The use of spirocyclic scaffolds in drug discovery

Gold(I)-Catalyzed Reactions of exo-Glycals with Propargyl Esters toward C-1 Alkenyl Spirocyclopropyl Carbohydrates

An atom-economic and diazo-free strategy for the construction of novel pseudo anomeric C-1 alkenyl spirocyclopropyl sugars is described. Leveraging the 1,2-migration pathway of propargyl esters under gold(I) catalysis, easily available exo-glycals undergo β-selective alkenylcarbenoid insertion in moderate to excellent yields. Preferential activation of propargyl moieties and concerted [2 + 1] insertion are both favored through ligand choice and electron enrichment of esters. Stereocontrol using conformational bias and rearrangement are also demonstrated.

Asymmetric Remote Aldol Cyclization Reaction to Synthesize Trifluoromethylated Heterospirocyclic Frameworks

The highly enantioselective organocatalytic synthesis of dihydropyran spirocyclic compounds bearing di- and trifluoromethyl groups by aldol cyclization reaction via trienamine using cyclic 2,5-dienones and different di- and trifluoromethylketones is described. Using a bifunctional aminothiourea catalyst, trifluoromethyl-functionalized dihydropyran spirocyclic products were obtained with good yields and enantioselectivities. Subsequent transformation with H2 and Pd/C has allowed the synthesis of the tetrahydropyran structure with three stereocenters. The plausible reaction mechanism was investigated by computational methods.

Microwave-assisted protocol towards synthesis of heterocyclic molecules: a comparative analysis with conventional synthetic methodologies (years 2019-2023): a review

Microwave-assisted protocols have become extensively accepted across various scientific and technological domains because of their numerous advantages, shorter reaction times, higher yields, and often milder reaction conditions. In this review, we focus on the synthesis of N, O, and S-containing heterocyclic structural cores, crucial in the development of pharmaceuticals, agrochemicals, and materials science following through conventional and microwave method via eliminating the side products and enhances the product yield that is nowadays the biggest barrier for a synthetic chemist. The major findings emphasizes the substantial advantages of microwave-assisted techniques over conventional synthetic protocols. This comparative study underscores the potential of microwave-assisted techniques to revolutionize heterocyclic compound synthesis, providing insights into optimizing reaction conditions and expanding the scope of chemical synthesis in industrial applications.

Palladium-Catalyzed (3+2) and (4+2) Cycloaddition Reactions of Sulfamidate Imine-Derived Azadienes: Synthesis of Spirocyclic Pyrrolidines and Tetrahydroquinolines

Diastereoselective Pd-catalyzed (3+2) and (4+2) cycloaddition reactions of sulfamidate imine-derived 1-azadienes with zwitterionic N-dipoles derived from 1-tosyl-2-vinylaziridine and 4-vinylbenzoxazinone have been developed. These reactions provide highly functionalized azaspirocycles featuring three contiguous stereocenters. The sulfonyl imine moiety of the cycloadducts can be fully reduced to access valuable β-amino alcohols.

One-Pot Tandem/Spirocyclization Reaction: Synthesis of Spiro[pyridine-thiazolidine] Ring Derivatives

An efficient one-pot, three-component approach was devised to synthesize spiro[pyridine-thiazolidine] ring skeletons using thiazole salts, aldehydes, and enaminones. This method allows the assembly of structurally diverse spiroazepines through [3 + 1 + 2] tandem/spirocyclization reactions. This spirocyclization reaction offers several advantages, including transition metal-free conditions, high chemoselectivity, and the ability to construct structurally novel polycyclic compounds.

Synthesis of fluorinated spiro-1,3-oxazines and thiazines via Selectfluor-mediated intramolecular cyclization

A fluorination-induced intramolecular cyclization for the synthesis of fluoro-substituted spiro-1,3-oxazine and spiro-1,3-thiazine derivatives is described. N-(2-(Cyclohex-1-en-1-yl)ethyl)benzamide and benzothioamide are used as the substrates, and the cationic fluorinating agent Selectfluor works as the fluoride source. The reaction yields a single diastereomer. The scope of this regioselective fluorination reaction is established by preparing thirty different spirooxazine and spirothiazine derivatives.

Catalyzed syntheses of novel series of spiro thiazolidinone derivatives with nano Fe2O3: spectroscopic, X-ray, Hirshfeld surface, DFT, biological and docking evaluations

