8,8 -Diapocarotene-8,8 -dial

Condensation of 15,15 -didehydro-8 -apo-p-caroten-8 -al (52) and of 8,8 -diapocarotene-8,8 -dial (54) with butylamine in the presence of triisobutyl borate gives the Schiff bases 53 and 55, respectively. These react with the boron oxide complex of acetylacetone to give the polyene P-diketones 56 and 57. 

A similar system, but with a more hindered porphyrin (tetramesitylporphyrin = tetraphenylporphyrin bearing three methyl substituents in ortho and para positions on each phenyl group), was tested for P-carotene oxidation by molecular oxygen. This system was chosen to slow the oxidation process and thus make it possible to identify possible intermediates by HPLC-DAD-MS analysis. The system yielded the same product families as with lycopene, i.e., (Z)-isomers, epoxides, and P-apo-carotenals, together with new products tentatively attributed to diapocarotene-dials and 5,6- and/or 5,8-epoxides of P-apo-carotenals. The oxidation mechanism appeared more complex in this set-up. 

As an example to the enol ether condensation, the synthesis of Cio-dialdehyde (12,12-diapocarotene-12,12 -dial) (4) was chosen because this compound is not available commercially, but is a building block of major importance in carotenoid synthesis (Chapter 3 Part I). The procedure described is based on previously published work [1-3] (Scheme 1). 

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