Ethyl-8’-apo-P-caroten-8’-oate

The commercial apo-p-carotenoids ethyl 8 -apo-p-caroten-8 -oate (286) and 8 -apo-p-caroten-8 -al (287) may be prepared from the Cig-aldehyde 53, used in the synthesis of p,p-carotene (2). By reaction of 53 with the Cg-acetal 288 the C25-aldehyde 15,15 -didehydro-12 -apo-p-caroten-12 -al (289) is obtained [115], This compound can be transformed into the Cso-aldehyde 287 by consecutive enol ether condensations first with vinyl ethyl ether (17), to give the C2/-aldehyde 290, and then with prop-1-enyl ether (18), followed by partial hydrogenation and isomerization [116] Scheme 59... 

Ethyl 8 -apo-p-caroten-8 -oate (286) can be synthesized by Wittig olefination of 15,15 -didehydro-C27-aldehyde 290 with 291 [117] Scheme 60... 

Only six of the approximately 600 naturally occurring carotenoids [6] have so far been produced industrially these are three symmetrical C4o-carotenoids, p,p-carotene (3), cantha-xanthin (380), and astaxanthin (403), and three apo-p-carotenoids, ethyl 8 -apo-p-caroten-8 -oate (7), 8 -apo-p-caroten-8 -al (482) and the C33-ketone citranaxanthin (466). Table 1 gives the structural formulae of these pigments and their main applications. 

The three-step enol ether condensation is again used in the chain lengthening of 36 (Scheme 9) [22]. Coupling to vinyl ethyl ether (31) leads to the C27-aldehyde 44. Repetition of the reaction sequence with prop-l-enyl ethyl ether (28) yields the corresponding C30-aldehyde. After partial hydrogenation and thermal isomerization in petroleum ether, 8 -apo-P-caroten-8 -al (482) is obtained in a yield of approximately 50% based on 27. The conversion of 44 into ethyl 8 -apo-p-caroten-8 -oate (7) is described in Section C. 

Ethyl 8 -apo-P"Caroten-8 -oate (1) was synthesized at an early date by Wittig olefination of the 15,15 -didehydro-C27-aldehyde 44 with 1-carbethoxyethylidene triphenylphosphorane (62) (Scheme 18). The ylide can either be isolated for use as an intermediate or generated in situ from the phosphonium salt by reaction with sodium ethoxide. The reaction is preferably carried out in dichloromethane. Exchange of this solvent for ethanol yields the 15,15 -didehydro... 

As has been shown in previous Chapters the Wittig and the Horner-Emmons reactions are of utmost importance for the coupling of carotenoid end groups with the polyene chain. In the following example, the synthesis of the naturally occurring C25-apocarotenal 507 (12 -apo-P-caroten-12 -al, (3-apo-12 -carotenal) and also ethyl 8 -apo-P-caroten-8 -oate (1) (P-apo-8-carotenoic acid ethyl ester), which is produced industrially by means of these reactions, is described. 

Carotenoids act as antioxidants in solution, micelles, and liposomes (Hill et al. 1995, Mortensen and Skibsted 1997, Mortensen et al. 1997, Woodall et al. 1997). The scavenging ability of the carotenoids P-carotene, 8 -apo-P-caroten-8 -al, can-thaxanthin, 7 -apo-7 ,7 -dicyano-p-carotene, ethyl 8 -apo-P-caroten-8 -oate, and 7,7 -diapo-7,7 -di-phenylcarotene towards radical HOO correlated with their redox properties (Polyakov et al. 2001). 

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