Bifunctional Organo/Metal Cooperatively Catalyzed [3 + 2] Annulation of para-Quinone Methides with Vinylcyclopropanes: Approach to Spiro[4.5]deca-6,9-diene-8-ones

Intermolecular Carbophosphination of Alkynes with Phosphole Oxides via Ni-Al Bimetal-Catalyzed C-P Bond Activation

Intermolecular carbophosphination reaction of alkynes or alkenes with unreactive C-P bonds remains an elusive challenge. Herein, we used a Ni-Al bimetallic catalyst to realize an intermolecular carbophosphination reaction of alkynes with 5-membered phosphole oxides, providing a series of 7-membered phosphepines in up to 94 % yield.

Electrophilic Halospirocyclization of N-Benzylacrylamides to Access 4-Halomethyl-2-azaspiro[4.5]decanes

An electrophilic spirocyclization of N-benzylacrylamides with N-halosuccinimides (NXS) as the halogenating reagents has been developed. This reaction is carried out at room temperature under simple conditions without relying on metal reagents, photochemistry, or electrochemistry, providing a fast and efficient route to synthesize a wide variety of 4-halomethyl-2-azaspiro[4.5]decanes with satisfactory yields. The approach is further highlighted through gram-scale synthesis and diverse transformations of the spiro products.

DMSO-promoted direct δ-selective arylation of p-quinone methenylpiperidine bearinides to generate fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources

Fuchsones have wide applications in modern society. Present methods for generating fuchsones have many disadvantages and there are significant limitations for further exploration of fuchsone applications. Herein, we describe a DMSO-promoted direct δ-selective arylation of p-QMs to synthesize symmetrical and unsymmetrical fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources. As unprecedented methods, these novel strategies present a great advantage and significance for further exploration of fuchsones and the development of new applications.

Palladium-Catalyzed Annulative Coupling of Spirovinylcyclopropyl Oxindoles with p-Quinone Methides

Pd-catalyzed annulative coupling of spirovinylcyclopropyl oxindoles with p-quinone methides has been accomplished via cascade carbon-carbon bond formation to afford bis-spirooxindole scaffolds. The mild reaction conditions, diastereoselectivity, functional group diversity, post-synthetic transformations, and mechanistic studies using DFT calculations are the important practical features.

Palladium-catalysed enantio- and regioselective (3 + 2) cycloaddition reactions of sulfamidate imine-derived 1-azadienes towards spirocyclic cyclopentanes

An enantio- and diastereoselective Pd-catalysed (3 + 2) cycloaddition of bis(trifluoroethyl) 2-vinyl-cyclopropane-1,1-dicarboxylate (VCP) with cyclic sulfamidate imine-derived 1-azadienes (SDAs) has been developed. These reactions provide highly functionalized spiroheterocycles having three contiguous stereocentres, including a tetrasubstituted carbon bearing an oxygen functionality. The two geminal trifluoroethyl ester moieties can be manipulated in a facially selective manner to afford more diversely decorated spirocycles with four contiguous stereocentres. In addition, diastereoselective reduction of the imine moiety can also afford a fourth stereocentre and exposes the important 1,2-amino alcohol functionality.

Synthesis of π-Extended Benzofulvenes: A Pd-Catalyzed Cascade Annulation Process Comprising C-C Bond Formation Followed by 1,6-Conjugate Addition

A Pd-catalyzed cascade annulation process comprising C-C bond formation followed by 1,6-conjugate addition for π-extended benzofulvenes is reported. This process is compatible with a wide range of functionalities for both the p-quinone methides and internal alkynes, leading to diverse π-extended benzofulvenes. Additionally, this strategy is also applicable for aryne annulation with p-quinone methides.

Addition of sulfonylphthalides to para-quinone methides: Selective 1,6-additions and oxidative annulations

KO^tBu mediated addition of sulfonylphthalides to p-quinone methides led to the selective synthesis of isochroman-1,4-diones and addition products. Interestingly, isochroman-1,4-diones were obtained via an unprecedented oxidative annulation pathway. The present work highlights a wide variety of substrates, good yields, shorter reaction time and ambient reaction conditions. Furthermore, a few addition products were transformed into functionalized heterocyclic molecules. Additionally, the scale-up experiment suggests the practical feasibility of preparing isochroman-1,4-diones in higher-scale reactions.

