CROCETIN 



273 



Stereochemistry of Crocetin 



The first observations on stereoisomers of crocetin are due to Kuhn and 

 WiNTERSTEiN^"^ who obtained, besides the known crocetin dimethyl ester of 

 m.p. 222°, an isomer of m.p. 141°. The lower melting isomer is converted to the 

 higher-melting, stable ester on illumination. The isomerisation can also be 

 brought about by other means, e.g. heat, iodine catalysis, and via the dihydro- 

 derivative. Kuhn and co-workers regarded the isomerisation as a cis-trans 

 change. 



From the ease of conversion of the lower into the high-melting form, Kuhn 

 and WiNTERSTEiN concluded that the former is a cis and the latter a trans 

 isomer of crocetin. The following nomenclature has been proposed^"^. 



TABLE 52 



NOMENCLATURE OF CROCETINS 



New Nomenclature 



M.p. 



Configuration 



Old name 



Stable crocetin 



Stable crocetin monomethyl ester 

 Stable crocetin dimethyl ester 

 Labile crocetin dimethyl ester 



285° 

 218° 

 222° 

 141° 



trans 

 trans 

 trans 

 cis 



a-Crocetin 

 /3-Crocetin 

 y-Crocetin 

 Pigment of Kuhn 



Preparatio/i 



a) Crocin: According to Karrer and Salomon^^^, saffron is dried at 90° and 

 pre-extracted with ether. The material is then extracted with ethanol and oily 

 components are precipitated with ether. After further addition of ether, crocin 

 gradually separates out in a microcrystalline form. Appreciable amounts of oily 

 material are also obtained; these consist largely of crocin, but their crystallisation 

 is rather difficult because of the presence of resinous components and takes a long 

 time. By repeated dissolution of this oil in hot ethanol, seeding with crocin and 

 standing, several crops of crystals can be obtained which are combined and re- 

 crystallised from the same solvent. 



b) Crocetin^''* : 500 g of dried saffron is pre-extracted with ether. The residue is 

 dried in air and extracted with 70% ethanol. About half the solvent is evaporated 

 and the residual solution is strongly diluted with water. It is saponified with a 

 solution of 30 g of potassium hydroxide and 500 ml of water. After acidification 

 with hydrochloric acid, a thick yellow precipitate separates which is dried on porous 

 tile and subsequently saponified with 10% ethanolic potassium hydroxide. The 

 potassium salt of crocetin thus obtained is filtered and treated with acetic acid. 

 For further purification, crude crocetin is recrystallised from pyridine. 



With regard to the isolation of crocetin from blossom leaves of Crocus luteus, 

 see the communication of Kuhn and co-workers^°^. The isolation of crocetin di- 

 methyl ester from saffron is described by Karrer and Helfenstein^"^. 

 References p. 2go-2g4. 



Carotenoids 18 



