Mimicking Nature to Make Unnatural Amino Acids and Chiral Diamines

Circular Dichroism Sensing: Strategies and Applications

The analysis of the absolute configuration, enantiomeric composition, and concentration of chiral compounds are frequently encountered tasks across the chemical and health sciences. Chiroptical sensing methods can streamline this work and allow high-throughput screening with remarkable reduction of operational time and cost. During the last few years, significant methodological advances with innovative chirality sensing systems, the use of computer-generated calibration curves, machine learning assistance, and chemometric data processing, to name a few, have emerged and are now matched with commercially available multi-well plate CD readers. These developments have reframed the chirality sensing space and provide new opportunities that are of interest to a large group of chemists. This review will discuss chirality sensing strategies and applications with representative small-molecule CD sensors. Emphasis will be given to important milestones and recent advances that accelerate chiral compound analysis by outperforming traditional methods, conquer new directions, and pioneering efforts that lie at the forefront of chiroptical high-throughput screening developments. The goal is to provide the reader with a thorough understanding of the current state and a perspective of future directions of this rapidly emerging field.

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.

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.

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 N-Substituted α-Amino Esters from α-Ketoesters via Imine Reductase-Catalyzed Reductive Amination

N-Substituted α-amino esters are widely used as chiral intermediates in a range of pharmaceuticals. Here we report the enantioselective biocatalyic synthesis of N-substituted α-amino esters through the direct reductive coupling of α-ketoesters and amines employing sequence diverse metagenomic imine reductases (IREDs). Both enantiomers of N-substituted α-amino esters were obtained with high conversion and excellent enantioselectivity under mild reaction conditions. In addition >20 different preparative scale transformations were performed highlighting the scalability of this system.

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.

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.

Process Optimisation Studies and Aminonitrile Substrate Evaluation of Rhodococcus erythropolis SET1, A Nitrile Hydrolyzing Bacterium

A comprehensive series of optimization studies including pH, solvent and temperature were completed on the nitrile hydrolyzing Rhodococcus erythropolis bacterium SET1 with the substrate 3-hydroxybutyronitrile. These identified temperature of 25 °C and pH of 7 as the best conditions to retain enantioselectivity and activity. The effect of the addition of organic solvents to the biotransformation mixture was also determined. The results of the study suggested that SET1 is suitable for use in selected organo-aqueous media at specific ratios only. The functional group tolerance of the isolate with unprotected and protected β-aminonitriles, structural analogues of β-hydroxynitriles was also investigated with disappointingly poor isolated yields and selectivity obtained. The isolate was further evaluated with the α- aminonitrile phenylglycinonitrile generating acid in excellent yield and ee (>99 % (S) - isomer and 50 % yield). A series of pH studies with this substrate indicated pH 7 to be the optimum pH to avoid product and substrate degradation.

Hydrogen Bond Assisted l to d Conversion of α-Amino Acids

l to d conversion of unactivated α-amino acids was achieved by solubility-induced diastereomer transformation (SIDT). Ternary complexes of an α-amino acid with 3,5-dichlorosalicylaldehyde and a chiral guanidine (derived from corresponding chiral vicinal diamine) were obtained in good yield as diastereomerically pure imino acid salt complexes and were hydrolysed to obtain enantiopure α-amino acids. A combination of DFT computation, NMR spectroscopy, and crystal structure provide detailed insight into how two types of strong hydrogen bonds assist in rapid epimerization of the complexes that is essential for SIDT.

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.

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.

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.

Enantioselective Carbonyl Catalysis Enabled by Chiral Aldehydes

Organocatalytic methods have achieved spectacular advancements for the preparation of chiral molecules in highly enantioenriched forms. The fast development of this field can mainly be attributed to the evolution of general and reliable activation modes. The discovery and identification of new activation modes are therefore highly desirable to push the boundaries of asymmetric reactions. In this Minireview, recent advances in enantioselective carbonyl catalysis, one useful subbranch of organocatalysis for the efficient activation of simple amines, will be summarized. With elegantly designed chiral aldehyde catalysts, highly enantioselective and efficient asymmetric reactions can be developed. Continued development of enantioselective carbonyl catalysis is expected in the future.

