The Curtius rearrangement: mechanistic insight and recent applications in natural product syntheses

Exploring the mechanism of the reductive amination of acetophenones via the Borch approach: the role of the acid catalyst

The energetic viability of several mechanistic variations of the reductive amination of acetophenones via the Borch approach was re-examined through density functional theory calculations. The crucial involvement of the acid catalyst is evident not only in the elimination of water, but also in the initial nucleophilic step. This role increases with the electron-donating capability of the substituent positioned at the para-position of acetophenone.

Decarboxylative stereoretentive C-N coupling by harnessing aminating reagent

In recent decades, strategies involving transition-metal catalyzed carbon-carbon or carbon-heteroatom bond coupling have emerged as potent synthetic tools for constructing intricate molecular architectures. Among these, decarboxylative carbon-nitrogen bond formation using abundant carboxylic acids or their derivatives has garnered notable attention for accessing alkyl- or arylamines, one of key pharmacophores. While several decarboxylative amination methods have been developed, the involvement of a common carboradical intermediate currently poses challenges in achieving stereospecific transformation toward chiral alkylamines. Herein, we present a base-mediated, stereoretentive decarboxylative amidation by harnessing 1,4,2-dioxazol-5-one as a reactive and robust amidating reagent under transition-metal-free ambient conditions, encompassing all types of primary, secondary and tertiary carboxylic acids, thereby providing access to the important pharmacophore, α-chiral amines. This method exhibits high functional group tolerance, convenient scalability, and ease of applicability for 15N-isotope labeling, thus accentuating its synthetic utilities. Experimental and computational mechanistic investigations reveal a sequence of elementary steps: i) nucleophilic addition of carboxylate to dioxazolone, ii) rearrangement to form a dicarbonyl N-hydroxy intermediate, iii) conversion to hydroxamate, followed by a Lossen-type rearrangement, and finally, iv) reaction of the in situ generated isocyanate with carboxylate leading to C-N bond formation in a stereoretentive manner.

Unlocking Amides: A General Method for the Self-Immolative Release of Amide-Containing Molecules

The controlled liberation of molecules from a constructed framework is a subject of profound interest across various chemical fields. It allows for the masking of a molecule's properties and precise deployment upon a single controllable release event. While numerous methodologies have been developed for amines, alcohols, and thiols, approaches for utilising amides as payload-release handles are still in their early stages of development, despite the prevalence of amides in therapeutic compounds and materials. Herein, is presented a comprehensive strategy for the controlled and selective release of a diverse range of amides with stable linkers. The versatility of this approach is demonstrated by its successful application in the targeted release of various amide-containing drugs in their natural form via the use of commonly used trigger motifs, such as dipeptides or glycosides. As a proof of concept, the FDA-approved antibiotic linezolid has been successfully converted into a prodrug form and released selectively only in the presence of the trigger event. Significantly, in its prodrug state, no activity against Mycobacterium tuberculosis was exhibited. Linezolid's full potential was achieved only upon controlled release, where an equipotent efficacy to the free linezolid control was demonstrated, thus emphasising the immense potential of this method.

A Disconnection for Rapid Access to Heterocyclic Benzylic Amines with Fully Substituted α-Carbons

2- or 4-Pyridyl benzylic amines represent a privileged motif in drug discovery. However, the formation of heterocyclic benzylic amines with fully substituted α-carbons can require the execution of lengthy synthetic routes, which limit their application. Addition of various nucleophilic agents to Ellman's imines has been well established; however, there is no precedented literature reported for pyridyl-type nucleophiles, which are very important for medicinal chemistry. In this letter, we disclose the development of a one-step synthesis of heterocyclic benzylic amines with fully substituted α-carbons from heteroaryl halides and sulfinyl imines. Starting from 2,4-dibromopyridine, regioselective synthesis of 2- or 4-pyridyl benzylic amines could be achieved by choosing toluene or MTBE as a solvent.

Design, synthesis and anticholinergic properties of novel α-benzyl dopamine, tyramine, and phenethylamine derivatives

Due to the important biological properties of dopamine, phenethylamine, and tyramine derivatives in the central nervous system, herein the synthesis of novel α-benzyl dopamine, phenethylamine, and tyramine derivatives is described. The title compounds were synthesized starting from 3-phenylpropanoic acids and methoxybenzenes in six or seven steps. Firstly, 3-(2,3-dimethoxyphenyl)propanoic acid (11) and 3-(3,4-dimethoxyphenyl)propanoic acid (12) were selectively brominated with N-bromosuccinimide (NBS). The Friedel-Crafts acylation of methoxylated benzenes with these brominated acids or commercially available 3-phenylpropanoic acid in polyphosphoric acid gave the desired dihydrochalcones. α-Carboxylation of dihydrochalcones, reduction of benzylic carbonyl groups, hydrolysis of esters to acid derivatives, and the Curtius rearrangement reaction of acids followed by in situ synthesis of carbamates from alkyl isocyanates and hydrogenolysis of the carbamates afforded the title compounds in good total yields. Alzheimer's disease (AD) and Parkinson's disease (PD) are chronic neurodegenerative diseases that become serious over time. However, the exact pathophysiology of both diseases has not been revealed yet. There have been many different approaches to the treatment of patients for many years, especially studies on the cholinergic system cover a wide area. Within the scope of this study, the inhibition effects of dopamine-derived carbamates and amine salts on the cholinergic enzymes AChE and BChE were examined. Dopamine-derived carbamate 24a-i showed inhibition in the micro-nanomolar range; compound 24d showed a Ki value of 26.79 nM against AChE and 3.33 nM against BChE, while another molecule, 24i, showed a Ki range of 27.24 nM and 0.92 nM against AChE and BChE, respectively. AChE and BChE were effectively inhibited by dopamine-derived amine salts 25j-s, with Ki values in the range of 17.70 to 468.57 µM and 0.76-211.23 µM, respectively. Additionally, 24c, 24e and 25m were determined to be 60, 276 and 90 times more selective against BChE than AChE, respectively.

