Homodimerization of 3-substituted-2-oxindoles for the construction of vicinal all-carbon quaternary centers: chemical, photochemical and electrochemical approaches

Advancements in organic synthesis are revolutionizing the synthesis of complex natural products, which are essential in biomedical research and drug discovery due to their intricate structures. Natural products such as chimonanthine, folicanthine, calycanthine, psychotriadine, etc., with vicinal all-carbon quaternary stereocenters, are particularly significant for their strong binding properties and biological activities. One common feature of these natural products is the presence of dimeric 3-substituted-2-oxindoles having vicinal all-carbon quaternary stereocenters. This review focuses on the chemical, photochemical, and electrochemical approaches for the homodimerization of 3-substituted-2-oxindoles employed by different researchers, with a strong focus on the mechanistic details of proton-coupled electron transfer (PCET). The article also demonstrates that PCET facilitates the reduction of kinetic barriers through the formation of low-energy intermediates and the expansion of synthetic possibilities. Furthermore, natural product syntheses (folicanthine and chimonanthine) from dimeric 3-substituted-2-oxindoles are discussed. Chemical syntheses are time-consuming and, even more importantly, generate significant waste due to the use of metal-based oxidants and catalysts. In this regard, electrochemical synthesis methods offer promising solutions by avoiding the use of chemical oxidants and metal catalysts, thus minimizing environmental impact. The article also outlines the advantages and disadvantages of different synthesis methods and proposes a new direction for future research in this field.

Transforming 2D azolium salts to 3D caged tertiary amines via stereoselective dearomative cascade annulation

Three-dimensional fused-ring frameworks, especially those incorporating heteroatoms, are fundamental to expanding chemical space and unlocking unique properties critical for drug discovery and functional materials, yet their synthesis remains a formidable challenge. Herein, we report for the first time the union of two distinct azolium salts as an efficient synthetic platform to access tertiary amine-caged frameworks under mild conditions. The strategy combines the masked nucleophilic and electrophilic properties of isoquinolinium and pyridinium salts, and avails double dearomatization guided inverse electron demand (4 + 2) or (3 + 2) annulation in a highly regio- and diastereoselective manner to construct the nitrogen caged motifs. Our methodology creates two new rings and four new bonds in a single operation and transforms flat-aromatic compounds into structurally unprecedented three-dimensional architectures with contiguous stereocenters in very high yields. DFT studies have shed light on the reaction mechanism, indicating that the annulation step is rate-limiting, with (4 + 2) annulation proceeding stepwise and (3 + 2) annulation following a concerted pathway.

Pd-catalyzed intramolecular C-H activation for the synthesis of fused-1,2,3-triazole quinolines and dihydroquinolines

A Pd-catalyzed, simple, and efficient approach toward structurally diverse fused polycyclic [1,2,3]triazolo[4,5-c]quinoline and 4,5-dihydro-[1,2,3]triazolo[4,5-c]quinoline has been developed. The method is based on intramolecular Pd-catalyzed C-H activation and features operational simplicity, high atom economy, broad substrate scope, excellent yields, and good functional group tolerance. Gram-scale experiments and post-synthetic modifications were performed to extend the synthetic applicability of the proposed methodology and enhance the structural complexity of the obtained derivatives.

Computational and Experimental Studies on the α-Functionalization of Ketones Using Domino Reactions: A Strategy to Increase Chemoselectivity at the α-Carbon of Ketones

A facile strategy to increase the chemoselectivity of domino reactions was proposed and successfully applied in the α-functionalization of ketones. The strategy involved widening the activation energy of the main reaction and side reaction through intermolecular interactions, thereby increasing the chemoselectivity of the domino reaction. In the proposed α-functionalization reaction, TMSCF3 acted as an excellent reagent which increased the nucleophilicity of DMF through the Van der Waals force and reduced the nucleophilicity of H2O through a hydrogen bond. We found that TMSCF3 can increase the activation energy difference between the main reaction and side reaction using DFT calculations, which greatly increased chemoselectivity and avoided the formation of by-products. TMSCF3 was recycled by rectification, and the average recovery rate was 87.2%. DFT calculations, XRD experiments, and control experiments were performed to support this mechanism. We are confident that this strategy has the potential to deliver significant practical advancements while simultaneously fostering broader innovation in the field of domino synthesis.

