Asymmetric Diels–Alder Reaction of 3-Vinylindoles and Nitroolefins Promoted by Multiple Hydrogen Bonds

Hydrogen Bond Donor-Catalyzed One-Pot Transformations of 2,2-Disubstituted Epoxides: Synthesis of Functionalized Nitrile-Rich Derivatives

A practically intriguing catalytic domino methodology has been developed for the synthesis of highly functionalized pyran and ethene-1,1,2-tricarbonitrile derivatives in a single-pot operation. The gem-dicyano olefins and the corresponding epoxide were taken as the reactive partners in the presence of a hydrogen bond donor (HBD)-catalyzed condition. The reaction was found to be highly efficient in terms of the formation of sequential C-C and O-C bonds along with an exceptional CSp2-CSp coupling step through a metal-free organocatalytic pathway. This strategy has been further utilized on ester-substituted epoxides, although the results differ from those with gem-dicyano epoxides. The process remains versatile and effective across a wide range of substrates. This catalytic protocol has been proven to be very generalized with varieties of substrate scope. A low catalyst loading, ambient reaction conditions, and satisfactory yields of all of the products are the vital features of this approach. Moreover, the overall atom-economic outcome along with the synergistic reactivity pattern between the activated epoxide and the malononitrile derivatives is also very significant to address the originality of this process. Spectroscopic analysis is utilized to validate the mechanistic interpretation.

A Facile Approach to Tetracyclic Indolines: Highly Diastereoselective [4+2] Annulation of Indoles with Bicyclic N-Substituted Cyclobutanes

A new stereoselective [4+2] annulation method for constructing tetracyclic indolines by reacting indoles with bicyclic N-substituted cyclobutanes has been developed. Using Sc(OTf)3 as a catalyst, a series of tetracyclic indolines with four continued stereogenic carbon centers have been obtained in ≤86% yields as single diastereomers. This reaction offers an accessible way for the rapid construction of the core structures of biologically active natural products like paucidirinine, deethylibophyllidine, and ibophyllidine.

Syntheses of 3-(2-Nitrovinyl)-indoles, Benzo[a]carbazoles, Naphtho[2,1-a]carbazoles, and 1-Hydroxy-β-carbolines Lead to Identification of Antiproliferative Compounds Active under Hypoxia

Herein, we describe a novel reaction between C-2-substituted indoles and 2-nitroacetophenones leading to a variety of indole-containing heterocyclic scaffolds. At 60 °C in AcOH with H2SO4 as catalyst, C-2 aryl indoles give 3-(2-nitrovinyl)-indoles with high Z or E geometric selectivity depending on the type of substrate utilized. These compounds undergo an electrocyclization process in a sealed vial in a microwave apparatus in DMF at 250 °C to give benzo[a]carbazoles and naphtho[2,1-a]carbazoles depending on whether the C-2 aromatic moiety is phenyl or naphthyl. Utilization of 2-methylindoles in the reaction with 2-nitroacetophenones and performing the reaction in a sealed vial in a microwave apparatus in AcOH at 200 °C leads to 1-hydroxy-β-carbolines. Selected compounds from each scaffold were tested for antiproliferative activities against MDA-MB-231 triple-negative breast cancer cells under normoxic and hypoxic conditions, and three compounds belonging to the 3-(2-nitrovinyl)-indole and 1-hydroxy-β-carboline series were identified to have single-digit micromolar IC50 values.

Indole as a Versatile Building Block in Cycloaddition Reactions: Synthesis of Diverse Heterocyclic Frameworks

Indole, a ubiquitous and structurally versatile aromatic compound, has emerged as a key player in the synthesis of diverse heterocyclic frameworks via cycloaddition reactions. These reactions are completely atom-economical and, hence, are considered as green reactions. This review article provides a comprehensive overview of the pivotal role played by indole in the construction of complex and biologically relevant heterocyclic compounds. Here we explore the chemistry of indole-based cycloadditions, highlighting their synthetic utility in accessing a wide array of heterocyclic architectures, including cyclohepta[b]indoles, tetrahydrocarbazoles, tetrahydroindolo[3,2-c]quinoline, and indolines, among others. Additionally, we discuss the mechanistic insights that underpin these transformations, emphasizing the strategic importance of indole as a building block. The content of this article will certainly encourage the readers to explore more work in this area.

Rapid Construction of Vinyl Indomorphans by Rhenium Catalysis

Here, an efficient route for accessing the vinylindomorphan skeleton is achieved by rhenium(I) catalysis. This transformation involves the condensation of indoles and alkyne-linked cyclohexanones, followed by intramolecular annulation to build the [3.3.1] bicyclic structure. This protocol complements the synthesis of the structurally complex heterocycles bearing a vinyl indole moiety. In addition, the selected products exhibited moderate cytotoxicity toward human A549 cells.

Synthesis of Functionalized Five-Membered Heterocycles from Epoxides: A Hydrogen-Bond Donor Catalytic Approach

The synthesis of highly functionalized five-membered oxa- and aza-heterocycles has been reported utilizing hydrogen-bond donor (HBD) catalysis. In this method, an epoxide was taken as a substrate and reacted with functionalized arylidene/alkylidene malononitrile derivatives in the presence of a newly designed HBD catalyst. In all the cases, the products 2,5-disubstituted tetrahydrofurans (2,5-THFs) were obtained in good to excellent yields (up to 86%) with high diastereoselectivity (dr up to 99:1) as a single regioisomer. The stereochemistry at the 2- and 5-positions of the five-membered ring has been confirmed by single-crystal X-ray analysis, and cis is found to be the major product. The same strategy has been further utilized to obtain substituted oxazolidines whenever the epoxide has been reacted with isocyanate as an electrophile. In order to induce enantioselectivity, a chiral epoxide has been reacted with both the electrophiles in the presence of the same catalyst system to afford the single stereoisomer of the final products. This synthetic methodology involves a low catalyst loading and ambient reaction condition and has been generalized with various substituents present in the starting electrophiles to produce the resultant products in acceptable yields and stereoselectivity.

