Desymmetrization of cyclohexadienones viad-camphor-derived triazolium salt catalyzed intramolecular Stetter reaction

A Domino Dearomative ipso-Annulation/Desymmetrization Approach: Stereoselective Access to Tricyclic Alkaloid Skeletons

Herein, we reveal an unprecedented domino annulation of N-benzyl-acrylamides with 1,3-dicarbonyls for the assembly of fused tricyclic alkaloid frameworks incorporating a spirocycle via an alkylation/dearomative ipso-annulation/Michael addition (desymmetrization) sequence. This conversion involves three carbon-carbon bond formations, generating four chiral carbons, including three quaternary carbon centers, in a single diastereomer in one pot under identical reaction conditions. The synthetic potential of this atom-economic method is illustrated by modifications of the functional groups present in the products obtained.

Enantioselective Desymmetrization Triggered by Iminium-Enamine Activation: Access to Complex Cyclohepta[b]indoles

The expeditious construction of complex molecules having multiple stereocentres is highly desirable in organic chemistry. In the present communication, we report the development of an organocatalytic asymmetric desymmetrization of prochiral enal-tethered cyclohexadienones via the C3-selective Friedel-Crafts alkylation of indoles triggered by LUMO-lowering iminium activation/HOMO-raising enamine activation. The reaction provides access to bicyclic enones, which further undergo acid-mediated intramolecular annulation from C2-position to afford highly strained cyclohepta[b]indoles with five contiguous stereocentres and three new C-C bonds in excellent enantioselectivity and diastereoselectivity.

Site-selective and stereoselective transformations on p-quinols & p-quinamines

The intermolecular transformation of simple substrates into highly functionalized scaffolds with multiple stereogenic centers is an attractive strategy in modern organic synthesis. Prochiral 2,5-cyclohexadienones, being stable and easily accessible, are privileged key building blocks for the synthesis of complex molecules and bioactive natural products. In particular, p-quinols and p-quinamines are important subclasses of cyclohexadienones, having both nucleophilic and electrophilic sites, and can undergo various intermolecular cascade annulations via formal cycloadditions and other transformations. This article highlights the recent developments of intermolecular transformations on p-quinols and p-quinamines along with plausible reaction mechanisms. We hope that this review will inspire the readers to explore the new potential applications of these unique prochiral molecules.

Solvent-Mediated Enantioselective Rauhut-Currier Cyclization via Iminium and Enamine Activation

In this work, we have developed an unconventional and highly enantioselective solvent-promoted Rauhut-Currier cyclization of enal-tethered cyclohexadienone by exploiting the reactivity of a simple Jørgensen-Hayashi catalyst through the merging of iminium and enamine activation. This asymmetric desymmetrization reaction has broad substrate scope in good yields with high to excellent enantioselectivity. DFT calculations suggest that the elimination of the alkoxy group is the rate-limiting step and that it proceeds through proton abstraction by solvent instead of a direct 1,3-proton shift.

Nitrogen-Containing Heterocyclic Compounds Obtained from Monoterpenes or Their Derivatives: Synthesis and Properties

Directed transformation of available natural compounds with native biological activity is a promising area of research in organic and medicinal chemistry aimed at finding effective drug substances. The number of scientific publications devoted to the transformation of natural compounds and investigations of their pharmacological properties, in particular, monoterpenes and their nearest derivatives, increases every year. At the same time, the chemistry of nitrogen-containing heterocyclic compounds has been actively developed since the 1950s after the news that the benzimidazole core is an integral part of the structure of vitamin B₁₂. At the time of writing this review, the data on chemical modifications of monoterpenes and their nearest derivatives leading to formation of compounds with a nitrogen-containing heterocycle core have not been summarized and systematized in terms of chemical transformations. In this review, we tried to summarize the literature data on the preparation and properties of nitrogen-containing heterocyclic compounds synthesized from monoterpenes/monoterpenoids and their nearest derivatives for the period from 2000 to 2021.

