Palladium-Catalyzed Synthesis of Nitrones Via Redox Cross-Coupling of Nitro Compounds and Alcohols

A novel methodology for the synthesis of nitrones via palladium-catalyzed redox cross-coupling of nitro compounds and alcohols is established. The protocol is a mild, convenient, ligand-free, and scalable synthesis method that can be compatible with various nitro compounds and alcohols. Nitrone is a significant multifunctional platform synthon which can be synthesized directly and efficiently via this tactic from commercially available and cheap raw materials.

Iron(III) and BF3·OEt2-Promoted O-Transfer Reaction of N-Aryl-α,β-Unsaturated Nitrones to Prepare Difluoroboron β-Ketoiminates

We described an iron(III) and BF3·OEt2-promoted oxygen transfer reaction of N-aryl-α,β-unsaturated nitrones to prepare various N,O-difluoroboron β-ketoiminates in good yields ranging from 24% to 87%. Control experiments revealed that the enaminone was the vital intermediate for the formation of N,O-difluoroboron β-ketoiminates, and iron(III) combined with BF3·OEt2 played as cocatalyst to promote the oxygen transfer reaction through intramolecular cyclization and N-O bond cleavage. More importantly, an estrone-derived N,O-difluoroboron β-ketoiminate was easily prepared in 40% yield from estrone in four steps.

Synthesis of Spirooxindole-1,2-oxazinan-5-ones through 2,2,2-Trifluoroethanol Promoted [3 + 3] Cycloaddition of N-Vinyl Oxindole Nitrones and Oxyallyl Cations

A variety of spirooxindole-1,2-oxazinan-5-one derivatives were prepared in moderate to excellent yields through 2,2,2-trifluoroethanol (TFE)-promoted [3 + 3] cycloaddition of N-vinyl oxindole nitrones with oxyallyl cations generated from α-tosyloxy ketones under mild reaction conditions. Mechanistic studies revealed that [3 + 3] cycloaddition might involve two possible reaction pathways, including direct [3 + 3] cycloaddition of N-vinyl oxindole ntirones with oxyallyl cations, or the addition of TFE to N-vinyl oxindole nitrones, sequential addition to oxyallyl cations, elimination, and cyclization. The present method features mild reaction conditions, broad substrate scope, good functional group tolerance, easy gram scalable preparation, and new applications of TFE.

Stable peri-Naphthoisatogens without C2 Protection: Synthesis via Aldrone Condensation, Optical Properties and 1,3-Dipolar Cycloaddition Reaction

peri-Annulation of naphthalane, an important tool for realization of wide range of functional materials, is presently accomplished with limited few functional groups like imide, amide and diamine-derivative (perimidine). To increase the diversity, we have incorporated α-keto aldonitrone as a new functional group, and herein report about five peri-naphthoisatogens (PNTIs) dyes. The synthesis were accomplished using an unusual reaction of aromatic nitro group, which is nucleophilic attack of a C-nucleophile (enol) to the N-atom of nitro group. In five different 5-alkylamino-8-nitro-1-acetylnaphthalenes, intramolecular acid-catalyzed nucleophilic attack of enol moiety to the N-atom of nitro group produced α-keto aldonitrone via addition-elimination mechanism. The PNTIs showed characteristics of 1,3-dipole and reacted with ethyl acrylate to produce isoxazolidine ring, which subsequently converted into aza phenalenone derivative via ring cleavage. Both the PNTI and the corresponding derivative strongly absorb in the visible region, displaying absorption maximum at 551 and 561 nm (in CHCl3 ) respectively. Compared to the popular analogous dye naphthalene monoimides, PNTIs showed bathochromic shift of absorption maximum by more than 100 nm. The emission maximum for the PNTI and its derivative in chloroform were observed at 594 and 635 nm respectively.

Stereoselective Photoredox Catalyzed (3+3) Dipolar Cycloaddition of Nitrone with Aryl Cyclopropane

By resorting to the principle of remote activation, we herein demonstrate the first photoredox catalyzed (3+3) dipolar cycloaddition of nitrones with aryl cyclopropanes. Key to the fidelity of the reaction resides in a facile manner of substrate activation by single-electron transfer (SET) oxidation with photoredox catalysis, and the reaction takes place through a stepwise cascade encompassing a three-electron-type nucleophilic substitution triggered cyclopropane ring-opening and a diastereoselective 6-endo-trig radical cyclization manifold. The reaction proceeds under mild conditions with excellent regio- and stereoselectivity, nicely complementing the well-developed Lewis acid catalyzed cycloaddition of donor-acceptor cyclopropanes. Other merits of the protocol include wide scope of aryl cyclopropanes with diversified substitution patterns and good functional-group compatibility. A mechanism involving an aryl radical cation promoted remote activation mode was also proposed and supported by mechanistic experiments.

