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

Introducing the Dihydro-1,3-azaboroles: Convenient Entry by a Three-Component Reaction, Synthetic and Photophysical Application

The (Fmes)BH₂·SMe₂ reagent (7) reacts sequentially with an acetylene and, e.g., xylylisonitrile in a convenient three-component reaction to give a series of unprecedented dihydro-1,3-azaborole derivatives 16. The tolane-derived example 16a was deprotonated and used as a ligand in organometallic chemistry. Compounds 16 served as the starting materials for the straightforward synthesis of various dihydro-1,3-azaborinine derivatives by treatment with an isonitrile. Several diaryldihydro-1,3-azaboroles showed interesting photophysical properties such as aggregation-induced emission and high fluorescence quantum yields.

Total Synthesis of Marine Alkaloids Motuporamines A and B via Ring Expansion of Cyclic β-Keto Esters

An expedient total synthesis of the title marine sponge alkaloids has been developed. The salient features of the synthesis are as follows: (i) preparation of the required 13- and 14-membered cyclic lactams with n + 4 ring-expansion strategy of cyclic β-keto esters and (ii) functional group manipulation of the resulted keto ester lactams. This approach used easily accessible and inexpensive materials/reagents, thus providing a promising alternative to the existing preparations.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

The Role of Total Synthesis in Structure Revision and Elucidation of Decanolides (Nonanolides)

Ten-membered lactones are commonly observed structures of natural products. They are mostly fungal metabolites, which often act as plant pathogens, but recently ten-membered lactones were identified as pheromones of frogs and termites. Although modern spectroscopic methods are nowadays routinely used to elucidate the structure of natural products, structural assignments of ten-membered lactones often remain incomplete or are surprisingly often erroneous. Most errors concern the absolute configuration. The examples discussed in this chapter demonstrate that enantioselective total synthesis is not only an efficient tool for corroborating or revising a proposed structure, but that the synthesis of different stereoisomers as references for gas chromatographic investigations can be a vital part of the structure elucidation process if only minute amounts of material are available. As a method of outstanding importance for the synthesis of ten-membered lactones olefin metathesis has emerged. Most of the examples discussed herein use one or more olefin metathesis reactions as key steps.

Recent advances in Rh(iii)/Ir(iii)-catalyzed C–H functionalization/annulation via carbene migratory insertion

The review highlighted diverse annulations, including nitrogen, oxygen, sulfur heterocycles and carbocylizations via Rh(iii)/Ir(iii)-catalyzed C–H functionalization/annulation with various arene and carbene precursors.

Recent advances in catalytic synthesis of medium-ring lactones and their derivatives

In light of the ever-increasing significance of medium-sized lactones across the range of chemical sciences, this review summarizes the recent advances in catalytic synthesis of medium-ring lactones with emphasis on reaction scope and mechanism.

Selective synthesis of acylated caprolactam via sequential Michael addition/palladium-catalyzed alpha-arylation of ketones

A formal [6 + 1] annulation reaction provides an efficient route enabling rapid access to diverse caprolactams by the double C–C bond formation of the same site from methyl ketones.

Brønsted acid-promoted hydroamination of unsaturated hydrazones: access to biologically important 5-arylpyrazolines

A novel and efficient Brønsted acid-promoted hydroamination of hydrazone-tethered olefins has been developed. A variety of pyrazolines have been easily obtained in good to excellent yields with high chemo- and regioselectivity under simple and mild conditions. This method represents a straightforward, facile, and practical approach toward biologically important 5-arylpyrazolines, which are difficult to access by previously reported radical hydroamination of β,γ-unsaturated hydrazones.

An efficient, chemo- and regioselective Brønsted acid-promoted hydroamination reaction of hydrazone-tethered olefins towards 5-arylpyrazolines was developed.

Radical Transformations towards the Synthesis of Quinoline: A Review

Quinoline is considered one of the most ubiquitous heterocycles due to its engaging biological activities and synthetic utility over organic transformations. Over the past few decades, numerous reports have been documented in the synthesis of quinolines. The classical methods including, Skraup, Friedlander, Doebner-von-Miller, Conrad-Limpach, Pfitzinger quinoline synthesis, and so forth, these are the well-known methods to construct principal quinoline scaffold with several advantages and limitations. Recently, radical insertion or catalyzed reactions have emerged as a powerful and efficient tool to construct heterocycles with high atom efficiency and step economy. In this concern, this minireview mainly focused on the developments of Quinoline synthesis via radical reactions. In addition, a brief description of the preparation procedure, reactivity, and mechanisms is also included, where as possible. Respectively, the synthesis of quinolines is classified and summarized based on its reactivity, so it will help the researchers to grab the information in this exploration area, as Quinolines are promising pharmacophores.

