Base-catalyzed synthesis of aryl amides from aryl azides and aldehydes

Pd/C-Catalyzed Carbonylative Amidation for the Synthesis of 2-Carboxamidocyclohexane-1,3-diones

Herein, a first-ever heterogeneous Pd/C-catalyzed single-step tandem approach for the synthesis of 2-carboxamidocyclohexane-1,3-diones via direct carbonylative C-H amidation of cyclohexane-1,3-diones is reported. The reaction progressed under base-, oxidant-, and ligand-free conditions employing oxalic acid as a CO surrogate and sodium azide as a nitrogen precursor in a double-layer vial system.

Ortho-Phosphinoarenesulfonamide-Mediated Staudinger Reduction of Aryl and Alkyl Azides

Conventional Staudinger reductions of organic azides are sluggish with aryl or bulky aliphatic azides. In addition, Staudinger reduction usually requires a large excess of water to promote the decomposition of the aza-ylide intermediate into phosphine oxide and amine products. To overcome the challenges above, we designed a novel triaryl phosphine reagent 2c with an ortho-SO₂NH₂ substituent. Herein, we report that such phosphine reagents are able to mediate the Staudinger reduction of both aryl and alkyl azides in either anhydrous or wet solvents. Good to excellent yields were obtained in all cases (even at a diluted concentration of 0.01 M). The formation of B-TAP, a cyclic aza-ylide, instead of phosphine oxide, eliminates the requirement of water in the Staudinger reduction. In addition, computational studies disclose that the intramolecular protonation of the aza-ylide by the ortho-SO₂NH₂ group is kinetically favorable and responsible for the acceleration of Staudinger reduction of the aryl azides.

A Selective Single Step Amidation of Polyfluoroarenes

This chemistry establishes a method for the synthesis of per- and poly-fluoroaryl acid amides, utilizing nucleophilic aromatic substitution. Traditionally, such amides are constructed in a two-step process, namely, ammonolysis and then N-acylation. Herein, good yields of N-polyfluoroaryl acid amides were achieved in a single step under mild reaction conditions. Key to achieving optimal yields is the use of two equivalents of the nucleophile. In addition, the mechanism of the reaction is discussed which has implications for other related nucleophilic substitutions.

Dihydroquinazolines enhance 20S proteasome activity and induce degradation of α-synuclein, an intrinsically disordered protein associated with neurodegeneration

Aggregates or oligomeric forms of many intrinsically disordered proteins (IDPs), including α-synuclein, are hallmarks of neurodegenerative diseases, like Parkinson's and Alzheimer's disease, and key contributors to their pathogenesis. Due to their disordered nature and therefore lack of defined drug-binding pockets, IDPs are difficult targets for traditional small molecule drug design and are often referred to as "undruggable". The 20S proteasome is the main protease that targets IDPs for degradation and therefore small molecule 20S proteasome enhancement presents a novel therapeutic strategy by which these undruggable IDPs could be targeted. The concept of 20S activation is still relatively new, with few potent activators having been identified thus far. Herein, we synthesized and evaluated a library of dihydroquinazoline analogues and discovered several promising new 20S proteasome activators. Further testing of top hits revealed that they can enhance 20S mediated degradation of α-synuclein, the IDP associated with Parkinson's disease.

One-Pot Tandem Assembly of Amides, Amines, and Ketones: Synthesis of C4-Quaternary 3,4- and 1,4-Dihydroquinazolines

A multicomponent tandem assembly procedure for the synthesis of diverse C4-quaternary 3,4-dihydroquinazolines from amides, amines, and ketones has been developed. The one-pot reaction involves successive triflic anhydride mediated amide dehydration, ketimine addition, and Pictet-Spengler-like cyclization processes and affords products in up to 92% yield. Conversion of 3,4-dihydroquinazolines to the corresponding 1,4-dihydroquinazolines via a two-step N1 dealkylation and regioselective N3 functionalization protocol, including computational rationale for the observed regioselectivity, is also described.

