Carotenoids 5,6-epoxide group

It must be underlined that independently of the MS equipment characteristics, no information about stereo-chemistry can be obtained. In fact, cis and trans isomers of the corresponding carotenoid showed identical mass spectra, as did carotenoids with epoxide groups at 5,6 and 5,8 positions. In addition, special care should be taken in assigning carotenoid molecular masses to avoid confusion due to the various ions that may be formed depending on measurement conditions. 

Fig. 5 RP-HPLC separation of (A) paprika extract and (B) saponified paprika extract on a Zorbax C l8 column at 460 nm. The solid peaks represent carotenoids containing a ketone group, and the hash-marked peaks represent carotenoids containing an epoxide group. (From Ref. 74a.)...

Mass Spectrometry. In an important and extensive survey, the mass spectrometric fragmentations of a wide range of carotenoids with specific deuterium labelling have been reported. On the basis of this work, the mechanisms of the well-known M—92, Af-106, Af-79, and Af-158 in-chain fragmentations, of some end-group fragmentations, and of the M— 80 and other fragmentations of carotenoid epoxides have been reassessed, and in several cases new mechanisms are proposed.38... 

Acids also induce chemical modifications of carotenoids. For instance, juice preparation requires acidic conditions, which trigger the spontaneous conversion of 5,6- and 5,6 -epoxide groups of violaxanthin, lutein epoxide, and antheraxanthin to 5,8- and 5, 8/-furanoid epoxides (Coultate, 1996 Gross, 1991). This conversion has a dramatic effect on the color because furanoids are essentially colorless. To be convinced about this fact, it is enough to compare the flesh color of fresh (solid line) and canned (dashed line) pineapples (Coultate, 1996) (Figure 5). 

Figure 22-5 does not indicate the stereochemistry of the allene group correctly the carotenoid chain protrudes behind the ring as drawn in the equation. Violaxanthin contains epoxide groups in the rings at... 

Epoxide groups, particularly at the 5,6-position of cyclic carotenoids, are fairly common. These are formed stereospecifically, and experiments with have confirmed that the oxygen is derived from molecular oxygen (Yamamoto and Chichester, 1%5). The mechanism of incorporation is unknown, however. 

The interaction of carotenoids with cigarette smoke has become a subject of interest since the results of the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study Group 1994 (ATBC) and CARET (Omenn et al. 1996) studies were released. P-Carotene has been hypothesized to promote lung carcinogenesis by acting as a prooxidant in the smoke-exposed lung. Thus, the autoxidation of P-carotene in the presence of cigarette smoke was studied in model systems (toluene) (Baker et al. 1999). The major product was identified as 4-nitro-P-carotene, but apocarotenals and P-carotene epoxides were also encountered. 

The key step of the synthesis is the rearrangement of the a-acetylenic alcohol 97 into the unsaturated carbonyl compound 124. This rearrangement was carried out with tris(triphenylsilyl)vanadate, triphenylsilanol and benzoic acid to give a mixture of the isomers 124 and 125. The latter was converted by iodine catalysis into the desired isomer 124. This key intermediate was afterwards transformed into the Cis-phosphonium salt 123 by standard procedures. The Wittig olefination of the Cio-dial 45 first with the fucoxanthin end group 123 and then with the peridinin end group 122 gave, in five steps, the C4o-carotenoid 126. Finally the epoxidation of this compound resulted in optically active fucoxanthin (121) and its (5S,6R)-isomer (Scheme 28). 

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