Catalytic Diastereoselective [5 + 2] Annulation of N-Acryloyl Indoles with Cyclic Sulfonyl Enamides: Facile Access to Isoeburnamonine

Synthesis of Azepinoindoles via Pd-Catalyzed C(sp2)-H Imidoylative Cyclization Reactions

An efficient and convenient method for the construction of diverse free (N-H)-benzazepinoindoles by Pd-catalyzed C(sp²)-H imidoylative cyclization of 3-(2-isocyanobenzyl)-1H-indoles was developed. The reaction shows a wide substrate scope and can be scaled up, providing a practical route to valuable bioactive azepinoindoles.

Recent advances in [5+2] cycloadditions

Abstract:

The existence of a seven-membered cyclic core in several natural products and biomolecules vitalized the research on its synthesis. [5+2] cycloaddition has become a promising strategy for the construction of seven-membered ring systems by the formation of carbon-carbon bonds in a single step, with strong regioselectivity and stereoselectivity. This review mainly focuses on recent developments in the area of [5+2] cycloaddition since 2019. Total synthesis of natural products involving [5+2] cycloaddition as key step leading to heptacyclic core is also discussed. Synthesis of fused and bridged ring systems via the reactions involving inter and intramolecular [5+2] cycloadditions like oxidopyrylium-mediated [5+2] cycloadditions, [5+2] cycloadditions of vinyl cyclopropanes (VCPs), vinyl phenols, etc is explained in the review with the latest examples. This review provides a useful guide for researchers exploring this powerful strategy to create more elegant heptacycles in their future research.

π-Lewis-Base-Catalyzed Asymmetric Vinylogous Umpolung Reactions of Cyclopentadienones and Tropone

We disclose that the carbonates of 4-hydroxy-2-cyclopentenones can form π-allylpalladium-based 1,2-carbodipoles, which isomerize to interesting η² -Pd⁰ -cyclopentadienone complexes. Compared with the labile parent cyclopentadienone, the HOMO energy of the related η² -complex was significantly raised via the back-bonding of Pd⁰ as a π-Lewis base, rendering the uncoordinated C=C bond an electron-richer dienophile in inverse-electron-demand aza-Diels-Alder-type reactions with diverse 1-azadienes. The vinylogous (aza)Morita-Baylis-Hillman or cross Rauhut-Currier addition to (imine)carbonyls or activated alkenes, respectively, was also realized to afford chiral [4+2] or [2+2] cycloadducts, respectively, after trapping the re-generated π-allylpalladium species. New C₁ -symmetric ligands from simple chiral sources were developed, exhibiting high stereoselectivity even with racemic substrates via an unusual dynamic kinetic resolution process. Besides, tropone could be similarly activated by a Pd⁰ complex.

Metal-free dearomative [5+2]/[2+2] cycloaddition of 1H-indoles with ortho-(trimethylsilyl)aryl triflates

Herein, we report a mild dearomative [5+2]/[2+2] cycloaddition of 1H-indoles with ortho-(trimethylsilyl)aryl triflates. The unique [5+2] cycloaddition enables the synthesis of a series of dibenzo[b,e]azepine derivatives in moderate to good yields. Increasing the steric hindrance at the C2-position of 1H-indoles leads to the [2+2] cycloaddition. Mechanistic investigations support that the reaction of 1H-indoles with arynes undergoes a [2+2] cycloaddition step, followed by a ring expansion to the [5+2] cycloaddition product.

Pd-Catalyzed asymmetric [5 + 2] cycloaddition of vinylethylene carbonates and cyclic imines: access to N-fused 1,3-oxazepines

A facile route to access enantioenriched N-fused 1,3-oxazepines via Pd-catalyzed asymmetric [5 + 2] cycloaddition of vinylethylene carbonates and cyclic imines has been developed.