Synthesis of Oxygen- and Nitrogen-Containing Heterocycles by Ring-Closing Metathesis

Synthesis of 2H-imidazoles via copper-catalyzed homo/cross-coupling of oxime acetates

A facile and practical protocol to construct 2H-imidazoles by applying an oxime acetate block as the sole component via oxidative homo/cross-coupling catalyzed by Cu(I) was developed. This strategy provides a straightforward method to produce a series of substituted 2H-imidazoles in moderate to excellent yields. The transformation process is straightforward to operate and is considered as a readily available catalytic system exhibiting good substrate compatibility, eliminating the necessity for pre-functionalization of azides or the use of additives.

Recent advances of 5-endo-trig radical cyclization: promoting strategies and applications

5-endo-trig radical cyclization has long been recognized as one of the most straightforward ways for the construction of densely functionalized five-membered rings. Nevertheless, according to Baldwin's rules, the 5-endo-trig radical cyclization is kinetically disfavored due to stereoelectronic effects and thus usually proceeds via a slow rate, which renders its application a challenging task. In recent years, with the emergence of efficient radical generation methods and effective cyclization strategies, 5-endo-trig radical cyclization has been successfully accelerated to a synthetically useful rate and has been utilized in the access of diverse five-membered carbo- and heterocyclic compounds. This review comprehensively summarizes the methodologies involving the 5-endo-trig radical cyclization process, with particular emphasis on the elucidation of the promoting strategies, which include the polar effect, geometrical constraints, spin delocalization effect, and persistent radical effect. Each of these strategies is discussed in detail, with illustrative examples from recent literature studies to highlight their practical applications and effectiveness. It is anticipated that the in-depth understanding of the 5-endo-trig radical cyclization provided by this review would inspire further innovation of this privileged reaction mode and expand its applications. Moreover, the potent ring-closure-promoting strategies revealed herein would also contribute to achieving other challenges of cyclizations with particular significance for organic synthesis.

Synthesis of N-Alkenylated Heterocycles via T3P-Promoted Condensation with Ketones

Herein, we describe a convenient protocol for the synthesis of N-alkenylated heterocycles using abundant ketone electrophiles and T3P as a water scavenger under microwave irradiation. The method can be applied to a diverse range of NH-heterocycles and ketones with good to excellent yields (up to 94%). This procedure is particularly attractive, as it is metal- and base-free, tolerates a variety of functional groups, and offers ease of product purification. The utility of the protocol was exemplified by synthesizing pharmaceutically relevant scaffolds containing the N-alkenyl motif and was further extended to a one-pot reductive amination sequence.

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

Total synthesis and structure-antifouling activity relationship of scabrolide F

An efficient synthetic strategy for scabrolide F (7), a norcembranolide diterpene that was isolated from the Taiwanese soft coral Sinularia scabra, has only recently been reported by our group. Herein, we report details of the first total synthesis of 7. The tetrahydrofuran domain of 7 was stereoselectively constructed via the 5-endo-tet cyclization of a hydroxy vinyl epoxide. The reaction of alkyl iodide 30 with dithiane 38, followed by the introduction of an alkene moiety, afforded allylation precursor 41. The coupling of alkyl iodide 42 and allylic stannane 43 was examined as a model experiment of allylation. Because the desired allylated product 44 was not obtained, an alternative synthetic route toward 7 was investigated instead. In the second synthetic approach, fragment-coupling between alkyl iodide 56 and aldehyde 58, macrolactonization, and transannular ring-closing metathesis were used as the key steps to achieve the first total synthesis of 7. We hope that this synthetic strategy provides access to the total synthesis of other macrocyclic norcembranolides. We also evaluated the antifouling activity and toxicity of 7 and its synthetic intermediates toward the cypris larvae of the barnacle Amphibalanus amphitrite. This study is the first to report the antifouling activity of norcembranolides as well as the biological activity of 7.

