Regioselective Synthesis of Pyrazolo[1,5-a]pyridine via TEMPO-Mediated [3 + 2] Annulation-Aromatization of N-Aminopyridines and α,β-Unsaturated Compounds

[3 + 2] Cycloaddition of N-Amino(iso)quinolinium Salts with Vinylsulfonium Salt: Synthesis of Pyrazolo[1,5-a]quinoline and Pyrazolo[5,1-a]isoquinoline Derivatives

In this study, we reported the utilization of vinylsulfonium salt as a highly efficient dipolarophile, leveraging the facile leaving ability of its sulfide moiety, to engage in a [3 + 2] cycloaddition with N-amino(iso)quinolinium salts. This approach facilitates the construction of various C1/C2-unsubstituted pyrazolo(iso)quinoline skeletons. The transformation is conducted under catalyst- and external oxidant-free conditions. Furthermore, the extended gram-scale reaction demonstrates the practical applicability of the developed protocol.

Transition metal- and oxidant-free [3 + 2] cycloaddition of N-amino(iso)quinolinium salts with vinyl acetate as an acetylene surrogate

A metal- and external oxidant-free [3 + 2] annulation of N-amino(iso)quinoline salts with vinyl acetate as the acetylene surrogate under simple and mild reaction conditions is described. A series of valuable pyrazolo(iso)quinoline scaffolds were synthesized through this process. Mechanistic investigations revealed that the reaction proceeds through a Mannich/cyclization/aromatization sequence. Furthermore, scale-up experiments and derivative syntheses of the product were conducted.

Direct β-C-H ketoalkylation of enaminoesters with cyclopropanols under metal-free conditions

A TEMPO-mediated β-ketoalkylation of enaminoesters with cyclopropanols under metal-free conditions is herein described. This reaction provides a straightforward and highly efficient route to β-keto alkyl substituted enaminoesters for the first time, which could be rapidly and efficiently converted into synthetically useful 2-alkoxycarbonyl functionalized 1,5-diketones. Moreover, the practicability of this protocol is successfully demonstrated by scale-up experiments and the late-stage functionalization of natural products and pharmaceutically relevant molecules.

Ligand-Free Iron-Catalyzed Carbonylation of Aryl Iodides with Alkenyl Boronic Acids: Access to α,β-Unsaturated Ketones

The application of earth-abundant and low-toxicity iron catalysts as replacements for palladium in carbonylative coupling reactions remains challenging and largely unexplored. Reported here is a highly efficient iron-catalyzed carbonylation of aryl iodides with alkenyl boronic acids under ligand-free conditions, enabling the synthesis of α,β-unsaturated ketones even at atmospheric CO pressure. The broad applicability, including its effectiveness with α-branched enones and biologically active molecules, along with high yields and selectivity, underlines the general applicability of this catalytic system.

Visible-light-promoted direct desulfurization of glycosyl thiols to access C-glycosides

C-Glycosides are essential for the study of biological processes and the development of carbohydrate-based drugs. Despite the tremendous hurdles, glycochemists have often fantasized about the efficient, highly stereoselective synthesis of C-glycosides with the shortest steps under mild conditions. Herein, we report a desulfurative radical protocol to synthesize C-alkyl glycosides and coumarin C-glycosides under visible-light induced conditions without the need of an extra photocatalyst, in which stable and readily available glycosyl thiols that could be readily obtained from native sugars are activated in situ by pentafluoropyridine. The benefits of this procedure include high stereoselectivity, broad substrate scope, and easy handling. Mechanistic studies indicate that the in situ produced tetrafluoropyridyl S-glycosides form key electron donor-acceptor (EDA) complexes with Hantzsch ester (for C-alkyl glycosides) or Et3N (for coumarin C-glycosides), which, upon irradiation with visible light, trigger a cascade of glycosyl radical processes to access C-glycosides smoothly.