Twelve spiro thiazolidinone compounds (A-L) were synthesized via either conventional thermal or ultrasonication techniques using Fe2O3 nanoparticles. The modification of the traditional procedure by using Fe2O3 nanoparticles led to enhancement of the yield of the desired candidates to 78-93% in approximately half reaction time compared with 58-79% without catalyst. The products were fully characterized using different analytical and spectroscopic techniques. The structure of the two derivatives 4-phenyl-1-thia-4-azaspirodecan-3-one (A) and 4-(p-tolyl)-1-thia-4-azaspirodecan-3-one (B) were also determined using single crystal X-ray diffraction and Hirshfeld surface analysis. The two compounds (A and B) were crystallized in the orthorhombic system with Pbca and P212121 space groups, respectively. In addition, the crystal packing of compounds revealed the formation of supramolecular array with a net of intermolecular hydrogen bonding interactions. The energy optimized geometries of some selected derivatives were performed by density functional theory (DFT/B3LYP). The reactivity descriptors were also calculated and correlated with their biological properties. All the reported compounds were screened for antimicrobial inhibitions. The two derivatives, F and J, exhibited the highest levels of bacterial inhibition with an inhibition zone of 10-17 mm. Also, the two derivatives, F and J, displayed the most potent fungal inhibition with an inhibition zone of 15-23 mm. Molecular docking investigations of some selected derivatives were performed using a B-DNA (PDB: 1BNA) as a macromolecular target. Structure and activity relationship of the reported compounds were correlated with the data of antimicrobial activities and the computed reactivity parameters.

Stereoselective Double Spirocyclization of 2-Benzyl-3-alkynyl Chromone with Nitrone via Gold-Catalyzed Cascade Reactions

A novel, highly stereoselective gold-catalyzed spirocyclization of 2-benzyl-3-alkynyl chromone with nitrone is described. This cascade reaction involves gold-catalyzed cycloisomerization, nitrone-olefin [3 + 2]-annulation, alkene oxidation, and rearrangement for the formation of spirocyclic products. Interestingly, the isoxazolidine ring generated from [3 + 2]-annulation donates oxygen to alkene to generate a new pyran-3(4H)-one and azetidine ring for dispiro-benzofuran formation upon heating. This work demonstrates the one-pot, gold-catalyzed, multiple-step reaction, and the reaction temperature directly affects the formation of spirocyclic products.

Enantioselective Synthesis of Spiro[cyclopentane-1,3'-oxindole] Scaffolds with Five Consecutive Stereocenters

A highly diastereo- and enantioselective cascade annulation reaction of Morita-Baylis-Hillman (MBH) maleimides of isatins with ortho-hydroxychalcones was achieved by a chiral N,N'-dioxide/Mg(II) complex Lewis acid catalyst. This strategy provides a concise and efficient route to densely functionalized spiro[cyclopentane-1,3'-oxindole] compounds with five consecutive stereocenters. The reaction itself features mild conditions, good functional group compatibility, and broad substrate scope (62 examples, up to 99% yield, up to >20:1 dr, 97% ee). In addition, an obvious ligand acceleration effect and chiral amplification effect were observed. DFT calculations were performed to elucidate the stereoselectivity observed. The gram-scale synthesis and the inhibitory effect of two products on the viability of A549 cells demonstrate the potential utility of the current method.

Organocatalytic Asymmetric Construction of Spirooxazines via Chemoselective Cascade Addition of N-Substituted Hydroxylamine with Keto-bis-enone

Spirooxazines represent a privileged heterocyclic scaffold having pronounced biological importance. Herein, we introduce a chiral bifunctional squaramide catalyzed highly chemoselective cascade reaction involving aza-Michael/1,2-addition/oxa-Michael addition of N-substituted hydroxylamine with keto-bis-enones. This strategy enables the synthesis of highly enantioenriched oxa-spirooxazines with a broad substrate tolerance. Scalability and synthetic transformation have demonstrated the feasibility of the protocol. Furthermore, control experiments provided insights into the reaction mechanism.

Electrochemical selenylative ipso-annulation of N-benzylacrylamides to construct seleno-azaspiro[4.5]decadienones

Herein, we present the electrochemical synthesis of selenylated azaspiro[4.5]decadienones through domino selenylation/ipso-annulation of N-benzylacrylamides with diselenides. The method showed a wide substrate scope under mild and external oxidant-free reaction conditions, involving the construction of C-Se and C-C bonds. Gram-scale synthesis and further functional group conversion of the product are also demonstrated.

Base-Mediated Divergent Synthesis of Spiro-heterocycles Using Pronucleophiles and Ethylene via Thianthrenation

Spirocyclic compounds are abundant in biologically active products. However, the divergent synthesis of spirocyclic compounds using low-cost and abundant available starting materials remains a challenge. Herein, we report an effective method for producing spirocyclic motifs using a cyclic β-carbonyl ester or amide and ethylene via thianthrenation. This strategy highlights the exciting possibility of utilizing abundant ethylene as a C-2 synthon and allows regulating the core structure of the spirocyclic compound by simply altering the base type.

Exploring the Reactivity of Rigid 1-Azadienes Derived from Methylene γ-Lactams. Applications to the Stereoselective Synthesis of Spiro-γ-Lactams

A study on the reactivity of rigid 1-azadienes derived from methylene γ-lactams is reported. Through the functionalization of 1-amino α,β-unsaturated γ-lactam derivatives, easily available from a multicomponent reaction of amines, aldehydes, and pyruvates, it is possible to in situ generate rigid 1-azadienes locked by a γ-lactam core. The 4π-electron system of those rigid 1-azadienes can behave as both diene and dienophile species through a spontaneous cyclodimerization reaction or exclusively as dienes or dienophiles if they are trapped with imines or cyclopentadiene, respectively. The use of chiral rigid 1-azadienes as dienophiles in the cycloaddition reaction with cyclopentadiene leads to the formation of spiro-γ-lactams bearing four stereogenic centers in a highly stereospecific manner, reporting the first example of the use of methylene-γ-lactams in the synthesis of spirocycles.

