Recent approaches for asymmetric synthesis of α-amino acids via homologation of Ni(II) complexes

Synthesis of metal-binding amino acids

The ability for amino acid residues to bind metals underpins the functions of metalloproteins to conduct a plethora of critical processes in living organisms as well as unnatural applications in the fields of catalysis, sensing and medicinal chemistry. The capability to access metal-binding peptides heavily relies on the ability to generate appropriate building blocks. This review outlines recently developed strategies for the synthesis of metal binding non-proteinogenic amino acids. The chemistries to access, as well as to incorporate these amino acids into peptides is presented herein.

Phosphine Catalyzed Michael-Type Additions: The Synthesis of Glutamic Acid Derivatives from Arylidene-α-amino Esters

The reaction of arylidene-α-amino esters with electrophilic alkenes to yield Michael-type addition compounds is optimized using several phosphines as organocatalysts. The transformation is very complicated due to the generation of several final compounds, including those derived from the 1,3-dipolar cycloadditions. For this reason, the selection of the reaction conditions is a very complex task and the slow addition of the acrylic system is very important to complete the process. The study of the variation in the structural components of the starting imino ester is performed as well as the expansion of other electron-poor alkenes. The crude products have a purity higher than 90% in most cases without any purification. A plausible mechanism is detailed based on the bibliography and the experimental results. The synthesis of pyroglutamate entities, after the reduction of the imino group and cyclization, is performed in high yields. In addition, the hydrolysis of the imino group, under acidic media, represents a direct access to glutamate surrogates.

A synthetic route to artificial chiral α-amino acids featuring a 3,4-dihydroisoquinolone core through a Rh(III)-catalyzed functionalization of allyl groups in chiral Ni(II) complexes

Currently, non-proteinogenic α-amino acids (α-AAs) have attracted increasing interest in bio- and medicinal chemistry. In this context, the first protocol for the asymmetric synthesis of artificial α-AAs featuring a 3,4-dihydroisoquinolone core with two stereogenic centers was successfully elaborated. A straightforward Rh(III)-catalysed C-H activation/annulation reaction of various aryl hydroxamates with a set of robust and readily available chiral Ni(II) complexes, which have allylic appendages derived from glycine (Gly), alanine (Ala) and phenylalanine (Phe), allowed incorporation of a 3,4-dihydroisoquinolone scaffold into the chiral amino acid residue. The reaction was performed in methanol and under mild conditions (at room temperature under air atmosphere), providing separable diastereomeric complexes with up to 94% total yield. The target α-AA with a 3,4-dihydroisoquinolone core in an enantiopure form was subsequently released from the obtained chiral Ni(II) complexes via an acidic decomposition in aqueous HCl, along with the recovery of the chiral auxiliary ligand.

Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

A new oxidatively stable ligand for the chiral functionalization of amino acids in Ni(II)-Schiff base complexes

A new oxidatively stable (S)-N-benzylproline-derived ligand ((S)-N-(2-benzoyl-5-tert-butylphenyl)-1-benzylpyrrolidine-2-carboxamide) and its Ni(II)-Schiff base complexes formed of glycine, serine, and dehydroalanine are reported. A bulky tert-butyl substituent in the phenylene fragment precludes unwanted oxidative dimerization of the Schiff base complex, making it suitable for targeted electrochemically induced oxidative modification of the amino acid side chain. Experimental and DFT studies showed that the additional tert-butyl group increases the dispersion interactions in the Ni coordination environment making the complexes more conformationally rigid and provides a higher level of thermodynamically controlled stereoselectivity as compared to the parent Belokon complex. Additionally, functionalization with the tert-butyl group significantly enhances the reactivity of the deprotonated glycine complex towards electrophiles as compared to the anionic species formed from the original Belokon complex. Solubility of the t-Bu-containing ligand and its Schiff base complexes is increased, facilitating scaling-up the reaction procedure and isolation of the functionalized amino acid.

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.

Pd(II) Complexes of Chiral Proline-Derived Ligands: Application for Dynamic Thermodynamic Resolution of α-Amino Acids and Their Antibacterial Activities

Novel type of Pd(II) complexes have been synthesized under operationally simple and convenient conditions and applied in the dynamic thermodynamic resolution of racemic N,C-unprotected α-amino acids. After rapid hydrolysis, these Pd(II) complexes produced the corresponding α-amino acids in satisfactory yields and enantioselectivities, accompanied by the recyclable proline-derived ligand. In addition, the method can be readily applied for S/R interconversion to obtain unnatural (R)-α-amino acids from readily available (S)-α-amino acids. Furthermore, biological assays showed that Pd(II) complexes (S,S)-3i and (S,S)-3m exhibited significant antibacterial activities similar to vancomycin, which may represent promising lead structures for further development of antibacterial agents.

