Acetylenic carotenoids

Structural elements such as allenic or acetylenic bonds, epoxydes, fnran-oxides, and C45 or C50 carotenoids are not found. 

T. Bjornland, A. Fiksdahl, T. Skjetne, J. Krane and S. Liaaen-Jensen, Gyroxanthin — the first allenic acetylenic carotenoid. Tetrahedron 56 (2000) 9047-9056. 

The transformations of compounds which are precursors for vitamin A and carotenoids have a special position among the rearrangements of the conjugated polyenes. Numerous isomerizations such as cw-fraws-isomerization, the dehydration of polyunsaturated acetylenic carbinols etc. were utilized to prepare the various carotenoides (e.g. /1-carotene, lycopene, cryptoxanthin, zeaxanthin) (for reviews, see References 146 and 147). However, one of these rearrangements turned out to be a considerable hindrance for the synthesis of target products. 

Other algal carotenoids contain acetylenic triple bonds. For example, alloxanthin has the following structure at both ends of the symmetric molecule. The symmetric carotenoids canthaxanthin and astaxan-thin have oxo groups at both ends ... 

The special potential for constructing double bonds stereoselectively, often necessary in natural material syntheses, makes the Wittig reaction a valuable alternative compared to partial hydrogenation of acetylenes. It is used in the synthesis of carotenoids, fragrance and aroma compounds, terpenes, steroides, hormones, prostaglandins, pheromones, fatty acid derivatives, plant substances, and a variety of other olefinic naturally occurring compounds. Because of the considerable volume of this topic we would like to consider only selected paths of the synthesis of natural compounds in the following sections and to restrict it to reactions of phosphoranes (ylides) only. 

The work is based on the idea of W. Reppe of applying the acetylene chemistry developed by him to the synthesis of terpenes. The 2-methylbutyn-2-ol (40, see page 14) formed by the addition of acetylene to acetone (39) was intended, as the C5 building block, as the starting point for terpene syntheses. A C5 building block appeared to be small enough to ensure the required flexibility for terpenoid vitamins and carotenoids and the extensive area of terpenoid flavors and fragrances. 

Several of the carotenoids are now commercially synthesized and used as food colors. A possible method of synthesis is described by Borenstein and Bunnell (1967). Beta-ionone is obtained from lemon grass oil and converted into a C14 aldehyde. The C14 aldehyde is changed to a C16 aldehyde, then to a C19 aldehyde. Two moles of the C19 aldehyde are condensed with acetylene dimagnesium bromide and, after a series of reactions, yield p-carotene. 

Other Degraded Carotenoids. The acetylenic diol (115), prepared by reaction of but-3-yn-2-ol dianion with 2,6,6-trimethyl-4,4-ethylenedioxycyclohex-2-en-l-one (116), afforded 3,5,5-trimethyl-4-(2-butenylidene)-cyclohex-2-en-l-one (117), a major constituent of Burley tobacco, on LiAlH4 reduction and hydrolysis.53... 

The steric stability of acetylenic carotenoids has been investigated. " Stereoisomerization of a -trans- and 9,9 -di-cw-alloxanthin [7,8,7, 8 -tetra-dehydro-jS,/S-carotene-3,3 -diol (94)] and all-tran5-7,8,7, 8 -tetradehydro-astaxanthin (49) in the presence of I2 gave mainly the 9,9 -di-cw- and 9-mono-c/s-isomers, with none of the all-trans-form present in the pseudo-equilibrium mixture. All-trans-7,8-didehydroastaxanthin (48), however, gave a mixture of the 9-mono-c/s- and all-trans-isomers. 

The end groups of acetylenic carotenoids like alloxanthin (8), found in algae and marine organisms, are structurally related to the end groups of fucoxanthin (9), the most abundant natural carotenoid . The allene and acetylene bonds are known to be biogenetically linked in polyacetylenes and the same seems likely to apply to... 

When asterinic acid was carefully isolated via its caroteno-protein, the only carotenoid obtained was 7,7, 8,8 -tetradehydroastaxanthin (36). As predicted in last year s report heteroxanthin (37) is an acetylenic carotenoid. Further spectral data confirmed this structure. ... 

Carotenoid Reactions.—Zeaxanthin (45) has previously been synthesized by Ci9 + C2 + Cl9 and C14 + C12 + C14 routes. A new versatile route has now been developed using a Cl5 + Cjo + Ci5 principle. Details are outlined in Scheme 3. Possibly the most notable features are the directed reduction of the acetylene to give a trans-double-bond, and the use of 1,2-epoxybutane for the... 

Acetylenic Grignard reagents are effective for reaction with conjugated compounds such as -ionone, which is an important starting material for the preparation of carotenoids that readily undergoes enolation under basic conditions. Some examples of synthetic processes are given [Eqs. (30-33) 46-49]. 

Method of manufacture all industrial processes for preparing carotenoids are based on P-ionone. This material can be obtained by total synthesis from acetone and acetylene via dehydrolinalol. The commercially available material is usually extended on a matrix such as acacia or maltodex-trin. These extended forms of beta-carotene are dispersible in aqueous systems. Beta-carotene is also available as micronized crystals suspended in an edible oil such as peanut oil. 

Carotenoids can also contain additional isoprene chains, homocarotenoids, or if less than 40 carbon atoms, apocarotenoids (Formulae 9.10 and 9.11). In some carotenoids allenic or acetylenic groups are found. 

Carotenoids and Polyterpenoids (Chapter 5).—The absolute configuration of a-carotene has been estabhshed as R. The list of acetylenic, allenic, and iso-prenylated (C45 and C50) carotenoids grows. A number of biologically important terpenoids of varying chain length appear to be degradation products of carotenoids. Notable among them is abscisic acid which has been chemically interrelated with violaxanthin and efficiently synthesised by oxidation of a-ionone. 

Other Degraded Carotenoids. Several syntheses of ionones and related compounds have been described. Some of these may prove useful for end-group construction in carotenoid synthesis. Thus 4-keto-j8-ionone (126) and 3,4-dehydro-jS-ionone (127) have been prepared by condensation of the sulphone (128) with propylene oxide, followed by elimination of phenylsulphinic acid and either oxidation or dehydration. A novel method utilizes a thermal acetylenic oxy-Cope rearrangement process to prepare the ionone compounds (129—131 R = Me) and analogues (R = Et or Pr ) from cyclohex-2-enylprop-2rynols (132). 

---