Synthesis of the Demospongic Compounds, (6Z, 11Z)-Octadecadienoic Acid and (6Z, 11Z)-Eicosadienoic Acid

A stereoselective synthesis of (6Z, 11Z)-octadecadienoic acid (1) and (6Z, 11Z)-eicosadienoic acid (2) from easily accessible pentane-1,5-diol (3) is described. Thus, compound 3 on pyranylation and oxidation gave the aldehyde 5 which was converted to the acid 7 by Wittig reaction with a suitable phosphorane. Its depyranylation and oxidation furnished the key aldehyde 9 which upon Wittig reaction with n-heptylidene and n-nonylidene phosphoranes, respectively followed by alkaline hydrolysis afforded the title acids.