Helical Structures of Cyclopentene-based α,α-Disubstituted α-Amino Acid Homopeptides

The cyclopentene-based α,α-disubstituted α-amino acid Ac₅c⁼ and its homopeptides, up to nonapeptides, were synthesized. The side-chain cyclopentene was expected to become symmetric, the C^α-carbon to be puckered, and other C^β, C^β', C^γ, C^γ'-carbons to be coplanar. As expected, side-chain cyclopentene conformations became symmetric and C^α-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 α,α-disupstituted α-amino acid homopeptides.