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

• Published in: Helvetica chimica acta Vol. 95; no. 10; pp. 1694 - 1713
• Main Authors: Anan, Kosuke, Demizu, Yosuke, Oba, Makoto, Kurihara, Masaaki, Doi, Mitsunobu, Suemune, Hiroshi, Tanaka, Masakazu
• Format: Journal Article
• Language: English
• Published: Zürich WILEY-VCH Verlag 01.10.2012; WILEY‐VCH Verlag; Wiley
• Subjects: Amino acids; Chemistry; Chemistry, Multidisciplinary; Foldamers; Helical structures; homochiral; Oligomers; Oligomers, homochiral; Physical Sciences; Science & Technology; LINEAR OLIGOPEPTIDES; Helical structures; CYCLIC ALPHA; HOMO-OLIGOPEPTIDES; INFRARED-ABSORPTION; Amino acids; X-RAY-DIFFRACTION; DIALKYLATED GLYCINES; CHIRAL CENTERS; BETA-PEPTIDES; Oligomers; Foldamers; homochiral; C-ALPHA,ALPHA-DIALKYLATED GLYCINES; SECONDARY STRUCTURE
• ISSN: 0018-019X; 1522-2675
• DOI: 10.1002/hlca.201200403

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.