Preparation of dehydro-l-ascorbic acid dimer by air oxidation of l-ascorbic acid in the presence of catalytic amounts of copper(II) acetate and pyridine

The catalytic system Cu(AcO)2–pyridine 1:4 mol % in methanol, slowly catalyses the air oxidation of ascorbic acid to the 2-methyl hemi-ketal of dehydroascorbic acid 5, and hydrogen peroxide. However, with Cu(AcO)2–pyridine 3:4 mol % the air oxidation is quite fast and no hydrogen peroxide is present at the end of the reaction. Removal of the catalyst and refluxing the foamy 5 in MeCN gives the oxidized, dimeric, dehydroascorbic acid in very good yields (∼70%) contaminated by ∼1–2% MeCN.