Recent synthetic strategies toward the synthesis of spirocyclic compounds comprising six-membered carbocyclic/heterocyclic ring systems

Construction of 1,6-anhydro-functionalized carbohydrate-fused spiro-chromanones via intramolecular domino/spirocyclization

The synthetic protocol for the first synthesis of carbohydrate-fused spiro-chromanones via intramolecular domino reaction between o-hydroxyacetophenones and dihydrolevoglucosenone (DHL) is described. The reactions proceed via the formation of an acetophenone (APh) enamine intermediate when performed in the presence of pyrrolidine in ethanol. The formation of a new carbon-carbon bond between nucleophilic carbon of the enamine of APh and C-2 of DHL is followed by cyclization, producing a new six-membered ring, as the result of an intramolecular domino process. Several substituted o-hydroxyacetophenones were reacted. The structure of products was determined using 1H and 13C NMR, GC-MS and X-ray crystallography. The yields are good to excellent (62-92 %), and the products are crystalline and easy to isolate. So far, the synthesized compounds were tested only for their antifungal properties. The results were disappointing.

Reaction strategies for the meta-selective functionalization of pyridine through dearomatization

The pyridine moiety is a crucial structural component in various pharmaceuticals. While the direct ortho- and para-functionalization of pyridines is relatively straightforward, the meta-selective C-H functionalization remains a significant challenge. This review highlights dearomatization strategies as a key area of interest in expanding the application of meta-C-H functionalization of pyridines. Dearomatization enables the meta-functionalization through various catalytic methods that directly generate dearomatization products, and some products can be rearomatized back to pyridine derivatives. Furthermore, this article also covers the dearomatization of multiple positions of pyridine in the synthesis of polycyclic compounds. It offers a comprehensive overview of the latest advancements in dearomatization at different positions of pyridine, aiming to provide a valuable resource for researchers in this field. It also highlights the advantages and limitations of existing technologies, aiming to inform a broader audience about this important field and foster its future development.

Synthesis And Photovoltaic Performance of Carbazole (Donor) Based Photosensitizers in Dye-Sensitized Solar Cells (DSSC): A Review

Carbazoles are nitrogen-containing aromatic heterocycles, having widespread applications in the field of photovoltaics. Carbazole-based photosensitizers have tunable features for absorption on semi-conductor (tellurium dioxide or zinc oxide) layers to create sufficient push-pull force in the conversion of sunlight into electrical energy, thus presenting as promising heterocyclic donor candidates to be used in dye-sensitized solar cells. For the synthesis of these dyes, various structural designs are available, namely, D-A, D-π-A, D-D-π-A, D-A-π-A, A-π-D-π-A-π-A, and D2-π-A that all involve incorporating carbazole as a donor (D), along with spacer (π-extender) moieties, such as thiophene, phenol, ethynylene, nitromethane, azine, thiadiazole, or acetonitrile. Additionally, acceptors (A) employed in the designs include cyanoacrylic acids, carboxylic acids, malononitrile, rhodanine-3-acetic acid, 4-aminobenzoic acid, or 4-amino salicylic acid. This comprehensive review explores the synthesis and photovoltaic performances of numerous carbazole-based photosensitizers tailored for dye-sensitized solar cells, covering the period of 2019-2023.

Accessing the synthesis of natural products and their analogues enabled by the Barbier reaction: a review

The Barbier reaction is significantly referred to as one of the efficient carbon-carbon bond forming reactions which involves the treatment of haloalkanes and carbonyl compounds by utilizing the catalytic role of a diverse range of metals and metalloids. The Barbier reaction is tolerant to a variety of functional groups, allowing a broad substrate scope with the employment of lanthanides, transition metals, amphoteric elements or alkaline earth metals. This reaction is also water-resistant, thereby overcoming the challenges posed by moisture sensitive organometallic species involving C-C bond formation reactions. The Barbier reaction has significantly found its applicability towards the synthesis of intricate and naturally occurring organic compounds. Our review provides an outlook on the synthetic applications of the Barbier reaction and its variants to accomplish the preparation of several natural products, reported since 2020.