Transition-Metal-Free Multicomponent Approach to the Regioselective Synthesis of Highly Substituted N-Alkylpyrazoles

An efficient transition-metal-free multicomponent approach to the regioselective synthesis of highly substituted N-alkylpyrazoles through 1,6-addition of pyrazole (in situ generated from α,β-unsaturated aldehyde and hydrazide) to para-Quinone Methides has been developed. The N-alkylpyrazole containing triarylmethanes having several heteroaryl rings (quinoline, pyridine, thiophene) at the central methine carbon atom was developed. This chemical process may be used for large-scale synthesis and provides a novel way to produce triarylmethanes with diverse functional groups.

Aromatization-driven cascade [1,5]-hydride transfer/cyclization for synthesis of spirochromanes

An aromatization-driven hydride transfer-involved α-C(sp3)–H bond functionalization of the oxygen atom was developed. Easily prepared p-quinone methides were applied to initiate [1,5]-hydride transfer/cyclization for generating spirochromanes.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Indane-1,3-Dione: From Synthetic Strategies to Applications

Indane-1,3-dione is a versatile building block used in numerous applications ranging from biosensing, bioactivity, bioimaging to electronics or photopolymerization. In this review, an overview of the different chemical reactions enabling access to this scaffold but also to the most common derivatives of indane-1,3-dione are presented. Parallel to this, the different applications in which indane-1,3-dione-based structures have been used are also presented, evidencing the versatility of this structure.

Palladium-Catalyzed Carbonylative [5+1] Cycloaddition of N-Tosyl Vinylaziridines: Solvent-Controlled Divergent Synthesis of α,β- and β,γ-Unsaturated δ-Lactams

A palladium-catalyzed carbonylative [5+1] cycloaddition of N-tosyl vinylaziridines with CO has been developed. This protocol affords an efficient and practical approach for solvent-controlled divergent synthesis of α,β-unsaturated δ-lactams in dimethylformamide and β,γ-unsaturated δ-lactams in tetrahydrofuran in good to excellent yields. Significantly, the step- and atom-economical reactions are more regioselective toward [5+1] cycloaddition than toward [3+1] cycloaddition.

Iron(III)-catalyzed tandem annulation of indolyl-substituted p-quinone methides with ynamides for the synthesis of cyclopenta[b]indoles

The unique reactivity of indolyl-substituted p-QMs as a new type of two-carbon synthon has been explored for the first time in a novel iron(III)-catalyzed tandem annulation. This (2+2) annulation/retro-4π electrocyclization/imino-Nazarov cyclization cascade reaction is characterized by an unusual structural reconstruction of indolyl-substituted p-QMs, leading to an expeditious assembly of synthetically important functionalized cyclopenta[b]indoles.

imethyl 2-Benzylmalonate

An electrochemical dearomatizing spirocyclization of alkynes with dimethyl 2-benzylmalonates for the preparation of spiro[4.5]deca-trienones has been developed. This approach adopts ferrocene (Cp₂Fe) as an electrocatalyst to produce carbon-centered radical intermediates from C-H-based malonates, which obviates the forthputting of noble-metal reagents, sacrificial chemical oxidants and 2-bromomalonates. A wide variety of spiro compounds are efficiently prepared with satisfactory results under mild conditions.

1,6-Addition of 1,2,3-NH triazoles to para-quinone methides: Facile access to highly selective N1 and N2 substituted triazoles

The regioselective syntheses of N¹ and N² substituted triazoles through a 1,6-addition reaction of 1,2,3-NH triazoles to p-quinone methide were achieved under mild reaction conditions. The present reactions showed superior results in terms of selectivity, mild reaction conditions, short reaction time and broad substrate scope with good functional-group compatibility. Considering the high synthetic value of N¹- and N²-substituted compounds and p-QM related research, the present strategy will greatly benefit researchers in various fields.

Tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles: construction of vicinal quaternary carbon centers

A novel tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles has been developed, providing dispirooxindole–cyclopentane–cyclohexadienones with vicinal quaternary carbon centers.

Pd-Catalyzed asymmetric decarboxylation for the construction of spiro[4.5]deca-6,9-dien-8-ones featuring vicinal quaternary carbons

A Pd-catalyzed decarboxylative strategy for the asymmetric construction of spiro[4.5]deca-6,9-dien-8-ones is reported. The epimerization of the resultant products occurred under otherwise Pd-catalysis.

Silver-Catalyzed Regioselective 1,6-Hydroarylation of para-Quinone Methides with Anilines and Phenols

A simple and efficient method for the silver-catalyzed regioselective 1,6-hydroarylation of para-quinone methides (p-QMs) with anilines and phenols has been established. Without the need for pre-protection, a broad range of...

