Recent Advances in Catalytic Enantioselective Synthesis of Fluorinated α‐ and β‐Amino Acids

Copper-Catalyzed Selective Amino-alkoxycarbonylation of Unactivated Alkenes with CO

1,2-Amino-difunctionalization reactions of alkenes allow the efficient introduction of different functional groups and the rapid construction of valuable functionalized amines. In this respect, we report a copper-catalyzed 1,2-amino-alkoxycarbonylation of unactivated alkenes with CO and alkylamine precursors in the presence of a Lewis acid additive. The novel protocol allows direct access to valuable β-amino acid derivatives from easily available starting materials. The presented methods feature high chemo- and regioselectivities, good functional group tolerance, and substrate scope including diverse bioactive compounds and drug-like molecules. Mechanistic studies indicate that the Lewis acid additive is the key to realizing the efficient umpolung addition of nucleophilic aminyl radicals to electron-rich alkenes, which represents an elegant activation strategy for aminyl radicals.

Stability studies of β-Amino- and β-Hydroxy difluoromethyl ketones in rat serum and rat liver microsomes

Although difluoromethyl ketones are used as tools in chemical biology and leads in drug discovery, the metabolic stability of these compounds is generally uncharacterized and must be inferred from in vivo pharmacological assays. In order to address this gap which impedes their wider use, we have synthesized and performed metabolic stability studies for thirty-nine β-amino and β-hydroxy difluoromethyl ketones. These investigations provide structure-stability relationships of the difluoromethyl ketones following incubation with rodent serum and liver microsomes.

Desymmetrization of Geminal Difluoromethylenes using a Palladium/Copper/Lithium Ternary System for the Stereodivergent Synthesis of Fluorinated Amino Acids

Fluorinated amino acids and related peptides/proteins have been found widespread applications in pharmaceutical and agricultural compounds. However, strategies for introducing a C-F bond into amino acids in an enantioselective manner are still limited and no such asymmetric catalysis strategy has been reported. Herein, we have successfully developed a Pd/Cu/Li ternary system for stereodivergent synthesis of chiral fluorinated amino acids. This method involves a sequential desymmetrization of geminal difluoromethylenes and allylic substitution with amino acid Schiff bases via Pd/Li and Pd/Cu dual activation, respectively. A series of non-natural amino acids bearing a chiral allylic/benzylic fluorine motif are easily synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities (up to >20 : 1 dr and >99 % ee). A density functional theory (DFT) study revealed the F-Cu interaction of the allylic substrate and the Cu catalyst significantly influence the stereoselectivity.

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.

Tinker, Tailor, Soldier, Spy: The Diverse Roles That Fluorine Can Play within Amino Acid Side Chains

Side chain-fluorinated amino acids are useful tools in medicinal chemistry and protein science. In this review, we outline some general strategies for incorporating fluorine atom(s) into amino acid side chains and for elaborating such building blocks into more complex fluorinated peptides and proteins. We then describe the diverse benefits that fluorine can offer when located within amino acid side chains, including enabling 19F NMR and 18F PET imaging applications, enhancing pharmacokinetic properties, controlling molecular conformation, and optimizing target-binding.

A transaminase with β-activity from Variovorax boronicumulans for the production of enantiopure β-amino acids

Enantioselective transamination of amino acids is a great challenge in biotechnology as suitable enzymes with wide substrate spectrum are rare. Here, we present a new transaminase from Variovorax boronicumulans (VboTA, Variovorax boronicumulansω-transaminase) which is specific for β-amino acids. The amino acid sequence of VboTA is similar to an ω-transaminase from Variovorax paradoxus, for which a crystal-structure is available. This similarity is allowing us to classify VboTA as a fold type 1 ω-transaminase (ω-TA). Although both enzymes have a high sequence similarity (86% identities, 92% positives), there are differences in the active center, which allow VboTA to accept a broader substrate spectrum. Both enzymes have also a different temperature stability and temperature optimum. VboTA deaminates the D-form of aromatic β-amino acids, such as β-homophenylalanine and β-phenylalanine as well as aliphatic β-amino acids, such as β-homoalanine and β-leucine. The optimal reaction conditions turned out to be 32 °C and pH 9. Kinetic resolution lead to high enantiomeric excess of 86.6% to >99.9%, depending on the amino donor/acceptor pair. In contrast to many other ω-TAs, VboTA has a broad substrate spectrum and uses both aromatic or aliphatic amino acids. With γ-amino acids as substrates, VboTA showed no activity at all.

Organocatalytic asymmetric nitroso aldol reaction of α-substituted malonamates

A practical enantioselective N-selective nitroso aldol reaction of α-methylmalonamates with a nitrosoarene is reported. The reaction employs the Takemoto thiourea catalyst for the induction of enantioselectivity, and the corresponding optically active oxyaminated malonamates were obtained in reasonably good yields.

