Synthesis of Chiral α‐Trifluoromethyl α,α‐Disubstituted α‐Amino Acids and Conformational Analysis of L‐Leu‐Based Peptides with (R)‐ or (S)‐α‐Trifluoromethylalanine

Abstract

AbstractVarious racemic α‐trifluoromethyl α,α‐disubstituted α‐amino acids were synthesized by the reaction of methyl 3,3,3‐trifluoropyruvate imines with Grignard reagents. The optical resolution of racemates using (R)‐1,1’‐bi‐2‐naphthol {(R)‐BINOL} esters gave optically active α‐trifluoromethylated α,α‐disubstituted α‐amino acids, such as α‐trifluoromethylalanine (αCF3Ala), α‐trifluoromethylleucine (αCF3Leu), and α‐trifluoromethylphenylalanine (αCF3Phe). The optically active (R)‐ or (S)‐αCF3Ala was incorporated into the L–Leu‐based pentapeptides, and their preferred conformation in solution and in the crystal state was studied by Fourier transform infrared (FT‐IR) absorption, nuclear Overhauser effect spectroscopy (NOESY) NMR, and circular dichroism (CD) spectra, as well as X‐ray crystallographic analysis. Both L–Leu‐based peptides with (R)‐ or (S)‐αCF3Ala formed right‐handed 310‐helical structures. Both peptide‐backbones at the N‐terminal residues 1–3 were very similar, but the φ and ψ torsion angles of residues 4 and 5 between peptides with (R)‐ or (S)‐ αCF3Ala were different.