638 



VI. CAROTENOIDS AND RELATED COMPOUNDS 



oxygen is avoided by insolation in closed quartz flasks where the air has 

 been replaced by carbon dioxide. 



f . Properties of Carotenoids Containing cis Bonds, (a) General Proper- 

 ties. The stereoisomeric configurations resulting from a spatial rear- 

 rangement of the double bonds of the all-trans carotenoids are characterized 

 by a markedly lower color intensity and an increased solubility. Although 

 such poly-as compounds as prolycopene and pro-7-carotene crystallize 

 readily, in general the cis isomers are less readily crystallizable than the 

 alI-/7-an.9 isomers. The melting points of the cis forms are considerably 

 lower than are those of the corresponding a\\-trans forms. Adsorption on 

 the chromatographic column shows a considerable variation from the all- 

 trans form. This property is of extreme importance, as it not only affords a 

 means of characterizing a stereochemical mixture, but also offers an excel- 

 lent method for the separation of the various components and for their 

 preparation in pure fonn. Some of the isomers, such as prolycopene and 

 pro-7-carotene, apparently have an increased thermal stability as com- 

 pared with the sdl-trans form, when their solutions are refluxed. One 

 excellent procedure for following stereochemical changes in the molecules 

 that exhibit optical activity is by alteration of their specific rotation. The 

 initial rotation may be changed in either direction if their molecules are 

 bent. Examples of this change are given in Table 28. 



Table 28 

 Comparative Optical Rotation of Ahh-trans Forms and cis Isomers of Some 



Carotenoids" 



« L. Zechmeister, Chem. Revs., 34, 267-344 (1944), p. 294. 



" L. Zechmeister and A. Polgdr, /. Am.. Chem. Soc, 66, 137-144 (1944). 



■= L. Zechmeister and W. A. Schroeder, J. Am. Chem. Soc, 65, 1535-1540 (1943). 



•^ L. Zechmeister, L. v. Cholnoky, and A. Polgdr, Ber., 73, 1678-1685 (1939). 



• L. Zechmeister and L. v. Cholnoky, Ann., 543, 248-257 (1940). 



(6) Spectral Characteristics as Affected hy trans-cis Isomerism. In all 

 cases, the extinction maxima for the all-/mns compounds are at longer wave 

 lengths than are those for the cis isomers. One of the most marked altera- 



J 