Chromatography Scrutiny, Molecular Docking, Clarifying the Selectivities and the Mechanism of [3 + 2] Cycloloaddition Reaction between Linallol and Chlorobenzene-Nitrile-oxide

Employing the Molecular Electron Density Theory, [3 + 2] cycloaddition processes between 4-chlorobenzenenitrileoxide and linalool, have been applied using the DFT/B3LYP/6-311(d,p) method, activation, reaction energies and the reactivity indices are calculated. In an investigation of conceptual DFT indices, LIL-1 will contribute to this reaction as a nucleophile, whilst NOX-2 will participate as an electrophile. This cyclization is regio, chemo and stereospecific, as demonstrated by the reaction and activation energies, in clear agreement with the experiment's results, in addition, ELF analysis revealed that the mechanism for this cycloaddition occurs in two steps. Furthermore, a docking study was conducted on the products studied, and the interaction with the protein protease COVID-19 (PDB ID: 6LU7), our results indicate that the presence of the -OH group increases the affinity of these products, moreover, adsorption study by chromatography was made on silica gel as support; our outcome reveals that the -OH group creates an intramolecular hydrogen bond in the product P2, while in the product P3 will create a hydrogen bond with the silica gel which makes the two products P2 and P3 are very easy to separate by chromatography, this result is in excellent agreement with the Rf retention value. The study might provide a fundamental for developing natural anti-viral compound in promoting human health.

Stereoselective synthesis of 6/7/6-fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder reaction sequence

A highly stereoselective synthesis of fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder (IEDHDA) reaction sequence is described. The present reaction sequence has three interesting features: (1) complete control of two potentially competitive processes, i.e., hetero-Diels-Alder reaction and [1,5]-hydride shift; (2) one-shot construction of the complicated 6/7/6-fused heterocyclic structure having multiple stereocenters; and (3) high control of its stereoselectivity. When alkenylidene barbiturates with an allyl benzyl ether moiety were treated with a catalytic amount of Sc(OTf)3 and 2,2'-bipyridine, the internal redox reaction/IEDHDA reaction proceeded successively to afford 6/7/6-fused heterocycles in good chemical yields with good to excellent diastereoselectivities.

Amine adducts of triallylborane as highly reactive allylborating agents for Cu(I)-catalyzed allylation of chiral sulfinylimines

The implementation of selective catalytic processes with highly active reagents is an attractive strategy that meets the modern principles of sustainable development of chemistry. In the current study, we for the first time describe the method and general principles of Cu(I)-catalyzed allylation of imines with amine adducts of allylic triorganoboranes. Triallylborane is an extremely reactive compound and cannot be used for the catalytic allylation of imines, whereas its amine adducts are ideal substrates for catalysis. The structure of the amine fragment successfully balances the safety, selectivity and stability of the allylboron reagent, allowing it to demonstrate high activity in catalytic allylation reactions, exceeding many times any known allylboranes. The obtained results are supported by quantitative kinetics data and DFT calculations. The catalytic efficacy of the system was demonstrated on model sulfinylimines (23 examples). High diastereoselectivity up to >99% was achieved, including for the gram-scale synthesis of 2-hydroxyphenyl-derivatives. Taking into account the high reactivity and unsurpassed atom-economy of amine adducts of triallylborane (AAT), they can be considered as prospective allylation reagents with Cu(I) and other appropriate metallocatalysts.

Recent Advances in Metal-Catalyzed Approaches for the Synthesis of Quinazoline Derivatives

Quinazolines are an important class of heterocyclic compounds that have proven their significance, especially in the field of organic synthesis and medicinal chemistry because of their wide range of biological and pharmacological properties. Thus, numerous synthetic methods have been developed for the synthesis of quinazolines and their derivatives. This review article briefly outlines the new synthetic methods for compounds containing the quinazoline scaffold employing transition metal-catalyzed reactions.