An asymmetric metal-templated route to amino acids with an isoquinolone core via a Rh(III)-catalyzed coupling of aryl hydroxamates with chiral propargylglycine Ni(II) complexes

A general protocol for the asymmetric synthesis of artificial amino acids (AAs) comprising an isoquinolone skeleton was successfully elaborated via a straightforward Rh(III)-catalyzed C-H activation/annulation of various aryl hydroxamates with a series of robust chiral propargylglycine Ni(II) complexes derived from glycine (Gly), alanine (Ala) and phenylalanine (Phe) in a green solvent (methanol) under mild conditions (at room temperature under air). Notably, in the case of phenylalanine-derived complexes, the formation of unfavorable 4-substituted isoquinolone regioisomers was achieved by a catalyst control for the first time. The subsequent acidic decomposition of the obtained Ni(II) complexes provides the target unnatural α- and α,α-disubstituted AAs with an isoquinolone core in an enantiopure form.

Design of Photoredox-Catalyzed Giese-Type Reaction for the Synthesis of Chiral Quaternary α-Aryl Amino Acid Derivatives via Clayden Rearrangement

Chiral quaternary α-aryl amino acids are biologically valued but synthetically challenging building blocks. Herein, we report a strategy for the synthesis of molecular architectures by unifying a photoredox catalytic asymmetric Giese-type reaction and Clayden rearrangement. A new class of chiral Karady-Beckwith dehydroalanines is designed and serves as a versatile handle for the photoredox-mediated highly stereoselective Giese-type reaction with feedstock carboxylic acids and tertiary amines. Subsequent Clayden rearrangement delivers chiral quaternary α-aryl amino acid derivatives with high stereoselectivity. The versatile approach offers a reliable source for the assembly of highly demanding chiral building blocks.

Rapid and efficient syntheses of tryptophans using a continuous-flow quaternization-substitution reaction of gramines with a chiral nucleophilic glycine equivalent

A continuous-flow quaternization reaction of gramines with MeI (<1 min) followed by a substitution reaction with a chiral nucleophilic glycine-derived Ni-complex (S)-2 (<1 min) has successfully been developed to afford the corresponding alkylated Ni-complexes 3 in good yields with excellent diastereoselectivity, based on the results of a one-pot quaternization-substitution reaction of gramines with (S)-2 in a batch process. The continuous-flow process allowed the safe and efficient scale-up synthesis of 3j (84% yield, 99% de, 540 g h^-1) to give 7-azatryptophan derivative (S)-4j readily by an acid-catalyzed hydrolysis reaction followed by protection with an Fmoc group. The present method for the rapid and efficient syntheses of enantiopure unnatural tryptophan derivatives from various gramines and (S)-2 will be useful to further promote peptide and protein drug discovery and development research.

Cu(II) Complexes with Proline-Derived Schiff Base Ligand: Chemical Resolution of N,C-Unprotected α-Amino Acids and Their Antibacterial Activity

An operationally simple and convenient resolution method via Cu(II) complexes was reported, efficiently providing valuable enantiopure N,C-unprotected α-amino acids. This protocol features synthetically attractive yields and a stereochemical outcome, using a recyclable Schiff base ligand and inexpensive easily accessible metal copper salts. These novel Cu(II) complexes can be obtained in an enantiopure state by means of column chromatography or recrystallization. Furthermore, all the Cu(II) complexes were evaluated for their antibacterial activities. Among them, complexes (S,2S)-3a, (S,2S)-3g, and (S,2S)-3o showed significant antibacterial activities against Staphylococcus aureus Mu50. Further biological evaluation indicated that they were effective against most of Gram-positive bacteria. It is the first study on the biological activities of transition metal complexes with this type of proline-derived Schiff base ligand.

