Catalytic asymmetric multiple dearomatizations of phenols enabled by a cascade 1,8-addition and Diels-Alder reaction

Asymmetric dearomatization of benzyl 1-naphthyl ethers via [1,3] O-to-C rearrangement

A catalytic asymmetric dearomatization reaction of benzyl 1-naphthyl ethers accelerated by a chiral N,N'-dioxide/Co(II) complex is disclosed. The reaction proceeds via an enantioselective [1,3] O-to-C rearrangement through a tight ion-pair pathway, providing a wide array of α-naphthalenone derivatives bearing an all-carbon quaternary center in high yields with excellent ee values.

Brønsted acid-catalyzed asymmetric dearomatization for synthesis of chiral fused polycyclic enone and indoline scaffolds

In the past two decades, substantial advances have been made on the asymmetric alkyne functionalization by the activation of inert alkynes. However, these asymmetric transformations have so far been mostly limited to transition metal catalysis, and chiral Brønsted acid-catalyzed examples are rarely explored. Here, we report a chiral Brønsted acid-catalyzed dearomatization reaction of phenol- and indole-tethered homopropargyl amines, allowing the practical and atom-economical synthesis of a diverse array of valuable fused polycyclic enones and indolines bearing a chiral quaternary carbon stereocenter and two contiguous stereogenic centers in moderate to good yields with excellent diastereoselectivities and generally excellent enantioselectivities (up to >99% enantiomeric excess). This protocol demonstrates Brønsted acid-catalyzed asymmetric dearomatizations via vinylidene-quinone methides.

C-O Coupling/[4+2] Cycloaddition Tandem Reactions via Oxidative Dearomatization of BINOLs: Access to Bridged Polycyclic Compounds

Intramolecular C-H activation/C-O coupling, dearomatization, and [4+2] cycloaddition of BINOL units have been well developed in a one-pot approach with maleimide derivatives as the dienophiles. This tandem catalytic system generates a variety of functionalized bridged polycyclic products in a step-economical manner, which greatly enriches the modification methods and strategies for the BINOL skeletons.

Bismuth(III)-Catalyzed 1,8-Addition/Cyclization/Rearrangement of Propargylic para-Quinone Methides with 2-Vinylphenol: Synthesis of Indeno[2,1-c]chromenes

The unique reactivity of in situ generated propargylic para-quinone methides as a new type of five-carbon synthon has been discovered by a novel bismuth(III)-catalyzed tandem annulation reaction. This 1,8-addition/cyclization/rearrangement cyclization cascade reaction is characterized by unusual structural reconstruction of 2-vinylphenol, involving cleavage of the C1'═C2' bond and formation of four new bonds. This method provides a convenient and mild approach to generate synthetically important functionalized indeno[2,1-c]chromenes. The mechanism of the reaction is proposed from several control experiments.

Design and synthesis of axially chiral aryl-pyrroloindoles via the strategy of organocatalytic asymmetric (2 + 3) cyclization

The catalytic asymmetric construction of axially chiral indole-based frameworks is an important area of research due to the unique characteristics of such frameworks. Nevertheless, research in this area is still in its infancy and has some challenges, such as designing and constructing new classes of axially chiral indole-based scaffolds and developing their applications in chiral catalysts, ligands, etc. To overcome these challenges, we present herein the design and atroposelective synthesis of aryl-pyrroloindoles as a new class of axially chiral indole-based scaffolds via the strategy of organocatalytic asymmetric (2 + 3) cyclization between 3-arylindoles and propargylic alcohols. More importantly, this new class of axially chiral scaffolds was derived into phosphines, which served as efficient chiral ligands in palladium-catalyzed asymmetric reactions. Moreover, theoretical calculations provided an in-depth understanding of the reaction mechanism. This work offers a new strategy for constructing axially chiral indole-based scaffolds, which are promising for finding more applications in asymmetric catalysis.

Catalyst-Controlled Divergent Reactions of 2,3-Disubstituted Indoles with Propargylic Alcohols: Synthesis of 3H-Benzo[b]azepines and Axially Chiral Tetrasubstituted Allenes

Catalyst-controlled divergent reactions of 2,3-disubstituted indoles with propargylic alcohols were developed for the first time. In the presence of TsOH or B(C₆F₅)₃ as catalyst, 2,3-disubstituted indoles reacted smoothly with 3-alkynyl-3-hydroxyisoindolinones to afford 3H-benzo[b]azepines by selective C2(sp²)-C3(sp²) ring expansion of indoles. In contrast, decreasing the catalyst strength (e.g., with chiral phosphoric acid) interrupted the cascade reactions, affording axially chiral tetrasubstituted allenes bearing an adjacent chiral quaternary carbon stereocenter. Control experiments provided insights into the reaction mechanism.

