Tautomers, ascorbate

It has been proved by X-ray analysis that, in the solid state, 1 is the tautomer present, but the claim has been made that,68 in solution, L-ascorbic acid exists as 2. Structures 3,4, and 5 were readily eliminated by a study of the l3C-n.m.r. spectrum,88,87 and, on the basis of the chemical shifts of the carbon resonances for C-l, C-2, and C-3, and the known chemistry of L-ascorbic acid, 1 is favored over 2 in solution. Berger98 claimed that the proton-carbon-coupled spectrum of L-ascorbic acid is consistent only with structure 1. Ogawa and coworkers87 studied the conformation of L-ascorbic acid and L-ascorbic acid-5-d in deuterium oxide by 13C-n.m.r. spectroscopy, and concluded that, in... 

Of the possible side-chain oxidized derivatives of ascorbic acid, all but 5-keto-ascorbic acid and 5-keto-6-formylascorbic acid have been reported. Bakke and Theander (66) formed 6-aldehydoascorbic acid, 37, as an unisolated intermediate in the reductive hydrolysis of 38 to ascorbic acid. Heyns and Linkies (67) synthesized 5-keto-saccharo-ascorbic acid, 40, via the oxidation and subsequent hydrolysis of man-narodilactone, 39. 5-Ketosaccharoascorbic acid appears to exist in solution as its enol tautomer. 

Methylation of L-ascorbic acid with diazomethane has thrown some light on the tautomeric nature of the vitamin. The greater acidity of the C-3-OH enables it to be titrated with ethereal diazomethane to give 3-0-methylascorbic acid. This reaction, however, is also accompanied by the formation of a small quantity of 1-methyl- -L-ascorbic acid, presumably from a minor tautomer. Both of these compounds undergo further methylation with ethereal diazomethane to give 2,3-di-O-methyl-L-ascorbic acid and 1,2-di-O-methyl- -L-ascorbic acid respectively (Figure 4.9). 

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