Ir(iii)/Ag(i)-catalyzed directly C-H amidation of arenes with OH-free hydroxyamides as amidating agents

A versatile Ir(iii)-catalyzed C-H amidation of arenes by employing readily available and stable OH-free hydroxyamides as a novel amidation source. The reaction occurred with high efficiency and tolerance of a range of functional groups. A wide scope of aryl OH-free hydroxyzamides, including conjugated and challenging non-conjugated OH-free hydroxyzamides, were capable of this transformation and no addition of an external oxidant is required. This protocol provided a simple, straightforward and economic method to a variety N-(2-(1H-pyrazol-1-yl)alkyl)amide derivates with good to excellent yield. Mechanistic study demonstrated that reversible C-H bond functionalisation might be involved in this reaction.

Simulating Non-Adiabatic Dynamics of Photoexcited Phenyl Azide: Investigating Electronic and Structural Relaxation en Route to the Formation of Phenyl Nitrene

Excited state molecular dynamics simulations of the photoexcited phenyl azide have been performed. The semi-classical surface hopping approximation has enabled an unconstrained analysis of the electronic and nuclear degrees of freedom which contribute to the molecular dissociation of phenyl azide into phenyl nitrene and molecular nitrogen. The significance of the second singlet excited state in leading the photodissociation has been established through electronic structure calculations, based on multi-configurational schemes, and state population dynamics. The investigations on the structural dynamics have revealed the N-N bond separation to be accompanied by synchronous changes in the azide N-N-N bond angle. The 100 fs simulation results in a nitrene fragment that is electronically excited in the singlet manifold.

CHNO - Formylnitrene, Cyanic, Isocyanic, Fulminic, and Isofulminic Acids and their Interrelationships at DFT and CASPT2 Levels of Theory

Fulminic and cyanic acids played a decisive role in the conception of isomerism 200 years ago. Cyanic (HOCN), isocyanic (HNCO), and fulminic (HCNO) acids have been detected in several interstellar sources, but isofulminic acid (HONC) is little known. Here we examine the interrelationships between the four acids and formylnitrene, HC(O)N, at the CASPT2 and three DFT levels. Formylnitrene has a triplet ground state, T0, a closed shell singlet (CSS), S0, and an open-shell singlet (OSS), S1, lying ∼7 and 27 kcal/mol above T0, respectively. The CSS is weakly stabilized by a 12 kcal/mol bond between the N and the O atoms. A conical intersection 12 kcal/mol above T0 permits easy T0-S0 interchange. Formyl azide and formylnitrene (T0 and S0) are isomerized thermally to HNCO. HOCN is best obtained via dissociation of the nitrene (or of HNCO) to H• + NCO• radicals ∼46 kcal/mol above the T0 nitrene. Isofulminic acid, HONC, isomerizes readily to cyanic acid, HOCN, in thermal and photochemical reactions. Fulminic acid, HCNO, can isomerize to HNCO via CSS formylnitrene. Easy tautomerization prevents the preparation of HOCN in quantity. The barrier to isomerization is strongly reduced in small hydrogen-bonded aggregates so that trace amounts of HOCN can exist in equilibrium with HNCO.

Nickel/Photoredox Dual-Catalyzed Regiodivergent Aminoalkylation of Unactivated Alkyl Halides

A mild and regiodivergent aminoalkylation of unactivated alkyl halides is disclosed via a dual photoredox/nickel catalysis. Bipyridyl-type ligands without an ortho-substituent control the site-selective coupling at the original position, while ortho-disubstituted ligands tune the site-selectivity at a remote, unprefunctionalized position. Mechanistic studies combined with DFT calculations give insight into the mechanism and the origins of the ligand-controlled regioselectivity. Notably, this redox-neutral, regiodivergent alkyl-alkyl coupling features mild conditions, broad substrate scope for both alkyl coupling partners, and excellent site-selectivity and offers a straightforward way for α-alkylation of tertiary amines to synthesize structurally diverse alkylamines and value-added amino acid derivatives.

