Organocatalyzed Dearomative Cycloaddition of 2-Nitrobenzofurans and Isatin-Derived Morita–Baylis–Hillman Carbonates: Highly Stereoselective Construction of Cyclopenta[b]benzofuran Scaffolds

Recent advances in the reactions of isatin-derived MBH carbonates for the synthesis of spirooxindoles

As one of the main fragments in medical drugs, spirooxindole has received considerable attention from organic and medicinal chemists. In the past few decades, chemists have been searching for more straightforward and efficient methods to produce compounds containing a spirooxindole fragment. In this regard, isatin-derived Morita-Baylis-Hillman (MBH) carbonates have been widely used as versatile building blocks for the synthesis of spirooxindole structures. This review summarizes the reactions reported in recent years for the construction of the spirooxindole skeleton or C3 disubstituted oxindole derivatives using isatin-derived MBH carbonates and demonstrates the role of isatin-derived MBH carbonates in these reactions.

Enzyme-Catalyzed Dynamic Kinetic Resolution for the Asymmetric Synthesis of 2,3-Dihydrobenzofuran Esters and Evaluation of Their Anti-inflammatory Activity

Utilizing enzymes as biocatalysts, an alternative strategy has been developed for the highly enantioselective synthesis of chiral 2,3-dihydrobenzofuran (2,3-DHB) esters via the dynamic kinetic resolution of 2,3-dihydro-3-benzofuranols, which are generated from an intramolecular Aldol reaction. This protocol provides easy access to a series of 2,3-DHB ester derivatives, prodrugs, and allows for functional group transformations. Biological evaluation also indicates that some of the products exhibit potent anti-inflammatory activity.

Synthesis of Benzofuro[3,2-b]indol-3-one Derivatives via Dearomative (3 + 2) Cycloaddition of 2-Nitrobenzofurans and para-Quinamines

An efficient dearomative (3 + 2) cycloaddition of para-quinamines and 2-nitrobenzofurans has been developed. This reaction proceeds smoothly under mild conditions and affords a series of benzofuro[3,2-b]indol-3-one derivatives in good to excellent yields (up to 98%) with perfect diastereoselectivities (all cases > 20:1 dr). The scale-up synthesis and versatile derivatizations demonstrate the potential synthetic application of the protocol. A plausible reaction mechanism is also proposed to account for the observed reaction process. This work represents the first instance of the N-triggered dearomative (3 + 2) cycloaddition of 2-nitrobenzofurans.

Copper(I)-Catalyzed Dearomatization of Benzofurans with 2-(Chloromethyl)anilines through Radical Addition and Cyclization Cascade

Herein, we described a copper(I)-catalyzed dearomatization of benzofurans with 2-(chloromethyl)anilines to prepare various tetrahydrobenzofuro[3,2-b]quinolines and 2-(quinolin-2-yl)phenols in good to excellent yields through radical addition and an intramolecular cyclization process. Mechanistic studies revealed that 2-(chloromethyl)anilines served as radical precursors. The present method features broad substrate scope, good functional group tolerance, quinoline scaffold diversity, and radical addition dearomatization of benzofurans.

Dithioallyl Cations in Stereoselective Dearomative (3 + 2) Cycloadditions of Benzofurans: Mechanism and Synthetic Applications

A stereoselective dearomative cyclopentannulation of benzofurans is reported. A previously reported dearomative (3 + 2) cycloaddition of indoles with 1,4-dithiane-fused allyl cations was found to lack stereoselectivity when more substituted cyclopentene rings are targeted. However, for benzofuran substrates, excellent levels of stereoselectivity were observed for the same allyl cation reagents under very similar reaction conditions. In this full account, we provide a mechanistic rationale and some design principles that govern the stereoselectivity of the intriguing dearomative transformations using dithioallyl cations and demonstrate how the stereoselectivity depends on electronic factors of the starting materials. The stereoselective methodology is also applied in a straightforward dearomative synthesis of the tricyclic sesquiterpenoid natural product aplysin and its analogues, starting from a simple benzofuran.

Phosphine-catalysed denitrative rearomatising (3 + 2) annulation of α,β-ynones and 3-nitroindoles

We describe a metal-free strategy to access various α-arylidene cyclopenta[b]indoles via phosphine-catalysed (3 + 2) annulation of α,β-ynones and 3-nitroindoles. For the first time, the rearomatisation of the indole nucleus was observed in such an annulative transformation. The method was extended to the synthesis of an antimalarial natural product, bruceolline E.