Reaction of 1,3,5-Triazinanes with Phosphoryl Diazomethanes: Access to 5-Phosphoryl-1,2,3,4-tetrahydropyrimidines

Conversion of 1,3,5-triazinanes into 5-phosporyl-1,2,3,4-tetrahydropyrimidines is achieved efficiently through the microwave-assisted reaction with phosphoryl diazomethanes. Both trialkyl and triaryl 1,3,5-triazinanes were converted by diazomethyldiarylphosphine oxides, dialkyl diazomethylphosphonates, and alkyl diazomethyl(aryl)phosphinates and functionalized simultaneously in good to excellent yields. The reaction is a sequence of 1,3,5-triazinane fragmentation, tandem nucleophilic addition of the generated formaldimines and phosphoryl diazomethanes, and final N,N-acetalization.

Construction of Seven-Membered Oxacycles Using a Rh(I)-Catalyzed Cascade C-C Formation/Cleavage of Cyclobutenol Derivatives

Herein, we describe the synthesis of substituted oxepane derivatives through the skeletal remodeling of 4-hydroxy-2-cyclobutenones, which are readily prepared from commercially available dialkyl squarates upon their reaction with acrylonitrile. Mechanistically, a Rh(I)-catalyzed C-C bond formation and cleavage cascade is proposed. Specifically, a fused [3.2.0] bicycle is proposed to form from dialkyl squarate-derived cyclobutenols via an unusual Rh(I)-catalyzed intermolecular oxa-Michael addition of a tertiary alcohol with acrylonitrile, followed by an intramolecular conjugate addition/migratory insertion. Subsequent C(sp3)-C(sp3) bond cleavage through a Rh-catalyzed β-carbon elimination is then theorized to furnish the oxepane scaffold. Computational studies support the formation of an intermediate [3.2.0] bicycle but also point to an alternative pathway for the formation of the oxepane products involving a Rh(III) intermediate. Additional studies have shown the overall process to be stereoretentive. The functional groups that are introduced in this process can be leveraged to form fused or bridged ring systems.

Pd-catalyzed divergent regioselective annulation of phosphinyl allenes accessing polyarylfurans and 2H-chromenes

A novel and efficient regioselective annulation of phosphinyl allenes with 2-bromophenol or 1-bromo-2-naphthol is achieved by palladium catalysis. The divergent pathway delivers structurally diverse polyarylfurans and 2H-chromene skeletons via two different Heck-type annulations. This protocol represents regioselectivity-tunable transformation of allenes into functionalized O-containing heterocycles with excellent group compatibility.

Synthesis of substituted 2H-Chromenes via Pd-catalyzed C-H activation and thermal cyclization

A proficient approach has been developed for the synthesis of substituted 2H-chromenes from C1-substituted glucal. The key step of our synthetic methodology was C-H activation in propylene carbonate solvent followed by 6π-electrocyclization aromatization in ethylene glycol as greener substitutes to toxic aprotic solvents, to obtain 2H-chromenes in a stepwise manner. The application of the developed methodology was further explored with the synthesis of a small library of substituted 2H-chromenes in good yields.

Electroreductive Radical Addition-Polar Cyclization Cascade to Access Cycloalkanes

Compared with flat aromatic scaffolds, three-dimensional aliphatic ring systems feature high structural complexity and topological diversity and, thus, have received increasing attention in drug discovery. Herein, we describe a mild and general electrochemical method for the modular synthesis of structurally distinct cyclic compounds, including monocyclic alkanes, benzo-fused ring systems, and spirocycles, from readily available alkenes and alkyl halides via a radical-polar crossover mechanism.

Unconventional Reactivity of a Grubbs Catalyst: Hydroalkylation Overriding Metathesis

Unprecedented reactivity of a Grubbs catalyst has been disclosed in a reaction between vinyl azaarenes and alkenylnitriles under standard metathesis conditions. No metathesis was observed; only hydroalkylation products were obtained in high yields. The practical utility of this method has been demonstrated by the application of the products in useful transformations, e.g., the formation of cyclic iminoesters and highly challenging medium-sized carbocycles.

