heptaenes of the marine diatom Rhizosolenia setigera were isolated and identified using NMR spectroscopy. They possess six methylene interrupted ( Z )‐double bonds starting at C‐3 and an additional terminal or n ‐2 ( Z )‐double bond. Structural and stable carbon isotopic evidence suggests that these polyenes are biosynthesized by chain elongation of the C22:6 n ‐3 fatty acid, followed by decarboxylation and introduction of double bonds at specific positions.