The Nitrofuran-Warhead-Equipped Spirocyclic Azetidines Show Excellent Activity against Mycobacterium tuberculosis

A series of 21 new 7'H-spiro[azetidine-3,5'-furo [3,4-d]pyrimidine]s substituted at the pyrimidine ring second position were synthesized. The compounds showed high antibacterial in vitro activity against M. tuberculosis. Two compounds had lower minimum inhibitory concentrations against Mtb (H37Rv strain) compared with isoniazid. The novel spirocyclic scaffold shows excellent properties for anti-tuberculosis drug development.

Photochemical dearomative skeletal modifications of heteroaromatics

Dearomatization has emerged as a powerful tool for rapid construction of 3D molecular architectures from simple, abundant, and planar (hetero)arenes. The field has evolved beyond simple dearomatization driven by new synthetic technology development. With the renaissance of photocatalysis and expansion of the activation mode, the last few years have witnessed impressive developments in innovative photochemical dearomatization methodologies, enabling skeletal modifications of dearomatized structures. They offer truly efficient and useful tools for facile construction of highly complex structures, which are viable for natural product synthesis and drug discovery. In this review, we aim to provide a mechanistically insightful overview on these innovations based on the degree of skeletal alteration, categorized into dearomative functionalization and skeletal editing, and to highlight their synthetic utilities.

Gold(I)-catalysed cyclisation of (E)-ketene-N,O-acetals: a synthetic route toward spiro-oxazole-γ-lactones

In this study, we developed a cascade 5,5-cyclisation of internal ketene-N,O-acetals utilizing homogeneous Au(I) catalysis. This process involves an initial 5-exo-dig carbocyclisation, followed by a 5-exo-dig heterocyclisation that stereoselectively incorporates the O-atom of a water molecule into an N-tethered propargyl alkyne. This sequential reaction results in the formation of one C-C, two C-O, and two C-I bonds, ultimately leading to the synthesis of spiro-α-iodo-γ-lactone structures featuring oxazole rings in good yields.

Microwave produced 8-methyl-1,2,4,8-tetraazaspiro[4.5]dec-2-en-3-amine derivatives: their in vitro and in silico analysis

Compound 1 is formed by a microwave-assisted multicomponent reaction of 1-methylpiperidin-4-one, 2-amino-4-methoxy-6-methyl-1,3,5-triazine, and thiosemicarbazide, followed by the synthesis of Schiff base 2a-l with a variety of aldehydes. A comparison was made between the conventional and microwave methods, and the microwave approach was shown to be considerably superior to the classical method since it takes less time and produces higher yields. Several spectral investigations, including 1H NMR, 13C NMR, Mass, and IR spectroscopy, are used to characterize the complete series. In vitro antibacterial testing suggests that compounds 2c, 2f, and 2g are promising antibacterial agents, although compounds 2d, 2e, and 2l are effective antimycobacterial agents when compared to the conventional medicine Rifampicin. The docking score from docking studies is considerable, which validates the results of the biological examination. Molecular docking was performed on Escherichia coli DNA gyrase. According to the in silico ADME analysis, each drug molecule is ideal for use in terms of drug solubility, hydrogen bonding, and cell permeability.

Eosin, blue LEDs and DIPEA are employed in a simple synthesis of (poly)cyclic O,O- and N,O-acetals

A simple procedure for the synthesis of (poly)cyclic O,O- and N,O-acetals from various enol ethers, N-acyl enamines or Boc-protected enamines has been developed. The key step is a photocatalytic Stork-Ueno-type cylization using the very simple metal-free conditions of catalytic eosin, diisopropylamine in the green solvent ethanol with blue LED irradition.

BBr3-mediated dearomative spirocyclization of biaryl ynones: facile access to spiro[5.5]dienones

This report covers boron tribromide (BBr3) mediated dearomative spirocyclization of biaryl ynones. The direct synthesis of spiro[5.5]dienones with a tri-substituted double bond is described for the first time in this paper. The scope of the reaction is broad and the spirocyclic products were obtained in moderate to good yields.

Diastereoselective synthesis of functionalized spiroindolines via intramolecular ipso-iodocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Herein, we describe a highly diastereoselective approach for synthesizing polyfunctionalized spiroindolines from indolyl-ynones involving an ipso-iodocyclization/nucleophile addition cascade. The developed strategy allows the formation of a spirocyclic core and the installation of two functional groups in a single operation. Also this strategy is accompanied by the generation of two C-C and one C-I bonds and two contiguous stereocenters.

A water mediated multicomponent reaction for the synthesis of novel spirooxindole derivatives and their antifungal activity

A water mediated three-component reaction of isatin, 4-aminocoumarin, and 1,3-cyclodicarbonyl compounds is reported for the synthesis of spiro[chromeno[4,3-b]cyclopenta[e]pyridine-7,3'-indoline]trione and the spiro[chromeno[4,3-b]quinoline 7,3'-indoline]trione. Up to 27 different spirooxindole derivatives were synthesized by this method. The bioactivity of these spirooxindole derivatives was evaluated and they were found to show antifungal activity against Cercospora arachidicola, Physalospora piricola, Rhizoctonia cerealis, and Fusarium moniliforme.

