Efficient synthesis of isoxazoles and isoxazolines from aldoximes using Magtrieve™ (CrO2)

Treatment of aldoximes 1 with Magtrieve (TM) (CrO2) in presence of dipolarophile 3 or 4, furnished a variety of isoxazolines 5a-u and isoxazoles 6a-q as 1,3-dipolar cycloaddition (1,3-DC) products (38 examples; 63-90% isolated yields). In situ formation of a nitrile oxide intermediate was confirmed through isolation of the dimerization product furoxane 2a in absence of any dipolarophile. The methodology has been extended to intramolecular nitrile oxide cycloaddition (INOC) reactions to access highly useful chromane derivatives 7-8 (75-80% isolated yields). Magtrieve (TM), as a new reagent for 1,3-DC reactions, has offered excellent substrate generality and at the same time demonstrated tolerance toward sensitive protecting groups and electron-rich functional groups. (C) 2009 Elsevier Ltd. All rights reserved.