Paving the Way towards Sustainability of Polyurethanes: Synthesis and Properties of Terpene-Based Diisocyanate

Due to growing concerns about environmental issues and the decline of petroleum-based resources, the synthesis of new biobased compounds for the polymer industry has become a prominent and timely topic. P-menthane-1,8-diamine (PMDA) is a readily available compound synthesized from turpentine, a cheap mixture of natural compounds isolated from pine trees. PMDA has been extensively used for its biological activities, but it can also serve as a source of valuable monomers for the polymer industry. In this work, commercial PMDA (ca. 85% pure) was purified by salinization, crystallization, and alkali treatment and then converted into p-menthane-1,8-diisocyanate (PMDI) through a phosgene-free synthesis at room temperature. A thorough analytical study using NMR techniques (1H, 13C, 13C-1H HSQC, 13C-1H HMBC, and 1H-1H NOESY) enables the characterization of the cis-trans isomeric mixtures of both PMDA and PMDI. These structural studies allowed for a better understanding of the spatial configuration of both isomers. Then, the reactivity of PMDI with a primary alcohol (benzyl alcohol) was studied in the presence of nine different catalysts exhibiting different activation modes. Finally, the use of PMDI in the synthesis of polyurethanes was explored to demonstrate that PMDI can be employed as a new biobased alternative to petrochemical-based isocyanates such as isophorone diisocyanate (IPDI).

Unified Divergent Total Synthesis of Discorhabdin B, H, K, and Aleutianamine via the Late-Stage Oxidative N,S-Acetal Formation

This study achieved the total syntheses of (+)-discorhabdin B, (-)-discorhabdin H, (+)-discorhabdin K, and (-)-aleutianamine. A phenethylamine fragment bearing a o-pivaloylthio group, corresponding to the D/E/G ring moiety, was prepared from benzothiophen-2-carboxylic acid methyl ester and condensed with a known pyrroloiminoquinone derivative. The adduct was subjected to [bis(trifluoroacetoxy)iodo]benzene (PIFA)-promoted oxidative spirocyclization to furnish the A/B/C/D/E spirocyclohexadienone fused with pyrroloiminoquinone. The total synthesis of (±)-discorhabdin B was completed via the key construction of the highly strained G ring with the N,S-acetal moiety featuring a newly developed CuBr2-mediated oxidative cascade cyclization. The stereocontrolled total synthesis of (+)-discorhabdin B was accomplished by a diastereoselective PIFA-promoted oxidative spirocyclization using a chiral thioester. (-)-Disocrhabdin H and (+)-discorhabdin K were synthesized by the site- and face-selective thia-Michael addition of l-ovothiol A to (+)-N-Ts-discorhabdin B with the concomitant formation of the F ring by forming the C2-N18 bond. The total synthesis of (-)-aleutianamine was achieved via a skeletal rearrangement initiated by the Luche reduction of the dienone moiety of (+)-N-Ts-discorhabdin B.

A Conformationally Restricted Gold(III) Complex Elicits Antiproliferative Activity in Cancer Cells

Diamine ligands are effective structural scaffolds for tuning the reactivity of transition-metal complexes for catalytic, materials, and phosphorescent applications and have been leveraged for biological use. In this work, we report the synthesis and characterization of a novel class of cyclometalated [C^N] Au(III) complexes bearing secondary diamines including a norbornane backbone, (2R,3S)-N2,N3-dibenzylbicyclo[2.2.1]heptane-2,3-diamine, or a cyclohexane backbone, (1R,2R)-N1,N2-dibenzylcyclohexane-1,2-diamine. X-ray crystallography confirms the square-planar geometry and chirality at nitrogen. The electronic character of the conformationally restricted norbornane backbone influences the electrochemical behavior with redox potentials of -0.8 to -1.1 V, atypical for Au(III) complexes. These compounds demonstrate promising anticancer activity, particularly, complex 1, which bears a benzylpyridine organogold framework, and supported by the bicyclic conformationally restricted diaminonorbornane, shows good potency in A2780 cells. We further show that a cellular response to 1 evokes reactive oxygen species (ROS) production and does not induce mitochondrial dysfunction. This class of complexes provides significant stability and reactivity for different applications in protein modification, catalysis, and therapeutics.

Fluorine-Assisted Rearrangement of Geminal Azidofluorides to Imidoyl Fluorides

Organoazide rearrangement constitutes versatile synthetic strategies but typically requires an extremely strong acid and/or a high reaction temperature. Our group recently discovered the remarkable accelerating effect of the geminal fluorine substituent that enables the facile rearrangement of azides into imidoyl fluorides without the aid of acid under much milder reaction conditions. The role of geminal fluorine was elucidated by both experimental and computational investigations. This new reactivity led to the development of a practical one-step tandem preparative method for potentially useful and bench-stable imidoyl fluorides from a wide range of structurally diverse geminal chlorofluorides. Our additional efforts to expand the reaction scope regarding the migrating group, halogen, and carbonyl function are described, and the synthetic utility of the imidoyl fluoride products was demonstrated in hopes of promoting the use of this under-appreciated functional group in the synthetic organic community.

Concise Enantioselective Total Synthesis of Isopavine Alkaloids

Herein, we report a concise asymmetric total synthesis of isopavine alkaloids, which feature a special azabicyclo[3.2.2]nonane tetracyclic skeleton. The key steps include iridium-catalyzed asymmetric hydrogenation of unsaturated carboxylic acids, Curtius rearrangement, and Eschweiler-Clarke methylation, which enable an enantioselective approach to isopavine alkaloids in 6-7 linear steps. Furthermore, for the first time, isopavine alkaloids, especially (-)-reframidine (3), are found to display effective antiproliferative effects on various cancer cell lines.