A Three-Component Synthesis of Spiropyrrolines

A rapid synthesis of medicinally important spiropyrrolines has been achieved by the reaction of 1,3-indanediones, benzaldehyde, and (1-azidovinyl)benzene under catalyst- and reagent-free conditions. This is the first report to synthesize diverse spiropyrroline compounds from readily available feedstock materials under metal-free conditions.

Photoinduced Alkylsulfonylation and Cyanoalkylsulfonylation of Morita-Baylis-Hillman Adducts via Multicomponent Insertion of Sulfur Dioxide

General and convenient visible-light-promoted alkylsulfonylation and cyanoalkylsulfonylation of MBH adducts have been developed through the multicomponent insertion of sulfur dioxide, enabling the assembly of two C-S bonds to generate structurally diverse allylic alkylsulfones (43 examples in total). The reaction of MBH adducts with potassium alkyltrifluoroborates and 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) adduct afforded sulfones with generally good yields. Notably, the addition of N,N,N',N'-tetramethylethylenediamine as a base into the photocatalytic system led to yielding an alkyl sulfonyl unit and cyano group-anchored trisubstituted alkenes by utilizing cycloketone oxime esters as C-radical precursors. Both of these reactions have constructed two C-S bonds, and all desired products were obtained in moderate to excellent yields with complete stereospecificity.

Catalytic Asymmetric Synthesis of 3,4'-Piperidinoyl Spirooxindoles via [3 + 3] Annulation of 3-Aminobenzofurans and Isatin-Derived Enals

A chiral NHC-catalyzed [3 + 3] cycloaddition reaction of 3-aminobenzofurans with isatin-derived enals has been documented, furnishing 3,4'-piperidinoyl spirooxindoles bearing a quaternary stereocenter with good yields and excellent enantioselectivities. Further gram-scale preparation and synthetic transformation of the cycloadducts to δ-amino acid derivative demonstrated good practicality and applicability of this reaction.

Novel spiroindoline derivatives targeting aldose reductase against diabetic complications: Bioactivity, cytotoxicity, and molecular modeling studies

Despite significant developments in therapeutic strategies, Diabetes Mellitus remains an increasing concern, leading to various complications, e.g., cataracts, neuropathy, retinopathy, nephropathy, and several cardiovascular diseases. The polyol pathway, which involves Aldose reductase (AR) as a critical enzyme, has been focused on by many researchers as a target for intervention. On the other hand, spiroindoline-based compounds possess remarkable biological properties. This guided us to synthesize novel spiroindoline oxadiazolyl-based acetate derivatives and investigate their biological activities. The synthesized molecules' structures were confirmed herein, using IR, NMR (1H and 13C), and Mass spectroscopy. All compounds were potent inhibitors with KI constants spanning from 0.186 ± 0.020 μM to 0.662 ± 0.042 μM versus AR and appeared as better inhibitors than the clinically used drug, Epalrestat (EPR, KI: 0.841 ± 0.051 μM). Besides its remarkable inhibitory profile compared to EPR, compound 6k (KI: 0.186 ± 0.020 μM) was also determined to have an unusual pharmacokinetic profile. The results showed that 6k had less cytotoxic effect on normal mouse fibroblast (L929) cells (IC50 of 569.58 ± 0.80 μM) and reduced the viability of human breast adenocarcinoma (MCF-7) cells (IC50 of 110.87 ± 0.42 μM) more than the reference drug Doxorubicin (IC50s of 98.26 ± 0.45 μM and 158.49 ± 2.73 μM, respectively), thus exhibiting more potent anticancer activity. Moreover, molecular dynamic simulations for 200 ns were conducted to predict the docked complex's stability and reveal significant amino acid residues that 6k interacts with throughout the simulation.

Brønsted Acid-Catalyzed Diastereoselective Synthesis of Spiroisoindolinones from Enamides

A highly diastereoselective synthesis of spiroisoindolinones from enamides and 3-hydroxy-isoindolinones is reported. The reaction proceeds rapidly in the presence of p-toluenesulfonic acid as a Brønsted acid catalyst and affords a variety of densely substituted spiroisoindolinones with three contiguous stereogenic centers in high yields (≤98%) and diastereoselectivities (up to dr >98:<2:0:0).

C-H activation-initiated spiroannulation reactions and their applications in the synthesis of spirocyclic compounds

Spirocyclic skeletons are prevalent in natural products, pharmaceuticals and organic functional materials. Meanwhile, transition-metal-catalyzed C-H activation reactions have demonstrated unparalleled advantages such as high efficiency, excellent atom-economy, good chemoselectivity and regioselectivity for the formation of target organic molecules. In recent years, C-H activation reactions have been creatively utilized in the synthesis of spirocyclic compounds. This review summarizes the most recent progress made in C-H activation-initiated spiroannulation reactions and their applications in the construction of structurally diverse and biologically valuable spirocyclic scaffolds by using alkynes, diazo compounds, maleimides, alkenes, quinones and cyclopropenones as the coupling partners.

