5 research outputs found
Incorporation of Trifluoromethylated Proline and Surrogates into Peptides: Application to the Synthesis of Fluorinated Analogues of the Neuroprotective Glycine-Proline-Glutamate (GPE) Tripeptide
The incorporation
into a peptide chain of highly hindered and weakly nucleophilic trifluoromethylated
prolines, pseudoprolines and oxazolidines has been achieved. As an
application, the synthesis of a new class of fluorinated analogues
of the neuroprotective tripeptide glycine-proline-glutamate (GPE)
is reported. These analogues have been elaborated from a panel of
five-membered ring trifluoromethylated amino acids (Tfm-AAs) through
the coupling reaction with a glutamate residue at the <i>C</i>-terminus and a glycine at the <i>N</i>-terminus. Although
the peptide coupling reaction at the <i>C</i>-terminal position
of the fluorinated amino acid was conveniently performed under standard
conditions, the very challenging coupling reaction at the highly deactivated <i>N</i>-terminal position proved to be much more problematic.
A methodological study was needed to identify suitable reaction conditions
for this difficult peptide coupling
Synthesis of Enantiopure <i>trans</i>-2,5-Disubstituted Trifluoromethylpyrrolidines and (2<i>S</i>,5<i>R</i>)‑5-Trifluoromethylproline
Enantiopure <i>trans</i>-2,5-disubstituted trifluoromethylpyrrolidines
were prepared on a several gram scale starting from a readily available
chiral fluorinated oxazolidine (Fox). A pure oxazolopyrrolidine intermediate
could be obtained after an efficient separation by selective diastereomer
destruction. The addition of various Grignard reagents on this oxazolopyrrolidine
provided disubstituted pyrrolidines with moderate to complete <i>trans</i> diastereoselectivity. The highly valuable compound
(2<i>S</i>,5<i>R</i>)-5-trifluoromethylproline
could be synthesized from the same oxazolopyrrolidine intermediate
via a Strecker-type reaction
Synthesis of Enantiopure <i>trans</i>-2,5-Disubstituted Trifluoromethylpyrrolidines and (2<i>S</i>,5<i>R</i>)‑5-Trifluoromethylproline
Enantiopure <i>trans</i>-2,5-disubstituted trifluoromethylpyrrolidines
were prepared on a several gram scale starting from a readily available
chiral fluorinated oxazolidine (Fox). A pure oxazolopyrrolidine intermediate
could be obtained after an efficient separation by selective diastereomer
destruction. The addition of various Grignard reagents on this oxazolopyrrolidine
provided disubstituted pyrrolidines with moderate to complete <i>trans</i> diastereoselectivity. The highly valuable compound
(2<i>S</i>,5<i>R</i>)-5-trifluoromethylproline
could be synthesized from the same oxazolopyrrolidine intermediate
via a Strecker-type reaction
Incorporation of CF<sub>3</sub>–Pseudoprolines into Peptides: A Methodological Study
The peptide coupling reactions allowing
the incorporation of trifluoromethyl
substituted oxazolidine-type pseudoprolines (CF<sub>3</sub>-ΨPro)
into peptide chains have been studied. While standard protocols can
be used for the peptide coupling reaction at the <i>C</i>-terminal position of the CF<sub>3</sub>-ΨPro, acid chloride
activation has to be used for the peptide coupling reaction at the <i>N</i>-terminal position to overcome the decrease of nucleophilicity
of the CF<sub>3</sub>-ΨPro. We demonstrate that the <i>N</i>-amidification of a diastereomeric mixture of CF<sub>3</sub>-ΨPro using Fmoc-protected amino acid chloride without base
gave the corresponding dipeptides as a single diastereomer (6 examples).
The ratio of the <i>cis</i> and <i>trans</i> 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 CF<sub>3</sub>-ΨPro has
been successfully synthesized
Local Control of the <i>Cis</i>–<i>Trans</i> Isomerization and Backbone Dihedral Angles in Peptides Using Trifluoromethylated Pseudoprolines
NMR studies and theoretical calculations have been performed on model peptides Ac-Ser(ΨPro)-NHMe, (<i>S</i>,<i>S</i>)Ac-Ser(Ψ<sup>H,CF3</sup>Pro)-NHMe, and (<i>R,S</i>)Ac-Ser(Ψ<sup>CF3,H</sup>Pro)-NHMe. Their thermodynamic and kinetic features have been analyzed in chloroform, DMSO, and water, allowing a precise description of their conformational properties. We found that trifluoromethyl C<sup>δ</sup>-substitutions of oxazolidine-based pseudoprolines can strongly influence the <i>cis</i>–<i>trans</i> rotational barriers with only moderate effects on the <i>cis</i>/<i>trans</i> population ratio. In CHCl<sub>3</sub>, the configuration of the CF<sub>3</sub>–C<sup>δ</sup> entirely controls the ψ-dihedral angle, allowing the stabilization of γ-turn-like or PPI/PPII-like backbone conformations. Moreover, in water and DMSO, this C<sup>δ</sup>-configuration can be used to efficiently constrain the ring puckering without affecting the <i>cis</i>/<i>trans</i> population ratio. Theoretical calculations have ascertained the electronic and geometric properties induced by the trifluoromethyl substituent and provided a rational understanding of the NMR observations