Synthetic Protocol for Pyrido[2,3-c]pyridazine and Pyrido[3,2-e][1,2]oxazine Scaffolds via a [4 + 2] Cycloaddition Reaction

Here, we revealed a convenient synthetic protocol for unique pyrido[2,3-c]pyridazine and pyrido[3,2-e][1,2]oxazine scaffolds with excellent regioselectvity and diastereoselectivity. The functionalized pyrido[2,3-c] pyridazines were successfully synthesized via a Cs2CO3-promoted [4 + 2] cycloaddition reaction of α-halogenated N-tosylhydrazones or N-acylhydrazones and 5,6-unsubstituted 1,4-dihydropyridines under mild conditions. Additionally, the similar base-promoted [4 + 2] cycloaddition reaction of α-chlorogenated oximes and 5,6-unsubstituted 1,4-dihydropyridines afforded functionalized pyrido[3,2-e][1,2]oxazines in satisfactory yields. The features of this reaction included mild reaction conditions, broad substrate scopes, high functional group tolerance, and significant atomic economy.

Synthesis of spiropyridazine-benzosultams by the [4 + 2] annulation reaction of 3-substituted benzoisothiazole 1,1-dioxides with 1,2-diaza-1,3-dienes

A simple and efficient method for the synthesis of spiropyridazine-benzosultams has been developed by means of [4 + 2] annulation reaction of 3-substituted benzoisothiazole 1,1-dioxides with 1,2-diaza-1,3-dienes. This approach displays advantages such as mild reaction conditions, wide substrate range tolerance, simple operation, compatibility with gram-scale preparation.

Rapid access to C2-quaternary 3-methyleneindolines via base-mediated post-Ugi Conia-ene cyclization

Highly efficient synthesis of diverse 2,2-disubstituted 3-methyleneindoline derivatives through a one-pot base-promoted post-Ugi 5-exo-dig "Conia-ene"-type cyclization has been disclosed. The mechanism study indicates that an intramolecular hydrogen bond may play a vital role in this process. The antiproliferative evaluation of cancer cell lines reveals that this protocol provides practical use in the green synthesis of bioactive compound libraries.

Recent Advances on Direct Functionalization of Indoles in Aqueous Media

Indoles and their derivatives have dominated a significant proportion of nitrogen-containing heterocyclic compounds and play an essential role in synthetic and medicinal chemistry, pesticides, and advanced materials. Compared with conventional synthetic strategies, direct functionalization of indoles provides straightforward access to construct diverse indole scaffolds. As we enter an era emphasizing green and sustainable chemistry, utilizing environment-friendly solvents represented by water demonstrates great potential in synthesizing valuable indole derivatives. This review aims to depict the critical aspects of aqueous-mediated indoles functionalization over the past decade and discusses the future challenges and prospects in this fast-growing field. For the convenience of readers, this review is classified into three parts according to the bonding modes (C-C, C-N, and C-S bonds), which focus on the diversity of indole derivatives, the prominent role of water in the chemical process, and the types of catalyst systems and mechanisms. We hope this review can promote the sustainable development of the direct functionalization of indoles and their derivatives and the discovery of novel and practical organic methods in aqueous phase.

Selective Synthesis of Pyrazolonyl Spirodihydroquinolines or Pyrazolonyl Spiroindolines under Aerobic or Anaerobic Conditions

Presented herein is a condition-controlled selective synthesis of pyrazolonyl spirodihydroquinolines or pyrazolonyl spiroindolines through formal [5 + 1] or [4 + 1] spiroannulation of 2-alkenylanilines with diazopyrazolones. Mechanistically, the formation of the title products involves initial generation of a pyrazolonyl spiro-fused seven-membered ruthenacycle species serving as a key intermediate through Ru(II)-catalyzed C-H/N-H bonds metalation, carbene formation, and its migratory insertion. When the reaction is carried out under air, the key intermediate undergoes reductive elimination to afford spirodihydroquinoline. When the reaction is run under argon, the key intermediate undergoes protonation and intramolecular nucleophilic addition to furnish spiroindoline. This work provides an atom-economical protocol for the effective functionalization of alkenyl C(sp²)-H bond, allowing rapid and selective assembly of valuable spiroscaffolds with a broad range of substrates.

The emerging role of radical chemistry in the amination transformation of highly strained [1.1.1]propellane: Bicyclo[1.1.1]pentylamine as bioisosteres of anilines

Bicyclo[1.1.1]pentylamines (BPCAs), emerging as sp3-rich surrogates for aniline and its derivatives, demonstrate unique structural features and physicochemical profiles in medicinal and synthetic chemistry. In recent years, compared with conventional synthetic approaches, the rapid development of radical chemistry enables the assembly of valuable bicyclo[1.1.1]pentylamines scaffold directly through the amination transformation of highly strained [1.1.1]propellane. In this review, we concisely summarize the emerging role of radical chemistry in the construction of BCPAs motif, highlighting two different and powerful radical-involved strategies including C-centered and N-centered radical pathways under appropriate conditions. The future direction concerning BCPAs is also discussed at the end of this review, which aims to provide some inspiration for the research of this promising project.

Construction of Oxo-Bridged Diazocines via Rhodium-Catalyzed (4+3) Cycloaddition of Carbonyl Ylides with Azoalkenes

Developing efficient strategies for synthesizing novel diazocine compounds is valuable because their use has been limited by their synthetic accessibility. This work describes the catalytic (4+3) cycloaddition reaction of carbonyl ylides with azoalkenes generated in situ. The rhodium-catalyzed cascade reaction features good atom and step economy, providing the first access to oxo-bridged diazocines. The product could be synthesized on a gram scale and converted into diversely substituted dihydroisobenzofurans.

Activator free diastereoselective 1,3-dipolar cycloaddition: a quick access to coumarin based spiro multi heterocyclic adducts

A formal diastereoselective 1,3-dipolar cycloaddition of azomethine ylide and coumarin derivatives to construct coumarin based spiro multi heterocyclics has been described. The in situ generation of azo-ylide was achieved for various heterocyclic carbonyls (indenoquinoxaline and isatin). This transformation is also suitable for maleimide dipolarophiles for the synthesis of hydro-maleimide derivatives. These decarboxylative annulations neither required any catalyst nor any activator. Further the pure products were isolated by filtration from the reaction mixture after the reaction under ambient conditions.