One-pot synthesis of polyfunctional pyrazoles: an easy access to α-diazoketones from arylglyoxal monohydrates and tosylhydrazine

Multicomponent syntheses of pyrazoles via (3 + 2)-cyclocondensation and (3 + 2)-cycloaddition key steps

Pyrazoles are rarely found in nature but are traditionally used in the agrochemical and pharmaceutical industries, while other areas of use are also actively developing. However, they have also found numerous other applications. The search for new and efficient syntheses of these heterocycles is therefore highly relevant. The modular concept of multicomponent reactions (MCR) has paved a broad alley to heteroaromatics. The advantages over traditional methods are the broader scope and increased efficiency of these reactions. In particular, traditional multistep syntheses of pyrazoles have considerably been extended by MCR. Progress has been made in the cyclocondensation of 1,3-dielectrophiles that are generated in situ. Limitations in the regioselectivity of cyclocondensation with 1,3-dicarbonyls were overcome by the addition-cyclocondensation of α,β-unsaturated ketones. Embedding 1,3-dipolar cycloadditions into a one-pot process has additionally been developed for concise syntheses of pyrazoles. The MCR strategy also allows for concatenating classical condensation-based methodology with modern cross-coupling and radical chemistry, as well as providing versatile synthetic approaches to pyrazoles. This overview summarizes the most important MCR syntheses of pyrazoles based on ring-forming sequences in a flashlight fashion.

Synthesis of Acyl Cyclopentaquinolinones through Simultaneous Construction of the Heterocyclic Scaffold and Introduction of the Acyl Group

Presented herein is an effective and concise synthesis of acyl cyclopentaquinolinone derivatives via the cascade reactions of N-(o-ethynylaryl)acrylamides with α-diazo carbonyl compounds. The formation of product involves a visible light-induced radical formation from α-diazo carbonyl compound followed by its addition onto the acrylamide moiety to trigger double radical annulation, single-electron oxidation, and β-elimination. To our knowledge, this is the first example in which the cyclopentaquinolinone scaffold was constructed along with the introduction of an acyl group under visible light irradiation conditions. Compared with literature methods for similar purpose, this newly developed protocol has advantages such as readily accessible substrates, mild reaction conditions, valuable products, concise synthetic procedure, and high sustainability. With all these merits, this method is expected to find wide applications in the construction of related acyl heterocyclic skeletons.

Diazo-Transfer Reaction of Nonactivated Ketones with 2-Azido-1,3-bis(2,6-diisopropylphenyl)imidazolium Hexafluorophosphate (IPrAP)

The diazo-transfer reaction of nonactivated ketone under mild reaction conditions was developed. Various nonactivated ketones such as aryl methyl ketones, sec-alkyl methyl ketones, and cyclic ketones were transformed into their corresponding α-diazoketones in one step by treating 2-azido-1,3-bis(2,6-diisopropylphenyl)imidazolium hexafluorophosphate (IPrAP) in the presence of iPr2NH in ethylene glycol. In the reaction of IPrAP with prim-alkyl methyl ketone and prim-alkyl aryl ketones, migratory amidation proceeded under the reaction conditions to afford the corresponding amides.

Divergent synthesis of two types of indolizines from pyridine-2-acetonitrile, (hetero)arylglyoxal, and TMSCN

Herein we describe the divergent synthesis of two types of indolizines via construction of the pyrrole moiety from pyridine-2-acetonitriles, arylglyoxals, and TMSCN. While one-pot three-component coupling provided 2-aryl-3-aminoindolizines via an unusual fragmentation process, a sequential two-step assembly protocol with these starting materials allowed efficient access to a wide range of new 2-acyl-3-aminoindolizines through an aldol condensation-Michael addition-cycloisomerization process. The subsequent manipulation of 2-acyl-3-aminoindolizines enabled direct access to novel polycyclic N-fused heteroaromatic skeletons.

Into the Blue: Ketene Multicomponent Reactions under Visible Light

For the first time, a detailed study on the photophysical properties of variously substituted diazoketones and on their photoreactivity under blue LED irradiation was carried out. Despite very limited absorbance in the visible region, we have demonstrated that, independently from their structure, α-diazoketones all undergo a very efficient Wolff rearrangement. Contrarily to the same UV-mediated reaction, where photons can give rise to side processes, in this case, almost all absorbed photons are selective and effective, and the quantum yield is close to 100%. If the rearrangement is carried out in the presence of isocyanides and carboxylic acids/silanols, the photoreactivity is not affected, and the resulting ketenes can afford α-acyloxy- and α-silyloxyacrylamides through two distinct multicomponent reactions, performed both in batch and under continuous flow, with improved selectivity and broader scope. These photoinduced multicomponent reactions can be coupled with other visible-light-mediated transformations, thus increasing the diversity of the molecules obtainable by this approach.

