Unusual Amino Acids in Medicinal Chemistry

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides

The catalytic hydroboration of imines, nitriles, and carbodiimides is a powerful method of preparing amines which are key synthetic intermediates in the synthesis of many value-added products. Imine hydroboration has perennially featured in notable reports while nitrile and carbodiimide hydroboration have gained attention recently. Initial developments in catalytic hydroboration of imines and nitriles employed precious metals and typically required harsh reaction conditions. More recent advances have shifted toward the use of base metal and main group element catalysis and milder reaction conditions. In this survey, we review metal and nonmetal catalyzed hydroboration of these unsaturated organic molecules and group them into three distinct categories: precious metals, base metals, and main group catalysts. The TON and TOF of imine hydroboration catalysts are reported and summarized with a brief overview of recent advances in the field. Mechanistic and kinetic studies of some of these protocols are also presented.

Remarkable enantioselectivity enhancement of the extractors with nonaxial chirality in liquid-liquid extraction of underivatized amino acids by introducing t-butyl group

A class of carbonyl extractors, (R)-3, (R)-4, and (R)-5, with nonaxial chirality containing asymmetric carbons has been synthesized and studied for their efficiencies in enantioselective liquid-liquid extraction for underivatized amino acids. The bulky t-butyl ketone extractors, (R)-4 and (R)-5, showed the stereoselectivities ranging 5.4-9.4 of l/d ratio much better than those of the aldehyde extractor, (R)-3, ranging 2.4-5.2. The imine formation rates and yields of the t-butyl ketones were not significantly affected by their bulkiness and even in the absence of resonance-assisted hydrogen bond. This work confirms that a bulky t-butyl ketone can be a good choice in the development of an extractor not only with axial chirality but also with nonaxial chirality for the enantioselective extraction of unprotected amino acids.

Stereocontrolled 1,3-nitrogen migration to access chiral α-amino acids

α-Amino acids are essential for life as building blocks of proteins and components of diverse natural molecules. In both industry and academia, the incorporation of unnatural amino acids is often desirable for modulating chemical, physical and pharmaceutical properties. Here we report a protocol for the economical and practical synthesis of optically active α-amino acids based on an unprecedented stereocontrolled 1,3-nitrogen shift. Our method employs abundant and easily accessible carboxylic acids as starting materials, which are first connected to a nitrogenation reagent, followed by a highly regio- and enantioselective ruthenium- or iron-catalysed C(sp³)-H amination. This straightforward method displays a very broad scope, providing rapid access to optically active α-amino acids with aryl, allyl, propargyl and alkyl side chains, and also permits stereocontrolled late-stage amination of carboxylic-acid-containing drugs and natural products.

N-Aminoacyl-3-amino-nido-carboranes as a Group of Boron-Containing Derivatives of Natural Amino Acids

A new group of nido-carboranyl derivatives of natural (S)-amino acids containing from 9 to 18 boron atoms was obtained in good yields as a result of acylation of 3-amino-1,2-dicarba-closo-dodecaborane followed by deboronation. The proposed approach is convenient and based on the use of readily available reagents and is suitable for the synthesis of enantiopure nido-carboranyl derivatives of amino acids with various side chains, including water-soluble boron-containing amino acids (17 examples).

Bioanalytical Methods and Strategic Perspectives Addressing the Rising Complexity of Novel Bioconjugates and Delivery Routes for Biotherapeutics

In recent years, an increase in the discovery and development of biotherapeutics employing new modalities, such as bioconjugates or novel routes of delivery, has created bioanalytical challenges. The inherent complexity of conjugated molecular structures means that quantification of the bioconjugate and its multiple components is critical for preclinical/clinical studies to inform drug discovery and development. Moreover, bioconjugates involve additional multifactorial complexity because of the potential for in vivo catabolism and biotransformation, which may require thorough investigations in multiple biological matrices. Furthermore, excipients that enhance absorption are frequently evaluated and employed for the development of oral and inhaled biotherapeutics. Risk-benefit assessments are required for novel or existing excipients that utilize dosages above previously approved levels. Bioanalytical methods that can measure both excipients and potential drug metabolites in biological matrices are highly relevant to these emerging bioanalysis challenges. We discuss the bioanalytical strategies for analyzing bioconjugates such as antibody-drug conjugates and antibody-oligonucleotide conjugates and review recent advances in bioanalytical methods for the quantification and characterization of novel bioconjugates. We also discuss bioanalytical considerations for both biotherapeutics and excipients through novel administration routes and review analyses in various biological matrices, from the extensively studied serum or plasma to tissue biopsy in the context of preclinical and clinical studies from both technical and regulatory perspectives.

