Asymmetric Synthesis Of α-Amino Acids Research Articles

Asymmetric Synthesis of α-Amino Acids by Organocatalytic Biomimetic Transamination.

A biomimetic enantioselective transamination of α-keto ester derivatives can be realized under mild conditions by using chiral quaternary ammonium arenecarboxylates in the absence of base additives. The corresponding α-amino acids can be used as versatile intermediates for further synthetic transformations that furnish chiral pyrrolidine and octahydroindolizine derivatives.

Rational Engineering of Bacillus cereus Leucine Dehydrogenase Towards α-keto Acid Reduction for Improving Unnatural Amino Acid Production.

Unnatural amino acids (UAAs) play a key role in modern medicinal chemistry such as small molecules and peptide-based drugs with fast-growing markets. Low efficiency for natural enzymes including leucine dehydrogenase (LeuDH, EC1.4.1.9) are one major challenge for UAA production. Here, rational engineering of LeuDH from Bacillus cereus with a structure-based design approach is studied. The results achieve higher enzymatic activity and stability toward α-keto acid reduction by improving the hydrophobic and rigidity of enzymatic substrate entrance tunnel. High catalytic efficiency for variant E116V is associated with the presence of more hydrophobic tunnels that allows easy substrate diffusion, which is confirmed in absorbance spectroscopy study. For variant T45M/E116V, melting temperature and half-lives of thermal inactivation at 60 °C is 62.8 °C and 29.2 h, respectively, much higher than 48.4 °C and 3.4 h of wild type. Structural analysis indicates that an additional hydrogen bond in β5 fold is formed in variant T45M, which results in a more rigid β5 fold leading to better stability. Furthermore, asymmetric synthesis of α-amino acids with coenzyme regeneration reveals higher productivities for variant T45M/E116V. This study indicates the importance of substrate entrance tunnel for enzymatic activities and stability, the engineered LeuDH would better serve UAA production.

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 (t1/2 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.

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

This review article critically discusses examples of asymmetric synthesis of tailor-made α-amino acids via homologation of Ni(II) complexes of glycine and alanine Schiff bases, reported in the literature from 2013 through the end of 2016. Where it is possible, reaction mechanism and origin of the stereochemical outcome is discussed in detail. Special attention is given to various aspects of practicality and scalability of the reported methods. Among the most noticeable developments in this area are novel designs of axially chiral ligands, application of electro- and mechano-chemical (ball-milling) conditions, and development of dynamic kinetic resolution procedures.

Frozen Chirality of Tertiary Aromatic Amides: Access to Enantioenriched Tertiary α-Amino Acid or Amino Alcohol without Chiral Reagent.

One of the fundamental and intriguing aspects of life is the homochirality of the essential molecules. In this field, the absolute asymmetric synthesis of α-amino acids is a major challenge. Herein, we report access, by chemical means, to tertiary α-amino acid derivatives in up to 96 % ee without using any chiral reagent. In our strategy, the dynamic axial chirality of tertiary aromatic amides is frozen in a crystal and is responsible for the stereoselectivity of the subsequent steps. Furthermore, we could control the configuration of the final product by manually sorting and selecting the initial crystals. Based on vibrational circular dichroism studies, we could rationalize the observed stereoselectivity.

Preparation of Enantiomerically Pure Perfluorobutanesulfinamide and Its Application to the Asymmetric Synthesis of α-Amino Acids.

A high yielding and practical two-step synthesis of enantiomerically pure perfluorobutanesulfinamide from Senanayake's 2-aminoindanol-derived sulfinyl transfer reagent was developed and carried out on a multigram scale. Straightforward condensation of this sulfinamide with ethyl glyoxylate provided the N-perfluorobutanesulfinyl imino ester. The utility of this activated N-sulfinyl imino ester was demonstrated for reactions that gave either no product or very low yields with the corresponding less electrophilic N-tert-butanesulfinyl derivative. Specifically, the Rh(III)-catalyzed C-H bond addition of aromatic compounds to the N-perfluorobutanesulfinyl imino ester provided arylglycines with very high diastereoselectivities for a range of directing groups including pyrrolidine amide, azo, sulfoximine, 1-pyrazole, and 1,2,3-triazole functionalities. Thermal asymmetric aza-Diels-Alder reactions also proceeded in good yields and with high selectivity, including for the substituted dienes (E)-1,3-pentadiene and (2E,4E)-2,4-hexadiene.

