Seco-carotenoids

As described in the preceding paragraphs, oxidation products of carotenoids can be formed in vitro as a result of their antioxidant or prooxidant actions or after their autoxidation by molecular oxygen. They can also be found in nature, possibly as metabolites of carotenoids. Frequently encountered products are the monoepoxide in 5,6- or 5, 6 -positions and the diepoxide in 5,6 5, 6 positions or rearrangement products creating furanoid cycles in the 5,8 or 5, 8 positions and 5,8 5, 8 positions, respectively. Products like apo-carotenals and apo-carotenones issued from oxidative cleavages are also common oxidation products of carotenoids also found in nature. When the fission occurs on a cyclic bond, the C-40 carbon skeleton is retained and the products are called seco-carotenoids. 

Some 117 naturally occurring apo-carotenoids, 88 of which have been fully identified and another 6 naturally occurring seco-carotenoids have been referenced as carotenoids, thus representing around 15% of the carotenoids numbered to date (see Figure 3.3.1). This subfamily of carotenoids would be even larger if we consider the retinoids andnorisoprenoids. However, these compounds are excluded by nomenclature rules that dictate that they are not deemed to be carotenoids because of the absence of two central methyl groups (at C20 and C20 ). Retinoic acid, retinal. 

FIGURE 11.1 Chemical structures of carotenoid oxidation products occurring in nature apocarotenoids 10 -apolycopen-lO -oic acid (504.4), apo-lO -violaxanthal (502), diapocarotenoid rosafluin (547.2), and seco-carotenoid 3-carotenone (562). The compound number corresponds to those in Britton et al. (2004). 

Yokoyama et al. have isolated a series of seco-carotenoids from Rutaceae. Semi-a-carotenone (3) was shown by c.d. studies to have the same absolute stereochemistry as a-carotene. The other examples isolated were semi-j -carotenone (33), jS-carotenone (34) and triphasiaxanthin (35). ... 

Further additions to the long list of tobacco constituents include 5(13),7 -megastigmadiene-6,9-diol (44), (3S,6i ,9i )-4,7 -megastigmadiene-3,9-diol (45) and its 95-epimer, and a seco-nor-carotenoid , 3,3-dimethyl-7-hydroxy-octan-2-one (46), all from Greek tobacco. The structures (47), (48), and (49) of pallescensin-1, -2, and -A have been proved by synthesis from a-cyclocitral (50)." ... 

FIGURE 3.10 Examples of oxygenated carotenoids (xanthophylls) (a) zeaxanthin (P,P-car-otene-3,3 -diol) (b) eschscholtzxanthin (4, 5 -didehydro-4,5 -refro-P,P-carotene-3,3 -diol) (c) 5,6-seco-P-carotene-5,6-dione and (d) 3 -hydroxy-8-apo-P-carotene-8-al. 

Carotenoids a large class of yellow and red pigments, which are highly unsaturated aliphatic and ali-cyclic hydrocarbons and their oxidation products. C. are biosynthesized from isoprene units (CsHj) and therefore have the methyl branches typical of isopre-noid compounds. Most C. have 40 C atoms and are thus tetraterpenes (consisting of 8 isoprene units). A few C. contain 45 and 50 C atoms, particularly in nonphotosynthetic bacteria. C. with fewer than 40 C atoms are called nor-, seco- or apocarotenoids. 

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