Synthesis of α,α-difluoro-β-amino amides using aldimines and bromodifluoroacetamides via the Reformatsky reaction

α,α-Difluoro-β-amino amides are attractive building blocks of biologically active compounds such as fluorinated pharmaceutical mimics and oligopeptides. Herein, we describe the zinc-promoted Reformatsky reaction of aldimines using bromodifluoroacetamides which provides a direct synthetic approach to α,α-difluoro-β-amino amides. This method gave various N-PMP protected α,α-difluoro-β-amino-β-aryl amides in 64-95% yields. Furthermore, these amides were efficiently converted into 2,2-difluoropropane-1,3-diamines under reductive conditions using a combination of NaBH4 and BF3.

Origami with small molecules: exploiting the C-F bond as a conformational tool

When present within an organic molecule, the C-F bond tends to align in predictable ways with neighbouring functional groups, due to stereoelectronic effects such as hyperconjugation and electrostatic attraction/repulsion. These fluorine-derived conformational effects have been exploited to control the shapes, and thereby enhance the properties, of a wide variety of functional molecules including pharmaceutical agents, liquid crystals, fragrance chemicals, organocatalysts, and peptides. This comprehensive review summarises developments in this field during the period 2010-2024.

Recent advances in the synthesis of fluorinated amino acids and peptides

The site-selective modification of amino acids, peptides, and proteins has always been an intensive topic in organic synthesis, medicinal chemistry, and chemical biology due to the vital role of amino acids in life. Among the developed methods, the site-selective introduction of fluorine functionalities into amino acids and peptides has emerged as a useful approach to change their physicochemical and biological properties. With the increasing demand for life science, the direct fluorination/fluoroalkylation of proteins has also received increasing attention because of the unique properties of fluorine atom(s) that can change the protein structure, increase their lipophilicity, and enable fluorine functionality as a biological tracer or probe for chemical biology studies. In this feature article, we summarized the recent advances in the synthesis of fluorinated amino acids and peptides, wherein two strategies have been discussed. One is based on the fluorinated building blocks to prepare fluorinated amino acids and peptides with diversified structures, including the transformations of fluorinated imines and nickel-catalyzed dicarbofunctionalization of alkenes with bromodifluoroacetate and its derivatives; the other is direct fluorination/fluoroakylation of amino acids, peptides, and proteins, in which the selective transformations of the functional groups on serine, threonine, tyrosine, tryptophan, and cysteine lead to a wide range of fluorinated α-amino acids, peptides, and proteins, featuring synthetic convenience and late-stage modification of biomacromolecules. These two strategies complement each other, wherein transition-metal catalysis and new fluoroalkylating reagents provide powerful tools to selectively access fluorinated amino acids, peptides, and proteins, showing the prospect of medicinal chemistry and chemical biology.

Efficient access to β-amino acid ester/β-amino ketone derivatives via photocatalytic radical alkoxycabonylimidation/carbonylimidation of alkenes

A photocatalytic protocol for the synthesis of β-amino acid ester and β-amino ketone derivatives is developed using simple and easy-to-synthesize oxime oxalate and oxime phenylglyoxylate as difunctionalization reagents.

TMSCF2 Br-Enabled Fluorination-Aminocarbonylation of Aldehydes: Modular Access to α-Fluoroamides

A protocol for the modular assembly of the α-fluoroamide motif has been developed, which provides a practical method for the efficient synthesis of structurally diverse α-fluoroamides from easily available aldehydes and tertiary amines through a three-component fluorination-aminocarbonylation process. The key to the success of this process is taking advantage of the multiple roles of the unique difluorocarbene reagent TMSCF₂ Br (TMS=trimethylsilyl). The mechanism of the process involves the 1,2-fluorine and oxygen migrations of the in situ formed TMS-protected α-aminodifluoromethyl carbinol intermediates, which represents a new type of deoxyfluorination reaction.