Construction of Fluorinated Amino Acid Derivatives via Cobalt-Catalyzed Oxidative Difunctionalization of Cyclic Ethers

Via difunctionalization of the α- and β-sites of cyclic ethers, we herein demonstrate a new synthetic method for the efficient construction of novel fluorinated γ-amino acid esters by employing a CoBr₂/m-CPBA catalyst system. Several cyclic ethers were transformed in combination with a vast range of amines and ethyl trifluoropyruvate into the desired products under mild conditions, making this method a practical platform to enrich the library of fluorinated amino acid derivatives from cost-effective and readily available feedstocks.

Access to enantioenriched molecules with diverse fluorinated tetrasubstituted stereocenters using hydroxy as a kinetic resolution auxiliary group

We describe in this paper that using secondary OH as the kinetic resolution auxiliary group, a series of previously unavailable fluorinated fully-substituted carbon molecules can be obtained with excellent level of enantioselectivities.

Catalytic asymmetric synthesis of α-stereogenic carboxylic acids: recent advances

Chiral carboxylic acids bearing an α-stereogenic center constitute the backbone of many natural products and therapeutic reagents as well as privileged chiral ligands and catalysts. Hence, it is not surprising that a large number of elegant catalytic asymmetric strategies have been developed toward the efficient synthesis of α-chiral carboxylic acids, such as α-hydroxy acids and α-amino acids. In this review, the recent advances in asymmetric synthesis of α-stereogenic free carboxylic acids via organocatalysis and transition metal catalysis are summarized (mainly from 2010 to 2020). The content is organized by the reaction type of the carboxyl source involved, including asymmetric functionalization of substituted carboxylic acids, cyclic anhydrides, α-keto acids, substituted α,β-unsaturated acids and so on. We hope that this review will motivate further interest in catalytic asymmetric synthesis of chiral α-substituted carboxylic acids.

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.

Enzymatic synthesis of fluorinated compounds

Fluorinated compounds are widely used in the fields of molecular imaging, pharmaceuticals, and materials. Fluorinated natural products in nature are rare, and the introduction of fluorine atoms into organic compound molecules can give these compounds new functions and make them have better performance. Therefore, the synthesis of fluorides has attracted more and more attention from biologists and chemists. Even so, achieving selective fluorination is still a huge challenge under mild conditions. In this review, the research progress of enzymatic synthesis of fluorinated compounds is summarized since 2015, including cytochrome P450 enzymes, aldolases, fluoroacetyl coenzyme A thioesterases, lipases, transaminases, reductive aminases, purine nucleoside phosphorylases, polyketide synthases, fluoroacetate dehalogenases, tyrosine phenol-lyases, glycosidases, fluorinases, and multienzyme system. Of all enzyme-catalyzed synthesis methods, the direct formation of the C-F bond by fluorinase is the most effective and promising method. The structure and catalytic mechanism of fluorinase are introduced to understand fluorobiochemistry. Furthermore, the distribution, applications, and future development trends of fluorinated compounds are also outlined. Hopefully, this review will help researchers to understand the significance of enzymatic methods for the synthesis of fluorinated compounds and find or create excellent fluoride synthase in future research.Key points• Fluorinated compounds are distributed in plants and microorganisms, and are used in imaging, medicine, materials science.• Enzyme catalysis is essential for the synthesis of fluorinated compounds.• The loop structure of fluorinase is the key to forming the C-F bond.

New pharmaceuticals approved by FDA in 2020: Small-molecule drugs derived from amino acids and related compounds

Amino acids (AAs) play an important role in the modern health industry as key synthetic precursors for pharmaceuticals, biomaterials, biosensors, and drug delivery systems. Currently, over 30% of small-molecule drugs contain residues of tailor-made AAs or derived from them amino-alcohols and di-amines. In this review article, we profile 12 AA-derived new pharmaceuticals approved by the FDA in 2020. These newly introduced drugs include Tazverik (epithelioid sarcoma), Gemtesa (overactive bladder), Zeposia (multiple sclerosis), Byfavo (induction and maintenance of procedural sedation), Cu 64 dotatate, and Gallium 68 PSMA-11 (both PET imaging), Rimegepant (acute migraine), Zepzelca (lung cancer), Remdesivir (COVID-19), Amisulpride (nausea and vomiting), Setmelanotide (obesity), and Lonafarnib (progeria syndrome). For each compound, we describe the spectrum of biological activity, medicinal chemistry discovery, and synthetic preparation.

