Asymmetric synthesis of phosphonotrifluoroalanine and its derivatives using N-tert-butanesulfinyl imine derived from fluoral

Chemistry and Pharmacology of Fluorinated Drugs Approved by the FDA (2016-2022)

Fluorine is characterized by high electronegativity and small atomic size, which provide this molecule with the unique property of augmenting the potency, selectivity, metabolic stability, and pharmacokinetics of drugs. Fluorine (F) substitution has been extensively explored in drug research as a means of improving biological activity and enhancing chemical or metabolic stability. Selective F substitution onto a therapeutic or diagnostic drug candidate can enhance several pharmacokinetic and physicochemical properties such as metabolic stability and membrane permeation. The increased binding ability of fluorinated drug target proteins has also been reported in some cases. An emerging line of research on F substitution has been addressed by using 18F as a radiolabel tracer atom in the extremely sensitive methodology of positron emission tomography (PET) imaging. This review aims to report on the fluorinated drugs approved by the US Food and Drug Administration (FDA) from 2016 to 2022. It cites selected examples from a variety of therapeutic and diagnostic drugs. FDA-approved drugs in this period have a variety of heterocyclic cores, including pyrrole, pyrazole, imidazole, triazole, pyridine, pyridone, pyridazine, pyrazine, pyrimidine, triazine, purine, indole, benzimidazole, isoquinoline, and quinoline appended with either F-18 or F-19. Some fluorinated oligonucleotides were also authorized by the FDA between 2019 and 2022.

Synthesis of Aminoalkyl Sclareolide Derivatives and Antifungal Activity Studies

Sclareolide was developed as an efficient C-nucleophilic reagent for an asymmetric Mannich addition reaction with a series of N-tert-butylsulfinyl aldimines. The Mannich reaction was carried out under mild conditions, affording the corresponding aminoalkyl sclareolide derivatives with up to 98% yield and 98:2:0:0 diastereoselectivity. Furthermore, the reaction could be performed on a gram scale without any reduction in yield and diastereoselectivity. Additionally, deprotection of the obtained Mannich addition products to give the target sclareolide derivatives bearing a free N-H group was demonstrated. In addition, target compounds 4-6 were subjected to an antifungal assay in vitro, which showed considerable antifungal activity against forest pathogenic fungi.

New Approved Drugs Appearing in the Pharmaceutical Market in 2022 Featuring Fragments of Tailor-Made Amino Acids and Fluorine

The strategic fluorination of oxidatively vulnerable sites in bioactive compounds is a relatively recent, widely used approach allowing us to modulate the stability, bio-absorption, and overall efficiency of pharmaceutical drugs. On the other hand, natural and tailor-made amino acids are traditionally used as basic scaffolds for the development of bioactive molecules. The main goal of this review article is to emphasize these general trends featured in recently approved pharmaceutical drugs.

Comparative study of different chiral ligands for dynamic kinetic resolution of amino acids

Dynamic kinetic resolution (DKR) of unprotected amino acids (AAs), via intermediate formation of Ni(II) complexes, is currently a leading methodology for preparation of natural and tailor-made AAs in enantiomerically pure form. In this work, we conduct a comparative case study of synthetic performance of four different ligands in DKR of six AAs representing aryl-, benzyl-, alkyl-, and long alkyl-type derivatives. The results of this study allow for rational selection of ligand/AA type to develop a practical procedure for preparation of target enantiomerically pure AAs.

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.

Asymmetric synthesis of the two enantiomers of β-phosphorus-containing α-amino acids via hydrophosphinylation and hydrophosphonylation of chiral Ni(ii)-complexes

A new approach for the synthesis of the two enantiomers of β-phosphorus-containing α-amino acids was developed via Michael addition of secondary phosphine oxides and dialkyl phosphites to chiral Ni(ii)-complexes of a dehydroalanine-Schiff base.

Asymmetric synthesis of (S)-3-methyleneglutamic acid and its N-Fmoc derivative via Michael addition-elimination reaction of chiral glycine Ni (II) complex with enol tosylates

The use of chiral Ni (II)-complexes of glycine Schiff bases has recently emerged as a leading methodology for asymmetric synthesis of structurally diverse Tailor-Made Amino Acids™, playing a key role in the design of modern pharmaceuticals. Here, we report first example of enantioselective preparation of (S)-3-methyleneglutamic acid and its N-Fmoc derivative via a new type of Michael addition-elimination reaction between chiral nucleophilic glycine equivalent and enol tosylates. This reaction was found to proceed with excellent yield (91%) and diastereoselectivity (>99/1 de) allowing straightforward asymmetric synthesis of (S)-3-methyleneglutamic acid derivatives and analogues. The observed results bode well for general application of this Ni (II) complex approach for preparation and biological studies of this previously unknown type of Tailor-Made Amino Acids™.

