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

Ascorbic acid in the presence of a catalytic amount of iodine reduces arsenic acid in methanol giving the arsenious acid bound to the 2-methyl hemi-ketal of dehydroascorbic acid, 5, in 1:1 and in a more stable 2:1 5/As(III) molar ratio. Removal of the As(III) and treating the 2-methyl hemi-ketal of dehydroascorbic acid with refluxing acetonitrile affords the pure, crystalline dehydroascorbic acid dimer in good yields. Ascorbic acid also binds to As(III) of H(3)AsO(3) in a 1:1 and 2:1 ascorbic acid/As(III) molar ratio. The 1:1 complex is not stable and by expulsion of H(3)AsO(3) is transformed to the more stable 2:1 complex. The data do not permit distinguishing the 2:1 complexes between AsL(2)(H(2)O)](-)H(+) or AsL(LH)(H(2)O) where L is the bis deprotonated and LH is the mono deprotonated 2-methyl hemi-ketal of dehydroascorbic acid or ascorbic acid. The 2:1 ascorbic acid/As(III) complex is oxidized by dioxygen, in a solvent-dependent manner, to dehydroascorbic acid implying dioxygen activation by the bound As(III). With thiophenol the same complex gives quantitatively triphenyl trithioarsenite, As(SPh)(3).