N-Heterocyclic Carbene-Catalyzed Activation of Esters of N-Hydroxyphthalimide: A Highly Enantioselective Route to Chiral Dihydropyridinones Bearing an All Carbon Quaternary Stereogenic Center

NHC-Activations on α-, β-, γ-, and Beyond

Over the recent decades, due to the special electronic characteristics and diverse reactivities, N-heterocyclic carbene (NHC) has received significant interest in organocatalyzed reactions. The formation of Breslow intermediates by NHC can convert into acyl anion equivalent, enolates, homoenolate, acyl azolium, and vinyl enolate etc., and the cycloaddition reactions of these species has attracted lots of attention. In this review, we focus on the summry of the development of NHC-activation of carbonyl carbon (or imine carbon) in situ, α-, β-, γ-, and beyond, and the cycloaddition reaction of these species.

Chiral NHC-Catalyzed [4+2] Cycloadditions: Highly Diastereo/Enantioselective Access to Spiro[cyclohex-4-ene-1,3'-indoles] and DFT Calculations

A highly efficient NHC-catalyzed cycloaddition of (E)-alkenylisatins and γ-chloroenals with a broad substrate scope has been developed to provide spiro[cyclohex-4-ene-1,3'-indole] in good yields (up to 99 % yield) with excellent diastereo- and enantioselectivities (up to >20 : 1 d.r., >99 % ee) under mild conditions without the use of metal and additives. Based on computational investigations, the role of the NHC on the diastereo- and enantioselectivity is discussed.

N-Heterocyclic Carbene-Catalyzed Atroposelective Synthesis of Pyrrolo[3,4-b]pyridines with Configurationally Stable C-N Axial Chirality

The first atroposelective synthesis of pyrrolo[3,4-b]pyridines catalyzed by N-heterocyclic carbene has been achieved. A wide range of chiral atropisomers of pyrrolo[3,4-b]pyridines were obtained in high yields with excellent enantioselectivities (96-99% enantiomeric excess). The experimental results and density functional theory calculations showed that the C-N axial chirality of the product had high thermal stability.

Stereoselective synthesis of chiral sultam-fused dihydropyridinones via photopromoted NHC catalyzed [4 + 2] annulation

A photopromoted NHC catalyzed asymmetric [4+2] annulation of saccharine-derived azadienes and α-diazoketones was developed, affording the corresponding sultam-fused dihydropyridinones efficiently (up to 80% yield, 99% ee and >20 : 1 d.r.).

Unmasking the reverse reactivity of cyclic N-sulfonyl ketimines: multifaceted applications in organic synthesis

The chemistry related to the exploration of cyclic N-sulfonyl ketimines and their derivatives has attracted significant attention in the last few decades because of their intriguing structures and properties. They serve broadly as reactive synthons in various reactions to create a diverse set of synthetically and biologically attractive molecules. Furthermore, these moieties, which possess multiple heteroatoms (N, O and S), display or can enhance many biological activities. In the case of synthetic reactions, chemists mainly focus on the chemical manipulation of the highly reactive prochiral CN bond of N-sulfonyl ketimines. Besides their traditional role as electrophiles, N-sulfonyl ketimines possess α-Csp³-H protons, and thus behave as potential carbonucleophiles, where they can undergo several C-X (X = C, N and O) bond-forming reactions with different types of electrophiles under various conditions to form a wide range of fascinating asymmetric and non-asymmetric versions of fused heterocycles, carbocycles, spiro-fused skeletons, pyridines, pyrroles, etc. Herein, we highlight the recent examples from our research work and others covering the scope of cyclic N-sulfonyl ketimines as useful carbonucleophiles. In addition, the detailed mechanistic studies of the above-mentioned reactions are also presented.

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones to access chromeno[2,3-b]pyridinones

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones has been successfully developed. A structurally diverse set of chromeno[2,3-b]pyridinones was efficiently constructed in acceptable to excellent yields. The reaction features mild reaction conditions, a broad substrate scope, and easy scale-up.

