Alkylation of Schiffbases 2 derived from 4-chlorobenzaldehyde and sultam-derived aminoacids 1 gave crystalline diastereomers 3 which after deprotection and re-crystallization, if necessary, afforded in good yield enantiomerically pure Cα′-methylated aminoacid (ex: αMePhe, αMeLeu, αMeMet).