Modular Assembly of Chalcones, N-Tosylhydrazones, and KSCN/KSeCN for the Synthesis of Trisubstituted Imidazo[2,1-b][1,3,4]thiadiazoles/selenadiazoles

A modular protocol that achieved the efficient synthesis of imidazo[2,1-b][1,3,4]thiadiazoles/selenadiazoles via using easily available N-tosylhydrazones, chalcone derivatives, and KSCN/KSeCN is proposed. The method overcomes the elongated synthesis steps and prefunctionalized synthons of previous methods. It solves the problem of traditional preparation methods, which makes it difficult to synthesize thickened selenium-containing heterocyclic molecules, further expanding the number of members in its family. The fluorescence of these compounds also reveals the potential values of the scaffolds we synthesized.

Asymmetric synthesis of spiro[4H-chromene-3,3'-oxindoles] via a squaramide-organocatalytic three-component cascade Knoevenagel/Michael/cyclization sequence

Asymmetric synthesis of spiro[4H-chromene-3,3'-oxindole] derivatives was realized through an organocatalytic cascade Knoevenagel/Michael/cyclization reaction using a quinidine-derived squaramide. Under the optimized conditions, the reactions of isatins, malononitrile, and sesamol yield the desired spirooxindoles in good yields (75-87%) and moderate to high ee values (up to 90% ee).

Potassium Phosphate-Mediated Synthesis of C4-Phosphorylated Quinolines via Cascade Cycloisomerization of Ynones

A cascade phosphorylation cycloisomerization of readily accessible ynones and diphenylphosphine oxides facilitated by potassium phosphate is described, allowing for the straightforward synthesis of C4-phosphorylated quinoline scaffolds. The formation of a C-P bond and a C-N bond is achieved in a single procedure without the need for pre-assembled quinoline cores prior to phosphorylation. This transformation operates without the requirement for metals or oxidants and exhibits excellent compatibility with various functional groups. Furthermore, antimicrobial activity evaluation demonstrated that the synthesized C4-phosphorylated quinoline derivatives exhibited potent inhibitory activity against Staphylococcus aureus.

A General Three-Component Nozaki-Hiyama-Kishi-Type Reaction Enabled by Delayed Radical-Polar Crossover

Nozaki-Hiyama-Kishi (NHK) reactions offer a mild approach for the formation of alcohol motifs through radical-polar crossover-based pathways from various radical precursors. However, the application of multicomponent NHK-type reactions, which allow the formation of multiple bonds in a single step, has been largely restricted to bulky alkyl radical precursors, thus limiting their expanded utilization. Herein, we disclose a general three-component NHK-type reaction enabled by delayed radical-polar crossover, which efficiently tolerates a plethora of radical precursors that were previously unavailable. This method enables the modular assembly of versatile homoallylic alcohols from feedstock chemicals with excellent chemo-, regio-, diastereo-, and enantioselectivities in a single step. Experimental studies and density functional theory (DFT) calculations reveal that the kinetically favored formation of an allylchromium(III) species is paramount for enforcing the delayed radical-polar crossover over direct radical addition. Finally, straightforward transformations and applications of the homoallylic alcohol products were demonstrated, showcasing the synthetic utility of this method.

Organocatalytic Asymmetric Construction of 2,6-Diazabicyclo-[2.2.2]octanes by Harnessing the Potential of an 3-Oxindolium Ion Intermediate

Due to its structural complexity and intrinsic sensitivity of bridged aminal junction, 2,6-diazabicyclo[2.2.2]octane (2,6-DABCO) has remained a highly desirable target in synthetic chemistry. However, the asymmetric access to this unit is still insufficient and hampered by the need for meticulously created functionalities for intricate double aza-cyclizations. Herein, we have developed a novel enantio- and diastereoselective protocol to access polycyclic chiral 2,6-DABCOs under metal-free conditions. This domino process involves the amine-catalyzed [4+2] annulation between glutaraldehyde and 2-arylindol-3-ones, followed by an acid-mediated Pictet-Spengler reaction/intramolecular aza-cyclization cascade sequence with tryptamine by trapping of in situ generated 3-oxindolium ion intermediate for the first time. Overall, 2,6-DABCOs fused with medicinally relevant scaffolds were isolated with good yield and excellent stereoselectivity by constructing five new bonds and four stereocenters in a one-pot operation.

