STRUCTURE OF THE VITAMINS A 691 



9.7 and 16.3% of C-methyl; this was demonstrated quantitatively by oxi- 

 dation with potassium permanganate and chromic acid, respectively. ''^ 

 Karrer et al^^ also proved that vitamin A is optically inactive; on hydro- 

 genation, it absorbs approximately 5 molecules of hydrogen (4.9), based 

 upon an assumed molecular weight of 286. These workers interpreted the 

 above data as affording evidence of two possible structural formulas for 

 \'itamin A. The formula which seemed the more probable, since it repre- 

 sents exactly one-half of the /3-carotene molecule, is given here. All sub- 

 sequent investigations have confii-med this structure, and it is generally ac- 

 cepted as the correct one for vitamin A (Ai). 



HiC CHj 



r C H 3 C H 3 



/\ hhIhhhIh 



HzC C'C:C'C:C"C:C'C:C' CHaOH 



I II 



H?C C 



\ / \ 



C CHj 



Hj 



N'itainiii A (AO alcohol 



Another observation which supports the general structural relationships 

 postulated above was made by Heilbron, ^Morton, and Webster. ^-^ These 

 workers found that vitamin A and a partial cyclization product produced 

 by treating vitamin A with mineral acids^^** are both transformed to 1,6- 

 dimethylnaphthalene on dehydrogenation with selenium. This reaction is 

 interpreted as giving definite proof of the terpenoid nature of the vitamin, 

 and is believed to afford confirmation of the accepted structure. 



Considerable support for the above structure of vitamin A is furnished by 

 absorption spectrum data; this evidence has indicated that the molecule 

 contains 5 double bonds and not 6 unsaturated linkages as given in the 

 second formula suggested by Karrer et al."^^ Von Euler and associates^^^ 

 found that when the absorption spectrum of vitamin A is compared with 

 those of octatrienol, decatetraenol, dihydrocrocetin, and dihydrobixin 

 (which contain 3, 4, 6, and 8 double bonds, respectively), it shows an ab- 

 sorption band intermediate between decatetraenol and dihydrocrocetin; 

 thus it seems to be evident that vitamin A has 5 and not 6 double bonds. 



The clinching proof of the ('20 skeleton for vitamin A is based upon the 

 establishment of the identity of synthetic perhydrovitamin A with a prod- 

 uct prepared by the hydrogenation of the natural vitamin A.^^ Karrer 



i"I. M. Heill)i()!i, R. A. Morton, mikI K. T. Wclister, Binrhem. ./., 26, 1194-1196 

 (1932). 



'28 J. R. Edisbuiv, A. E. Gillani, I. M. HtMli)roii, and R. A. Morton, Biochem. ./., 26, 

 1164-1173(1932). " 



129 H. V. EultT, P. Karrer, E. Klnssniann, .•uid I{. Morf, Helv. Chim. Acta, 15. 502-507 

 (1932). 



