Molecular sieve dependent absolute stereoselectivity in asymmetric catalytic 1,3-dipolar cycloaddition reactions

The absolute stereochemistry of the enantioselective 1,3-dipolar cycloaddition of alkenes with nitrones catalyzed by chiral magnesium(II)-bisoxazoline complexes is shown to be dependent on the presence of molecular sieves. In the presence of powdered molecular sieves 4 Angstrom, the reaction proceeds to give the endo product with up to 82% ee, whereas in the absence of molecular sieves the opposite enantiomer of the endo product is obtained in up to 73% ee. The influence of additives such as waterer different drying agents on the absolute stereoselectivity of the reaction was also studied. The absolute stereochemistry of the products was determined on the basis of an X-ray analysis of a product containing a chiral center with a known configuration. The influence of molecular sieves on the different possible intermediates in the 1,3-dipolar cycloaddition, consisting of the alkenoyloxazolidinone coordinated to the magnesium(II)-bisoxazoline catalyst, is discussed, and it is proposed that molecular sieves are a part of the catalytic system.