To obtain, in a simple manner and in high yield while retaining its optical purity essentially and completely, the subject compound useful as a herbicide, by reacting a specific pyridine compound with a specific amount or more of a specific aminodimethylbutyramide, in the presence of a strong base, etc. followed by treatment with an aqueous acid.

This compound, a chiral free carboxylic acid of formula II, is obtained by reacting a compound of formula I (Y and Z are each H, a 1-4C alkyl or the like substitutable with one 1-4C alkoxy; R1 and R2 are each a 1-8C alkyl, phenyl or the like) (e.g. dimethyl 5-methylpyridine-2,3-dicarboxylate) with at least 1 molar equivalent of (R) 2-amino-2,3-dimethylbutyramide in the presence of a strong base (pref. an alkali metal alkoxide such as potassium t-butoxide) and a nonpolar solvent essentially with no water (pref. aromatic hydrocarbon or the like such as toluene) to produce a salt of the compound of formula II, and then treating the thus produced salt with an aqueous acid (pref. HCI or the like).