Oligo(Leu)s containing Pro or (Dmob) Leu residues were prepared by the stepwise elongation and fragment condensation methods. The peptides prepared were the following: Boc‐Leun‐OBzl (n = 3–6 and 9), Boc‐Pro‐Leu3‐OBzl, Boc‐Leun‐Pro‐OBzl (n = 3–5), Boc‐Leun‐Pro‐Leu3‐OBzl (n = 3 and 4), Boc‐(Leu4‐Pro)2‐OBzl, Boc‐Leu3‐Pro‐Leun‐Pro‐Leu3‐OBzl (n = 6 and 7), Boc‐Leu3‐Gly‐(Dmob) Leu‐OMe, Boc‐[Leu3‐Gly‐(Dmob) Leu‐Leu3]n‐OBzl (n = 1 and 2). The insertion of the Pro and Gly‐(Dmob) Leu residues into a peptide chain caused “the peptide segment separation” and achieved remarkable solubility improvement of N‐ and C‐protected oligo(Leu) derivatives in a variety of organic solvents. The conformations of typical peptides in the solid state and in DMSO were also investigated using i.r. spectroscopy. The incorporation of tertiary peptide bonds in a central position of the β‐sheet structure induces the onset of an unordered structure in the solid state. In DMSO, peptides are efficiently subjected to solvation to have the unordered structure.

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