Rh2(II)-Catalyzed Intermolecular N-Aryl Aziridination of Olefins Using Nonactivated N Atom Precursors.

The development of the first intermolecular Rh2(II)-catalyzed aziridination of olefins using anilines as nonactivated N atom precursors and an iodine(III) reagent as the stoichiometric oxidant is reported. This reaction requires the transfer of an N-aryl nitrene fragment from the iminoiodinane intermediate to a Rh2(II) carboxylate catalyst; in the absence of a catalyst only diaryldiazene formation was observed. This N-aryl aziridination is general and can be successfully realized by using as little as 1 equiv of the olefin. Di-, tri-, and tetrasubstituted cyclic or acylic olefins can be employed as substrates, and a range of aniline and heteroarylamine N atom precursors are tolerated. The Rh2(II)-catalyzed N atom transfer to the olefin is stereospecific as well as chemo- and diastereoselective to produce the N-aryl aziridine as the only amination product. Because the chemistry of nonactivated N-aryl aziridines is underexplored, the reactivity of N-aryl aziridines was explored toward a range of nucleophiles to stereoselectively access privileged 1,2-stereodiads unavailable from epoxides, and removal of the N-2,4-dinitrophenyl group was demonstrated to show that functionalized primary amines can be constructed.

3-Arylaziridine-2-carboxylic Acid Derivatives and (3-Arylaziridin-2-yl)ketones: The Aziridination Approaches.

Aziridination reactions represent a powerful tool in aziridine synthesis. Significant progress has been achieved in this field in the last decades, whereas highly functionalized aziridines including 3-arylated aziridine-2-carbonyl compounds play an important role in both medical and synthetic chemistry. For the reasons listed, in the current review we have focused on the ways to obtain 3-arylated aziridines and on the recent advances (mainly since the year 2000) in the methodology of the synthesis of these compounds via aziridination.

Synthesis and Antiproliferative Activity of Phosphorus Substituted 4-Cyanooxazolines, 2-Aminocyanooxazolines, 2-Iminocyanooxazolidines and 2-Aminocyanothiazolines by Rearrangement of Cyanoaziridines.

Several phosphorus-substituted N-acylated cyanoaziridines 2 and N-carbamoylated cyanoziridines 5 were prepared in good to high yields. N-Acylated cyanoaziridines 2 were used, after ring expansion, in an efficient synthesis of oxazoline derivative 3a and in a completely regio-controlled reaction in the presence of NaI. Conversely, N-carbamoyl cyanoaziridines 5 reacted with NaI to obtain a regioisomeric mixture of 2-aminocyanooxazolines 7. Mild acidic conditions can be used for the isomerization of N-thiocarbamoyl cyanoaziridine 6a into a 2-aminocyanothiazoline derivative 8a by using BF3·OEt2 as a Lewis acid. Likewise, a one pot reaction of NH-cyanoaziridines 1 with isocyanates obtained 2-iminocyanooxazolidines 9 regioselectively. This synthetic methodology involves the addition of isocyanates to starting cyanoaziridines to obtain N-carbamoyl cyanoaziridines 5, which after the ring opening, reacts with a second equivalent of isocyanate to give the final 2-imino cyanooxazolidines 9. In addition, the cytotoxic effect on the cell lines derived from human lung adenocarcinoma (A549) was also screened. 2-Iminooxazolidines 9 exhibited moderate activity against the A549 cell line in vitro. Furthermore, a selectivity towards cancer cells (A549) over non-malignant cells (MCR-5) was detected.

Synthesis of novel sugar derived aziridines, as starting materials giving access to sugar amino acid derivatives.

D-Erythrosyl aziridines were obtained from D-erythrosyl triazoles either by photolysis or through diazirine intermediates. These were found to undergo rich, high yielding chemistry by reaction with protic acids (HCl, BiI3/H2O and trifluoroacetic acid) leading to two types of furanoid sugar α-amino acids, and polyhydroxylprolines. Based on experimental evidence, reaction mechanisms have been proposed for the syntheses.

