Asymmetric α-Fluoroalkyl-α-Amino Acids: Recent Advances in Their Synthesis and Applications.

Due to the specific properties provided by fluorine atoms to biomolecules, amino acids with fluorinated side chains are of great interest for medicinal chemistry and chemical biology. Among them, α-fluoroalkyl-α-amino acids constitute a unique class of compounds. In this review, we outline the strategies adopted for their syntheses in enantiopure or enantioenriched forms and their incorporation into peptides. We then describe the consequences of the introduction of fluorine atoms in these compounds for the modulation of their hydrophobicity and the control of their conformation. Emerging applications are presented in the areas of enzyme inhibition, medicinal chemistry, hydrolytic stability of peptides, antimicrobial peptides, PET, and 19F NMR probes.

Trifluoromethylthiolation of Tryptophan and Tyrosine Derivatives: A Tool for Enhancing the Local Hydrophobicity of Peptides.

The incorporation of fluorinated groups into peptides significantly affects their biophysical properties. We report herein the synthesis of Fmoc-protected trifluoromethylthiolated tyrosine (CF3S-Tyr) and tryptophan (CF3S-Trp) analogues on a gram scale (77-93% yield) and demonstrate their use as highly hydrophobic fluorinated building blocks for peptide chemistry. The developed methodology was successfully applied to the late-stage regioselective trifluoromethylthiolation of Trp residues in short peptides (66-80% yield) and the synthesis of various CF3S-analogues of biologically active monoamines. To prove the concept, Fmoc-(CF3S)Tyr and -Trp were incorporated into the endomorphin-1 chain (EM-1) and into model tripeptides by solid-phase peptide synthesis. A remarkable enhancement of the local hydrophobicity of the trifluoromethylthiolated peptides was quantified by the chromatographic hydrophobicity index determination method, demonstrating the high potential of CF3S-containing amino acids for the rational design of bioactive peptides.

Difluoroalanine: Synthesis, Incorporation in Peptides, and Hydrophobic Contribution Assessment.

The straightforward synthesis of chiral (R)- and (S)-difluoroalanine is reported. The key step is a Strecker-type reaction followed by hydrogenolysis, Fmoc protection, and acidic hydrolysis. Peptide coupling reactions at its N- and C-terminal positions provide diastereomerically pure tripeptides. On the basis of hydrophobicity index measurements, the hydrophobic contribution of difluoroalanine in a peptide chain was found to be similar to that of isoleucine for a smaller van der Waals volume of the side chain.

Inviting C5-Trifluoromethylated Pseudoprolines into Collagen Mimetic Peptides.

Numerous collagen mimetic peptides (CMPs) have been engineered using proline derivatives substituted at their C(3) and/or C(4) position in order to stabilize or functionalize collagen triple-helix mimics. However, no example has been reported so far with C(5) substitutions. Here, we introduce a fluorinated CMP incorporating trifluoromethyl groups at the C(5) position of pseudoproline residues. In tripeptide models, our CD, NMR, and molecular dynamics (MD) studies have shown that, when properly arranged, these residues meet the structural requirements for a triple-helix assembly. Two host-guest CMPs were synthesized and analyzed by CD spectroscopy. The NMR analysis in solution of the most stable confirmed the presence of structured homotrimers that we interpret as triple helices. MD calculations showed that the triple-helix model remained stable throughout the simulation with all six trifluoromethyl groups pointing outward from the triple helix. Pseudoprolines substituted at the C(5) positions appeared as valuable tools for the design of new fluorinated collagen mimetic peptides.

Introducing the Chiral Constrained α-Trifluoromethylalanine in Aib Foldamers to Control, Quantify and Assign the Helical Screw-Sense.

Oligomers of α-aminoisobutyric acid (Aib) are achiral peptides that adopt 310 helical structures with equal population of left- and right-handed conformers. The screw-sense preference of the helical chain may be controlled by a single chiral residue located at one terminus. 1 H and 19 F NMR, X-ray crystallography and circular dichroism studies on new Aib oligomers show that the incorporation of a chiral quaternary α-trifluoromethylalanine at their N-terminus induces a reversal of the screw-sense preference of the 310 -helix compared to that of a non-fluorinated analogue having an l-α-methyl valine residue. This work demonstrates that, among the many particular properties of introducing a trifluoromethyl group into foldamers, its stereo-electronic properties are of major interest to control the helical screw sense. Its use as an easy-to-handle 19 F NMR probe to reliably determine both the magnitude of the screw-sense preference and its sign assignment is also of remarkable interest.

Synthesis of enantiopure α-Tfm-proline and α-Tfm-pipecolic acid from oxazolo-pyrrolidines and -piperidines.

