Asymmetric 1,3-dipolar cycloaddition reactions of benzonitrile oxide mediated by a chiral Lewis acid

摘要

In this paper we report on the asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides mediated by pybox/ytterbium triflates, -/magnesium bromide, and -/magnesium perchlorate. It was confirmed that the reactions proceed smoothly to give isoxazoline derivatives in high enantionteric excesses with Mg2+ or Yb3+ complexes and acrylamide dipolarophiles bearing an oxazolidinone or imidazolidinone coordination auxiliary as well as a pybox ligand. In reactions with a dipolarophile bearing 4,4-dimethyloxazolidinone as the coordination auxiliary, an enantiomeric excess (59 % ee) of the corresponding cycloaddition product was achieved by using a slow addition technique to generate nitrile oxides in the presence of Yb(OTf)(3)/ph-pybox. The cycloaddition reaction of a dipolarophile with an unsubstituted oxazolidinone group in the presence of Yb(OTf)(3)/ph-pybox at a relatively high temperature (35 degrees C) produced a higher enantiomeric excess. A relatively low temperature (-78 degrees C) was required to obtain enantiomeric excesses such as 69 and 87 % ee in the reactions of a dipolarophile bearing an imidazolidinone group to afford cycloadducts in the presence of MgBr2/ ip-pybox. ((c) Wiley-VCH Verlag GmbH & Co.