[3 + 2] Cycloaddition of Nitrile Imines with Enamides: An Approach to Functionalized Pyrazolines and Pyrazoles

An efficient [3 + 2] cycloaddition of in situ generated nitrile imines with enamides has been established. A wide range of functionalized pyrazoline derivatives (53 examples) were obtained in moderate to good yields (up to 96%) under very mild conditions. This protocol features broad substrate scope, good functional group tolerance, and operational simplicity. Practical transformation of the products into useful pyrazoles via a one-pot process and the scalability of this protocol highlight the utility of this synthetic methodology.

Dithiane-Induced [3+2] Cycloaddition Tactic for the Convergent Synthesis of Dihydropyrrole and Pyrrole Derivatives

An efficient transition-metal-free tactic for the convergent synthesis of substituted dihydropyrroles and pyrroles by β-chloro-vinyl dithiane cyclization with a broad range of imines was developed. [3+2] Cyclization and aromatization occur under these reaction conditions providing biologically relevant dihydropyrroles and pyrroles in good yields.

Synthetic Study toward the Total Synthesis of Lycoclavatumide

The synthetically challenging and highly functionalized azabicyclo[6.4.1] ring system, which is found in lycopodium alkaloid lycoclavatumide and some natural molecules, was synthesized for the first time. The key reaction was a diastereoselective type II [5+2] cycloaddition with excellent functional group compatibility. We tried to install the desired eight-membered ring in the final product by RCM reaction.

Synthesis of organoselenyl isoquinolinium imides via iron(III) chloride-mediated tandem cyclization/selenation of N'-(2-alkynylbenzylidene)hydrazides and diselenides

This report describes the synthesis of organoselenyl isoquinolinium imides through a tandem cyclization between N'-(2-alkynylbenzylidene)hydrazides and diselenides. The reaction was carried out at room temperature under an ambient atmosphere using cheap iron(iii) chloride as the metallic source. The strategy shows good tolerance to a broad range of N'-(2-alkynylbenzylidene)hydrazides and diselenides, and forms C-N and C-Se bonds in one step. The obtained product is further transformed into a bioactive H-pyrazolo[5,1-a]isoquinoline skeleton easily via a silver catalyzed [3 + 2] cycloaddition.

Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization of norbornene and norbornadiene

A highly enantioselective Pd/Xiang-Phos-catalyzed carbohetero-functionalization of norbornene is described, giving a direct access to various chiral norbornane-fused dihydrofurans and dihydro-pyrroles. This synthetic methodology provides the first example of asymmetric carboetherification of norbornene, and also tolerates norbornadiene well.

Challenging Metathesis Catalysts with Nucleophiles and Brønsted Base: Examining the Stability of State-of-the-Art Ruthenium Carbene Catalysts to Attack by Amines

Critical to advancing the uptake of olefin metathesis in leading contexts, including pharmaceutical manufacturing, is identification of highly active catalysts that resist decomposition. Amines constitute an aggressive challenge to ruthenium metathesis catalysts. Examined here is the impact of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), morpholine, n-butylamine, and triethylamine on Ru metathesis catalysts that represent the current state of the art, including cyclic alkyl amino carbene (CAAC) and N-heterocyclic carbene (NHC) complexes. Accordingly, the amine-tolerance of the nitro-Grela catalyst RuCl2(H2IMes)(=CHAr) (nG; Ar = C6H4-2-O i Pr-5-NO2) is compared with that of its CAAC analogues nGC1 and nGC2, and the Hoveyda-class catalyst RuCl2(C2)(=CHAr') HC2 (Ar' = C6H4-2-O i Pr). In C1, the carbene carbon is flanked by an N-2,6-Et2C6H3 group and a CMePh quaternary carbon; in C2, by an N-2- i Pr-6-MeC6H3 group and a CMe2 quaternary carbon. The impact of 1 equiv amine per Ru on turnover numbers (TONs) in ring-closing metathesis of diethyl diallylmalonate was assessed at 9 ppm Ru, at RT and 70 °C. The deleterious impact of amines followed the trend NEt3 ∼ NH2 n Bu ≪ DBU ∼ morpholine. Morpholine is shown to decompose nGC1 by nucleophilic abstraction of the methylidene ligand; DBU, by proton abstraction from the metallacyclobutane. Decomposition was minimized at 70 °C, at which nGC1 enabled TONs of ca. 60 000 even in the presence of morpholine or DBU, vs ca. 80 000 in the absence of base. Unexpectedly, H2IMes catalyst nG delivered 70-90% of the performance of nGC1 at high temperatures, and underwent decomposition by Brønsted base at a similar rate. Density functional theory (DFT) analysis shows that this similarity is due to comparable net electron donation by the H2IMes and C1 ligands. Catalysts bearing the smaller C2 ligand were comparatively insensitive to amines, owing to rapid, preferential bimolecular decomposition.