Diastereoselective Synthesis of Dihydrobenzofuran-Fused Spiroindolizidines via Double-Dearomative [3 + 2] Cycloadditions

Spiroindolizidine oxindoles represent a kind of privileged scaffold in many biologically active natural alkaloids. 2,3-Dihydrobenzofuran derivatives exhibit significant bioactivities in a variety of pharmaceuticals. Herein, we assembled these two privileged fragments into a small molecule via double-dearomative [3 + 2] cycloadditions with pyridinium ylides and 2-nitrobenzofurans. This protocol features remarkable advantages including wide substrate scope, mild condition, high level of diastereoselectivities and yields. Thus, a collection of spiroindolizidine-fused dihydrobenzofurans/indolines were facilely produced efficiently.

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Substituted β-Naphthols with Morita-Baylis-Hillman (MBH) Adducts

Pd-catalyzed intermolecular asymmetric allylic dearomatization of substituted β-naphthol derivatives with Boc-protected Morita-Baylis-Hillman (MBH) adducts was developed. The reaction occurs smoothly in 1,4-dioxane at room temperature in the presence of [Pd(C₃H₅)Cl]₂ (2.5 mol %), (S, S_p)-PHOX ligand (5.5 mol %), and Li₂CO₃ (1.0 equiv). A series of dearomatized products were afforded in moderate to excellent yields and enantioselectivity (up to 99% yield, 97% ee). Furthermore, the compatibility with gram-scale reaction and mild conditions make the current method synthetically useful.

An Access to Highly Functionalized Dihydrobenzofuran Spirooxindole Scaffolds

We have developed an efficient protocol for the construction of polycyclic dihydrobenzofuran spirooxindole scaffolds via base promoted cascade annulation of Morita-Baylis-Hillman (MBH) carbonates of isatins with ortho-hydroxychalcones or ortho-hydroxy-β-nitrostyrenes. The complex polycyclic compounds were conveniently synthesized in satisfactory yields and with high diastereoselectivity. This protocol provides a swift and convenient approach for the assembly of diverse highly functionalized dihydrobenzofuran spirooxindoles and also features broad substrate scope, high molecular convergence, and excellent atomic economy.

Organocatalyzed asymmetric dearomative 1,3-dipolar cycloaddition of 2-nitrobenzofurans and N-2,2,2-trifluoroethylisatin ketimines

A readily available chiral cyclohexanediamine-derived bifunctional tertiary amine-squaramide catalyst is more effective for the asymmetric dearomative 1,3-dipolar cycloaddition of 2-nitrobenzofurans and N-2,2,2-trifluoroethylisatin ketimines. A range of structurally diverse spiro-fused polyheterocyclic compounds containing oxindole, pyrrolidine, and hydrobenzofuran motifs were smoothly obtained in excellent results (up to 99% yield, >20:1 dr in all cases and up to 99% ee). This method features high efficiency, mild reaction conditions, exquisite asymmetric induction, wide functional group tolerance, great potential for scale-up synthesis, and attractive product diversification.

Metal-Free Diastereo- and Enantioselective Dearomative Formal [3 + 2] Cycloaddition of 2-Nitrobenzofurans and Isocyanoacetate Esters

The diastereo- and enantioselective dearomative formal [3 + 2] cycloaddition of 2-nitrobenzofurans and α-aryl-α-isocyanoacetate esters provides tricyclic compounds bearing the 3a,8b-dihydro-1H-benzofuro[2,3-c]pyrrole framework with three consecutive stereogenic centers. The reaction was enabled by a cupreine-ether organocatalyst. The reaction products were obtained with almost full diastereoselectivity and with excellent enantiomeric excesses for a number of substituted 2-nitrobenzofurans and isocyanoacetates.