Electrophilic Azides for Materials Synthesis and Chemical Biology

Organic azides are involved in a variety of useful transformations, including nitrene chemistry, reactions with nucleophiles and electrophiles, and cycloadditions. The 1,3-dipolar cycloadditions of azides constitute a major class of highly reliable and versatile reactions, as shown by the development and rapid adoption of click chemistry and bioorthogonal chemistry. Metal-catalyzed azide-alkyne cycloaddition (Cu/RuAAC), the prototypical click reaction, has found wide utility in pharmaceutical, biomedical, and materials sciences. The strain-promoted, or distortion-accelerated, azide-alkyne cycloaddition eliminates the need for a metal catalyst.In the azide-mediated 1,3-dipolar cycloaddition reactions, azides are ambiphilic, i.e., HOMO-LUMO-controlled dipoles where both the HOMO and LUMO interact strongly with the dipolarophile. Azide-alkyne cycloaddition proceeds primarily through the HOMO_azide-LUMO_{dipolarophile} interaction, and electron-deficient dipolarophiles react more readily. The inverse-electron-demand reaction, involving the LUMO_azide-HOMO_{dipolarophile} interaction, is less common because of the low stability of electron-deficient azides such as acyl, sulfonyl, and phosphoryl azides. Nevertheless, there have been reports since the 1960s showing enhanced reaction kinetics between electron-poor azides and electron-rich dipolarophiles. Our laboratory has developed the use of perfluoroaryl azides (PFAAs), a class of stable electron-deficient azides, as nitrene precursors and for reactions with nucleophiles and electron-rich dipolarophiles. Perfluorination on the aryl ring also facilitates the synthesis of PFAAs and quantitative analysis of the products by ¹⁹F NMR spectroscopy.In this Account, we summarize key reactions involving electrophilic azides and applications of these reactions in materials synthesis and chemical biology. These electron-deficient azides exhibit unique reactivity toward nucleophiles and electron-rich or strained dipolarophiles, in some cases leading to new transformations that do not require any catalysts or products that are impossible to obtain from the nonelectrophilic azides. We highlight work from our laboratories on reactions of PFAAs with enamines, enolates, thioacids, and phosphines. In the reactions of PFAAs with enamines or enolates, the triazole or triazoline cycloaddition products undergo further rearrangement to give amidines or amides as the final products at rates of up to 10⁵ times faster than their non-fluorinated anlogues. Computational investigations by the distortion/interaction activation strain model reveal that perfluorination lowers the LUMO of the aryl azide as well as the overall activation energy of the reaction by decreasing the distortion energies of the reactants to reach the transition states. The PFAA-enamine reaction can be carried out in a one-pot fashion using readily available starting materials of aldehyde and amine, making the reaction especially attractive, for example, in the functionalization of nanomaterials and derivatization of antibiotics for the preparation of theranostic nanodrugs. Similar fast kinetics was also observed for the PPAA-mediated Staudinger reaction, which proceeds at 10⁴ times higher rate than the classic Staudinger ligation, giving stable phosphoimines in high yields. The reaction is biorthogonal, allowing cell-surface labeling with minimal background noise.

Iron(ii)-chloride-catalyzed regioselective azidation of allenamides with TMSN3

The first report of iron catalyzed azidation of allenamides via radical process to provide an efficient route for accessing allyl azides.

Direct Amidation of Carboxylic Acids with Nitroarenes

N-Aryl amides are an important class of compounds in pharmaceutical and agrochemical chemistry. Rapid and low-cost synthesis of N-aryl amides remains in high demand. Herein, we disclose an operationally simple process to access N-aryl amides directly from readily available nitroarenes and carboxylic acids as coupling substrates. This method involves the in situ activation of carboxylic acids to acyloxyphosphonium salt for one-pot amidation, without the need for isolation of the corresponding synthetic intermediates. Furthermore, the ease of preparation and workup allow the quick and efficient synthesis of a wide range of N-aryl amides, including several amide-based druglike and agrochemical molecules.