The Latest FDA-Approved Pharmaceuticals Containing Fragments of Tailor-Made Amino Acids and Fluorine

Nowadays, the selective introduction of fluorine into bioactive compounds is a mature strategy in the design of drugs allowing to increase efficiency, biological half-life and bio-absorption. On the other hand, amino acids (AAs) represent one of the most ubiquitious classes of naturally occurring organic compounds, which are found in over 40% of newly marked small-molecule pharmaceutical drugs and medical formulations. The primary goal of this work is to underscore two major trends in the design of modern pharmaceuticals. The first is dealing with the unique structural characteristics provided by the structure of amino acids featuring an abundance of functionality and the presence of a stereogenic center, all of which bodes well for the successful development of targeted bioactivity. The second is related to fine-tuning the desired activity and pharmacokinetics by selective introduction of fluorine. Historically, both trends were developed separately as innovative and prolific approaches in modern drug design. However, in recent decades, these approaches are clearly converging leading to an ever-increasing number of newly approved pharmaceuticals containing both structural features of amino acids and fluorine.

Nickel-Catalyzed Cross-Coupling of Amino-Acid-Derived Alkylzinc Reagents with Alkyl Bromides/Chlorides: Access to Diverse Unnatural Amino Acids

Unnatural α-amino acids are important synthetic targets in the field of peptide science. Herein we report an efficient, versatile, and straightforward strategy for the synthesis of homophenylalanine derivatives via the nickel-catalyzed Csp3-Csp3 cross-coupling of (fluoro)benzyl bromides/chlorides with natural α-amino-acid-derived alkylzinc reagents. The current protocol features the advantages of a low-cost nickel catalyst system, synthetic convenience, and the tolerance of rich functionality and stereochemistry.

Hexafluoroisobutylation of Enolates Through a Tandem Elimination/Allylic Shift/Hydrofluorination Reaction

The isobutyl side chain is a highly prevalent hydrophobic group in drugs, and it notably constitutes the side chain of leucine. Its replacement by a hexafluorinated version containing two CF3...

Tailor-Made Amino Acids in Pharmaceutical Industry: Synthetic Approaches to Aza-Tryptophan Derivatives

Over the recent years there has been a noticeable upsurge of interest in aza-analogs of tryptophan which are isosteric to the latter and found numerous applications in medicinal, bioorganic chemistry, and peptide research. In the present review article, five aza-tryptophan derivatives are profiled, including aza-substitution in the positions 2, on the five-membered ring, as well as in positions 4, 5, 6, and 7 on the six-membered ring. A detailed and comprehensive literature overview of the synthetic methods for the preparation of these aza-tryptophans is presented and general facets of the biological properties and most promising applications are discussed.

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.

Rhodium-Catalyzed Asymmetric Hydrogenation of 3-Benzoylaminocoumarins for the Synthesis of Chiral 3-Amino Dihydrocoumarins

An asymmetric hydrogenation of 3-benzoylaminocoumarins was achieved for the first time using our BridgePhos-Rh catalytic system, providing chiral 3-amino dihydrocoumarins in high yields (up to 98 %) and with excellent enantioselectivities (up to 99.7 % ee). The relationship between the enantioselectivities of the hydrogenations and the dihedral angles and the resulting π-π stacking effects of the BridgePhos-Rh complexes, which were determined by X-ray diffraction analysis, are discussed. The corresponding hydrogenated products allow for many transformations, providing several chiral skeletons with important physiological and pharmacological activities.

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.

Synthesis of β-Hydroxy α-Amino Acids Through Brønsted Base-Catalyzed syn-Selective Direct Aldol Reaction of Schiff Bases of Glycine o-Nitroanilide

Here we report the highly enantio- and syn-selective synthesis of β-hydroxy α-amino acids from glycine imine derivatives under Brønsted base (BB) catalysis. The key of this approach is the use of benzophenone-derived imine of glycine o-nitroanilide as a pronucleophile, where the o-nitroanilide framework provides an efficient hydrogen-bonding platform that accounts for nucleophile reactivity and diastereoselectivity.

A general asymmetric synthesis of artificial aliphatic and perfluoroalkylated α-amino acids by Luche's cross-electrophile coupling reaction

Aliphatic artificial α-amino acids (α-AAs) have attracted great interest in biochemistry and pharmacy. In this context, we developed a promising practical protocol for the asymmetric synthesis of these α-AAs through the selective and efficient intermolecular cross-electrophile coupling of Belokon's chiral dehydroalanine Ni(ii) complex with different alkyl and perfluoroalkyl iodides mediated by a dual Zn/Cu system. The reaction afforded diastereomeric complexes with dr up to 21.3 : 1 in 24-95% yields (19 examples). Exemplarily, three enantiomerically pure aliphatic α-AAs were obtained through acidic decomposition of (S,S)-diastereomers of Ni(ii) complexes. Importantly, the chiral auxiliary ligand (S)-BPB ((S)-2-(N-benzylprolyl)aminobenzophenone) was easily recycled by simple filtration after acidic complex decomposition and reused for the synthesis of the initial dehydroalanine Ni(ii) complex.

