α-Aryl-substituted allenamides in an imino-Nazarov cyclization cascade catalyzed by Au(I)

High-Pressure Promoted Nazarov-like Electrocyclization Enables Access to trans-4,5-Diamino-cyclopent-2-enones Bearing Electron-Poor Anilines

Unlike secondary alkyl amines and electron-rich anilines, secondary electron-poor anilines are challenging amine sources to explore the chemical space of Lewis acid-catalyzed condensation-based transformations with furfural. In this work, we report the efficient synthesis of trans-4,5-diamino cyclopentenones (DCP) using a high-pressure promoted Nazarov-like electrocyclization of Stenhouse salts arising from the Sc(III)-catalyzed condensation of furfural with secondary electron-poor anilines. The reaction enables access to otherwise difficult-to-access DCP and compatibility with a large scope of alkyl and aryl secondary amines. A 2- to 18-fold increase in yields for electron-poor anilines was highlighted using this approach in the synthesis of a pharmacologically active compound.

Recent Advances in the Catalytic Synthesis of the Cyclopentene Core

Five-membered carbocycles are ubiquitously found in natural products, pharmaceuticals, and other classes of organic compounds. Within this category, cyclopentenes deserve special attention due to their prevalence as targets and as well as key intermediates for synthesizing more complex molecules. Herein, we offer an overview summarizing some significant recent advances in the catalytic assembly of this structural motif. A great variety of synthetic methodologies and strategies are covered, including transition metal-catalyzed or organocatalyzed processes. Both inter- and intramolecular transformations are documented. On this ground, our expertise in the application of C-H functionalization reactions oriented towards the formation of this ring and its subsequent selective functionalization is embedded.

Titanium-Mediated aza-Nazarov Annulation for the Synthesis of N-Fused Tricycles: A General Method to Access Lamellarin Analogues

Fused heterocycles with nitrogen incorporation are of particular bioactive use and high importance in many research fields, especially isoquinoline-based [6/6/5] tricycles. Here, we report a unique strategy to access multifunctional N-fused tricycles from α,β-unsaturated isoquinoline ketone and sulfonamide under mild reaction conditions. The methodology features wide substrate tolerance, and a set of N-fused heteroarenes including quinoline, phthalazine, quinazoline, quinoxaline, and benzothiazole cores are furnished efficiently. Moreover, the protocol is easy to scale up to synthesize lamellarin analogues, and the amide group of the product is also easy to transfer to other functional groups.

Characterization and Utilization of the Elusive α,β-Unsaturated N-Tosyliminium: the Synthesis of Highly Functionalizable Skipped Halo Enynes

A formal haloalkynylation of allenamides has been described for the synthesis of highly stereo- and regioselective skipped halo enynes. Exclusive γ-regioselectivity is achieved through the intermediacy of a conjugated N-tosyliminium intermediate-direct evidence for the formation of which was validated by NMR and HRMS. Quantum mechanical computations reveal that the reactive intermediate geometry is key to controlling the 1,2- or 1,4-regioselectivity of alkyne interception. Divergent access to elusive unsaturated systems has also been reported.

Denitrative imino-diaza-Nazarov cyclization: synthesis of pyrazoles

An iodine-catalyzed denitrative imino-diaza-Nazarov cyclization (DIDAN) methodology has been developed for the synthesis of pyrazoles with high to excellent yields by using α-nitroacetophenone derivatives and in situ generated hydrazones. The key transformation of this oxidative 4π-electrocyclization proceeds through an enamine-iminium ion intermediate. This rapid one-pot DIDAN protocol results in the selective generation of C-C and C-N bonds and cleavage of a C-N bond.

Biomimetic 2-Imino-Nazarov Cyclizations via Eneallene Aziridination

Amidoallyl cations are appealing three-carbon synthons for the preparation of complex amine-containing carbocycles; however, methods to generate and utilize these reactive species are limited and underexplored compared to those for oxallyl cations. Here we disclose a bioinspired strain-driven ring opening of bicyclic methyleneaziridines to 2-amidopentadienyl cation intermediates that readily engage in Nazarov cyclizations. Advantages of this strategy include ease of generation and improved reactivity compared to 3-pentadienyl cations, control over the ultimate position of the alkene, the potential for high dr between vicinal stereocenters, and the ability to further elaborate the products to fully substituted aminocyclopentanes. Experimental and computational studies support a dual role for the Rh₂L_n complex as both a nitrene transfer catalyst and a Lewis acid promoter, insight that provides a framework for the future development of asymmetric 2-imino-Nazarov cyclizations.