Construction of 2-pyridones via oxidative cyclization of enamides: access to Pechmann dye derivatives

An efficient protocol for the construction of structurally diverse 2-pyridone derivatives from imines and α,β-unsaturated acid chlorides in a single operation is reported. The target compounds, including coumarin-8-oxoprotoberbine analogues and lamellarin G isomers, were prepared via thermal cyclization of the in situ generated enamides followed by thermal dehydrogenation. The cyclization of enamides was achieved by the introduction of an electron-withdrawing group on the α-carbon of acid chlorides. This methodology allows quick access to polycyclic Pechmann dyes via rare double oxidative cyclizations of dienamides under mild conditions.

N-Heterocyclic carbene catalyzed asymmetric [3 + 3] cycloaddtion of β,β-disubstituted, α,β-unsaturated carboxylic esters with 3-aminobenzofurans

An NHC-catalyzed β-carbon functionalization reaction to afford enantioenriched benzofuran fused δ-lactams bearing an all-carbon quaternary stereocenter is documented.

Imines as acceptors and donors in N-heterocyclic carbene (NHC) organocatalysis

The synthetic potential of imines as electrophiles or as a source of nucleophilic coupling partner in N-heterocyclic carbene (NHC) catalysis for the synthesis of various nitrogen heterocycles and functionalized amines is highlighted in this Feature Article. Electrophilic imines are suitable candidates for intercepting the NHC-derived acyl anions, homoenolate equivalents, and (di)enolates for the synthesis of α-amino ketones and a variety of lactam derivatives. Moreover, enamines generated from imines bearing α-hydrogen could be trapped with α,β-unsaturated acylazoliums for the synthesis of functionalized dihydropyridinones. NHCs are also useful for the umpolung of imines for the generation of aza-Breslow intermediates thus leading to the synthesis of indoles, quinolines, dihydroquinoxalines etc. A concise account of the diverse reactivity of imines in NHC catalysis has been presented.

Highly Chemoselective and Enantioselective Synthesis of 3,4-2H-Pyrindin-2-ones by an NHC-Catalyzed [3 + 3] Cyclization

A highly chemoselective and enantioselective cyclization of γ-chloroenals and ketimines has been developed to synthesize enantiopure 3,4-2H-pyrindin-2-ones as major products. It is proposed that the intermediate enone IV reacted with an enamine to proceed with a [3 + 3] cyclization, thereby affording 3,4-2H-pyrindin-2-ones as major products. Interestingly, the addition of LiCl promoted the formation of the enamine and accelerated the [3 + 3] cyclization. In contrast, the [4 + 2] cycloaddition reaction between the intermediate vinyl enolate VIII and an imine offered 5,6-2H-pyrindin-2-ones as minor products. This protocol represents the exceptional potential of N-heterocyclic carbene (NHC) catalytic reactions in accessing biologically active 3,4-2H-pyrindin-2-one derivatives in good yield with high chemoselectivities and excellent enantiomeric purities.

Enantioselective NHC-Catalyzed [3+3] Annulation of α-Bromoenals with 2-Aminobenzimidazoles

A chiral carbene-catalyzed [3+3] annulation of α-bromoenals with 2-aminobenzimidazoles providing pyrimido[1,2-a]benzimidazoles has been described. This protocol features a broad scope and good functional group tolerance. Biological studies indicated that the formed pyrimido[1,2-a]benzimidazole exhibited moderate cytotoxic activity against tumor cells.

Iodine(iii) promoted ring-rearrangement reaction of 1-arylamino-2-oxocyclopentane-1-carbonitriles to synthesize N-aryl-δ-valerolactams

Environmentally benign hypervalent iodine(iii) has been utilised selectively for carbocyclic to heterocyclic ring transformation under various electronic conditions with detailed structural and mechanistic investigation.

Facile Synthesis of 2,2-Diacyl Spirocyclohexanones via an N-Heterocyclic Carbene-Catalyzed Formal [3C + 3C] Annulation

A novel strategy for the construction of 2,2-diacyl spirocyclohexanones 3 has been demonstrated on the basis of an NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates with 2-ethylidene 1,3-indandiones. Furthermore, enantioenriched 3 was obtained in good to excellent yields with good enantioselectivities when chiral N-heterocyclic carbene (NHC) was employed. Notably, ring opening of the resulting 2,2-diacyl spirocyclohexanones 3 with hydrazine led to the formation of phthalazinones in good to excellent yields.

