The invention relates to a process for the stereoselective preparation of amino acid derivatives, comprising a hydrogenation reaction of the compound of formula (III), alternatively its enantiomer, wherein R is (C1-C8)-alkyl; followed by a hydrolysis reaction to obtain L-mesityl alanine, alternatively its enantiomer D-mesityl alanine and, optionally, subjecting said compound to an amino group protection reaction, particularly as Fmoc. It also comprises Fmoc-L- or Fmoc-D-mesityl alanine as products per se, useful as intermediates in preparing peptides or peptide analogs with therapeutic or biological activity.