2-Oxindole and related heterocycles: synthetic methodologies for their natural products and related derivatives

Natural goods, medications, and pharmaceutically active substances all contain substituted oxindoles. Generally, the C-3 stereocenter of the substituents of oxindoles and their absolute arrangement have a substantial impact on the bioactivity of these substances. In this case, the desire for contemporary probe and drug-discovery programs for the synthesis of chiral compounds using desirable scaffolds with high structural diversity further drives research in this field. Also, the new synthetic techniques are generally simple to apply for the synthesis of other similar scaffolds. Herein, we review the distinct approaches for the synthesis of diverse useful oxindole scaffolds. Specifically, the research findings on the naturally existing 2-oxindole core and a variety of synthetic compounds having a 2-oxindole core are discussed. We present an overview of the construction of oxindole-based synthetic and natural products. In addition, the chemical reactivity of 2-oxindole and its related derivatives in the presence of chiral and achiral catalysts are thoroughly discussed. The data compiled herein provides broad information related to the bioactive product design, development, and applications of 2-oxindoles and the reported techniques will be helpful for the investigation of novel reactions in the future.

Synthesis of 1,2,4-Oxadiazolines through Deoxygenative Cyclization of N-Vinyl-α,β-Unsaturated Nitrones with in Situ Generated Nitrile Oxides from Hydroxamoyl Chlorides

A variety of 1,2,4-oxadiazoline derivatives were synthesized in moderate to good yields through a deoxygenative cyclization cascade reaction of N-vinyl-α,β-unsaturated nitrones and hydroxamoyl chlorides. Mechanistic studies revealed that the reaction underwent double additions of nitrile oxides to N-vinyl-α,β-unsaturated nitrones, sequential elimination, and intramolecular cyclization to afford 1,2,4-oxadiazolines. Alternatively, 1,2,5-oxadiazolines were also obtained as major products in i-PrOH solvent through [3 + 3] cycloaddition and selective [3,3]-rearrangement. Moreover, the prepared 1,2,4-oxadiazolines were easily converted to polysubstituted pyrroles under thermal conditions.

Synthesis of Azetidine Nitrones and Exomethylene Oxazolines through a Copper(I)-Catalyzed 2,3-Rearrangement and 4π-Electrocyclization Cascade Strategy

A variety of azetidine nitrones are prepared in moderate to good yields through copper(I) combined with 2-aminopyridine to catalyze skeletal rearrangement of O-propargylic oximes. Mechanistic studies reveal that the reaction undergoes a copper(I)-catalyzed tandem [2,3]-rearrangement, 4π-electrocyclization, ring opening, and recyclization over four steps in one pot. Substituents at the terminus of alkyne and oxime moieties have a significant impact on the formation of azetidine nitrones and exomethylene oxazolines, respectively. Furthermore, the obtained azetidine nitrone could easily participate in [3 + 2] cycloaddition with alkynoates, and a [2.2]-paracyclophane-derived azetidine nitrone is synthesized in 45% yield over five steps from bromo[2.2]-paracyclophane.

Synthesis of N-Vinyl Cinnamaldehyde Nitrones through Atropisomeric Quinoxaline-Derived N,N,O-Ligand-Promoted Chan-Lam Reaction

A type of quinoxaline-derived tridentate N,N,O-ligand was synthesized in good to excellent yields over three steps through iodination of 2-aryl indoles, sequential Kornblum-type oxidation with DMSO, and capture by 1,2-diaminobenzenes. The prepared atropisomeric N,N,O-ligand was successfully applied in the synthesis of N-vinyl cinnamaldehyde nitrones as only Z-isomers in good yields through the Chan-Lam reaction. The method features an easily accessed tunable tridentate N,N,O-ligand, broad substrate scope, good functional group tolerance, and high Z-isomer for N-vinyl nitrones.

