Process for the stereo-selective preparation of L stereoisomers of unnatural aromatic amino acids optionally substituted on the aromatic ring of formula: wherein the compound of formula is 2',6'-dimethyl-L-Tyrosine, 2',6'-dimethyl-L-Phenylalanine. Providing the following steps: a) coupling amino acid L-valine having a protective group on the amine with a glycine methyl ester to obtain dipeptide of formula: wherein R 1 is methoxy and R 2 is isopropyl b) Removing the protective group from the compound of formula as prepared in step a) by reacting with a suitable reactive to obtain dipeptide of formula or salt thereof: wherein R 1 is methoxy and R 2 is isopropyl c) Intramolecular condensation of dipeptide of formula as prepared in step b) to obtain compound of formula: wherein R 2 is isopropyl d) Acetylation of compound of formula as prepared in step c) to obtain compound of formula: wherein R 2 is isopropyl and Ac is acetyl e) Condensation of compound of formula as prepared in step d) with aldehyde of formula to obtain Z-alkene of formula: wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is benzyloxy or H f) Removal of acetyl residue from compound of as prepared in step e) to obtain compound of formula: wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is benzyloxy or H g) Catalytic hydrogenation of compound of formula as prepared in step f) in presence of a achiral catalyst, to obtain compound of formula (VIII): wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is H or OH h) Hydrolysis of compound of formula (VIII) as prepared in step g) to obtain unnatural aromatic amino acids optionally substituted on the aromatic ring of formula in mixture with a amino acid of formula: H2N-CH -COOH wherein R2 is isopropyl, a) coupling amino acid L-valine having a protective group on the amine with a glycine methyl ester to obtain dipeptide of formula: wherein R 1 is methoxy and R 2 is isopropyl b) Removing the protective group from the compound of formula as prepared in step a) by reacting with a suitable reactive to obtain dipeptide of formula or salt thereof: wherein R 1 is methoxy and R 2 is isopropyl c) Intramolecular condensation of dipeptide of formula as prepared in step b) to obtain compound of formula: wherein R 2 is isopropyl d) Acetylation of compound of formula as prepared in step c) to obtain compound of formula: wherein R 2 is isopropyl and Ac is acetyl e) Condensation of compound of formula as prepared in step d) with aldehyde of formula to obtain Z-alkene of formula: wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is benzyloxy or H wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is benzyloxy or H f) Removal of acetyl residue from compound of as prepared in step e) to obtain compound of formula: wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is benzyloxy or H wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is benzyloxy or H g) Catalytic hydrogenation of compound of formula as prepared in step f) in presence of a achiral catalyst, to obtain compound of formula (VIII): wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is H or OH wherein R 2 is isopropyl R3 and R7 are CH 3 R4 and R6 are H R5 is H or OH h) Hydrolysis of compound of formula (VIII) as prepared in step g) to obtain unnatural aromatic amino acids optionally substituted on the aromatic ring of formula in mixture with a amino acid of formula: H2N-CH -COOH wherein R2 is isopropyl,