While engaged in the preparation of pure fatty acids for use in medical research during the late war under the auspices of the National Research Council, the writers had occasion to prepare pure clupanodonic acid, C18H28O2. The information contained in the literature regarding this acid was vague, and a number of preliminary studies were made in order to divise a cheap and convenient method for its preparation. The method finally adopted was a modification of that of Riedel.1 The methyl esters, which were prepared by debromination with zinc of the polybromides of the methyl esters of menhaden oil, had a molecular weight too high for methyl clupanodonate, and furthermore distilled over too wide a range to be a pure compound. These facts indicated that we were dealing with some acids other than clupanodonic. Since practically none of the properties of clupanodonic acid has been described, the investigation was continued with the idea of determining the nature of the pure acid, and it has finally developed into an attempt at a characterization of the highly unsaturated acids of fish oils.

1 Riedel, Ger. pat., 266,350.

Hofstaedter in 18542 was the first to discuss in detail the presence of an unsaturated acid in a marine oil. Investigations by Fahrion,3 Bull,4 Tolman,5 Twitchell6 and others have demonstrated beyond question the presence of highly unsaturated acids in both fish and marine animal oils. By brominating the acids from Japanese sardine oil and reducing the resulting bromides with zinc and alcoholic hydrochloric acid, Tsujimoto7 obtained an acid with an iodine number of 344, which he called clupanodonic acid. Majida and Okada8 reported that the acids obtained in this fashion had a molecular weight too high for clupanodonic acid, and that on hydrogenation they obtained an apparent mixture of arachidic and behenic acids.

When the problem had been under way for 18 months, we read the interesting preliminary report of Tsujimoto in which he describes the separation of the acids of fish oils by means of variations in the solubility of their lithium salts in acetone.9 From the analysis of 4 fish oils he concluded that the highly unsaturated acids were chiefly C20H32O2 and C22H34O2, the latter of which he proposes to call clupanodonic acid instead of that to which he had formerly given the name.10 In view of the fact that this name has been used for the compound, C18H28O2, for sixteen years, we prefer to retain it and are using, therefore, the following nomenclature11 for the unsaturated acids throughout this paper.