Palladium-Catalyzed [3 + 2] Cycloaddition of 4-Vinyl-4-Butyrolactones with Alkenes: Synthesis of Spirocyclopentane Compounds

A palladium-catalyzed [3 + 2] cycloaddition of 4-vinyl-4-butyrolactones (VBLs) with alkenes has been developed to afford various spirocyclopentane products in high yields with excellent diastereoselectivities. The scale-up reaction and further derivations of the product worked well, demonstrating the potential application of the current reaction in organic synthesis.

Carbene-Catalyzed [3 + 3] Annulation of Enals and Vinyl Sulfoxonium Ylides

Carbene-catalyzed [3 + 3] annulation of enals and vinyl sulfoxonium ylides has been demonstrated. This method efficiently synthesizes a range of 2-sulfenylidene-3-cyclohexen-1-ones with high atom economy. Notably, the presence of the sulfoxonium ylide moiety in the obtained products significantly enhances their potential for further synthetic transformations.

Baeyer-Villiger oxidation: a promising tool for the synthesis of natural products: a review

Baeyer-Villiger oxidation is a well-known reaction utilized for the synthesis of lactones and ester functionalities from ketones. Chiral lactones can be synthesized from chiral or racemic ketones by employing asymmetric Baeyer-Villiger oxidation. These lactones act as key intermediates in the synthesis of most of the biologically active natural products, their analogues, and derivatives. Various monooxygenases and oxidizing agents facilitate BV oxidation, providing a broad range of synthetic applications in organic chemistry. The variety of enzymatic and chemoselective Baeyer-Villiger oxidations and their substantial role in the synthesis of natural products i.e., alkaloids, polyketides, fatty acids, terpenoids, etc. (reported since 2018) have been summarized in this review article.

Fluorescent Materials Based on Spiropyran for Advanced Anti-Counterfeiting and Information Encryption

In daily life, counterfeit and substandard products, particularly currency, medicine, food, and confidential documents, are capable of bringing about very serious consequences. The development of anti-counterfeiting and authentication technologies with multilevel securities is a powerful means to overcome this challenge. Among various anti-counterfeiting technologies, fluorescent anti-counterfeiting technology is well-known and commonly used to fight counterfeiters due to its wide material source, low cost, simple usage, good concealment, and simple response mechanism. Spiropyran is favored by scientists in the fields of anti-counterfeiting and information encryption due to its reversible photochromic property. Here, we summarize the current available spiropyran-based fluorescent materials from design to anti-counterfeiting applications. This review will be help scientists to design and develop fluorescent anti-counterfeiting materials with high security, high performance, quick response, and high anti-counterfeiting level.

Development of novel transition metal-catalyzed synthetic approaches for the synthesis of a dihydrobenzofuran nucleus: a review

The synthesis of dihydrobenzofuran scaffolds bears pivotal significance in the field of medicinal chemistry and organic synthesis. These heterocyclic scaffolds hold immense prospects owing to their significant pharmaceutical applications as they are extensively employed as essential precursors for constructing complex organic frameworks. Their versatility and importance make them an interesting subject of study for researchers in the scientific community. While exploring their synthesis, researchers have unveiled various novel and efficient pathways for assembling the dihydrobenzofuran core. In the wake of extensive data being continuously reported each year, we have outlined the recent updates (post 2020) on novel methodological accomplishments employing the efficient catalytic role of several transition metals to forge dihydrobenzofuran functionalities.

Skeletal Editing: Ring Insertion for Direct Access to Heterocycles

Skeleton editing has rapidly advanced as a synthetic methodology in recent years, significantly streamlining the synthesis process and gaining widespread acceptance in drug synthesis and development. This field encompasses diverse ring reactions, many of which exhibit immense potential in skeleton editing, facilitating the generation of novel ring skeletons. Notably, reactions that involve the cleavage of two distinct rings followed by the reformation of new rings through ring insertion play a pivotal role in the construction of novel ring skeletons. This article aims to compile and systematize this category of reactions, emphasizing the two primary reaction types and offering a thorough exploration of their associated complexities and challenges. Our endeavor is to furnish readers with comprehensive reaction strategies, igniting research interest and injecting fresh impetus into the advancement of this domain.

