Base-Promoted Denitrogenative/Deoxygenative/Deformylative Benzannulation of N-Tosylhydrazones with 3-Formylchromones for Diverse and Polyfunctionalized Xanthones

Benzannulation with Et3N as 1,3-Diene Variants

With triethylamine as a 1,3-diene variant, a simple and practical process for the synthesis of phthalimides has been developed from readily available maleimide. The transformation can be performed in the absence of a metal catalyst with high levels of functional group tolerance. Various phthalimide compounds were constructed in moderate to good yields under mild conditions. Mechanism research indicates that oxygen and acid also play crucial roles in this reaction.

Palladium-Catalyzed Chemo- and Regiocontrolled Tandem Cyclization/Cross-Coupling of 2-Benzyl-3-alkynyl Chromones with Aryl Iodides for the Synthesis of 4H-Furo[3,2-c]chromenes and Xanthones

A novel Pd-catalyzed chemo- and regiocontrolled tandem cyclization/cross-coupling reaction of 3-alkynyl chromone with aryl iodide was developed for the synthesis of 4H-furo[3,2-c]chromenes and xanthones. The difunctionalization of alkynes through O-attack/5-exo-dig and C-attack/6-endo-dig cyclization was reported by this rare approach, which was selectively controlled by the addition of KF or a bidentate phosphine ligand. A one-pot tandem process was demonstrated directly from γ-alkynyl-1,3-diketone for this method.

Microwave-assisted annulation for the construction of pyrido-fused heterocycles and their application as photoluminescent chemosensors

A catalyst-free microwave-assisted annulation protocol for the preparation of biologically interesting pyrido-fused quinazolinones and pyrido[1,2-a]benzimidazoles is developed. This reaction involves the [3 + 3] annulation of various quinazolinones or benzimidazoles with 3-formylchromones to yield functionalized 11H-pyrido[2,1-b]quinazolin-11-one and pyrido[1,2-a] benzimidazole derivatives. This approach is successfully extended to the construction of various pyrazolo[4,3-d]pyrido[1,2-a]pyrimidin-10(1H)-ones. The present approach is complementary to the existing synthetic methodologies and offers a rapid and facile approach with a broad substrate scope, good yields, catalyst-free conditions, and a high functional group tolerance. The optimal synthesized compound is also employed as an "on-off" photoluminescent probe for the selective detection of Fe³⁺ and Ag⁺ metal ions.

Rearrangement-Driven Molecular Diversity: Synthesis of Functionalized Pyrones, Orthoesters, and Xanthones from Spiroketals

The reaction of tricyclic 5,5-benzannulated spiroketals with trifluoroacetic acid (TFA) and AlCl₃ furnished benzopyranobenzopyrans, benzofuro-orthoesters, and benzofuroxanthones. Whereas the reaction of tricyclic 5,5-benzannulated spiroketals with TFA produced the pyrones, the reaction with AlCl₃ furnished densely functionalized orthoesters and xanthones. The formation of these products was rationalized by fascinating mechanistic pathways involving semipinacol/α-ketol molecular rearrangements.

Construction of Highly Functionalized Xanthones via Rh-Catalyzed Cascade C-H Activation/O-Annulation

A facile and efficient strategy for obtaining functionalized and multihydroxylated xanthones via Rh catalysis under redox-neutral conditions is developed. Diverse salicylaldehydes bearing heterocycles, aromatics, and fused aromatics can be rapidly coupled with 1,4-benzoquinones or 1,4-hydroquinones to afford valuable xanthones via cascade C-H/O-H functionalization and annulation. This protocol provides a rapid synthetic approach to obtain biologically active materials through late-stage functionalization and prepares natural products such as subelliptenone, pruniflorone N, and ravenelin.

Catalyst- and Substituent-Controlled Regio- and Stereoselective Synthesis of Indolyl Acrylates by Lewis-Acid-Catalyzed Direct Functionalization of 3-Formylindoles with Diazo Esters

A facile and efficient In(OTf)₃- and BF₃·OEt₂-catalyzed direct transformation of 3-formylindoles with diazo esters has been developed for synthesizing diverse and functionalized indolyl acrylates. This one-pot protocol furnishes various (Z)-α-hydroxy-β-indolyl acrylates, (E)-β-(2-alkoxy-2-oxoethoxy)-α-indolyl acrylates, and (Z)-3-hydroxy-2-indolyl acrylates by a catalyst- and substituent-controlled, regio- and stereoselective cascade reaction. The protocol has several advantages, including low loading of the catalyst, mild reaction conditions, broad scope, and high functional group tolerance. The synthesized compounds can be further converted into diversely functionalized materials.

Indium-Catalyzed Aromative Spiro Coupling of Quinones with Oxindoles for Highly Functionalized Xanthenes as Efficient Fluorophores

A convenient and an efficient protocol for the assembly of diverse xanthenes bearing a biologically interesting oxindole nucleus is developed by utilizing the In(III)-catalyzed spiro coupling of 1,4-benzoquinones or 1,4-naphthoquinones with oxindoles. This novel protocol proceeds via a cascade of double Michael additions and intramolecular cyclization. The synthesized compounds have potential use as fluorophores for the selective imaging of heavy metals in living cells.

Base-Mediated Denitrogenative Sulfonylation/Benzannulation of Conjugated N-Sulfonylhydrazones with 3-Formylchromones for the Construction of Polyfunctionalized Biaryl Sulfones

Base-promoted benzannulation of conjugated N-sulfonylhydrazones and 3-formylchromones for the synthesis of diverse biaryl sulfones is described. The approach facilitates new C-C and C-S bond formation via the cascade diazo formation/Michael addition/ring opening/denitrogenative sulfonylation/intramolecular cycloaddition/dehydration and introduces diverse functional groups onto biaryl sulfones. The synthesized compounds are converted to aryl sulfones bearing bioactive benzisoxazole and benzofuran frameworks. Moreover, the synthesized biaryl sulfones possess potent turn-on fluorescence sensing and UV absorbance properties.

Lewis Acid Catalyzed [4 + 2] Cycloaddition of N-Tosylhydrazones with ortho-Quinone Methides

A formal [4 + 2] cycloaddition of N-tosylhydrazones with ortho-quinone methides was developed, affording the facile synthesis of diverse 1,3-oxazine derivatives under mild conditions. In this transformation, N-tosylhydrazones are used as a 1,2-dipole synthon under base-free conditions. Moreover, the substrate scope is broad, and the products are formed with high diastereoselectivities in most of the cases.

Palladium-Catalyzed Aerobic Benzannulation of Amines, Benzaldehydes, and β-Dicarbonyls

A palladium-catalyzed aerobic multicomponent benzannulation of Hantzsch reactants, i.e., amines, aldehydes, and β-dicarbonyls, has been developed. Hence, a viable entry to highly functionalized anilines has been accessed under solvent-free neat conditions. Mechanistically, the palladium chelating with an imine intermediate was proposed to be the key for this novel carbocyclization.