Redox-Neutral Radical Cascade Cyclization of N-Arylacrylamides with Unactivated Alkyl and Aryl Chlorides

Herein, we report the first metal-free, redox-neutral strategy for radical cascade alkylative radical addition, cyclization of N-arylacrylamides with unactivated alkyl chlorides to give corresponding 3,3-disubstituted oxindoles in moderate to good yields. This transformation's salient features are the utilization of an organo photocatalyst, mild reaction conditions, and broad substrate scope. Moreover, this methodology is suitable for hetero cycle derived acrylamides and further allowed to utilize aryl chlorides for radical cyclization reaction. Finally, DFT studies allow us to shed light on the reaction mechanism.

Regioselective Synthesis of 3-Substituted Isocoumarin-1-imines via Palladium-Catalyzed Denitrogenative Transannulation of 1,2,3-Benzotriazin-4(3H)-ones and Terminal Alkynes

A palladium-catalyzed denitrogenative transannulation strategy to access various 3-substituted isocoumarin-1-imine frameworks using 1,2,3-benzotriazin-4(3H)-ones and terminal alkynes is described. The reaction is highly regioselective and tolerates a wide range of functional groups. The reaction is believed to proceed via a five-membered palladacycle intermediate extruding environmentally benign molecular nitrogen as a by-product. The utility of this method was showcased through the one-pot synthesis of biologically relevant 3-substituted isocoumarin scaffolds.

SOCl2-Catalyzed Meyer-Schuster Rearrangement of 3°-Propargylic Alcohols: Synthesis of Densely Arene-Substituted Pyrazolines Bearing Quaternary Centers from α,β-Unsaturated Carbonyl Compounds and Arylhydrazines

Meyer-Schuster rearrangement of 3°-propargyl alcohol to the corresponding α,β-unsaturated carbonyl compounds under SOCl2 catalysis has been reported. Terminal and internal propargyl alcohols efficiently participated in the reaction. Furthermore, we have demonstrated the synthetic utility of conjugated carbonyl compounds to access densely arene-substituted pyrazolines bearing quaternary centers by reacting with arylhydrazine hydrochloride. Isotopic labeling studies were carried out to gain insights into the reaction mechanism.

Brønsted-Acid-Catalyzed In Situ Formation of Acyclic Tertiary Enamides and Its Application to the Preparation of Diverse Nitrogen-Containing Heterocyclic Compounds

A Brønsted acid-catalyzed cascade process, involving in situ formation of acyclic tertiary enamides and intramolecular Michael reaction, is developed for the synthesis of functionalized cyclic tertiary enamides. Based on the dual reactivities of the enamide moiety, several reaction sequences were realized by using rationally designed substrates, leading to biologically relevant nitrogen-containing heterocyclic compounds with diverse structural skeletons in a concise and diastereocontrolled manner.

DBU-Promoted Tandem Cyclization of Ynones and Diazo Compounds: Direct Synthesis of Eight-Membered Cyclic Ethers

A novel DBU-promoted tandem cyclization reaction of ynones with diazo compounds as the N-terminal electrophiles has been developed. The reaction provides a simple and efficient method for the synthesis of fused eight-membered oxocino[2,3-c] pyrazoles from readily available acyclic starting materials in a single step. This reaction allows the formation of four new bonds and two rings in a highly regio- and diastereoselective manner, where two adjacent stereocenters are created simultaneously in an atom-economic manner.

[RhCp*Cl2]2-Catalyzed Indole Functionalization: Synthesis of Bioinspired Indole-Fused Polycycles