Kitamura Electrophilic Fluorination Using HF as a Source of Fluorine

This review article focused on the innovative procedure for electrophilic fluorination using HF and in situ generation of the required electrophilic species derived from hypervalent iodine compounds. The areas of synthetic application of this approach include fluorination of 1,3-dicarbonyl compounds, aryl-alkyl ketones, styrene derivatives, α,β-unsaturated ketones and alcohols, homoallyl amine and homoallyl alcohol derivatives, 3-butenoic acids and alkynes.

Asymmetric Synthesis of 4,4-(Difluoro)glutamic Acid via Chiral Ni(II)-Complexes of Dehydroalanine Schiff Bases. Effect of the Chiral Ligands Structure on the Stereochemical Outcome

Four differently substituted chiral Ni(II)-complexes of dehydroalanine Schiff base were prepared and reacted with BrCF2COOEt/Cu under the standard reaction conditions. The observed diastereoselectivity was found to depend on the degree and pattern of chlorine substitution for hydrogen in the structure of the dehydroalanine complexes. The unsubstituted complex gave the ratio of diastereomers (S)(2S)/(S)(2R) of 66/34. On the other hand, introduction of chlorine atoms in the strategic positions on the chiral ligands allowed to achieve a practically attractive diastereoselectivity of (∼98.5/1.5). Diastereomerically pure major product was disassembled to prepare 9-fluorenylmethyloxycarbonyl (Fmoc) derivative of (S)-4,4-difluoroglutamic acid.

Preparative Method for Asymmetric Synthesis of (S)-2-Amino-4,4,4-trifluorobutanoic Acid

Enantiomerically pure derivatives of 2-amino-4,4,4-trifluorobutanoic acid are in great demand as bioisostere of leucine moiety in the drug design. Here, we disclose a method specifically developed for large-scale (>150 g) preparation of the target (S)-N-Fmoc-2-amino-4,4,4-trifluorobutanoic acid. The method employs a recyclable chiral auxiliary to form the corresponding Ni(II) complex with glycine Schiff base, which is alkylated with CF3-CH2-I under basic conditions. The resultant alkylated Ni(II) complex is disassembled to reclaim the chiral auxiliary and 2-amino-4,4,4-trifluorobutanoic acid, which is in situ converted to the N-Fmoc derivative. The whole procedure was reproduced several times for consecutive preparation of over 300 g of the target (S)-N-Fmoc-2-amino-4,4,4-trifluorobutanoic acid.

Applications of fluorine-containing amino acids for drug design

Fluorine-containing amino acids are becoming increasingly prominent in new drugs due to two general trends in the modern pharmaceutical industry. Firstly, the growing acceptance of peptides and modified peptides as drugs; and secondly, fluorine editing has become a prevalent protocol in drug-candidate optimization. Accordingly, fluorine-containing amino acids represent one of the more promising and rapidly developing areas of research in organic, bio-organic and medicinal chemistry. The goal of this Review article is to highlight the current state-of-the-art in this area by profiling 42 selected compounds that combine fluorine and amino acid structural elements. The compounds under discussion represent pharmaceutical drugs currently on the market, or in clinical trials as well as examples of drug-candidates that although withdrawn from development had a significant impact on the progress of medicinal chemistry and/or provided a deeper understanding of the nature and mechanism of biological action. For each compound, we present features of biological activity, a brief history of the design principles and the development of the synthetic approach, focusing on the source of tailor-made amino acid structures and fluorination methods. General aspects of the medicinal chemistry of fluorine-containing amino acids and synthetic methodology are briefly discussed.