Enantioselective Total Synthesis of (+)-Stephadiamine

An asymmetric synthesis of (+)-stephadiamine has been accomplished featuring (a) an enantioselective dearomatizative Michael addition to generate a quaternary stereocenter; (b) a domino sequence involving reductive generation of nitrone from γ-nitro ketone followed by a highly regio- and diastereo-selective intramolecular [3 + 2] cycloaddition to construct the aza[4,3,3]propellane core with concurrent generation of two quaternary stereocenters and two functional groups ready for subsequent transformations; (c) the Curtius rearrangement of the sensitive α,α-disubstituted malonic acid mono ester for the installation of α,α-disubstituted amino ester moiety; (d) a benzylic C-H oxidation under photoredox catalytic conditions; and (e) a highly diastereoselective ketone reduction affording δ-hydroxyester preorganized for lactonization.

Photoredox-Nickel Dual-Catalyzed C-Alkylation of Secondary Nitroalkanes: Access to Sterically Hindered α-Tertiary Amines

The preparation of tertiary nitroalkanes via the nickel-catalyzed alkylation of secondary nitroalkanes using aliphatic iodides is reported. Previously, catalytic access to this important class of nitroalkanes via alkylation has not been possible due to the inability of catalysts to overcome the steric demands of the products. However, we have now found that the use of a nickel catalyst in combination with a photoredox catalyst and light leads to much more active alkylation catalysts. These can now access tertiary nitroalkanes. The conditions are scalable as well as air and moisture tolerant. Importantly, reduction of the tertiary nitroalkane products allows rapid access to α-tertiary amines.

Copper-Catalyzed C(sp3 )-Amination of Ketone-Derived Dihydroquinazolinones by Aromatization-Driven C-C Bond Scission

Herein, we describe the development of a copper-catalyzed C(sp³ )-amination of proaromatic dihydroquinazolinones derived from ketones. The reaction is enabled by the intermediacy of open-shell species arising from homolytic C-C bond-cleavage driven by aromatization. The protocol is characterized by its operational simplicity and generality, including chemical diversification of advanced intermediates.

Digital micelles of encoded polymeric amphiphiles for direct sequence reading and ex vivo label-free quantification

Identification and quantification of synthetic polymers in complex biological milieu are crucial for delivery, sensing and scaffolding functions, but conventional techniques based on imaging probe labellings only afford qualitative results. Here we report modular construction of precise sequence-defined amphiphilic polymers that self-assemble into digital micelles with contour lengths strictly regulated by oligourethane sequences. Direct sequence reading is accomplished with matrix-assisted laser desorption/ionization (MALDI) tandem mass spectrometry, facilitated by high-affinity binding of alkali metal ions with poly(ethylene glycol) dendrons and selective cleavage of benzyl-carbamate linkages. A mixture of four types of digital micelles could be identified, sequence-decoded and quantified by MALDI and MALDI imaging at cellular, organ and tissue slice levels upon in vivo administration, enabling direct comparison of biological properties for each type of digital micelle in the same animal. The concept of digital micelles and encoded amphiphiles capable of direct sequencing and high-throughput label-free quantification could be exploited for next-generation precision nanomedicine designs (such as digital lipids) and protein corona studies.

Dendritic Quaternary-Encoded Oligourethanes for Data Encryption

The use of sequence-defined digital polymers for data storage and encryption has received increasing attention due to their precision structures similar to natural biomacromolecules (e.g., DNA) but increased stability. However, the rapid development of sequencing techniques raises the concern of information leakage. Herein, dendritic quaternary-encoded oligourethanes bearing a photoresponsive trigger, self-immolative backbones, and a mass spectrometry tag of PEG dendron have been developed for data encryption. Although the sequence information in linear analogs can be readily deciphered by mass spectrometry, sequencing of dendritic oligourethanes cannot be achieved by either primary MS or tandem MS/MS owing to the unique spatial conformation. Intriguingly, the fragmentation pathways of a quaternary dendrimer under MS/MS conditions can be converted to 2772-bit 2D matrices with ≈1.98×10⁸⁷ permutations, serving as high-strength encryption keys for highly reliable data encryption.

Continuous Flow Generation of Acylketene Intermediates via Nitrogen Extrusion

A flow chemistry process for the generation and use of acylketene precursors through extrusion of nitrogen gas is reported. Key to the development of a suitable continuous protocol is the balance of reaction concentration against pressure in the flow reactor. The resulting process enables access to intercepted acylketene scaffolds using volatile amine nucleophiles and has been demonstrated on the gram scale. Thermal gravimetric analysis was used to guide the temperature set point of the reactor coils for a variety of acyl ketene precursors. The simultaneous generation and reaction of two reactive intermediates (both derived from nitrogen extrusion) is demonstrated.