Enantioselective Construction of Chiral Bispiro[Oxindole-Pyrrolidine-Pyrazolone] Derivatives via Sequential and One-Pot Mannich/Hydroamination Reaction

The atom-economic method for the preparation of novel bispirocyclic compounds containing three fused heterocyclic scaffolds privileged in drug discovery was developed by using a chiral amine-squaramide Mannich reaction and Au(I)-catalyzed hydroamination. The developed synthetic strategy performed either stepwise or as a one-pot process allows the formation of chiral bispirocyclic [oxindole-pyrrolidine-pyrazolones] in high yields (up to 75%) with excellent enantioselectivities (up to 99%).

Stereoselective Synthesis of Tri- and Tetrasubstituted Olefins via 1,6-Additions of Diazo Compounds and Their Precursors to p-Quinone Methides

Reactions of para-quinone methides (p-QMs) with α-diazo-β-ketosulfones and their corresponding esters as well as simple β-dicarbonyl compounds and β-ketosulfones have been carried out under basic conditions. While the reaction of diazosulfone with p-QMs afforded trisubstituted olefins via deacylative 1,6-addition and elimination, α-diazo-β-ketoesters and various active methylene compounds such as 1,3-dicarbonyls and β-ketosulfones afforded tetrasubstituted olefins via 1,6-addition and aerial oxidation. These simple, environmentally benign, and mechanistically diverse protocols provided the products in moderate to excellent yields and selectivities.

Silver-Catalyzed Regioselective Phosphorylation of para-Quinone Methides with P(III)-Nucleophiles

A simple and efficient method for the silver-catalyzed regioselective phosphorylation of para-quinone methides (p-QMs) with P(III)-nucleophiles (P(OR)₃, ArP(OR)₂, Ar₂P-OR) has been established via Michaelis-Arbuzov-type reaction. A broad range of P(III)-nucleophiles and para-quinone methides are well tolerated under the mild conditions, giving the expected diarylmethyl-substituted organophosphorus compounds with good to excellent yields. Moreover, a series of corresponding enantiomers can be obtained by employing dialkyl arylphosphonite (ArP(OR)₂) as substrates. The control experiments and ³¹P NMR tracking experiments were also performed to gain insights for the plausible reaction mechanism. This protocol may have significant implications for the formation of C(sp³)-P bonds in Michaelis-Arbuzov-type reactions.

Recent synthetic strategies toward the synthesis of spirocyclic compounds comprising six-membered carbocyclic/heterocyclic ring systems

Spirocyclic compounds fascinate the synthetic chemists due to their privileged ring system and efficacy in drug discovery. Many natural compounds comprise spirocyclic moiety in their skeleton and are effective in pharmaceutical industry. Over the years, many synthetic methodologies have been established for the construction of spirocyclic compounds. In this review, recent synthetic approaches to accessing various spirocompounds comprising six-membered carbocycles/heterocycles have been summarized.

Metal-Free, Acid-Catalyzed 1,6-Conjugate Addition of NH-Sulfoximines to para-Quinone Methides: Accessing to Diarylmethine Imino Sulfanone

We have reported an efficient and metal-free method for the construction of α-diarylmethine imino sulfanone using acid-catalyzed 1,6-conjugate addition of sulfoximines on para-quinine methides (p-QMs). This method showed broad functional group tolerance and a wide range of substrate scope with good to excellent yield. The excellent protocol exhibits mild reaction conditions with high atom economy. The practicability of the present method was supported by a Gram-scale reaction.

Enantioselective Domino Reaction of 3-Vinylindole and p-Quinone Methides Enabled by Chiral Imidodiphosphoric Acids: Asymmetric Synthesis of Multisubstituted 3-Indolyl Cyclopenta[b]indoles

The development of a stereoselective method for the rapid assembly of structurally complex molecules remains fascinating and challenging in synthetic organic chemistry. Here, we report an enantioselective domino reaction between 3-vinylindole and p-quinone methide for the preparation of 3-indolyl cyclopenta[b]indoles containing multiple chiral centers. Chiral imidodiphosphoric acids enable this cascade asymmetric process, delivering a series of products with excellent yields (≤99%), enantioselectivities (≤99%), and diastereoselectivities (≤20:1 dr).

Metal-Free, Acid/Phosphine-Induced Regioselective Thiolation of p-Quinone Methides with Sodium Aryl/Alkyl Sulfinates

A simple and efficient method for the regioselective thiolation of p-quinone methides with sodium aryl/alkyl sulfinates has been established using an acid/phosphine-induced radical route under transition-metal-free conditions. A broad range of sodium aryl/alkyl sulfinates and p-quinone methides (p-QMs) are compatible for the reaction, giving the expected products with good to excellent yields. Control experiments were also performed to gain insights into the generation mechanism of thiyl radicals and hydrogen-atom transfer process. This protocol provides a safe and feasible way for the formation of carbon-sulfur bonds.