Enantioselective Synthesis of Spiro[Indoline-3,4-Pyrrolo[3,4-b]Pyridines] Via an Organocatalysed Three-Component Cascade Reaction

Asymmetric synthesis of derivatives of spiro[indoline-3,4-pyrrolo[3,4-b]pyridines] were first developed through the organocatalytic cascade of Knoevenagel/Michael/cyclization reactions using a quinidine-derived squaramide. Under the optimized conditions, the three-component reactions of isatins, cyanoacetates, and 3-aminomaleimides yield the desired heterocycle-fused spirooxindoles in good yields (78-91 %) with 53 %-99 % enantiomeric excess (ee). Notably, this reaction enables a broad substrate scope under mild conditions and provides a convenient method for the enantioselective construction of diverse spirooxindoles combined with dihydropyridine and maleimide skeletons, which has great potential for the construction of new bioactive chemical entities.

Transition-Metal-Free Radical Relay Cascade Annulation of Amides: Access to Antitumor Active Benzo[b]azepine and Oxindole Derivatives

Efficient transition-metal-free synthesis of benzo[b]azepines and oxindoles is achieved via a radical relay cascade strategy employing halogen atom transfer (XAT) for aryl radical generation followed by intramolecular hydrogen atom transfer (HAT). Optimization yielded moderate to substantial yields under visible light irradiation. Preliminary biological assessments revealed promising anti-tumor activity for select compounds. This study underscores the potential of XAT-mediated radical relay cascades in medicinal chemistry and anticancer drug discovery.

Chemodivergent Reactions of Aromatic Ring-Annulated Hexahydrocyclopentafurans with Various Aldehydes

Hexahydro-2H-cyclopenta[b]furan fused to electron-rich aromatic rings reacts with various aromatic aldehydes in different modes to build diverse frameworks. The reaction of a dimethoxybenzene-fused cyclopentafuran generated diquinanes fused with a hydrofuran ring, indenopyrans, or diarylindanes depending upon the type of aromatic aldehydes, whereas an indole-annulated cyclopentafuran generated another type of diquinane fused with hydrofuran ring or benzylidenecyclopentafuran. The chemodivergence is due to the different properties between indole- and dimethoxybenzene-fused hydrocyclopentafurans. Namely, Lewis acid-promoted furan-ring opening of the substrates resulted in the formation of an electrophilic or a nucleophilic intermediate, respectively. Additionally, the observed chemodivergence can be attributed to the distinctive electronic properties of three classified aromatic aldehydes. Of particular interest is that 2,4-dimethoxybenzaldehyde reacted with the dimethoxybenzene-fused cyclopentafuran at the benzene ring, whereas it reacted with the indole-fused cyclopentafuran at the formyl group.

Selective Synthesis of Mono- and Bis-Phosphorylated (Dihydro)pyrans via TMSCl-Mediated Cascade Phosphorylation Cycloisomerization of Enynones

A chlorotrimethylsilane (TMSCl)-mediated cascade phosphorylation and cycloisomerization of enynones with diphenylphosphine oxides is presented. This methodology enables the highly selective synthesis of monophosphorylated 2H-pyrans and bisphosphorylated dihydropyrans through precise solvent-reagent stoichiometry control. The strategy demonstrated excellent functional group compatibility and high yields (up to 96%), providing facile access to structurally diverse phosphorylated heterocycles with potential applications in medicinal chemistry and materials science.

Mn-Catalyzed Electrooxidative Radical Cascade Cyclization for the Synthesis of 6-Phosphorylated Quinoxalino[2,1-b]quinazolin-12-ones

Due to their important potential medicinal value, chemists are pursuing mild and efficient methods to synthesize structurally diverse quinazolinone derivatives. In this paper, a series of isocyano-tethered N-aryl quinazolinones were designed and synthesized to conduct electrocatalytic radical cascade cyclization reactions with phosphine oxides by utilizing inexpensive MnII salt as the catalyst. The desired 6-phosphorylated quinoxalino[2,1-b]quinazolin-12-ones were obtained in moderate to good yields.

Triple Nucleophilic Head-to-Tail Cascade Polycyclization of Diazodienals via Combination Catalysis: Direct Access to Cyclopentane Fused Aza-Polycycles with Six-Contiguous Stereocenters

Reported herein are the bench stable (2E,4E)-diazohexa-2,4-dienals (diazodienals) and their unprecedented polycyclization with aldimine and arylamines enabled by Rh(II)/Brønsted acid relay catalysis. This scalable and atom-economical reaction provides direct access to the biologically important azatricyclo[6.2.1.04,11]undecane fused polycycles having six-contiguous stereocenters. Mechanistic studies revealed that polycyclization proceeds through an unusual triple-nucleophilic cascade initiated by aldimine attack on remote Rh-carbenoid, 6π-electrocyclization of aza-trienyl azomethine ylide, stereoselective aza-Michael addition via iminium activation, and inverse electron-demand intramolecular aza Diels-Alder reaction. The π-π secondary interactions play a crucial role in the preorganization of reactive intermediates for the pericyclic reactions and, hence, the overall efficiency of the polycyclization.

