Redox-Triggered Switchable Synthesis of 3,4-Dihydroquinolin-2(1H)-one Derivatives via Hydride Transfer/N-Dealkylation/N-Acylation

Diversity-oriented synthesis of indole-fused scaffolds and bis(indolyl)methane from tosyl-protected tryptamine

An efficient, diversity-oriented synthesis of indole-1,2-fused 1,4-benzodiazepines, tetrahydro-β-carbolines, and 2,2'-bis(indolyl)methanes was established starting from tosyl-protected tryptamine. These diverse privileged skeletons were controllably constructed by adjusting different hydride donors and Brønsted acids. A variety of indole-1,2-fused 1,4-benzodiazepines were facilely accessed using benzaldehydes bearing cyclic amines as hydride donors via a cascade N-alkylation/dehydration/[1,5]-hydride transfer/Friedel-Crafts alkylation sequence. The reaction site could be switched when benzaldehydes bearing an alkoxy moiety as hydride donors were used for the generation of tetrahydro-β-carbolines. On the other hand, the switchable synthesis of 2,2'-bis(indolyl)methanes could be achieved as well by applying p-TsOH·H2O as a catalyst. The reactions feature mild conditions, simple and practical operation, excellent efficiency and the use of EtOH as a green solvent. Using the concept of diversity-oriented, reagent-based synthesis, the inexpensive feedstock tryptamine was efficiently converted to three different types of privileged scaffolds, which facilitates rapid compound library synthesis for accelerating drug discovery.

Controllable Synthesis of N- and O-Containing Heterocycles via Formal [3 + 2] and [5 + 2] Cyclizations

The controllable synthesis of spirooxindole-dihydrofurans and spirooxindole-benzazepines was developed through formal [3 + 2] and [5 + 2] cyclization reactions from 2-(2-oxoindolin-3-yl)malononitriles and ortho-aminobenzaldehydes, respectively. A variety of spirooxindole-benzazepines were facilely constructed via a furan ring-open-involved hydride transfer/cyclization process. It is noteworthy that the application of the hydride-transfer-involved [5 + 2] cyclization strategy for construction of spirobenzazepines was unprecedented. In addition, the spiro N- and O-containing heterocycles were highly functionalized by amino, amide, and cyano groups, which were conducive to late-stage functionalization.

Controllable Synthesis of N-Heterocycles via Hydride Transfer Strategy-Enabled Formal [5 + 1] and [5 + 2] Cyclizations

The Brønsted acid-controlled switchable synthesis of indoline-fused tetrahydroquinolines and indole-fused benzazepines was developed through hydride transfer-enabled formal [5 + 1] and [5 + 2] cyclization reactions from indoles and N-alkyl o-aminobenzoketones. Indoline, furanone, and tetrahydroquinoline hybridized pentacyclic products were unprecedentedly accessed via a cascade condensation/hydride transfer/dearomatization-cyclization/deethylation/nucleophilic addition process. In addition, the undeveloped hydride transfer-involved [5 + 2] cyclizations were also realized for direct construction of indole-fused benzazepines.

Alkyl amines and ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions

The use of alkyl amines and ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions represents a promising strategy that greatly enriches redox-neutral hydride transfer chemistry. This review summarizes the remarkable progress made in this field, and focuses on (1) alkyl amines as traceless hydride donors in cascade [1,5]-hydride transfer/elimination reactions and (2) alkyl ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions. The reaction mechanisms, features, scope, limitations, and synthetic applications are included, where appropriate. Importantly, its powerful ability in allene synthesis and the combination with [Re]-vinylidene and carbocation chemistries render this strategy attractive enough to inspire chemists to develop colorful reactions for building molecular complexity.

Aromatization-driven cascade [1,5]-hydride transfer/cyclization for synthesis of spirochromanes

An aromatization-driven hydride transfer-involved α-C(sp3)–H bond functionalization of the oxygen atom was developed. Easily prepared p-quinone methides were applied to initiate [1,5]-hydride transfer/cyclization for generating spirochromanes.

Lewis Acid-Catalyzed Chemodivergent and Regiospecific Reaction of Phenols with Quaternary Peroxyoxindoles

The indium-catalyzed regiospecific coupling of substituted phenol derivatives and quaternary peroxyoxindoles for the synthesis of C2 or C4 benzoxazin-3-one-substituted phenols via skeletal rearrangement is described. This reaction is demonstrated with 17 examples with good yields and diverse aryl substituents. In contrast to the indium-catalyzed reaction, the Cu(OTf)₂-catalyzed reaction of the phenol with quaternary peroxyoxindoles afforded C2 or C4 2-oxindole-substituted phenol derivatives. This diverse catalytic reaction generated various biologically important phenol-substituted 2-oxindole derivatives directly without any skeleton rearrangement and was demonstrated with 19 examples in high yield. The regiospecificity and the reaction pathways were explained with the support of density functional theory (DFT).