Palladium-catalysed cyclisation of vinylethylene carbonates and anhydrides: a new approach to diverse medium-sized bislactones

Efficient construction of medium-sized lactones has attracted considerable interest over several decades, but remains a formidable challenge in synthetic chemistry. Here, we describe an unprecedented palladium-catalysed regioselective [5 + n] cyclisation (n = 5, 6, and 7) between vinylethylene carbonates and various anhydrides. Catalytic transformation occurs under mild, room-temperature conditions and offers an exceptional substrate scope. A broad spectrum of medium-sized bislactones with skeletal diversity can be obtained easily.

Gold(i)-catalyzed tandem cyclization of cyclopropylidene-tethered propargylic alcohols: an approach to functionalized naphtho[2,3-c]pyrans

An unprecedented gold(i)-catalyzed cyclization of cyclopropylidene-tethered propargylic alcohols via a 6-endo-dig enyne cyclization followed by a ring-opening of cyclopropane and nucleophilic closure reaction to construct naphtho[2,3-c]pyrans was developed. This transformation represents a highly efficient method for the synthesis of polycyclic compounds in one-pot, and two new rings are formed in an atom economic manner.

Pd-catalyzed stereoselective tandem ring-opening amination/cyclization of vinyl γ-lactones: access to caprolactam diversity

A stereoselective amination/cyclization cascade process has been developed that allows for the preparation of a series of unsaturated and substituted caprolactam derivatives in good yields. This conceptually novel protocol takes advantage of the easy access and modular character of vinyl γ-lactones that can be prepared from simple precursors. Activation of the lactone substrate in the presence of a suitable Pd precursor and newly developed phosphoramidite ligand offers a stereocontrolled ring-opening/allylic amination manifold under ambient conditions. The intermediate (E)-configured ε-amino acid can be cyclized using a suitable dehydrating agent in an efficient one-pot, two-step sequence. This overall highly chemo-, stereo- and regio-selective transformation streamlines the production of a wide variety of modifiable and valuable caprolactam building blocks in an operationally attractive way.

Ruthenium Metathesis: A Key Step To Access a New Cyclic Tetrasubstituted Olefin Platform

An rapid and mild synthetic route for the preparation of cyclic tetrasubstituted platforms via ruthenium-catalyzed ring-closing metathesis (RCM) has been developed. This process tolerates a wide range of functionalities such as nitrogen, oxygen, sulfur, silicon, and carbon tethered groups, as well as very challenging fluorine and boron atoms (36 derivatives, up to 96%). This diversity-oriented method was further demonstrated by the postfunctionalization reactions, such as Pd-couplings, N-substitution, and reductive amination introducing a morpholine moiety.

Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019)

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.

Preparation of Nanofungicides Based on Imidazole Drugs and Their Antifungal Evaluation

In spite of modern crop protection measures, the overall crop losses due to pests and pathogens are huge. Rhizoctonia solani, Macrophomina phaseolina, Sclerotium rolfsii, and Fusarium oxysporum are one of the most devastating soil-borne fungi and cause numerous plant diseases. Therefore, the present study aimed to systematically design and develop new nanofungicides based on imidazole drugs, clotrimazole, econazole nitrate, and miconazole nitrate, for effective and efficient management of plant diseases. The assessment of these antifungal medicines for their fungicide likeness using Hao's rule and their enzyme inhibitory potential by molecular docking was helpful in ensuring their utility as antifungal agents in managing phytopathogenic fungi. Nanotechnological strategies were used to develop nanoformulations of test compounds in poly(ethylene glycol) 300 for further augmenting their bioactivity. Transmission electron microscopy studies confirmed the nanosize of the prepared products. Analysis of their in vitro and in vivo antifungal properties revealed their usefulness in controlling the test fungi, R. solani, M. phaseolina, S. rolfsii, and F. oxysporum. Excellent in vitro antifungal activities were displayed by the clotrimazole nanoformulation with a median effective dose (ED₅₀) of 1.18 μg/mL against R. solani, the econazole nitrate nanoformulation with an ED₅₀ of 5.25 μg/mL against S. rolfsii, and the miconazole nitrate nanoformulation with an ED₅₀ of 1.49 and 1.82 μg/mL against M. phaseolina and F. oxysporum. Furthermore, in vivo studies against test fungi demonstrated the antifungal potency of all the nanoformulations with disease incidences ranging from 11.11 to 27.38% in plants treated with nanoformulations of test chemicals as compared to the inoculated control (39.68-72.38%).

