Helical l-Leu-Based Peptides Having Chiral Five-Membered Carbocyclic Ring Amino Acids with an Ethylene Acetal Moiety

l–Leu-based heteropeptides having (R)- or (S)-chiral five-membered carbocyclic ring amino acids (Ac5c3EG) with an ethylene acetal moiety were prepared. A conformational analysis using FT-IR absorption, 1H NMR, and circular dichroism (CD) spectra revealed that l–Leu-based hexapeptides and nonapeptides having (R)- or (S)-Ac5c3EG formed right-handed (P) helical structures in solution. An X-ray crystallographic analysis of nonapeptides 5 a and 5 b showed similar right-handed (P) α-helical structures, without an intramolecular hydrogen bond of the peptide N–H⋅⋅⋅–O– (acetal) type