Cinchonidine-Catalyzed Synthesis of Oxazabicyclo[4.2.1]nonanones from N-Aryl-α,β-unsaturated Nitrones and 1-Ethynylnaphthalen-2-ols

A variety of oxazabicyclo[4.2.1]nonanone derivatives were prepared in good yields through a cinchonidine-catalyzed cascade reaction of N-aryl-α,β-unsaturated nitrones and 1-ethynylnaphthalen-2-ols. Mechanistic studies show that the reaction undergoes a [4 + 3] cycloaddition of nitrones to vinylidene o-quinone methide generated in situ from 1-ethynylnaphthalen-2-ols in the presence of cinchonidine, 1,3-rearrangement of N-O vinyl moieties, ring-opening, and finally double intramolecular cyclizations to afford oxazabicyclo[4.2.1]nonanones over five steps in a one-pot reaction.

Catalytic asymmetric transformation of nitrones and allenes to dihydropyridoindoles via chiral N,N'-dioxide/cobalt(II) catalysis

A chiral N,N'-dioxide/cobalt(II) complex catalytic system is developed to promote the multistep cascade reaction of α,β-unsaturated-N-aryl nitrones with allenes, giving a variety of chiral dihydropyridoindoles in moderate to good yields with excellent dr and ee values. Mechanistic studies support a [3+2] cycloaddition/[3,3]-rearrangement/retro-Mannich process.

Nickel(II)-Catalyzed [3 + 2] Cycloaddition of Nitrones and Allenoates to Access N-Vinylindoles and N-Vinylpyrroles

A variety of N-vinylindoles and N-vinylpyrroles were prepared in moderate to good yields through the nickel(II)-catalyzed [3 + 2] cycloaddition of α,β-unsaturated nitrones with allenoates under mild reaction conditions. A rational mechanism for the formation of N-vinylindoles was proposed based on the ¹⁸O-labeled experiments and key intermediates detected by high-resolution mass spectrometry trace experiments. The present method highlights a nickel(II)-controlled cyclization, atom-economical reaction, broad substrate scope, good functional group tolerance, and high Z-stereoselectivity for the enamine bond.

Single-Step Approach toward Nitrones via Pyridinium Ylides: The DMAP-Catalyzed Reaction of Benzyl Halides with Nitrosoarenes

A single-step approach is reported for the preparation of nitrones from benzyl halides and nitrosoarenes via pyridinium ylides, utilizing 4-dimethylaminopyridine (DMAP) catalyst and mild reaction conditions (Li₂CO₃, dimethylacetamide, and room temperature). The reaction provides both keto- and aldonitrones in high yields with a wide scope for benzyl halides and nitrosoarenes. In the same reaction system, 2-methyl-2-nitrosopropane, which does not have an aryl group, also affords the corresponding N-tert-butyl nitrones from primary benzyl bromides that have an electron-withdrawing group. As an application of the reaction, methyl 2-bromo-2-phenylacetate was used to prepare the corresponding isoxazolidine by a sequential one-pot synthesis.

Synthesis of α-aminooxy amides through [3 + 3] cycloaddition and Sc(OTf)3-catalyzed double C-N bond cleavage in a one-pot reaction

Various α-aminooxy amides bearing a quaternary carbon at the α-position were prepared in good to excellent yields under mild reaction conditions from N-vinyl nitrones and α-bromohydroxamates. The N-vinyl nitrones tolerate a wide range of N-vinyl fluorenone nitrones and N-vinyl isatin nitrones. Mechanistic studies show that the reaction initially proceeds through [3 + 3] cycloaddition between N-vinyl nitrones and aza-oxyallyl cations generated from α-bromohydroxamates to afford six-membered N,O-heterocycles, followed by double C-N bond cleavage in the presence of the Sc(OTf)3 catalyst. A selective N-O bond cleavage of the obtained α-aminooxy amides is also realized under Fe/NH4Cl conditions. Furthermore, gram-scalable preparations of α-aminooxy amides are easily achieved.

Synthesis of polycyclic indoles via organocatalytic bicyclization of α-alkynylnaphthalen-2-ols with nitrones

A new organocatalytic bicyclization of α-alkynylnaphthalen-2-ols with nitrones was first reported, leading to the convergent synthesis of polycyclic indoles with substantial substitution diversity in generally good yields through scission of the N-O bond of nitrones via [3,3]-sigmatropic rearrangement. This transformation showcases the use of a quinine catalyst in a complicated cascade system that has been shown to effectively construct polycyclic heterocycles via alkyne difunctionalization.