Diastereoselective trans Cyclopropanation of 3-Alkylidene Oxindoles with In Situ Generated α-Diazo Carbonyls or α,β-Unsaturated Diazo Compounds

An efficient diastereoselective trans cyclopropanation of 3-alkylidene oxindoles with in situ generated α-diazo carbonyl compounds or α,β-unsaturated diazo compounds under metal-free conditions has been developed to synthesize 3-spirocyclopropyl-2-oxindole derivatives. The procedure is based on the 1,3-dipolar character of the corresponding diazo compounds under base-catalyzed conditions. The method has a wide substrate scope and uses easily available starting materials.

Iodine-Catalyzed Regioselective Oxidative Cyclization of Aldehyde Hydrazones with Electron-Deficient Olefins for the Synthesis of Mefenpyr-Diethyl

A regioselective synthesis of polysubstituted dihydropyrazoles and pyrazoles through an iodine-catalyzed oxidative cyclization strategy of aldehyde hydrazones with electron-deficient olefins is described. The protocol adopts very mild reaction conditions and provides desirable yields. The reaction is supposed to proceed via a cascade C-H functionalization, C-N bond formation, and oxidation sequential processes. The overall simplicity and regioselectivity of the catalytic system make this approach a valuable and step-economical tool to construct a C-C bond for the synthesis of Mefenpyr-Diethyl.

Contractive Annulation: A Strategy for the Synthesis of Small, Strained Cyclophanes and Its Application in the Synthesis of [2](6,1)Naphthaleno[1]paracyclophane

A new synthetic strategy (contractive annulation) for the synthesis of highly strained cyclophanes has been conceived and its viability has been demonstrated through a nine-step synthesis of [2](6,1)naphthaleno[1]paracyclophane from [2.2]paracyclophane.

Synthesis of functionalized pyrazole derivatives by regioselective [3+2] cycloadditions of N-Boc-α-amino acid-derived ynones

[3+2] cycloadditions of ynones derived from glycine and (S)-alanine and some other dipolarophiles with azomethine imines, nitrile oxides, diazoacetate, and azidoacetate were studied. The dipolarophiles were obtained from α-amino acids, either by the reduction of the carboxy function with ethynylmagnesium bromide or by propiolation of the amino function. Cu-catalyzed cycloadditions of ynones to azomethine imines were regioselective and gave the expected cycloadducts as inseparable mixtures of diastereomers. In some instances, further oxidative hydrolytic ring-opening took place to afford 3,3-dimethyl-3-(1H-pyrazol-1-yl)propanoic acids. Acid-catalyzed cycloadditions of 3-butenone were also regioselective and provided mixtures of diastereomeric cycloadducts, which were separated by chromatography. In the reactions of title ynones with alkyl diazoacetates, in situ-formed benzonitrile oxides, and tert-butyl azidoacetate, all cycloadducts were obtained as single regioisomers. The structures of all novel compounds were established by nuclear magnetic resonance and X-ray diffraction.

Pyrazole amino acids: hydrogen bonding directed conformations of 3-amino-1H-pyrazole-5-carboxylic acid residue

A series of model compounds containing 3-amino-1H-pyrazole-5-carboxylic acid residue with N-terminal amide/urethane and C-terminal amide/hydrazide/ester groups were investigated by using NMR, Fourier transform infrared, and single-crystal X-ray diffraction methods, additionally supported by theoretical calculations. The studies demonstrate that the most preferred is the extended conformation with torsion angles ϕ and ψ close to ±180°. The studied 1H-pyrazole with N-terminal amide/urethane and C-terminal amide/hydrazide groups solely adopts this energetically favored conformation confirming rigidity of that structural motif. However, when the C-terminal ester group is present, the second conformation with torsion angles ϕ and ψ close to ±180° and 0°, respectively, is accessible. The conformational equilibrium is observed in NMR and Fourier transform infrared studies in solution in polar environment as well as in the crystal structures of other related compounds. The observed conformational preferences are clearly related to the presence of intramolecular interactions formed within the studied residue. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.