Process for the preparation of pyridine-2,3-dicarboxylic acids

Process for the preparation of pyridine-2,3-dicarboxylic acid of the formula:

• ##STR17## in which:
• R.sup.1 is hydrogen, alkyl of 1 to 12 carbon atoms, alkoxy of 1 to 12 carbon atoms, halogeno, mercapto, phenyl, or phenoxy;
• R.sup.2, taken independently of R.sup.3, is hydrogen, alkyl of 1 to 12 carbon atoms, alkoxy of 1 to 12 carbon atoms, halogeno, mercapto, phenyl, or phenoxy;
• and
• R.sup.3, taken independently of R.sup.2, is hydrogen or alkyl of 1 to 12 carbon atoms, unsubstituted or substituted with halogeno or alkoxy of 1 to 12 carbon atoms, or R.sup.2 and R.sup.3, taken together, are the residue of an aromatic ring fused to the depicted pyridine ring which comprises subjecting a 2,3-dihydroxybenzoic acid of the formula:
• ##STR18## in which R.sup.1, R.sup.2, and R.sup.3 are as herein defined, to the action of 2,3-dihydroxybenzoate-3,4-dioxygenase in a liquid medium lacking active decarboxylase and having a pH from 4 to 9, an ionic strength below about 1 molar, and a low concentration of metal cations and complex anions, to accumulate 2-hydroxy-3-carboxymuconic acid semialdehyde, and in sufficiently close temporal proximity to avoid substantial decarboxylation, reacting the 2-hydroxy-3-carboxymuconic acid semialdehyde thus formed with a source of ammonia or a primary amine.