Facile Synthesis of Azetidine Nitrones and Diastereoselective Conversion into Densely Substituted Azetidines

Synthesis of Isoindole N-Oxides by Palladium-Catalyzed C-H Functionalization of Aldonitrones

A palladium-catalyzed strategy for isoindole N-oxide ring construction by C-H functionalization of aldonitrones is described. Our protocol is of general character, providing isoindole N-oxides with a variety of functional groups, including very sterically congested products. Further transformations into spirocyclic isoindolines, isoindoles, or a polycyclic isoquinolinium salt have been demonstrated as well. A mechanistic study suggests that the catalytic process proceeds via a Heck-type addition to the double C═N bond.

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.

Copper-Catalyzed Reactions of Alkenyl Boronic Esters via Chan-Evans-Lam Coupling/Annulation Cascades: Substrate Selective Synthesis of Dihydroquinazolin-4-ones and Polysubstituted Quinolines

Copper-catalyzed cascade cyclization reactions between alkenyl boronic esters and N-H-based nucleophiles have been established, providing new approaches for one-pot assembly of azacycles. Following the Chan-Evans-Lam C-N couplings, the cyclization processes occur via divergent pathways based on the utilized substrates, affording hydroamination product dihydroquinazolin-4-ones or aromatization product quinolines. Via this one-pot C-N coupling/annulation cascade, the target substituted azacycles can be obtained in moderate to good yields in each case.

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.

Palladium-Catalyzed Access to Benzocyclobutenone-Derived Ketonitrones via C(sp2)-H Functionalization

The palladium-catalyzed C(sp²)–H functionalization of bromoaryl aldonitrones leading to benzocyclobutenone-derived ketonitrones is described. This method allows for the preparation of a wide range of strained, four-membered ketonitrones with broad functional group tolerance. Downstream transformations of the formed products were readily demonstrated, illustrating the synthetic utility of the obtained benzocyclobutenone-derived nitrones for the construction of polycyclic nitrogen-containing scaffolds.

A general synthesis of azetidines by copper-catalysed photoinduced anti-Baldwin radical cyclization of ynamides

A general anti-Baldwin radical 4-exo-dig cyclization from nitrogen-substituted alkynes is reported. Upon reaction with a heteroleptic copper complex in the presence of an amine and under visible light irradiation, a range of ynamides were shown to smoothly cyclize to the corresponding azetidines, useful building blocks in natural product synthesis and medicinal chemistry, with full control of the regioselectivity of the cyclization resulting from a unique and underrated radical 4-exo-dig pathway.

The construction of four-membered rings via a 4-exo-dig cyclization was originally theorized to be unfavourable and only recently shown in sparse examples. Here the authors present a photochemical, radical 4-exo-dig cyclization of ynamides to form azetidines, promoted by copper photoredox catalysis.

Recent Advances in the Green Synthesis of Heterocycles: From Building Blocks to Biologically Active Compounds

Recent advances in the environmentally benign synthesis of common heterocycles are described. This account features three main parts; the preparation of non-aromatic heterocycles, one-ring aromatic heterocycles and their condensed analogs. Due to the great variety of and high interest in these compounds, this work focuses on providing representative examples of the preparation of the target compounds.

Copper-Catalyzed β-Lactam Formation Initiated by 1,3-Azaprotio Transfer of Oximes and Methyl Propiolate

A copper(II)-catalyzed protocol to construct trans-configured β-lactams and spirocyclic β-lactams from oximes and methyl propiolate has been developed, which features excellent substrate flexibility and diastereoselectivity (up to >99:1 dr). In situ FT-IR mechanistic experiments support that ketene species might be involved in the formation of β-lactams.

Visible-light-promoted nitrone synthesis from nitrosoarenes under catalyst- and additive-free conditions

A green and sustainable nitrone formation reaction via visible-light-promoted reaction of aryl diazoacetates with nitrosoarenes is described. This protocol exhibits good functional group tolerance and broad substrate scope for both aryl diazoacetates with nitrosoarenes. Comparing the reported methods for the synthesis of nitrones from nitrosoarenes, the reaction described herein occurs under sole visible-light irradiation without the need of any catalysts and additives.

Rhodium-Catalyzed Denitrogenative Diazole-Triazole Coupling toward Aza-Bridged Structures and Imidazole-Based Chelating Ligands

1,4,8-Triazaocta-1,3,5,7-tetraenes, generated in situ by Rh₂(Piv)₄-catalyzed denitrogenative coupling of pyrazoles with 1-sulfonyl-1,2,3-triazoles, smoothly form 2,6,8-triazabicyclo[3.2.1]octa-3,6-dienes via intramolecular aza-Diels-Alder cycloaddition. This domino reaction, combined with the subsequent thermal or acid-catalyzed rearrangement of the cycloadducts, provides direct and flexible access to N-sulfonylated (Z)-2-(2-aminovinyl)imidazoles.

