692 VII. VITAMINS A 



and Morf^^ used, as a starting material for the synthesis of perhydrovita- 

 min A, /3-ionylidene ethylacetate,i^° which Karrer et aZ.^" had previously 

 prepared from ;S-ionone and ethyl bromoacetate. After hydrogenation of 

 /3-ionylidene ethylacetate, the ester was changed to the alcohol by the 

 Bouveault-Blanc reduction. This alcohol was then converted to the bro- 

 mide, which in turn was condensed with diethyl malonate to form a dicar- 

 boxylic acid ester; the latter was readily changed into a d? monocarboxylic 

 acid. When the corresponding acid chloride was treated with zinc methyl 

 chloride, a Cis ketone was formed. By treatment of the ketone with ethyl 

 bromoacetate and subsequent reduction of the ester, perhydrovitamin A 

 was finally formed. 



HjC CHj 



C CHj CH3 



/xhhhIhhhIh 



HzC C-C-C'C'C-C'C'C-C- CHsOH 



I |HHHHHHHH 



HzC CH 



\ / \ 

 C CH3 



Hz 



Perhydrovitamin A 



The synthetic alcohol and the product obtained by the hydrogenation of 

 the vitamin A concentrate are both viscous oils. They do not yield solid 

 derivatives which will permit a definite proof of identity. However, the 

 evidence was obtained by conversion of both products to a C23 ketone ; the 

 semicarbazones prepared from the ketones obtained from the two sources 

 were identical, as indicated by their melting points (67 °C.) and other 

 properties. These various experimental data would seem to leave no 

 doubt that vitamin A (Ai) alcohol has the structure represented earlier. 



(2) Vitamin Ai 



The elucidation of the structure of vitamin A2 has been difficult because 

 of the impossibility of obtaining it in crystalline form or of separating it 

 from the usual form of vitamin A by chromatography, or by molecular dis- 

 tillation; however, certain reactions are known which give us some informa- 

 tion as to its structure. In the first place, it presumably has a close rela- 

 tionship to vitamin Ai, as it produces about the same amount of geronic 

 acid on ozonolysis as does halibut oil having a similar blue value; this 

 would indicate the presence of like amounts of /S-ionone. The shift in the 

 absorption maximum toward the visible spectrum in the case of vitamin 

 A2, i.e., 328 m/Li to 340-345 vciji,''^ is interpreted to mean that vitamin A2 

 possesses 6 double bonds in contrast to the 5 unsaturated linkages in vitamin 

 A:. 



130 P. Karrer, R. Morf, and K. Schopp, Helv. Chim. Acta, 16, 557-561 (1933). 



131 P. Karrer, H. Salomon, R. Morf, and O. Walker, Helv. Chim. Acta, 15, 878-889 

 (1932). 