Carbonylative cyclization of biaryl enones with aldehydes and oxamic acids

An oxidative radical-promoted carbonylative cyclization strategy for the synthesis of phenanthren-9-(10H)-one frameworks from biaryl enones using aldehydes as the carbonyl radical sources is disclosed. The reaction proceeds through a sequential addition of a carbonyl radical to the olefin followed by cyclization with an aryl ring. The method is further extended to carbamoyl radicals generated from oxamic acids to access the corresponding phenanthrenones with amide functionalities.

Boryl Radical as a Catalyst in Enabling Intra- and Intermolecular Cascade Radical Cyclization Reactions: Construction of Polycyclic Molecules

Cascade radical cyclization constitutes an atom- and step-economic route for rapid assembly of polycyclic molecular skeletons. Although an array of redox-active metal catalysts has recently shown robust applications in enabling various catalytic cascade radical processes, the use of free organic radical as the catalyst, which is capable of triggering strategically distinct cascades, has rarely been developed. Here, we disclosed that the benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical is capable of catalyzing cascade cyclization reactions in both intra- and intermolecular pathways, assembling [5,5] fused bicyclic and [6,6,6] fused tricyclic molecules, respectively. The catalytic reactions start with the chemo- and regioselective addition of the boryl radical catalyst to a tethered alkene or alkyne moiety, followed by either an intramolecular formal [3+2] or an intermolecular [2+2+2] cycloaddition process to construct bicyclo[3.3.0]octane or tetrahydrophenanthridine skeletons, respectively. Eventually, a β-elimination occurs to release the boryl radical catalyst, completing a catalytic cycle. High to excellent diastereoselectivity is achieved in both catalytic reactions under substrate control.

Access to CF2COR-Containing Quinazolinones via Visible-Light-Induced Domino Difluoroalkylation/Cyclization of N-Cyanamide Alkenes

A green and highly efficient visible-light-induced radical cascade difluoroalkylation/cyclization reaction of N-cyanamide alkenes has been developed. A variety of CF2COR-containing quinazolinones have been obtained in high yields with cheap non-metallic 4CzIPN as the photocatalyst. This photocatalytic reaction provides rapid, facile, and practical access to valuable polycyclic quinazolinone, and it is amenable to the gram scale.

Organocatalyzed [4 + 2] cycloaddition of α,β-unsaturated ketones and isatylidene malononitrile: accessing spiro[3-arylcyclohexanone]oxindole derivatives

Herein, we developed a series of compounds featuring spiro[3-arylcyclohexanone]oxindoles through Barbas [4 + 2] cycloaddition reactions between isatylidene malononitrile and α,β-unsaturated ketones using l-proline as an organocatalyst. The reported methodology offers many advantages such as mild reaction conditions, diverse substrate scope with high yields, easy reaction setup, and use of easily synthesizable starting materials.

Photoredox-Catalyzed Divergent Radical Cascade Annulations of 1,6-Enynes via Pyridine N-Oxide-Promoted Vinyl Radical Generation

The divergent organophotoredox-catalyzed radical cascade annulation reactions of 1,6-enynes were developed. A series of cyclopropane-fused hetero- and carbo-bicyclic, tricyclic, and spiro-tetracyclic compounds were facilely synthesized from a broad scope of 1,6-enynes and 2,6-lutidine N-oxide under mild and metal-free conditions with blue light-emitting diode light irradiation. The cascade annulation reaction occurs with the intermediacy of a β-oxyvinyl radical, which is produced from photocatalytically generated pyridine N-oxy radical addition to the carbon-carbon triple bond.

Catalytic Asymmetric Polycyclization of Tertiary Enamides with Silyl Enol Ethers: Total Synthesis of (-)-Cephalocyclidin A

A catalytic enantioselective polycyclization of tertiary enamides with terminal silyl enol ethers has been developed by virtue of Cu(OTf)2 catalysis with a novel spiropyrroline-derived oxazole (SPDO) ligand. This tandem reaction offers an effective approach to assemble bicyclic and tricyclic N-heterocycles bearing both aza- and oxa-quaternary stereogenic centers, which are primal subunits in a range of natural alkaloids. Strategic application of this methodology and a late-stage radical cyclization as key steps have been showcased in the concise total synthesis of (-)-cephalocyclidin A.

