A method for chiral resolution of a mixture of stereoisomers of a compound of formula, wherein each of R 1, R 2, R 3, R 4, R 5, R 6 and R 7 is independently any one selected from a group consisting of H, -NH 2, a C 1-6 alkyl group, a C 2-6 alkenyl group, a C 2-6 alkynyl group and a halogen, and R 1 and R 2 are different from each other, comprising mixing said mixture of stereoisomers of the compound of formula with (i) a chiral auxiliary, preferably one or more selected from a group consisting of 2,3-dibenzoyltartaric acid, O,O' -di- p -toluoyltartaric acid, a stereoisomer thereof, and a combination thereof; and (ii) an auxiliary salt-forming compound, preferably one or more selected from a group consisting of mandelic acid, camphorsulfonic acid, a stereoisomer thereof, and a combination thereof, more preferably D-mandelic acid, L-mandelic acid, -(-)-10-camphorsulfonic acid, -(+)-10-camphorsulfonic acid or a combination thereof; in the presence of a solvent, preferably one or more selected from a group consisting of water, a C 1-14 alcohol, acetic acid, nitromethane, propionic acid, formic acid and a combination thereof, more preferably one or more selected from a group consisting of water, methanol, ethanol and isopropyl alcohol, much more preferably methanol, isopropyl alcohol or a combination thereof; thereby precipitating a diastereomeric salt of said chiral auxiliary (i) with the compound of formula in enantiomeric excess. wherein each of R 1, R 2, R 3, R 4, R 5, R 6 and R 7 is independently any one selected from a group consisting of H, -NH 2, a C 1-6 alkyl group, a C 2-6 alkenyl group, a C 2-6 alkynyl group and a halogen, and R 1 and R 2 are different from each other, comprising mixing said mixture of stereoisomers of the compound of formula with (i) a chiral auxiliary, preferably one or more selected from a group consisting of 2,3-dibenzoyltartaric acid, O,O' -di- p -toluoyltartaric acid, a stereoisomer thereof, and a combination thereof; and (ii) an auxiliary salt-forming compound, preferably one or more selected from a group consisting of mandelic acid, camphorsulfonic acid, a stereoisomer thereof, and a combination thereof, more preferably D-mandelic acid, L-mandelic acid, -(-)-10-camphorsulfonic acid, -(+)-10-camphorsulfonic acid or a combination thereof; (i) a chiral auxiliary, preferably one or more selected from a group consisting of 2,3-dibenzoyltartaric acid, O,O' -di- p -toluoyltartaric acid, a stereoisomer thereof, and a combination thereof; and (ii) an auxiliary salt-forming compound, preferably one or more selected from a group consisting of mandelic acid, camphorsulfonic acid, a stereoisomer thereof, and a combination thereof, more preferably D-mandelic acid, L-mandelic acid, -(-)-10-camphorsulfonic acid, -(+)-10-camphorsulfonic acid or a combination thereof; in the presence of a solvent, preferably one or more selected from a group consisting of water, a C 1-14 alcohol, acetic acid, nitromethane, propionic acid, formic acid and a combination thereof, more preferably one or more selected from a group consisting of water, methanol, ethanol and isopropyl alcohol, much more preferably methanol, isopropyl alcohol or a combination thereof; thereby precipitating a diastereomeric salt of said chiral auxiliary (i) with the compound of formula in enantiomeric excess.