Synthesis of carotenoids

For syntheses of carotenoids (4), where R is a large group, disconnection (a) achieves the most simplification. How would you make the phosphonium salt (5) needed for this strategy ... 

Numerous other syntheses of carotenoids making use of the Wittig reaction have yielded p,y-carotene, optically active y,y-carotene 281), (+)-a-carotene 282), and some... 

Other syntheses of carotenoids via direct olefination have been recently reported [140]. 

In this series the achievement of the now well-established industrial syntheses of carotenoids notably that of fl-carotene has been of practical interest because of the permitted colourant nature of the compound and for the legally stipulated vitaminisation... 

Carotenoids.—number of excellent reviews on carotenoids have appeared, based on the main lectures given at the 4th International Symposium on Carotenoids, at Basle, Switzerland. They include reviews on new carotenoid structures, syntheses of carotenoids and related polyenes (including new routes to tunaxanthin and lutein, and various industrial syntheses of vitamin A and related carotenoids. Wittig reactions have been used in the synthesis of a non-isoprenoid ether pigment, including condensations with the phosphonium salts (77) and (78). 

Haeck, H.H., Kralt, T., and van Leeuwen, P.H., Syntheses of carotenoidal compounds. Part 1. Preparation of some substituted polyenes with a cross-conjugation. Reel. Trav. Chim. Pays-Bas, 85, 334, 1966. 

Table 1. Representative examples of intermediate or product identification in total syntheses of carotenoids...

The first syntheses of specifically C-labelled carotenoids to be reported were those of ([14 - C] Spheroidene and [15 - C]-spheroidene) (97) [20]. These syntheses and the synthesis of C-labelled (3,p-carotene (3) and astaxanthin (403) are described in this Chapter. All the other reports describe syntheses of carotenoids labelled with or radioactive labels ( " C and H). Two syntheses of p,p-carotene (3) labelled with have been reported, namely [6,6 - " C2]-p,P-carotene [21] and [15,15 - C2]-P,p-carotene [22]. p,p-Carotenes with deuterium or tritium at the central C(15) and C(15 ) positions have been prepared by reduction of 15,15 -didehydro-p,p-carotene with deuterium or tritium gas over a Lindlar catalyst [23]. In 1989, the synthesis of [10,19,19,19,10, 19, 19, 19 - Hg]-p,p-carotene was published [24]. The synthesis of [16,16,16,16, 16, 16 - H6]-P,p-carotene has also been reported [25]. 

Several syntheses of carotenoids isotopically labelled with deuterium have been reported [65-68]. The total synthesis of spheroidenes (97) specifically labelled with deuterium in the central part is based on the synthetic scheme discussed above for the C-labelled spheroidenes [68]. When deuterium-enriched compounds are used, a few modifications are necessary to avoid scrambling and isotope dilution (Scheme 28). 

Several Cs-units which have functional groups attached regioselectively, notably 2-methylbut-2-ene-l,4-dial-1-acetal (64), [3-(5,5-dimethyl-l,3-dioxan-2-yl)-but-2-enyl] triphenylphos-phonium chloride (65), 3-methylbut-2-ene-l,4-dial-1-acetal (66) and 3-ethoxycarbonylbut-2-en-l-yl diethylphosphonate (67) are key materials in the industrial syntheses of carotenoids. Methods for the preparation of these synthons are described in Chapter 3 Part I. 

In the following Appendix new syntheses of carotenoids published since 1986 are listed. Included are new syntheses of carotenoids already listed in the Key to Carotenoids as well as syntheses of new carotenoids. According to the general policy of the Editors the reader is advised to use both the Key to Carotenoids and this Appendix for complete information. 

Total syntheses of carotenoids until 1970 were expertly reviewed by Mayer and Isler (133) and up-dated in 1975 by Weedon (176). A complete account of total syntheses of optically active carotenoids is given elsewhere by Mayer (131). 

It is noteworthy that the 4,6-retro structure is retained despite the drastic condensation conditions employed here for the synthesis of (286). C15 Phosphonium salts with the 4,6-retro and 18,6-retro structures have been employed as intermediates in syntheses of carotenoids (Mayer, 1979). 