Synthesis of 3(2H)-furanone derivatives: p-TsOH/halotrimethylsilane promoted cycloketonization of γ-hydroxyl ynones

A p-TsOH/halotrimethylsilane facilitated cycloketonization of γ-hydroxyl ynones is detailed. This methodology enables the one-step synthesis of polysubstituted 3(2H)-furanone products. It is remarkable that the reaction exhibits excellent regio- and chemoselectivity by the addition of very small quantities of p-toluenesulfonic acid and water.

Applying green chemistry principles to iron catalysis: mild and selective domino synthesis of pyrroles from nitroarenes

An efficient and general cascade synthesis of pyrroles from nitroarenes using an acid-tolerant homogeneous iron catalyst is presented. Initial (transfer) hydrogenation using the commercially available iron-Tetraphos catalyst is followed by acid catalysed Paal-Knorr condensation. Both formic acid and molecular hydrogen can be used as green reductants in this process. Particularly, under transfer hydrogenation conditions, the homogeneous catalyst shows remarkable reactivity at low temperatures, high functional group tolerance and excellent chemoselectivity transforming a wide variety of substrates. Compared to classical heterogeneous catalysts, this system presents complementing reactivity, showing none of the typical side reactions such as dehalogenation, debenzylation, arene or olefin hydrogenation. It thereby enhances the chemical toolbox in terms of orthogonal reactivity. The methodology was successfully applied to the late-stage modification of multi-functional drug(-like) molecules as well as to the one-pot synthesis of the bioactive agent BM-635.

Rapid Assembly of Functionalized 2H-Chromenes and 1,2-Dihydroquinolines via Microwave-Assisted Secondary Amine-Catalyzed Cascade Annulation of 2-O/N-Propargylarylaldehydes with 2,6-Dialkylphenols

An efficient, secondary amine-catalyzed cascade annulation of 2-O/N-propargylarylaldehydes with 2,6-dialkylphenols was established to access biologically relevant functionalized 2H-chromenes and 1,2-dihydroquinolines tethered with a synthetically useful p-quinone methide scaffold in high yields under microwave irradiation and conventional heating conditions. The microwave-assisted strategy was convenient, clean, rapid, and high yielding in which the reactions were completed in just 15 min, and the yields obtained were up to 95%. This highly atom-economical domino process constructed two new C-C double bonds and a six-membered O/N-heterocyclic ring in a single synthetic operation. Its mechanism process was rationalized as involving sequential iminium ion formation, nucleophilic addition, and intramolecular annulation steps. Furthermore, the synthesized 2H-chromene derivatives were transformed into valuable indeno[2,1-c]chromenes, 5H-indeno[2,1-c]quinolines, and oxireno[2,3-c]chromene via a palladium-catalyzed double C-H bond activation process and epoxidation, respectively.

Diastereoselective Synthesis of trans-anti-Hydrophenanthrenes via Ti-mediated Radical Cyclization and Total Synthesis of Kamebanin

Ent-kaurenes consist of an ABC-ring based on a trans-anti-hydrophenanthrene skeleton and a D ring with an exomethylene. Highly oxygen-functionalized ent-kauren-15-ones have promising antiinflammatory pharmacological activity. In this study, we developed a novel diastereoselective synthesis of trans-anti-hydrophenanthrenes via a Ti-mediated reductive radical cyclization. We also demonstrated the applicability of this method by developing the first total synthesis of (±)-kamebanin (longest linear sequence; 17 steps, overall yield; 6.5 %). Furthermore, this synthesis provided a formal semi-pinacol rearrangement for the construction of the quaternary carbon at C8 and a novel Thorpe-Ziegler-type reaction for the construction of the D-ring.

Synthesis of fused bis-indazoles/indazoles via a one-pot sequential strategy

We report here an efficient synthesis of fused bis-indazoles/indazoles via a one-pot sequential strategy starting from o-azido aldehydes and amines. This novel method involves the sequential formation of 2H-indazole followed by a Pd-catalyzed intramolecular cross-dehydrogenative coupling reaction. Overall, this one-pot sequential reaction involved the formation of new five bonds, resulting in the formation of three heterocyclic rings.

