Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2-Oxazolidinones: Catalyst-Free Synthesis Under Ambient Conditions

The development of sustainable synthetic routes to access valuable oxazolidinones via CO₂ fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction features a high activation barrier and thus requires a catalytic system, and may present some other critical issues. Here, the straightforward gram-scale synthesis of a series of 5-aryl-2-oxazolidinones was developed at ambient temperature and atmospheric CO₂ pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO₂ adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO₂ . The reaction mechanism was investigated by NMR spectroscopy and DFT calculations applied to model cases.

Synthesis of α-Aminophosphonic Acid Derivatives Through the Addition of O- and S-Nucleophiles to 2H-Azirines and Their Antiproliferative Effect on A549 Human Lung Adenocarcinoma Cells

This work reports a straightforward regioselective synthetic methodology to prepare α-aminophosphine oxides and phosphonates through the addition of oxygen and sulfur nucleophiles to the C–N double bond of 2H-azirine derivatives. Determined by the nature of the nucleophile, different α-aminophosphorus compounds may be obtained. For instance, aliphatic alcohols such as methanol or ethanol afford α-aminophosphine oxide and phosphonate acetals after N–C3 ring opening of the intermediate aziridine. However, addition of 2,2,2-trifluoroethanol, phenols, substituted benzenthiols or ethanethiol to 2H-azirine phosphine oxides or phosphonates yields allylic α-aminophosphine oxides and phosphonates in good to high general yields. In some cases, the intermediate aziridine attained by the nucleophilic addition of O- or S-nucleophiles to the starting 2H-azirine may be isolated and characterized before ring opening. Additionally, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549) and non-malignant cells (MCR-5) was also screened. Some α-aminophosphorus derivatives exhibited very good activity against the A549 cell line in vitro. Furthermore, selectivity towards cancer cell (A549) over non-malignant cells (MCR-5) has been detected in almost all compounds tested.

Asymmetric synthesis of CF2-functionalized aziridines by combined strong Brønsted acid catalysis

A diastereo- and enantioselective approach to access chiral CF2-functionalized aziridines from difluorodiazoethyl phenyl sulfone (PhSO2CF2CHN2) and in situ-formed aldimines is described. This multicomponent reaction is enabled by a combined strong Brønsted acid catalytic platform consisting of a chiral disulfonimide and 2-carboxyphenylboronic acid. The optical purity of the obtained CF2-substituted aziridines could be further improved by a practical dissolution-filtration procedure.

Mechanical Force Induces Ylide-Free Cycloaddition of Nonscissible Aziridines

The application of aziridines as nonvulnerable mechanophores is reported. Upon exposure to a mechanical force, stereochemically pure nonactivated aziridines incorporated into the backbone of a macromolecule do not undergo cis-trans isomerization, thus suggesting retention of the ring structure under force. Nonetheless, aziridines react with a dipolarophile and seem not to obey conventional reaction pathways that involve C-C or C-N bond cleavage prior to the cycloaddition. Our work demonstrates that a nonvulnerable chemical structure can be a mechanophore.

Synthesis of Oxazolidinones by using Carbon Dioxide as a C1 Building Block and an Aluminium-Based Catalyst

Oxazolidinone synthesis through the coupling of carbon dioxide and aziridines was catalysed by an aluminium(salphen) complex at 50-100 °C and 1-10 bar pressure under solvent-free conditions. The process was applicable to a variety of substituted aziridines, giving products with high regioselectivity. It involved the use of a sustainable and reusable aluminium-based catalyst, used carbon dioxide as a C₁ source and provided access to pharmaceutically important oxazolidinones as illustrated by a total synthesis of toloxatone. This protocol was scalable, and the catalyst could be recovered and reused. A catalytic cycle was proposed based on stereochemical, kinetic and Hammett studies.

Enantioselective Reaction of 2H-Azirines

2H-Azirines are useful precursors for the synthesis of a variety of chiral aziridine and amine derivatives with a range of biological activities. Owing to the ring strain and the presence of a C=N double bond, 2H-azirines are more reactive than other types of ketimine, and undergo a range of enantioselective reactions, including reduction and Diels-Alder reactions, as well as nucleophilic addition to the C=N double bond. Therefore, the enantioselective reactions of 2H-azirines has become a hot topic, in particular within the last few years. In this Minireview, we focus on the enantioselective reactions of 2H-azirines by using catalytic or stoichiometric amounts of chiral additives, the reaction mechanisms, and the applications of these reactions of 2H-azirines and related compounds in organic synthesis.

