Turn-Induction in Peptides Incorporating Novel Cyrene-Derived α,α-Disubstituted Amino Acid

Cyrene-derived α,α-disubstituted amino acids (H-Cyr-OH) are introduced into peptides using the multiple component Ugi reaction. Conformational analysis of the Cyr monomer and dimer was performed by X-ray crystallography and NMR spectroscopy. In the solid state, the backbone dihedral angle values of (R)- and (S)-Cyr were respectively characteristic of left- and right-handed α-helical conformers; however, the dimer appeared to adopt a ten-membered hydrogen bond in a distorted type II′ beta-turn. In solution, solvent shielded amide NH hydrogens suggested that the Cyr residue could adopt the central position of γ-turn conformations. With conformational properties indicative of α-, beta-, and γ-turns, the Cyr residue offers interesting potential as a novel chiral α,α-disubstituted amino acid for exploring chemical space.