186 



ASCORBIC ACID 



by Micheel when he demonstrated that the di-p-nitrobenzoate of the 

 dimethyl ether of ascorbic acid furnished, upon ozonization, oxahc acid 

 and di-p-nitrobenzoyl-L-threonic acid.^^ The acid character was traced to 

 the dienoUc system which was present in a ring.^^' ^^ Two enolic hydroxyl 

 groups were involved in this system because they disappeared upon oxida- 

 tion of ascorbic acid to dehydroascorbic acid and they underwent methyla- 

 tion with diazomethane. The two enolic hydroxyl groups located on carbon 

 atoms 2 and 3 do not display the same acidity, for a monomethyl ether 

 could readily be isolated by titration of L-ascorbic acid with ethereal di- 

 azomethane.^®- '*^ The postulation of formula I for ascorbic acid, showing it 



CO— I 



I 

 HO— C 



II 

 HO— C 



I 

 H— CO— ' 



I 

 HO— C— H 



CH2OH 

 I 



CO— 1 



I 



CO 



I 



CO 



I 



H— CO — 

 I 

 HO— C— H 



CHjOH 

 II 



COOH 



C— OH 



II 

 C— OH 



I 

 COOH 



VII 



HO 90^ 

 HO— C 



HO io_l 



I 

 HO— C— H 



CH2OH 

 III 



COOH 

 I 

 COOH 



COOH 

 I 

 H— C— OH 

 I 

 HO— C— H 

 I 

 CH2OH 



IV 



COOH 



I 

 CO 



I 



CO 



I 



COOH 

 Vila 



COOH 

 I 

 H— C— OH 

 I 

 HO— C— H 

 1 

 ^ COOH 



V 



"^ COOH 



I 

 H— C— OMe 

 I 

 MeO— C— H 

 I 

 CHjOMe 



VI 



to be a member of the l series, then became possible. ^^ Support for this 

 theory was forthcoming from parallel experiments with dihydroxymaleic 

 acid (VII). The latter displayed light absorption properties similar to ascor- 

 bic acid, and, moreover, it was found to undergo reversible oxidation with 

 iodine to give the dehydro compound (Vila) from which VII could be 

 regenerated by reduction with hydrogen sulfide. ^- 



The tentative structure assigned to ascorbic acid (I) and the 7-lactone 

 structure postulated for its first oxidation product, dehydroascorbic acid 

 (II), were also in accord with the fact that the latter readily formed hy- 



^« T. Reichstein, A. Griissner, and R. Oppenauer, Helv. Chim. Acta 17, 510 (1934); 



W. N. Haworth and E. L. Hirst, ibid. 17, 520 (1934). 

 4' W. N. Haworth, I']. L. Hirst, and F. Smith, J. Chem. Soc. 1934, 1556. 



