Copper Catalysis for Nicotinate Synthesis through β- Alkenylation/Cyclization of Saturated Ketones with β -Enamino Esters

Iron-Catalyzed One-Pot Cascade Reactions of Oximes with Inactivated Saturated Ketones: Entry to Highly Substituted Pyridines

An iron-catalyzed oxidative [3 + 3] annulation of oxime esters with inactivated saturated ketones is described. This cascade strategy allows one-step rapid synthesis of various structurally important pyridines through an oxidative dehydrogenation/annulation/oxidative aromatization sequence via direct α,β-dehydrogenation of simple saturated ketones followed by annulation with oximes. This method shows good functional group tolerance, readily accessible starting materials, a wide substrate scope, high chemoselectivity, and no need for extra stoichiometric oxidant and is also applicable to the late-stage functionalization of natural products.

Synthesis of polysubstituted pyridazines via Cu-mediated C(sp3)-C(sp3) coupling/annulation of saturated ketones with acylhydrazones

Pyridazine is a significant skeleton that widely exists in drugs and bioactive molecules. We herein describe expeditious approaches to access polysubstituted pyridazines from readily accessible unactivated ketones and acylhydrazones via Cu-promoted C(sp3)-C(sp3) coupling/cyclization sequences in a single-step fashion. Notably, the disparate 3,4,6-trisubstituted pyridazines and 3,5-disubstituted pyridazines could be obtained by tailoring the ketone's structure and reaction conditions. These transformations feature good functional group compatibility, excellent step-economy, and chemoselectivity. The potential synthetic utility of these conversions is illustrated by scale-up reactions and late-stage derivatizations of the as-prepared pyridazine products.

Iron-Catalyzed Oxidative Rearrangement of Cyclopropanone Hemiaminals: General Access to Pyrroloindolones from Indoles

A concise synthetic approach to medicinally relevant pyrroloindolones and related fused heterocycles is reported via the diastereoselective N-addition of unprotected indoles to readily accessible cyclopropanone equivalents. The resulting stable hemiaminals are shown to smoothly rearrange to pyrroloindolones in mild conditions using Fe(III) catalysis in the presence of inexpensive ammonium persulfate as a stoichiometric oxidant. Experimental evidence points toward the formation of a β-carboxylic radical intermediate prone to cyclization and oxidative rearomatization as the operative mechanistic pathway.

Iodine-Catalyzed Diversity-Oriented Synthesis of 3,4-Heterocycle-Fused Coumarins from 4-Aminocoumarins and Aurones in Different Solvent

An unprecedented protocol has been developed for the synthesis of 3,4-heterocycle-fused coumarins from 4-aminocoumarins and aurones through iodine-catalyzed cascade reactions. Dihydropyridine-fused coumarin, pyridine-fused coumarin, and pyrrole-fused coumarin derivatives were achieved in good yields with high selectivity when CH3CN, AcOH, and DMSO were used as the solvent, respectively. This protocol provides several advantages, such as easily available starting materials, high atom economy, friendly environment, and simple procedure.

Transition metal-free [3 + 3] annulation of cyclopropanols with β-enamine esters to assemble nicotinate derivatives

A metal-free and efficient approach for the synthesis of structurally important nicotinates through 4-HO-TEMPO-mediated [3 + 3] annulation of cyclopropanols with β-enamine esters is presented. This protocol features high atom efficiency, green waste, simple operation and broad substrate scope. Moreover, the experiments of gram-scale synthesis and recovery of oxidants make this strategy more sustainable and practical.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Recent Advances in Organic Synthesis of 3-Amino- or 4-Aminocoumarins

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Coumarin is a privileged scaffold that contains the unique 2H-chromen-2-one motif, and its derivatives are widely distributed in nature, especially in plants. In recent years, due to their diverse pharmacological activities and remarkable photochemical properties, they have attracted significant attention from scientists, which has also prompted the research on the synthesis approaches and the availability of substrates for these compounds. This article is a brief description of the methods for the synthesis of various coumarin derivatives via two- or multi-component reactions involving 3-amino or 4-aminocoumarin reported during 2015-2021. This review may help expand the development of various analogues with coumarin as the basic unit.