N-Alkyl Carbamoylimidazoles as Isocyanate Equivalents: Exploration of the Reaction Scope for the Synthesis of Ureas, Hydantoins, Carbamates, Thiocarbamates, and Oxazolidinones

The reaction of the HCl or trifluoroacetic acid salts of primary amines with carbonyldiimidazole (CDI) is shown to be a preparatively useful method for forming monosubstituted carbamoylimidazoles (28 examples) without the formation of symmetrical urea side products. The utility of these air- and water-stable crystalline carbamoylimidazole reagents was demonstrated by their reactions as blocked or masked isocyanate equivalents. Reaction with various classes of nucleophiles provides access to useful functional groups including ureas, carbamates, thiocarbamates, hydantoins, and oxazolidinones. A parallel synthesis library of 30 ureas was generated by the reaction of 6× carbamoylimidazole intermediates with 5× amines and triethylamine. The unsymmetrical urea-containing natural products macaurea A and pygmaniline A were also prepared in good yields (95% over four steps and 79% over three steps, respectively) using this approach. The reaction of carbamoylimidazoles with amino acid methyl esters followed by microwave irradiation in aqueous media gives hydantoins in high yields, further demonstrating the ability of carbamoylimidazoles as isocyanate surrogates. Three hydantoin-containing natural products including macahydantoin D and meyeniihydantoin A were prepared in nearly quantitative yields from proline methyl ester and carbamoylimidazoles. The reaction of carbamoylimidazoles with alcohols and thiols under basic conditions affords carbamates and thiocarbamates, respectively, in good yields. Lastly, a method for the preparation of chiral oxazolidinone heterocycles from chiral epoxy alcohols is demonstrated using a double displacement approach. The reactions occur with high regio- and stereoselectivity (dr ≥ 15:1 by ¹H NMR) via a domino attack of the corresponding alkoxides with carbamoylimidazoles followed by an intramolecular attack of the in situ generated urea anion at the proximal position of the epoxide group.

Carboxylate-Assisted Iridium (III)-Catalyzed C(sp2)-H Amidation of 2-Aroylimidazoles With Dioxazolones

The Ir(III)-catalyzed ortho C-H amidation of 2-aroylimidazoles with 3-aryldioxazolones as an amidating reagent is reported. The method provides a broad substrate scope with wide functional group compatibility. Mechanistic studies indicate that C-H bond cleavage is reversible and appears not to be the rate-determining step. The presence of an electron-donating group in the 2-aroylimidazoles and an electron-withdrawing group in the 3-aryldioxazoles significantly accelerates the reaction, suggesting that nitrene insertion is the rate-determining step.

Features of ZED1227: The First-In-Class Tissue Transglutaminase Inhibitor Undergoing Clinical Evaluation for the Treatment of Celiac Disease

ZED1227 is a small molecule tissue transglutaminase (TG2) inhibitor. The compound selectively binds to the active state of TG2, forming a stable covalent bond with the cysteine in its catalytic center. The molecule was designed for the treatment of celiac disease. Celiac disease is an autoimmune-mediated chronic inflammatory condition of the small intestine affecting about 1–2% of people in Caucasian populations. The autoimmune disease is triggered by dietary gluten. Consumption of staple foods containing wheat, barley, or rye leads to destruction of the small intestinal mucosa in genetically susceptible individuals, and this is accompanied by the generation of characteristic TG2 autoantibodies. TG2 plays a causative role in the pathogenesis of celiac disease. Upon activation by Ca²⁺, it catalyzes the deamidation of gliadin peptides as well as the crosslinking of gliadin peptides to TG2 itself. These modified biological structures trigger breaking of oral tolerance to gluten, self-tolerance to TG2, and the activation of cytotoxic immune cells in the gut mucosa. Recently, in an exploratory proof-of-concept study, ZED1227 administration clinically validated TG2 as a “druggable” target in celiac disease. Here, we describe the specific features and profiling data of the drug candidate ZED1227. Further, we give an outlook on TG2 inhibition as a therapeutic approach in indications beyond celiac disease.

Synthesis of Substituted 2-Pyridones via 6π-Electrocyclization of Dienyl Isocyanates

A one-pot Curtius rearrangement of dienyl carboxylic acids followed by a 6π-electrocyclization process to form substituted 2-pyridone products has been developed. Dienyl isocyanates generated from aliphatic acids were more reactive than their aromatic counterparts. Additionally, substitution patterns of the carboxylic acids had an impact on the efficiency of the cyclization.

Organic Isocyanates and Isothiocyanates: Versatile Intermediates in Agrochemistry

In recent decades, organic isocyanates and isothiocyanates have been often applied as reactive intermediates in research syntheses or manufacturing routes of many agrochemicals. These heterocumulenes allowed the installation of crucial carboxylic functions, such as carbamates, ureas, and semicarbazones, but have also been used for the construction of five- and six-membered heterocycles, such as tetrazolones, thiazoles, and uracils.

1 Introduction

2 Preparation of Carboxylic Acid Functions

3 Preparation of Heterocyclic Rings

4 Conclusion

Cutting edge of diphenyl phosphorazidate (DPPA) as a synthetic reagent – A fifty-year odyssey

Recent developments of diphenyl phosphorazidate (DPPA, (C6H5O)2P(O)N3) has been reviewed.

CHNO isomers and derivatives – a computational overview

A state-of-the-art computational study of CHNO isomers is presented, including substituted derivatives and cyclotrimerization and deprotonation products.