ChemInform Abstract: Asymmetric Synthesis of Quaternary α‐Amino Acids and Their Phosphonate Analogues

AbstractReview: [117 refs.

ChemInform Abstract: Asymmetric Synthesis of α‐Amino Acids via Homologation of Ni(II) Complexes of Glycine Schiff Bases. Part 3. Michael Addition Reactions and Miscellaneous Transformations

AbstractReview: 131 refs.

Asymmetric synthesis of α-amino acids via homologation of Ni(II) complexes of glycine Schiff bases. Part 3: Michael addition reactions and miscellaneous transformations.

The major goal of this review is a critical discussion of the literature data on asymmetric synthesis of α-amino acids via Michael addition reactions involving Ni(II)-complexes of amino acids. The material covered is divided into two conceptually different groups dealing with applications of: (a) Ni(II)-complexes of glycine as C-nucleophiles and (b) Ni(II)-complexes of dehydroalanine as Michael acceptors. The first group is significantly larger and consequently subdivided into four chapters based on the source of stereocontrolling element. Thus, a chiral auxiliary can be used as a part of nucleophilic glycine Ni(II) complex, Michael acceptor or both, leading to the conditions of matching vs. mismatching stereochemical preferences. The particular focus of the review is made on the practical aspects of the methodology under discussion and mechanistic considerations.

Catalytic enantioselective silylation of N-sulfonylimines: asymmetric synthesis of α-amino acids from CO2 via stereospecific carboxylation of α-amino silanes.

A catalytic enantioselective silylation of N-tert-butylsulfonylimines using a Cu-secondary diamine complex was demonstrated. The resulting optically active α-amino silanes could be carboxylated under a CO2 atmosphere (1 atm) to afford the corresponding α-amino acids in a stereoretentive manner. This two-step sequence provides a new synthetic protocol for optically active α-amino acids from gaseous CO2 and imines in the presence of a catalytic amount of a chiral source.

ChemInform Abstract: Asymmetric Synthesis of α‐Amino Acids via Homologation of Ni(II) Complexes of Glycine Schiff Bases. Part 1. Alkyl Halide Alkylations

AbstractReview: 141 refs.

Asymmetric synthesis of α-amino acids via homologation of Ni(II) complexes of glycine Schiff bases. Part 2: Aldol, Mannich addition reactions, deracemization and (S) to (R) interconversion of α-amino acids

This review provides a comprehensive treatment of literature data dealing with asymmetric synthesis of α-amino-β-hydroxy and α,β-diamino acids via homologation of chiral Ni(II) complexes of glycine Schiff bases using aldol and Mannich-type reactions. These reactions proceed with synthetically useful chemical yields and thermodynamically controlled stereoselectivity and allow direct introduction of two stereogenic centers in a single operation with predictable stereochemical outcome. Furthermore, new application of Ni(II) complexes of α-amino acids Schiff bases for deracemization of racemic α-amino acids and (S) to (R) interconversion providing additional synthetic opportunities for preparation of enantiomerically pure α-amino acids, is also reviewed. Origin of observed diastereo-/enantioselectivity in the aldol, Mannich-type and deracemization reactions, generality and limitations of these methodologies are critically discussed.

Asymmetric synthesis of α-amino acids via homologation of Ni(II) complexes of glycine Schiff bases; Part 1: alkyl halide alkylations

Alkylations of chiral or achiral Ni(II) complexes of glycine Schiff bases constitute a landmark in the development of practical methodology for asymmetric synthesis of α-amino acids. Straightforward, easy preparation as well as high reactivity of these Ni(II) complexes render them ready available and inexpensive glycine equivalents for preparing a wide variety of α-amino acids, in particular on a relatively large scale. In the case of Ni(II) complexes containing benzylproline moiety as a chiral auxiliary, their alkylation proceeds with high thermodynamically controlled diastereoselectivity. Similar type of Ni(II) complexes derived from alanine can also be used for alkylation providing convenient access to quaternary, α,α-disubstituted α-amino acids. Achiral type of Ni(II) complexes can be prepared from picolinic acid or via recently developed modular approach using simple secondary or primary amines. These Ni(II) complexes can be easily mono/bis-alkylated under homogeneous or phase-transfer catalysis conditions. Origin of diastereo-/enantioselectivity in the alkylations reactions, aspects of practicality, generality and limitations of this methodology is critically discussed.