Silver-Assisted [3 + 2] Annulation of Nitrones with Isocyanides: Synthesis of 2,3,4-Trisubstituted 1,2,4-Oxadiazolidin-5-ones

A silver-assisted method for [3 + 2] annulation of nitrones with isocyanides has been developed. The developed protocol allows access to a variety of 2,3,4-trisubstituted 1,2,4-oxadiazolidin-5-one derivatives as single diastereomers in good to excellent yields using silver oxide as the catalyst and molecular oxygen as the terminal oxidant. A plausible mechanism involving a nucleophilic addition/cyclization/protodeargentation/oxidation pathway is proposed on the basis of experimental results.

Recent advances in the catalytic synthesis of 3-aminooxindoles: an update

3-Substituted-3-aminooxindoles are versatile scaffolds and these motifs constitute the core structure of number of natural products and biologically active compounds.

Catalyst-controlled formal [4 + 1] annulation of N-vinyl fluorenone nitrones and allenoates to prepare spirofluorenylpyrrolines

A simple gimeracil-controlled formal [4 + 1] annulation was developed to prepare various spirofluorenylpyrrolines in good yields with high diastereoselectivity from N-vinyl fluorenone nitrones and allenoates.

Ambient access to a new family of pyrrole-fused pyrazine nitrones via 2-carbonyl-N-allenylpyrroles

The chemo-, regio- and stereoselective synthesis of pyrrole-fused pyrazine nitrones via the direct reaction of 2-carbonyl-N-allenylpyrroles (readily accessible from the corresponding NH-pyrroles) with hydroxyl amine hydrochloride has been developed.

Carbonyl-Directed Addition of N-Alkylhydroxylamines to Unactivated Alkynes: Regio- and Stereoselective Synthesis of Ketonitrones

A variety of ketonitrones were synthesized in moderate to excellent yields with high chemo-, regio-, and stereoselectivity by using carbonyl-directed addition of N-alkylhydroxylamines to unactivated alkynes under mild conditions. The product diverisity could be controlled by the use of different bases, and EtN( n-Pr)₂ could promote the formation of ketonitrones while using EtONa as base led to indanone-derived nitrones. Control experiments indicated that the carbonyl group of the substrate acted as an H-bond acceptor except for an electron-withdrawing group, and conjugated enone skeleton accounted for the high selectivity.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.

Development of Synthetic Methodologies via Catalytic Enantioselective Synthesis of 3,3-Disubstituted Oxindoles

3,3-Disubstituted oxindoles are widely distributed in natural products, drugs, and pharmaceutically active compounds. The absolute configuration and the substituents on the fully substituted C3 stereocenter of the oxindole often significantly influence the biological activity. Therefore, tremendous efforts have made to develop catalytic enantioselective syntheses of this prominent structural motif. Research in this area is further fueled by the ever-increasing demand for modern probe- and drug-discovery programs for synthetic libraries of chiral compounds that are derived from privileged scaffolds with high structural diversity. Notably, the efficient construction of fully substituted C3 stereocenters of oxindole, tetrasubstituted or all-carbon quaternary, spirocyclic or not, also becomes a test ground for new synthetic methodologies. We have been engaged in developing efficient methods for diversity-oriented synthesis of chiral 3,3-disubstituted oxindoles from readily available starting materials. We have systematically developed catalytic enantioselective methods to prepare 3-substituted 3-hydroxyoxindoles, 3-aminooxindoles, and 3-thiooxindoles, quaternary oxindoles, and spirocyclic oxindoles. These protocols can be classified into six approaches: (1) enantioselective addition of nucleophiles to isatins or isatin ketimines; (2) unprotected 3-substituted oxindoles as nucleophiles; (3) functionalization of oxindole-derived tetrasubstituted alkenes; (4) desymmetrization of oxindole-based diynes; (5) spirocyclopropyl oxindoles as donor-acceptor (D-A) cyclopropanes; and (6) elaboration of diazooxindoles. By the use of these methods, chiral oxindoles with rich structural diversity are readily accessed with high to excellent enantioselectivity. Some methods have been used for the enantioselective formal or total synthesis of natural products, bioactive compounds, or their analogues. On the basis of these studies, we developed synthetic methodologies that have potential application. We designed phosphoramide-based bifunctional catalysts for the efficient construction of quaternary oxindoles: a cinchona-alkaloid-derived phosphoramide for the Michael addition of unprotected 3-substituted oxindoles to nitroolefins with broad substrate scope and a chiral 1,2-cyclohexanediamine-derived bifunctional phosphoramide for the activation of fluorinated enol silyl ethers for the addition to isatylidene malononitrile. The phosphoramide-based catalysts achieved better enantiofacial control than the analogous H-bond-donor-derived catalysts in these reactions, suggesting the potential of the former in new chiral catalyst development. We identified chiral Au(I) and Hg(II) catalysts for olefin cyclopropanation of diazooxindoles. We further disclosed the effective activation of spirocyclopropyl oxindoles by using electron-withdrawing N-protecting groups for enantioselective [3 + 3] cycloaddition, offering the promise of constructing a diverse range of spirocyclic oxindoles by the use of such monoactivated D-A cyclopropanes. We developed tandem sequences that allow the facile synthesis of 3,3-disubstituted oxindoles from simple starting materials in a one-pot operation, including a tandem Morita-Baylis-Hillman/bromination/[3 + 2] annulation sequence, a hydrogenation/ketimine formation/asymmetric 6π electrocyclization sequence, a C-H functionalization/Michael addition or amination sequence, and an aza-Wittig/Strecker sequence. We designed oxindole-based diynes to realize a highly enantioselective Cu-catalyzed alkyne-azide cycloaddition (CuAAC), outlining the desymmetrization of prochiral diynes as an effective strategy to exploit asymmetric CuAAC. This Account focuses on the synthetic methodologies developed in our group for the catalytic enantioselective synthesis of 3,3-disubstituted oxindoles and provides an overview of our research on the design, development, and applications of these methods that will provide useful insights for the exploration of new reactions.