Domino synthesis of functionalized pyridine carboxylates under gallium catalysis: Unravelling the reaction pathway and the role of the nitrogen source counter anion

The catalytic potential of various metal Lewis acid catalysts have been assessed to derive a high-yielding, multi-component domino synthesis of functionalized pyridines from (E)-3-(dimethylamino)-1-aryl/heteroaryl-prop-2-en-1-ones, 1,3-dicarbonyl compounds, and an ammonium salt (as the nitrogen precursor). Amongst the various metal halides, tetrafluoroborates, perchlorates, and triflates used as the catalyst GaI3 proved to be the most effective. The mechanistic course of the most plausible pathway has been outlined as the intermediate formation of imine/enamine by the reaction of the 1,3-dicarbonyl compound with ammonia (liberated in situ from the ammonium salt used as the nitrogen source), which participates in the domino nucleophilic Michael reaction to the (2E)-3-(dimethylamino)-1-aryl/hetroarylprop-2-en-1-one by its active methylene carbon through its enamine form followed by intramolecular cyclization and aromatization. The effect of different ammonium salts as the nitrogen source has been investigated and NH4OAc was found to be best. The influence of the acetate counter anion of NH4OAc on the progress of the reaction was studied and its specific role in the cyclization and subsequent aromatization has been revealed. This work offers distinct advantages compared to the literature reported methodologies on the count of several green index parameters.

Ketones as strategic building blocks for the synthesis of natural product-inspired compounds

Natural product-inspired compound collections serve as excellent sources for the identification of new bioactive compounds to treat disease. However, such compounds must necessarily be more structurally-enriched than traditional screening compounds, therefore inventive synthetic strategies and reliable methods are needed to prepare them. Amongst the various possible starting materials that could be considered for the synthesis of natural product-inspired compounds, ketones can be especially valuable due to the vast variety of complexity-building synthetic transformations that they can take part in, their high prevalence as commercial building blocks, and relative ease of synthesis. With a view towards developing a unified synthetic strategy for the preparation of next generation bioactive compound collections, this review considers whether ketones could serve as general precursors in this regard, and summarises the opulence of synthetic transformations available for the annulation of natural product ring-systems to ketone starting materials.

Synthesis of acyloxy-2H-azirine and sulfonyloxy-2H-azirine derivatives via a one-pot reaction of β-enamino esters, PIDA and carboxylic acid or sulfonic acid

PIDA mediated oxidative acyloxylation/azirination and sulfonyloxylation/azirination of β-enamino esters were investigated. A series of functionalized acyloxy-2H-azirine and sulfonyloxy-2H-azirine derivatives was synthesized in moderate to good yields. This represents the first oxidative sulfonyloxylation/azirination of β-enamino esters with PIDA and sulfonic acid for access to sulfonyloxy-2H-azirine. Hypervalent iodine reagents enable cascade C-O/C-N bond formation. Furthermore, a possible reaction pathway was proposed based on the experimental results.

Efficient access to β-amino acid ester/β-amino ketone derivatives via photocatalytic radical alkoxycabonylimidation/carbonylimidation of alkenes

A photocatalytic protocol for the synthesis of β-amino acid ester and β-amino ketone derivatives is developed using simple and easy-to-synthesize oxime oxalate and oxime phenylglyoxylate as difunctionalization reagents.

An Isoxazole Strategy for the Synthesis of Fully Substituted Nicotinates

A four-step quasi one-pot procedure for the preparation of fully substituted nicotinates from ketone enamines and 4-methylideneisoxazol-5-ones has been developed. The reaction sequence involves (1) reaction of 4-methylideneisoxazol-5-ones with ketone enamines with the formation of isoxazole-5-ols, (2) their O-methylation with diazomethane, (3) hydrogenative cleavage of the O-N bond in 5-methoxyisoxazoles under action of Mo(CO)₆/H₂O and simultaneous isomerization and condensation of the formed enamines, with the formation of dihydropyridines, and (4) aromatization of the latter.

Continuous-Flow Synthesis of β-Amino Acid Esters by Lipase-Catalyzed Michael Addition of Aromatic Amines

A continuous-flow procedure for the synthesis of β-amino acid esters has been developed via lipase-catalyzed Michael reaction of various aromatic amines with acrylates. Lipase TL IM from Thermomyces lanuginosus was first used to catalyze Michael addition reaction of aromatic amines. Compared with other methods, the salient features of this work include green reaction conditions (methanol as reaction medium), short residence time (30 min), readily available catalyst and a reaction process that is easy to control. This enzymatic synthesis of β-amino acid esters performed in continuous-flow microreactors is an innovation that provides a new strategy for the fast biotransformation of β-amino acid esters.

DABCO- and DBU-promoted one-pot reaction of N-sulfonyl ketimines with Morita-Baylis-Hillman carbonates: a sequential approach to (2-hydroxyaryl)nicotinate derivatives

An intriguing DABCO-catalyzed and DBU-promoted one-pot synthesis of an important class of (2-hydroxyaryl)pyridine derivatives bearing a carboxylate or a nitrile group suitably placed at C3 position of the aza-ring has been achieved in acceptable chemical yields with a broad functional group tolerance. This sequential C-C/C-N bond making process proceeds through a regioselective allylic alkylation/aza-Michael reaction between MBH carbonates derived from an acrylate/acrylonitrile and N-sulfonyl ketimines as C,N-binucleophiles catalyzed by DABCO, followed by elimination of SO2 under the influence of base and subsequent aromatization in an open atmosphere.