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.