Highly Enantioselective Synthesis of Fused Tri- and Tetrasubstituted Aziridines: aza-Darzens Reaction of Cyclic Imines with α-Halogenated Ketones Catalyzed by Bifunctional Phosphonium Salt

The first enantioselective aza-Darzens reaction of cyclic imines with α-halogenated ketones was realized under mild reaction conditions by using amino-acid-derived bifunctional phosphonium salts as phase-transfer promoters. A variety of structurally dense tri- and tetrasubstituted aziridine derivatives, containing benzofused heterocycles as well as spiro-structures, were readily synthesized in high yields with excellent diastereo- and enantioselectivities (up to >20:1 d.r. and >99.9 % ee). The highly functionalized aziridine products could be easily transformed into different classes of biologically active compounds.

Regiodivergent and Stereospecific Aziridine Opening by Copper-Catalyzed Addition of Silicon Grignard Reagents

A stereospecific ring opening of various 2-aryl-substituted aziridines with silicon Grignard reagents under copper catalysis is reported. The regiochemical outcome is governed by the steric demand of the silicon nucleophile. The LiCl introduced with the magnesium reagent R₃ SiMgX⋅2 LiCl is shown to enhance the S_N 2-type displacement of the carbon-nitrogen bond by coordination to the aziridine nitrogen atom.

Telescoped, Divergent, Chemoselective C1 and C1-C1 Homologation of Imine Surrogates: Access to Quaternary Chloro- and Halomethyl-Trifluoromethyl Aziridines

A conceptually novel, high‐yielding, mono‐ or bis‐homologation was realized with lithium halocarbenoids and enables the one‐step, fully chemocontrolled assembly of a new class of quaternary trifluoromethyl aziridines. Trifluoroacetimidoyl chlorides (TFAICs) act as convenient electrophilic platforms, enabling the addition of either one or two homologating elements by simply controlling the stoichiometry of the process. Mechanistic studies highlighted that the homologation event, carried out with two different carbenoids (LiCH₂Cl and LiCH₂F), leads to fluoromethyl analogues in which the first nucleophile is employed for constructing the cycle and the second for decorating the resulting molecular architecture.

Bach Goes to Town

Donald A. Tomalia is one of the pioneers in the field of dendrimers, who is still active at the age of 80 years-old. The present contribution is a journey through his scientific contributions from the early beginning until his discovery of the poly(amidoamine), (PAMAM)-dendrimers.

Modular One-Step Three-Component Synthesis of Tetrahydroisoquinolines Using a Catellani Strategy

Reported is a modular one-step three-component synthesis of tetrahydroisoquinolines using a Catellani strategy. This process exploits aziridines as the alkylating reagents, through palladium/norbornene cooperative catalysis, to enable a Catellani/Heck/aza-Michael addition cascade. This mild, chemoselective, and scalable protocol has broad substrate scope (43 examples, up to 90 % yield). The most striking feature of this protocol is the excellent regioselectivity and diastereoselectivity observed for 2-alkyl- and 2-aryl-substituted aziridines to access 1,3-cis-substituted and 1,4-cis-substituted tetrahydroisoquinolines, respectively. Moreover, this is a versatile process with high step and atom economy.

Design of Trifluoroalkenyl Iodonium Salts for a Hypervalency-Aided Alkenylation-Cyclization Strategy: Metal-Free Construction of Aziridine Rings

The synthesis of fluorinated compounds and their use as pharmaceutical ingredients or synthetic building blocks have been in the focus of chemical and medicinal research. However, the efficient synthesis of trifluoromethylated nitrogen heterocycles is sometimes challenging. Herein, we disclose a simple aziridination process that relies on the use of amines and novel alkenyl iodonium reagents for the synthesis of strained, trifluoromethylated heterocycles. With the utilization of a newly designed and bench-stable but highly reactive hypervalent alkenyl iodonium species, these three-membered-ring heterocyclic compounds can be efficiently constructed from simple amines under mild conditions in the absence of transition-metal catalysts. The special reactivity of the new trifluoropropenyl synthon towards nucleophilic centers could be exploited in more general cyclization and alkenylation reactions in the future.