Hal-Py-SO3H as a novel and recyclable catalyst for highly efficient synthesis of xanthene and spiropyran derivatives

The aim of this research is to synthesize a new sulfonic acid catalyst based on halloysite nanotubes (Hal-Py-SO3H) and characterize it as a solid acid nanocatalyst by various analytical techniques such as Fourier-Transformed Infrared spectroscopy (FTIR), Thermal gravimetric Analysis (TGA), X-ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), Vibrating Energy-Dispersive X-ray analysis (EDX), Transmission electron microscopy (TEM) and X-ray atomic mapping. Furthermore, this new catalyst was evaluated in synthesizing spiropyran derivatives via multicomponent reactions (MCRs) and Xanthen derivatives under environmentally sustainable conditions. The main advantages of this approach include green conditions, excellent yields, quick reaction rates, and ease of preparation. Additionally it was observed that the catalyst exhibited robust stability even after multiple recycling processes, indicating its potential for practical applications in sustainable chemical transformations.

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.

CAN-Promoted Thiolative ipso-Annulation of Unactivated N-Benzyl Acrylamides: Access to SCN/SCF3/SO2Ar Containing Azaspirocycles

A variety of acrylamides holding an unactivated N-benzyl group underwent dearomative ipso-cyclization induced by sulfur-centered radicals (SCN/ SCF₃/ SO₂Ar) in the presence of ceric ammonium nitrate (CAN) as the oxidant to furnish azaspirocycles in good yields. This is the first report on ipso-dearomatization of N-benzyl acrylamides that proceeds without a substituent at the para-position of the aromatic ring. The developed conditions are also found to be suitable for substrates holding substituents such as F, NO₂, OMe, OH, and OAc at the para-position. The reaction features water as the source of oxygen, is compatible with a variety of functional groups, and proceeds in a short time.

Unveiling the Chemistry and Synthetic Potential of Catalytic Cycloaddition Reaction of Allenes: A Review

Allenes with two carbon-carbon double bonds belong to a unique class of unsaturated hydrocarbons. The central carbon atom of allene is sp hybridized and forms two σ-bonds and two π-bonds with two terminal sp² hybridized carbon atoms. The chemistry of allenes has been well documented over the last decades. They are more reactive than alkenes due to higher strain and exhibit significant axial chirality, thus playing a vital role in asymmetric synthesis. Over a variety of organic transformations, allenes specifically undergo classical metal catalyzed cycloaddition reactions to obtain chemo-, regio- and stereoselective cycloadducts. This review briefly describes different types of annulations including [2+2], [2+2+1], [3+2], [2+2+2], [4+2], [5+2], [6+2] cycloadditions using titanium, cobalt, rhodium, nickel, palladium, platinum, gold and phosphine catalyzed reactions along with a mechanistic study of some highlighted protocols. The synthetic applications of these reactions towards the synthesis of natural products such as aristeromycin, ent-[3]-ladderanol, waihoensene(-)-vindoline and (+)-4-epi-vindoline have also been described.

Diazo Tetramic Acids Provide Access to Natural-Like Spirocyclic Δα,β-Butenolides through Rh(II)-Catalyzed O-H Insertion/Base-Promoted Cyclization

3-Diazotetramic acids were found to be valid substrates for the recently discovered approach toward natural-like Δα,β-spirobutenolides via Rh(II)-catalyzed O-H insertion into propiolic acids followed by base-promoted intramolecular Michael addition. The target Δα,β-spirobutenolides were obtained in generally high yields and, in the case of chiral 5-monosubstituted 3-diazotetramic acids, high diastereoselectivity. The synthesis of Δα,β-spirobutenolides that we report here was virtually insensitive to the structure of the propiolic acids though it was somewhat sensitive to the structure of the 3-diazotetramic acids, thereby demonstrating quite a large scope. Thus, a new class of α-diazocarbonyl compounds suitable for the realization of the approach outlined above was identified.