Enantioselective Cu(I)-catalyzed borylative cyclization of enone-tethered cyclohexadienones and mechanistic insights

The catalytic asymmetric borylation of conjugated carbonyls followed by stereoselective intramolecular cascade cyclizations with in situ generated chiral enolates are extremely rare. Herein, we report the enantioselective Cu(I)-catalyzed β-borylation/Michael addition on prochiral enone-tethered 2,5-cyclohexadienones. This asymmetric desymmetrization strategy has a broad range of substrate scope to generate densely functionalized bicyclic enones bearing four contiguous stereocenters with excellent yield, enantioselectivity, and diastereoselectivity. One-pot borylation/cyclization/oxidation via the sequential addition of sodium perborate reagent affords the corresponding alcohols without affecting yield and enantioselectivity. The synthetic potential of this reaction is explored through gram-scale reactions and further chemoselective transformations on products. DFT calculations explain the requirement of the base in an equimolar ratio in the reaction, as it leads to the formation of a lithium-enolate complex to undergo C-C bond formation via a chair-like transition state, with a barrier that is 22.5 kcal/mol more favourable than that of the copper-enolate complex.

Rapidly building molecular structures with both elements of complexity and flexibility is a key goal of organic synthesis. Here the authors show a tandem copper-catalyzed β-borylation/Michael addition on prochiral enone-tethered 2,5-cyclohexadienones, to generate bicyclic borylated products in high yield and enantioselectivity.

Selectfluor-mediated construction of 3-arylselenenyl and 3,4-bisarylselenenyl spiro[4.5]trienones via cascade annulation of N-phenylpropiolamides with diselenides

A cascade annulation of N-phenylpropiolamides with diselenides leading to the construction of 3-arylselenenyl spiro[4.5]trienones was realized under mild conditions with Selectfluor as the sole oxidant.

Visible-Light-Induced Tandem Difluoroalkylated Spirocyclization of N-Arylpropiolamides: Access to C3-Difluoroacetylated Spiro[4,5]trienones

A visible-light-catalyzed difluoroacetylated spirocyclization of N-arylpropiolamides with ethyl bromodifluoroacetate as a CF2CO2Et radical precursor is described. This approach allows the formation of two carbon-carbon bonds and one carbon-oxygen bond in...

Transition-metal catalyzed oxidative spirocyclization of N-aryl alkynamides with methylarenes under microwave irradiation

A practical synthetic route to construct a variety of 3-benzyl spiro[4,5]trienones was developed via transition-metal Cu/Ag-catalyzed oxidative ipso-annulation of activated alkynes with unactivated toluenes using TBPB as an oxidant under microwave irradiation. This method allows the formation of two carbon-carbon bonds and one carbon-oxygen bond in a single reaction through a sequence of C-H oxidative coupling, ipso-carbocyclization and dearomatization. The advantages of this protocol are its operational simplicity and broad substrate scope, and the ability to afford the desired products in moderate to good yields.

Chalcogenative spirocyclization of N-aryl propiolamides with diselenides/disulfides promoted by Selectfluor

A practical and efficient synthetic route to construct a variety of 3-arylselenenyl/3-arylthio spiro[4.5]trienones was developed using Selectfluor reagent as a mild oxidant. This reaction proceeds via a sequence of electrophilic cation addition, spirocyclization and dearomatization, then offers an approach to introduce Se/S-centered cation into the C–C triple bonds. The utility of this protocol were justified by the excellent compatibility of a wide range of functional groups, good yields and scalability under mild reaction conditions.

Acyclic Twisted Amides

In this contribution, we provide a comprehensive overview of acyclic twisted amides, covering the literature since 1993 (the year of the first recognized report on acyclic twisted amides) through June 2020. The review focuses on classes of acyclic twisted amides and their key structural properties, such as amide bond twist and nitrogen pyramidalization, which are primarily responsible for disrupting nN to π*C═O conjugation. Through discussing acyclic twisted amides in comparison with the classic bridged lactams and conformationally restricted cyclic fused amides, the reader is provided with an overview of amidic distortion that results in novel conformational features of acyclic amides that can be exploited in various fields of chemistry ranging from organic synthesis and polymers to biochemistry and structural chemistry and the current position of acyclic twisted amides in modern chemistry.

Transition-Metal Catalyzed Stereoselective Desymmetrization of Prochiral Cyclohexadienones

The development of transition-metal catalyzed enantioselective and diastereoselective transformations has contributed many advances in the field of synthetic organic chemistry. Particularly, stereoselective desymmetrization of prochiral cyclohexadienones represents a powerful strategy for accessing highly functionalized and stereochemically enriched scaffolds, which are often found in biologically active compounds and natural products. In recent years, several research groups including our group have made a significant progress on transition-metal catalyzed stereoselective desymmetrizations of 2,5-cyclohexadienones. In this account, we will provide an overview of the recent developments in this area employing Pd, Cu, Rh, Au, Ag, Ni, Co, and Mn-catalysts.

