TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The reaction mixture is heated to a temperature of 80-130 degrees C, preferably 100-125 degrees C at a pressure of oxygen or air of 800-1000 psi for a time sufficient to form the predominant reaction product. The pressure is the pressure of oxygen. In the method, at least 90% of the furan aldehyde is oxidized into reaction products, and the predominant reaction product is at least 80% of the reaction products. The predominant reaction product is a 5-ester furan 2-carboxylic acid and is collected as a precipitate from the reaction mixture in a first purification step. The precipitate is dissolved in a solvent in which the predominant product has higher solubility than 2,5-furan dicarboxylic acid (FDCA) in a second purification step. The oxygen is provided by oxygen gas or air dissolved in the solvent at a pressure of at least 200 psi and carbon dioxide is expanded in the solvent at a pressure of at least 100 psi. Preferred Components: The reaction mixture contains acetic acid as a principle solvent. The reaction mixture also includes bromide and FDCA is formed as co-product of the oxidizing. The predominant reaction product is diformyl furan, and the reaction mixture does not contain bromide. The 5-ether of the furan aldehyde is a 5-(alkoxymethyl)furfural, a 5-(aryloxymethyl)furfural, a 5-(cycloalkoxy-methyl)furfural, 5-(acetoxymethyl)furfural or 5-(butoxymethyl)furfural. The furan aldehyde is 5-(hydroxymethyl)furfural (HMF). The solvent is ethyl acetate, dimethylformamide, dimethylacetamide, tetrahydrofuran, dioxane, methyl ethyl ketone, acetonitrile, methyltetrahydrofuran, methyl isobutyl ketone, or 1-6C-alcohols. The catalyst salt has an anion comprising an acetate, acetate hydrate, bromide, chloride, fluoride, iodide, alkoxide, azide, oxalate, carbonate, carboxylate, hydroxide, nitrate, borate, oxide and/or acetylacetonate. The reaction mixture includes carbon dioxide expanded in a principle solvent of the reactions mixture. The principle solvent is acetic acid. ACTIVITY - Plant Protectant.    MECHANISM OF ACTION - None given.    EXAMPLE - A reaction mixture containing 5-(hydroxymethyl)furfural (5 g), acetic acid (50 ml), cobalt acetate (0.97 g), manganese acetate (0.98 g) and methyl ethyl ketone (1.90 ml) was placed in a reactor (100 ml) and subjected to 1000 psi oxygen at 120 degrees C for 3.5 hours, and worked up to afford 5-(hydroxymethyl)furfural (4201 g/kg).