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

The cyclopentene-based α,α-disubstituted α-amino acid Acc 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, H NMR spectra, and X-ray crystallographic analyses revealed that Acc homopeptides did not form a planar conformation, but assumed a 3-helical structure, similar to cyclopentane-based α,α-disupstituted α-amino acid homopeptides.