4 VITAMINS A AND CAROTENES 



II. Chemistry and Industrial Preparation 



NICHOLAS A. MILAS 



The chemistry of vitamin A and that of the closely related carotenoid 

 pigments which act as precursors of this vitamin has been the subject of 

 numerous investigations during the past three decades. Much information 

 of lasting value has accumulated, and an attempt will be made to summarize 

 it in the following sections. 



A. THE ANTIXEROPHTHALMIC PROVITAMINS 

 1. Introduction 



The provitamins A are carotenoid pigments^ which are converted to 

 vitamin A in vivo, usually in the intestinal tract of animals,- • ^ by some as 

 yet unknown mechanism, or in vitro by careful oxidative degradation.^"'' 

 Chemically, provitamin A carotenoids belong to a class of organic com- 

 pounds known as polyenes which are built up of isoprene units, the most 

 common of which contain eight such units or forty carbon atoms. Caroten- 

 oids are, as a rule, hydrocarbons, but some are found to contain hydroxyl, 

 carbonyl, and oxirane or epoxide groups. 



There are some well-knoAvn characteristics common to all provitamin 

 A carotenoids: (1) all are crystalline solid pigments; (2) in all of them one 

 finds the arrangement of isoprene units reversed in the center of the mole- 

 cule so that the methyl groups occupy the 1 ,6 instead of the 1 ,5 position, 

 and it is this arrangement which makes it possible for the cleavage to oc- 

 cur in the center, thereby producing vitamin A; (3) all contain a large 

 number of carbon-carbon double bonds, most of which are conjugated and 

 are thereby responsible for the production of color and the characteristic 

 absorption spectra of these molecules; (4) all contain at least one trimethyl- 

 cyclohexenyl or jS-ionone ring, usually in conjugation with the open carbon 

 chain double bonds; (5) although it is now well known that all naturally 

 occurring provitamin A carotenoids have the all-^rans configuration, they 



^ P. Karrer and E. Jucker, Carotenoids (trans, and revised by E. A. Braude). El- 

 sevier Publishing Co., New York, 1950. 



2 (a) E. L. Sexton, J. W. Mehl, and H. J. Deuel, Jr., ./. Nutrition 31, 299 (1946); 



(b) F. H. Mattson, J. W. Mehl, and H. J. Deuel, .Jr., Arch. Biochem. 15, 65 (19-47); 



(c) C. E. Wiese, J. W. Mehl, and H. J. Deuel, Jr., ibid., 15, 75 (1947); A. L. S. 

 Cheng and H. J. Deuel, Jr., .7. Nutrition 41, 619 (1950). 



3 S. Y. Thompson, J. Ganguly, and S. K. Kon, Brit. J. Nutrition 3, 50 (1949). 

 * R. F. Hunter and N. E. Williams, /. Chem. Soc. 1945, 554. 



6 Ct. C. L. Goss and W. D. McFarlane, Science 106, 375 (1947). 



6 N. L. Wendler, C. Rosenblum, and M. Tishler, J. Am. Chem. Soc. 72, 234 (1950). 



^ P. Meunier, J. Jouanneteau, and G. Zwingelstein, Compt. rend. 231, 1170 (1950). 