Amino acids in inflammatory bowel diseases: Modern diagnostic tools and methodologies

Amino acids are crucial building blocks of living organisms. Together with their derivatives, they participate in many intracellular processes to act as hormones, neuromodulators, and neurotransmitters. For several decades amino acids have been studied for their potential as markers of various diseases, including inflammatory bowel diseases. Subsequent improvements in sample pretreatment, separation, and detection methods have enabled the specific and very sensitive determination of these molecules in multicomponent matrices-biological fluids and tissues. The information obtained from targeted amino acid analysis (biomarker-based analytical strategy) can be further used for early diagnostics, to monitor the course of the disease or compliance of the patients. This review will provide an insight into current knowledge about inflammatory bowel diseases, the role of proteinogenic amino acids in intestinal inflammation and modern analytical techniques used in its diagnosis and disease activity monitoring. Current advances in the analysis of amino acids focused on sample pretreatment, separation strategy, or detection methods are highlighted, and their potential in clinical laboratories is discussed. In addition, the latest clinical data obtained from the metabolomic profiling of patients suffering from inflammatory bowel diseases are summarized with a focus on proteinogenic amino acids.

Mass spectrometric-based investigation of differentially protected azatryptophan derivatives using Orbitrap mass spectrometry: Differentiation of positional isomers under protonation and alkali-cationization conditions

RATIONALE

Differentiation and structural characterization of positional isomers of differentially protected azatryptophan derivatives using electrospray ionization high-resolution tandem mass spectrometry (ESI-HRMS/MS) is important from the perspective of drug discovery research. Also, these derivatives can be used as building blocks for the synthesis of various biologically active compounds and have attracted significant attention in the field of modern drug discovery, especially peptide-based drugs, protein folding and protein-protein interactions because of their interesting spectral properties.

METHODS

ESI-HRMS/MS in positive ionization mode was used to differentiate and characterize positional isomers of protected azatryptophan derivatives.

RESULTS

ESI-HRMS/MS of [M + H]⁺ and [M + Na]⁺ ions of positional isomers of differentially protected azatryptophan derivatives display distinct fragmentation patterns. The MS/MS of [M + H]⁺ ion of isomer 1 showed an additional ion at m/z 358.0846 ([M + H-Boc-C₁₄ H₁₀ -HF]⁺ ) which was not present for 4. The fragment ion at m/z 332.0857 was observed for 1 and not for 4 which would be formed by the expulsion of butyloxycarbonyl (Boc) and fluorenylmethyloxycarbonyl (Fmoc) groups. Moreover, the ions 422.0812 and 378.0912 are found to be relatively more abundant for isomer 4 which could be probably attributed to the formation of stable ions. Similarly, other positional isomers exhibited distinct fragmentation from one another.

CONCLUSIONS

The present study demonstrates that ESI-HRMS/MS can be used for differentiation and structural characterization of positional isomers of protected azatryptophan derivatives. The MS/MS of [M + H]⁺ and [M + Na]⁺ ions of these positional isomers displayed differences in their fragmentation behaviour. The impact of different substitutions at different positions (1 and 6) of protected azatryptophan derivatives (1-6) on their fragmentation behaviour was also investigated in detail. Also, the nitrogen atom at different positions in the pyrrolopyridine ring led to different fragmentation patterns.

An expeditious route to sterically encumbered nonproteinogenic α-amino acid precursors using allylboronic acids

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yields and diastereoselectivities. Gram-scale synthesis, broad tolerance of functional groups, excellent stereodivergence, post-synthetic modifications, and easy removal of the chiral auxiliary are some of the key highlights. The protocol is applicable to various amino acids and short peptides, resulting in the incorporation of these precursors at the N-terminal position.