Enantioenriched γ-Aminoalcohols, β-Amino Acids, β-Lactams, and Azetidines Featuring Tetrasubstituted Fluorinated Stereocenters via Palladacycle-Catalyzed Asymmetric Fluorination of Isoxazolinones

Enantiopure fluorine containing β-amino acids are of large biological and pharmaceutical interest. Strategies to prepare β-amino acid derivatives possessing a F-containing tetrasubstituted stereocenter at the α-C atom in a catalytic asymmetric sense are rare, in particular using an enantioselective electrophilic C-F bond formation. In the present study, a highly enantioselective palladacycle-catalyzed fluorination of isoxazolinones was developed. It is demonstrated that isoxazolinones are useful precursors toward enantiopure β-amino acid derivatives by diastereo- and chemoselective reduction. The formed γ-aminoalcohols served as valuable precursors toward β-amino acids, β-amino acid esters, and β-lactams, all featuring tetrasubstituted fluorinated stereocenters. In addition, by this work, enantioenriched fluorinated azetidines were accessible for the first time.

Chemical Aspects of Human and Environmental Overload with Fluorine

Over the last 100-120 years, due to the ever-increasing importance of fluorine-containing compounds in modern technology and daily life, the explosive development of the fluorochemical industry led to an enormous increase of emission of fluoride ions into the biosphere. This made it more and more important to understand the biological activities, metabolism, degradation, and possible environmental hazards of such substances. This comprehensive and critical review focuses on the effects of fluoride ions and organofluorine compounds (mainly pharmaceuticals and agrochemicals) on human health and the environment. To give a better overview, various connected topics are also discussed: reasons and trends of the advance of fluorine-containing pharmaceuticals and agrochemicals, metabolism of fluorinated drugs, withdrawn fluorinated drugs, natural sources of organic and inorganic fluorine compounds in the environment (including the biosphere), sources of fluoride intake, and finally biomarkers of fluoride exposure.

β-Aminopeptidases: Insight into Enzymes without a Known Natural Substrate

β-Aminopeptidases have the unique capability to hydrolyze N-terminal β-amino acids, with varied preferences for the nature of β-amino acid side chains. This unique capability makes them useful as biocatalysts for synthesis of β-peptides and to kinetically resolve β-peptides and amides for the production of enantiopure β-amino acids. To date, six β-aminopeptidases have been discovered and functionally characterized, five from Gram-negative bacteria and one from a fungus, Aspergillus Here we report on the purification and characterization of an additional four β-aminopeptidases, one from a Gram-positive bacterium, Mycolicibacterium smegmatis (BapAMs), one from a yeast, Yarrowia lipolytica (BapAYlip), and two from Gram-negative bacteria isolated from activated sludge identified as Burkholderia spp. (BapABcA5 and BapABcC1). The genes encoding β-aminopeptidases were cloned, expressed in Escherichia coli, and purified. The β-aminopeptidases were produced as inactive preproteins that underwent self-cleavage to form active enzymes comprised of two different subunits. The subunits, designated α and β, appeared to be tightly associated, as the active enzyme was recovered after immobilized-metal affinity chromatography (IMAC) purification, even though only the α-subunit was 6-histidine tagged. The enzymes were shown to hydrolyze chromogenic substrates with the N-terminal l-configurations β-homo-Gly (βhGly) and β3-homo-Leu (β3hLeu) with high activities. These enzymes displayed higher activity with H-βhGly-p-nitroanilide (H-βhGly-pNA) than previously characterized enzymes from other microorganisms. These data indicate that the new β-aminopeptidases are fully functional, adding to the toolbox of enzymes that could be used to produce β-peptides. Overexpression studies in Pseudomonas aeruginosa also showed that the β-aminopeptidases may play a role in some cellular functions.IMPORTANCE β-Aminopeptidases are unique enzymes found in a diverse range of microorganisms that can utilize synthetic β-peptides as a sole carbon source. Six β-aminopeptidases have been previously characterized with preferences for different β-amino acid substrates and have demonstrated the capability to catalyze not only the degradation of synthetic β-peptides but also the synthesis of short β-peptides. Identification of other β-aminopeptidases adds to this toolbox of enzymes with differing β-amino acid substrate preferences and kinetics. These enzymes have the potential to be utilized in the sustainable manufacture of β-amino acid derivatives and β-peptides for use in biomedical and biomaterial applications. This is important, because β-amino acids and β-peptides confer increased proteolytic resistance to bioactive compounds and form novel structures as well as structures similar to α-peptides. The discovery of new enzymes will also provide insight into the biological importance of these enzymes in nature.