Divergent Synthesis of F- and CF3-Containing N-Fused Heterocycles Enabled by Fragmentation Cycloaddition of β-CF3-1,3-Enynes with N-Aminopyridiniums Ylides

The two novel cyclization modes of β-CF3-1,3-enynes are presented herein for the divergent construction of F- and CF3-containing N-fused heterocycles. Fluorinated pyrazolo[1,5-a]pyridines were afforded from β-CF3-1,3-enynes with N-aminopyridiniums ylides via detrifluoromethylative [2 + 3] cyclizations, followed by fluorine transfer from a CF3 unit. Whereas reaction with N-aminoisoquinoliniums ylides gave CF3-substituted pyrrolo[2,1-a]isoquinoline by unprecedented fragmentation [3 + 2]-cycloadditions. Additionally, gram-scale experiments and synthetic utility are demonstrated by further derivatization of fluorinated heterocycles.

One-pot cascade synthesis of dibenzothiophene-based heterobiaryls from dibenzothiophene-5-oxide

A sulfoxide directed C-H metalation/boration/B2Pin2 mediated reduction/Suzuki coupling process to synthesize 4-substituted dibenzothiophene (DBT) in one-pot from dibenzothiophene-5-oxide (DBTO) was developed. A variety of DBT-based heterobiaryls were prepared in satisfactory to good yields. A mechanism was proposed. The application of this methodology was demonstrated by synthesizing a luminescent material.

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.

Ruthenium(II)-Catalyzed Sterically Hindered C-H Acyloxylation to Synthesize Biaryl Isoquinoline Derivatives via Peresters

A novel C-H acyloxylation method of 1-(1-naphthalen-1-yl)isoquinoline derivatives with peresters in the presence of [Ru(p-cymene)Cl₂]₂ has been developed. The combination of ruthenium(II), AgBF₄, CoI₂, and 2,2,6,6-tetramethyl-1-piperidinyloxy is found to be an effective catalytic system to provide various biaryl compounds in satisfactory yields within minutes. Notably, steric hindrance is a very important determinant of the reaction.

Synthesis of Pyrazolo[1,5-a]pyridinyl, Pyrazolo[1,5-a]quinolinyl, and Pyrazolo[5,1-a]isoquinolinyl Sulfonyl Fluorides via a [3 + 2] Annulation

A [3 + 2] cycloaddition reaction of N-aminopyridines, N-aminoquinolines, and N-aminoisoquinolines with 1-bromoethene-1-sulfonyl fluoride (BESF) was performed to obtain optimum yields of various useful pyrazolo[1,5-a]pyridinyl, pyrazolo[1,5-a]quinolinyl, and pyrazolo[5,1-a]isoquinolinyl sulfonyl fluorides (43-90% yield). The transformation process showed broad substrate specificity, mild reaction conditions, and operational simplicity. Therefore, the reaction has great applicable value in the field of medicinal chemistry and other disciplines.

Controllable transformation of indoles using iodine(III) reagent

Herein, an efficient and highly functional group-compatible procedure for controllable transformation of indoles by the combination of phenyliodine bis(trifluoroacetate) (PIFA) with n-Bu₄NCl·H₂O (TBAC) was exploited. Through controlling the amount of PIFA and TBAC from one to three equivalents, 3-chloro-indoles, 3-chloro-2-oxindoles, and 3,3-dichloro-2-oxindoles were obtained, respectively, in satisfactory to excellent yields. The advantages of the protocol include mild conditions, facile process with short reaction time, high yields, satisfactory functional group tolerance, and the use of PIFA, which is an air- and moisture-stable promoter. The mechanism studies showed that the reaction may proceed through a halonium ion species-mediated halogenation-elimination-halogenation stepwise process.

Modular Access to 2-(Trifluoromethyl)pyrazolo[1,5-a]pyridines and Their Benzo Analogues through a Copper(I)-Catalyzed Radical Annulation

A mechanistically distinctive copper-catalyzed radical annulation to valuable 2-(trifluoromethyl)pyrazolo[1,5-a]pyridines and their benzo analogues has been described for the first time. Notably, the newly developed complementary process allows the synthesis of 4- or 6-substituted target molecular entities as a single product, which was previously challenging to access by existing methods. The utility of this process is further demonstrated by the facile construction of four different ring systems, a gram-scale synthesis, and the late-stage functionalization of bioactive molecules.