Design, synthesis and application of spiro[4.5]cyclohexadienones via one-pot sequential p-hydroxybenzylation/oxidative dearomatization

One-pot sequential p-hydroxybenzylation/oxidative dearomatization/spiroannulation has been designed for the efficient construction of tetrahydrofuran containing spiro-cyclohexadienones. This reaction proceeds through the p-hydroxybenzylation of 1,3-diketones with p-hydroxybenzyl alcohol via quinone methide formation followed by oxidative dearomatization/spiroannulation with suitable alcohols. The Friedel-Crafts alkylation of spiro[4.5]cyclohexadienones with indoles provided a broad array of highly diastereoselective C-3 alkylated spirocycles and cyclohepta[b]indoles depending upon the ring size of the fused cyclic ketones.

N-Heterocyclic Carbene-Catalyzed Cyclization of Aldehydes with α-Diazo Iodonium Triflate: Facile Access to 2,5-Disubstituted 1,3,4-Oxadiazoles

Herein, we report a novel organocatalytic process for synthesis of complex 1,3,4-oxadiazoles from readily accessible aldehydes. By exploiting the nucleophilicity of the putative Breslow intermediate and the inherent electrophilicity of α-diazo iodonium triflate, we have found that N-heterocyclic carbene catalyst promotes efficient cyclization of various aldehydes and α-diazo iodonium triflates. The reaction proceeds under mild conditions with a wide range of functional group tolerance. The heterocyclic products can be readily further functionalized, rendering the protocol highly valuable.

Enamides and dienamides in phosphoric acid-catalysed enantioselective cycloadditions for the synthesis of chiral amines

This feature article describes how enamides and dienamides can participate in chiral phosphoric acid catalyzed enantioselective cycloadditions to prepare a wide range of cyclic amines.

A dearomative ipso-iodocyclization/desymmetrization sequence leading to optically active tricyclic piperazine scaffolds

Dearomative ipso-iodocyclization of 4-(1-ethoxyethoxy)-N-propargylanilines leading to 1-azaspiro[4.5]deca-3,6,9-trien-8-ones has been developed. This reaction is characterized by the yield of fewer toxic byproducts and is conducted by a more user-friendly protocol compared to other reported methods. The synthetic application of these spiro products was demonstrated by transformation of the iodo component into other functionalities. The desymmetrization of the resulting cyclohexadienones was also achieved by the diastereoselective aza-Michael addition of internal amino acid moieties to yield tricyclic piperazine scaffolds with two newly formed contiguous chiral centers.

Electrochemical Synthesis of Spiro[4.5]trienones through Radical-Initiated Dearomative Spirocyclization

A novel and green route has been developed for the electrochemical synthesis of spiro[4.5]trienones through radical-initiated dearomative spirocyclization of alkynes with diselenides. This metal-free and oxidant-free electrosynthesis reaction was performed in an undivided cell under mild conditions. A variety of selenation spiro[4.5]trienones products were prepared in moderate-to-good yields, showing a broad scope and functional group tolerance. Moreover, the developed continuous-flow system combined with electrosynthesis possesses the potential to achieve scaled-up reactions, overcoming the low efficiency of conventional electrochemical scaled-up reactions.

Photo-mediated synthesis of halogenated spiro[4,5]trienones of N-aryl alkynamides with PhI(OCOCF3)2 and KBr/KCl

A novel and convenient photo-mediated halogenated spirocyclization of N-(p-methoxyaryl)propiolamides has been developed. The photolysis of phenyliodine bis(trifluoroacetate) (PIFA) as an iodination reagent led to iodinated ipso-cyclization under the irradiation of a xenon lamp, while brominated ipso-cyclization or chlorinated ipso-cyclization was achieved by irradiating a mixture of PIFA and KBr/KCl under a blue LED. The present protocol simply utilizes light as the safe and clean energy source and doesn't require any external photocatalyst providing various 3-halospiro[4,5]trienones in good to excellent yields (up to 93%).