[1,5]-Hydride Shift Triggered N-Dealkylative Cyclization into 2-Oxo-1,2,3,4-tetrahydroquinoline-3-carboxylates via Boronate Complexes

A new simple one-pot two-step protocol for the synthesis of 2-oxo-1,2,3,4-tetrahydroquinoline-3-carboxylate from 2-(2-(benzylamino)benzylidene)malonate under the action of BF3·Et2O was developed. It was shown that the reaction proceeds through the formation of a stable iminium intermediate containing a difluoroboryl bridge in the dicarbonyl fragment of the molecule.

Selective Reduction of Quinolinones Promoted by a SmI2/H2O/MeOH System

The selective reduction of quinolin-2(1H)-ones promoted by a SmI₂/H₂O/MeOH system is reported for the first time. The reaction is effectively carried out to afford 3,4-dihydroquinoline-2(1H)-ones under mild conditions in a one-pot fashion with good to excellent yields.

Catalytic asymmetric Michael/cyclization reaction of 3-isothiocyanato thiobutyrolactone: an approach to the construction of a library of bispiro[pyrazolone-thiobutyrolactone] skeletons

Here, we demonstrate the first example of 3-isothiocyanato thiobutyrolactone serving as a useful building block in the Michael/cyclization reaction with alkylidene pyrazolones for the enantioselective construction of optically active structural bispiro[pyrazolone-thiobutyrolactone] skeletons containing three contiguous stereocenters with two spiroquaternary stereocenters. These products were smoothly afforded in up to 90% yield, >20 : 1 dr and >99% ee with chiral squaramide as the catalyst under mild conditions. Notably, this is also the first example of the merger of a spirocyclic pyrazolone scaffold with a spirocyclic thiobutyrolactone scaffold, potentially useful in medicinal chemistry.

Advances in chromone-based reactants in the ring opening and skeletal reconstruction reaction: access to skeletally diverse salicyloylbenzene/heterocycle derivatives

Salicyloylbenzene/heterocycles are privileged scaffolds found in many natural products and bioactive molecules. Numerous useful approaches for the preparation of these privileged scaffolds have been developed in recent years. Among these approaches, chromone-based reactants have demonstrated their importance in the synthesis of these salicyloylbenzene/heterocycle scaffolds with structural complexity and potential biological appeal. In this review, the recent advances in the synthesis of salicyloylbenzene/heterocycles are summarized and discussed according to the chromone-based reactants which could be achieved in one step via ring-opening and skeletal reconstruction reactions. Both the mechanisms and the applications of the corresponding products in organic and medicinal chemistry are also described.

A combination of polarity reversal, Diels-Alder cycloaddition and skeletal remodeling to access pyridine-fused nitrones

An unprecedented cascade strategy consisting of polarity reversal, normal electron-demand Diels-Alder cycloaddition and skeletal remodeling was developed to construct novel pyridine-fused nitrones in up to 82% yield. The key to the success was the umpolung process, which transformed the electron-deficient 3-nitropyridinium ring into a reactive, π-extended cyclic nitroalkene, serving as a rarely reported hetero-diene to participate in normal Diels-Alder cycloadditions.

The Cascade [1,5]-Hydride Shift/Intramolecular C(sp3)–H Activation: A Powerful Approach to the Construction of Spiro-Tetrahydroquinoline Skeleton

The direct functionalization of inert C–H bonds is regarded as one of the most powerful strategies to form various chemical bonds and construct complex structures. Although significant advancements have been witnessed in the area of transition metal-catalyzed functionalization of inert C–H bonds, several challenges, such as the utilization and removal of expensive transition metal complexes, limited substrate scope and large-scale capacity, and poor atom economy in removing guiding groups coordinated to the transition metal, cannot fully fulfill the high standard of modern green chemistry nowadays. Over the past decades, due to its inherent advantage compared with a transition metal-catalyzed strategy, the hydride shift activation that applies “tert-amino effect” into the direct functionalization of the common and omnipresent C(sp³)–H bonds adjacent to tert-amines has attracted much attention from the chemists. In particular, the intramolecular [1,5]-hydride shift activation, as the most common hydride shift mode, enables the rapid and effective production of multifunctionally complex frameworks, especially the spiro-tetrahydroquinoline derivatives, which are widely found in biologically active natural products and pharmaceuticals. Although great accomplishments have been achieved in this promising field, rarely an updated review has systematically summarized these important progresses despite scattered reports documented in several reviews. Hence, in this review, we will summarize the significant advances in the cascade [1,5]-hydride shift/intramolecular C(sp³)-H functionalization from the perspective of “tert-amino effect” to build a spiro-tetrahydroquinoline skeleton, and the content is categorized by structure type of final spiro-tetrahydroquinoline products containing various pharmaceutical units. Besides, current limitations as well as future directions in this field are also pointed out. We hope our review could provide a quick look into and offer some inspiration for the research on hydride shift strategy in the future.