Enantiopure α-Tfm-proline and α-Tfm-pipecolic acid were synthesized starting from commercially available diesters and ethyl trifluoroacetate. A Strecker type reaction on intermediate chiral Tfm-oxazolo-pyrrolidine and -piperidine provided the corresponding nitrile precursor of enantiopure (R) and (S) α-Tfm-proline and α-Tfm-pipecolic acid. The C-terminal peptide coupling reaction of α-Tfm-pipecolic acid has been successfully achieved.

Trifluoromethylated proline analogues as efficient tools to enhance the hydrophobicity and to promote passive diffusion transport of the l-prolyl-l-leucyl glycinamide (PLG) tripeptide.

The synthesis of four CF3-proline analogues of the PLG peptide is reported. Our results show that the incorporation of trifluoromethylated amino acids (Tfm-AAs) at the N-terminal position of a peptide significantly increases its hydrophobicity. In addition, depending on the relative configuration and the position of the CF3 group, Tfm-AAs can also promote passive diffusion transport.

Homochiral versus Heterochiral Trifluoromethylated Pseudoproline Containing Dipeptides: A Powerful Tool to Switch the Prolyl-Amide Bond Conformation.

The design of constrained peptides is of prime importance in the development of bioactive compounds and for 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 that changing the configuration of the Cα in Xaa or in ΨPro is sufficient to invert the cis:trans populations while changing the nature of the Xaa side chain finely tuned the conformers ratio. Moreover, a strong correlation is found between the puckering of the oxazolidine ring and the peptide bond conformation. This finding highlights the role of the trifluoromethyl group in the stabilization of the peptide bond geometry. We anticipate that such templates will be very useful to constrain the backbone geometry of longer peptides.

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 C-terminus and a glycine at the N-terminus. Although the peptide coupling reaction at the C-terminal position of the fluorinated amino acid was conveniently performed under standard conditions, the very challenging coupling reaction at the highly deactivated N-terminal position proved to be much more problematic. A methodological study was needed to identify suitable reaction conditions for this difficult peptide coupling.

Straightforward synthesis of novel enantiopure α-trifluoromethylated azetidine 2-carboxylic acid and homoserines.

The straightforward syntheses of enantiopure (2R)-2-trifluoromethyl-2-carboxyazetidine and (R)- and (S)-trifluoromethylhomoserines are reported. The key step is a Strecker-type reaction on a common chiral CF3-containing bicyclic oxazolidine intermediate obtained by a condensation reaction of (R)-phenylglycinol and ethyl-4,4,4-trifluoroacetoacetate (ETFAA).

Incorporation of CF3-pseudoprolines into peptides: a methodological study.

The peptide coupling reactions allowing the incorporation of trifluoromethyl substituted oxazolidine-type pseudoprolines (CF3-ΨPro) 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-ΨPro, 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-ΨPro. We demonstrate that the N-amidification of a diastereomeric mixture of CF3-ΨPro 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-ΨPro has been successfully synthesized.

Synthesis of an MIF-1 analogue containing enantiopure (S)-α-trifluoromethyl-proline and biological evaluation on nociception.

The synthesis and the effect of a novel MIF-1 analogue on nociception during acute pain in rat model are reported. The synthesis of this enantiopure trifluoromethyl group containing tripeptide was performed through a peptide coupling reaction between the HCl. Leu-Gly-NH2 and the (S)-α-Tfm-proline. The analgesic effect of the CF3-(MIF-1) 2 has been evaluated in vivo on rat model by paw pressure (PP) and hot plate (HP) tests and compared to the native peptide MIF-1. Highest analgesic effect was observed with CF3-(MIF-1) 2 only in PP test. In order to study the mechanisms of nociception induced by the studied peptides, the involvement of the opioid and the nitric oxideergic systems was investigated. The results are in favor of a participation of both system since pretreatment, 20 min before injection of the CF3-(MIF-1) 2, with the non-competitive antagonist of opiate receptors naloxone, the nitric oxide synthase (NOS) inhibitor l-N(G)-nitroarginine ester (l-NAME) or the nitric oxide (NO) donor l-arginine (l-Arg) significantly decreased the pain perception in PP and HP tests.

Exclusive alpha-coupling in the aldol reaction of unsaturated trimethylsilyl esters: an efficient and practical direct synthesis of unsaturated beta-hydroxy acids.

The lithium enolates of trimethylsilyl but-3-enoate and 3-methylbut-3-enoate reacted with aldehydes and saturated or aromatic ketones at -70 degrees C to give exclusively the alpha-condensation products in excellent yields. The unsaturated beta-hydroxy acids thus obtained were directly identified, and the usual conversion into their methyl esters with diazomethane was not necessary. Unsaturated ketones underwent Michael reaction through alpha-addition leading to the unsaturated 5-oxo acids.