MOLECULAR-SIEVE DEPENDENT ABSOLUTE STEREOSELECTIVITY IN ASYMMETRIC CATALYTIC 1,3-DIPOLAR CYCLOADDITION REACTIONS

The absolute stereochemistry of the enantioselective 1,3-dipolar cyclo addition of alkenes with nitrones catalyzed by chiral magnesium(II)-bi soxazoline complexes is shown to be dependent on the presence of molec ular sieves. In the presence of powdered molecular sieves 4 Angstrom, the reaction proceeds to give the endo product with up to 82% ee, wher eas in the absence of molecular sieves the opposite enantiomer of the endo product is obtained in up to 73% ee. The influence of additives s uch as waterer different drying agents on the absolute stereoselectivi ty of the reaction was also studied. The absolute stereochemistry of t he products was determined on the basis of an X-ray analysis of a prod uct containing a chiral center with a known configuration. The influen ce of molecular sieves on the different possible intermediates in the 1,3-dipolar cycloaddition, consisting of the alkenoyloxazolidinone coo rdinated to the magnesium(II)-bisoxazoline catalyst, is discussed, and it is proposed that molecular sieves are a part of the catalytic syst em.