P-apo-8 -carotenoic acid ethyl ester

There are several provitamins A these belong to the carotenoid pigments. The most important one is P-carotene, and some of the pigments that can be derived from it are of practical importance. These are P-apo-8 -carotenal and P-apo-8 -carotenoic acid ethyl ester (Figure 9-2). Other provitamins are a- and y-carotene and cryptoxanthin. 

Fig. 3. Commercially important carotenoids P-carotene (10), canthaxanthin [514-78-3] (11), astaxanthin [472-61-7] (12), p-apo-8 -carotenal [1107-26-2] (13), p-apo-8 -carotenoic acid ethyl ester [1109-11-1] (14), and citranaxanthin [3604-90-8] (15).

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. 

To a suspension of 3.5 g (10 mmol) crystalline 507 in 50 ml heptane are added, at 25°C and under N2, 90 mg (0.5 mmol) 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (4-OH-TEMPO) in 1 ml EtOH. Then 3 g (11 mmol) 5 are added and, within 4 h, 5 ml of a 20% solution of NaOEt (12.8 mmol in EtOH) is added at 25-30°C. Stirring is continued for 1 h. The organic phase is washed with iM H2SO4 and 60% MeOH. Upon addition of MeOH the p-apo-8 -carotenoic acid ethyl ester (/) precipitates, is filtered off and washed twice with MeOH. After drying at 50°C in vacuo 3.9 g (84.7%) of 1 are obtained [5]. 

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