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yield and diastereoselectivities.

Biocatalytic synthesis of non-standard amino acids by a decarboxylative aldol reaction

Enzymes are renowned for their catalytic efficiency and selectivity. Despite the wealth of carbon-carbon bond forming transformations in traditional organic chemistry and nature, relatively few C-C bond forming enzymes have found their way into the biocatalysis toolbox. Here we show that the enzyme UstD performs a highly selective decarboxylative aldol addition with diverse aldehyde substrates to make non-standard, γ-hydroxy amino acids. We increased the activity of UstD through three rounds of classic directed evolution and an additional round of computationally-guided engineering. The enzyme that emerged, UstD^v2.0, is efficient in a whole-cell biocatalysis format. The products are highly desirable, functionally rich bioactive γ-hydroxy amino acids that we demonstrate can be prepared stereoselectively on gram-scale. The X-ray crystal structure of UstD^v2.0 at 2.25 Å reveals the active site and provides a foundation for probing the mechanism of UstD.

Enantioselective synthesis of indole-based unnatural β-Alkynyl α-amino acid derivatives via chiral phosphoric acid catalysis

The synthesis of unnatural α-amino acid derivatives has attracted considerable interest in recent years, as they are ubiquitous in protein synthesis and peptide-based drug discovery. Herein, we reported the chiral phosphoric acid catalyzed asymmetric reaction of indoles with β,γ-alkynyl-α-imino esters for the enantioselective synthesis of unnatural indole-based α-amino acid derivatives. This asymmetric organocatalysis protocol enables efficient synthesis of unnatural α-amino acid derivatives with a tetrasubstituted stereogenic center and an alkyne moiety with up to 99% yield and 98% ee, resulting in operationally simple conditions, short reaction time, and broad substrate scope.

Iridium-Catalyzed Amidation of In Situ Prepared Silyl Ketene Acetals to Access α-Amino Esters

Disclosed herein is a convenient Ir-catalyzed amidation of esters to access α-amido esters. Initially prepared silyl ketene acetals are directly employed, without separate purification, for subsequent amidation with an oxycarbonylnitrenoid precursor using the Cp*(LX)Ir(III) catalyst. The α-amidation was facile for both α-aryl and α-alkyl esters. Density functional theory studies revealed that the generation of a putative Ir-nitrenoid is facilitated by the chelation of the countercation additive during the N-O bond cleavage of the nitrene precursor.

Scalable and Selective β-Hydroxy-α-Amino Acid Synthesis Catalyzed by Promiscuous l-Threonine Transaldolase ObiH

Enzymes from secondary metabolic pathways possess broad potential for the selective synthesis of complex bioactive molecules. However, the practical application of these enzymes for organic synthesis is dependent on the development of efficient, economical, operationally simple, and well-characterized systems for preparative scale reactions. We sought to bridge this knowledge gap for the selective biocatalytic synthesis of β-hydroxy-α-amino acids, which are important synthetic building blocks. To achieve this goal, we demonstrated the ability of ObiH, an l-threonine transaldolase, to achieve selective milligram-scale synthesis of a diverse array of non-standard amino acids (nsAAs) using a scalable whole cell platform. We show how the initial selectivity of the catalyst is high and how the diastereomeric ratio of products decreases at high conversion due to product re-entry into the catalytic cycle. ObiH-catalyzed reactions with a variety of aromatic, aliphatic and heterocyclic aldehydes selectively generated a panel of β-hydroxy-α-amino acids possessing broad functional-group diversity. Furthermore, we demonstrated that ObiH-generated β-hydroxy-α-amino acids could be modified through additional transformations to access important motifs, such as β-chloro-α-amino acids and substituted α-keto acids.