Polycyclic fused indoles are ubiquitous in natural products and pharmaceuticals due to their immense structural diversity and biological inference, making them suitable for charting broader chemical space. Indole-based polycycles continue to be fascinating as well as challenging targets for synthetic fabrication because of their characteristic structural frameworks possessing biologically intriguing compounds of both natural and synthetic origin. As a result, an assortment of new chemical processes and catalytic routes has been established to provide unified access to these skeletons in a very efficient and selective manner. Transition-metal-catalyzed processes, in particular from rhodium(III), are widely used in synthetic endeavors to increase molecular complexity efficiently. In recent years, this has resulted in significant progress in reaching molecular scaffolds with enormous biological activity based on core indole skeletons. Additionally, Rh(III)-catalyzed direct C-H functionalization and benzannulation protocols of indole moieties were one of the most alluring synthetic techniques to generate indole-fused polycyclic molecules efficiently. This review sheds light on recent developments toward synthesizing fused indoles by cascade annulation methods using Rh(III)-[RhCp*Cl2]2-catalyzed pathways, which align with the comprehensive and sophisticated developments in the field of Rh(III)-catalyzed indole functionalization. Here, we looked at a few intriguing cascade-based synthetic designs catalyzed by Rh(III) that produced elaborate frameworks inspired by indole bioactivity. The review also strongly emphasizes mechanistic insights for reaching 1-2, 2-3, and 3-4-fused indole systems, focusing on Rh(III)-catalyzed routes. With an emphasis on synthetic efficiency and product diversity, synthetic methods of chosen polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro cages are reviewed. The newly created synthesis concepts or toolkits for accessing diazepine, indol-ones, carbazoles, and benzo-indoles, as well as illustrative privileged synthetic techniques, are included in the featured collection.

Oxidative transformations of olefins employing persulfate/visible light irradiation in water

We present a green and economical approach for the photooxidation of diverse olefins through the use of ammonium persulfate and blue light irradiation, resulting in the formation of vicinal diols from styrenes and aliphatic alkenes, and vinyl esters and diacids from α,β-unsaturated ketones. The involvement of sulfate radicals in the reaction medium was established as the primary species responsible for the selective generation of the products. A significant advantage of the method lies in its broad substrate scope and economic feasibility, making it a promising alternative to conventional transition metal photocatalysis.

Regioselective Reaction of 2-Indolylmethanols with Enamides

A highly regioselective reaction of 2-indolylmethanols with enamides has been developed at room temperature by using AlCl₃ as a catalyst. A wide range of hybrids (40 examples) of indoles and enamides were obtained in moderate to good yields (up to 98% yield). This transformation represents the efficient way to introduce biologically important indoles and enamides skeleton into structurally complex hybrids.

Discovery of a simple iron catalyst reveals the intimate steps of C-H amination to form C-N bonds

Formation of ubiquitous C-N bonds traditionally uses prefunctionalized carbon precursors. Recently, metal-catalyzed amination of unfunctionalized C-H bonds with azides has become an attractive and atom-economic strategy for C-N bond formation, though all catalysts contain sophisticated ligands. Here, we report Fe(HMDS)2 (HMDS = N(SiMe3)2 -) as an easy-to-prepare catalyst for intramolecular C-H amination. The catalyst shows unprecedented turnover frequencies (110 h-1 vs. 70 h-1 reported to date) and requires no additives. Amination is successful for benzylic and aliphatic C-H bonds (>80% yield) and occurs even at room temperature. The simplicity of the catalyst enabled for the first time comprehensive mechanistic investigations. Kinetic, stoichiometric, and computational studies unveiled the intimate steps of the C-H amination process, including the resting state of the catalyst and turnover-limiting N2 loss of the coordinated azide. The high reactivity of the iron imido intermediate is rationalized by its complex spin system revealing imidyl and nitrene character.

From the bottle: simple iron salts for the efficient synthesis of pyrrolidines via catalytic C-H bond amination

Commercially available iron salts FeX2 are remarkably active catalysts for pyrrolidine formation from organic azides via direct C-H bond amination. With FeI2, amination is fast and selective, (<30 min for 80% yield at 2 mol% loading), TONs up to 370 are reached with just 0.1 mol% catalyst, different functional groups are tolerated, and a variety of C-H bonds were activated.

