Metabolites of ß, ß-Carotene in the Stick Insect, Carausius morosus Br.: Compounds with 2-One and 3,4-Didehydro-2-one Structure

Six novel keto carotenoids have been isolated from the stick insect. The structure of the pre­ dominant red pigment has been assigned to 3,4,3',4'-tetradehydro-/?,/?-carotene-2,2'-dione on the basis of electronic, infrared, and mass spectra. Minor pigments are 3,4-didehydro-/?,^-carotene-2,2'dione, 2'-hydroxy-3,4-didehydro-/?,/?-caroten-2-one, and 2'-hydroxy-/?,/?-caroten-2-one. Two other com­ pounds, /?,/?-caroten-2-one and /?„/?-carotene-2,2'-dione, have been tentatively identified. The ad­ ditional occurrence of /?,/?-caroten-2-ol and /?,/?-carotene-2,2'-diol has been reported recently. All hydroxylated pigments are mainly present as fatty-acid esters. During hydride reduction of the 3,4-didehydro-2-one compounds hydrogenation of the 3,4-double bond was observed as a side reaction. A hypothetical scheme of carotenoid metabolism in the stick insect is proposed. According to this pathway the keto groups are introduced by a pseudo one-step reaction; the hydroxylated pig­ ments are thought to be reduction products of the corresponding ketones. The proposed pathway is fitting more to physiological data than an alternative one based on the hydroxyl —> carbonyl sequence.