Extraction of Senecio lomentows affords a new alkaloid, ClsHmOlN, isomeric with the previously described otosenine. Hydrogenation with platinum or palladium as catalyst gives a tetrahydro derivative. Hydrolysis of the alkaloid yields an acid, CloHleOe, identical with jaconecic acid first obtained by the hydrolysis of jacobine. Tomentosine with hydrochloric acid gives a chlorinated dehydrated jaconecic acid, presumably the same compound as that derived from otosenine by similar treatment. A synthetic approach to jaconecic acid involved the oxidation of isoseneciphyllic acid by perbenzoic acid. From the product, a pure dilactone and a mixture of isomeric dilactones were isolated. The first was reduced to two dihydro derivatives which were isolated in pure form. The dihydro compounds from the first were stable to thionyl chloride, the dihydro product from the second reacted with replacement of an hydroxyl with chlorine. The infrared spectrum of the resulting material is very similar to that of the product obtained by treatment of tomentosine with hydrochloric acid. The structure of jaconecic acid is discussed in light of its physical and chemical properties. The second was reduced to a mixture of dihydro compounds. The structures of the synthetic lactones are discussed.