CHAPTER VIII 

 The synthesis of carotenoids 



In spite of many attempts, no total synthesis of a natural carotenoid has 

 been achieved up to the present time. Kuhn and his co-workers^ have synthe- 

 sized many carotenoid-like diphenylpolyenes and polyene dicarboxylic acids 

 and thus provided valuable material for the comparison of structure and colour 

 in polyenes. Karrer and his co-workers have synthesized the perhydro- 

 derivatives of three natural carotenoids and thus established the constitution 

 of the natural pigments. The compounds involved are perhydro-lycopene^, 

 perhydro-norbixin^ and perhydro-crocetin*. 



The first conversion of one natural carotenoid into another was achieved 

 by Karrer and Solmssen^ who reduced dihydrorhodoxanthin to zeaxanthin 

 by means of aluminium zsopropoxide and zsopropyl alcohol (p. 182). Partial 

 syntheses of natural carotenoids are also represented by the oxidative degra- 

 dation of zeaxanthin and xanthophyll to /?-citraurin* and by the conversion of 

 lycopene into norbixin^. By the action of N-bromsuccinimide on lycopene, 

 Karrer and Rutschmann obtained dehydrolycopene, a carotenoid pigment 

 with 15 conjugated double bonds (cf. p. 121). 



Recently, Karrer and Jucker" have succeeded in converting natural 

 carotenoids containing isolated double bonds into other naturally occurring 

 pigments. Thus, a-carotene can be converted into /5-carotene by the action of 

 sodium ethoxide at elevated temperatures, and similarly, xanthophyll can be 

 converted into zeaxanthin. These conversions are of interest in showing that the 

 isolated double bonds can be brought into conjugation. 



Thus, until recently, only very few natural carotenoids had been partially 

 synthesized. Within the last few years, however, Karrer and Jucker^ 

 succeeded in preparing about 20 carotenoids by the introduction of oxygen 

 into different carotenoid pigments by means of monoperphthalic acid. Some 

 of these carotenoids were known to occur in nature, but their constitution had 

 previously been unknown. 



The addition of oxygen by means of perbenzoic acid was employed by 



* Cf. p. 184. Later L. Zechmeister and L. v. Cholnoky obtained |3-citraurin by the 

 hydrol3rtic fission of capsanthin, cf. p. 248. 



References p. 64-65. 