Solvent directed chemically divergent synthesis of β-lactams and α-amino acid derivatives with chiral isothiourea

A protocol for the chemically divergent synthesis of β-lactams and α-amino acid derivatives with isothiourea (ITU) catalysis by switching solvents was developed. The stereospecific Mannich reaction occurring between imine and C(1)-ammonium enolate generated zwitterionic intermediates, which underwent intramolecular lactamization and afforded β-lactam derivatives when DCM and CH₃CN were used as solvents. However, when EtOH was used as the solvent, the intermediates underwent an intermolecular esterification reaction, and α-amino acid derivatives were produced. Detailed mechanistic experiments were conducted to prove that these two kinds of products came from the same intermediates. Furthermore, chemically diversified transformations of β-lactam and α-amino acid derivatives were achieved.

A protocol for the solvent directed chemically divergent synthesis of β-lactam and α-amino acid derivatives with chiral isothiourea was reported.

Synthesis of γ-amino acids via photocatalyzed intermolecular carboimination of alkenes

We report a direct approach to achieve the energy transfer-driven carboimination of alkenes for the synthesis of a diverse collection of valuable γ-amino acids.

Concise enantioselective synthesis of non-proteinogenic α-aminoacids via an organocatalytic Mannich-type reaction

Scalable asymmetric synthesis of non-proteinogenic (S)-aminoacids and their derivatives based on an organocatalyzed Mannich-type reaction involving allomaltol applicable to subsequent RuIII-catalyzed oxidative cleavage has been developed.

Luminescent Covalent Organic Frameworks for Biosensing and Bioimaging Applications

Luminescent covalent organic frameworks (LCOFs) have attracted significant attention due to their tunability of structures and photophysical properties at molecular level. LCOFs are built to highly ordered and periodic 2D or 3D framework structures through covalently assembling with various luminophore building blocks. Recently, the advantages of LCOFs including predesigned properties of structure, unique photoluminescence, hypotoxicity and good biocompatibility and tumor penetration, broaden their applications in biorelated fields, such as biosensing, bioimaging, and drug delivery. A specific review that analyses the advances of LCOFs in the field of biosensing and bioimaging is thus urged to emerge. Here the construction of LCOFs is reviewed first. The synthetic chemistry of LCOFs highlights the key role of chemical linkages, which not only concrete the building blocks but also affect the optical properties and even can act as the responsive sites for potential sensing applications. How to brighten LCOFs are clarified through description of structure managements. The ability to utilize the luminescence of LCOFs for applications in biosensing and bioimaging is discussed using state-of-the-art examples of varied practical goals. A prospect finally addresses opportunities and challenges the development of LCOFs facing from chemistry, physics to the applications, according to their current progress.

Design, synthesis and biological evaluation of P2-modified proline analogues targeting the HtrA serine protease in Chlamydia

High temperature requirement A (HtrA) serine proteases have emerged as a novel class of antibacterial target, which are crucial in protein quality control and are involved in the pathogenesis of a wide array of bacterial infections. Previously, we demonstrated that HtrA in Chlamydia is essential for bacterial survival, replication and virulence. Here, we report a new series of proline (P2)-modified inhibitors of Chlamydia trachomatis HtrA (CtHtrA) developed by proline ring expansion and Cγ-substitutions. The structure-based drug optimization process was guided by molecular modelling and in vitro pharmacological evaluation of inhibitory potency, selectivity and cytotoxicity. Compound 25 from the first-generation 4-substituted proline analogues increased antiCtHtrA potency and selectivity over human neutrophil elastase (HNE) by approximately 6- and 12-fold, respectively, relative to the peptidic lead compound 1. Based on this compound, second-generation substituted proline residues containing 1,2,3-triazole moieties were synthesized by regioselective azide-alkyne click chemistry. Compound 49 demonstrated significantly improved antichlamydial activity in whole cell assays, diminishing the bacterial infectious progeny below the detection limit at the lowest dose tested. Compound 49 resulted in approximately 9- and 22-fold improvement in the inhibitory potency and selectivity relative to 1, respectively. To date, compound 49 is the most potent HtrA inhibitor developed against Chlamydia spp.