Switchable multipath cascade cyclization to synthesize bicyclic lactams and succinimides via chemodivergent reaction

Herein, a novel switchable multipath cascade cyclization via chemodivergent reaction between readily available ketoamides and deconjugated butenolides was developed to efficiently synthesize γ-lactone fused γ-lactams and succinimide fused hemiketals. The Aldol/aza-Michael reaction and Aldol/imidation/hemiketalization reaction were enabled by catalytic amounts of two bases, namely tetramethyl guanidine and NaOAc. A wide range of substrate scope with diverse functional group compatibility was demonstrated to deliver the corresponding products with good yield and excellent diastereoselectivity (>60 examples).

Multisite-Sequential Cyclization To Construct 1,2,4-Triazole-Based N-Fused Heterocyclics

A feasible protocol that uses atomic groups (KSCN, KSeCN, and NH₂CN), o-bromobenzoyl hydrazides, and formyls as reaction factors to synthesize N-fused 1,2,4-triazole with benzothiazides, benzoselenazinones, and quinazolinones was proposed. The method overcomes the lengthy multistep synthesis, narrow substrate scope, and toxicity challenge induced by the use or production of hazardous substances. It also enables the development of fused-heterocyclic selenium and quinazolinone derivatives. Their fluorescent performance further demonstrates the practicability of this methodology.

Cucurbit[10]uril-based supramolecular radicals: Powerful arms to kill facultative anaerobic bacteria

Two water-soluble supramolecular complexes (CB[10]⊃PSA and CB[10]⊃TPE-cyc) are constructed based on the host-guest interaction between cucurbit[10]uril (CB[10]) and perylene diimide derivative (PSA) or tetracationic cyclophane (TPE-cyc). Attributing to the matched redox potential, both supramolecular complexes can be specifically reduced into corresponding supramolecular radical cations or anions by facultative anaerobic E. coli. Benefiting from the strong near-infrared (NIR) absorption, CB[10]⊃PSA radical anions and CB[10]⊃TPE-cyc radical cations act as efficient NIR photosensitizers and perform an excellent antimicrobial activity (close to 100%) via PTT. In addition, the biocompatibility of TPE-cyc is notably improved under the protection of CB[10], guaranteeing its biosafety for in vivo application. CB[10]⊃PSA radical anions and CB[10]⊃TPE-cyc radical cations are in situ generated in the E. coli-infected abscess of mice and effectively inhibit the bacterial infection without obvious system toxicity. It is anticipated that this supramolecular strategy may pave a new way for the selective bacteria inhibition to regulate the balance of different bacterial flora.

A Diastereoselective Cascade Annulation Approach to Bridged Polycyclic Heterocycles Involving an Unexpected Rearrangement

A diastereoselective cascade annulation between allenoates and in-situ generated isoquinoline N-oxides generating sp³-rich bridged polycyclic heterocycles is disclosed. The reaction proceeds through an unprecedented non-rearomatized rearrangement and allows access to a broad range of bridged heterocycles in 38-93% yields with excellent functional group tolerance and high diastereoselectivity. Density functional theory calculations provided key insights into the possible reaction pathway and the stereoselectivity of this procedure.

Structural Elucidation and Total Synthesis for the Pair of Unprecedented Polypyridines with Anti-AChE and HIV-1 Protease Activities from Alangium chinense

Unlike reported pyridine hybrids, 2S (1a) and 2R-alanginenmine A (1b) from Alangium chinense featuring an unprecedented piperidine-bridged polypyridine skeleton represented a pair of alkaloid subtypes with a unique multiple pyridine scaffold. Enlightened by the rare structural characteristics and possible biosynthetic pathway, (±)-alanginenmine A (1) have been achieved in ideal yield by gram-class total synthesis with four steps. In addition, both compounds 1a and 1b exhibited anti-acetylcholinesterase (AChE) and HIV-1 protease activities in the biological activity evaluation. Further, molecular docking was investigated for the mechanism of action between the isolated compounds and HIV-1 protease. The stronger Coulomb interactions and van der Waals interaction, as well as the hydrogen bond interactions of 1a, might be the main cause for its better anti-HIV-1 protease activity than 1b. This work provided a comprehensive research including natural product discovery, bioactivity evaluation, and total synthesis for the new type of leading anti-HIV-1 protease.