Asymmetric organocatalyzed reaction sequence of 2-hydroxy cinnamaldehydes and acyclic N-sulfonyl ketimines to construct diverse chiral bridged polycyclic aminals

2-Hydroxy cinnamaldehydes and acyclic N-sulfonyl ketimines were used as multisite substrates in an iminium catalysis triggered sequential process, leading to diverse chiral bridged polycyclic aminals in a highly regio- and stereoselective manner.

(Dynamic) Kinetic Resolution of Enamines/Imines: Enantioselective N-Heterocyclic Carbene Catalyzed [3+3] Annulation of Bromoenals and Enamines/Imines

The enantioselective N-heterocyclic carbene catalyzed [3+3] annulation of α-bromoenals by dynamic kinetic resolution (DKR) of enamines and normal resolution of α,α-disubstituted imines were developed. The corresponding substituted dihydropyridones were isolated in good yields with excellent diastereo- and enantioselectivities, and a high selective factor (up to 83) was realized for the resolution of α,α-disubstituted imines.

Chiral NHC-catalyzed 1,3-dipolar [3 + 2] cycloaddition of azomethine imines with α-chloroaldehydes for the synthesis of bicyclic pyrazolidinones

An NHC-catalyzed [3 + 2] cycloaddition reaction of azomethine imines and α-chloroaldehydes was developed for the synthesis of chiral bicyclic pyrazolidinone derivatives.

Cycloadduct formation of α,β-unsaturated esters with azides catalyzed by NHC systems

NHC-catalyzed cycloadduct formation of α,β-unsaturated esters with azides has been developed. This strategy could generate 1,2,3-triazoles and dihydropyrazoles with high yields and regioselectivities. The broad substrate scope and mechanistic survey of this process are also presented.

Carbene-catalyzed LUMO activation of alkyne esters for access to functional pyridines

A carbene-catalyzed activation of alkyne ester for quick access to functional pyridines is reported. The reaction involves a previously unexplored unsaturated acyl azolium intermediate bearing a carbon–carbon triple bond that shall exhibit rich chemical reactivities.

1-Hydroxybenzotriazole-Assisted, N-Heterocyclic Carbene Catalyzed β-Functionalization of Saturated Carboxylic Esters: Access to Spirooxindole Lactones

A 1-hydroxybenzotriazole-assisted, N-heterocyclic carbene catalyzed direct β-functionalization of saturated carboxylic esters is disclosed. This formal [3 + 2] annulation reaction of carboxylic esters with isatins affords optically pure spirooxindole lactones (on gram scale) bearing two vicinal stereogenic centers. A dual role of HOBt is proposed based on controlled experiments to rationalize the enhancement of diastereoselectivity and enantioselectivity.

Enantioselective Nucleophilic β-Carbon-Atom Amination of Enals: Carbene-Catalyzed Formal [3+2] Reactions

An enantioselective β-carbon amination for enals is disclosed. The nitrogen atom from a protected hydrazine with suitable electronic properties readily behaves as a nucleophile. Addition of the nitrogen nucleophile to a catalytically generated N-heterocyclic-carbene-bound α,β-unsaturated acyl azolium intermediate constructs a new carbon-nitrogen bond asymmetrically. The pyrazolidinone products from our catalytic reactions are common scaffolds in bioactive molecules, and can be easily transformed into useful compounds such as β(3) -amino-acid derivatives.

Formal [3 + 3] annulation of isatin-derived 2-bromoenals with 1,3-dicarbonyl compounds enabled by Lewis acid/N-heterocyclic carbene cooperative catalysis

Several novel isatin-derived 2-bromoenals were applied for formal [3 + 3] annulation with 1,3-dicarbonyl compounds enabled by NHC/Lewis acid cooperative catalysis.

Enantioselective N-heterocyclic carbene-catalyzed synthesis of saccharine-derived dihydropyridinones with cis-selectivity

The N-heterocyclic carbene-catalyzed [2 + 4] cyclocondensation of α-chloroaldehydes and saccharine-derived 1-azadienes was developed, giving dihydropyridinones in good yields with exclusive cis-selectivities and excellent enantioselectivities.