808 IX. THE VITAMIN E GROUP 



The differences between a- and /3-tocopherol can readily be proved by 

 comparing the products formed on pyrolysis and as a result of chromic acid 

 oxidation. In the case of /3-tocopherol, the main product is trimethylhy- 

 droquinone*^^^ ~*^ or i/'-cumoquinol (XIV) , instead of duroquinone. When 

 pyrolysis occurs in the presence of hydrogen iodide, p-xylenol (XV) is 

 formed in place of trimethylhydroquinone. 



CH3 CHj 



/^\ /■\ 



HOC CCH, HOC CH 



HC. COH "' HC, CH 



\/ \/ 



CHj CH3 



Pyrolysis of |S-Tocopherol 



On the other hand, treatment with chromic acid produces the same C21 

 lactone (VIII) which had been obtained with a-tocopherol.^^ These data 

 would seem to indicate that |8- and a-tocopherols are identical, except that 

 only 2 methyl groups instead of 3 are attached to the aromatic portion of 

 the chroman nucleus. Since these are in the paraposition to each other, 

 they are obviously in positions 5 and 8. The final proof of the structure of 

 /3-tocopherol has been obtained by synthesis.^^-*^ 



{3) y-Tocopherol 



7-Tocopherol, or 7,8-dimethyltocol, has the same empirical formula as 

 /3-tocopherol, namely C28H48O2. The structural formula which has been 

 proved by synthesis is shown in (XVI). On pyrolysis, the same trimethyl- 

 hydroquinone (XIV) originates as with /3-tocopherol. The same trimethyl- 

 hydroquinone would originate from 5,8-, 5,7, or 7,8-dimethyltocol. 



"^f ^ T' CH3 CH3 CH3 



H,CCn C C— CHjCH2CH,CHCH2CHjCHjCHCHj.CHjCHjCHCHj 



C^ CH,, 



CH3 



XE[ 



The fact that 7-tocopherol represents the 7,8-dimethyltocol rather than 

 5,7-dimethyltocol can best be proved by synthesis. Thus, when o-xylo- 

 hydroquinone is used as the starting material in the synthesis of Karrer 

 and Fritzsche,^-'*^ 7-tocopherol is the isomer which results. Since the 

 methyl groups are on adjacent carbons in o-xylohydroquinone, they can be 

 only in positions 7 and 8. 



8^ F. Bergel, A. R. Todd, and T. S. Work, J. Soc. Chem. Ind., 56, 1054 (1937). 

 8» P. Karrer and H. Fritzsche, Helv. Chim. Acta, 22, 260-263 (1939). 