Recent developments in one-pot stepwise synthesis (OPSS) of small molecules

One-pot synthesis is an active topic in organic chemistry due to its intrinsic advantages of simple operation, high mass efficiency, low cost, and less amount of waste disposal. Among three kinds of one-pot syntheses, 1) cascade reactions, 2) multicomponent reactions (MCRs), and 3) one-pot stepwise synthesis (OPSS), OPSS could be more flexible and practical since it is carried out stepwisely and have variable reaction conditions for different steps. This perspective article uses selected examples to highlight the recent development in OPSS involving cyclization, cycloaddition, rearrangement, and catalytic reactions for the synthesis of heterocyclic scaffolds, asymmetric molecules, natural products, and bioactive compounds.

Copper-Catalyzed Reactions of Alkenyl Boronic Esters via Chan-Evans-Lam Coupling/Annulation Cascades: Substrate Selective Synthesis of Dihydroquinazolin-4-ones and Polysubstituted Quinolines

Copper-catalyzed cascade cyclization reactions between alkenyl boronic esters and N-H-based nucleophiles have been established, providing new approaches for one-pot assembly of azacycles. Following the Chan-Evans-Lam C-N couplings, the cyclization processes occur via divergent pathways based on the utilized substrates, affording hydroamination product dihydroquinazolin-4-ones or aromatization product quinolines. Via this one-pot C-N coupling/annulation cascade, the target substituted azacycles can be obtained in moderate to good yields in each case.

Stereoselective Cyclization Cascade of Dihydroquinoxalinones by Visible-Light Photocatalysis: Access to the Polycyclic Quinoxalin-2(1H)-ones

An intriguing stereoselective visible-light photocatalysis of dihydroquinoxalinone derivatives has been realized via cyclization with or without the solvolysis cascade. The reactions provided the polycyclic ring structures with efficient formation of multiple bonds and with high stereoselectivity. X-ray crystallography unequivocally determined the structures of five polycyclic products.

Recent Advances in Transition-Metal-Catalyzed Reactions of N-Tosylhydrazones

N-Tosylhydrazones are highly versatile precursors for in situ carbene formation and are frequently used in metal-catalyzed cross-coupling reactions. Due to their many applications in organic synthesis, including C–C, C–O, C–N, and C–S bond formation, N-tosylhydrazones have recently received much interest. They can be simply synthesized by reacting an aldehyde or ketone with N-tosylhydrazine to produce a solid N-tosylhydrazone, which is a ‘green’ precursor of diazo compounds. Using a suitable metal catalyst, N-tosylhydrazones show versatile substrate scope for the synthesis of substituted diaminopyrroles, chromenopyrazoles, alkenylpyrazoles, benzofuran thioethers, tetrahydropyridazines, sulfur-containing heterocycles, and benzofurans with potent biological activities and even regioselective N-functionalization reactions. Metal-catalyzed reactions of N-tosylhydrazones for the construction of bioactive heterocycles are still highly in demand. Hence, this review focuses on the recent synthetic application of N-tosylhydrazones influenced by different transition metals with notable features like simple workup procedures, gram-scale synthesis, broad substrate scope, multicomponent processes, cyclization, and carbon–heteroatom bond formation.

1 Introduction

2 Applications of N-Tosylhydrazones

3 Conclusion

Microwave-Assisted Copper(II)-Catalyzed Cascade Cyclization of 2-Propargylamino/Oxy-Arylaldehydes and O-Phenylenediamines: Access to Densely Functionalized Benzo[f]Imidazo[1,2-d][1,4]Oxazepines and Benzo[f]Imidazo[1,2-d][1,4]Diazepines

A highly efficient microwave-assisted copper(II)-catalyzed cyclization cascade was established starting from readily accessible O/N-propargylated 2-hydroxy or 2-aminobenzaldehydes and o-phenylenediamines to synthesize densely functionalized imidazo[1,2-d][1,4]oxazepines and imidazo[1,2-d][1,4]diazepines in high yields (up to 93%). This one-pot two-step process was found to be highly atom economical (-H₂O, -H₂) and operationally simple and enabled the generation of two new heterocycle rings (seven- and five-membered) and three new C-N bonds in a single synthetic operation. These reactions well tolerated a variety of substituents including electron-donating and electron-withdrawing groups and furnished the desired fused heterocycles in high yields under microwave irradiation in a very short reaction time. The mechanism of the established protocol involves sequential imine formation-intramolecular cyclization-air oxidation followed by 7-exo-dig cyclization steps. A comparative study between the microwave-assisted approach and conventional heating was also performed to demonstrate the advantages of the microwave-assisted protocol in terms of high yield and shorter reaction time.