Entry to new spiroheterocycles via tandem Rh(II)-catalyzed O-H insertion/base-promoted cyclization involving diazoarylidene succinimides

A facile approach to novel medicinally relevant spiro heterocyclic scaffolds (namely furan-2(5H)-ones, tetrahydrofurans and pyrans spiro-conjugated with the succinimide ring) has been developed. The protocol consists of Rh(II)-catalyzed insertion of heterocyclic carbenes derived from diazoarylidene succinimides (DAS) into the O-H bond of propiolic/allenic acids or brominated alcohols, followed by base-promoted cyclization to afford the target spirocyclic compounds in good to high yields.

Electrophile-Controlled Regiodivergent Palladium-Catalyzed Imidoylative Spirocyclization of Cyclic Alkenes

An intermolecular controllable Pd-catalyzed spirocyclization of isocyano cycloalkenes has been developed, offering efficient and selective approaches toward spirocyclic hydropyrrole scaffolds. 2-Azaspiro-1,7-dienes could be obtained through a "chain-walking" process with aryl/vinyl iodides as electrophiles, while the normal Heck product 2-azaspiro-1,6-dienes were selectively generated when aryl triflates were used as the coupling partner of isocyanides. Mechanistic studies suggested that the counteranion of the Pd(II) intermediate played a crucial role in the regioselectivity control. Dihydropyrrole-fused 5,6,7-membered spirocycles were switchably accessed under mild conditions with wide functional group tolerance.

Facile Synthesis of Quinoline-Substituted 3-Hydroxy-2-oxindoles and 3-Amino-2-oxindoles via a Palladium-Catalyzed Cascade Intramolecular Cyclization/Intermolecular Nucleophilic Addition Reaction

An efficient cascade intramolecular cyclization/intermolecular nucleophilic addition reaction of allenyl benzoxazinone with isatin or isatin-derived ketimine has been established by using Pd0-π-Lewis base catalysis. A series of 3-hydroxy-2-oxindoles and 3-amino-2-oxindoles with quaternary carbon atoms at the C3 position were synthesized in good yields under mild conditions through this protocol.

Access to Spiro Bis-β-lactams via a Metal-Free Microwave-Assisted Wolff Rearrangement/Staudinger [2+2] Cycloaddition Cascade Involving 3-Diazotetramic Acids and Imines

Herein, we report the study of the thermally promoted reaction of 3-diazotetramic acids with imines as a rapid route to a novel spiro heterocyclic scaffold, spiro bis-β-lactams (2,6-diazaspiro[3.3]heptane-1,5-diones). The transformation proceeds via metal-free microwave-assisted Wolff rearrangement of the diazo reagent followed by Staudinger [2+2] cycloaddition of the heterocyclic ketenes with Shiff bases. This methodology enables the preparation of diastereomerically pure spiro bis-β-lactams in high yields and provides an avenue for exploring new versions of the privileged β-lactam core for drug design.

Strain-enabled radical spirocyclization cascades: rapid access to spirocyclobutyl lactones and - lactams

Spirocyclobutane derivatives have gained significant attention in drug discovery programs due to their broad spectrum of biological activities and clinical applications. Ring-strain in organic molecules is a powerful tool to promote reactivity by releasing strain energy, allowing the construction of complex molecules selectively and efficiently. Herein, we report the first strain-enabled radical spirocyclization cascades for the synthesis of functionalized spirocyclobutyl lactones and - lactams, which are finding increasing applications in medicinal chemistry. The reaction of interelement compounds with bicyclobutane (BCB) allyl esters and - amides proceeds with high chemoselectivity under simple, catalyst-free conditions using blue light irradiation. The reaction has been successfully extended to synthesize bis-spirocycles. To introduce a more diverse set of functional groups, we have developed a dual photoredox/nickel catalytic system capable of mediating the carbosulfonylation of BCB allyl amides. The reaction shows broad applicability across various (hetero)aryl halides, aryl sulfinates, and BCB allyl amides, operates under mild conditions and demonstrates excellent functional group compatibility. The functional groups introduced during the cascade reactions served as versatile handles for further synthetic elaboration.

Electrochemical Oxidative Dearomatization Strategy for Accessing Spiro[4.5]dienones and Derivatives

Electrochemical dearomatization has been recognized as an attractive tool for the rapid construction of structurally diverse molecules. The designed methodology encompasses an eco-friendly and efficient electrochemical approach to synthesizing spiro[4.5]dienones under mild reaction conditions. Furthermore, detailed mechanistic studies strongly bolster our hypothesis and emphasize the role of HFIP in the mechanism. The protocol is scalable and showcases a broad substrate scope with tolerance toward numerous functional groups. Henceforth, this strategy can be deployed as an alternative and sustainable tool for accessing spiro[4.5]dienones.

