52 VITAMINS A AND CAROTENES 



H3C CH3 ^jj^ (.jj^ 



/ I I 



-CH=CH— C=CH— CH=CH— C=CH— CHoOH 



-CH3 



XCVIII Vitamin A, 



suggested ^^'* to account for the shift of the absorption spectrum of vitamin 

 A2 toward longer wavelengths. A vitamin A of this structure has now been 

 synthesized^*^ following the routes outhned in Fig. 7, except that 3-methy]- 

 heptadien-4,6-yn-l-ol-3 (XCIX) and 3-methylheptadien-3,5-yn-l-ol-7 (C) 

 were used instead of LXIX and LXVI. Although the pure C22-vitamin A 

 has its main maximum at the same position as vitamin A2, the second maxi- 

 mum at 287 mju is absent, and the maximum of the antimony trichloride 

 color occurs at 642 m/x instead of at 693 m/z, the maximum of vitamin A2. 

 From a study of the relative volatilities of natural vitamin Ai and vitamin 

 A2, Gray and Cawley^^^ found that the two vitamins must have nearly the 

 same molecular weight and suggested the simple dehydrovitamin Ai struc- 

 ture, XCVIII, for vitamin A2. This was later supported by Morton et al.,^^^ 

 although Karrer and his students^^^ supported the open-chain structure, 

 XCVII, on the basis of their ozonolysis experiments which yielded acetone 

 in appreciable amounts. The open-chain structure was also supported by 

 Meunier,^^^ who claims to have obtained from the oxidation of lycopene 

 (see Table IV, p. 12) with manganese dioxide an aldehyde which had 

 growth-promoting properties. On ozonolysis of a purer sample of vitamin 

 A2, Karrer and Schneider^*^ subsequently failed to obtain acetone and there- 

 fore supported structure XCVIII. This structure was finally confirmed 

 by synthesis"^ and by the preparation from the synthetic vitamin of the 

 various derivatives shown in Table XIV. 



Starting from dehydro-/3-ionone (CI)^^^ and building up the side chain 



1" M. K. Salah and R. A. Morton, Biochem. J. 43, LVI (1948) ; also, private communi- 

 cation to E. R. H. Jones by R. A. Morton in ref. 173. 



'88 N. A. Milas and D. D. Grassetti, Paper presented before the 2nd Intern. Congr. 

 Biochem., Paris. July, 1952. 



18S E. M. Shantz and J. H. Brinkman, J. Biol. Chem. 183, 467 (1950). 



"0 G. Wald, Science 109, 482 (1949). 



'" G. Wald, Biochem. and Biophys. Acta 4, 215 (1950). 



•92 E. LeB. Gray and J. D. Cawley, J. Biol. Chem. 131, 317 (1939) ; 134, 397 (1940). 



153 R. A. Morton, M. K. Salah, and A. L. Stubbs, Nature 159, 744 (1947). 



19^ P. Karrer, A. Geiger, and E. Bretscher, Helv. Chim. Acta 24, 161 E (1941); 26, 

 1758 (1943). 



185 L. Fieser, J. Org. Chem. 15, 930 (1950). 



196 p Meunier, Rev. intern, vitaminol. 23, 21 (1951). 



1" H. B. Henbest, J. Chem. Soc. 1951, 1074. 