The self-disproportionation of enantiomers (SDE) via column chromatography of β-amino-α,α-difluorophosphonic acid derivatives

This work presents the first study of the self-disproportionation of enantiomers via chromatography (SDEvC) of β-aminophosphonic acid esters, several of which have been synthesized for the first time. Three types of structures were examined, N-acetylated, dipeptide construction with N-Cbz glycine, and a free amine. In the latter case, this is the first time that SDEvC has been reported for free amine amino acids. In all the three types of structures, significant SDE magnitudes (Δee's up to 55%) were exhibited underscoring the ubiquitous nature of the SDE phenomenon. Chemical models of homo- versus heterochiral intermolecular interactions are proposed to rationalize the SDE magnitude differences amongst these new β-aminophosphonic acid derivatives. In addition, the incorporation of additional, competing binding modes to a molecule, was found to lead to a reduction of the SDE magnitude by shifting the intermolecular binding away from the stereogenic center and/or by leading to a convoluted binding system that disrupts the structured and relatively stable assemblies that give rise to the SDE.

Practical Method for Preparation of (S)-2-Amino-5,5,5-trifluoropentanoic Acid via Dynamic Kinetic Resolution

This work reports an operationally convenient ∼20 g scale synthesis of (S)-2-amino-5,5,5-trifluoropentanoic acid and its Fmoc-derivative via dynamic kinetic resolution of the corresponding racemate.

Large-Scale Asymmetric Synthesis of Fmoc-(S)-2-Amino-6,6,6-Trifluorohexanoic Acid

Here we report the first large-scale synthesis of Fmoc-(S)-2-amino-6,6,6-trifluorohexanoic acid via asymmetric alkylation of chiral Ni(II)-complex of glycine Schiff base with CF3(CH2)3I. The synthesis was performed on over 100 g scale and can be recommended as the most advanced procedure for reliable preparation of large amounts of enantiomerically pure Fmoc-(S)-2-amino-6,6,6-trifluorohexanoic acid for protein engineering and drug design. Chiral auxiliary used in this protocol can be >90 % recovered and reused.

Optical Resolution of Rimantadine

This work discloses a new procedure for the resolution of commercially available racemic rimantadine hydrochloride to enantiomerically pure (S)-rimantadine using (R)-phenoxypropionic acid as a recyclable resolving reagent. Good chemical yields, operational ease, and low-cost structure underscore the preparative value of this method for the production of enantiomerically pure rimantadine for medicinal or synthetic studies.

Convenient Asymmetric Synthesis of Fmoc-(S)-6,6,6-Trifluoro-Norleucine

In this work we report a convenient asymmetric synthesis of Fmoc-(S)-6,6,6-trifluoro-norleucine via alkylation reaction of chiral glycine equivalent. The target amino acid of 99% enantiomeric purity was prepared with 82.4% total yield (three steps).

Effect of substituents on the configurational stability of the stereogenic nitrogen in metal(II) complexes of α-amino acid Schiff bases

Herein, we report a general method for quantitative measurement of the configurational stability of the stereogenic nitrogen coordinated to M (II) in the corresponding square planar complexes. This stereochemical approach is quite sensitive to steric and electronic effects of the substituents and shown to work well for Ni(II), Pd(II), and Cu(II) complexes. Structural simplicity of the compounds used, coupled with high sensitivity and reliability of experimental procedures, bodes well for application of this approach in evaluation of chemical stability and stereochemical properties of newly designed chiral ligands for general asymmetric synthesis of tailor-made amino acids.

Mannich-type addition of 1,3-dicarbonyl compounds to chiral tert-butanesulfinyltrifluoroacetaldimines. Mechanistic aspects and chiroptical studies

Mannich-type addition of 1,3-dicarbonyl compounds to (SS)-N-tert-butanesulfinyltrifluoroacetaldimine has been carried out under dramatically different conditions. The stereochemical outcome was quite different when the reaction was carried out under solvent-free conditions, at high temperature and without any catalyst or additive, compared with the DBU catalyzed reaction in dichloromethane solution at low temperature. Mechanistic aspects of the reaction under both the conditions are discussed. In this respect, vibrational (VCD) and electronic circular dichroism (ECD) and optical rotatory dispersion (ORD) experiments proved to be valuable tools for determining the absolute configurations of the reaction products.

Second-order asymmetric transformation and its application for the practical synthesis of α-amino acids

We report a discovery of a new rimantadine [1-(1-adamantyl)ethanamine]-derived chiral ligand and its application for the preparation of α-amino acids using the second-order asymmetric transformation approach. The operational ease of experimental procedures coupled with excellent chemical yields and stereochemical outcome suggests some potential synthetic generality of this approach.