2,2'-Biphenol/B(OH)3 Catalyst System for Nazarov Cyclization

A novel catalyst system-a combination of the readily available 2,2'-biphenol with the inexpensive, nontoxic, and eco-friendly B(OH)₃-promoted the Nazarov cyclization of activated and inactivated divinyl ketones to afford the corresponding cyclopentenones up to 96% yield under, in a cis-selective manner. Compared with the conventional harsh conditions with hazardous reagents, user-friendly method was established with bench-stable and easy-to-handle reagents.

Enantioselective Functionalization of Enamides at the β-Carbon Center with Indoles

We report an enantioconvergent approach for the functionalization of enamides at the β-carbon atom, which involves a chiral Brønsted acid induced tautomerization of 2-amidoallyl into 1-amidoallyl cations. These putative reactive intermediates were produced by ionization of racemic α-hydroxy enamides with a chiral Brønsted acid and captured with substituted indoles in a highly regio- and enantioselective manner.

Versatile One-Pot Synthesis of Polysubstituted Cyclopent-2-enimines from α,β-Unsaturated Amides: Imino-Nazarov Reaction

The imino-Nazarov cyclization of the polysubstituted pentan-1,4-diene-3-imines was realized. To this aim, a one-pot procedure involving reductive alkenyliminylation of α,β-unsaturated secondary amides with potassium organotrifluoroborates, followed by acid-catalyzed imino-Nazarov cyclization of the polysubstituted pentan-1,4-diene-3-imine intermediates, was studied systematically. This mild, operationally simple, flexible, and high-yielding protocol efficiently affords polysubstituted pentan-1,4-diene-3-imines, cyclopentenimines, and α-amino cyclopentenones, which are useful scaffolds in organic synthesis. The substituent effect at the C2 position of the polysubstituted pentan-1,4-diene-3-imines was studied by means of density-functional theory calculations. Results suggested that the electron-donating group facilitates the imino-Nazarov cyclization process.

One-Pot Generation of Bicyclo[3.1.0]hexanols and Cyclohexanones by Double Interrupted Nazarov Reactions

Organoaluminum-mediated double interrupted Nazarov cyclization to access bicyclo[3.1.0]hexanols via nucleophilic methyl attack followed by Simmons-Smith-type electrophilic cyclopropanation is reported. These alcohols can undergo ring opening to afford cyclohexanones or cyclohexenones, broadening the range of scaffolds available via interrupted Nazarov reaction beyond the usual cyclopentanoid products. Throughout the sequence, a total of four new C-C bonds are formed, along with four new stereogenic centers.

Gold-catalyzed [4+3]- and [4+2]-annulations of 3-en-1-ynamides with isoxazoles via novel 6π-electrocyclizations of 3-azahepta trienyl cations

New gold-catalyzed [4+3]-annulations of 3-en-1-ynamides with isoxazoles afford 4H-azepines efficiently; this process involves 6π electrocyclizations of gold-stabilized 3-azaheptatrienyl cations. In the presence of Zn(OTf)2, the resulting 4H-azepines undergo skeletal rearrangement to furnish substituted pyridine derivatives. We subsequently develop new catalytic [4+2]-annulations between the same 3-en-1-ynamides and isoxazoles to deliver substituted pyridine products using Au(i)/Zn(ii) catalysts. This work reports the first success of the 6π electrocyclizations of heptatrienyl cations that are unprecedented in literature reports.

Brønsted Acids Enable Three Molecular Rearrangements of One 3-Alkylidene-2H-1,2-oxazine Molecule into Distinct Heterocyles

This work describes three different strategies to structurally rearrange one 3-alkylidene-2H-1,2-oxazine molecule into three distinct heterocycles using HOTf, propiolic acid, and silica gel, respectively. The mechanisms of these rearrangement reactions involve three independent routes, including (i) Brønsted acid catalysis, (ii) a synergetic action of Brønsted acids and anions, (iii) a surface-directed chemoselectivity.