Organocatalytic Enantioselective 1,10-Addition of Alkynyl Indole Imine Methides with Thiazolones: An Access to Axially Chiral Tetrasubstituted Allenes

An asymmetric organocatalytic remote 1,10-addition of alkynyl indole imine methides generated in situ from α-(6-indolyl) propargylic alcohols with thiazolones has been developed for the first time, affording axially chiral tetrasubstituted allenes featuring vicinal sulfur-containing quaternary carbon stereocenters in high yields with excellent stereoselectivities. The representative scale-up reaction and transformations of the 1,10-adduct were examined. The reaction mechanism was expounded by control experiments and DFT calculations.

Dearomative Michael addition involving enals and 2-nitrobenzofurans realized under NHC-catalysis

In this manuscript, the first enantioselective dearomative Michael addition between α,β-unsaturated aldehydes and 2-nitrobenzofurans realized under N-heterocyclic carbene activation has been described. The reaction proceeds via addition of homoenolate to Michael acceptors leading to the formation of biologically important heterocycles with high yields and stereoselectivities. Their functionalization potential has been confirmed in selected, diastereoselective transformations.

Recent Applications of the Diels–Alder Reaction in the Synthesis of Natural Products (2017–2020)

The Diels–Alder reaction has long been established as an extremely useful procedure in the toolbox of natural product chemists. It tolerates a wide spectrum of building blocks of different complexity and degrees of derivatization, and enables the formation of six-membered rings with well-defined stereochemistry. In recent years, many total syntheses of natural products have been reported that rely, at some point, on the use of a [4+2]-cycloaddition step. Among classic approaches, several modifications of the Diels–Alder reaction, such as hetero-Diels–Alder reactions, dehydro-Diels–Alder reactions and domino-Diels–Alder reactions, have been employed to extend the scope of this process in the synthesis of natural products. Our short review covers applications of the Diels–Alder reaction in natural product syntheses between 2017 and 2020, as well as selected methodologies which are inspired by, or that can be used to access natural products.

1 Introduction

2 Syntheses from 2017

3 Syntheses from 2018

4 Syntheses from 2019

5 Syntheses from 2020

6 Conclusion

Chiral phosphoric acid-catalyzed regio- and enantioselective reactions of functionalized propargylic alcohols

Chiral phosphoric acid has been utilized for covalent activation of propargylic alcohols to act as pre-catalyst. With this activation mode, a range of highly regio- and enantioenriched heterocyclic products could be generated efficiently from racemic propargylic alcohols.

Metal-free dearomatization reactions of naphthol-ynamides for the divergent and enantioselective synthesis of azaspirocycles

An efficient Brønsted acid (BA) catalyzed intramolecular dearomatization cyclization of naphthol-ynamides has been developed, enabling the practical and divergent synthesis of two azaspirocycles in high yields.

Dearomatization of 2,3-Disubstituted Indoles via 1,8-Addition of Propargylic (Aza)-para-Quinone Methides

Dearomatization of indole is a useful strategy to access indolimines: a motif widely exists in biologically active molecules and natural products. Herein, an efficient method for the dearomatization of 2,3-disubstituted indoles to generate diverse indolimines with tetrasubstituted allenes is described. This work accomplishes dearomatization of 2,3-disubstituted indoles through 1,8-addition of (aza)-para-quinone methides, which are generated in situ from propargylic alcohols. A series of synthetically useful indolimines containing quaternary carbon centers and tetrasubstituted allenes can be accessed in good yields (up to 99%). Additionally, the separability of product isomers, diversified product transformations, and easy scale-up of the reaction demonstrate the potential application of this method.

Brønsted Acid Catalyzed Dearomatization by Intramolecular Hydroalkoxylation/Claisen Rearrangement: Diastereo- and Enantioselective Synthesis of Spirolactams

Described herein is a novel Brønsted acid catalyzed intramolecular hydroalkoxylation/Claisen rearrangement, allowing the practical and atom-economic synthesis of a range of valuable spirolactams from readily available ynamides in generally good to excellent yields with excellent diastereoselectivities and broad substrate scope. Importantly, an unexpected dearomatization of nonactivated arenes and heteroaromatic compounds is involved in this tandem sequence. Moreover, an asymmetric version of this tandem cyclization was also achieved by efficient kinetic resolution by chiral phosphoric acid catalysis. In addition, the [3,3]-rearrangement is shown to be kinetically preferred over the related [1,3]-rearrangement by theoretical calculations.

Asymmetric dearomatization catalysed by chiral Brønsted acids via activation of ynamides

Chiral Brønsted acid-catalysed asymmetric synthesis has received tremendous interest over the past decades, and numerous efficient synthetic methods have been developed based on this approach. However, the use of chiral Brønsted acids in these reactions is mostly limited to the activation of imine and carbonyl moieties, and the direct activation of carbon-carbon triple bonds has so far not been invoked. Here we show that chiral Brønsted acids enable the catalytic asymmetric dearomatization reactions of naphthol-, phenol- and pyrrole-ynamides by the direct activation of alkynes. This method leads to the practical and atom-economic construction of various valuable spirocyclic enones and 2H-pyrroles that bear a chiral quaternary carbon stereocentre in generally good-to-excellent yields with excellent chemo-, regio- and enantioselectivities. The activation mode of chiral Brønsted acid catalysis revealed in this study is expected to be of broad utility in catalytic asymmetric reactions that involve ynamides and the related heteroatom-substituted alkynes.

