Enantioselective 1,3-dipolar cycloaddition reaction of nitrones with α,β-unsaturated aldehydes catalyzed by cationic 3-oxobutylideneaminatocobalt(III) complexes

The enantioselective 1,3-dipolar cycloaddition reaction of nitrones with alpha,beta-unsaturated aldehydes was realized using 3-oxobutylideneaminatocobalt complex catalysts. Varieties of the cobalt(II) and cobalt(III) complexes were screened and the cationic cobalt(III) complex with hexafluoroantimonate was found to be the most effective for the catalytic enantioselective 1,3-dipolar cycloaddition reaction. In the presence of the cobalt(III) hexafluoroantimonate complex, the enantioselective 1,3-dipolar cycloaddition reaction of various nitrones with alpha,beta-unsaturated aldehydes afforded the corresponding isoxazolidines in high yields and with high enantioselectivities. The absolute configuration of the optically active products was determined by X-ray analysis. Reasonable explanations for the enantioselection in the present 1,3-dipolar cycloaddition reaction catalyzed by the 3-oxobutylideneaminatocobalt complex were proposed.