Ring opening and skeletal reconstruction of 3-vinyl benzofuranone-chromone synthons: catalyst-free access to skeletally-diverse 2-pyridone and optically active imidazoline derivatives

Herein is reported the first example of ring opening and skeletal reconstruction of 3-vinyl benzofuranone-chromones 1 as versatile synthons, which can react with ammonia or primary aliphatic amines as binucleophiles, for the eco-friendly and atom-economical synthesis of diverse and functionalized 2-pyridones 3 with potential biological activity in good to excellent yields (77-93%). When using optically active 1,2-diphenylethylenediamine 2 as the binucleophile, the in situ generated 2-pyridone intermediates are successfully transformed to novel optically active functionalized imidazoline derivatives 4 with high efficiency (up to 87% yield). In particular, this is the first report on the catalyst-free intramolecular cyclization occurring between an amide and a primary aliphatic amine for the construction of imidazoline molecules.

Switchable construction of oxa-heterocycles with diverse ring sizes via chemoselective cyclization controlled by dibrominated compounds

Switchable construction of oxa-heterocycles with diverse ring sizes has been developed by performing dibrominated-compound-controlled chemoselective cyclization and subsequent derivatization.

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...

Regio- and stereoselective synthesis and evaluation of densely functionalized bispiro[oxindole-isoxazole-indandione] hybrids as anticancer agents

This is the combination of three key pharmacophores through bispiro quaternary carbon atoms and evaluation of their anticancer activity.

3-Vinyl oxindole-chromone synthon as a skeletal reconstruction reactant for the synthesis of 2-hydroxy benzoyl pyridones

The first example of 3-vinyl oxindole-chromones in the ring opening and recyclization reaction.

Redox-triggered dearomative [5 + 1] annulation of indoles with O-alkyl ortho-oxybenzaldehydes for the synthesis of spirochromanes

The dearomative [5 + 1] annulation of 2-methylindoles with new five-membered synthon was developed through cascade [1,5]-hydride transfer/dearomative cyclization in HFIP for the synthesis of spirochromanes bearing the 2-methylindolenine skeleton.

HFIP-mediated three-component imidization of electron-rich arenes with in situ formed spiroindolenines for facile construction of 2-arylspiroindolenines

The three-component reaction of o-aminobenzaldehydes with 5-hydroxyindole and electron-rich arenes has been achieved through HFIP-mediated cascade hydride transfer/dearomative cyclization/CDC-type imidization at room temperature under air.

Design, synthesis and evaluation of structurally diverse ortho-acylphenol-diindolylmethane hybrids as anticancer agents

A highly efficient synthesis of structurally diverse ortho-acylphenol–diindolylmethane hybrids 3 using carboxylic acid-activated chromones as versatile synthetic building blocks is reported here for the first time, through 1,4-nucleophilic addition and followed by a decarboxylation and pyrone ring opening reaction process.

Merging dearomatization with redox-neutral C(sp3)–H functionalization via hydride transfer/cyclization: recent advances and perspectives

This review highlights the encouraging advances in hydride transfer-involved dearomatization reaction during the past decade, the content of which is categorized according to the hydride acceptors, namely vinylogous imines and quinone methides.

Visible-Light Mediated Metal-Free 6π-Photocyclization of N-Acrylamides: Thioxanthone Triplet Energy Transfer Enables the Synthesis of 3,4-Dihydroquinolin-2-ones

An efficient thioxanthone-catalyzed triplet energy transfer process for the synthesis of 3,4-dihydroquinolin-2-ones via a 6π-photocyclization is reported. Featuring a rare example of a metal-free formal C(sp²)-H/C(sp³)-H arylation mediated by visible-light, this work hopes to inspire further interest in these small molecules as sustainable alternatives to existing transition-metal photocatalysts in related processes.

Synthesis of Dihydroquinolinone Derivatives via the Cascade Reaction of o-Silylaryl Triflates with Pyrazolidinones

Presented herein is a novel synthesis of dihydroquinolinone derivatives through an unprecedented cascade reaction of o-silylaryl triflates with pyrazolidinones. Mechanistically, the formation of the title products is believed to involve a cascade procedure including in situ formation of aryne and its addition with pyrazolidinone followed by N-N bond cleavage and intramolecular C-C bond formation/annulation. Compared with literature methods for the synthesis of dihydroquinolinones, this protocol has advantages such as multistep transformations accomplished in one pot, broad substrate scope, mild reaction conditions, and good tolerance of diverse functional groups. In addition, the products thus obtained demonstrated significant in vitro antiproliferative activity in selected human cancer cell lines.

Intermolecular Interception of α-Oxo Gold Carbenes of Nitroalkyne Cycloisomerization with 1,2-Benzo[d]isoxazole: Synthesis of Functionalized Quinazoline 1-Oxides

The known nitrogen-transfer reagent 1,2-benzo[d]isoxazole has been used to trap the postulated α-oxo gold carbene intermediate involved in the [Au]-catalyzed internal redox process of 2-alkynylnitrobenzenes. This process led us to develop a general convergent method for the synthesis of highly functionalized quinazoline 1-oxides.

Facile syntheses of tetrahydroquinolines and 1,2-dihydroquinolines via vinylogous cascade hydride transfer/cyclization

The medicinally significant 3-monosubstituted tetrahydroquinolines and 1,2-dihydroquinolines were controllably constructed via redox-neutral vinylogous cascade condensation/[1,5]-hydride transfer/cyclization in EtOH.