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.

N-Alkyl-α-amino acids in Nature and their biocatalytic preparation

N-Alkylated-α-amino acids are useful building blocks for the pharmaceutical and fine chemical industries. Enantioselective methods of N-alkylated-α-amino acid synthesis are therefore highly valuable and widely investigated. While there are a variety of chemical methods for their synthesis, they often employ stoichiometric quantities of hazardous reagents such as pyrophoric metal hydrides or genotoxic alkylating agents, whereas biocatalytic routes can provide a greener and cleaner alternative to existing methods. This review highlights the occurrence of the N-alkyl-α-amino acid motif and its role in nature, important applications towards human health and biocatalytic methods of preparation. Several enzyme classes that can be used to access chiral N-alkylated-α-amino acids and their substrate selectivities are detailed.

The self-disproportionation of enantiomers (SDE) of α-amino acid derivatives: facets of steric and electronic properties

α-Amino acids (α-AAs) are in extremely high demand in nearly every sector of the food and health-related chemical industries and continue to be the subject of intense multidisciplinary research. The self-disproportionation of enantiomers (SDE) is an emerging and one of the least studied areas of α-AA or enantiomeric properties, critically important for their production and application. In the present work, we report a detailed study of the SDE via achiral, gravity-driven column chromatography for a set of N-acylated, N-carbonylated, N-fluoroacylated, and N-thioacylated α-amino acid esters. As well as thioacylation, attention was paid to the effect of altering the R group of the ester functionality, the side chain, or that of the acyl group attached to the amide nitrogen, whereby it was found that electron-withdrawing groups in the latter moiety had a pronounced effect on the magnitude and behavior of the resulting SDE phenomenon. Intriguingly, in the case of N-fluoroacylated derivatives, by favoring the formation of dimeric associates and effecting a strong bias toward homochiral associates over heterochiral associates, the SDE magnitude was greatly reduced contrary to intuitive expectations. Energy estimates resulted from DFT calculations.

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.

Optical Analysis of Reaction Yield and Enantiomeric Excess: A New Paradigm Ready for Prime Time

This Perspective highlights the advances of optical methods for asymmetric reaction discovery. Optical analysis allows for the determination of absolute configuration, enantiomeric excess and reaction yield that is amenable to high-throughput experimentation. Thus, the synthetic organic community is encouraged to incorporate the methods discussed to expedite the development of high-yielding, enantioselective transformations.

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.

Synthesis of Enantiopure Mixed Alkyl-Aryl Vicinal Diamines by the Diaza-Cope Rearrangement: A Synthesis of (+)-CP-99,994

The stereoselective synthesis of mixed alkyl-aryl vicinal diamines was demonstrated by the use of 1,2-bis(2-hydroxyphenyl)-1,2-diaminoethene (hpen). A sequential addition of aryl and alkyl aldehyde to hpen gave a fused imidazolidine-dihydro-1,3-oxazine ring stereoselectively, which undergoes the diaza-Cope rearrangement to provide mixed vicinal diimines at elevated temperature in good yields and excellent stereoselectivity. We also showed that (+)-CP-99,994 can be readily prepared by the diaza-Cope rearrangement in overall 42% yield.

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.

Catalytic Stereoinversion of L-Alanine to Deuterated D-Alanine

A combination of an achiral pyridoxal analogue and a chiral base has been developed for catalytic deuteration of L-alanine with inversion of stereochemistry to give deuterated D-alanine under mild conditions (neutral pD and 25 °C) without the use of any protecting groups. This system can also be used for catalytic deuteration of D-alanine with retention of stereochemistry to give deuterated D-alanine. Thus a racemic mixture of alanine can be catalytically deuterated to give an enantiomeric excess of deuterated D-alanine. While catalytic deracemization of alanine is forbidden by the second law of thermodynamics, this system can be used for catalytic deracemization of alanine with deuteration. Such green and biomimetic approach to catalytic stereocontrol provides insights into efficient amino acid transformations.