Base-Promoted SNAr Reactions of Fluoro- and Chloroarenes as a Route to N-Aryl Indoles and Carbazoles

KOH/DMSO-promoted C-N bond formation via nucleophilic aromatic substitution (S_NAr) between chloroarenes or fluoroarenes with indoles and carbazole under transition metal-free conditions affording the corresponding N-arylated indoles and carbazoles has been developed.

A Versatile Catalyst-Free Perfluoroaryl Azide-Aldehyde-Amine Conjugation Reaction

A tri-component reaction, involving an electrophilically-activated perfluoroaryl azide, an enolizable aldehyde and an amine, reacts readily at room temperature without any catalysts in solvents including aqueous conditions to yield a stable amidine conjugate. The versatility of this reaction is demonstrated in the conjugation of an amino acid without prior protection of the carboxyl group, and in the synthesize antibiotic-nanoparticle conjugates.

Synthesis of Carbohydrate-Grafted Glycopolymers Using a Catalyst-Free, Perfluoroarylazide-Mediated Fast Staudinger Reaction

Glycopolymers have gained increasing importance in investigating glycan-lectin interactions, as drug delivery vehicles and in modulating interactions with proteins. The synthesis of these glycopolymers is still a challenging and rigorous exercise. In this regard, the highly efficient click reaction, copper (I)-catalyzed alkyne-azide cycloaddition, has been widely applied not only for its efficiency but also for its tolerance of the appended carbohydrate groups. However, a significant drawback of this method is the use of the heavy metal catalyst which is difficult to remove completely, and ultimately toxic to biological systems. In this work, we present the synthesis of carbohydrate-grafted glycopolymers utilizing a mild and catalyst-free perfluorophenyl azide (PFPA)-mediated Staudinger reaction. Using this strategy, mannose (Man) and maltoheptaose (MH) were grafted onto the biodegradable poly(lactic acid) (PLA) by stirring a PFAA-functionalized PLA with a phosphine-derivatized Man or MH in DMSO at room temperature within an hour. The glycopolymers were characterized by ¹H-NMR, 19F-NMR, 31P-NMR and FTIR.

Methods for direct C(sp2)–H bonds azidation

Direct functionalization of C–H bonds has attracted great attention in recent years from the perspectives of atom and step economy.

Direct Amidation of Carboxylic Acids through an Active α-Acyl Enol Ester Intermediate

The development of a highly efficient and simple protocol for the direct amidation of carboxylic acids is described employing ynoates as novel coupling reagents. The transformation proceeds in good to excellent yields via in situ α-acyl enol ester intermediates formation under mild reaction conditions. This useful method has been demonstrated for a range of substrates to provide a succinct access to structurally diverse amides, including key intermediates of glibenclamide, tiapride hydrochloride, and nateglinide, and can be conducted on a mole scale.

Intramolecular Schmidt Reaction of Vinyl Azides with Cyclic Ketones

Cyclic ketones tethered with a vinyl azide group undergo a Schmidt-hydrolysis sequence to give secondary lactams bearing a ketone side chain. Secondary lactams are obtained in a regioselective manner that is not possible in a conventional Schimdt reaction. In addition to the well-documented C-2 nucleophilicity, the N nucleophilicity of vinyl azide disclosed in this work opens a new direction for reaction invention involving vinyl azides.

Perfluoroaryl Azide Staudinger Reaction: A Fast and Bioorthogonal Reaction

We report a fast Staudinger reaction between perfluoroaryl azides (PFAAs) and aryl phosphines, which occurs readily under ambient conditions. A rate constant as high as 18 m-1  s-1 was obtained between methyl 4-azido-2,3,5,6-tetrafluorobenzoate and methyl 2-(diphenylphosphanyl)benzoate in CD3 CN/D2 O. Furthermore, the iminophosphorane product was stable toward hydrolysis and aza-phosphonium ylide reactions. This PFAA Staudinger reaction proved to be an excellent bioothorgonal reaction. PFAA-derivatized mannosamine and galactosamine were successfully transformed into cell-surface glycans and efficiently labeled with phosphine-derivatized fluorophore-conjugated bovine serum albumin.

