Allenic carotenoids synthesis

Neoxanthin, a precursor of the plant hormone abscisic acid, is an allenic xantho-phyll recognized as the last product of carotenoid synthesis in green plants. A cDNA for neoxanthin synthase (NSY) was isolated from tomato cv. Philippino using a molecular approach based on the mechanistic and structural similarities of NSY to two other closely related carotenogenic enzymes, lycopene cyclase (LCY) and capsanthin-capsorubin synthase (CCS) (Bouvier et al., 2000). 

The synthesis of fucoxanthin (121), reported by Ito is clearly a highlight in the field of carotenoid synthesis [32-74], The carotenoid was synthesized according to the strategy Cis + Cio + Ci5 = C40 with the Cio-dialdehyde 45 as central unit. As the allenic precursor 103 for the Ci5-end group 122 had previously been prepared for the synthesis of peridinin (108) the emphasis was on the preparation of the 3-hydroxy-8-oxo-p end group 123. 

The acetylenic diphosphonate 73 is easily obtained by reaction of dibromide 4 with two equivalents of triethylphosphite under Arbusov conditions [5]. The specific advantages of the diphosphonate 13 have been demonstrated in the field of the synthesis of allenic carotenoids [10]. 

A representative allenic carotenoid is mimulaxanthin (202), a symmetrical structure with allenic end groups. For the total synthesis of mimulaxanthin (202), the most suitable candidate starting compound is the grasshopper ketone (11a) which has been synthesized by several groups [7-13]. 

No allenic carotenoids have yet been prepared by total synthesis, but the allenic ketones (55) and (56) have been synthesized as racemates (92). In a recent preliminary note the total synthesis of optically inactive 2,6-trans-2, 6 -trans decaprenoxanthin has been outlined. Natural deca-prenoxanthin (8) has 2/ ,6R,2 R,6 7 -configuration (11) with the 2,6(2, 6 )-substituents in a cis relationship. 

By using the same method, Katsumura and coworkers isolated the enantiomerically pure allene segment in 97a and 97b (Scheme 26)". The one-step singlet oxygen ene reaction pathway, to produce the allene moiety from the twisted l,3,3-trimethylcyclohexenyl-2-vinyl derivatives, was considered as a possible biomimetic route towards the synthesis of carotenoids. 

SCEIEME 26. Synthesis of the allene moiety of carotenoids via biomimetic photosensitized oxygenation... 

The synthesis of 6 -ep/-peridinin (31), the allene epimer of the carotenoid butenohde peridinin, is representative of the state-of-the-art in this field (Scheme 54.55 xhe new strategy developed by... 

Propynols are useful synthons in the synthesis of carotenoids and may be prepared by the zinc-mediated condensation of aldehydes and ketones with propargylic bromides. However, this condensation also produces isomeric allenic byproducts, as is illustrated by the condensation of 3-ionone (3) with propargyl bromide (63) x the presence of zinc to give 64 and 65 [35] (Scheme 14). Suppression of such allenic byproducts is possible by replacing zinc with magnesium or aluminium in the presence of mercuric salts [35,36],... 

Part IV Synthesis of Acetylenic, Allenic and In-chain Substituted Carotenoids... 

In some important examples of naturally occurring carotenoids, the polyene chain is modified by the presence of one or two acetylenic or allenic groups. These and other interesting features are illustrated by peridinin (558) and pyrrhoxanthin (556) which contain allenic and acetylenic groups, respectively, as well as unusual modifications such as a C37-skeleton with an abnormal arrangement of side-chain methyl groups, and the presence of a butenolide structure. The synthesis of such carotenoids, particularly in the natural optically active form, provides a major challenge and the syntheses that have been developed are described in this Chapter. 

Murakami, Y, Nakano, M., Shimofusa, T, Furuichi, N and Katsumura, S. (2005) Total synthesis of paracentrone, Cji-allenic opo-carotenoid. Org. Biomd. Chem., 3,1372—1374. 

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