Synthesis of N -Sulfonyl Pyrazoles Through Cyclization Reactions of Sulfonyl Hydrazines with Enaminones Promoted by p -TSA

p-Toluenesulfonic acid-promoted organic transformations for the generation of molecular complexity

Since the beginning of this century, p-toluenesulfonic acid (p-TSA) catalysed organic transformations have been an active area of research for developing efficient synthetic methodologies. Often, catalysis using p-TSA is associated with many advantages, such as operational simplicity, high selectivity, excellent yields, and ease of product isolation, which make organic synthesis convenient and versatile. Notably, p-TSA is a non-toxic, commercially available, inexpensive solid organic compound that is soluble in water, alcohols, and other polar organic solvents. p-TSA is a strong acid compared to many protic or mineral acids and its high acidity helps activate different organic functional groups. p-TSA-promoted conversions are fast, have a high atom and pot economy, and feature a multiple bond-forming index. Therefore, the utilization of p-TSA enables the synthesis of many important structural scaffolds without any hazardous metals, making it desirable in numerous applications of sustainable and green chemistry. Recently, this emerging area of research has become one of the pillars of synthetic organic chemistry to synthesise biologically relevant, complex carbocycles and heterocycles. This study provides a comprehensive summary of methods, applications, and mechanistic insights into p-TSA-catalysed organic transformations, covering the literature reports that have appeared since 2012.

Crystal structure of 2-[(5-amino-1-tosyl-1H-pyrazol-3-yl)-oxy]-1-(4-meth-oxy-phen-yl)ethan-1-one 1,4-dioxane monosolvate

In the structure of the title compound, C19H19N3O5S·C4H8O2, the two independent dioxane mol-ecules each display inversion symmetry. The pyrazole ring is approximately parallel to the aromatic ring of the oxy-ethanone group and approximately perpendicular to the tolyl ring of the sulfonyl substituent. An extensive system of classical and 'weak' hydrogen bonds connects the residues to form a layer structure parallel to (201), within which dimeric subunits are conspicuous; neighbouring layers are connected by classical hydrogen bonds to dioxanes and by 'weak' hydrogen bonds from Htol-yl donors.

Copper-Promoted Aerobic Oxidative [3+2] Cycloaddition Reactions of N,N-Disubstituted Hydrazines with Alkynoates: Access to Substituted Pyrazoles

A copper-promoted aerobic oxidative [3+2] cycloaddition reaction for the synthesis of various substituted pyrazoles from N,N-disubstituted hydrazines with alkynoates in the presence of bases is developed. This work involves a direct C(sp³)-H functionalization and the formation of new C-C/C-N bonds. In this strategy, inexpensive and easily available Cu₂O serves as the promoter and air acts as the green oxidant. The reaction exhibits the advantages of high atom and step economy, high regioselectivity, and easy operation.

Thiazole-5-carbaldehyde Synthesis by Cascade Annulation of Enaminones and KSCN with Dess-Martin Periodinane Reagent

The Dess-Martin periodinane (DMP) reagent-mediated reactions of tertiary enaminones with potassium thiocyanate for the synthesis of thiazole-5-carbaldehydes are developed. The product formation involves cascade hydroxyl thiocyanation of the C═C double bond, intramolecular hydroamination of the C≡N bond, and thiazole annulation by condensation on the ketone carbonyl site, representing novel reaction pathways in the reactions between enaminones and thiocyanate salt. DMP plays dual roles in mediating the free radical thiocyanation and inducing the unconventional selective thiazole-5-carbaldehyde formation by masking the in situ generated formyl group during the reaction process.

Recent advances in pyrazole synthesis employing diazo compounds and synthetic analogues

This review summarizes the recent developments in the construction of pyrazoles using diazo compounds, nitrile imines and their synthetic equivalents.

Recent advances in transition metal-free annulation toward heterocycle diversity based on the C-N bond cleavage of enaminone platform

The enaminones and analogous stable enamines are well known as platform building blocks in organic synthesis for construction of heterocyclic compounds, especially N-heterocycles. To date, especially enaminones have been successfully...