Design and synthesis of theranostic antibiotic nanodrugs that display enhanced antibacterial activity and luminescence

We report the modular formulation of ciprofloxacin-based pure theranostic nanodrugs that display enhanced antibacterial activities, as well as aggregation-induced emission (AIE) enhancement that was successfully used to image bacteria. The drug derivatives, consisting of ciprofloxacin, a perfluoroaryl ring, and a phenyl ring linked by an amidine bond, were efficiently synthesized by a straightforward protocol from a perfluoroaryl azide, ciprofloxacin, and an aldehyde in acetone at room temperature. These compounds are propeller-shaped, and upon precipitation into water, readily assembled into stable nanoaggregates that transformed ciprofloxacin derivatives into AIE-active luminogens. The nanoaggregates displayed increased luminescence and were successfully used to image bacteria. In addition, these nanodrugs showed enhanced antibacterial activities, lowering the minimum inhibitory concentration (MIC) by more than one order of magnitude against both sensitive and resistant Escherichia coli The study represents a strategy in the design and development of pure theranostic nanodrugs for combating drug-resistant bacterial infections.

Poly(HEMA-co-HEMA-PFPA): Synthesis and preparation of stable micelles encapsulating imaging nanoparticles

We report the preparation of stable micelles from random copolymers of 2-hydroxyethyl methacrylate (HEMA) and perfluorophenyl azide (PFPA)-derivatized HEMA (HEMA-PFPA). The copolymers were synthesized by RAFT polymerization at room temperature under mild conditions without affecting the azide functionality. Upon addition of water to the copolymer solution in DMSO, the random copolymers self-assembled into micelles even at the percentage of HEMA-PFPA as low as 4.5%. The size of the micelles can be controlled by the molecular weight and the concentration of the copolymer, and the percentage of HEMA-PFPA in the copolymer. In addition, iron oxide nanoparticles and quantum dots were successfully encapsulated into the micelles with high encapsulation efficiency (∼80%). These nanoparticles, which were hydrophobic and formed agglomerates in water, became fully dispersed after encapsulating into the micelles. The micelles were stable and the size remained unchanged for at least 6months.

Reaction of azides and enolisable aldehydes under the catalysis of organic bases and Cinchona based quaternary ammonium salts

This work describes a two-step sequence to prepare amides in moderate enantioselectivity from azides and enolisable aldehydes that proceeds under chiral phase transfer catalysis.

Recent Developments in Amide Synthesis Using Nonactivated Starting Materials

Amides are unquestionably one of the most important functional groups in organic chemistry because of their presence in numerous interesting molecules such as peptides, pharmaceutical agents, naturally occurring molecules, proteins and alkaloids, among others. This synopsis surveys the diverse recent approaches to amide synthesis from nonactivated carboxylic acids and derivatives as well as noncarboxylic compounds, highlighting the most innovative methodologies and those that are more eco-friendly compared to traditional methods while focusing on recent developments during the past two years.

Catalyst-Free Cycloaddition Reaction for the Synthesis of Glyconanoparticles

A new conjugation method for the immobilization of carbohydrates on nanomaterials was demonstrated simply by mixing perfluorophenyl azide-functionalized silica nanoparticles (SNPs), an amine-derivatized carbohydrate, and phenylacetaldehyde under ambient conditions without any catalyst. The density of carbohydrates on the glyconanoparticles was determined using the quantitative ¹⁹F NMR (¹⁹F qNMR) technique; for example, the density of d-mannose (Man) on Man-SNPs was 2.5 ± 0.2 × 10^-16 nmol/nm². The glyconanoparticles retained their binding affinity and selectivity toward cognate lectins. The apparent dissociation constant of the glyconanoparticles was measured by a fluorescence competition assay, where the binding affinity of Man-SNPs was almost 4 orders of magnitude higher than that of Man with concanavalin A. Moreover, even with a ligand density of 2.6 times lower than Man-SNPs synthesized by the copper-catalyzed azide-alkyne cycloaddition, the binding affinity of Man-SNPs prepared by the current method was more than 4 times higher.