Allenic and Acetylenic Carotenoids

The conjugated polyene system typical of carotenoids gives rise to only very weak bands in IR spectroscopy. However, this technique has proved to be of value for detecting certain special structural features such as acetylenic, allenic, hydroxy and unreactive keto groups, e.g. like those in fucoxanthin and capsanthin (for a comprehensive discussion of carotenoid chemistry see Vetter et aL, 1971). The structures of allenic and acetylenic carotenoids may be determined with the help of IR spectroscopy, because of the unusual peak at 1928 cm" indicative of allenic groups, and the weak band at 2170 cm, characteristic for acetylenic carotenoids (Vetter et al., 1971). 

Allenic and acetylenic carotenoids. Rev. Pure Appl. Chem. (Australia) 20,51 (1970). 

Areas of acetylenic chemistry reviewed recently include the base-catalysed isomerization of acetylenes, nucleophilic additions to acetylenes, additions to activated triple bonds, synthetic and naturally occurring acetylene compounds as drugs, allenic and acetylenic carotenoids, linear polymers from acetylenes, carbonylation of mono-olefinic and monoacetylenic hydrocarbons, and the combustion and oxidation of acetylene. Several books have also appeared. ... 

The end groups of acetylenic carotenoids like alloxanthin (8), found in algae and marine organisms, are structurally related to the end groups of fucoxanthin (9), the most abundant natural carotenoid . The allene and acetylene bonds are known to be biogenetically linked in polyacetylenes and the same seems likely to apply to... 

In some important examples of naturally occurring carotenoids, the polyene chain is modified by the presence of one or two acetylenic or allenic groups. These and other interesting features are illustrated by peridinin (558) and pyrrhoxanthin (556) which contain allenic and acetylenic groups, respectively, as well as unusual modifications such as a C37-skeleton with an abnormal arrangement of side-chain methyl groups, and the presence of a butenolide structure. The synthesis of such carotenoids, particularly in the natural optically active form, provides a major challenge and the syntheses that have been developed are described in this Chapter. 

Most carotenoids are xanthophylls, not carotenes. These compounds are formed by the introduction of one or more oxygen functions into the molecule. Other modifications of carotenoids involve the formation of allenic and acetylenic groups, additions to the polyene chain, and the loss of carbon atoms. Little is known concerning the biosynthesis of these compounds. 

Structural elements such as allenic or acetylenic bonds, epoxydes, fnran-oxides, and C45 or C50 carotenoids are not found. 

T. Bjornland, A. Fiksdahl, T. Skjetne, J. Krane and S. Liaaen-Jensen, Gyroxanthin — the first allenic acetylenic carotenoid. Tetrahedron 56 (2000) 9047-9056. 

Carotenoids can also contain additional isoprene chains, homocarotenoids, or if less than 40 carbon atoms, apocarotenoids (Formulae 9.10 and 9.11). In some carotenoids allenic or acetylenic groups are found. 

Carotenoids and Polyterpenoids (Chapter 5).—The absolute configuration of a-carotene has been estabhshed as R. The list of acetylenic, allenic, and iso-prenylated (C45 and C50) carotenoids grows. A number of biologically important terpenoids of varying chain length appear to be degradation products of carotenoids. Notable among them is abscisic acid which has been chemically interrelated with violaxanthin and efficiently synthesised by oxidation of a-ionone. 

Functional groups include allene, acetylene, hydroxy, methoxy, epoxy, keto, aldehyde, carboxylate, lactone, acyl ester, glycoside, glycosyl ester and sulphate. Carotenoids with elements other than oxygen directly attached to the carbon skeleton have not been found in Nature. 

Acetylenic and Allenic Carotenoids.—The apo-acetylenic carotenoid hopkinsi-oxanthin (40), identified by McBeth" in the Nudibranch Hopkinsia rosacea, was also isolated from its food, the Bryozoan Eurystomella bilabiata. Other Nudi-branchs contain the related carotenoid triophaxanthin (41). ... 

Part IV Synthesis of Acetylenic, Allenic and In-chain Substituted Carotenoids... 

Swift IE, Milborrow BV (1981) Stereochemistry of allene biosynthesis and the formation of the acetylenic carotenoid diadinoxanthin and peridinin (C37) from neoxanthin. Biochem J 199 69-74... 

Bjornland, T, Fiksdahl, A., Skjetne, T, Krane, J., and Liaaen-Jensen, S. (2000) Gyroxanthin-the first allenic acetylenic carotenoid. Tetrahedron, 56, 9047-9056. 

Besides changes in the number of double bonds, other changes in the hydrogenation level of the compound may occur. Allenic bonds, as in XVIII and XXI, and also acetylenic bonds, are found in some carotenoids. Finally, in some cases there is a shift by one position of each of the single and double bonds in the conjugated system. Such a carotenoid is designated by the prefix retro. ... 

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