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.

Probing α-Amino Aldehydes as Weakly Acidic Pronucleophiles: Direct Access to Quaternary α-Amino Aldehydes by an Enantioselective Michael Addition Catalyzed by Brønsted Bases

The high tendency of α-amino aldehydes to undergo 1,2-additions and their relatively low stability under basic conditions have largely prevented their use as pronucleophiles in the realm of asymmetric catalysis, particularly for the production of quaternary α-amino aldehydes. Herein, it is demonstrated that the chemistry of α-amino aldehydes may be expanded beyond these limits by documenting the first direct α-alkylation of α-branched α-amino aldehydes with nitroolefins. The reaction produces densely functionalized products bearing up to two, quaternary and tertiary, vicinal stereocenters with high diastereo- and enantioselectivity. DFT modeling leads to the proposal that intramolecular hydrogen bonding between the NH group and the carbonyl oxygen atom in the starting α-amino aldehyde is key for reaction stereocontrol.

Enantioselective extraction of unprotected amino acids coupled with racemization

Scalable and economical methods for the production of optically pure amino acids, both natural and unnatural, are essential for their use as synthetic building blocks. Currently, enzymatic dynamic kinetic resolution (DKR) underpins some of the most effective processes. Here we report the development of enantioselective extraction coupled with racemization (EECR) for the chirality conversion of underivatized amino acids. In this process, the catalytic racemization of amino acids in a basic aqueous solution is coupled with the selective extraction of one enantiomer into an organic layer. Back-extraction from the organic layer to an acidic aqueous solution then completes the deracemization of the amino acid. The automation of the EECR process in a recycling flow reactor is also demonstrated. Continuous EECR is made possible by the sterically hindered chiral ketone extractant 5, which prevents the coextraction of the copper racemization catalyst because of its nonplanar geometry. Furthermore, the extractant 5 unexpectedly forms imines with amino acids faster and with greater enantioselectivity than less bulky derivatives, even though 5 cannot participate in intramolecular resonance-assisted hydrogen bonding. These features may allow EECR to challenge the preponderance of enzymatic DKR in the production of enantiomerically enriched amino acids.

Dynamic kinetic resolution is a common approach for the preparation of optically pure amino acids using enzymes. Here the authors report an alternative method based on enantioselective extraction coupled with racemization, in which a bulky extractant affords hydrophobic extractable imines with amino acids rapidly, reversibly and enantioselectively.

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™.

Tailor-made amino acid-derived pharmaceuticals approved by the FDA in 2019

Amino acids (AAs) are among a handful of paramount classes of compounds innately involved in the origin and evolution of all known life-forms. Along with basic scientific explorations, the major goal of medicinal chemistry research in the area of tailor-made AAs is the development of more selective and potent pharmaceuticals. The growing acceptance of peptides and peptidomimetics as drugs clearly indicates that AA-based molecules become the most successful structural motif in the modern drug design. In fact, among 24 small-molecule drugs approved by FDA in 2019, 13 of them contain a residue of AA or di-amines or amino-alcohols, which are commonly considered to be derived from the parent AAs. In the present review article, we profile 13 new tailor-made AA-derived pharmaceuticals introduced to the market in 2019. Where it is possible, we will discuss the development form drug-candidates, total synthesis, with emphasis on the core-AA, therapeutic area, and the mode of biological activity.

Cyclic tailor-made amino acids in the design of modern pharmaceuticals

Tailor-made AAs are indispensable components of modern medicinal chemistry and are becoming increasingly prominent in new drugs. In fact, about 30% of small-molecule pharmaceuticals contain residues of tailor-made AAs or structurally related diamines and amino-alcohols. Cyclic tailor-made AAs present a particular value to rational structural design by virtue of their local conformational constraints and are widely used in lead optimization programs. The present review article highlights 34 compounds, all of which are derived from cyclic AAs, representing recently-approved, small-molecule pharmaceuticals as well as promising drug candidates currently in various phases of clinical study. For each compound, the discussion includes the discovery, therapeutic profile and optimized synthesis, with a focus on the preparation of cyclic tailor-made AA as the principal structural feature. The present review article is intended to serve as a reference source for organic, medicinal and process chemists along with other professionals working in the fields of drug design and pharmaceutical discovery.