Visible-Light Photoredox-Catalyzed Formal [5 + 1] Cycloaddition of N-Tosyl Vinylaziridines with Difluoroalkyl Halides

A visible-light photoredox-catalyzed formal [5 + 1] cycloaddition of N-tosyl vinylaziridines with difluoroalkyl halides as unique C1 synthons was developed. The procedure provides an efficient and practical method to synthesize diverse pyridines in moderate to good yields. The reaction underwent a radical-initiated kinetically controlled ring-opening of vinylaziridines and involved a key α,β-unsaturated imine intermediate, followed by an E2 elimination, a 6π electrocyclization, and defluorinated aromatization.

Catalyst-free 1,6-conjugate addition of indoles and 4-hydroxycoumarins to para-quinone methides: synthesis of unsymmetrical triarylmethanes

The catalyst-free 1,6-conjugate addition of indoles and 4-hydroxycoumarins to para-quinone methides is reported. This protocol allowed us to access a range of unsymmetrical triarylmethanes in good to excellent yields. The outlined procedure is operationally simple, efficient, atom and step economical. The synthesized heterocyclic triarylmethanes were further converted into highly substituted indoloisoquinolines and pyranochromenones via metal-catalyzed C-H activation/annulation.

Ruthenium(II)-Catalyzed Regioselective [3 + 2] Spiroannulation of 2H-Imidazoles with 2-Alkynoates

The C═N double bond of 2H-imidazole has been employed as a C-electrophile for the ruthenium(II)-catalyzed [3 + 2] spiroannulation reaction of 4-phenyl-2H-imidazoles and 2-alkynoates to synthesize spiroimidazole-4,1'-indenes. This strategy features high regioselectivity, broad functional group tolerance, and use of ruthenium as a catalyst, providing a new method to synthesize spirocycles with potential applications in pharmaceuticals.

CF3-Containing para-Quinone Methides for Organic Synthesis

A new family of CF3-containing para-quinone methides (CF3-QMs) was systematically investigated for its suitability in organic synthesis. Addition of different nucleophiles gives access to target molecules with a benzylic CF3-containing stereogenic center straightforwardly. The electrophilicity parameter E of the prototypical CF3-QM 2,6-di-tert-butyl-4-(2,2,2-trifluoroethylidene)cyclohexa-2,5-dien-1-one was determined to be -11.68 according to the Mayr scale, making it one of the most reactive quinone methides known so far.

A thermal decarboxylative Cloke–Wilson rearrangement of dispirocyclopropanes derived from para-quinone methides and bromo-Meldrum's acids: an approach to synthesize spirobutyrolactone para-dienones

An unprecedented approach to synthesize spirobutyrolactone para-dienones from para-quinone methides and bromo-Meldrum's acids has been developed.

Recent developments in 1,6-addition reactions of para-quinone methides (p-QMs)

In this review, we provide a comprehensive overview of recent progress in this rapidly growing field by summarizing the 1,6-conjugate addition and annulation reactions of p-QMs with consideration of their mechanisms and applications.

1,6-Conjugate addition initiated formal [4+2] annulation of p-quinone methides with sulfonyl allenols: a unique access to spiro[5.5]undeca-1,4-dien-3-one scaffolds

An expedient one-pot synthesis of carbocyclic spiro[5.5]undeca-1,4-dien-3-ones via 1,6-conjugate addition initiated formal [4+2] annulation sequences by employing p-quinone methides and sulfonyl allenols.

InCl3-catalyzed 5-exo-dig cyclization/1,6-conjugate addition of N-propargylamides with p-QMs to construct oxazole derivatives

An InCl₃-catalyzed tandem intramolecular 5-exo-dig cyclization/1,6-conjugate addition/aromatization of N-propargylamides with p-QMs to produce oxazoles tethering diarylmethane has been successfully developed.

Oxidation of Enones for Regioselective [3+2] Cycloaddition through γ-Enone Radical Intermediates

Herein, an oxidization reaction of enones with a Cu^II complex that leads to a new type of regioselective [3+2] cycloaddition is reported. Highly functionalized cyclopentenes and spirocyclic compounds are obtained in moderate-to-good yields. This cycloaddition reaction occurred through the formation of γ-enone radicals, providing a rarely explored reactivity pattern for enones.