An ultrasound-assisted solvent and catalyst-free synthesis of structurally diverse pyrazole centered 1,5-disubstituted tetrazoles via one-pot four-component reaction

Abstract:

1,5-Disubstituted tetrazoles are vital drug-like scaffold usually encountered as valuable bioisosteres of cis-amide bond. In this article, we reported synthesis of some novel medicinally relevant pyrazole centered 1,5-disubstituted tetrazoles using ultrasound irradiation via a one-pot 4-C reaction from various pyrazole originated aldehyde, amine, isocyanide, and sodium azide. All the synthesized derivatives were characterized by IR, 1H NMR, 13C NMR, spectroscopic techniques, and mass analysis. This ultrasound-assisted green protocol has several advantages like mild reaction condition, high yield, catalyst and solvent-free reaction protocol, 15 minutes reaction time and easy workup.

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.

Mild and Expeditious Synthesis of Sulfenyl Enaminones of l-α-Amino Esters and Aryl/Alkyl Amines through NCS-Mediated Sulfenylation

Sulfenylation or selenylation of enaminones of l-α-amino esters requires mild reaction conditions due to the presence of a racemization-prone chiral center and reactive side chains. An N-chlorosuccinimide (NCS)-mediated methodology has been developed for rapid sulfenylation of enaminones of l-α-amino esters and aryl/alkyl amines at room temperature in open air under metal-free conditions. Enaminones of l-α-amino esters bearing aliphatic, aromatic, and heterocyclic side chains react efficiently with diverse aryl/alkyl/heteroaryl thiols (R1SH) in the presence of NCS to afford a library of biologically important sulfenyl enaminones in good-to-excellent yields (71-90%). Under similar reaction conditions, the enaminones also react with benzeneselenol to produce selenyl enaminones in good yield (73-83%). The NCS-mediated pathway generates sulfenyl chloride (R1SCl) as an intermediate which leads to rapid sulfenylation of enaminones through cross-dehydrogenative coupling (CDC) under mild reaction conditions.

Amine- and Amino Acid-Based Compounds as Carbonic Anhydrase Activators

After being rather neglected as a research field in the past, carbonic anhydrase activators (CAAs) were undoubtedly demonstrated to be useful in diverse pharmaceutical and industrial applications. They also improved the knowledge of the requirements to selectively interact with a CA isoform over the others and confirmed the catalytic mechanism of this class of compounds. Amino acid and amine derivatives were the most explored in in vitro, in vivo and crystallographic studies as CAAs. Most of them were able to activate human or non-human CA isoforms in the nanomolar range, being proposed as therapeutic and industrial tools. Some isoforms are better activated by amino acids than amines derivatives and the stereochemistry may exert a role. Finally, non-human CAs have been very recently tested for activation studies, paving the way to innovative industrial and environmental applications.

Design, Synthesis and Late-Stage Modification of Indane-Based Peptides via [2+2+2] Cyclotrimerization

Here, we prepared several dipeptides containing 2-aminoindane-2-carboxylic acid (Aic) and carried out further synthetic transformations. Synthesis and purification of modified peptides by using [2+2+2] cyclotrimerization is a challenging task. We are able to modify the unusual amino acids and peptide derivatives by late-stage incorporation of benzylhalo functionality. To incorporate benzylhalo moiety we used [2+2+2] cyclotrimerization in the presence of Mo(CO)₆ . These halo derivatives are potential substrates for further modification by Sonogashira reaction, Suzuki-Miyaura cross-coupling, sultine formation, and the Diels-Alder reaction sequence.

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.

Reprogramming CBX8-PRC1 function with a positive allosteric modulator

Canonical targeting of Polycomb repressive complex 1 (PRC1) to repress developmental genes is mediated by cell-type-specific, paralogous chromobox (CBX) proteins (CBX2, 4, 6, 7, and 8). Based on their central role in silencing and their dysregulation associated with human disease including cancer, CBX proteins are attractive targets for small-molecule chemical probe development. Here, we have used a quantitative and target-specific cellular assay to discover a potent positive allosteric modulator (PAM) of CBX8. The PAM activity of UNC7040 antagonizes H3K27me3 binding by CBX8 while increasing interactions with nucleic acids. We show that treatment with UNC7040 leads to efficient and selective eviction of CBX8-containing PRC1 from chromatin, loss of silencing, and reduced proliferation across different cancer cell lines. Our discovery and characterization of UNC7040 not only reveals the most cellularly potent CBX8-specific chemical probe to date, but also corroborates a mechanism of Polycomb regulation by non-specific CBX nucleotide binding activity.