Practical and modular construction of benzo[c]phenanthridine compounds

Here, we describe a general and modular strategy for the rapid assembly of benzo[c]phenanthridine (BCP) derivatives using homogeneous gold catalysis. Notably, in contrast to traditional methods based on the specially preformed substrates that have an inherent preference for the formation of this class of compounds with limited flexibility, this protocol is achieved via a selectively intramolecular cascade of a diazo-tethered alkyne and subsequently an intermolecular cyclization with a nitrile to facilitate the successive C-N and C-C bonds formation. This methodology uses readily available nitriles as the nitrogen source to deliver the products in good yield with excellent functional group compatibility. A preliminary anti-tumor activity study of these generated products exhibits high anticancer potency against five tumor cell lines, including HeLa, Mel624, SW-480, 8505C, LAN-1. Besides, we report a catalyst-controlled intermolecular cycloaddition/intramolecular insertion of the substrate with a fulvene to provide fused polycarbocycles containing a seven-membered ring.

One-Pot Double Intramolecular Cyclization Approach to Tetralin-Based Cis-Fused Tetracyclic Compounds

Presented herein is a BF₃·OEt₂-mediated, diastereoselective one-pot double cyclization of 4-aryl-2-[(arylthio)methyl]butanals leading to the formation of cis-tetrahydro-6H-naphtho[2,1-c]thiochromenes for the first time. Mechanistically, the formation of the title products involves the one-pot intramolecular Friedel-Crafts hydroxyalkylation/intramolecular Friedel-Crafts alkylation cascade. This synthetic methodology is featured by its high atom economy, broad substrate scope, mild transition-metal-free reaction conditions, capability to assemble two new rings in one pot, and moderate to high yields (up to 94% yield). It was then applied in the synthesis of a thia analogue of brazilane and a chromeno[3,4-c]chromene derivative. Moreover, the methodology was successfully extended to the synthesis of cis-hexahydrobenzo[c]phenanthrenes. Specifically, 1,5-diarylpentan-3-ones were first subjected to the Corey-Chaykovsky reaction, and the resulting epoxides, without being chromatographically isolated, were treated with BF₃·OEt₂ to afford the cyclized products in high yields (up to 84% yield over two steps).

Encapsulating Electron-Rich Pd NPs with Lewis Acidic MOF: Reconciling the Electron-Preference Conflict of the Catalyst for Cascade Condensation via Nitro Reduction

Cascade reactions take advantage of step-saving and facile operation for obtaining chemicals. Herein, catalytic hydrogenation of nitroarene coupled condensation with β-diketone to afford β-ketoenamines is achieved by an integrated nanocatalyst, Pd-e@UiO-66. The catalyst has the structure of an acid-rich metal-organic framework (MOF), UiO-66-encapsulated electron-rich Pd nanoparticles, and it reconciles the electron-effect contradiction of cascade catalytic reactions: catalytic hydrogenation requires an electron-rich catalyst, while condensation requires electron-deficient Lewis acid sites. The catalyst showed good activity, high chemoselectivity, and universal applicability for the synthesis of β-ketoenamines using nitroarenes. More than 30 β-ketoenamines have been successfully prepared with up to 99% yield via the methodology of relay catalysis. The catalyst exhibited excellent stability to maintain its catalytic performance for more than five cycles. Furthermore, we conducted an in-depth exploration of the reaction mechanism with theoretical calculations.

Organic radical reactions confined to containers in supramolecular systems

Radical chemistry and host-guest chemistry have each developed rapidly over the past decades and their intersection offers an attractive opportunity for modern applications. Radicals can be introduced into the frameworks of supramolecular hosts or radicals can be guests, generated in and confined to host containers. In this highlight we outline research achievements in both approaches, photoinduced and external reagent-initiated radicals in the host. Specific topics include rearrangement and fragmentation reactions, hydrocarbon oxidation and alkyl halide reductions of molecules confined to various supramolecular complexes. Applications to challenging problems in chemical synthesis are suggested.

Electrocatalysis as a key strategy for the total synthesis of natural products

Electrochemical strategies have been a powerful approach for the synthesis of valuable intermediates, in particular heterocyclic motifs. Because of the mild nature, a wide range of nonclassical bond disconnections have been achieved via in situ-generated radical intermediates in a highly efficient manner. In particular, anodic electrochemical oxidative strategies have been utilized for the total synthesis of many structurally intriguing natural products. In this review article, we have discussed a number of total syntheses of structurally intriguing alkaloids and terpenoids in which electrochemical processes play an important role as a key methodology.