D3Rings: A Fast and Accurate Method for Ring System Identification and Deep Generation of Drug-Like Cyclic Compounds

Continuous exploration of the chemical space of molecules to find ligands with high affinity and specificity for specific targets is an important topic in drug discovery. A focus on cyclic compounds, particularly natural compounds with diverse scaffolds, provides important insights into novel molecular structures for drug design. However, the complexity of their ring structures has hindered the applicability of widely accepted methods and software for the systematic identification and classification of cyclic compounds. Herein, we successfully developed a new method, D3Rings, to identify acyclic, monocyclic, spiro ring, fused and bridged ring, and cage ring compounds, as well as macrocyclic compounds. By using D3Rings, we completed the statistics of cyclic compounds in three different databases, e.g., ChEMBL, DrugBank, and COCONUT. The results demonstrated the richness of ring structures in natural products, especially spiro, macrocycles, and fused and bridged rings. Based on this, three deep generative models, namely, VAE, AAE, and CharRNN, were trained and used to construct two data sets similar to DrugBank and COCONUT but 10 times larger than them. The enlarged data sets were then used to explore the molecular chemical space, focusing on complex ring structures, for novel drug discovery and development. Docking experiments with the newly generated COCONUT-like data set against three SARS-CoV-2 target proteins revealed that an expanded compound database improves molecular docking results. Cyclic structures exhibited the best docking scores among the top-ranked docking molecules. These results suggest the importance of exploring the chemical space of structurally novel cyclic compounds and continuous expansion of the library of drug-like compounds to facilitate the discovery of potent ligands with high binding affinity to specific targets. D3Rings is now freely available at http://www.d3pharma.com/D3Rings/.

Synthesis of Spirocyclohexadienones via Palladium-Catalyzed Dearomatization of Dibenzoxaborins

The development of Pd(II)-catalyzed dearomatization transformation of dibenzoxaborins with alkynes triggered by transmetalation from boron to palladium has been achieved, leading to the synthesis of spirocyclohexadienones, an important skeleton demonstrating potential biomedical utility. The [3 + 2] spiroannulation exhibits remarkable regioselectivity and broad substrate scope under mild reaction conditions. This methodology employs dibenzoxaborin as a substrate to establish the formal dearomatization of 2-phenylphenol, which poses a formidable energy barrier to the destruction of aromaticity.

Recent advances in microwave-assisted multicomponent synthesis of spiro heterocycles

Spiro heterocycle frameworks are a class of organic compounds that possesses unique structural features making them highly sought-after targets in drug discovery due to their diverse biological and pharmacological activities. Microwave-assisted organic synthesis has emerged as a powerful tool for assembling complex molecular architectures. The use of microwave irradiation in synthetic chemistry is a promising method for accelerating reaction rates and improving yields. This review provides insights into the current state of the art and highlights the potential of microwave-assisted multicomponent reactions in the synthesis of novel spiro heterocyclic compounds that were reported between 2017 and 2023.

Switchable Synthesis of Spirodihydroindolizines and Indolizines from Aurones and Pyridin-2-yl Active Methylene Compounds

A novel and flexible domino reaction of aurones with pyridin-2-yl active methylene compounds promoted by I2/BF3 has been developed to afford spirodihydroindolizines and indolizines in a controllable manner. When the reaction was performed in 1,2-dichloroethane at 80 °C, a variety of spirodihydroindolizines were obtained, whereas it almost exclusively provided a series of indolizines when the reaction was performed in a mixed solvent of 1,2-dichloroethane and N,N-dimethylformamide at a relatively higher temperature of 100 °C. Being metal-free, excellent product selectivity, high atom economy, good functional group tolerance, and feasibility for large-scale synthesis are the salient features of the developed methodology.

Formation of bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids

A catalyst-free synthesis of stable bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids in MeCN in 75-95% yield is described. The antioxidant and antimicrobial properties of the synthesized compounds are investigated. For this purpose, the radical scavenging activities of four derivatives were studied by radical trapping of diphenylpicrylhydrazine and ferric reduction power experiments. Disk diffusion test on Gram-positive and Gram-negative bacteria was employed to investigate antibacterial activities of five derivatives.

Access to pyrrolines and fused diaziridines by selective radical addition to homoallylic diazirines

Pyrroline derivatives are common in bioactive natural products and therapeutic agents. We report here a synthesis of pyrrolines and fused diaziridines by divergent radical cyclization of homoallylic diazirines, which can serve as an internal radical trap and a nitrogen source. This reaction proceeds by selective radical addition to C[double bond, length as m-dash]C or N[double bond, length as m-dash]N bonds followed by intramolecular cyclization. Frontier molecular orbital analysis provides a deep insight into the origin of the selectivity. The reaction demonstrates a new cyclization mode, broad functional group compatibility and high product diversity, and reveals a much broader chemical space for diazirine studies.

Brook Rearrangement as Trigger for Dearomatization Reaction: Synthesis of Non-Aromatic N-Heterocycles

The [1,2]-Brook rearrangement stands as a potent technique for constructing complex molecules. In this study, we showcase its power in the dearomatization of aromatic N-heterocycles. Through a concise four-step process that integrates lithiation, nucleophilic addition, Brook rearrangement and dearomatization reaction, we demonstrate a versatile strategy for generating diverse non-aromatic N-heterocycles which exhibit ambident reactivities. Various acyl silanes, halo-pyridines, and quinolines have been explored within this context. The synthetic utility of this methodology is demonstrated through the construction of complex architectures.