Total synthesis of haploscleridamine, villagorgin A and an approach towards lissoclin C

An investigation of asymmetric total syntheses of three indole-imidazole alkaloids from histidine are described. A common advanced piperidinone was contructed via a ring-closing metathesis which was then subjected to a modified Fischer indole synthesis. Deprotection of an N-tosyl group via a dissolving metal reduction affords haploscleridamine which upon reaction with aqueous formaldehyde in trifluoroethanol provided villagorgin A. On closer examination, it was found that villagorgin A was produced as a byproduct during the reductive detosylation in the presence of magnesium and methanol. Attempts to obtain the brominated haploscleridamine congener, lissoclin C through use of bromophenyl hydrazone were thwarted by reductive debromination during deprotection efforts. Investigation of the enantiopurity of the synthetic natural products revealed production of almost racemic materials in some batches as the result of partial racemization of an early stage intermediate. A revised approach routinely provided scalemic haploscleridamine and villagorgin in 30% ee. Analysis of the enantiomer composition of all intermediates by HPLC using columns with chiral stationary phases; this analysis revealed several steps where erosion of enantiomer composition occurred.

Expanding the Synthetic Toolbox through Metal-Enzyme Cascade Reactions

The combination of metal-, photo-, enzyme-, and/or organocatalysis provides multiple synthetic solutions, especially when the creation of chiral centers is involved. Historically, enzymes and transition metal species have been exploited simultaneously through dynamic kinetic resolutions of racemates. However, more recently, linear cascades have appeared as elegant solutions for the preparation of valuable organic molecules combining multiple bioprocesses and metal-catalyzed transformations. Many advantages are derived from this symbiosis, although there are still bottlenecks to be addressed including the successful coexistence of both catalyst types, the need for compatible reaction media and mild conditions, or the minimization of cross-reactivities. Therefore, solutions are here also provided by means of catalyst coimmobilization, compartmentalization strategies, flow chemistry, etc. A comprehensive review is presented focusing on the period 2015 to early 2022, which has been divided into two main sections that comprise first the use of metals and enzymes as independent catalysts but working in an orchestral or sequential manner, and later their application as bionanohybrid materials through their coimmobilization in adequate supports. Each part has been classified into different subheadings, the first part based on the reaction catalyzed by the metal catalyst, while the development of nonasymmetric or stereoselective processes was considered for the bionanohybrid section.

Iodonium ylides: an emerging and alternative carbene precursor for C-H functionalizations

The metal-catalyzed successive activation and functionalization of arene/heteroarene is one of the most fundamental transformations in organic synthesis and leads to privileged scaffolds in natural products, pharmaceuticals, agrochemicals, and fine chemicals. Particularly, transition-metal-catalyzed C-H functionalization of arenes with carbene precursors via metal carbene migratory insertion has been well studied. As a result, diverse carbene precursors have been evaluated, such as diazo compounds, sulfoxonium ylides, triazoles, etc. In addition, there have been significant developments with the use of iodonium ylides as carbene precursors in recent years, and these reactions proceed with high efficiencies and selectivities. This review provides a comprehensive overview of iodonium ylides in C-H functionalizations, including the scope, limitations, and their potential synthetic applications.

One-pot double annulations to confer diastereoselective spirooxindolepyrrolothiazoles

A novel four-component reaction in one pot as an atom- and step-economic process was developed to synthesize diastereoselectively spirooxindolepyrrolothiazoles through sequential N,S-acetalation of aldehydes with cysteine and decarboxylative [3 + 2] cycloaddition with olefinic oxindoles. High synthetic efficiency, operational simplification and reaction process economy using EtOH as solvent, and only releasing CO2 and H2O as side products confer this approach favorable in green chemistry metrics analysis.

Silicon Catalyzed C-O Bond Ring Closing Metathesis of Polyethers

The Lewis superacid bis(perchlorocatecholato)silane catalyzes C-O bond metathesis of alkyl ethers with an efficiency outperforming all earlier reported systems. Chemoselective ring contractions of macrocyclic crown ethers enable substrate-specific transformations, and an unprecedented ring-closing metathesis of polyethylene glycols allows polymer-selective degradation. Quantum chemical computations scrutinize a high Lewis acidity paired with a simultaneous low propensity for polydentate substrate binding as critical for successful catalysis. Based on these mechanistic insights, a second-generation class of silicon Lewis superacid with enhanced efficacy is identified and demonstrated.

