Abstract An optically active (S)-α-ethylleucine ((S)-αEtLeu) as a chiral α-ethylated α, α- disubstituted α-amino acid was synthesized by means of a chiral acetal auxiliary of (R, R)- cyclohexane-1, 2-diol. The chiral α-ethylated α, α-disubstituted amino acid (S)-αEtLeu was introduced into the peptides constructed from 2-aminoisobutyric acid (= dimethylglycine, Aib), and also into the peptide prepared from diethylglycine (Deg). The X-ray ...