Synthesis of α-CF3-proline derivatives by means of a formal (3 + 2)-cyclisation between trifluoropyruvate imines and Michael acceptors

We herein report the first formal (3 + 2)-cyclisation between 3,3,3-trifluoropyruvate-derived imines and indandione-based Michael acceptors. This reaction gives access to a novel class of spirocyclic α-CF3-α-proline derivatives with complete control of the diastereoselectivity under phase transfer-catalysed reaction conditions.

Polyaniline Doping by α,α-Difluoro-β-amino Acids

Currently, polyaniline (PANI) is considered as a promising polymer that can be used in biosensors, drug delivery systems, bioelectronics, etc. Its biocompatibility can be strongly improved by using dopants with biofunctionality. This study reveals the protonation/doping of PANI by fluorinated analogs of natural amino acids, namely, α,α-difluoro-β-amino acids (DFAAs) with alkyl and aromatic tails in N-methylpyrrolidone solutions. We find that these acids can dope PANI due to both the weakened basicity of their amino groups because of two fluorine atoms in α,α-positions and specific intermolecular interactions (π-π stacking, alkyl-π, F-π) of their tails with units of PANI chains. These interactions did not give the doped PANI salts with high conductivity but led to formation of stable PANI-DFAA complexes, which were confirmed both by clear changes in the UV-Vis and Fourier transform infrared spectra of the protonated/doped PANI and by their conductivity of ∼10^-6 S/cm. Our results suggest an applicability of such PANI complexes as carriers of DFAA for their biomedical applications.

Fluoro-Aryl Substituted α,β2,3-Peptides in the Development of Foldameric Antiparallel β-Sheets: A Conformational Study

α,β2,3-Disteroisomeric foldamers of general formula Boc(S-Ala-β-2R,3R-Fpg)nOMe or Boc(S-Ala-β-2S,3S-Fpg)nOMe were prepared from both enantiomers of syn H-2-(2-F-Phe)-h-PheGly-OH (named β-Fpg) and S-alanine. Our peptides show two appealing features for biomedical applications: the presence of fluorine, attractive for non-covalent interactions, and aryl groups, crucial for π-stacking. A conformational study was performed, using IR, NMR and computational studies of diastereoisomeric tetra- and hexapeptides containing the β2,3-amino acid in the R,R- and S,S-stereochemistry, respectively. We found that the stability of peptide conformation is dependent on the stereochemistry of the β-amino acid. Combining S-Ala with β-2R,3R-Fpg, a stable extended β-strand conformation was obtained. Furthermore, β-2R,3R-Fpg containing hexapeptide self-assembles to form antiparallel β-sheet structure stabilized by intermolecular H-bonds and π,π-interactions. These features make peptides containing the β2,3-fluoro amino acid very appealing for the development of bioactive proteolytically stable foldameric β-sheets as modulators of protein-protein interaction (PPI).

Access to Unprotected β-Fluoroalkyl β-Amino Acids and Their α-Hydroxy Derivatives

Unprotected β-(het)aryl-β-fluoroalkyl β-amino acids and their α-hydroxy derivatives can be readily obtained using a decarboxylative Mannich-type reaction without protection/deprotection steps. This protocol utilizes lithium hexamethyldisilazide and (het)arylfluoroalkyl ketones to generate NH-ketimine intermediates. The mild reaction conditions allow the preparation of original fluorinated β-amino acids as useful building blocks in a practical and scalable manner.

Palladium-catalyzed cross-coupling of unactivated alkylzinc reagents with 2-bromo-3,3,3-trifluoropropene and its application in the synthesis of fluorinated amino acids

A palladium-catalyzed cross-coupling of unactivated alkylzinc reagents with 2-bromo-3,3,3-trifluoropropene (BTP) has been developed, which was used as a key step to prepare a series of trifluoromethylated and difluoromethylated amino acids.

Combined experimental and theoretical study of long-range H–F interactions in α-fluoro amides

A combined experimental and computational approach provided insight into the nature and conformational dependence of long-range ⁴J_HF couplings in α-fluoro amides.

