Recent Advances on Epoxide- and Aziridine-Based [3+2] Annulations

[3+2] Annulations are a powerful method for the synthesis of five-membered heterocyclic compounds. The annulations have concerted cycloaddition and formal (stepwise) cycloaddition reaction pathways. In addition to the well-established O-centered and N-centered ylides, epoxides and aziridines could serve as synthetic equivalent of 1,3-dipoles for [3+2] annulation with dipolarophiles for making functionalized tetrahydrofuran, pyrrolidine, and associated compounds. This review article covers recent development on epoxide- and aziridine-based [3+2] annulation reactions. The reactions are classified based on the ring opening conditions, including acid/base catalysis, organocatalysis, and transitional-metal catalysis.

Palladium-Catalyzed (3+2) and (4+2) Cycloaddition Reactions of Sulfamidate Imine-Derived Azadienes: Synthesis of Spirocyclic Pyrrolidines and Tetrahydroquinolines

Diastereoselective Pd-catalyzed (3+2) and (4+2) cycloaddition reactions of sulfamidate imine-derived 1-azadienes with zwitterionic N-dipoles derived from 1-tosyl-2-vinylaziridine and 4-vinylbenzoxazinone have been developed. These reactions provide highly functionalized azaspirocycles featuring three contiguous stereocenters. The sulfonyl imine moiety of the cycloadducts can be fully reduced to access valuable β-amino alcohols.

Visible-Light-Induced Dearomative Annulation of Indoles toward Stereoselective Formation of Fused- and Spiro Indolines

Dearomatization approaches are attractive for their abilities to transform simple, planar arenes into complex, three-dimensional architectures. In particular, visible-light driven dearomatization strategies are significant because of their mild, green, and sustainable nature, enabling the fabrication of new chemical bonds via an electron transfer or energy transfer process. Indole compounds, being potentially bioactive and readily accessible, can be employed efficiently as building blocks for constructing diverse annulated frameworks under photocatalysis. Highly stereoselective radical cascade reactions of appropriate indole systems can provide complex cyclic scaffolds bearing multiple stereocenters. In fact, the past few years have witnessed the renaissance of dearomative cycloadditions of indoles via visible-light-induced photocatalysis. The present review highlights recent advances (2019-mid 2024) in visible-light-driven dearomative annulation of indoles leading to formation of polycyclic indolines, including angularly fused and spiro indolines. Most of the reactions described in this review are simple, providing quick access to the desired products. Additionally, characteristic reaction mechanisms are offered to provide an understand of how indole scaffolds show distinctive reactivity under photocatalytic conditions.

Recent updates on vinyl cyclopropanes, aziridines and oxiranes: access to heterocyclic scaffolds

This present review delineates the repertoire of vinyl cyclopropanes and their stuctural analogues to accomplish a wide array of oxa-cycles, aza-cycles, and thia-cycles under transition metal catalysis and metal-free approaches from early 2019 to the present date. The generation of electrophilic π-allyl intermediates and 1-3/1-5-dipolarophile chemistry originating from VCPs are always green, step- and atom-economical and sustainable strategies in comparsion with prefunctionalized and/or C-H activation protocols. Here, the strained ring-system extends its applicability by relieving the strain to undergo a ring-expansion reaction to accomplish 5-9 membered carbo- and heterocyclic systems. The availability of chiral ligands in the ring-expansion reaction of VCPs and their analogues has paved the way to realizing asymmetric synthetic transformations.

NBS-induced intramolecular annulation reactions for the divergent synthesis of fused- and spirocyclic indolines

An NBS-induced intramolecular annulation of 3-(1H-indol-3-yl)-N-alkoxypropanamide is described. The reactions proceed well and quickly under mild conditions with the help of a base. It was found that C2-substituents on the indole ring in 3-(1H-indol-3-yl)-N-alkoxypropanamide have a great influence upon the reaction. By using C2-methyl- and C2-phenyl-3-(1H-indol-3-yl)-N-alkoxypropanamide as templates, practical protocols for the divergent synthesis of fused- and spirocyclic indoline compounds were studied and established.

Synthesis of Cyclopenta[b]indoles via Sc(III)-Catalyzed Annulation of Vinyl Diazoacetates with Indole-Derived Unsaturated Imines

A Lewis acid Sc(OTf)₃-catalyzed annulation reaction of vinyl diazoacetates with in situ formed α,β-unsaturated imines for the preparation of indole-fused tricyclic rings has been reported. This strategy involves a sequential addition/rebound addition process of vinyl diazoacetates and an in situ dedinitrogenation. This annulation protocol features low-cost catalysts, mild reaction conditions, and facilely prepared substrates.

Cyclization of Vinyl Diazo Compounds with Benzofuran-Derived Azadienes Enabled by NaBArF4

An unprecedented cycloaddition of vinyl diazo compounds with benzofuran-derived azadienes catalyzed by rarely independently used NaBAr^F₄ has been established. Benzofuran-fused hydropyridines were constructed with excellent yields and high diastereoselectivity via a Na⁺-catalyzed inverse-electron-demand aza-Diels-Alder reaction. Notably, this transformation also features good compatibility with a one-pot protocol to deliver the spiro[benzofuran-cyclopentene] skeleton, as well as perfect atom economy and simple reaction conditions.

Synthesis of Spiroindolenine Bearing Chroman Scaffolds Based on Cylization Reaction to para-Methylated Methides

An efficient diastereoselective cyclization reaction of ortho-hydroxyphenyl-substituted para-quinone methides with α,β-unsaturated imines generated in situ from aryl sulfonyl indoles was developed, which afforded spiroindolenine bearing chroman scaffolds in good to...