Retro-carotenes

Eleven apo-carotenoids (1-11), including five new compounds, 4, 6, 9,10 and 11, were isolated from the fruits of the red paprika collected from Japan by Maoka et al. (2001b). The structures of new apocarotenoids were determined to be apo-14 -zeaxanthinal (4), apo-13-zeaxanthinone (6), apo-12 -capsorubinal (9), apo-8 -capsorubinal (10) and 9,9 -diapo-10,9 -retro-carotene-9,9 -dione (11) by spectroscopic analysis. The other six known apocarotenoids were identified to be apo-8 -zeaxanthinal (1), apo-lO -zeaxanthinal (2), apo-12 -zeaxan-thinal (3), apo-15-zeaxanthinal (5), apo-11-zeaxanthinal (7) and apo-9-zeaxanthinone (8), which had not been found previously in paprika. These apocarotenoids were assumed to be oxidative cleavage products of C40 carotenoid, such as capsanthin in paprika. 

N-Bromosuccinimide added at -50° to a stirred soln. of dehydro-retro-carotene in chloroform-acetic acid, stirred 7 min., N,N-diethyIaniline added, stirring continued 2 hrs. at 0°, the crude product stirred 2 hrs. at 40° with benzene and methanolic 10%-KOH, the resulting crude isozeaxanthin dissolved in benzene, Al-isopropoxide and acetone added, stirred and refluxed 16 hrs. under Ng -> canthaxanthin. Y 74%. F. e. s. J. D. Surmatis et al., Helv. 55, 974 (1970). 

Because of the presence of an extended polyene chain, the chemical and physical properties of the retinoids and carotenoids are dominated by this feature. Vitamin A and related substances are yellow compounds which are unstable in the presence of oxygen and light. This decay can be accelerated by heat and trace metals. Retinol is stable to base but is subject to acid-cataly2ed dehydration in the presence of dilute acids to yield anhydrovitamin A [1224-18-8] (16). Retro-vitamin A [16729-22-9] (17) is obtained by treatment of retinol in the presence of concentrated hydrobromic acid. In the case of retinoic acid and retinal, reisomerization is possible after conversion to appropriate derivatives such as the acid chloride or the hydroquinone adduct. Table 1 Hsts the physical properties of -carotene [7235-40-7] and vitamin A. 

For example, the ion of [M-69]+, which is observed in the tandem mass spectra of lycopene, neurosporene, and y-carotene but not a-carotene, p-carotene, lutein, or zeaxanthin, indicates the presence of a terminal acyclic isoprene unit. Elimination of a hydroxyl group or a molecule of water, [M-17]+ or [MH-18]+, from carotenoids such as astaxanthin or zeaxanthin is characteristic of the presence of a hydroxyl group. Also, tandem mass spectrometry can be used to distinguish between isomeric carotenoids such as a-carotene and p-carotene, or lutein and zeaxanthin. For example, the ring of a-carotene containing the double bond that i s not conj ugated to the rest of the polyene chain shows unique retro-Diels-Alder fragmentation to form the ion of [M-56]+. In a similar manner, isomeric lutein and zeaxanthin differ by the... 

Reduction of zeaxanthin ditosylate gave a complex mixture of products including /5-carotene, zeaxanthin, and retro-products. Oxidation of... 

Further syntheses of zeaxanthin (27) and rhodoxanthin [4, 5 -didehydro-4,5 -retro-p,p-carotene-3,3 dione (48)] have been described.42 In a simplification of a previous procedure, the acetylenic triol (49) was reduced (LiAlH4) to the allenic diol (50), which with PPh3Br underwent rearrangement to the 3-hydroxy-p-ring... 

In retro-carotenoids, all the single or double bonds of the conjugated polyene system have shifted by one position. The carbon atoms defining the new conjugated system are indicated in the nomenclatnre as a prefix prior to the term -retro-. For example, 4, 5 -didehydro-4,5 -retro-p,p-carotene-3,3 -diol (eschscholtzxanthin) (Fignre 3.10b). 

Nevertheless, based on relatively simple carotenoids, virtually a true kaleidoscope of colours, up to blue shades, can be found in Nature. A few oxidation products of natural carotenoids, such as violerythrin - 549 run in acetone) from actinioerythrol (isolated from sea anemones, A, ax = 500 run in acetone) and 3,4-dihydro-4,3-retro-jS-carotene-3,3-dione (A, = 563 nm in acetone) from 3,3 -dihydroxyisorenieratene (A ax = 460 nm in acetone) have a distinguished blue colour (Fig. 7.18 and Fig. 7.19). [39] While it takes just one reaction step, oxidation is not a preferred route how Nature affords blue hues. 

Carotenoids with rran -disubstituted ethylenes show a strong band near 965 cm due to C-H out-ofplane deformation vibrations. For all-trans carotenes this is a sharp singlet retro-carotcnoids as well as some carbonyl-substituted or cw-polyenes show a doublet in this region. This double peak seems to be conclusive evidence for the retro system. Splitting of a peak into a doublet is often observed with central-cw carotenoids. The C-H out-of-plane deformation vibrations of cis disubstituted double bands give rise to a strong absorption at 780 cm . A disubstituted cis double band shows weak absorption near 830 cm and sometimes a peak is also observed at 1380 cm (VettQT etal., 1971). 

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