Nitrone and Alkyne Cascade Reactions for Regio- and Diastereoselective 1-Pyrroline Synthesis

The synthesis of 1-pyrrolines from N-alkenylnitrones and alkynes has been explored as a retrosynthetic alternative to traditional approaches. These cascade reactions are formal [4+1] cycloadditions that proceed through a proposed dipolar cycloaddition and N-alkenylisoxazoline [3,3']-sigmatropic rearrangement. A variety of cyclic alkynes and terminal alkynes have been shown to undergo the transformation with N-alkenylnitrones under mild conditions to provide the corresponding spirocyclic and densely substituted 1-pyrrolines with high regio- and diastereoselectivity. Mechanistic studies provide insight into the balance of steric and electronic effects that promote the cascade process and control the diastereo- and regioisomeric preferences of the 1-pyrroline products. Diastereoselective derivatization of the 1-pyrrolines prepared by the cascade reaction demonstrate the divergent synthetic utility of the new method.

Synthesis of polysubstituted azetidines via cascade trifluoromethylation/cyclization of N-allyl ynamides

A cascade trifluoromethylation/cyclization of N-allyl sulfonylynamides is developed, providing a direct access to azetidine-fused tricyclic compounds at room temperature.

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.

Functionalized azetidines via visible light-enabled aza Paternò-Büchi reactions

Azetidines are four-membered nitrogen-containing heterocycles that hold great promise in current medicinal chemistry due to their desirable pharmacokinetic effects. However, a lack of efficient synthetic methods to access functionalized azetidines has hampered their incorporation into pharmaceutical lead structures. As a [2+2] cycloaddition reaction between imines and alkenes, the aza Paternò-Büchi reaction arguably represents the most direct approach to functionalized azetidines. Hampered by competing reaction paths accessible upon photochemical excitation of the substrates, the current synthetic utility of these transformations is greatly restricted. We herein report the development of a visible light-enabled aza Paternò-Büchi reaction that surmounts existing limitations and represents a mild solution for the direct formation of functionalized azetidines from imine and alkene containing precursors.

Synthesis and Transformations of Nitrones for Organic Synthesis

Nitrones are important compounds and are highly useful in many aspects. The first part describes the methods for synthesis of nitrones, which are useful and environmentally friendly. Catalytic oxidations, condensations, and other useful reactions are described. The nitrones thus obtained are key intermediates for the synthesis of biologically important nitrogen compounds. The second part describes the fundamental transformations of nitrones, which will provide the strategies and means for the construction of nitrogen compounds. The reactions with nucleophiles or radicals, C-H functionalization, and various addition reactions are described. The last reactions are particularly important for highly selective carbon-carbon bond formations. 1,3-Dipolar cycloaddition reactions are excluded because the size of the review is limited and excellent reviews have been published in Chemical Reviews.

Generation and Rearrangement of N,O-Dialkenylhydroxylamines for the Synthesis of 2-Aminotetrahydrofurans

A new diastereoselective route to 2-aminotetrahydrofurans has been developed from N,O-dialkenylhydroxylamines. These intermediates undergo a spontaneous C-C bond-forming [3,3]-sigmatropic rearrangement followed by a C-O bond-forming cyclization. A copper-catalyzed N-alkenylation of an N-Boc-hydroxylamine with alkenyl iodides, and a base-promoted addition of the resulting N-hydroxyenamines to an electron-deficient allene, provide modular access to these novel rearrangement precursors. The scope of this de novo synthesis of simple nucleoside analogues has been explored to reveal trends in diastereoselectivity and reactivity. In addition, a base-promoted ring-opening and Mannich reaction has been discovered to covert 2-aminotetrahydrofurans to cyclopentyl β-aminoacid derivatives or cyclopentenones.

Organocatalytic Asymmetric Synthesis of α-Oxetanyl and α-Azetidinyl Tertiary Alkyl Fluorides and Chlorides

Asymmetric thiourea and squaramide catalysis provides access to synthetically versatile α-oxetanyl and α-azetidinyl alkyl halides exhibiting a tetrasubstituted chiral carbon center with high yields and enantioselectivities. The products are readily transformed with negligible erosion of enantiopurity and excellent diastereoselectivity to a diverse group of multifunctional compounds including fluorooxindoles with two contiguous chirality centers, fluorinated heterocyclic spiranes, and polyspiro compounds.