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.

Atroposelective Dynamic Kinetic Resolution via In Situ Hemiaminals Catalyzed by N-Heterocyclic Carbene

Axially chiral amino acids and its derivatives are vital building blocks of bioactive molecules, artificial peptides, and asymmetric catalysts. Herein, we report an unprecedented carbene-catalyzed atroposelective dynamic kinetic resolution to access axially chiral amino esters via in situ hemiaminals. This protocol features a broad substrate scope and good functional group tolerance and allows the rapid assembly of axially chiral amino esters in good to high yields with high enantioselectivities.

Photocatalysis in the Life Science Industry

In the pursuit of new pharmaceuticals and agrochemicals, chemists in the life science industry require access to mild and robust synthetic methodologies to systematically modify chemical structures, explore novel chemical space, and enable efficient synthesis. In this context, photocatalysis has emerged as a powerful technology for the synthesis of complex and often highly functionalized molecules. This Review aims to summarize the published contributions to the field from the life science industry, including research from industrial-academic partnerships. An overview of the synthetic methodologies developed and strategic applications in chemical synthesis, including peptide functionalization, isotope labeling, and both DNA-encoded and traditional library synthesis, is provided, along with a summary of the state-of-the-art in photoreactor technology and the effective upscaling of photocatalytic reactions.

Morphology Transformation of Foldamer Assemblies Triggered by Single Oxygen Atom on Critical Residue Switch

The synthesis of morphologically well-defined peptidic materials via self-assembly is challenging but demanding for biocompatible functional materials. Moreover, switching morphology from a given shape to other predictable forms by molecular modification of the identical building block is an even more complicated subject because the self-assembly of flexible peptides is prone to diverge upon subtle structural change. To accomplish controllable morphology transformation, systematic self-assembly studies are performed using congener short β-peptide foldamers to find a minimal structural change that alters the self-assembled morphology. Introduction of oxygen-containing β-amino acid (ATFC) for subtle electronic perturbation on hydrophobic foldamer induces a previously inaccessible solid-state conformational split to generate the most susceptible modification site for morphology transformation of the foldamer assemblies. The site-dependent morphological switching power of ATFC is further demonstrated by dual substitution experiments and proven by crystallographic analyses. Stepwise morphology transformation is shown by modifying an identical foldamer scaffold. This study will guide in designing peptidic molecules from scratch to create complex and biofunctional assemblies with nonspherical shapes.

Oxa-spirocycles: synthesis, properties and applications

A general approach to a new generation of spirocyclic molecules – oxa-spirocycles – was developed. The key synthetic step was iodocyclization. More than 150 oxa-spirocyclic compounds were prepared. Incorporation of an oxygen atom into the spirocyclic unit dramatically improved water solubility (by up to 40 times) and lowered lipophilicity. More potent oxa-spirocyclic analogues of antihypertensive drug terazosin were synthesized and studied in vivo.

A general practical approach to a new generation of spirocyclic molecules – oxa-spirocycles – is developed.

Organocatalytic asymmetric synthesis of α-amino esters from sulfoxonium ylides

Described here is the first organocatalytic asymmetric N–H insertion reaction of α-carbonyl sulfoxonium ylides. Without a metal catalyst, this reaction represents an attractive complement to the well-established carbene insertion reactions. As a stable surrogate of diazocarbonyl compounds, sulfoxonium ylides reacted with a range of aryl amines to provide efficient access to α-aryl glycines with excellent enantiocontrol in the presence of a suitable chiral phosphoric acid catalyst. The high stability and weak basicity of sulfoxonium ylides not only enable this protocol to be user-friendly and practically useful, but also preclude catalyst decomposition, which is crucial to the excellent amenability to electron-poor amine nucleophiles. Detailed mechanistic studies indicated that the initial protonation is reversible and the C–N bond formation is rate-determining.

