Incorporation of CF3–Pseudoprolines into Peptides: A Methodological Study
摘要:
The peptide coupling reactions allowing the incorporation of trifluoromethyl substituted oxazolidine-type pseudoprolines (CF3-Psi Pro) into peptide chains have been studied. While standard protocols can be used for the peptide coupling reaction at the C-terminal position of the CF3-Psi Pro, acid chloride activation has to be used for the peptide coupling reaction at the N-terminal position to overcome the decrease of nucleophilicity of the CF3-Psi Pro. We demonstrate that the N-amidification of a diastereomeric mixture of CF3-Psi Pro using Fmoc-protected amino acid chloride without base gave the corresponding dipeptides as a single diastereomer (6 examples). The ratio of the cis and trans amide bond conformers was determined by NMR study, highlighting the role of the Xaa side chains in the control of the peptide backbone conformation. Finally a tripeptide bearing a central CF3-Psi Pro has been successfully synthesized.
We have recently reported the synthesis of enantiomerically pure CF3-oxazolidine pseudoprolines (CF3-ΨPro). Complete NMR studies, together with DFT calculations, have highlighted the marked stereoelectronic effects of the CF3 group on these new proline surrogates. In this paper, we describe for the first time the conformational features of dipeptides incorporating one CF3-ΨPro residue. Extensive NMR analyses have been carried out in solution and revealed the presence of a stable type-VI β-turn in a pseudotetrapeptide sequence.
applications in supramolecular chemistry. Due to its nature, the peptide bond undergoes a spontaneous cis–trans isomerism, and the cis isomers are much more difficult to stabilize than the trans forms. By using oxazolidine-based pseudoprolines (ΨPro) substituted by a trifluoromethyl group, we show that the cis peptide bond can be readily switched from 0% to 100% in Xaa-ΨPro dipeptides. Our results prove
The peptide coupling reactions allowing the incorporation of trifluoromethyl substituted oxazolidine-type pseudoprolines (CF3-Psi Pro) into peptide chains have been studied. While standard protocols can be used for the peptide coupling reaction at the C-terminal position of the CF3-Psi Pro, acid chloride activation has to be used for the peptide coupling reaction at the N-terminal position to overcome the decrease of nucleophilicity of the CF3-Psi Pro. We demonstrate that the N-amidification of a diastereomeric mixture of CF3-Psi Pro using Fmoc-protected amino acid chloride without base gave the corresponding dipeptides as a single diastereomer (6 examples). The ratio of the cis and trans amide bond conformers was determined by NMR study, highlighting the role of the Xaa side chains in the control of the peptide backbone conformation. Finally a tripeptide bearing a central CF3-Psi Pro has been successfully synthesized.