DMSO as a C1 Source for [2 + 2 + 1] Pyrazole Ring Construction via Metal-Free Annulation with Enaminones and Hydrazines

A cascade reaction between enaminones, hydrazines, and dimethyl sulfoxide (DMSO) for the synthesis of 1,4-disubstituted pyrazoles catalyzed by molecular iodine in the presence of Selectfluor has been realized. DMSO plays a dual role as the C₁ source and the reaction medium. In addition, the synthesis of 1,3,4-trisubstituted pyrazoles using aldehydes as alternative C₁ building blocks has also been achieved.

Crystal structure of ethyl 2-(3-amino-5-oxo-2-tosyl-2,5-di-hydro-1H-pyrazol-1-yl)acetate

In the title compound, C14H17N3O5S, the five-membered ring is essentially planar. The substituents at the nitro-gen atoms subtend a C-N-N-S torsion angle of -95.52 (6)°. The amino group forms an intra-molecular hydrogen bond to a sulfonyl oxygen atom; two inter-molecular hydrogen bonds from the amino group, to the other S=O group and to the oxo substituent, form a layer structure parallel to the ab plane. The structure determination confirms that the title compound is N- rather than O-alkyl-ated.

Highly Regioselective and Chemoselective [3 + 3] Annulation of Enaminones with ortho-Fluoronitrobenzenenes: Divergent Synthesis of Aposafranones and Their N-Oxides

A base-promoted unprecedented strategy for the regioselective and chemoselective divergent synthesis of highly functionalized aposafranones and their N-oxides has been developed from the [3 + 3] annulation of enaminones with o-fluoronitrobenzenenes. This novel synthetic strategy offers an alternative method for the construction of aposafranones and their N-oxides are meaningful in the fields of both biology and organic synthesis. The established protocol explores the annulation scope of enaminones, and it expands the application of nitro-based cyclization.

New Reaction Products of Acetylacetone with Semicarbazide Derivatives

A new approach is suggested herein for the synthesis of pyrazole derivatives by reacting 4-nitrosemicarbazide with acetylacetone. Additional studies were done on the reaction of acetylacetone with semicarbazide and its derivatives (4-aminosemicarbazide, methylsemicarbazide, and dimethylsemicarbazide). The study on the reaction with acetylacetone resulted in monocyclic 3,5-dimethyl-N-nitropyrazole-1-carboxamide, monocyclic 5-hydroxy-3,5-dimethyl-2-pyrazoline, and bicyclic bis(3,5-dimethylpyrazole-1-carbonyl)hydrazine, and conditions for the formation of acetone semicarbazone were identified. The structures of the resultant compounds were validated by physicochemical analytical methods, including X-ray diffraction. The computer-aided screening in the PASS prediction software discovered a high biological activity of the newly obtained compounds.

Recent Advances in Organic Synthesis Based on N,N-Dimethyl Enaminones

Enaminones are gaining increasing interest because of their unique properties and their importance in organic synthesis as versatile building blocks. N,N-Dimethyl enaminones offer a better leaving group (a dimethylamine group) than other enaminones, and allow further elaboration via a range of facile chemical transformations. Over the past five years, there have been an increasing number of reports describing the synthetic applications of N,N-dimethyl enaminones. This review provides a comprehensive overview on the synthetic applications of N,N-dimethyl enaminones that have been reported since 2016.

1 Introduction

2 Direct C(sp2)–H α-Functionalization

2.1 Synthesis of α-Sulfenylated N,N-Dimethyl Enaminones

2.2 Synthesis of α-Thiocyanated N,N-Dimethyl Enaminones

2.3 Synthesis of α-Acyloxylated N,N-Dimethyl Enaminones

3 Functionalization Reactions via C=C Double Bond Cleavage

3.1 Synthesis of Functionalized Methyl Ketones

3.2 Synthesis of α-Ketoamides, α-Ketoesters and 1,2-Diketones

3.3 Synthesis of N-Sulfonyl Amidines

4 Construction of All-Carbon Aromatic Scaffolds

4.1 Synthesis of Benzaldehydes

4.2 Synthesis of the Naphthalenes

5 Construction of Heterocyclic Scaffolds

5.1 Synthesis of Five-Membered Heterocycles

5.2 Synthesis of Six-Membered Heterocycles

5.3 Synthesis of Quinolines

5.4 Synthesis of Functionalized Chromones

5.5 Synthesis of Other Fused Polycyclic Heterocycles

6 Conclusions and Perspectives