The cyclopentene-based ?,?-disubstituted ?-amino acid Ac5c= and its homopeptides, up to nonapeptides, were synthesized. The side-chain cyclopentene was expected to become symmetric, the C?-carbon to be puckered, and other C?, C?', C?, C?'-carbons to be coplanar. As expected, side-chain cyclopentene conformations became symmetric and C?-carbons were puckered. Conformational studies using FT-IR absorption, 1H NMR spectra, and X-ray crystallographic analyses revealed that Ac5c= homopeptides did not form a planar conformation, but assumed a 310-helical structure, similar to cyclopentane-based ?,?-disupstituted ?-amino acid homopeptides.