An organocatalytic asymmetric N–H insertion reaction of α-carbonyl sulfoxonium ylides has been developed to provide efficient access to α-amino esters without involving a metal carbenoid intermediate.

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.

Truce-Smiles Rearrangements by Strain Release: Harnessing Primary Alkyl Radicals for Metal-Free Arylation

The ring-opening of 3-aminocyclobutanone oximes enables easy generation of primary alkyl radicals, capable of undergoing an unprecedented strain-release, desulfonylative radical Truce-Smiles rearrangement, providing divergent access to valuable 1,3 diamines and unnatural β-amino acids. Characterized by mild conditions and wide scope of migrating species, this protocol allows the modular assembly of sp³ -aryls under transition metal-free, room-temperature conditions.

Visible Light Induced Cu-Catalyzed Asymmetric C(sp3)-H Alkylation

The asymmetric functionalization of C-H is one of the most attractive strategies in asymmetric synthesis. In the past decades, catalytic enantioselective C(sp³)-H functionalization has been intensively studied and successfully applied in various asymmetric bond formations, whereas asymmetric C(sp³)-H alkylation was not well developed. Photoredox catalysis has recently emerged as an efficient way to synthesize organic compounds under mild conditions. Despite many photoinduced stereoselective reactions that have been achieved, the related enantioselective C(sp³)-C(sp³) coupling is challenging, especially of the photocatalytic asymmetric C(sp³)-H radical alkylation. Here, we report a visible light induced Cu catalyzed asymmetric sp³ C-H alkylation, which is effective for coupling with unbiased primary, secondary, and tertiary alkyl fragments in high enantioselectivities. This reaction would provide a new approach for the synthesis of important molecules such as unnatural α-amino acids and late-stage functionalization of bioactive compounds, and will be useful for modern peptide synthesis and drug discovery.

Rhodaelectro-catalyzed access to chromones via formyl C-H activation towards peptide electro-labeling

Chromones represent a privileged scaffold in medicinal chemistry and are an omnipresent structural motif in natural products. Chemically encoded non-natural peptidomimetics feature improved stability towards enzymatic degradation, cell permeability and binding affinity, translating into a considerable impact on pharmaceutical industry. Herein, a strategy for the sustainable assembly of chromones via electro-formyl C–H activation is presented. The rational design of the rhodaelectro-catalysis is guided by detailed mechanistic insights and provides versatile access to tyrosine-based fluorogenic peptidomimetics.

The chromone scaffold is present in drugs and bioactive natural products, but conventional approaches to access chromones require stoichiometric amounts of oxidants. Here, the authors report rhodaelectro-catalyzed assembly of chromones by electrochemical formyl C–H activations, providing the basis for late-stage peptide diversification.

Chiral covalent organic framework-monolith as stationary phase for high-performance liquid chromatographic enantioseparation of selected amino acids

The separation of amino acid (AA) enantiomers shows significance for chemistry, food, and biology, but remains challenging due to their similar properties. A promising nanoporous chiral covalent organic framework (COF) as a stationary phase for high-performance liquid chromatography (HPLC) suffers from the irregularity and widely distributed particle size of the chiral COF. Herein, we show the facile preparation of a chiral COF-monolith as a stationary phase for HPLC enantiomeric separation of AAs via orthogonal experiments. The CTzDa-monolith is prepared by the incorporation of the model chiral COF named CTzDa into the porous poly(ethylene dimethacrylate-co-methacrylate) monolith and reveals great permeability and mechanical stability. The corresponding CTzDa-monolithic column gives better chiral HPLC separation of AAs than the commercial Poroshell 120 chiral-T column. Thermal dynamic analysis and molecular docking calculations imply the involvement of stereoscopic hydrogen, π-π, and van der Waals interactions between the CTzDa and AAs during HPLC enantioseparation. The facile incorporation of the chiral COF into the porous monolith will promote the potential of a chiral COF as a stationary phase for HPLC.