Synthesis, Structure and Stereochemistry of Dispirocompounds Based on Imidazothiazolotriazine and Pyrrolidineoxindole

Methods for the synthesis of two types of isomeric dispirocompounds based on imidazothiazolotriazine and pyrrolidineoxindole, differing in the structure of imidazothiazolotriazine fragment, namely, linear dispiro[imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazine-6,3'-pyrrolidine- 4',3″-indolines] and angular dispiro[imidazo[4,5-e]thiazolo[2,3-c][1,2,4]triazine-7,3'-pyrrolidine-4',3″-indolines] were proposed. The first method relies on a 1,3-dipolar cycloaddition of azomethine ylides generated in situ from paraformaldehyde and N-alkylglycine derivatives to the corresponding oxindolylidene derivatives of imidazothiazolotriazine. The cycloaddition leads to a mixture of two diastereomers resulted from anti- and syn-approaches of azomethine ylide in approximately a 1:1 ratio, which were separated by column chromatography. Another method consists in rearrangement of linear dispiro[imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazine-6,3'-pyrrolidine-4',3″-indolines] into hitherto unavailable angular dispiro[imidazo[4,5-e]thiazolo[2,3-c]-[1,2,4]triazine-7,3'-pyrrolidine-4',3″-indolines] upon treatment with KOH. It was found that the anti-diastereomer of linear type underwent rearrangement into the isomeric angular syn-diastereomer, while the rearrangement of the linear syn-diastereomer gave the angular anti-diastereomer.

Dearomatization of Cyclic Diphenylhydrazines: Harnessing the o-Semidine Rearrangement for the Synthesis of Spirocyclic Tetrahydroquinolines

The synthesis of novel tetrahydroquinoline-containing spirocycles has been achieved through an acid-promoted dearomatization of cyclic diarylhydrazines. The reaction, proceeding through a dearomative o-semidine rearrangement, furnishes a stable, yet reactive spirocyclohexadieniminium ion, which can further be used as an electrophile or a diene in a one-pot sequence. These transformations efficiently produce novel diazaspirocycles and allow for further synthetic elaboration of the cyclohexadienamine products.

Spirocyclic Compounds in Fragrance Chemistry: Synthesis and Olfactory Properties

In this Review, both the scientific literature and patents have been analysed to gather, examine and classify synthetic strategies developed to access spirocyclic molecules having olfactory properties of interest for the perfume industry. New structures have been reported at a steady state since 1961 with a particularly intense activity in the 80's. In the diversity of olfactory properties observed, spirocyclic molecules were found to be quite frequently woody, as well as fruity and ambery, among other tonalities. More recent data reflect the evolution of modern synthetic organic chemistry towards bioinspiration, enantioselective reactions, green and sustainable chemistry and biotechnology.

Spirocarbocycle Synthesis from Chloromethylarenes via Transition-Metal-Catalyzed Allylative Dearomatization and Ring Closure Metathesis

A strategy for the synthesis of spirocarbocycles by using chloromethyl arenes as starting materials is described in this paper. The palladium-catalyzed allylative dearomatization and the subsequent ruthenium-catalyzed ring closure metathesis proceeded smoothly under mild conditions to produce the corresponding spirocarbocycle products with moderate to high yields. Benzene-ring-, naphthalene-ring-, and anthracene-ring-containing substrates can be easily transformed into spirocarbocycles by using the proposed method.

Highly Selective Synthesis of Seven-Membered Azaspiro Compounds by a Rh(I)-Catalyzed Cycloisomerization/Diels-Alder Cascade of 1,5-Bisallenes

The synthesis of spiro compounds featuring seven- and six-membered rings in the spirobicyclic motif is successfully achieved through a cascade process encompassing a rhodium(I)-catalyzed cycloisomerization followed by a highly selective Diels–Alder homodimerization. The scope of the reaction is analyzed based on a series of synthetic substrates, and control experiments and DFT calculations led us to justify the exquisite degree of selectivity observed.

