Basil C. L. Weedon

Carotenoids and related compounds. Part XXX. Stereochemistry and synthesis of phytoene

Studies on models synthesised by stereochemically controlled routes have been used to elucidate the configurations of the central triene chromophores in the phytoenes (7,8,11,12,7′,8′,11′,12′-octahydro-ψ,ψ-carotenes). The common natural phytoene has been shown to be cis about the 15,15′-double bond only, and the main phytoene from Flavobacterium dehydrogenans to be the all-trans-isomer. Both these isomers have been synthesised, and their stereomutation has been studied.

Carotenoids and related compounds. Part XXXII. Synthesis of astaxanthin, phoenicoxanthin, hydroxyechinenone, and the corresponding diosphenols

Autoxidation of 4-oxo-carotenoids in the presence of potassium t-butoxide gives the corresponding diosphenols (3,4-didehydro-3-hydroxy-4-ones). These on reduction with potassium borohydride give the 3,4-dihydroxycompounds which, on carefully controlled oxidation, yield the 3-hydroxy-4-ones.Astaxanthin (3,3′-dihydroxy-ββ-carotene-4,4′-dione), phoenicoxanthin (3-hydroxy-β,β-carotene-4,4′-dione), and hydroxyechinenone (3-hydroxy-β,β-caroten-4-one) have been synthesised in this way from canthaxanthin (β,β-carotene-4,4′-dione), echinenone (β,β-caroten-4-one), and their 15,15′-acetylenic analogues.

Electro-organic reactions. Part V. Cathodic pinacolisation of α- and β-ionones

Controlled potential electrolysis at a mercury cathode of α- and β-ionone gives, in aqueous dimethylformamide, the corresponding pinacols in ca. 50% chemical yield. By electrolysis in acetonitrile with acetic acid as proton donor β-ionone is converted smoothly into the pinacol (70% isolated yield), and retinal is pinacolised in ca. 10% yield.

CAROTENOID RESEARCH ‐ PAST, PRESENT AND FUTURE

Various features of carotenoid research over the last 50 years are discussed, and attention is drawn to some areas where further significant advances are to be expected in the future.

Carotenoids and related compounds. Part XXIX. Stereochemistry and synthesis of the allenic end group. Absolute configuration of zeaxanthin

Three of the four racemates of the allenic ketone, 4-(2,4-dihydroxy-2,6,6-trimethylcyclohexylidene)but-3-en-2-one, have been synthesised and their configurations determined. Except for optical activity, one of these racemates is identical with the allenic ketone from the grasshopper Romalea microptera, and its mono-(secondary) acetate with a degradation product of fucoxanthin. The latter has been converted into an optically active allenic ketone identical with that from grasshoppers. The absolute configuration of the product was established by X-ray crystallographic analysis of its p-bromobenzoate.These results reveal that zeaxanthin (3,3′-dihydroxy-β-carotene) has the 3R,3′R-configuration.

Carotenoids and related compounds. Part XXXIII. Synthesis of dehydroflexixanthin and deoxyflexixanthin

The structures assigned to the two Flexibacter pigments flexixanthin (3,1′-dihydroxy-3′,4′-didehydro-1′,2′-dihydro-β,ψ-caroten-4-one) and deoxyflexixanthin (1′-hydroxy-3′,4′-didehydro-1′,2′-dihydro-β,ψ-caroten-4-one) have been confirmed by synthesis of the latter, and of dehydroflexixanthin (3,1′-dihydroxy-2,3,3′,4′-tetradehydro-1′,2′-dihydro-β,ψ-caroten-4-one), the autoxidation product of the former.

Electro-organic reactions. Part II. Mechanism of the kolbe electrolysis of substituted phenylacetate ions

For the anodic oxidation of phenylacetate and substituted phenylacetate ions changes in the distribution of products have been related to changes in electrochemical parameters, nuclear substitution, and the concentration of added perchlorate ions. The results support a reaction mechanism which involves the important adsorption of carboxylate perchlorate ions and according to which added anions and bulky substituents cause surface effects which significantly effect the competition between radical and carbenium ion pathways. For p-methoxyphenylacetate ions oxidation is probably initiated by electron transfer from the aromatic nucleus (pseudo-Kolbe reaction). From a steady state kinetic analysis expressions have been derived which relate product distribution with structural, electrochemical, and adsorption parameters.

Natural and synthetic pyrrol-2-ylpolyenes

By structural studies, and comparisons with synthetic models, the two main pigments of Wallemia sebi have been identified as 2,10-dimethyl-5-hydroxy-11-(pyrrol-2-yl)undeca-4,6,8,10-tetraen-3-one (11) and 2,2-dimethyl-5-[5-methyl-6-(pyrrol-2-yl)hexa-1,3,5-trienyl]furan-3(2H)-one (12). Geometrical isomers and chloro-derivatives of these pigments were also observed.

Anodic syntheses. Part XVI. Oxidation of phenylenediacetic acids and benzylic ethers

Electrolysis of o-, m-, and p-phenylenediacetic acids gives the diethers C6H4(CH2OMe)2 and the aldehydes CHO·C6H4·CH2·OMe, the latter resulting from anodic oxidation of the diethers. The ortho-diacid also affords a lactone. The anodic oxidation of ethers of this type to the corresponding aldehydes has been shown to be a general reaction of benzylic ethers having an electron-releasing substituent attached to the ring.