(3E,5Z)-1,3,5-Undecatriene () having 98.2% stereoisomeric purity has been prepared by a new and expeditious route involving two selective Pd-catalyzed carbon-carbon bond forming reactions. The key step of this synthesis is a highly diastereoselective Pd-catalyzed cross-coupling reaction between 1-heptynylzinc chloride () and a molar excess of (E)/(Z)-1,2-dibromoethylene (). Another highly diastereoselective Pd-catalyzed reaction, which involves a cross-coupling between trimethylsilylethynylzinc chloride () and molar excesses of stereoisomeric mixtures of 1-bromo-1-alkenes (), has been used to prepare (E)-1-trimethylsilyl-3-nonen-1-yne (), a key intermediate for the synthesis of (3E,5E)- and (3Z,5E)-1,3,5-undecatriene, () and (). Compounds and , isolated from the essential oil of , also occur together with compound in the male attracting oils of seaweeds. These substances have odors highly appreciated in perfumery.