Process for preparing pyridine-2,3- dicarboxylic acid compounds

Claim 1

A process for preparing pyridine-2,3-­dicarboxylic acid compounds of the formula: wherein R¹ and R³ are the same or different and are each a hydrogen atom, a substituted or unsubstituted straight chain or branched chain alkyl having 1 to 4 carbon atoms, or a substituted or unsubstituted phenyl; R² is hydrogen atom, a substituted or unsubstituted straight chain or branched chain alkyl having 1 to 4 carbon atoms, a substituted or unsubstituted phenyl, a straight chain or branched chain alkylthio having 1 to 4 carbon atoms, a straight chain or branched chain alkoxy having 1 to 4 carbon atoms, a substituted or unsubstituted phenylthio, a substituted or unsubstituted phenoxy, a halogen atom, a straight chain or branched chain alkoxycarbonyl having 1 to 4 carbon atoms in the alkoxy moiety, an aminocarbonyl which may optionally have a substituent, or cyano; or both of R¹ and R² or both R² and R³ may together form a divalent alkylene selected from trimethylene, tetramethylene and penta­methylene; and R⁴ and R⁵ are the same or different and are each a straight chain or branched chain alkyl having 1 to 4 carbon atoms, which comprises reacting an enone of the formula (II): wherein R¹, R² and R³ are as defined above with an aminodiester of the formula (III): R⁴OOCCH=C(NH₂)COOR⁵ wherein R⁴ and R⁵ are as defined above.