Total Synthesis of Scabrolide F

The first total synthesis of scabrolide F, a norcembranolide isolated from the soft coral Sinularia scabra, is described. Hydroxycarboxylic acid, which is the key synthetic intermediate, was synthesized in a convergent manner by fragment coupling. The obtained hydroxycarboxylic acid was subjected to macrolactonization and subsequent transannular ring-closing metathesis (RCM) to furnish scabrolide F. The synthetic protocol can be extended to the total synthesis of other norcembranolides.

Enantioselective Synthesis of Atropisomers via Vinylidene ortho-Quinone Methides (VQMs)

Atropisomers, arising from conformational restriction, are inherently chiral due to the intersecting dissymmetric planes. Since there are numerous applications of enantiopure atropisomers in catalyst design, drug discovery, and material science, the asymmetric preparation of these highly prized molecules has become a flourishing field in synthetic chemistry. A number of catalysts, synthetic procedures, and novel concepts have been developed for the manufacture of the atropisomeric molecules. However, due to the intrinsic properties of different types of atropisomers featuring biaryl, hetero-biaryl, or non-biaryl architectures, only very few methods pass the rigorous inspection and are considered generally applicable. The development of a broadly applicable synthetic strategy for various atropisomers is a challenge. In this Account, we summarize our recent studies on the enantioselective synthesis of atropisomers using the vinylidene ortho-quinone methides (VQMs) as pluripotent intermediates.The most appealing features of VQMs are the disturbed aromaticity and axial chirality of the allene fragment. At the outset, the applications of VQMs in organic synthesis have been neglected due to their principal liabilities: ephemeral nature, extraordinary reactivity, and multireaction sites. The domestication of this transient intermediate was demonstrated by in situ catalytic asymmetric generation of VQMs, and the reactivity and selectivity were fully explored by judiciously modifying precursors and tuning catalytic systems. A variety of axially chiral heterocycles were achieved through five-, six-, seven- and nine-membered ring formation of VQM intermediates with different kinds of branched nucleophilic functional groups. The axially chiral C-N axis could be constructed from VQM intermediates via N-annulation or desymmetrization of preformed C-N scaffolds. We take advantage of the high electrophilicity of VQMs toward a series of sulfur and carbon based nucleophiles leading to atropisomeric vinyl arenes. Furthermore, chiral helical compounds were realized by cycloaddition or consecutive annulation of VQM intermediates. These achievements demonstrated that the VQMs could work as a nuclear parent for the collective synthesis of distinct and complex optically active atropisomers. Recently, we have realized the isolation and structural characterization of the elusive VQMs, which were questioned as putative intermediates for decades. The successful isolation of VQMs provided direct evidence for their existence and an unprecedented opportunity to directly investigate their reactivity. The good thermal stability and reserved reactivity of the isolated VQMs demonstrated their great potential as synthetic reagents and expanded the border of VQM chemistry.

B2pin2-Mediated Cascade Cyclization/Aromatization Reaction: Facial Access to Functionalized Indolizines

Herein, a B₂pin₂-mediated radical cascade cyclization/aromatization reaction of enaminone with pyridine is described. This strategy provides a practical way for the construction of valuable functionalized indolizines under metal-, external oxidant-, and base-free conditions, which could be compatible with various kinds of functional groups, such as halogen, π-system, heterocycle, ferrocenyl, etc. A preliminary mechanism investigation indicated that the pyridine-boryl radical formed in situ triggered the reaction to occur.

Radical annulation of a designed diene system: access to nitro-benzo[b]azepines

Herein, we describe a novel O₂N˙-triggered ordered addition 7-endo cyclization reaction with excellent chemo- and regioselectivity. With such a strategy, structurally diverse nitro-benzo[b]azepines were prepared with 28 examples. Large-scale operation and handy N-Ts and N-Cbz deprotection reveal the promising utility of this methodology. Mechanistic studies suggest that the reaction proceeds through a radical pathway.

