Helical Structures of Bicyclic <i>α</i>‐Amino Acid Homochiral Oligomers with the Stereogenic Centers at the Side‐Chain Fused‐Ring Junctions

Abstract

Abstract Chiral bicyclic α ‐amino acid ( R , R )‐Ab 5,6= c with stereogenic centers at the γ ‐position of fused‐ring junctions, and its enantiomer ( S , S )‐Ab 5,6= c, were synthesized. The CD spectra of ( R , R )‐Ab 5,6= c oligomers indicated that the ( R , R )‐Ab 5,6= c hexapeptide formed a mixture of right‐handed ( P )‐ and left‐handed ( M )‐ 3 10 ‐helices, while, in the ( R , R )‐Ab 5,6= c nonapeptide, a right‐handed ( P )‐ 3 10 ‐helix slightly dominated over the ( M )‐helix. X‐Ray crystallographic analyses of ( S , S )‐tripeptide and ( R , R )‐hexapeptide revealed that both the tripeptide and hexapeptide formed a mixture of ( P )‐ and ( M )‐ 3 10 ‐helices, respectively. These results indicated that the side‐chain environments around the stereogenic centers are particularly important to control the helical‐screw handedness of foldamers.