Apocarotenoids structure

N.m.r. correlation (270 MHz) with synthetic products has confirmed that the citrus C30 pigments such as 8 -apo-/3-caroten-8 -al have the proposed unsym-metrical C20-C10 apocarotenoid structures (15) and not the alternative symmetrical C15-C15 diapocarotenoid structure (18). 

Plant apocarotenoids have a wide variety of structures and functions. As expected, there is a small gene family of CCDs with different cleavage sites and somewhat promiscuous substrate selection. Some CCDs are stereo-specific, for example, 9-cis epoxycarotenoids are the substrates for NCEDs (9-cis expoxy dioxygenases) that produce the precursor of ABA biosynthesis, xanthoxin. Both linear carotenoids (lycopene) and cyclic carotenoids are substrates for cleavage at various double bonds including the central 15-15 and eccentric 5-6, 7-8, 9-10, 9 -10, and 11-12 bonds. Some CCDs cleave both linear and cyclic carotenoids and may cleave the same molecule twice, e.g., both 9-10 and 9 -10 positions. 

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). 

Some carotenoids have structures containing fewer than 40 carbon atoms and derived formally by loss of part of the C40 skeleton. These compounds are referred to as apocarotenoids when carbon atoms have been lost from the ends of the molecule or as norcarotenoids when carbon atoms have been lost formally from within the chain. These modifications are caused by oxidative degradation at the level of the terminal rings either by nonspecific mechanisms (lipoxygenase, photo-oxidation) or by... 

Figure 11.8 Structures of c/s-bixin and norbixin, apocarotenoid pigments in annatto.

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. 

With their extended system of conjugated double bonds, the carotenoids contain a reactive electron-rich system that is susceptible to reactions with electrophilic compounds. This structure is responsible for high sensitivity of carotenoids to oxygen and light. The central chain of conjugated double bonds can be oxidatively cleaved at various points, giving rise to the family of apocarotenoids. 

New Structures and Stereochemistry Bicyclic Carotenoids Monocyclic Carotenoids Acyclic Carotenoids Apocarotenoids Degraded Carotenoids Synthesis and Reactions Carotenoids Retinoids... 

The unsymmetrical structure of apocarotenoids poses particular technical and economic challenges. It is remarkable that these compounds can be constructed economically in linear syntheses from the appropriate building blocks. The viability of the process has therefore to be attributed to the fact that nearly all of the reaction steps are shared with and can be integrated into other production processes. However, for the apocarotenoid syntheses, additional Cj-phosphonium salts and Cs-phosphonate esters are required. A multitude of different synthetic routes have been described. [61] Only the more recent developments are discussed here. 

Many compounds with fewer than 40 carbon atoms, but with carotenoid-like structures, are found in nature. Synthesis of these compounds, sometimes called apocarotenoids, appears to occur mainly by catabolism of carotenoids (Parry and Horgan, 1991). Apocarotenoids in the range of C9-C13 are found in plant essential oils, but others, ranging up to C30 compounds, are essentially nonvolatile. However, knowledge of the biochemistry of carotenoid metabolism is limited (Parry and Horgan, 1991). In vitro, photooxidation of carot-... 

Order according to the handbook systematic names in brackets, for structural comparison see Figs. 7.35 and 7.36 degradation products (apocarotenoids) are characterized by the prefix apo (see also Fig. 7.37). 

Davies (1976) has reviewed the new system of nomenclature of carotenoids. Figure 1 shows the basic structure and numbering system for (3-, a-, and y-carotenes and for cryptoxanthin, the major carotenoids contributing to the vitamin A activity in animal and human foods. Apocarotenoids are compounds that have been shortened by removal of at least one end of the molecule beyond a designated location. In general, they have less biological activity than P-carotene (see Table I). 

On the basis of structure alone, Simpson and Chichester (1981) estimate that 50-60 carotenoids and apocarotenoid compounds could have provitamin A activity. Only a few of the identified carotenoids, however, have both vitamin A activity and occur in significant amounts in natural foods as commonly eaten by vertebrate animals (Bauemfeind, 1972). Of these, p-carotene has by far the most provitamin A activity. The a- and y-carotenoids and cryptoxanthin (3-hy-droxy-P-carotene), also found in substantial amounts in commonly consumed foods, have about one-half the activity of the P form. Table I contains a list of the provitamin A activity found in the most important mammalian food sources of carotenoids. Table II lists vitamin A-inactive carotenoids commonly found in food. 

The main formation pathway of C9 apocarotenoids with cyclohexanone and cyclohexenone structures is conversion of hydroperoxides derived from P-damascol (Figure 9.29). Hydroperoxides generated by autoxidation of carotenoids are further oxidised, reduced and hydrated to form a variety of different structures. The most important compound of this apocarotenoid... 

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