Preparation of α‐Amino Acids by Oxidative Oxazoline–Oxazinone Rearrangement–Hydrogenation (OOOH). Scope and Limitations

The range and scope of the oxidative oxazoline-oxazinone rearrangement-hydrogenation sequence (OOOH)--a short, direct asymmetric synthesis of α-amino acids from carboxylic acids--was explored. The highest yet reported diastereoselectivity for hydrogenation of the oxazinone C=N bond (d.r.=>80:1) is disclosed and rationalized with the aid of ab initio molecular calculations.

Asymmetric Synthesis of α-Amino Acids by Reduction of N-tert-Butanesulfinyl Ketimine Esters

A highly regio- and diastereoselective reduction of various N-tert-butanesulfinyl ketimine esters with L-Selectride resulting in the formation of α-amino acids is reported. This method is quite general and also practical for the preparation of both enantiomers of aryl or aliphatic α-amino acids in high yields.

Asymmetric syntheses of α-amino acids via double chiral induction in alkylation of the ketimine derived from 2-hydroxypinan-3-one and menthyl glycinate

Asymmetric syntheses of (S)-α-amino acids in 28–98% optical yields via double chiral induction in alkylations of ketimine 1 derived from (+)-2-hydroxypinan-3-one and (-)-menthyl glycinate which is a chiral match pair have been studied. The factors controlling the diastereoselectivities in alkylation reactions of the ketimine, the properties of alkylating agents and various alkylation conditions are examined.

ChemInform Abstract: Transition Metal Complexes. A New Class of Catalysts for the Asymmetric Synthesis of α-Amino Acids by Phase-Transfer Alkylation.

ChemInform Abstract: Transition Metal Complexes. A New Class of Catalysts for the Asymmetric Synthesis of α-Amino Acids by Phase-Transfer Alkylation.

ChemInform Abstract: Claisen Rearrangements of Enantiomerically Pure C3-(Acyloxy)-E- vinylsilanes.

The Ireland ester Claisen rearrangement of chiral (R)- and (S)-C3-(acyloxy)-(E)-vinylsilanes gives access to a wide range of α-chiral-β-silyl-(E)-hexenoic acids with useful levels of diastereoselectivity for both the 2,3-syn and 2,3-anti diastereomers as illustrated in eqs 1 and 2. The vicinal diastereoselectivities for simple propionate esters rac-1b, (S)-1e, and (R)-1g can be varied from 1:12 to 16:1 (syn:anti) depending on reaction conditions. The employment of appropriately protected glycolate esters yields Claisen products with diastereoselectivities ranging from 23:1 (syn:anti), resulting from chelation control of enolate geometry, to 1:3.6 (syn:anti) via the use of a nonchelating, silicon-protecting group. The reaction of α-azido acetate (R)-1i is the first example of a Claisen rearrangement involving an α-azido ester and results in a new approach toward the asymmetric synthesis of α-amino acids

ChemInform Abstract: The Asymmetric Synthesis of α-Amino Acids via the Addition of Grignard Reagents to Imine Derivatives.

Abstract The ester-8-phenylmenthyl N-Boc-glycinate 5a, undergoes free radical bromination by N-bromosuccinimide to give 8-phenylmenthyl N-Boc-bromoglycinate 8. Treatment of the bromide 8 with a variety of Grignard reagents at low temperature gave 8-phenylmenthyl (S-N-Boc-2-alkylglycinates with high diastereoselectivity. Conditions were found for the hydrolysis of these derivatives with no racemization of the resultant amino acid.

ChemInform Abstract: Molecular Design of a C2-Symmetric Chiral Phase-Transfer Catalyst for Practical Asymmetric Synthesis of α-Amino Acids.

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