Green synthesis and anti-tumor efficacy via inducing pyroptosis of novel 1H-benzo[e]indole-2(3H)-one spirocyclic derivatives

Pyroptosis induction is anticipated to be a new approach to developing anti-tumor medications. A novel class of spirocyclic compounds was designed by hybridization of 1H-Benzo[e]indole-2(3H)-one with 1,4-dihydroquinoline and synthesized through a new green "one-pot" synthesis method using 10 wt% SDS/H2O as a solvent to screen novel tumor cell pyroptosis inducers. The anti-tumor activity of all compounds in vitro was determined by the MTT method, and a fraction of the compounds showed good cell growth inhibitory activity. The quantitative structure-activity relationship models of the compounds were established by artificial intelligence random forest algorithm (R2 = 0.9656 and 0.9747). The ideal compound A9 could, in a concentration-dependent manner, prevent ovarian cancer cells from forming colonies, migrating, and invading. Furthermore, A9 could significantly induce pyroptosis and upregulate the expression of pyroptosis-related proteins GSDME-N, in addition to inducing apoptosis and mediating the expression of apoptosis-related proteins in ovarian cancer cells. A9 (5 mg/kg) significantly reduced tumor volume and weight of ovarian cancer in vivo, decreased caspase-3 expression in tumor tissue, and induced the production of GSDME-N. This study provides a green and efficient atom-economic synthesis method for 1H-Benzo[e]indole-2(3H)-one spirocyclic derivatives and a promising pyroptosis inducer with anti-tumor activity.

Recent developments using malononitrile in ultrasound-assisted multicomponent synthesis of heterocycles

Ultrasonic irradiation serves as a vigorous and environmentally sustainable approach for augmenting multicomponent reactions (MCRs), offering benefits such as thermal enhancement, agitation, and activation, among others. Malononitrile emerges as a versatile reagent in this context, participating in a myriad of MCRs to produce structurally diverse heterocyclic frameworks. This review encapsulates the critical role of malononitrile in the sonochemical multicomponent synthesis of these heterocyclic structures. The paper further delves into the biochemical and pharmacological implications of these heterocycles, elucidating their reaction mechanisms as well as delineating the method's scope and limitations. We furnish an overview of the merits and challenges inherent to this synthetic approach and offer insights for potential avenues in future research.

Electrochemical Arene Radical Cation Promoted Spirocyclization of Biaryl Ynones: Access to Alkoxylated Spiro[5,5]trienones

Herein, a novel electrochemical arene radical cation promoted dearomative spirocyclization of biaryl ynones with alcohols is described, providing a conceptually novel transformation mode for producing diverse alkoxylated spiro[5,5]trienones. The catalyst- and chemical-oxidant-free spirocyclization protocol features broad substrate scope and high functional group tolerance. Mechanistic studies reveal that the generation of arene radical cation via anodic single-electron oxidation is crucial, with sequential 6-endo-dig cyclization, dissociation of hemiketal, anodic oxidation, and nucleophilic attack of alcohols.

Synthesis of Highly Functionalized Spirocycles and Pentafulvene-Containing Dyes Involving 2-(2'-ketoalkyl)-1,3-indandiones

Synthesis of highly functionalized spiro[4.4]nonane and spiro[4.5]decane motifs by the reaction of dimethylacetylenedicarboxylate (DMAD) with 2-(2'-ketoalkyl)-1,3-indandiones and 2-(3'-ketoalkyl)-1,3-indandiones, respectively, has been developed by utilizing a catalytic amount of DABCO. The tertiary hydroxy-containing spiro[4.4]nonane products were converted into fully conjugated pentafulvene π-systems in an acidic medium through dehydration and unprecedented C-C bond rearrangement.

N-Biphenyl Pyrrolinones and Dibenzofurans as RNA-Binding Protein LIN28 Inhibitors Disrupting the LIN28-Let-7 Interaction

The RNA-binding protein LIN28 is a regulator of miRNA let-7 biogenesis. Inhibitors of LIN28 are highly sought after given the central role that LIN28 plays in tumorigenesis and development of cancer stem cells as well as LIN28's association with poor clinical prognosis. Although LIN28 inhibitors of different scaffolds have been reported, the potential of most LIN28 inhibiting small molecules was not fully explored since very limited structure-activity relationship (SAR) studies have been performed. We previously identified trisubstituted pyrrolinones as a new class of LIN28 inhibitors disrupting the LIN28-let-7 interaction. Here, we performed extensive SAR by evaluating 95 small molecules and identified new trisubstituted pyrrolinones featuring either an N-biphenyl or N-dibenzofuran substituent, overthrowing the existing conclusion that a salicylic acid moiety is indispensable for activity. Exchange of the negatively charged salicylic acid moiety in LIN28 inhibitors with a heterocyclic substituent is beneficial for membrane permeability, leading to increased activity in a cellular assay, and will potentially reduce toxicity.

