Carotenoids continued synthesis

Thorough biochemical analysis of carotenoid biosynthesis, classical genetics, and more recently molecular genetics resulted in the elucidation of the main routes for the synthesis of acyclic and cyclic carotenoids at a molecular level (Sandmann 2001). Little is known, however, about the biosynthesis of carotenoids containing additional modifications of the end groups, the polyene chain, the methyl groups, or molecular rearrangements that contribute to the tremendous structural diversity of carotenoids. At present, hundreds of individual carotenoids have been characterized (Britton et al. 1998), and novel carotenoids continue to be isolated. All carotenoids are derived from the isoprenoid or terpenoid pathway. 

The continuous development of carotenoid research is reflected in various monographs and reviews published over the years and three generations of books under the title Carotenoids have been published by Birkhauser. The first one, by Karrer and Jucker, published in 1948 [1] and the second, edited by Isler and published in 1971 [2], became classics which are still widely used as valuable sources of information. The book by Isler contains a major chapter of 250 pages on total synthesis and this comprehensive and authoritative review describes systematically the construction of many synthons and... 

Carotenoids, Retenoids, Pheromones and Polyenes. - (Ali- )-(3iS, 5R,6R)-Paracentrone (199), first isolated from the sea urchin Paracentrotus lividus, has been synthesised using Wittig reactions of ylides (197) and (198) as key steps. The Wittig reaction continues to be used in the synthesis of retinals, for example, side-chain methyl-shifted analogues, but little or no new chemistry has been reported. ( )-Selective Wadsworth-Emmons olefination using the a-sulfonylalkylphosphonate (200) followed by stereospecific desulfonylation has been used to synthesise the sex pheromone (201) of the potato tuberworm moth Phthorimaea operculella. ... 

Inhibition and Regulation. Evidence has been obtained for the existence of two /S-carotene pools and two biosynthetic sites in the chloroplast. Light-induced carotene synthesis has been studied in mutants of Phycomyces blakesleeanus with abnormal phototropism. The use of substances which affect carotenoid biosynthesis continues. Nicotine has been used in studies of C50 carotenoid formation in Halobacteria and of arylcarotene synthesis in Chlorobium. ... 

The pioneering synthetic work quickly led to the synthesis of carotenoids on an industrial scale. The industrial production of p,p-carotene (3) began in 1954, only four years after its first synthesis on a laboratory scale. This extremely rapid development was made possible by the enthusiasm and perseverance of Isler and his colleagues at Roche in Basel. Since then, commercial synthesis of carotenoids has continuously advanced and today the two major industrial producers Roche and BASF produce six different carotenoids, namely p,p-carotene (3), canthaxanthin (380), optically inactive astaxanthin (403) and the apocarotenoids 8 -apo-p-caroten-8 -al (482), 8-apo-p-caroten-8 -oic acid (486) ethyl ester, and citranaxanthin (466). The total annual sale is now in the region of US 300 million, and the commercially produced carotenoids are used mainly as food and feed additives. 

There are thus both continuing and new challenges to the ingenuity of the synthetic organic chemist. This book presents the current state of the art of carotenoid synthesis. It also provides practical guidance for the newcomer and describes the most effective procedures currently available. 

Future research will lead to the development of new, more economical routes to the end groups. Even though the standard coupling methods have been radically improved and firmly established in industrial syntheses, new coupling methods such as those based on the use of transition metal catalysis may continue to revolutionise the technical synthesis of carotenoids. 

The situation is complicated even further because several different phases of carotenoid biosynthesis must be considered, (i) bulk synthesis during the initial construction of the photosynthetic apparatus (ii) synthesis in the mature leaf as part of turnover (iii) synthesis in response to changes in environmental conditions, especially light intensity (iv) synthesis by the enzymes that were present in etiolated tissues, and may continue to function in the light (v) synthesis of abnormal compounds in response to stress. 

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