Tailor-Made Amino Acids and Fluorinated Motifs as Prominent Traits in Modern Pharmaceuticals

Structural analysis of modern pharmaceutical practices allows for the identification of two rapidly growing trends: the introduction of tailor-made amino acids and the exploitation of fluorinated motifs. Curiously, the former represents one of the most ubiquitous classes of naturally occurring compounds, whereas the latter is the most xenobiotic and comprised virtually entirely of man-made derivatives. Herein, 39 selected compounds, featuring both of these traits in the same molecule, are profiled. The total synthesis, source of the corresponding amino acids and fluorinated residues, and medicinal chemistry aspects and biological properties of the molecules are discussed.

Solvent-triggered stereoselectivity of α,α-cyclopropanation of amino acids in the Ni(ii) chiral coordination environment

Experimental and DFT investigation of the α,α-cyclopropanation of amino acids via nucleophilic addition of the deprotonated glycine Ni(ii)-Schiff-base complex, containing the (S)-proline derivatives as an auxiliary chiral moiety, to alkyl α-bromoacrylates was performed. It was demonstrated that the predominant configuration of the newly formed α-stereocenter is (S), regardless of the solvent used but the smart choice of solvent allows high diastereoselectivity at the removed β-stereocenter to be obtained, which commonly is rather rare. DFT analysis of the reaction path provides a rationale for the stereochemical outcome observed. The cyclopropanated complexes exhibit stereodependent redox activity, thus supporting that this is a general phenomenon inherent to this class of Ni Schiff-base derivatives, accounting for the influence of the peripheral groups in the metal coordination environment on the relative impact of different parts of the molecule in the frontier orbitals via conformational changes.

Asymmetric Synthesis of Tailor-Made Amino Acids Using Chiral Ni(II) Complexes of Schiff Bases. An Update of the Recent Literature

Tailor-made amino acids are indispensable structural components of modern medicinal chemistry and drug design. Consequently, stereo-controlled preparation of amino acids is the area of high research activity. Over last decade, application of Ni(II) complexes of Schiff bases derived from glycine and chiral tridentate ligands has emerged as a leading methodology for the synthesis of various structural types of amino acids. This review article summarizes examples of asymmetric synthesis of tailor-made α-amino acids via the corresponding Ni(II) complexes, reported in the literature over the last four years. A general overview of this methodology is provided, with the emphasis given to practicality, scalability, cost-structure and recyclability of the chiral tridentate ligands.

Which Stereoinductor Is Better for Asymmetric Functionalization of α-Amino Acids in a Nickel(II) Coordination Environment? Experimental and DFT Considerations

The results of extended comparative investigation of nickel(II) Schiff base complexes (containing various auxiliary chiral moieties) commonly used as a methodological platform for the asymmetric synthesis of tailor-made α-amino acids are provided. The following issues are addressed: 1) redox activity (determining the possibility for electrochemically induced reactions); 2) quantitative estimation of the reactivity of deprotonated complexes towards electrophiles; and 3) quantum-chemical estimation of noncovalent interactions in the metal coordination environment (which shed light on the origin of the stereochemical outcome observed for different stereoinductors). Possible mechanisms that determine the relationship between the stereochemical configuration of a molecule and its electronic structure are discussed. The DFT-calculated HOMO-LUMO energies and localization, as well as relative energies for the (S)- and (R)-alanine derivatives, that determine the stereoinduction efficiency in thermodynamically controlled reactions in nickel(II) coordination are provided. The computational data are supported by experimental results on the monobenzylation of glycine derivatives.

Chiral N,N'-dioxide/Mg(OTf)2 complex-catalyzed asymmetric [2,3]-rearrangement of in situ generated ammonium salts

Catalytic enantioselective [2,3]-rearrangements of in situ generated ammonium ylides from glycine pyrazoleamides and allyl bromides were achieved by employing a chiral N,N'-dioxide/MgII complex as the catalyst. This protocol provided a facile and efficient synthesis route to a series of anti-α-amino acid derivatives in good yields with high stereoselectivities. Moreover, a possible catalytic cycle was proposed to illustrate the reaction process and the origin of stereoselectivity.

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.