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.

FeTPPCl/FeCl3 Co-Catalyzed One-Pot Green Synthesis of α-Diaryl-β-alkynol Derivatives via Propargylic Carbocation Chemistry

A green and highly efficient one-pot method for α-diaryl-β-alkynol derivatives in water at room temperature was developed using the cocatalysis of a Lewis acid and meso-tetraphenylporphyrin iron(III) chloride (FeTPPCl). The unprecedented transformation was promoted by a modulation of the charge properties of propargylic carbocation chemistry and the use of an in situ-generated oxonium ylide as a matching nucleophile. The reaction was performed in water at room temperature with a highly step-economic manipulation in good to excellent yields and with a broad substrate scope. Water also acts as the third reactant for the one-pot transformation. Notably, the FeTPPCl catalyst can be directly reused four times with a slight discount in yields.

Tyrosine-Specific Modification via a Dearomatization-Rearomatization Strategy: Access to Azobenzene Functionalized Peptides

Azobenzene functionalized peptides are of great importance in photoresponsive biosystems and photopharmacology. Herein, we report an efficient approach to prepare azobenzene functionalized peptides through late-stage modification of tyrosine-containing peptides using a dearomatization-rearomatization strategy. This approach shows good chemoselectivity and site selectivity as well as sensitive group tolerance to various peptides. This method enriches the postsynthetic modification toolbox of peptides and has great potential to be applied in medicinal chemistry and chemical biology.

Formal (3 + 4)-Annulation of Propargylic p-Quinone Methides with 2-Indolylmethanols: Synthesis of Polysubstituted Indole-Fused Oxepines

A novel Brønsted acid catalyzed 1,8-addition mediated (3 + 4)-annulation of in situ generated propargylic p-quinone methides with 2-indolylmethanols is described. This method provides a convenient and mild approach to structurally interesting and synthetically important polysubstituted indole-fused oxepines in high yields. Moreover, 2-indolylmethanols as four-atom synthons in the (3 + 4)-annulations under Brønsted acid conditions have been explored for the first time.

Brønsted Acid-Catalyzed Formal (3+3)-Annulation of Propargylic (Aza)-para-Quinone Methides with 4-Hydroxycoumarins and 1,3-Dicarbonyl Compounds

Herein, we describe a highly effective 1,8-conjugate-addition-mediated formal (3+3)-annulation of (aza)-para-quinone methides in situ generated from propargylic alcohols with 4-hydroxycoumarins and 1,3-dicarbonyl compounds under the catalysis of a Brønsted acid. This methodology affords efficient and practical access to synthetically important and highly functionalized pyranocoumarins and pyrans in excellent yields under mild conditions. Importantly, these products exhibit impressive inhibitory activity toward α-glucosidase.

Asymmetric Intramolecular Dearomatization of Nonactivated Arenes with Ynamides for Rapid Assembly of Fused Ring System under Silver Catalysis

Arene dearomatization is a straightforward method for converting an aromatic feedstock into functionalized carbocycles. Enantioselective dearomatizations of chemically inert arenes, however, are quite limited and underexplored relative to those of phenols and indoles. We developed a method for diazo-free generation of silver-carbene species from an ynamide and applied it to the dearomatization of nonactivated arenes. Transiently generated norcaradiene could be trapped by intermolecular [4 + 2] cycloaddition, synthesizing polycycles with five consecutive stereogenic centers. This protocol constitutes the first highly enantioselective reaction based on the diazo-free generation of silver-carbene species. Mechanistic investigations revealed a dearomatization followed by two different classes of pericyclic reactions, as well as the origin of the chemo- and enantioselectivity.

Synthesis of Pyrrolo[1,2-a]indoles via (3+2)-Annulations of (Aza)-para-Quinone Methides with Indoles

An efficient and straightforward Brønsted acid mediated (3+2)-annulation of (aza)-para-quinone methides, generated in situ from propargylic alcohols and indoles, has been developed involving 1,8-conjugate addition/5-endo annulation cascade. This protocol affords a mild and effective method for the construction of synthetically important and structurally interesting functionalized pyrrolo[1,2-a]indoles.

Organocatalytic site- and stereoselective 1,6-additions of N-aryl-3-oxobutanamides to propargylic aza-p-quinone methides

A chiral phosphoric acid catalyzed site-selective 1,6-conjugate addition of N-aryl-3-oxobutanamides to in situ formed propargylic aza-p-quinone methides from propargylic alcohols has been established for the first time.

Syntheses of α-CF3-α-quaternary ketones via p-quinone methides and their derivatization to compounds with successively congested stereogenic centers

NHC-catalyzed 1,6-conjugate additions of a variety of aldehydes to δ-CF₃-δ-substituted p-quinone methides and their highly diastereoselective derivatization to compounds with successively congested stereogenic centers were performed.