Initial steps of the metal-catalysed degradation of L -dehydroascorbic acid in acidic aqueous solutions

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

“…However, in DMF the ascorbic acid in the complex was nearly completely oxidized by air in 3 days mainly giving the compounds 4 and 8, identified from the 13 C NMR spectrum of the mixture in D 2 O [13]. Hydrogen peroxide was detected but oxalic or arsenic acids were not detected.…”

“…Part of the solution did not precipitate calcium oxalate (pK sp 8.6) nor calcium arsenate (pK sp 18.2) after adding two drops of concentrated aqueous calcium chloride. Its 13 3406 s, broad, 2956 w, 1796 vs, 1636 w, 1468 w, 1348 m, 1218 m, 1138 s, 1088 s, 1030 s, 995 s, 832 w, 764 m, 680 m, 614 m, 550 m. The samples for 1 H and 13 C NMR spectra did not contain precipitated As 2 O 3 and the chemical shifts for the complex 5/As(III) 2:1 are shown in Tables 1 and 2. 3. Results and discussion 3.1.…”

“…Obtaining pure 4 is difficult because the dissolution of 6 in water gives 4 contaminated by 7 [21] and by a small amount of 2,3-hexodiulosonic acid [13].…”

“…It acts as an one-electron donor to various electron acceptors, especially inorganic compounds [10,11], giving, through the highly acidic radical AH Å , a comparatively stable [12] radical anion A ÅÀ , which upon donation of the single electron gives the first oxidation product, dehydroascorbic acid [10]. In alkaline solutions, dehydroascorbic acid can be further oxidized to oxalic acid and trihydroxybutyric acid (threonic acid) [9], while in acidic solutions various rearranged products are obtained [13].…”

“…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].…”