Copper-Catalyzed Synthesis of Isoxazoles

Significance

Reported is the copper(II)-catalyzed cycloaddition/condensation of activated methylene nitro compounds 1 and 1,3-dicarbonyl compounds 2 to give polysubstituted isoxazoles 3. The best results were achieved using a combination of N-methylpiperidine (NMP) as a base in combination with Cu(OAc)₂ as a catalyst in chloroform or in toluene (depending on the nitro substrates). In the absence of the dipolarophiles, the nitro compounds 1 were found to undergo self-condensation to lead to the corresponding fur­oxans 4. In the case of the reaction of benzoyl­nitromethane (1a), 3-benzoyl-4-nitro-5-phenylisoxazole 5 was also obtained due to the stronger enolic character of the benzoylnitromethane. The structure of 5 was confirmed by X-ray crystallographic structure analysis.