Catalytic Enantioselective Aza-Benzoin Reactions of Aldehydes with 2H-Azirines

The unprecedented enantioselective aza-benzoin reaction of aldehydes with 2H-azirines was developed by utilizing a chiral N-heterocyclic carbene as the catalyst. A wide range of corresponding aziridines can be obtained in good yields with high enantioselectivities. The obtained optically active aziridines should be useful in the synthesis of other valuable molecules.

Synthesis and Evaluation of Biological Activities of Aziridine Derivatives of Urea and Thiourea

In the present paper, we report the synthesis and evaluation of in vitro antimicrobial activities of aziridine-thiourea derivatives. A series of aziridines in reaction with isocyanates and isothiocyanates to obtain urea and thiourea derivatives were used. The structures of all new products were confirmed based on spectroscopic data (¹H-NMR, ¹³C-NMR, HR-MS). These compounds were screened for their in vitro antimicrobial activity against a panel of Gram-positive and Gram-negative strains of bacteria. Six of the tested compounds appeared to be promising agents against reference strains of Escherichia coli, Staphylococcus aureus and Staphylococcus epidermidis. Subsequently, compounds exhibiting promising antibacterial activity were tested against twelve clinical isolates of S. aureus from three different sources of infection. The most bactericidal compounds (MIC = 16-32 µg/mL) showed better antibacterial activity against MRSA than ampicillin and streptomycin. The in vitro cytotoxicity analysis on L929 murine fibroblast and HeLa human tumor cell line using the MTT assay allowed us to select the least toxic compounds for future investigation.

Titanocene-Catalyzed Radical Opening of N-Acylated Aziridines

Aziridines activated by N-acylation are opened to the higher substituted radical through electron transfer from titanocene(III) complexes in a novel catalytic reaction. This reaction is applicable in conjugate additions, reductions, and cyclizations and suited for the construction of quaternary carbon centers. The concerted mechanism of the ring opening is indicated by DFT calculations.

Chemo- and Enantioselective Intramolecular Silver-Catalyzed Aziridinations

Asymmetric nitrene-transfer reactions are a powerful tool for the preparation of enantioenriched amine building blocks. Reported herein are chemo- and enantioselective silver-catalyzed aminations which transform di- and trisubstituted homoallylic carbamates into [4.1.0]-carbamate-tethered aziridines in good yields and with ee values of up to 92 %. The effects of the substrate, silver counteranion, ligand, solvent, and temperature on both the chemoselectivity and ee value were explored. Stereochemical models were proposed to rationalize the observed absolute stereochemistry of the aziridines, which undergo nucleophilic ring opening to yield enantioenriched amines with no erosion in stereochemical integrity.

Enantioselective Reaction of 2H-Azirines with Phosphite Using Chiral Bis(imidazoline)/Zinc(II) Catalysts

The first highly enantioselective nucleophilic addition reaction of phosphites with 2H-azirines has been developed. The reaction was applied to various 3-substituted 2H-azirines using novel chiral bis(imidazoline)/Zn^II catalysts to afford products in good yield with high enantioselectivity. The transformation of the obtained optically active aziridines showed that 2H-azirines act as either α,β- or β,β-dicarbocationic amine synthons.

Catalyst-Controlled Multicomponent Aziridination of Chiral Aldehydes

A highly diastereoselective and enantioselective method for the multicomponent aziridination of chiral aldehydes has been developed with BOROX catalysts of the VANOL (3,3'-diphenyl-2,2'-bi-1-naphthol) and VAPOL (2,2'-diphenyl-(4-biphenanthrol)) ligands. Very high to perfect catalyst control is observed with most all substrates examined including aldehydes with chiral centers in the α- and β-positions. High catalyst control was also observed for a number of chiral heterocyclic aldehydes allowing for the preparation of epoxy aziridines, bis(aziridines) and ethylene diaziridines. Application of this reaction in the synthesis of β³ -homo-d-alloisoleucine and β³ -homo-l-isoleucine is reported.

Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes

The visible-light-promoted diastereodivergent intramolecular oxyamination of alkenes is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon of highly diastereoselective ring-opening of aziridines controlled by electron sacrifices was observed. Highly diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti-aziridine intermediates were generated quickly from primary amidyl radicals and the diastereoselectivities were controlled by pK_a values of the electron sacrifices.