Intramolecular Csp3-H/C-C bond amination of alkyl azides for the selective synthesis of cyclic imines and tertiary amines

The intramolecular Csp³–H and/or C–C bond amination is very important in modern organic synthesis due to its efficiency in the construction of diversified N-heterocycles. Herein, we report a novel intramolecular cyclization of alkyl azides for the synthesis of cyclic imines and tertiary amines through selective Csp³–H and/or C–C bond cleavage. Two C–N single bonds or a C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N double bond are efficiently constructed in these transformations. The carbocation mechanism differs from the reported metal nitrene intermediates and therefore enables metal-free and new transformation.

A novel intramolecular cyclization of alkyl azides for the synthesis of cyclic imines and tertiary amines has been developed. The aliphatic C–H or C–C bond was selectively cleaved with the efficient formation of two C–N single bonds or a CN double bond.

Production of Hydroxy Acids: Selective Double Oxidation of Diols by Flavoprotein Alcohol Oxidase

Flavoprotein oxidases can catalyze oxidations of alcohols and amines by merely using molecular oxygen as the oxidant, making this class of enzymes appealing for biocatalysis. The FAD-containing (FAD=flavin adenine dinucleotide) alcohol oxidase from P. chrysosporium facilitated double and triple oxidations for a range of aliphatic diols. Interestingly, depending on the diol substrate, these reactions result in formation of either lactones or hydroxy acids. For example, diethylene glycol could be selectively and fully converted into 2-(2-hydroxyethoxy)acetic acid. Such a facile cofactor-independent biocatalytic route towards hydroxy acids opens up new avenues for the preparation of polyester building blocks.

Tandem Construction of Indole-Fused Phthalazines from (2-Alkynylbenzylidene)hydrazines under Metal-Free Conditions

An efficient approach to invent diversely substituted indole-fused phthalazines from in situ formed (2-alkynylbenzylidene)hydrazines under metal-free conditions via selective radical cyclization has been developed. Notably, this 6-exo-dig addition-cyclization tandem procedure proceeds under air atmosphere and shows a broad substrate suitability, as well as avoids harmful byproducts, which complies with the concept of green synthesis.

Fluorescence-labeled neopeltolide derivatives for subcellular localization imaging

Design, synthesis and functional analysis of fluorescent derivatives of neopeltolide, an antiproliferative marine macrolide, are reported herein. Live cell imaging using the fluorescent derivatives showed rapid cellular uptake and localization within the endoplasmic reticulum as well as the mitochondria.

New frontiers in the transition-metal-free synthesis of heterocycles from alkynoates: an overview and current status

This review highlights the successful utilization of transition-metal-free approaches for the modular assembly of various heterocycles from alkynoates.

Access to cyano-substituted pyrazolines through copper-catalyzed cascade cyanation/cyclization of unactivated olefins

A mild copper-catalyzed cascade cyanation/cyclization of hydrazone-tethered unactivated olefins was developed for the efficient and practical synthesis of cyano-containing pyrazolines.

Metal-free syntheses of new azocinesviaaddition reactions of enaminones with acenaphthoquinone followed by oxidative cleavages of the corresponding vicinal diols

A one-pot, clean and green procedure is described for the syntheses of new azocine derivatives via addition reactions of enaminones with acenaphthoquinone followed by periodic acid-mediated oxidative cleavages of the corresponding vicinal diols. Various derivatives of azocine were prepared and well characterized. The excellent yields, simple synthesis procedure, lack of a need to carry out any tedious work-up and column chromatography, metal-free catalysis, and mild reaction conditions are important features of this protocol.

One-pot metal-free periodic acid-mediated syntheses of azocine rings with high yields under mild reaction conditions via the addition reactions of enaminones and acenaphthoquinone.

Formal [5+1] annulation reactions of dielectrophilic peroxides: facile access to functionalized dihydropyrans

Peroxides, functioning as unique five-atom bielectrophilic synthons, enable the new [5+1] annulation reactions to access dihydropyrans in high yields.

Facile access to versatile aza-macrolides through iridium-catalysed cascade allyl-amination/macrolactonization

Direct access to benzo-fused aza-macrolides was successfully realised via the first iridium-catalysed intermolecular decarboxylative couplings of vinylethylene carbonates with isatoic anhydrides under relatively mild conditions.