Organocatalytic Regio- and Enantioselective [3 + 2]-Annulations of Ninhydrin-Derived Morita-Baylis-Hillman Carbonates with 3-Methyleneoxindoles

A type of Morita-Baylis-Hillman (MBH) carbonates has been developed from ninhydrin. These MBH carbonates have been successfully employed as 3C-synthons in the organocatalytic asymmetric [3 + 2]-annulations of the isatin-derived electron-deficient olefins, affording structurally diverse spirooxindoles in high yield with excellent stereoselectivity. In particular, the regioselectivity of MBH carbonates was controlled by the reaction partner, 3-methyleneoxindoles with carbonyl groups (R = ArCO), affording β-selective products and 3-methyleneoxindoles with ester groups (R = CO₂Me) furnishing γ-selective products. The representative scale-up reactions and transformation of product were examined. The reaction mechanism was expounded by control experiments.

Ag-Catalyzed Asymmetric Interrupted Barton-Zard Reaction Enabling the Enantioselective Dearomatization of 2- and 3-Nitroindoles

We disclose a Ag-catalyzed asymmetric interrupted Barton-Zard reaction of α-aryl-substituted isocyanoacetates with 2- and 3-nitroindoles, which enables the dearomatization of nitroindoles and hence offers rapid access to an array of optically active tetrahydropyrrolo[3,4-b]indole derivatives bearing three contiguous stereogenic centers, including two tetrasubstituted chiral carbon atoms with pretty outcomes (up to 99% yield, 91:9 dr, and 96% ee). The synthetic potential of the protocol was showcased by the gram-scale reaction and versatile transformations of the product.

Asymmetric Decarboxylative [3+2] Cycloaddition for the Diastereo- and Enantioselective Synthesis of Spiro[2.4]heptanes via Cyclopropanation

Asymmetric cycloaddition reaction has emerged as one of the powful and reliable strategies for the construction of enantioenriched molecules, especially those with polycyclic frameworks. Herein, we report the asymmetric decarboxylative...

Higher-order [10 + 2] cycloaddition of 2-alkylidene-1-indanones enables the dearomatization of 3-nitroindoles: access to polycyclic cyclopenta[b]indoline derivatives

The higher-order [10 + 2] cycloaddition of 3-nitroindoles and 2-alkylidene-1-indanones enables the dearomatization of 3-nitroindoles and affords a range of structurally diverse cyclopenta[b]indolines with excellent results.

DBU-catalyzed dearomative annulation of 2-pyridylacetates with α,β-unsaturated pyrazolamides for the synthesis of multisubstituted 2,3-dihydro-4H-quinolizin-4-ones

DBU-catalyzed dearomative [3 + 3] annulation of 2-pyridylacetates and α,β-unsaturated pyrazolamides for the synthesis of multisubstituted 2,3-dihydro-4H-quinolizin-4-ones was developed.

Diastereoselective construction of a library of structural bispiro[butyrolactone/valerolactone-pyrrolidine-indanedione] hybrids via 1,3-dipolar cycloaddition reactions

Herein, a highly efficient strategy that allows the diversity synthesis of a library of structural bispiro[butyrolactone-pyrrolidine-indanedione] hybrids is achieved effectively by means of 1,3-dipolar cycloadditions of α,β-unsaturated butyrolactones/valerolactones as dipolarophiles...

Organocatalytic asymmetric synthesis of multifunctionalized α-carboline-spirooxindole hybrids that suppressed proliferation in colorectal cancer cells

An asymmetric organocatalytic cascade reaction has been reported for the rapid assembly of multifunctionalized α-carboline-spirooxindole hybrids, which suppressed proliferation in colorectal cancer cells.

Construction of Hydrodibenzo[b,d]furan Frameworks from Morita-Baylis-Hillman Carbonates of Isatins and o-Hydroxy Enones via Palladium and Brønsted Base Relay Catalysis

A cascade assembly between isatin-derived Morita-Baylis-Hillman carbonates and o-hydroxybenzylideneacetones has been developed under the relay catalysis of Pd(PPh₃)₄ and DBU, affording a spectrum of 1,2,3,4-tetrahydrodibenzo[b,d]furan architectures incorporating a spirooxindole motif in moderate to good yields with excellent diastereoselectivity. The fused indole analogues were similarly furnished by employing the benzylideneacetones having an o-TsNH group.

