Patent ID: 7700802

Claim:
A method of separating an endo isomer and an exo isomer of a dicarboxylic acid represented by formula (1), wherein, R 1 to R 8 represent a hydrogen atom, methyl group, ethyl group, or butyl group, or an anhydride thereof, or formula (2), wherein, R′ 1 to R′ 10 represent a hydrogen atom, methyl group, ethyl group, or butyl group, or an anhydride thereof, the method comprising the steps of: (a) providing a mixture comprising mainly the endo isomer of the dicarboxylic acid represented by formula (1) or (2) or an anhydride thereof, and the exo isomer of the dicarboxylic acid represented by formula (1) or (2) or an anhydride thereof; (b) mixing the mixture with an alkali metal hydroxide and a solvent; and (c) filtering the mixture obtained in step (b) to separate an aqueous phase and a solid phase, thereby separating the endo isomer from the exo isomer.