A process for producing pyridine-2,3-dicarboxylic acid derivatives of the formula IMAGE (I) is described. The process is based on the cleavage of HNR10R11 from 1-amino-1,2,3,4-tetrahydropyridine-2,3-dicarboxylic acid derivatives of the formula II IMAGE (II) to give 1,4-dihydropyridine-2,3-dicarboxylic acid derivatives of the formula III IMAGE (III) and subsequent oxidation of these compounds to obtain pyridine-2,3-dicarboxylic acid derivatives of the formula I. According to one variant of the process, the 1,4-dihydropyridine-2,3-dicarboxylic acid derivatives of the formula III are obtained directly, without isolation of the intermediately formed 1-amino-1,2,3,4-tetrahydropyridine-2,3-dicarboxylic acid derivatives of the formula II, by reaction of alpha , beta -unsaturated hydrazones of the formula IV IMAGE (IV) with ethene-1,2-dicarboxylic acid derivatives of the formula V IMAGE (V) and subsequently oxidized to pyridine-2,3-dicarboxylic acid derivatives of the formula I. In the above formulae, R1, R2, R3, R4, R5, R10 and R11 have the meanings defined in claim 1. The pyridine-2,3-dicarboxylic acid derivatives of the formula I are intermediates for producing herbicidal active substances.
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