CONVERSION OF CAROTENOIDS INTO VITAMIN A 



two a-ionone residues. ^ ^ Alteration of the side chain such as occurs 

 in [3-dihydro-carotene ^ ^ destroys vitamin A activity. 



M? M? Me 



— V° I . i 



He 



Me Me Me 



Me / ■■> 



/ ''^y\AWv\^AyV^^ /""^JUaW^^^ 



y-Me 

 He 



P-carotenone 



— /-Me Me Me Me 



Me 



P-hydroxy-carotene 



Me Mp M« 



Isocaroter.e 



p-d i hyd ro-carotene 



Recent work by Karrer and his associates on the production of caro- 

 tenoid epoxides, has led to the necessity of modifying somewhat the 

 statement than an unsubstituted ^-ionone residue is necessary for 

 activity because 5 : 6-diepoxides, e.g., (3-carotene-5 : 6-5' : 6'-diepoxide 

 are vitamin A precursors ; ^ ^ the body can presumably convert the 

 epoxide into [3-carotene. 



Me 



Me 



Me-y- 



Me 



Me 



Me 



3-carotene-5 : 6-5' : 6' diepoxide 



However, when 5 : 6-epoxides are isomerized to 5 : 8-epoxides such 

 compounds, e.g., aurochrome, are inactive. ^^ The presence of a 

 furanoid grouping does not interfere with activity of a molecule 

 which contains in addition an unoxidized p-ionone or a 5 : 6-epoxy 

 group, for example, mutatochrome ^ ^' ^ ° and luteochrome ^ ' are 

 active : see next page. 



The work on the apo-carotenes revealed another interesting fact 

 that, from the point of view of vitamin A activity, it matters little 

 whether the terminal group is aldehydic, carboxy, or carbonyl. This 

 ability of the body to deal with varying terminal groups has been 

 further demonstrated recently with derivatives of vitamin A itself: 



271 