Biomimetic enantioselective synthesis of β,β-difluoro-α-amino acid derivatives

Although utilization of fluorine compounds has a long history, synthesis of chiral fluorinated amino acid derivatives with structural diversity and high stereoselectivity is still very appealing and challenging. Here, we report a biomimetic study of enantioselective [1,3]-proton shift of β,β-difluoro-α-imine amides catalyzed by chiral quinine derivatives. A wide range of corresponding β,β-difluoro-α-amino amides were achieved in good yields with high enantioselectivities. The optically pure β,β-difluoro-α-amino acid derivatives were further obtained, which have high application values in the synthesis of fluoro peptides, fluoro amino alcohols and other valuable fluorine-containing molecules.

Asymmetric Synthesis of 3-Phenyl-2,3-dihydro-1H-pyrrolo[3,2-b]pyridine-3-carbonitriles Catalyzed by Phase-Transfer Catalyst Derived from tert-Leucine

Chiral phase-transfer catalysts derived from tert-leucine were synthesized and used in the asymmetric synthesis of 4-azaindoline derivatives. By this method, both enantiomers of the corresponding products were obtained in excellent yield (up to 99%) with high enantio­selectivities (up to 91% ee) and diastereoselectivities (up to >99: 1 dr).

Synthesis and Biological Evaluation of CF3 Se-Substituted α-Amino Acid Derivatives

Several CF₃ Se-substituted α-amino acid derivatives, such as (R)-2-amino-3-((trifluoromethyl)selanyl)propanoates (5 a/6 a), (S)-2-amino-4-((trifluoromethyl)selanyl)butanoates (5 b/6 b), (2R,3R)-2-amino-3-((trifluoromethyl)selanyl)butanoates (5 c/6 c), (R)-2-((S)-2-amino-3-phenylpropanamido)-3-((trifluoromethyl)selanyl)propanoates (11 a/12 a), and (R)-2-(2-aminoacetamido)-3-((trifluoromethyl)selanyl)propanoates (11 b/12 b), were readily synthesized from natural amino acids and [Me₄ N][SeCF₃ ]. The primary in vitro cytotoxicity assays revealed that compounds 6 a, 11 a and 12 a were more effective cell growth inhibitors than the other tested CF₃ Se-substituted derivatives towards MCF-7, HCT116, and SK-OV-3 cells, with their IC₅₀ values being less than 10 μM for MCF-7 and HCT116 cells. This study indicated the potentials of CF₃ Se moiety as a pharmaceutically relevant group in the design and synthesis of novel biologically active molecules.

Copper-Catalyzed Enantioselective Difluoromethylation of Amino Acids via Difluorocarbene

Difluoromethyl amino acids (DFAA) exhibit intriguing biological properties, making them highly desirable motifs in agrochemical and pharmaceutical science. However, stereochemical control of direct difluoromethyl transformation via the difluorocarbene species has not been demonstrated. Here we describe an efficient copper-catalyzed asymmetric difluoromethylation reaction that systematically delivers chiral DFAA as rationally designed mechanism-based inhibitors of PLP-dependent amino acid decarboxylases. The reaction employs difluoromonochloromethane, an abundant raw material, as the direct precursor of difluorocarbene species, enabling the unprecedentedly direct conversion of amino esters into corresponding valuable DFAA products in good yields with excellent enantioselectivities. This de novo synthesis creates opportunities to integrate an asymmetric catalytic platform for the preparation of diverse libraries of biologically important DFAA derivatives and will support efforts in both drug discovery and development.

Tailor-made amino acids in the design of small-molecule blockbuster drugs

The role of amino acids (AAs) in modern health industry is well-appreciated. Residues of individual AAs, or their chemical modifications, such as diamines and amino alcohols, are frequently found in the structures of modern pharmaceuticals. The goal of this review article, is to emphasize that, currently, tailor-made AAs serve as key structural features in many most successful pharmaceuticals, so-called blockbuster drugs. In the present article, we profile 14 small-molecule drugs, underscoring the breadth of structural variety of AAs applications in numerous therapeutic areas. For each compound, we provide spectrum of biological activity, medicinal chemistry discovery, and synthetic approaches.

Efficient Synthesis of New Fluorinated β-Amino Acid Enantiomers through Lipase-Catalyzed Hydrolysis

An efficient and novel enzymatic method has been developed for the synthesis of β-fluorophenyl-substituted β-amino acid enantiomers through lipase PSIM (Burkholderia cepasia) catalyzed hydrolysis of racemic β-amino carboxylic ester hydrochloride salts 3a–e in iPr₂O at 45 °C in the presence of Et₃N and H₂O. Adequate analytical methods were developed for the enantio-separation of racemic β-amino carboxylic ester hydrochlorides 3a–e and β-amino acids 2a–e. Preparative-scale resolutions furnished unreacted amino esters (R)-4a–e and product amino acids (S)-5a–e with excellent ee values (≥99%) and good chemical yields (>48%).