Organocatalytic Asymmetric [2 + 4] Cycloadditions of 3-Vinylindoles with ortho-Quinone Methides

Catalytic asymmetric [2 + 4] cycloadditions of 3-vinylindoles with ortho-quinone methides and their precursors were carried out in the presence of chiral phosphoric acid to afford a series of indole-containing chroman derivatives with structural diversity in overall high yields (up to 98%), good diastereoselectivities (up to 93:7 dr) and moderate to excellent enantioselectivities (up to 98% ee). This approach not only enriches the chemistry of catalytic asymmetric cycloadditions involving 3-vinylindoles but is also useful for synthesizing chiral chroman derivatives.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.

Application of 3-Alkyl-2-vinylindoles in Catalytic Asymmetric Dearomative (2+3) Cycloadditions

The first application of 3-alkyl-2-vinylindoles in catalytic asymmetric dearomative cycloadditions was established by chiral phosphoric acid (CPA)-catalyzed (2+3) cycloaddition with azoalkenes, leading to the generation of chiral pyrroloindolines bearing two tetrasubstituted stereogenic centers in good yields (61-96%) and excellent stereoselectivities (all >95:5 dr, 86-99% ee). This reaction has realized the first enantioselective dearomative cycloaddition of 3-alkyl-2-vinylindoles, which brings a new reactivity to this class of vinylindoles and will enrich the chemistry of 3-alkyl-2-vinylindoles. In addition, this approach has provided a useful strategy for the construction of enantioenriched pyrroloindoline skeletons bearing two tetrasubstituted stereogenic centers. More importantly, the bioassay of these chiral pyrroloindolines has revealed that some compounds exhibit strong anti-cancer activity against Hela and MCF-7 cell lines, which will be helpful for discovering anti-cancer drug candidates.

Advances in organocatalytic asymmetric reactions of vinylindoles: powerful access to enantioenriched indole derivatives

This review summarizes advances in vinylindole-based organocatalytic asymmetric reactions since 2008 and includes the applications of some methodologies in the total synthesis of natural products, points out remaining challenges in this research area.

Catalytic Asymmetric Formal [3+2] Cycloaddition of Azoalkenes with 3-Vinylindoles: Synthesis of 2,3-Dihydropyrroles

Chiral phosphoric acid-catalyzed highly enantioselective formal [3 + 2] cycloaddition reaction of azoalkenes with 3-vinylindoles has been established. Under mild conditions, the projected cycloaddition proceeded smoothly, affording a variety of 2,3-dihydropyrroles in high yields and excellent enantioselectivities, and also in a diastereospecific manner. As opposed to the common 4-atom synthons in the previous literature reports, azoalkenes served as 3-atom synthons. Besides, the observed selectivity was supported by primary theoretical calculation. The unique chemistry of azoalkenes disclosed herein will empower asymmetric synthesis of nitrogen-containing ring structural motifs in a broader context.

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Chiral phosphoric acid catalyzed formal [3 + 2] cycloaddition reaction

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2,3-Dihydropyrroles were enantioselectively synthesized

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Azoalkenes served as 3-atom synthons

Mechanistic Insight toward Understanding the Role of Charge in Thiourea Organocatalysis

Pyranylation and glycosylation are pivotal for accessing a myriad of natural products, pharmaceuticals, and drug candidates. Catalytic approaches for enabling these transformations are of utmost importance and integral to advancing this area of synthesis. In exploring this chemical space, a combined experimental and computational mechanistic study of pyranylation and 2-deoxygalactosylation catalyzed by a cationic thiourea organocatalyst is reported. To this end, a thiourea-cyclopropenium organocatalyst was employed as a model system in combination with an arsenal of mechanistic techniques, including ¹³C kinetic isotope effect experiments, deuterated labeling studies, variable-temperature ¹H NMR spectroscopy, and density functional theory calculations. From these studies, two distinct reaction pathways were identified for this transformation corresponding to either dual hydrogen bond (H-bond) activation or Brønsted acid catalysis. The former involving thiourea orchestrated bifurcated hydrogen bonding proceeded in an asynchronous concerted fashion. In contrast, the latter stepwise mechanism involving Brønsted acid catalysis hinged upon the formation of an oxocarbenium intermediate accompanied by subsequent alcohol addition.

Construction of chiral chroman scaffolds via catalytic asymmetric (4 + 2) cyclizations of para-quinone methide derivatives with 3-vinylindoles

The title reaction has been established in the presence of chiral phosphoric acid, affording chiral chroman derivatives bearing an indole moiety in high yields and with moderate to good stereoselectivities.

Efficiently diastereoselective synthesis of functionalized hydro-carbazoles by base-mediated tandem annulation of 1-(2-amino-aryl)prop-2-en-1-ones and sulfur ylide

The base-promoted [3 + 3]/[1 + 4] tandem reaction of tosyl-protected o-amino α,β-unsaturated ketones and crotonate-derived sulfur ylide is developed for efficiently diastereoselective synthesis of functionalized hydrocarbazoles.

Progresses in organocatalytic asymmetric dearomatization reactions of indole derivatives

This review summarizes the progresses in organocatalytic asymmetric dearomatization reactions of indole derivatives and their applications in total synthesis of natural products, and gives some insights into challenging issues in this research field.