Construction of α-methoxyimidoyl ketonitrones via phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes

A dimethyl phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes was developed. Nitrosoarenes were successfully used as electrophiles to trap aza-enolate intermediates that were generated from nucleophilic addition of deprotonated phosphite to α-keto N-tert-butanesulfinyl imidates and following phospha-Brook rearrangement, allowing efficient construction of ketonitrones with excellent (Z)-geometries.

Diastereo- and enantioselective [3 + 3] cycloaddition of spirocyclopropyl oxindoles using both aldonitrones and ketonitrones

Optically active spirocyclic compounds play an important role in drug discovery, and new synthetic strategies for the efficient generation of spiro stereocenters are in much demand. Here we report a catalytic enantioselective cycloaddition using spirocyclic donor–acceptor cyclopropanes as a promising approach for the generation of spiro stereocenters. A diastereo- and enantioselective [3 + 3] cycloaddition of spirocyclopropyl oxindoles with both aldonitrones and ketonitrones is developed. The key to reaction development is the activation of spirocyclopropyl oxindoles by a suitable electron-withdrawing N-protecting group. This activation approach offers the promise of a general solution to enable spirocyclopropyl oxindoles as synthons for catalytic enantioselective synthesis of spirocyclic oxindoles featuring a C3 spiro stereocenter, a prominent structural motif in drugs and pharmaceutically active compounds. This protocol also constitutes the catalytic enantioselective reaction using unactivated achiral ketonitrones to construct tetrasubstituted carbon stereocenters.

Chiral spirocyclic compounds are important structural motifs for drug discovery. Here, the authors report a synthetic route to spirocycles based on enantioselective cycloaddition of activated spirocyclopropyl oxindoles, which act as donor-acceptor cyclopropanes.

Copper-Catalyzed Selective N-Vinylation of 3-(Hydroxyimino)indolin-2-ones with Alkenyl Boronic Acids: Synthesis of N-Vinyl Nitrones and Spirooxindoles

A copper-catalyzed selective cross-coupling reaction of 3-(hydroxyimino)indolin-2-ones with alkenyl boronic acids to access (E)-N-vinyl oxindole nitrones has been achieved under mild conditions. The studies showed that catalytic copper salt selectively gave mono N-vinylation products, while 2.0 equiv of copper salt provided double N-vinylation products. The control experiments revealed that the carbonyl group in 3-(hydroxyimino)indolin-2-one played important roles on N-vinylation. Furthermore, the prepared N-vinyl oxindole nitrones could be converted to spirooxindoles in good yields under thermal conditions.

Trinuclear (aminonitrone)ZnII complexes as key intermediates in zinc(ii)-mediated generation of 1,2,4-oxadiazoles from amidoximes and nitriles

Amidoximes react with zinc salts in undried EtOAc furnishing the trinuclear species, where amidoximes are stabilized in the aminonitrone form.