Pure Dehydro-l-ascorbic Acid Prepared by O₂-Oxidation ofl-Ascorbic Acid with Active Charcoal as Catalyst

Preparation of dehydro-l-(+)-ascorbic acid dimer by oxidation of ascorbic acid with arsenic acid/iodine and formation of complexes between arsenious acid and ascorbic acid

“…It darkens at $190°C and turns black at 224-227°C. IR (KBr): same as published [18,29] but without a band at 1637 cm À1 . The acetonitrile supernatant was concentrated to $10 ml and to the orange solution, ether (20 ml) was added slowly while stirring.…”

“…Dimerization in boiling acetonitrile gave pure crystalline 6 in 45-55% yield. Thus, acetonitrile was as effective as ethyl methyl ketone [18] for the dimerization of the mixture of 4 and 5 to 6. Prior ''baking'' of the mixture of 4 and 5 at 100°C/2 mm Hg and refluxing in acetonitrile gave 46% of crystalline 6, a yield essentially similar to that obtained by Pecherer [16].…”

“…Its preparation from an aqueous solution of 4 is nearly impossible [15,16], whereas it can be prepared from the syrupy 5 by thermal elimination of methanol either by ''baking'' at 100°C in vacuum, with yields 23% [15] or $40% [16] or by refluxing in methyl ethyl ketone (64%) [18] or heating in nitromethane (90%) [17]. In the latter case, the product did not show the correct molecular weight [17], it had a pinkish color and was not free of solvent [18]. The precipitation of the insoluble anomer of 6 from the DMF solution requires the addition of a cocktail of formic acid, oxalic acid, and phthalic anhydride and the yields were $60% [20].…”

“…However, oxidation in non-protic solvents, e.g., N,N-dimethylformamide (DMF) [20], produces the two anomers of the dimerized dehydroascorbic acid, 6 [21]. The oxidants used are: iodine in wet methanol [15], chlorine in dry methanol [16], p-benzoquinone, chloranil or mercuric(II) acetate in DMF or dimethyl sulfoxide (DMSO) or N,N-dimethylacetamide [20], and air or dioxygen in the presence of palladium on carbon [17] or activated charcoal [18] suspended in methanol.…”

“…When ascorbic acid is oxidized in aqueous solution, compound 4 is formed [14] which has been isolated in 79% purity [13]. When the oxidation is done in methanol [15][16][17][18], then two isomers of the 2-methyl hemi-ketal 5 are formed [19] which can be isolated as a solid [16]. However, oxidation in non-protic solvents, e.g., N,N-dimethylformamide (DMF) [20], produces the two anomers of the dimerized dehydroascorbic acid, 6 [21].…”