Enantiopure 5-CF3-Proline: Synthesis, Incorporation in Peptides, and Tuning of the Peptide Bond Geometry.

The straightforward synthesis of enantiopure 5-(R)-and 5-(S)-trifluoromethylproline is reported. The key steps are a Ruppert-Prakash reagent addition on l-pyroglutamic esters followed by an elimination reaction and a selective reduction. The solution-phase and solid-phase incorporation of this unprotected enantiopure fluorinated amino acid in a short peptide chain was demonstrated. Compared to proline, the CF3 group provides a decrease of the trans to cis amide bond isomerization energy and an increase of the cis conformer population.