Chiral Auxiliary Approach for Gold(I)-Catalyzed Cyclizations

Two different classes of stereoselective cyclizations have been developed using a chiral auxiliary approach with commercially available [JohnPhosAu(MeCN)SbF6 ] as catalyst. First, a stereoselective cascade cyclization of 1,5-enynes was achieved using the Oppolzer camphorsultam as chiral auxiliary. In this case, a one-pot cyclization-hydrolysis sequence was developed to directly afford enantioenriched spirocyclic ketones. Then, the stereoselective alkoxycyclization of 1,6-enynes was mediated by an Evans-type oxazolidinone. A reduction-hydrolysis sequence was selected to remove the auxiliary to give enantioenriched β-tetralones. DFT studies confirmed that the steric clash between the chiral auxiliary and alkene accounts for the experimentally observed diastereoselective cyclization through the Si face.

Leveraging cascade alkynyl Prins cyclization towards the stereoselective synthesis of spiro-furan quinazolinone scaffolds

A TfOH-promoted, metal-free protocol has been unveiled for the synthesis of spiro-furan quinazolinones employing alkynol urea derivatives utilizing alkynyl Prins cyclization reaction. This methodology produces highly functionalized spiro-heterocycles in excellent yields with exclusive E-selectivity under ambient conditions. Furthermore, late-stage modifications incorporate bromide and acetyl functionalities into the synthesized spiro-heterocycles.

Palladium-Catalyzed Aminoalkylative Cyclization Enables Modular Synthesis of Exocyclic 1,3-Dienes

A novel and efficient palladium-catalyzed regioselective and stereodivergent ring-closing reaction of aminoenynes with aldehydes and boronic acids or hydrosilane is developed. This three-component reaction allows for the modular synthesis of a series of exocyclic 1,3-dienes bearing 5- to 8-membered saturated N-heterocycles. The reactions utilize a simple Pd-catalyst and work with broad range of aminoenynes, aldehydes and organometallic reagents under mild reaction conditions. The products represent useful intermediates for chemical synthesis due to the versatility of the conjugated diene. Preliminary mechanistic details of the method are also revealed.

Diastereoselective Synthesis of Dispiro[Imidazothiazolotriazine-Pyrrolidin-Oxindoles] and Their Isomerization Pathways in Basic Medium

Highly diastereoselective methods for the synthesis of two series of regioisomeric polynuclear dispyroheterocyclic compounds with five or six chiral centers, comprising moieties of pyrrolidinyloxindole and imidazo[4,5-e]thiazolo[3,2-b]-1,2,4-triazine of linear structure or imidazo[4,5-e]thiazolo[2,3-c]-1,2,4-triazine of angular structure, have been developed on the basis of a [3+2] cycloaddition of azomethine ylides to functionalized imidazothiazolotriazines. Depending on the structure of the ethylenic component, cycloaddition proceeds as an anti-exo process for linear derivatives, while cycloaddition to angular ones resulted in a syn-endo diastereomer. Novel pathways of isomerization for the synthesized anti-exo products upon treatment with sodium alkoxides have been found, which resulted in two more series of diastereomeric dispiro[imidazothiazolotriazine-pyrrolidin-oxindoles] inaccessible with the direct cycloaddition reaction. For the first series, the inversion of the configuration of one stereocenter, i.e., C-4' atom of the pyrrolidine cycle, (epimerization) was established. For the second one, configuration of the obtained diastereomer formally corresponded to the syn-endo approach of the azomethine ylide in the case of cycloaddition to the ethylenic component.

Bioevaluation of Spiro N-Propargylic β-Enaminones as Anti-Breast Cancer Agents: In Vitro and Molecular Docking Studies

The study aimed to investigate the in vitro inhibitory activities of spiro N-propargylic β-enaminones, SPEs 1-31, against BCa cells, to perform in silico molecular docking studies to understand the nature of the interaction between the compounds and the ERα, PR, EGFR, and Her2, and to determine the ADMET and drug-likeness properties. Cytotoxic activity was investigated via MTT assay. DNA fragmentation was evaluated via ELISA assay. Cell cycle distributions were investigated by flow cytometry. Expression levels of Bcl-2, Bax, p21 and Cyclin D1 were measured by qRT-PCR and western blot analysis. Molecular docking was done using Autodock/vina software. ADMET analysis was calculated using the ADMETlab 2.0 tool. SPEs 1, 22, and 28 showed selective cytotoxic activity against all BCa cells with SI values >2. SPEs induced apoptosis and caused significant changes in Bcl-2 and Bax levels. The cell cycle was arrested at the S phase and levels of p21 and Cyclin D1 were induced in all BCa cells. Molecular docking analysis revealed that SPE1, SPE22, and SPE28 showed high binding affinities with ERα, PR, EGFR, and Her2. ADMET analysis revealed that SPEs are drug-like compounds as they obey the five rules of Lipinsky and are not toxic. Therefore, these potential anticancer compounds should be further validated by in vivo studies for their appropriate function in human health with a safety profile, and a comprehensive drug interaction study should be performed.