Interrupted Curtius Rearrangements of Quaternary Proline Derivatives: A Flow Route to Acyclic Ketones and Unsaturated Pyrrolidines

Conversion of N-Boc-protected quaternary proline derivatives under thermal Curtius rearrangement conditions was found to afford a series of ring-opened ketone and unsaturated pyrrolidine products instead of the expected carbamate species. The nature of the substituent on the quaternary carbon thereby governs the product outcome due to the stability of a postulated N-acyliminium species. A continuous flow process with in-line scavenging was furthermore developed to streamline this transformation and safely create products on a gram scale.

Synthesis, in vitro anticancer activity and in silico studies of certain isoxazole-based carboxamides, ureates, and hydrazones as potential inhibitors of VEGFR2

The ensuing research presents the results of in vitro anticancer activity of novel 28 compounds of isoxazole-based carboxamides 3(a-d); ureates 4(a-g), 5, 6, 7a,b, 8; and hydrazones 9(a-f), 10(a-d), 11a,b as potential inhibitors of VEGFR2. The carboxamides and ureates were synthesized by converting 5-(aryl)-isoxzaole-3-carbohydrazides 1a,b to the corresponding carbonylazides 2a,b followed by treatment with the appropriate amines. The hydrazones were directly obtained through condensation of the carbohydrazide 1a,b with aldehydes and/or ketones. The structures of the target compounds were confirmed by elemental and spectral analyses. A preliminary in vitro anticancer screening of solutions (10^-5M) on 60 cancer cell lines (NCI, USA) revealed that the carboxamide 3c is the most promising growth inhibitor. Explicitly, 3c showed potent anticancer activity at 10µ M against leukemia (HL-60(TB), K-562 and MOLT-4), colon cancer (KM12) and melanoma (LOX IMVI) cell lines with %GI range = 70.79-92.21. Evaluation of growth inhibitory activity of the synthesized compounds against hepatocellular carcinoma (HepG2), that overexpresses VEGFR2, showed superior activity of compounds 8, 10a and 10c with IC₅₀ in sub micromolar concentrations of 0.84, 0.79 and 0.69 μM, respectively, which is better than that of the reference drug, Sorafenib (IC₅₀ = 3.99 µM). Moreover, these compounds displayed high selective cytotoxicity for HepG2 cancer cells over the nontumorigenic THLE2 liver cells (SI range = 26.37-38.60) which reflect their safety. The results of VEGFR2 kinase inhibition assay demonstrate that, compounds 8 and 10a are the most active inhibitors with IC₅₀ = 25.7 and 28.2 nM, respectively, (Sorafenib IC₅₀ = 28.1 nM). Molecular docking of the synthesized derivatives to VEGFR2 (PDB: 3WZE) showed similar binding modes to that of the co-crystallized ligand, sorafenib. Moreover, the results of computational assessment of ADME and drug-likeness characteristics inspire further investigations of the new isoxazole-based derivatives to afford more potent, safe and orally active VEGFR2 inhibitors as potential anticancer drug candidates.

Concise and Stereoselective Total Syntheses of Annotinolides C, D, and E

The annotinolides are one of the most recent additions to the Lycopodium family of alkaloids, with its members possessing challenging, caged structures that include a [3.2.1]-bicyclic core bearing six contiguous stereocenters, including four that are fully substituted. Herein, we document a concise and stereoselective route that achieves the first total syntheses of three of its members: annotinolides C, D, and E. Key operations include a gold(I)-catalyzed Conia-ene reaction that fashions much of the main core in a single operation, as well as a number of other challenging and chemoselective transformations to generate the remaining elements. Moreover, efforts utilizing the natural products themselves, seeking adjustments in their oxidation states and the rearrangement of individual ring systems, shed light on their potential biogenesis with some outcomes counter to those originally proposed. Finally, formal enantioenriched syntheses of the target molecules are also presented.

A Novel Photoinduced Ligation Approach for Cross-Linking Polymerization, Polymer Chain-End Functionalization, and Surface Modification Using Benzoyl Azides

Various ligation processes have recently become a powerful tool in synthetic polymer chemistry. Herein, the use of a new photochemical ligation process as a versatile approach for the cross-linking polymerization, functionalization of polymer chain ends, and surface modification of various materials such as silica and graphene oxide, is demonstrated. The process is based on the formation of urethane linkages by the reaction of photochemically in situ generated isocyanates from benzoyl azides with hydroxyl moieties in the presence of organobase, bicyclo[2.2.2]-1,4-diazaoctane (DABCO) under ambient conditions. The intermediates and obtained materials are characterized by NMR, FTIR, TGA, and TEM analyses. It is believed that this simple and efficient ligation process will expand future applications to fabricate complex macromolecular structures, biomaterials, and gels.

