Helical Structures of Bicyclic α-Amino Acid Homochiral Oligomers with the Stereogenic Centers at the Side-Chain Fused-Ring Junctions

Chiral bicyclic α‐amino acid (R,R)‐Ab5,6=c with stereogenic centers at the γ‐position of fused‐ring junctions, and its enantiomer (S,S)‐Ab5,6=c, were synthesized. The CD spectra of (R,R)‐Ab5,6=c oligomers indicated that the (R,R)‐Ab5,6=c hexapeptide formed a mixture of right‐handed (P)‐ and left‐handed (M)‐310‐helices, while, in the (R,R)‐Ab5,6=c nonapeptide, a right‐handed (P)‐310‐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)‐310‐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.