Stereocontrolled transformations of cyclohexadienone derivatives to access stereochemically rich and natural product-inspired architectures

The last two decades or so have witnessed an upsurge in defining the art of designing complex natural products and nature-inspired molecules. Throughout these decades, fundamental insights into stereocontrolled, step-economic and atom-economical synthesis principles were achieved by the numerous synthetic accomplishments particularly in diversity-oriented synthesis (DOS). This has empowered the visualization of the third dimension in synthetic design and thus has resulted in a dramatic increase with today's diversity-oriented synthesis (DOS) at the forefront enabling access to diverse scaffolds with a high degree of stereochemical and skeletal complexity. To this end, a starting material-based approach is one of the powerful tools utilized in DOS that allows rapid access to molecular architectures with a high sp3 content. Skeletal and stereochemical diversity is often paramount for the selective modulation of the biological function of a complementary protein in the biological space. In this context, stereocontrolled transformation of cyclohexadienone scaffolds has positioned itself as a powerful platform for the rapid generation of stereochemically enriched and natural product-inspired compound collections. In this review, we cover multidirectional synthetic strategies that utilized cyclohexadienone derivatives as pluripotent building blocks en route for the construction of novel chemical space.

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.

Synthesis of (−)-Verbenone-derived Triazolium Salts and Their Application in Enantioselective Intramolecular Stetter Reaction

Two novel chiral verbenone-derived triazolium salts have been synthesized from readily available (−)-verbenone and found to be efficient for the enantioselective intramolecular Stetter reaction. The approach, based on the intramolecular annulation between acyl anion equivalents and Michael acceptors, benefits from broad substrate scope, high chemical and stereochemical efficiency, and operational simplicity. Mono-, and disubstituded chromanone derivatives have been obtained in excellent yields and in a highly stereochemical manner.

Palladium-catalyzed highly diastereoselective cascade dihalogenation of alkyne-tethered cyclohexadienones via Umpolung of palladium enolate

A highly regio- and diastereoselective Pd(ii)-catalyzed cascade dihalogenation of alkyne-tethered cyclohexadienones proceeding via an electrophilic oxa-π-allyl palladium-species was demonstrated.

Enantioselective synthesis of 3-amino-hydrobenzofuran-2,5-diones via Cu(i)-catalyzed intramolecular conjugate addition of imino esters

The copper-catalyzed enantioselective intramolecular conjugate addition of imino esters for desymmetrization of cyclohexadienones was described, providing access to enantioenriched 3-amino-hydrobenzofuran-2,5-diones.

N-Heterocyclic Carbene (NHC)-Organocatalyzed Kinetic Resolutions, Dynamic Kinetic Resolutions, and Desymmetrizations

The last couple of decades have witnessed tremendous development within N-heterocyclic carbene (NHC) organocatalysis. NHCs have been used as powerful organic catalysts in asymmetric synthesis. Although great achievements have been made in asymmetric NHC catalysis, their applications in kinetic resolution (KR), dynamic kinetic resolution (DKR), and desymmetrization processes have been relatively less developed. Moreover, limited activation modes have been involved in these processes. This review provides an overview of the NHC-organocatalyzed KR, DKR, and desymmetrization reactions in the preparation of chiral functional molecules. The aim is to highlight both the importance and elegance of these methods in the construction of challenging chiral compounds and to provide our own perspective on future development in this direction.

Asymmetric synthesis of functionalized tetrahydrofluorenones via an NHC-catalyzed homoenolate Michael addition

The first example of an N-heterocyclic carbene-catalyzed asymmetric desymmetrization of enal-tethered cyclohexadienones via an intramolecular homoenolate Michael addition/esterification reaction is described.

Enantioselective nickel-catalyzed arylative intramolecular 1,4-allylations

The enantioselective desymmetrization of allenyl cyclohexa-2,5-dienones by nickel-catalyzed arylative intramolecular 1,4-allylations is described.

One-Pot Consecutive Sulfonamidation/ipso-Cyclization Strategy for the Construction of Azaspirocyclohexadienones

Harnessing of Morita-Baylis-Hillman (MBH) carbonates of acetylenic aldehydes as handy synthons has allowed a facile synthesis of azaspirocyclohexadienones by sequential DABCO-promoted sulfonamidation/ICl-mediated ipso-iodocyclization reactions. A variety of MBH-carbonates having aryl or heteroaryl groups on the alkyne functionality fruitfully participated in the one-pot ipso-annulation reaction to provide the corresponding 3-iodo spirocyclohexadienones. The sulphonamide functionality was further utilized to construct the tricyclic fused-sultam framework.