Mechanistic Insight into Asymmetric Hetero-Michael Addition of α,β-Unsaturated Carboxylic Acids Catalyzed by Multifunctional Thioureas

Carboxylic acids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylic acids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH₂ to α,β-unsaturated carboxylic acids with moderate to good enantioselectivities, the reaction mechanism remains to be clarified. Detailed investigation of the reaction using spectroscopic analysis and kinetic studies identified tetrahedral borate complexes, comprising two carboxylate anions, as reaction intermediates. We realized a dramatic improvement in product enantioselectivity with the addition of 1 equiv of benzoic acid. In this aza-Michael reaction, the boronic acid not only activates the carboxylate ligand as a Lewis acid, together with the thiourea NH-protons, but also functions as a Brønsted base through a benzoyloxy anion to activate the nucleophile. Moreover, molecular sieves were found to play an important role in generating the ternary borate complexes, which were crucial for obtaining high enantioselectivity as demonstrated by DFT calculations. We also designed a new thiourea catalyst for the intramolecular oxa-Michael addition to suppress another catalytic pathway via a binary borate complex using steric hindrance between the catalyst and substrate. Finally, to demonstrate the synthetic versatility of both hetero-Michael additions, we used them to accomplish the asymmetric synthesis of key intermediates in pharmaceutically important molecules, including sitagliptin and α-tocopherol.

Backbone-Fluorinated 1,2,3-Triazole-Containing Dipeptide Surrogates

The 1,2,3-triazole moiety can be incorporated as a peptide bond bioisostere to provide protease resistance in peptidomimetics. Herein, we report the synthesis of peptidomimetic building blocks containing backbone-fluorinated 1,4-disubstituted 1,2,3-triazole moieties. Synthetic protocols for the preparation of various Xaa-Gly dipeptide surrogates in the form of Xaa-ψ[triazole]-F₂Gly building blocks were established, and selected examples were introduced into the endogenous peptide opioid receptor ligand Leu-enkephalin as a model compound.

Chemoselective, Substrate-directed Fluorination of Functionalized Cyclopentane β-Amino Acids

This work describes a substrate-directed fluorination of some highly functionalized cyclopentane derivatives. The cyclic products incorporating CH₂ F or CHF₂ moieties in their structure have been synthesized from diexo- or diendo-norbornene β-amino acids following a stereocontrolled strategy. The synthetic study was based on an oxidative transformation of the ring carbon-carbon double bond of the norbornene β-amino acids, followed by transformation of the resulted "all cis" and "trans" diformyl intermediates by fluorination with "chemodifferentiation".

Diastereoselective Synthesis and Conformational Analysis of (2R)- and (2S)-Fluorostatines: An Approach Based on Organocatalytic Fluorination of a Chiral Aldehyde

Stereoselectively fluorinated analogues of the amino acid statine have been efficiently synthesized. The key step is an organocatalytic electrophilic fluorination of a chiral β-oxygenated aldehyde, which provided a test of both diastereoselectivity and chemoselectivity. The target statine analogues were found to adopt unique conformations influenced by the fluorine gauche effect, rendering them potentially valuable building blocks for incorporation into bioactive peptides.

Conformational preferences in the β-peptide oligomers of cis-2-amino-1-fluorocyclobutane-1-carboxylic acid

These oligomers adopt a regular zig-zag strand-like secondary structure which does not rely on intra-residue 6-ring hydrogen bonds for stability.

Trifluoromethyl-modified dipeptides by ZrCl4-promoted aza-Henry reactions

Chiral (R)-1-phenylethylamine was successfully employed in a tandem aza-Henry addition-reduction reaction to give chiral β-nitro α-trifluoromethyl amines. A subsequent coupling reaction with N-Boc-protected amino acids leads to obtain optically pure CF3-modified dipeptides carrying two different N-protecting groups. These peptidomimetic units are characterized by the presence of the [CH(CF3)NH] group as mimetic of the natural [CONH] peptidic bond and can be used for the synthesis of more complex CF3-modified peptides after selective deprotection of one of the two amine functions. 2D NMR spectral analyses were employed to determine the absolute configurations of all newly synthesized chiral compounds.