Other approaches to direct C2Q couplings have been reported (9,30—35). Based on their knowledge of sulfone chemistry, Rhc ne-Poulenc has patented many syntheses of P-carotene which use this olefination chemistry (36—41). Homer-Emmons chemistry has also been employed for this purpose (42). The synthetic approaches to the carotenoids have been reviewed (43). 

Carotenoids are predominantly synthesized in nature by photosynthetic plants, algae, bacteria, and some fungi. - Animals can metabolize carotenoids in a characteristic manner, but they are not able to synthesize carotenoids. The total global biosynthesis of carotenoids is estimated to be in excess of 100 million tons per year. ... 

The photoprotective role of carotenoids is demonstrated in plant mutants that cannot synthesize essential leaf carotenoids. These mutants are lethal in nature since without carotenoids, chlorophylls degrade, their leaves are white in color, and photosynthesis cannot occur. Generally, the carotenoids are effective for visible light but have no effects in ultraviolet, gamma, or x-radiation. The reactions are listed as follows ... 

Carotenoid oxidation products were not only formed from the parent molecules in order to elucidate structure, they were also obtained by partial or total synthesis or by direct oxidation of carotenoid precursors. Thus, apo-8 -lycopenal was synthesized in 1966 more recently, the ozonide of canthaxanthin was obtained by chemical oxidation of canthaxanthin. ... 

The late emergence of hydrophilic, synthetically modified carotenoids is probably the result of a too well-respected principle by traditional carotenoid chemists synthesize carotenoids—don t synthesize with carotenoids Indeed, except for some early reported functional group transformations... 

There are two approaches to synthesizing hydrophilic carotenoids (1) appending a hydrophilic group to the carotenoid scaffold (Foss et al. 2006a) or (2) joining a carotenoid to a hydrophilic compound, Scheme 3.3 (Foss et al. 2003). Whereas the Scheme 3.3 intuitively explains the difference, these techniques cannot be clearly separated in praxis the distinction may appear more emotional than conceptual. Both methods are habitually hampered by low yields, find their limits in the availability of functionalized carotenoids, and cause problems in the work-up procedure due to the amphiphilic character of the products. 

Hertzberg S and Liaaen-Jensen S. 1985. Carotenoid sulfates 4. Syntheses and properties of carotenoid sulfates. Acta Chemica Scandinavica Series B—Organic Chemistry and Biochemistry 39(8) 629-638. 

Carotenoids are one of the most abundant groups of pigments found in nature. Every year more than 100 million tonnes of them are being synthesized in the biosphere. Nearly 600 molecular species of carotenoids are currently identified (Del Campo et al., 2007). As powerful antioxidants, vitamin precursors, natural colorants, and odorants they became a serious global market commodity accounting for almost 1 billion dollars of the yearly trade (BCC research, 2007). 

Animals cannot synthesize carotenoids de novo. To deposit carotenoids in the proper tissues in the proper amounts, they must acquire carotenoids from dietary sources and transport them to target sites. Knowledge of the molecular mechanisms of carotenoid transport, however, is still... 

Because plants are able to synthesize carotenoids de novo, the carotenoid composition of plant foods is enriched by the presence of small or trace amounts of biosynthetic precursors, along with derivatives of the main components. Although commonly thought of as plant pigments, carotenoids are also encountered in some animal foods. Animals are incapable of carotenoid biosynthesis thus their carotenoids need to be derived from the diet. Selectively or unselectively absorbed, carotenoids accumulate in animal tissues unchanged or slightly modified into typical animal carotenoids. 

Some carotenoids contain the 2,5-dihydrofuran ring, but these compounds maybe artefacts (B-71MI31200). The syntheses of 2,5-dihydrofurans (489) and (490) occurring in hops have been accomplished. Similar spirans occur in Santalina species (73JOC3652). 

Some bacteria synthesize C50 carotenoids such as decaprenoxanthin (Fig. 22-5), the extra carbon atoms at each end being donated from additional prenyl groups, apparently at the stage of cyclization of lycopene.134 Thus, a carbocation derived by elimination of pyrophosphate from dimethylallyl-PP could replace the H+ shown in the first step of Eq. 22-11. The foregoing descriptions deal with only a few of the many known structural modifications of carotenoids.2 135 136... 

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