Stereoselective synthesis of highly functionalized (Z)-chloroalkene dipeptide isosteres containing an α,α-disubstituted amino acid.

Described here is the first stereoselective synthesis of highly functionalized chloroalkene dipeptide isosteres containing an α,α-disubstituted amino acid (ααAA). This synthesis requires the construction of a quaternary carbon center, and this challenge was overcome by the Aza-Darzens condensation of ketimine with α,α-dichloroenolate, producing 2-chloroaziridines with quaternary carbon centers including spirocyclic motifs, which are valuable for the previously elusive synthesis of various ααAA-containing chloroalkene isosteres.

Aziridine synthesis by coupling amines and alkenes via an electrogenerated dication.

Aziridines-three-membered nitrogen-containing cyclic molecules-are important synthetic targets. Their substantial ring strain and resultant proclivity towards ring-opening reactions makes them versatile precursors of diverse amine products1-3, and, in some cases, the aziridine functional group itself imbues important biological (for example, anti-tumour) activity4-6. Transformation of ubiquitous alkenes into aziridines is an attractive synthetic strategy, but is typically accomplished using electrophilic nitrogen sources rather than widely available amine nucleophiles. Here we show that unactivated alkenes can be electrochemically transformed into a metastable, dicationic intermediate that undergoes aziridination with primary amines under basic conditions. This new approach expands the scope of readily accessible N-alkyl aziridine products relative to those obtained through existing state-of-the-art methods. A key strategic advantage of this approach is that oxidative alkene activation is decoupled from the aziridination step, enabling a wide range of commercially available but oxidatively sensitive7 amines to act as coupling partners for this strain-inducing transformation. More broadly, our work lays the foundations for a diverse array of difunctionalization reactions using this dication pool approach.

Synthesis of ß-Phenethylamines via Ni/Photoredox Cross-Electrophile Coupling of Aliphatic Aziridines and Aryl Iodides.

A photoassisted Ni-catalyzed reductive cross-coupling between tosyl-protected alkyl aziridines and commercially available (hetero)aryl iodides is reported. This mild and modular method proceeds in the absence of stoichiometric heterogeneous reductants and uses an inexpensive organic photocatalyst to access medicinally valuable ß-phenethylamine derivatives. Unprecedented reactivity was achieved with the activation of cyclic aziridines. Mechanistic studies suggest that the regioselectivity and reactivity observed under these conditions are a result of nucleophilic iodide ring opening of the aziridine to generate an iodoamine as the active electrophile. This strategy also enables cross-coupling with Boc-protected aziridines.

Enantiopure α-Trifluoromethylated Aziridine-2-carboxylic Acid (α-TfmAzy): Synthesis and Peptide Coupling.

A straightforward synthesis of enantiopure α-trifluoromethyl aziridine-2-carboxylic acid (α-TfmAzy) is reported from a trifluoropyruvate derived enantiopure oxazolidine. A key Strecker-type synthetic step and a late cyanide basic hydrolysis gave the target compounds in six steps and 41% yield. A final peptide coupling was performed to demonstrate the usefulness of this highly constrained fluorinated unnatural amino acid.

Rational Design of Mechanism-Based Inhibitors and Activity-Based Probes for the Identification of Retaining α-l-Arabinofuranosidases.

Identifying and characterizing the enzymes responsible for an observed activity within a complex eukaryotic catabolic system remains one of the most significant challenges in the study of biomass-degrading systems. The debranching of both complex hemicellulosic and pectinaceous polysaccharides requires the production of α-l-arabinofuranosidases among a wide variety of coexpressed carbohydrate-active enzymes. To selectively detect and identify α-l-arabinofuranosidases produced by fungi grown on complex biomass, potential covalent inhibitors and probes which mimic α-l-arabinofuranosides were sought. The conformational free energy landscapes of free α-l-arabinofuranose and several rationally designed covalent α-l-arabinofuranosidase inhibitors were analyzed. A synthetic route to these inhibitors was subsequently developed based on a key Wittig-Still rearrangement. Through a combination of kinetic measurements, intact mass spectrometry, and structural experiments, the designed inhibitors were shown to efficiently label the catalytic nucleophiles of retaining GH51 and GH54 α-l-arabinofuranosidases. Activity-based probes elaborated from an inhibitor with an aziridine warhead were applied to the identification and characterization of α-l-arabinofuranosidases within the secretome of A. niger grown on arabinan. This method was extended to the detection and identification of α-l-arabinofuranosidases produced by eight biomass-degrading basidiomycete fungi grown on complex biomass. The broad applicability of the cyclophellitol-derived activity-based probes and inhibitors presented here make them a valuable new tool in the characterization of complex eukaryotic carbohydrate-degrading systems and in the high-throughput discovery of α-l-arabinofuranosidases.