Rational design of 1,2,3-triazole hybrid structures as novel anticancer agents: synthesis, biological evaluation and molecular docking studies

New hybrid compounds belonging to the class of 1,4-disubstituted 1,2,3-triazoles were synthesized. The structural characterization of the synthesized compounds was performed using IR, ¹H-NMR, ¹³C NMR and elemental analysis techniques. Diarylketones 1a and 1b were used as starting compounds for the synthesis of triazoles. The corresponding diarylmethanol derivatives (2a,b) were obtained from reduction of ketone units with NaBH₄. Oxyalkynes (3a,b) were obtained by treating the hydroxyl group with NaH in anhydrous THF and then with propargylbromide. The target hybrid structures 6a-n were obtained from the metal-catalyzed "click reaction" of the arylazide and alkyne units. The newly synthesized compounds were structurally analysed using ¹H-NMR, ¹³C-NMR, elemental analysis, LC-MS and FT-IR. The antioxidant and anticancer activities of all compounds were investigated. It has been determined that the new hybrid structures have very good antioxidant and anticancer activities according to the standards. In particular, compounds 6b, 6h, 6i and 6j (IC₅₀: 1.87, 12.5, 7.22, 8.04 µM) showed excellent activity compared to standard 5-Fu (IC₅₀: 40.89 µM). According to the results of molecular docking of compounds 6b and 6i with the highest cancer activity, MetAP-2 was found to have a high affinity through exposed polar and apolar contacts with fundemental residues in the binding pocket.Communicated by Ramaswamy H. Sarma.

Synthetic strategies for the preparation of γ-phostams: 1,2-azaphospholidine 2-oxides and 1,2-azaphospholine 2-oxides

γ-Phostams include γ-phosphonolactams and γ-phosphinolactams and their fused derivatives, phosphorus analogues of γ-lactams. They are 1,2-azaphospholidine 2-oxides and 1,2-azaphospholine 2-oxides and important biological five-membered azaphosphaheterocycles. They have been prepared through two major strategies of cyclizations and annulations. Cyclizations achieve ring construction through the formation of any bond in the ring, while annulations build the ring via [4 + 1] and [3 + 2] fashions with the simultaneous formation of two bonds. The review includes the synthesis of 1,2-azaphospholidine and 1,2-azaphospholine 2-oxides/sulfides and their fused derivatives.

Design and synthesis of unprecedented 9- and 10-membered cyclonucleosides with PRMT5 inhibitory activity

Synthesis of medium-sized rings is known to be challenging due to high transannular strain especially for 9- and 10-membered rings. Herein we report design and synthesis of unprecedented 9- and 10-membered purine 8,5'-cyclonucleosides as the first cyclonucleoside PRMT5 inhibitors. The cocrystal structure of PRMT5:MEP50 in complex with the synthesized 9-membered cyclonucleoside 1 revealed its binding mode in the SAM binding pocket of PRMT5.

Dual-target Janus kinase (JAK) inhibitors: Comprehensive review on the JAK-based strategies for treating solid or hematological malignancies and immune-related diseases

Janus kinases (JAKs) are the non-receptor tyrosine kinases covering JAK1, JAK2, JAK3, and TYK2 which regulate signal transductions of hematopoietic cytokines and growth factors to play essential roles in cell growth, survival, and development. Dysregulated JAK activity leading to a constitutively activated signal transducers and activators of transcription (STAT) is strongly associated with immune-related diseases and cancers. Targeting JAK to interfere the signaling of JAK/STAT pathway has achieved quite success in the treatment of these diseases. However, inadequate clinical response and serious adverse events come along by the treatment of monotherapy of JAK inhibitors. With better and deeper understanding of JAK/STAT pathway in the pathogenesis of diseases, researchers start to show huge interest in combining inhibition of JAK and other oncogenic targets to realize a broader regulation on pathological processes to block disease development and progression, which has hastened extensive research of dual JAK inhibitors over the past decades. Until now, studies of dual JAK inhibitors have added BTK, SYK, FLT3, HDAC, Src, and Aurora kinases to the overall inhibitory profile and demonstrated significant advantage and superiority over single-target inhibitors. In this review, we elucidated the possible mechanism of synergic effects caused by dual JAK inhibitors and briefly describe the development of these agents.