274 CAROTENOID CARBOXYLIC ACIDS XIII 



Chemical Constitution of Crocetin 



The constitution of crocetin was elucidated by Karrer, Salomon and co- 

 workers^^- ^''' ^^. As a result of this work (and the investigations on bixin) a real 

 insight into the principles governing the structure of carotenoids was obtained 

 for the first time. 



CH3 ^^3 ^113 '-'H3 



HOOC-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-COOH 

 Crocetin 



The main features of the constitution of crocetin were determined by 

 Karrer and Salomon^"'. These authors recognised the polyene nature of the 

 pigment and the presence of a system of conjugated double bonds with side- 

 chain methyl groups, and identified perhydrocrocetin as an aliphatic, saturated, 

 dicarboxylic acid. The molecular formula C20H24O4 was proposed by Kuhn and 

 L'Orsa^"^ and confirmed by Karrer and co-workers. Karrer and Salomon^"^ 

 established the presence of 7 double bonds in the crocetin molecule by means 

 of catalytic hydrogenation and Kuhn and L'Orsa^"^ established the presence 

 of 4 side-chain methyl groups by chromic acid degradation. The latter authors 

 proposed an 'unsymmetrical' structure for crocetin, whereas Karrer and co- 

 workers^^" put forward the formula shown above, the correctness of which 

 was proved by the degradation of perhydrocrocetin to 6:ii-dimethylhexa- 

 decane-2 : 15-dione^i^ and by the total synthesis of perhydrocrocetin^^^. 

 Degradation of perhydrocrocetin to 6:ii-dimethylhexadecane-2:5-dione: 



CH3 CH3 CHj CH3 



HOOCCHCHaCHaCHaCHCHjCHaCHjCHaCHCHaCHjCHgCH • COOH 

 Perhydrocrocetin 



\ 

 CHo CHo CH3 CH3 



II II 



HOOCC- CHaCHjCHaCHCHaCHjCHjjCHaCHCHaCHaCHaC- COOH 



Bi Br 



CH3 CH3 CH3 CH3 



H3COOC • CCHgCHaCHaCHCHaCHaCHaCHaCHCHaCHaCHaCOOCHs 



OH OH 



a:a'-Dihydroxyperhydrocrocetin dimethyl ester 



References p. 290-294. 