A Porous Metal-Organic Framework Assembled by [Cu30] Nanocages: Serving as Recyclable Catalysts for CO2 Fixation with Aziridines

Based on a novel ligand 5-(2,6-bis(4-carboxyphenyl)pyridin-4-yl)isophthalic acid (H4BCP) with large skeletons, a unique porous framework {[Cu2(BCP)(H2O)2]·3DMF} n (1) assembled by nano-sized and censer-like [Cu30] cages is successfully obtained and structurally characterized. In 1, the large 1D channel in frameworks and window size in the nanocages can enrich methylene blue and capture CO2, exhibiting the promising applications in environmental protection. More importantly, the explorations on the cycloaddition reaction of CO2 and aziridines with various substituents suggest that 1 can serve as an efficient heterogeneous catalyst for CO2 conversion with aziridines in a solvent-free system, which can be reused at least ten times without any obvious loss in catalytic activity. This is the first example of metal-organic framework (MOF)-based catalysts in converting CO2 into high-value oxazolidinones through activating aziridines and CO2, further extending the applications of MOFs materials in catalysis.

Three-component synthesis of highly functionalized aziridines containing a peptide side chain and their one-step transformation into β-functionalized α-ketoamides

A sequential three-component process is described, starting from 3-arylmethylene-2,5-piperazinediones and involving a one-pot sequence of reactions achieving regioselective opening of the 2,5-diketopiperazine ring and diastereoselective generation of an aziridine ring. This method allows the preparation of N-unprotected, trisubstituted aziridines bearing a peptide side chain under mild conditions. Their transformation into β-trifluoroacetamido-α-ketoamide and α,β-diketoamide frameworks was also achieved in a single step.

Continuous-Flow Synthesis and Derivatization of Aziridines through Palladium-Catalyzed C(sp(3) )-H Activation

A continuous‐flow synthesis of aziridines by palladium‐catalyzed C(sp³)−H activation is described. The new flow reaction could be combined with an aziridine‐ring‐opening reaction to give highly functionalized aliphatic amines through a consecutive process. A predictive mechanistic model was developed and used to design the C−H activation flow process and illustrates an approach towards first‐principles design based on novel catalytic reactions.

Ring Contraction of 3-Hydroxy-3-(trifluoromethyl)piperidines: Synthesis of 2-Substituted 2-(Trifluoromethyl)pyrrolidines

A ring contraction of 3-hydroxy-3-(trifluoromethyl)piperidines was achieved via an aziridinium intermediate. This contraction facilitates the synthesis of a series of 2-substituted 2-(trifluoromethyl)pyrrolidines incorporating a quaternary center at the C2 position.

Efficient regioselective ring opening of activated aziridine-2-carboxylates with [(18)f]fluoride

Aziridines can undergo a range of ring-opening reactions with nucleophiles. The regio- and stereochemistry of the products depend on the substituents on the aziridine. Aziridine ring-opening reactions have rarely been used in radiosynthesis. Herein we report the ring opening of activated aziridine-2-carboxylates with [¹⁸F]fluoride. The aziridine was activated for nucleophilic attack by substitution of various groups on the aziridine nitrogen atom. Fluorine-18 radiolabelling was followed by ester hydrolysis and removal of the activation group. Totally regioselective ring opening and subsequent deprotection was achieved with tert-butyloxycarbonyl- and carboxybenzyl-activated aziridines to give α-[¹⁸F]fluoro-β-alanine in good radiochemical yield.

Anomalous nuclear Overhauser effects in carbon-substituted aziridines: scalar cross-relaxation of the first kind

Anomalous NOESY cross-peaks that cannot be explained by dipolar cross-relaxation or chemical exchange are described for carbon-substituted aziridines. The origin of these is identified as scalar cross-relaxation of the first kind, as demonstrated by a complete theoretical description of this relaxation process and by computational simulation of the NOESY spectra. It is shown that this process relies on the stochastic modulation of J-coupling by conformational transitions, which in the case of aziridines arise from inversion at the nitrogen center. The observation of scalar cross-relaxation between protons does not appear to have been previously reported for NOESY spectra. Conventional analysis would have assigned the cross-peaks as being indicative of a chemical exchange process occurring between correlated spins, were it not for the fact that the pairs of nuclei displaying them cannot undergo such exchange.