Structure-Guided Design and In-Cell Target Profiling of a Cell-Active Target Engagement Probe for PARP Inhibitors.

Inhibition of the poly(ADP-ribose) polymerase (PARP) family of enzymes has become an attractive therapeutic strategy in oncology and beyond; however, chemical tools to profile PARP engagement in live cells are lacking. Herein, we report the design and application of PARPYnD, the first photoaffinity probe (AfBP) for PARP enzymes based on triple PARP1/2/6 inhibitor AZ9482, which induces multipolar spindle (MPS) formation in breast cancer cells. PARPYnD is a robust tool for profiling PARP1/2 and is used to profile clinical PARP inhibitor olaparib, identifying several novel off-target proteins. Surprisingly, while PARPYnD can enrich recombinant PARP6 spiked into cellular lysates and inhibits PARP6 in cell-free assays, it does not label PARP6 in intact cells. These data highlight an intriguing biomolecular disparity between recombinant and endogenous PARP6. PARPYnD provides a new approach to expand our knowledge of the targets of this class of compounds and the mechanisms of action of PARP inhibitors in cancer.

One-pot construction of functionalized aziridines and maleimides via a novel pseudo-Knoevenagel cascade reaction.

An Ugi, novel pseudo-Knoevenagel, ring expansion cascade reaction was discovered and utilized for the synthesis of aziridinyl succinimides in one-pot. Subsequently, densely functionalized aziridines and maleimides have been designed and synthesized through similar cascade reactions. The target compounds were prepared by means of a mild reaction and a simple operation procedure, which could be applicable to a broad scope of starting materials. This series of novel cascade reactions generates opportunities for the tailored synthesis of a wide range of biologically active scaffolds through tuneable Ugi inputs. Discovery of compound 8i with comparable potency to sorafenib in liver cancer cell lines could provide a new avenue for liver cancer drug discovery.

New phosphazine and phosphazide derivatives as multifunctional ligands targeting acetylcholinesterase and ß-Amyloid aggregation for treatment of Alzheimer's disease.

Phosphazine and phosphazide derivatives are described herein as a new class of selective and potent acetylcholinesterase (AChE) inhibitors and ß-amyloid aggregation inhibitors. Phosphazines (5-7) were synthesized smoothly via a redox-condensation reaction of 1,2-bis(diphenylphosphino)ethane with different amines derivatives in the presence of dialkyl azodicarboxylate (Staudinger reaction) while phosphazides (8) via electrophilic attack of azido derivatives. Structures of the synthesized compounds were justified on the basis of compatible elementary and spectroscopic analyses. All the compounds were evaluated for their acetylcholinesterase inhibitory activity. The most three potent compounds (5b-c and 8b) showing AChE IC50 values (29.85-34.96 nM) comparable to that of donepezil (34.42 nM) were subjected to further investigation by testing their butyrylcholinesterase, MMP-2 and self-induced Aß aggregation inhibition activity. Especially, the coumarin phosphazide derivative (8b) presented the best AChE inhibition selectivity index (IC50 = 34.96 nM, AChE/BuChE; 3.81) together with good inhibition ability against MMP-2 (IC50 = 441.33 nM) and self-induced Aß1-42 aggregation (IC50 = 337.77 nM). In addition, the inhibition of metal-induced Aß aggregation by 8b was confirmed by thioflavine T fluorescence. The most potent effect of 8b was observed on the Zn2+-induced Aß42 aggregation. Kinetic study of compound 8b suggested it to be a competitive AChE inhibitor. Also, it specifically chelates metal and is predicted to be permeable to BBB. It also possesses low toxicity on SH-SY5Y neuroblastoma cells with a safety index of 15.37. In addition, it was demonstrated that compound 8b can improve the cognitive impairment of scopolamine-induced model in mice with % alternations and transfer latency time comparable to that of donepezil. Also, a docking study was carried out and it was in accordance with the in vitro results. These promising in vitro and in vivo findings highlight compound 8b as a possible drug candidate in searching for new multifunctional AD drugs.

