II. CHEMISTRY AXD INDUSTRIAL PREPARATION 



17 



of the various isomers are different, they can be separated chromatographi- 

 eally. 



The I'hange from an all-/m/is to a cis-trans configuration of a provitamin 

 A molecule usually causes a decrease in the biological potency of the mole- 

 cule. This may be interpreted as being due to the change of shape of the 

 molecule which may not fit into the biological systems that cause the break- 

 down of the provitamin A into vitamin A. Usually the all-^rans forms of 

 prox'itamins A show a higher biological potency than their geometrical 





300 



400 



500 



Wavelength, m// 



Fig. 3. Molecular extinction curves of 7-carotene in hexane: , fresh solution 



of the all-trans form; , mixture of stereoisomers after refluxing in the dark 



for 45 minutes, , mixture of stereoisomers after iodine catalj^sis in light at room 



temperature. [From J. Am. Chem. Soc. 65, 1941 (1943).] 



isomers. Table VI gives the relative biological potencies of some of the 

 well-known stereoisomeric provitamin A carotenoids. 



4. Biologically Active Derivatives of Provitamins A 



Provitamin A carotenoids can, under specified conditions, be converted 

 into certain derivatives which retain their biological activity. For example, 

 a- and jS-carotenes react with cold concentrated hydrogen iodide to give, 

 among other products, 5,G-dihydro-a- and /3-carotenes.'**' In this case one 

 of the trimethylcyclohexenyl rings has become fully hydrogenated. With 

 monoperphthalic acid''^ ■ ''^ both a- and /S-carotenes form monoepoxides 



*o A. Polgdr and L. Zechmcistcr, ./. Am. Chem. Soc. 65, 1528 (1943). 

 ^1 P. Karrcr and coworkers, Helv. Chim. Acta 28, 300, 427, 471, 474, 717, 1143, 1156 

 (1945); 30, 531, 5.36 (1947). 



