Organocatalysts promote enantioselective 1,3-dipolar cycloadditions of nitrones with 1-cycloalkene-1-carboxaldehydes

In the presence of enantiopure organocatalysts, 1-cycloalkene-1-carboxaldehydes and various nitrones furnished fused isoxazolidines. Thus, some chiral pyrrolidinium salts catalyzed the formation of such cycloadducts in high diastereo- and enantioselectivity (up to 92% ee). The predominant diastereomer, the exo one, was mostly obtained in excellent diastereoselectivity (> 99:1 dr). Furthermore, after recrystallization of one of the cycloadducts, this was obtained enantiomerically pure (> 99% ee). The absolute configuration of one of the cycloadducts was determined. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.