Synthesis of Enantiomerically Pure N-Boc-Protected 1,2,3-Triaminopropylphosphonates and 1,2-Diamino-3-Hydroxypropylphosphonates.

All possible isomers of 1,2,3-tri(N-tert-butoxycarbonylamino)propylphosphonate 6 were synthesized from the respective diethyl [N-(1-phenylethyl)]-1-benzylamino-2,3-epiiminopropylphosphonates 5 via opening the aziridine ring with trimethylsilyl azide (TMSN3) followed by hydrogenolysis in the presence of di-tert-butyl dicarbonate (Boc2O). [N-(1-phenylethyl)]-1-benzylamino-2,3-epiiminopropylphosphonates (1R,2R,1'S)-5a and (1S,2S,1'R)-5c were smoothly transformed into diethyl 3-acetoxy-1-benzylamino-2-[N-(1-phenylethyl)amino]propylphosphonates (1R,2R,1'S)-9a and (1S,2S,1'R)-9c, respectively by the opening of the aziridine ring with acetic acid. Transformations of [N-(1-phenylethyl)]-1-benzylamino-2,3-epiiminopropylphosphonates (1S,2R,1'S)-5b and (1R,2S,1'R)-5d into diethyl 3-acetoxy-1-benzylamino-2-[(1-phenylethyl)amino]propylphosphonates (1S,2R,1'S)-9b and (1R,2S,1'R)-9d were accompanied by the formation of ethyl {1-(N-benzylacetamido)-3-hydroxy-2-[(1-phenylethyl)amino]propyl}phosphonate (1S,2R,1'S)-10b and (1R,2S,1'R)-10d and 3-(N-benzylacetamido)-4-[N-(1-phenylethyl)]amino-1,2-oxaphospholane (3S,4R,1'S)-11b and (3R,4S,1'R)-11d as side products. Diethyl (1R,2R)-, (1S,2S)-, (1S,2R)- and (1R,2S)-3-acetoxy-1,2-di(N-tert-butoxycarbonylamino)propylphosphonates 7a-7d were obtained from the respective 3-acetoxy-1-benzylamino-2-[N-(1-phenylethyl)amino]propylphosphonates 9a-9d by hydrogenolysis in the presence of Boc2O.

Synthesis, Characterization and Evaluation of Cytotoxic Activities of Novel Aziridinyl Phosphonic Acid Derivatives.

New aziridine 2-phosphonic acids were prepared by monohydrolysis of the aziridine 2-phosphonates that were obtained by the modified Gabriel-Cromwell reaction of vinyl phosphonate or α-tosylvinyl phosphonate with a primary amine or a chiral amine. The cellular cytotoxicity of these compounds was tested against the HCT-116 colorectal cancer cell lines and the CCD-18Co normal colon fibroblast lines using the MTT assay. Three of the synthesized phosphonic acid derivatives 2e (ethyl hydrogen {(2S)-1-[(1S)-1-(naphthalen-2-yl)ethyl]aziridin-2-yl}phosphonate), 2h (ethyl hydrogen (1-benzylaziridin-2-yl)phosphonate), and 2i (ethyl hydrogen (1-cyclohexylaziridin-2-yl)phosphonate) showed higher cytotoxicity than the reference cancer treatment agent etoposide. Cell death was through a robust induction of apoptosis even more effectively than etoposide, a well-known apoptosis inducing agent.

A Rh(III)-Catalyzed Formal [4+1] Approach to Pyrrolidines from Unactivated Terminal Alkenes and Nitrene Sources.

