This report describes the synthesis of the two enantiomerically pure α-methylated β-branched phenylalanine derivatives, (S)- and (R)-α-methyl-β,β-diphenylalanine-(αMe)Dip-starting from the chiral building blocks (R)- and (S)-N-Boc-N,O-isopropylidene-α-methylserine methyl esters, respectively. The key step involves a double alkylation with a Grignard reagent on an ester group. The use of the same protocol for the preparation of other α-methylated β-branched serine derivatives is also described.