Lewis acid-catalyzed [3 + 2] annulations of oxindole based spirocyclic donor-acceptor cyclopropanes with ynamides

[3 + 2] annulations of oxindole based spirocyclic donor-acceptor cyclopropanes and ynamides catalyzed by copper triflate have been developed for the synthesis of biologically important spirocyclopenteneoxindoles. These reactions tolerated a wide scope of substrates and provided the desired products in good to high yields (up to 90%) with up to >40 : 1 diastereoselectivities under mild conditions.

Dinuclear Zinc Catalyzed Enantioselective Dearomatization [3+2] Annulation of 2-Nitrobenzofurans and 2-Nitrobenzothiophenes

The application of dinuclear zinc catalysts in a dearomatization reaction has been developed. Catalytic asymmetric dearomatization [3+2] annulations of 2-nitrobenzofurans or 2-nitrobenzothiophenes with CF₃ -containing N-unprotected isatin-derived azomethine ylides catalyzed by dinuclear zinc catalysts are realized with excellent diastereomer ratios (dr) of >20 : 1 and enantiomeric excess (ee) of up to 99 %. This protocol provides a practical, straightforward access to structurally diverse pyrrolidinyl spirooxindoles containing a 2,3-fused-dihydrobenzofuran (or dihydrobenzothiphene) moiety, and four contiguous stereocenters. Reactions can be performed on a gram scale. The absolute configuration of products is confirmed by X-ray single crystal structure analysis, and a possible mechanism is proposed.

Stereoselective Access to Spirooxindoles and Bisoxindoles Through Organocatalyzed Asymmetric Divergent Transformations of Isatin-derived MBH Carbonates

An interesting β-isoquinidine catalyzed divergent reaction was developed to produce either spirocyclopentene oxindoles, spirocyclopentadiene oxindoles or bisoxindoles in a high enantioselective fashion. The utility of this protocol was demonstrated by the versatile transformations of the products. This work not only represents the first highly stereoselective intermolecular catalytic asymmetric allylic alkylation reaction between two isatin-derived MBH carbonate molecules but also constitutes a rare example of isatin-derived MBH carbonate-based enantioselective and α-regioselective [3+2] cycloaddition reactions.

Silver-Catalyzed Asymmetric Dearomatization of Electron-Deficient Heteroarenes via Interrupted Barton-Zard Reaction

Herein we report a catalytic asymmetric dearomatization reaction of electron-deficient heteroarenes with α-substituted isocyanoacetates through an interrupted Barton-Zard reaction. A range of optically active pyrrolo[3,4-b]indole derivatives was obtained in good yields (up to 97 %) with high stereoselectivities (up to >20:1 dr and 97 % ee), using a catalytic system consisting of a cinchona-derived amino-phosphine and silver oxide. This reaction features wide substrate scope and mild conditions, and provides a new strategy for developing asymmetric dearomatization reactions.

Asymmetric Dearomative (3+2)-Cycloaddition Involving Nitro-Substituted Benzoheteroarenes under H-Bonding Catalysis

In our studies, the organocatalytic 1,3-dipolar cycloaddition between 2-nitrobenzofurans or 2-nitrobenzothiophene and N-2,2,2-trifluoroethyl-substituted isatin imines has been developed. The reaction has been realized by employing bifunctional organocatalysis, with the use of squaramide derivative being crucial for the stereochemical efficiency of the process. The usefulness of the cycloadducts obtained has been confirmed in selected transformations, including aromative and non-aromative removal of the nitro group.

Organocatalytic Asymmetric Cyclopropanation of 3-Acylcoumarins with 3-Halooxindoles: Access to Spirooxindole-cyclopropa[c]coumarin Compounds

A highly diastereo- and enantioselective cyclopropanation reaction of 3-acylcoumarins with 3-halooxindoles catalyzed by an organocatalyst through a [2 + 1] Michael/intramolecular cyclization process was developed. This scenario provides a facile strategy to access spirooxindole-cyclopropa[c]coumarin compounds bearing three continuous stereocenters, including two vicinal quaternary all-carbon stereocenters with high to excellent diastereo- and enantioselectivities. The HRMS study revealed the vital importance of the ammonium ylide intermediate in the catalytic process.

Dearomative 1,6-addition of P(O)–H to in situ formed p-QM-like ion pairs from 2-benzofuryl-ols to C3-phosphinoyl hydrobenzofurans

We report a dearomative C3-phosphorylation and a tandem C3-phosphorylation/aromatization of 2-benzofuryl-ols with P(O)–H species to afford C3-phosphinoyl hydrobenzofurans and benzofurans, respectively.

Catalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones: stereoselective construction of dihydrobenzofuran-bridged polycyclic skeletons

An organocatalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones is developed for the construction of dihydrobenzofuran-bridged polycyclic skeletons with good results.

Enantioselective [3 + 2] annulation of 3-hydroxymaleimides with quinone monoimines

Enantioselective [3 + 2] annulation of 3-hydroxymaleimides with quinone monoimines provided a large variety of succinimide fused indolines in moderate to good yields with moderate to good enantioselectivities.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Reaction condition-dependent divergent synthesis of spirooxindoles and bisoxindoles

DABCO-catalyzed divergent transformations of isatin-derived MBH carbonates leading to either spirocyclopentene oxindoles, spirocyclopentadiene 2-oxindoles or bisoxindoles are described.

Candida antarctica lipase-B-catalyzed kinetic resolution of 1,3-dialkyl-3-hydroxymethyl oxindoles

Candida antarctica (CAL-B) lipase-catalyzed resolution of 1,3-dialkyl-3-hydroxymethyl oxindoles has been performed to obtain (R)-1,3-dialkyl-3-acetoxymethyl oxindoles with up to 99% ee and (S)-1,3-dialkyl-3-hydroxymethyl oxindoles with up to 78% ee using vinyl acetate as acylating agent and acetonitrile as solvent transforming (S)-3-allyl-3-hydroxymethyl oxindole to (3S)-1'-benzyl-5-(iodomethyl)-4,5-dihydro-2H-spiro[furan-3,3'-indolin]-2'-one. The optically active 3-substituted-3-hydroxymethyl oxindoles and spiro-oxindoles are among the key synthons in the synthesis of potentially biologically active molecules.

Organocatalytic enantioselective tandem sulfa-Michael/aldol reaction to access dihydrothiopyran-fused benzosulfolane skeletons bearing three contiguous stereocenters

The first organocatalytic diastereo- and enantioselective tandem sulfa-Michael/aldol reaction of 2-mercaptoindole-3-carbaldehydes and 2-mercaptobenzaldehydes with benzo[b]thiophene sulfones was developed. With multiple hydrogen-bonding thiourea as a catalyst, a wide range of polycyclic dihydrothiopyran-fused benzosulfolanes were smoothly obtained with excellent results (up to 99% yield, >20 : 1 dr and 99% ee) under mild reaction conditions.

[4 + 2] Annulation Reaction of In Situ Generated Azoalkenes with Azlactones: Access to 4,5-Dihydropyridazin-3(2H)-Ones

An unprecedented [4 + 2] annulation reaction between in situ formed azoalkenes and azlactones has been developed. This reaction provides a facile access to an array of 4,5-dihydropyridazin-3(2H)-one derivatives, which are very promising in medicinal applications as potential biologically active candidates. Notably, these dihydropyridazinones could also be synthesized via a one-pot reaction protocol by using the in situ formed azlactones from N-acyl amino acids and in situ generated azoalkenes from α-halogeno hydrazones. The potential applications of the methodology were also demonstrated by gram-scale experiments and the versatile conversions of the products into other nitrogen-containing compounds.

Optically Active Flavaglines-Inspired Molecules by a Palladium-Catalyzed Decarboxylative Dearomative Asymmetric Allylic Alkylation

With the aid of a class of newly discovered Trost-type bisphosphine ligands bearing a chiral cycloalkane framework, the Pd-catalyzed decarboxylative dearomative asymmetric allylic alkylation (AAA) of benzofurans was achieved with high efficiency [0.2-1.0 mol% Pd₂(dba)₃/L], good generality, and high enantioselectivity (>30 examples, 82-99% yield and 90-96% ee). Moreover, a diversity-oriented synthesis (DOS) of previously unreachable flavaglines is disclosed. It features a reliable and scalable sequence of the freshly developed Tsuji-Trost-Stoltz AAA, a Wacker-Grubbs-Stoltz oxidation, an intra-benzoin condensation, and a conjugate addition, which allows the efficient construction of the challenging and compact cyclopenta[b]benzofuran scaffold with contiguous stereocenters. This strategy offers a new avenue for developing flavagline-based drugs.