Palladium-Catalyzed Cascade Arene/Alkyne Annulation: Synthesis of Fluorene-Benzoxazine Derivatives

Synthesis of 2,4-Dicyanoalkylated Benzoxazines through the Radical-Mediated Cascade Cyclization of Isocyanides with AIBN under Metal- and Additive-Free Conditions

A general and novel method for the radical cascade cyclization of aryl isocyanides with AIBN has been described. This strategy provides straightforward access to various 2,4-dicyanoalkylated benzoxazines in moderate to good yields under metal- and additive-free conditions. The reaction can apply to a gram scale and tolerate diverse functional groups. 2,4-Dicyanoalkylated benzoxazine derivatives feature a large Stokes shift and intramolecular charge transfer properties.

Palladium-catalyzed asymmetric [4+2] annulation of vinyl benzoxazinanones with pyrazolone 4,5-diones to access spirobenzoxazine frameworks

Herein, a palladium-catalyzed general synthetic strategy to access an attractive and decorated set of chiral spiro derivatives of benzoxazine compounds is unveiled utilizing vinyl benzoxazinanones reacted with pyrazolone 4,5-diones, which extends the application of vinyl benzoxazinanones with ketones. This asymmetric catalytic [4+2] cycloaddition reaction demonstrates a broad substrate scope with functional group tolerance in yields of up to 76% and up to 96% ee. A facile scale-up and straightforward conversion to diversely substituted products verify the synthetic utility of this method.

Palladium-Catalyzed trans-Hydroalkoxylation: Counterintuitive Use of an Aryl Iodide Additive to Promote C–H Bond Formation

We report an enantioselective palladium-catalyzed trans-hydroalkoxylation of propargylic amines with a trifluoroacetaldehyde-derived tether to build chiral oxazolidines. Diastereoselective hydrogenation using a heterogeneous palladium catalyst then gave access to protected benzylic amino alcohols in 45–87% yields and 84–94% ee values. Hydroalkoxylation of the alkynes required a catalytic amount of aryl iodide, highlighting the counterintuitive key role played by a putative Pd(II)/ArI oxidative addition complex to promote oxypalladation/protodemetalation.

Copper Catalyzed Inverse Electron Demand [4+2] Cycloaddition for the Synthesis of Oxazines

A copper catalyzed tandem CuAAC/ring cleavage/[4+2] annulation reaction of terminal ynones, sulfonyl azides, and imines has been developed to synthesize the functionalized oxazines under mild conditions. Particularly, the intermediate N-sulfonyl acylketenimines undergo cycloaddition of an inverse electron demand Diels–Alder reaction with imines and a series of 1,3-oxazine derivatives were obtained successfully in good yields.

Gold(I)-Catalyzed Synthesis of 4H-Benzo[d][1,3]oxazines and Biological Evaluation of Activity in Breast Cancer Cells

The first gold(I)-catalyzed cycloisomerization procedure applied to the synthesis of substituted 4H-benzo[d][1,3]oxazines has been developed starting from N-(2-alkynyl)aryl benzamides. The chemoselective oxygen cyclization via the 6-exo-dig pathway yielded the observed heterocycles in modest to good chemical yields under very mild reaction conditions. The obtained oxazines were assayed on the breast cancer (BC)-derived cell lines MCF-7 and HCC1954 with differential biological activity. The newly synthesized 4H-benzo[d][1,3]oxazine compounds showed several degrees of cell proliferation inhibition with a remarkable effect for those compounds having a substituted aryl at C-2 of the molecules. The 4H-benzo[d][1,3]oxazines showed an IC₅₀ ranking from 3.1 to 95 μM in MCF-7 and HCC1954 cells. These compounds represent potential drug candidates for BC treatment. However, additional assays are needed to elucidate their complete effect over the cellular and molecular hallmarks of cancer.

Synthesis of Spiro[benzo[d][1,3]oxazine-4,4'-isoquinoline]s via [4+1+1] Annulation of N-Aryl Amidines with Diazo Homophthalimides and O2

Synthesis of spiro[benzo[d][1,3]oxazine-4,4'-isoquinoline]s through a unique [4+1+1] annulation of N-aryl amidines with diazo homophthalimides and O₂ is presented. This unprecedented spirocyclization reaction features readily obtainable substrates, structurally and pharmaceutically attractive products, a cost-free and clean oxygen source, sustainable reaction medium, tolerance of a broad spectrum of functional groups, and an interesting reaction mechanism based on sequential C(sp²)-H/C(sp³)-H bond cleavage and oxygen insertion.

Enantioselective Syntheses of 4H-3,1-Benzoxazines via Catalytic Asymmetric Chlorocyclization of o-Vinylanilides

The catalytic asymmetric halocyclization of alkene is a powerful and straightforward strategy for the synthesis of chiral heterocyclic compounds. Herein, an effective approach to chiral benzoxazine derivatives through organocatalyzed chlorocyclization of o-vinylanilides was reported. This method provides facile access to a series of chiral benzoxazines in good to excellent yields (up to 99% yield) and with high-level enantiocontrol (up to 92% ee).

A denitrogenative palladium-catalyzed cascade for regioselective synthesis of fluorenes

We herein report a denitrogenative palladium-catalyzed cascade for the modular and regioselective synthesis of polysubstituted fluorenes. Hydrazone facilitates the Pd(ii) to Pd(iv) oxidative addition in a Catellani pathway and is also the methylene synthon in the proposed reaction. Aryl iodides and 2-bromoarylaldehyde hydrazones undergo a norbornene-controlled tandem reaction sequence to give a broad scope of fluorenes in the presence of a palladium catalyst. The method described is scalable and adaptable to a three-component reaction with in situ generation of the hydrazone group. Preliminary mechanistic investigations have been conducted.

Cyclization of ortho-ethynylbiaryls as an emerging versatile tool for the construction of polycyclic arenes

Cyclization and cycloisomerization of ortho-aryl(ethynyl)arenes provide an easy direct access to fused polycyclic aromatic carbo- and heterocycles. This methodology has demonstrated an impressive progress in the recent years. The goal of this review is to give a comprehensive outlook on the synthetic potential, scope, limitations, and mechanistic aspects of the cyclization reactions. The material is arranged according to the activation method that can be used to induce cyclization: pyrolysis, metal catalysis, electrophilic activation, radical induction, base catalysis. Particular attention is paid to the specificity of ortho-ethynylbiaryls with a heterocyclic central core.

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Palladium-Catalyzed Cyclization of N-Acyl- o-alkynylanilines with Isocyanides Involving a 1,3-Acyl Migration: Rapid Access to Functionalized 2-Aminoquinolines

A simple and expedient approach for the synthesis of functionalized 2-aminoquinolines via palladium-catalyzed annulation of N-acyl- o-alkynylanilines with isocyanides has been developed with high atom economy, in which an unconventional 6- endo-dig cyclization process is observed. Further investigations of the mechanism demonstrated that an intramolecular acyl migration of the N-protecting groups was involved in this transformation.

Base-Promoted Synthesis of Quinoline-4(1H)-thiones from o-Alkynylanilines and Aroyl Isothiocyanates

A base-promoted synthesis of quinoline-4(1H)-thiones has been accomplished from the in situ generated o-alkynylthiourea, obtained by reacting o-alkynylanilines with aroyl/acyl isothiocyanates. A 6-exo-dig S-cyclization of the in situ generated thiourea is followed by a rearrangement to give quinoline-4(1H)-thiones.