Synthesis of Seleno-Dibenzocycloheptenones/Spiro[5.5]Trienones by Radical Cyclization of Biaryl Ynones

We report herein an alternative method for the synthesis of seleno-dibenzocycloheptenones and seleno-spiro[5.5]trienones through the radical cyclization of biaryl ynones in the presence of diorganyl diselenides, using Oxone as a green oxidizing agent. The reactions were conducted using acetonitrile as the solvent in a sealed tube at 100 °C. The protocol is operationally simple and scalable, exhibits high regioselectivity, and allows the synthesis of 24 dibenzocycloheptenones/spiro[5.5]trienones in yields of up to 99%, 17 of which are unpublished compounds. Additionally, synthetic transformations of the prepared compounds, such as oxidation and reduction reactions, are demonstrated.

Transient Chelating Group-Controlled Stereoselective Rh(I)-Catalyzed Silylative Aminocarbonylation of 2-Alkynylanilines: Entry to (Z)-3-(Silylmethylene)indolin-2-ones

A new, mild acryl transient chelating group-controlled stereoselective Rh(I)-catalyzed silylative aminocarbonylation of 2-alkynylanilines with CO and silanes for producing (Z)-3-(silylmethylene)indolin-2-ones is presented. By using an acryl transient chelating group 2-alkynylanilines...

Recent advances in continuous flow synthesis of heterocycles

In the current scenario, flow chemistry is emerging as a significant technology in the field of organic synthesis. This miniaturized protocol including microreactors facilitates excellent heat transfer, low solvent wastage, lesser reaction time, a safer environment for reagent handling and appreciable yields of desired products. Thus, this "enabling technology" has a great scope in the synthesis and preparation of a variety of heterocycles that require toxic reagents as starting materials. This review discusses the recent advances (2020-2021) in continuous flow strategy for synthesis and derivatization of variety of heterocyclic entities, of different ring size, using different approaches. This also highlights the advantages of different combined techniques like Microwave assisted heating, electrochemical flow cell, LED light source, NMR and FT-IR analysis, etc., that enables utilization of various mechanisms and real-time monitoring of reactions leading to improved results.

Unmasking the reverse reactivity of cyclic N-sulfonyl ketimines: multifaceted applications in organic synthesis

The chemistry related to the exploration of cyclic N-sulfonyl ketimines and their derivatives has attracted significant attention in the last few decades because of their intriguing structures and properties. They serve broadly as reactive synthons in various reactions to create a diverse set of synthetically and biologically attractive molecules. Furthermore, these moieties, which possess multiple heteroatoms (N, O and S), display or can enhance many biological activities. In the case of synthetic reactions, chemists mainly focus on the chemical manipulation of the highly reactive prochiral CN bond of N-sulfonyl ketimines. Besides their traditional role as electrophiles, N-sulfonyl ketimines possess α-Csp³-H protons, and thus behave as potential carbonucleophiles, where they can undergo several C-X (X = C, N and O) bond-forming reactions with different types of electrophiles under various conditions to form a wide range of fascinating asymmetric and non-asymmetric versions of fused heterocycles, carbocycles, spiro-fused skeletons, pyridines, pyrroles, etc. Herein, we highlight the recent examples from our research work and others covering the scope of cyclic N-sulfonyl ketimines as useful carbonucleophiles. In addition, the detailed mechanistic studies of the above-mentioned reactions are also presented.

Chemoselective synthesis of 5,4'-imidazolinyl spirobarbiturates via NBS-promoted cyclization of unsaturated barbiturates and amidines

A selective cyclization of unsaturated barbiturates and amidines promoted by N-bromosuccinimide has been successfully developed to afford a vast variety of 5,4'-imidazolinyl spirobarbiturates in moderate to good yields. The present protocol features broad substrate scope, facile work-up procedure and mild reaction conditions, providing a novel strategy for the highly selective and efficient construction of structurally diverse spiroimidazolines.