STEREOSPECIFIC SYNTHESES AND SPECTROSCOPIC PROPERTIES OF ISOMERIC 2,4,6,8-UNDECATETRAENES - NEW HYDROCARBONS FROM THE MARINE BROWN ALGA-GIFFORDIA-MITCHELLAE .4.

Giffordene (= (2Z,4Z,6E,8Z)-2,4,6,8-undecatetraene; 9f) and five stereoisomers are new C11H16 hydrocarbons from the marine brown alga Giffordia mitchellae. Their synthesis is based on non-stereoselective Wittig reactions of (E)-2-alkenals with appropriate acetylenic phosphoranes and subsequent chromatographic separation of the resulting (E/Z)-pairs. The uniform eynes (> 98% purity) are then stereospecifically reduced to (Z)-alkenes with Zn(Cu/Ag) in aq. MeOH at r.t. 13C and 1H-NMR data of the new tetraenes are presented. Biosynthetically, giffordene (9f) originates from dodeca-3,6,9-trienoic acid via an untable (3Z,5Z,8Z)-1,3,5,8-undecatetraene followed by a thermally allowed antarafacial 1,7-sigmatropic hydrogen shift to the (2Z,4Z,6E,8Z)-isomer 9f.