Oxidative decarboxylation of aldonolactones by cerium(IV) sulphate in aqueous sulphuric acid; synthesis of D-arabinose

Abstract

Oxidations of D-glucono-1,5-lactone, D-glucono-1,4-lactone, calcium D-gluconate, and 3-deoxy-D-arabino-hexono-1,4-lactone by cerium(IV) sulphate in sulphuric acid were studied at 25 and at 37 °C, with various concentrations of reactants and acidities. The results indicated that it is the aldonic acid that undergoes oxidation; the stoicheiometry requires two cerium(IV) ions for the oxidation of one molecule of aldonic acid to the next lower sugar. The rate of disappearance of cerium(IV) ions depended directly on its concentration, but the dependence on the organic substrate concentration suggested the existence of an intermediate complex; this was proved by a spectrophotometric method. The complex undergoes slow unimolecular decomposition to a free radical, which reacts with Ce^IV to afford the products. The reaction rate was proportional to 1/[SO₄H₂]². A preparative experiment with D-glucono-1,5-lactone gave crystalline D-arabinose in 94% yield.