Ethynyl-B(dan) in [3+2] Cycloaddition and Larock Indole Synthesis: Synthesis of Stable Boron-Containing Heteroaromatic Compounds

The cycloaddition of nitrile oxides with ethynyl-B(dan) (dan=naphthalene-1,8-diaminato) allowed the facile preparation of diverse isoxazolyl-B(dan) compounds, all of which displayed excellent protodeborylation-resistant properties. The dan-installation on the boron center proves vital to the high stability of the products as well as the perfect regioselectivity arising from hydrogen bond-directed orientation in the cycloaddition. The diminished boron-Lewis acidity of ethynyl-B(dan) also renders it amenable to azide-alkyne cycloaddition, Larock indole synthesis and related heteroannulations. The obtained boron-containing triazole, indoles, benzofuran and indenone exhibit sufficient resistance toward protodeborylation. Despite the commonly accepted transmetalation-inactive property derived from the diminished Lewis acidity, the synthesized heteroaryl-B(dan) compound was still found to be convertible to the oligoarene via sequential Suzuki-Miyaura coupling.

Solvent-free synthesis of 3,5-isoxazoles via 1,3-dipolar cycloaddition of terminal alkynes and hydroxyimidoyl chlorides over Cu/Al2O3 surface under ball-milling conditions

Scalable, solvent-free synthesis of 3,5-isoxazoles under ball-milling conditions has been developed. The proposed methodology allows the synthesis of 3,5-isoxazoles in moderate to excellent yields from terminal alkynes and hydroxyimidoyl chlorides, using a recyclable Cu/Al₂O₃ nanocomposite catalyst. Furthermore, the proposed conditions are reproducible to a 1.0-gram scale without further milling time variations.

A practical and scalable mechanochemical 1,3-dipolar cycloaddition between hydroxyimidoyl chlorides and terminal alkynes catalyzed by Cu/Al₂O₃ allows a quick access to 3,5-isoxazole derivatives.

Synthesis of 5-(Fluoroalkyl)isoxazole Building Blocks by Regioselective Reactions of Functionalized Halogenoximes

A comprehensive study on the synthesis of 5-fluoroalkyl-substituted isoxazoles starting from functionalized halogenoximes is reported. One-pot metal-free [3 + 2] cycloaddition of CF3-substituted alkenes and halogenoximes bearing ester, bromo, chloromethyl, and protected amino groups was developed for the preparation of 5-trifluoromethylisoxazoles. The target 3,5-disubstituted derivatives were obtained in a regioselective manner in good to excellent yield on up to 130 g scale. 5-Fluoromethyl- and 5-difluoromethylisoxazoles were synthesized by late-stage deoxofluorination of the corresponding 5-hydroxymethyl or 5-formyl derivatives, respectively, in turn prepared via metal-free cycloaddition of halogenoximes and propargylic alcohol. An alternative approach based on nucleophilic substitution in 5-bromomethyl derivatives was found to be more convenient for the preparation of 5-fluoromethylisoxazoles. Reaction of isoxazole-5-carbaldehydes with the Ruppert-Prakash reagent was used for the preparation of (β,β,β-trifluoro-α-hydroxyethyl)isoxazoles. Utility of described approaches was shown by multigram preparation of side-chain functionalized mono-, di-, and trifluoromethylisoxazoles, for example, fluorinated analogues of ABT-418 and ESI-09.

Copper-Catalyzed Cascade Cyclization Reactions of Diazo Compounds with tert-Butyl Nitrite and Alkynes: One-Pot Synthesis of Isoxazoles

A novel copper-catalyzed [3 + 2] cycloaddition reaction of alkynes with nitrile oxides generated in situ from the coupling reaction of copper carbene and nitroso radical has been developed. The three-component reaction provides a simple and efficient method for the construction of isoxazoles in a highly regioselective manner in a single step. On the basis of the experimental results and density functional theory calculations, a catalytic cycle (Cu^I-Cu^II-Cu⁰-Cu^I) for this cascade cyclization reaction is proposed.

Novel valdecoxib derivatives by ruthenium(ii)-promoted 1,3-dipolar cycloaddition of nitrile oxides with alkynes - synthesis and COX-2 inhibition activity

Novel valdecoxib-based cyclooxygenase-2 inhibitors were synthesized in one step via 1,3-dipolar cycloaddition of nitrile oxides with a series of eleven aryl alkynes, six of them described for the first time. Application of Ru(ii)-catalysis leads preferably to the formation of the 3,4-diaryl-substituted isoxazoles, while under thermal heating with base the 3,5-diaryl substitution pattern is favoured. The new the 3,4-diaryl-substituted isoxazoles possessing a small substituent (H and Me) displayed high COX-2 inhibition affinity (IC₅₀ = 0.042-0.073 μM) and excellent selectivity (COX-2 SI > 2000). In contrast, the 3,5-diaryl substituted compounds displayed almost no COX activity. The introduction of a 4-fluorophenyl substituent resulted in retained high COX-2 affinity, making these compounds together with the feasible one step reaction promising candidates for the development of fluorine-18 labelled radiotracers.

Metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides

In this review recent advances in the metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides are highlighted, covering references from the period 2000 until August 2018.

Exposure to air boosts CuAAC reactions catalyzed by PEG-stabilized Cu nanoparticles

The catalytic activity of Cu(0) NPs is boosted upon aerobic oxidation, forming Cu₂O NPs, and further improved on an SBA-15 support.

A base promoted multigram synthesis of aminoisoxazoles: valuable building blocks for drug discovery and peptidomimetics

An easy access to a large panel of aminoisoxazoles starting from commonly available amino acids.

Simple and Highly Efficient Synthesis of 3,5-Disubstituted Isoxazoles Using Cu/Graphene/Clay Nanohybrid as a New Heterogeneous Nano Catalyst

A facile and convenient protocol for ‘click’ cycloaddition of structurally diverse alkynes with in situ generated nitrile oxides catalysed by Cu/aminoclay/reduced graphene oxide nanohybrid (Cu/AC/r-GO nanohybrid) as a highly efficient heterogeneous nano-catalyst is described. In this method, Cu/AC/r-GO nanohybrid catalyses the 1,3-dipolar cycloaddition of alkynes and nitrile oxides in the presence of NaHCO3 in H2O/THF (50:50, V/V) to afford the corresponding 3,5-disubstituted isoxazoles. The Cu/AC/r-GO nanohybrid is a low cost, non-hygroscopic, chemically and thermally stable catalyst that can be reused for many consecutive reaction runs without significant loss in its reactivity.