210 



ASCORBIC ACID 



COOH 



CO 



CHOH 



CHOH 



COOR 



CO 



CHOH 



COOH 



i 



co- 



HO— C 



HO— C 



CHO- 



Ri 



Ri 



II 



Ri 

 III 



these procedures are not so valuable from a commercial point of view as 

 those utilizing L-sorbose. The latter, once a rare sugar, can be efficiently 

 produced on a large scale by the bacterial oxidation of n-glucitol (sor- 

 bitol),'^- ^^ which is readily obtainable by the catalytic hydrogenation of 

 D-glucose. 



Attention has therefore been focused on the oxidation of L-sorbose to 

 2-keto-L-gulonic acid. This transformation can be brought about directly 

 with nitric acid'^''' or with other oxidizing agents such as hypochlorite, 

 peroxide, and permanganate at neutral pH^*. Electrolytic oxidation of 

 L-sorbose to 2-keto-L-gulonic acid has also been recommended.'^ 



A better direct process seems to be that in which the L-sorbose is oxidized 

 with air in the presence of a noble metal catalyst such as platinum at a 

 neutral or slightly alkaline pH.^"' ^^ The general field of catalytic oxidations 

 of this type has not been investigated as much as it deserves for it not 

 only holds out considerable promise in the case under discussion but it is 

 worthy of wide application in organic chemistry. 



The indirect transformation of L-sorbose into 2-keto-L-gulonic acid, by 

 making use of a derivative in which only the CH2OH group at Ci is free to 

 be oxidized, provides an excellent and probably the best approach to the 

 manufacture of L-ascorbic acid. To achieve this, L-sorbose is converted 

 into 2,3,4,6-diisopropylidene-L-sorbofuranose; analogous derivatives may 

 be made with other carbonyl compounds such as cyclohexanone. Such 



12 p. A. Wells, J. J. Stubbs, L. B. Lockwood, and E. T. Roe, Ind. Eng. Chem. 29, 1385 



(1937). 

 "U. S. Pat. 2,121,533 (1938). 

 " W. N. Haworth, E. L. Hirst, J. K. N. Jones, and F. Smith, British Pat. 443.901 



(1936). 

 16 J. Overhoff and W. Huyser, U. S. Pat. 2,467,442 (1949). 

 1" Dutch Pat. 59,301 (1947). 

 " Dutch Pat. 59,584 (1947). 



18 German Pat. 644,962 (1937). 



19 Danish Pat. 68,836 (1949). 

 2»K. Heyns, Ann. 558, 177 (1947). 



21 O. Dalmer and K. Heyns, German Pat. 692,897 (1940); U. S. Pat. 2,190,377 (1940) 