A Model for Late-Stage Modification of Polyurethane Dendrimers Using Thiol-Ene Click Chemistry

Dendritic materials possessing urethane linkage are surprisingly more stable than similar structures having functional groups such as ether, ester, amide, or carbosilane. This generates profound interest in dendritic polyurethanes. Construction of a well-defined polyurethane dendrimer is, however, challenging because of isocyanates' high reactivity. As a model of our ongoing dendrimer-research, herein, we report a protecting group-free one-pot multicomponent Curtius reaction to furnish a robust and versatile AB₂-type dendron, which ensures late-stage modification of both the dendron and dendritic macromolecule yielding a surface functionalized polyurethane dendrimer. While 5-hydroxyisophthalic acid, 11-bromoundecanol, and 4-penten-1-ol were utilized in the construction of the dendron, thiol-ene click chemistry was employed for the late-stage modification. Novel dendrons and dendrimers synthesized were characterized by NMR (1D and 2D) and high-resolution MALDI-TOF analysis. This strategy allows an easy late-stage modification of dendritic macromolecules and is highly useful in the synthesis of both symmetrical and unsymmetrical dendrimers (Janus dendrimers).

Well-Defined Aryl-FeII Complexes in Cross-Coupling and C–H Activation Processes

Herein we explore the intrinsic organometallic reactivity of iron embedded in a tetradentate N₃C macrocyclic ligand scaffold that allows the stabilization of aryl-Fe species, which are key intermediates in Fe-catalyzed cross-coupling and C–H functionalization processes. This study covers C–H activation reactions using ^MeL_H and FeCl₂, biaryl C–C coupling product formation through reaction with Grignard reagents, and cross-coupling reactions using ^MeL_Br or ^HL_Br in combination with Fe⁰(CO)₅. Synthesis under light irradiation and moderate heating (50 °C) affords the aryl-Fe^II complexes [Fe^II(Br)(^MeL)(CO)] (1^Me) and [Fe^II(^HL)(CO)₂]Br (1^H). Exhaustive spectroscopic characterization of these rare low-spin diamagnetic species, including their crystal structures, allowed the investigation of their intrinsic reactivity.

Synthesis of Novel Pyridine Bearing Biologically Active Imidiazolyl, Pyrazolyl, Oxa/thiadiazolyl and Urea Derivatives as Promising Anticancer Agents

BACKGROUND

A novel series of pyridine containing 1,3,4-oxa/thiadiazol derivatives 4a,b, pyrazole derivatives 5-7, thiazole derivatives 9a,b and 17a-c, urea derivatives 12a-c, imidiazole derivative 16, imidazo[1,2-a]pyridine derivatives 18a, b, tetrazole 19, pyrane 20 and pyridine derivatives 21 has been synthesized.

OBJECTIVE

This research aims to synthesize 6-(Trifluoromethyl)-2-{[3-(trifluoromethyl)phenyl] amino} nicotinohydrazide 2 and 6-(trifluoromethyl)-2-{[3-(trifluoromethyl)phenyl]amino} pyridin-3-carboaldhyde 15 as key intermediate for the synthesis of novel pyridine derivatives bearing different heterocyclic rings in order to study the additive effect of this ring toward tumor cell lines.

METHODS

6-(Trifluoromethyl)-2-{[3-(trifluoromethyl)phenyl]amino} nicotinohydrazide 2 was synthesized in a series of synthetic steps and was used as key intermediate for the synthesis of compounds 3-(1,3,4- oxa/thiadiazol-2-yl)-6-(trifluoromethyl)-N-(3- trifluoromethyl) phenyl) pyridin-2-amine 4a,b, (3,5-dimethyl- 1H-pyrazol-1-yl derivatives) [6-(trifluoromethyl)-2-{[3- trifluoromethyl) phenyl] amino} pyridin-3- yl]methanone 5a,b, 6-8, 9a,b and 12a-c. Also, 6-(trifluoromethyl)-2-{[3-(trifluoromethyl)phenyl]amino} pyridin-3-carboaldhyde (15) was used as a key intermediate for the synthesis of novel series of pyridine derivatives with different heterocyclic ring (16-21).

RESULTS

Structures of the newly synthesized compounds were established by elemental analysis and spectral data. All the synthesized compounds were screened for their in vitro anticancer activity against liver cancer (HepG2), human colon cancer (HT-29) and human breast adenocarcinoma cell lines (MCF-7).

CONCLUSION

All the synthesized compounds were investigated for their in vitro antitumor activity. Compounds 4b, 9a,b and 19 showed higher antitumor activity than the doxorubicin. Interestingly, pyridine with pfluorophenyl urea 12a demonstrated the most potent antitumor activity. The activity of these compounds is strongly dependent on the basic skeleton of the molecules and the nature of the heterocyclic ring attached to the pyridine moiety.

Catalytic Decarboxylative C-N Formation to Generate Alkyl, Alkenyl, and Aryl Amines

Transition-metal-catalyzed sp² C-N bond formation is a reliable method for the synthesis of aryl amines. Catalytic sp³ C-N formation reactions have been reported occasionally, and methods that can realize both sp² and sp³ C-N formation are relatively unexplored. Herein, we address this challenge with a method of catalytic decarboxylative C-N formation that proceeds through a cascade carboxylic acid activation, acyl azide formation, Curtius rearrangement and nucleophilic addition reaction. The reaction uses naturally abundant organic carboxylic acids as carbon sources, readily prepared azidoformates as the nitrogen sources, and 4-dimethylaminopyridine (DMAP) and Cu(OAc)₂ as catalysts with as low as 0.1 mol % loading, providing protected alkyl, alkenyl and aryl amines in high yields with gaseous N₂ and CO₂ as the only byproducts. Examples are demonstrated of the late-stage functionalization of natural products and drug molecules, stereospecific synthesis of useful α-chiral alkyl amines, and rapid construction of different ureas and primary amines.