We have developed a formal [4+1] approach to pyrrolidines from readily available unactivated terminal alkenes as 4-carbon partners. The reaction provides a rapid construction of various pyrrolidine containing structures, especially for the diastereoselective synthesis of spiro-pyrrolidines. Mechanistic investigation suggests a Rh(III)-catalyzed intermolecular aziridination of the alkene and subsequent acid-promoted ring expansion for the pyrrolidine formation.

Cu(II)-Mediated N-H and N-Alkyl Aryl Amination and Olefin Aziridination.

Cu(II)-mediated direct NH2 and NH alkyl aryl aminations and olefin aziridinations are described. These room-temperature, one-pot, environmentally friendly procedures replace costly Rh2 catalysts and, in some instances, display important differences with comparable Rh2- and Fe-supported reactions.

Peptides Containing meso-Oxa-Diaminopimelic Acid as Substrates for the Cell-Shape-Determining Proteases Csd6 and Pgp2.

The enzymes Csd6 and Pgp2 are peptidoglycan (PG) proteases found in the pathogenic bacteria Helicobacter pylori and Campylobacter jejuni, respectively. These enzymes are involved in the trimming of non-crosslinked PG sidechains and catalyze the cleavage of the bond between meso-diaminopimelic acid (meso-Dap) and d-alanine, thus converting a PG tetrapeptide into a PG tripeptide. They are known to be cell-shape-determining enzymes, because deletion of the corresponding genes results in mutant strains that have lost the normal helical phenotype and instead possess a straight-rod morphology. In this work, we report two approaches directed towards the synthesis of the tripeptide substrate Ac-iso-d-Glu-meso-oxa-Dap-d-Ala, which serves as a mimic of the terminus of an non-crosslinked PG tetrapeptide substrate. The isosteric analogue meso-oxa-Dap was utilized in place of meso-Dap to simplify the synthetic procedure. The more efficient synthesis involved ring opening of a peptide-embedded aziridine by a serine-based nucleophile. A branched tetrapeptide was also prepared as a mimic of the terminus of a crosslinked PG tetrapeptide. We used MS analysis to demonstrate that the tripeptide serves as a substrate for both Csd6 and Pgp2 and that the branched tetrapeptide serves as a substrate for Pgp2, albeit at a significantly slower rate.

Stereoselective Multicomponent Reactions in the Synthesis or Transformations of Epoxides and Aziridines.

Small ring heterocycles, such as epoxides and aziridines, are present in several natural products and are also highly versatile building blocks, frequently involved in the synthesis of numerous bioactive products and pharmaceuticals. Because of the potential for increased efficiency and selectivity, along with the advantages of environmentally benign synthetic procedures, multicomponent reactions (MCRs) have been explored in the synthesis and ring opening of these heterocyclic units. In this review, the recent advances in MCRs involving the synthesis and applications of epoxides and aziridines to the preparation of other heterocycles are discussed emphasizing the stereoselectivity of the reactions.

Synthesis and biological evaluation of cyanoaziridine phosphine oxides and phosphonates with antiproliferative activity.

This work reports an efficient diastereoselective synthetic methodology for the preparation of phosphorus substituted cyanoaziridines through the nucleophilic addition of TMSCN, as cyanide source, to the C-N double bond of 2H-azirine derivatives. The aziridine ring, in these novel cyanoaziridines, can be activated by simple N-tosylation or N-acylation. In addition, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549) and human embryonic kidney (HEK293) was also screened. N-H and N-Substituted cyanoaziridines showed excellent activity against the A549 cell line in vitro. Moreover, selectivity towards cancer cell (A549) over (HEK293), and non-malignant cells (MCR-5) has been observed.

Photo-Induced Aziridination of Alkenes with N-Sulfonyliminoiodinanes.

Activation of N-sulfonyliminiodinanes was achieved by photo-irradiation at 375 nm, which enabled the reaction with several alkenes to afford the corresponding aziridines. Mechanistic studies suggested that the reaction would proceed through a stepwise mechanism via radical intermediates rather than through a concerted process.