The diaza-Nazarov cyclization involving a 2,3-diaza-pentadienyl cation for the synthesis of polysubstituted pyrazoles

Functionally-rich pyrazoles in a cascade reaction from in situ generated hydrazones and acetophenones under aerobic conditions were synthesized through novel enamine–imino diaza-Nazarov 4π-electrocyclization which is supported by DFT calculations.

Nazarov reaction: current trends and recent advances in the synthesis of natural compounds and their analogs

Recent applications of the Nazarov reaction to stereoselective synthesis of pharmacology-relevant cyclopentenone scaffolds are summarized.

Assembly of functionalized carbocycles or N-heterocycles through a domino electrocyclization-[1,2] migration reaction sequence

The development of processes that streamline the synthesis of complex, functionalized carbocycles and heterocycles remains a hotly pursued topic because their scaffolds are present in a range of bioactive molecules and electronic materials. Although the Nazarov reaction has emerged to be useful in the synthesis of carbocycles and heterocycles, using an electrocyclization to trigger a migration remains underdeveloped. By constructing several bonds in one operation, domino reaction sequences are particularly effective at improving the efficiency of synthesis. The use of transition metal catalysts has the potential to render these processes stereoselective. This review examines the use of electrocyclization-[1,2] migrations to construct molecules and is organized by the type of ring constructed and the order of the two steps in this process.

The first intermolecular interrupted imino-Nazarov reaction: expeditious access to carbocyclic nucleoside analogues

The first intermolecular interrupted imino-Nazarov reaction with silylated pyrimidine derivatives as nucleophile has been developed, furnishing carbocyclic nucleoside analogues in a one-pot operation.

The analogous imino-variant of the Nazarov reaction suffers from unfavorable energetics associated with the ring closure process, thereby limiting the utility of this class of reactions. In this report, the realization of the first intermolecular interrupted imino-Nazarov reaction utilizing silylated pyrimidine derivatives as nucleophilic trapping partners culminated in an expedient synthetic route to carbocyclic nucleoside analogues. The potential application of silylated nucleobases to intercept the oxyallyl cation in other variants of the Nazarov reaction provides vast opportunities to develop new strategies for the formation of carbocyclic nucleoside analogues.

Gold(I)-catalyzed dearomative Rautenstrauch rearrangement: enantioselective access to cyclopenta[b]indoles

A highly enantioselective dearomative Rautenstrauch rearrangement catalyzed by cationic (S)-DTBM-Segphosgold(I) is reported. This reaction provides a straightforward method to prepare enantioenriched cyclopenta[b]indoles. These studies show vast difference in enantioselectivity in the reactions of propargyl acetates and propargyl acetals in the chiral ligand-controlled Rautenstrauch reaction.

Nazarov cyclization of dienylaziridines: synthesis of cyclopentadienyl/hydrinedienyl/indenyl glycines

Cyclopentadienyl, hydrinedienyl, and indenyl glycines were synthesized by the Nazarov cyclization of dienylaziridines for the first time. Several substrates were synthesized to demonstrate the compatibility of this reaction. Asymmetric synthesis of these amino acids was also developed to show the additional scope of this method.

Regio- and stereoselective synthesis of ensulfonamides/enamides via catalyst-free intermolecular addition of indoles/pyrroles/imidazole to allenamides

A catalyst-free intermolecular addition of indoles, pyrroles, and imidazole to allenamides is reported. The reaction proceeds smoothly and provides a series of (E)-ensulfonamide/enamide derivatives in high yields with excellent regioselectivity.

Central-axial-central chirality transfer: asymmetric synthesis of highly substituted indenes bearing a stereogenic quaternary carbon center from optically active propargyl alcohols

An asymmetric synthesis of highly substituted indenes 3, bearing a quaternary stereogenic carbon center, has been developed via the central-axial-central chirality transfer from optically active propargyl alcohols 1. This transformation involves the addition/rearrangement of 1 and ynamides 2 to give tetra-substituted allenes 4 and further cyclization of 4.