Quarternary α-cyanobenzylsulfonamides: A new subclass of CAA fungicides with excellent anti-Oomycetes activity

A bioisosteric carboxamide - sulfonamide replacement explored during the optimization of an insecticide lead compound led to the surprising discovery of a formerly unknown subclass of the Carboxylic Acid Amide (CAA) fungicides, which is the very first CAA fungicide group without a carboxamide function. In this paper we present invention pathway, racemic and stereoselective synthesis routes, structure-activity relationship studies as well as resistance profile of this novel family of fungicides.

N,N-Diacylaminals as Emerging Tools in Synthesis: From Peptidomimetics to Asymmetric Catalysis

N,N-Diacylaminals are flexible molecular scaffolds that have commonly been utilized as amide surrogates in peptidomimetics. The singularities of this motif as an N-acyl imine equivalent and as hydrogen-bond donor have recently opened new synthetic opportunities, especially in the field of asymmetric catalysis. This concept article highlights this diverse synthetic potential and provides the elements necessary for further developments.

Transition-metal-free decarboxylative ipso amination of aryl carboxylic acids

An unprecedented DMAP-catalysed decarboxylative amination of carboxylic acids has been achieved under metal free conditions, enabling the convenient synthesis of structurally diverse aryl and alkyl amines.

A pyridinium anionic ring-opening reaction applied to the stereodivergent syntheses of Piperaceae natural products

A stereodivergent strategy has been devised to access the diene motif found in biologically active compounds from the Piperaceae family. Herein the first total syntheses of 2E,4E configured piperchabamide E (2) and its enantiomer (ent-2), as well as 2E,4Z configured scutifoliamide B (3), are narrated. The mainstay in the adopted approach is the gram-scale conversion of quaternized pyridine in a practical three-step sequence to access isomerically pure conjugated bromodiene esters 2E,4E8 and 2E,4Z9 by differential crystallization. Even though the developed oxidation protocol forms the basis of the entailed divergent strategy, the geometrical integrity of the involved bromodiene motive can be controlled by the choice of solvent. Thus, while oxidation of pure bromodienal 2E,4Z7 in methanol yields equal amounts of bromodiene esters 2E,4E8 and 2E,4Z9, only bromodiene ester 2E,4Z10 is formed in isopropanol. Subseqently, capitalizing on a stereoretentive Suzuki cross-coupling and direct amidation of the corresponding esters, the featured natural products can be accessed in five and six steps, respectively. The somewhat surprising (R)-configured amine portion, which has been assigned to piperchabamide E (2), is facilitated by a Curtius rearrangement. Following this, the actual amine portion is shown to be (S)-configured.

A pyrone remodeling strategy to access diverse heterocycles: application to the synthesis of fascaplysin natural products

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described. The pyrido[1,2-a]indole core was demonstrated to be a versatile scaffold that can be site-selectively functionalized. The utility of this novel annulation strategy was showcased in a concise formal synthesis of three fascaplysin congeners.

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described.

Tracking down the brominated single electron oxidants in recent organic red-ox transformations: photolysis and photocatalysis

A wide range of organic and inorganic brominated compounds including molecular bromine have been extensively used as oxidants in many organic photo-redox transformations in recent years, an area of ever growing interest because of greener and milder approaches. The oxidation power of these compounds is utilized through both mechanistic pathways (by hydrogen atom transfer or HAT in the absence of a photocatalyst and a combination of single electron transfer or SET and/or HAT in the presence of a photocatalyst). Not only as terminal oxidants for regeneration of photocatalysts, but brominated reactants have also contributed to the oxidation of the reaction intermediate(s) to carry on the radical chain process in several reactions. Here in this review mainly the non-brominative oxidative product formations are discussed, carried out since the last two decades, skipping the instances where they acted as terminal oxidants only to regenerate photocatalysts. The reactions are used to generate natural products, pharmaceuticals and beyond.

Divergent Synthesis of γ-Amino Acid and γ-Lactam Derivatives from meso-Glutaric Anhydrides

The first divergent synthesis of both γ-amino acid and γ-lactam derivatives from meso-glutaric anhydrides is described. The organocatalytic desymmetrisation with TMSN₃ relies on controlled generation of a nucleophilic ammonium azide species mediated by a polystyrene-bound base to promote efficient silylazidation. After Curtius rearrangement of the acyl azide intermediate to access the corresponding isocyanate, hydrolysis/alcoholysis provided uniformly high yields of γ-amino acids and their N-protected counterparts. The same intermediates were shown to undergo an unprecedented decarboxylation-cyclisation cascade in situ to provide synthetically useful yields of γ-lactam derivatives without using any further activating agents. Mechanistic insights invoke the intermediacy of an unconventional γ-N-carboxyanhydride (γ-NCA) in the latter process. Among the examples prepared using this transformation are 8 APIs/molecules of considerable medicinal interest.