Combining flavin photocatalysis with parallel synthesis: a general platform to optimize peptides with non-proteinogenic amino acids

Most peptide drugs contain non-proteinogenic amino acids (NPAAs), born out through extensive structure-activity relationship (SAR) studies using solid-phase peptide synthesis (SPPS). Synthetically laborious and expensive to manufacture, NPAAs also can have poor coupling efficiencies allowing only a small fraction to be sampled by conventional SPPS. To gain general access to NPAA-containing peptides, we developed a first-generation platform that merges contemporary flavin photocatalysis with parallel synthesis to simultaneously make, purify, quantify, and even test up to 96 single-NPAA peptide variants via the unique combination of boronic acids and a dehydroalanine residue in a peptide. We showcase the power of our newly minted platform to introduce NPAAs of diverse chemotypes-aliphatic, aromatic, heteroaromatic-directly into peptides, including 15 entirely new residues, and to evolve a simple proteinogenic peptide into an unnatural inhibitor of thrombin by non-classical peptide SAR.

Biocatalytic enantioselective hydroaminations enabling synthesis of N-arylalkyl-substituted L-aspartic acids

N-Substituted l-aspartic acids are important chiral building blocks for pharmaceuticals and food additives. Here we report the asymmetric synthesis of various N-arylalkyl-substituted l-aspartic acids using ethylenediamine-N,N'-disuccinic acid lyase (EDDS lyase) as a biocatalyst. This C-N lyase shows a broad non-natural amine substrate scope and outstanding enantioselectivity, allowing the efficient addition of structurally diverse arylalkylamines to fumarate to afford the corresponding N-arylalkyl-substituted l-aspartic acids in good isolated yield (up to 79%) and with excellent enantiopurity (>99% ee). These results further demonstrate that C-N lyases working in reverse constitute an extremely powerful synthetic tool to prepare difficult noncanonical amino acids.

Stereoretentive N-Arylation of Amino Acid Esters with Cyclohexanones Utilizing a Continuous-Flow System

The N-arylation of chiral amino acid esters with minimal racemization is a challenging transformation because of the sensitivity of the α-stereocenter. A versatile synthetic method was developed to prepare N-arylated amino acid esters using cyclohexanones as aryl sources under continuous-flow conditions. The designed flow system, which consists of a coil reactor and a packed-bed reactor containing a Pd(OH)₂ /C catalyst, efficiently afforded the desired N-arylated amino acids without significant racemization, accompanied by only small amounts of easily removable co-products (i. e., H₂ O and alkanes). The efficiency and robustness of this method allowed for the continuous synthesis of the desired product in very high yield and enantiopurity with high space-time yield (74.1 g L^-1  h^-1 ) and turnover frequency (5.9 h^-1 ) for at least 3 days.

Novel Cyclic Lipopeptides Fusaricidin Analogs for Treating Wound Infections

Both acute and chronic cutaneous wounds are often difficult to treat due to the high-risk for bacterial contamination. Once hospitalized, open wounds are at a high-risk for developing hospital-associated infections caused by multi drug-resistant bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa. Treating these infections is challenging, not only because of antibiotic resistance, but also due to the production of biofilms. New treatment strategies are needed that will help in both stimulating the wound healing process, as well as preventing and eliminating bacterial wound infections. Fusaricidins are naturally occurring cyclic lipopeptides with antimicrobial properties that have shown to be effective against a variety of fungi and Gram-positive bacteria, with low toxicity. Continuing with our efforts toward the identification of novel cyclic lipopeptides Fusaricidin analogs, herein we report the synthesis and evaluation of the antimicrobial activity for two novel cyclic lipopeptides (CLP), CLP 2605-4 and CLP 2612-8.1 against methicillin resistant S. aureus and P. aeruginosa, respectively, in in vivo porcine full thickness wound model. Both CLPs were able to reduce bacterial counts by approximately 3 log CFU/g by the last assessment day. Peptide 2612-8.1 slightly enhanced the wound healing, however, wounds treated with peptide 2605-4, have shown higher levels of inflammation and impaired wound healing process. This study highlights the importance of identifying new antimicrobials that can combat bacterial infection while not impeding tissue repair.