Peptide foldamers composed of six-membered ring α,α-disubstituted α-amino acids with two changeable chiral acetal moieties

Chiral cyclic α,α-disubstituted α-amino acids with four chiral centers at their acetal moieties were synthesized. An X-ray crystallographic analysis of homo-chiral tripeptide with (2R,3R)-butane-2,3-diol acetal moieties revealed that the tripeptide formed both (P) and (M) helical structures, and all peptide main-chain N(i)-H were intramolecularly hydrogen-bonded with the side-chain acetal -O- of the same amino acid residues (i). The effect of the four chiral centers in the amino acid residue on the peptide backbone helical-screw control was very weak