Left-Handed Helix of Three-Membered Ring Amino Acid Homopeptide Interrupted by an N–H···Ethereal O‑Type Hydrogen Bond

A chiral three-membered ring Cα,α-disubstituted α-amino acid (R,R)-Ac3cdMOM, in which the α-carbon is not a chiral center, but two side chain β-carbons are chiral centers, was synthesized from dimethyl l-(+)-tartrate, and its homopeptides were prepared. X-ray crystallographic analysis of (R,R)-Ac3cdMOM pentapeptide showed bent left-handed (M) 310-helical structures with an unusual intramolecular hydrogen bond of the N–H···O (ethereal) type. The left-handedness of the bent helices was exclusively controlled by the side-chain β-carbon chiral centers.