Synthesis of Ethyl Isoxazole-3-Carboxylates Using an Ionic Liquid as a Soluble Support

1,3-Dipolar Cycloaddition Reactions of Nitrile Oxides under "Non-Conventional" Conditions: Green Solvents, Irradiation, and Continuous Flow

The 1,3-dipolar cycloaddition reactions (DCs) of nitrile oxides (NOs) to alkenes and alkynes are useful methods for the synthesis of 2-isoxazolines and isoxazoles respectively, which are important classes of heterocyclic compounds in organic and medicinal chemistry. Most of these reactions are carried out in organic solvents and under thermal activation. Nevertheless the use of supercritical carbon dioxide (scCO₂ ) and ionic liquids (Ils) as alternative solvents and the application of microwave (MW) and ultrasound (US) as alternative activation procedures have evident advantages from the "Green Chemistry" point of view. The critical discussion on the applications of these "unconventional" activation methods and reaction conditions in the 1,3-DCs of NOs is the objective of the present Review.

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.

Harnessing the TEMPO-Catalyzed Aerobic Oxidation for Machetti-De Sarlo Reaction toward Sustainable Synthesis of Isoxazole Libraries

A practical synthesis of isoxazole/isoxazoline derivatives via Machetti-De Sarlo reaction under sustainable conditions has been accomplished. This protocol involves the use of readily available 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) to catalyze the cyclocondensation of primary nitroalkanes with alkynes/alkenes to afford a library of isoxazole/isoxazoline products. From an eco-benign perspective, notable advantages of this method are as follows: (i) water as the solvent, (ii) air as the oxidant, (iii) transition metal-free, (iv) no base required, (v) no toxic byproduct, (vi) no need of solvent extraction, (vii) diverse substrate scope, (viii) high chemical yields, (ix) excellent chemo- and regioselectivity, (x) short reaction time, (xi) gram-scale synthesis, (xii) extension to heterogeneous version, and (xiii) catalyst recyclability. For these reasons, the developed method is appropriate for safe laboratory use and can be expected to inspire the progress of TEMPO-based organocatalysis for the preparation of isoxazole/isoxazoline moieties in an environmentally benign fashion.