Dicyclic carotenoids

The Roche group extended this work and in 1981, at the 6th International Symposium on Carotenoids in Liverpool, reported the total synthesis of several of the ten optical isomers of e,8-carotene-3,3 -diol (tunaxanthin, 149) and of four diastereoisomers of p,e-carotene-3,3-diol, including the most common (3R,3 / ,6 / )-isomer, lutein (133). The starting material for these syntheses was 6-oxoisophorone, which the Roche scientists went on to use to synthesize a large number of dicyclic carotenoids in optically inactive and active form, as reported at the 7th International Symposium on Carotenoids in Munich in 1984. 

Usually, almost all carotenoids in organisms are synthesized from lycopene 7. Most carotenoids are dicyclic carotenoids, i.e., p-carotene 13, cx-carotene 10, and their derivatives. Some eubacteria and some fungi produce the monocyclic carotenoids, y-carotene 12, and its derivatives. Acyclic carotenoids are mostly found in purple bacteria and some archaea [1-3]. 

There are basically two types of carotenoids those that contain one or more oxygen atoms are known as xanthophylls those that contain hydrocarbons are known as carotenes. Common oxygen substituents are the hydroxy (as in p-cryptoxanthin), keto (as in canthaxanthin), epoxy (as in violaxanthin), and aldehyde (as in p-citraurin) groups. Both types of carotenoids may be acyclic (no ring, e.g., lycopene), monocyclic (one ring, e.g., y-carotene), or dicyclic (two rings, e.g., a- and p-carotene). In nature, carotenoids exist primarily in the more stable all-trans (or all-E) forms, but small amounts of cis (or Z) isomers do occur. - ... 

Both carotenoids and xanthophylls have been found in all leaf tissues examined. Cyclic carotenes and xanthophylls from leaf tissues have both P- and e-ring types. Only dicyclic xanthophylls, and particularly those with C-3 or C-3, have been found in the biosynthetic tissues of higher plants. Acyclic carotenoids normally are not present (Young, 1993a). The same four major carotenoids, namely P-carotene (1), lutein (2), violaxanthin (3), and neoxanthin (4), have been found as the major carotenoid constituents of leaf tissues of all plants examined to date (Young, 1993a). Purple and green phototrophic bacteria rarely contain any of the monocyclic... 

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