208 ASCORBIC ACID 



1 -methyl hetero ether (LXIX) shows an absorption band at a much longer 

 wavelength than the normal 3-methyl ether (LXVIII), and it also has a 

 larger specific rotation. Upon further methylation of the 3-methyl ether it 

 gives the normal 2,3-dimethyl ether (VIII), whereas the 1-methyl ether 

 (LXIX) yields the corresponding 1 , 2-dimethyl heteroascorbic acid (LXX) . 

 In aqueous solution VIII is stable, but the 1, 2-dimethyl hetero compound 

 (LXX) loses the methyl group from Ci to give a 2-methyl ether (LXXI) 

 which must have the normal ascorbic acid structure, since upon methyla- 

 tion it gives rise to 2 ,3-dimethyl-L-ascorbic acid (VIII).'''* The latter crystal- 

 lizes as a monohydrate, but the melting point is rather low and it is ad- 

 visable to characterize it as the 5 , 6-monoisopropylidene derivative*^ or the 

 5,6-di-p-nitrobenzoate*^ or as the 6-trityl ether which occurs in two crystal- 

 line modifications.'*'* • ■'** 



Another curious type of isomerization is displayed by 2,3-dimethyl-L- 

 ascorbic acid (VIII) when it is allowed to react at room temperature with 

 dilute alkalies in which treatment one equivalent of base is consumed. By 

 analogy with simple lactones LXX VI would be expected, but evidently 

 such a system, having its electrons displaced from the double bond by the 

 ionized carboxyl group, is unstable, and stabilization is attained by satura- 

 tion of the double bond through closure of the ring between C3 and Ce to 

 give a salt (LXXV) of the methyl furanoside of a 3-keto acid, a compound 

 which shows no selective absorption in the ultraviolet. Upon acidification 

 of LXXV it affords the free acid furanoside (LXXIV), which readily 

 undergoes further ring closure to give the furanoside 7-lactone (LXIII). 

 Treatment of the latter with ammonia readily gives, by opening of the 

 1 ,4-lactone ring, a crystalline amide which may also be obtained in smaller 

 yield directly from the normal 2,3-dimethyl-L-ascorbic acid (VIII). The 

 cycle of isomerizations may now be completed, for by boiling LXXIII with 

 methyl alcoholic hydrogen chloride the furanoside methyl group is elimi- 

 nated from C3 with the formation of the 2-methyl-L-ascorbic acid. Further 

 structural information is derived from the fact that methylation of VIII 

 with methyl sulfate followed by esterification was found to give a penta- 

 methyl compound (LXXII)."* 



The type of isomerization which takes place when 2,3-dimethyl-L-as- 

 corbic acid is transformed into its iso compound by the agency of alkalies 

 has also been shown to occur with the 2 , 3 , 5-trimethyl- but not Avith the 

 2 , 3 , 5 , 6-tetramethyl-L-ascorbic acid, thus showing that the — CH2OH 

 group at C 6 is involved in the isomerization.'*^ 



The transformations and isomerizations discussed above in connection 

 with 2,3-dimethyl-L-ascorbic acid are evidently general reactions inasmuch 

 as 2,3-dimethyl-D-glucoascorbic acid behaves in an analogous manner. ^^ 