Chromatographic approach to study the configurational stability of Ni(II) complexes of amino-acid Schiff bases possessing stereogenic nitrogen

Herein, we disclose the design of a model Ni(II) complex of glycine Schiff base possessing single-nitrogen stereogenic center, which was successfully used for high-performance liquid chromatography (HPLC)-assisted assessment of its configurational stability. The major finding is that the configurational stability of the Ni(II)-coordinated nitrogen is profoundly dependent on the reaction conditions used, in particular the solvent, and can range from inconsequential (t_½ less than 5 min) to virtually completely stable (t_½ 90 y). The discovery reported in this study most likely to be of certain theoretical and synthetic value.

Tandem Alkylation-Second-Order Asymmetric Transformation Protocol for the Preparation of Phenylalanine-Type Tailor-Made α-Amino Acids

In this work, we disclose an advanced general process for the synthesis of tailor-made α-amino acids (α-AAs) via tandem alkylation-second-order asymmetric transformation. The first step is the alkylation of the chiral Ni(II) complex of glycine Schiff base, which is conducted under mild phase-transfer conditions allowing the structural construction of target α-AAs. The second step is based on the methodologically rare second-order asymmetric transformation, resulting in nearly complete precipitation of the corresponding (S_C,R_N,R_C)-configured diastereomer, which can be collected by a simple filtration. The operational convenience and potential scalability of all experimental procedures, coupled with excellent stereochemical outcome, render this method of high synthetic value for the preparation of various tailor-made α-AAs.

Modern Approaches for Asymmetric Construction of Carbon-Fluorine Quaternary Stereogenic Centers: Synthetic Challenges and Pharmaceutical Needs

New methods for preparation of tailor-made fluorine-containing compounds are in extremely high demand in nearly every sector of chemical industry. The asymmetric construction of quaternary C-F stereogenic centers is the most synthetically challenging and, consequently, the least developed area of research. As a reflection of this apparent methodological deficit, pharmaceutical drugs featuring C-F stereogenic centers constitute less than 1% of all fluorine-containing medicines currently on the market or in clinical development. Here we provide a comprehensive review of current research activity in this area, including such general directions as asymmetric electrophilic fluorination via organocatalytic and transition-metal catalyzed reactions, asymmetric elaboration of fluorine-containing substrates via alkylations, Mannich, Michael, and aldol additions, cross-coupling reactions, and biocatalytic approaches.

Axially chiral Ni(II) complexes of α-amino acids: Separation of enantiomers and kinetics of racemization

Herein we present design, synthesis, chiral HPLC resolution, and kinetics of racemization of axially chiral Ni(II) complexes of glycine and di-(benzyl)glycine Schiff bases. We found that while the ortho-fluoro derivatives are configurationally unstable, the pure enantiomers of corresponding axially chiral ortho-chloro-containing complexes can be isolated by preparative HPLC and show exceptional configurational stability (t_1/2 from 4 to 216 centuries) at ambient conditions. Synthetic implications of this discovery for the development of new generation of axially chiral auxiliaries, useful for general asymmetric synthesis of α-amino acids, are discussed.

Analysis of crystallographic structures of Ni(ii) complexes of α-amino acid Schiff bases: elucidation of the substituent effect on stereochemical preferences

This work disclosed the significance of a parallel displaced type of aromatic interactions between o-amino-benzophenone and N-benzyl rings in Ni(ii) complexes.

Asymmetric synthesis of α-deuterated α-amino acids

A generalized approach for the preparation of α-²H-α-amino acids in enantiomerically pure form and with up to 99% deuteration is disclosed.

Detrifluoroacetylative in Situ Generation of Free 3-Fluoroindolin-2-one-Derived Tertiary Enolates: Design, Synthesis, and Assessment of Reactivity toward Asymmetric Mannich Reactions

The discovery of detrifluoroacetylative in situ generation of a new type of fluorinated amide enolates derived from 3-fluoroindolin-2-one and their asymmetric Mannich additions with sulfinylaldimines bearing fluoroalkyl groups is reported, which afforded α-fluoro-β-(fluoroalkyl)-β-aminoindolin-2-ones containing C-F quaternary stereogenic centers with excellent yields and high diastereoselectivities.

Asymmetric synthesis of quaternary α-fluoro-β-keto-amines via detrifluoroacetylative Mannich reactions

Efficient asymmetric detrifluoroacetylative Mannich addition reactions between 2-fluoro-1,3-di-ketones/hydrates and chiral N-sulfinyl-imines via C-C bond cleavage were reported, which afforded C-F quaternary α-fluoro-β-keto-amines with excellent yields and high diastereoselectivity.