Synergistic effect of pybox substituents and lanthanide cations in reversing the asymmetric induction in the catalysed Diels-Alder reaction between 3-acryloyl-1,3-oxazolidin-2-one and cyclopentadiene

The enantioselectivity in the pybox/lanthanide(III) triflate catalysed Diels-Alder reaction between cyclopentadiene and 3-acryloyl-1,3-oxazolidin-2-one is profoundly influenced by the presence of a phenyl group in the 5'-position of the ligand, since it sometimes amplifies, and sometimes reverses the enantioselectivity induced by the 4'-substituent. The results can be summarised in three points. Of the catalysts derived from (4'R,5'R)-4-Me-5-Ph-pybox (6), the Sc-III-based one is both strongly endo-selective and strongly enantiose-lective. Several catalysts derived from (4'R,5'R)-trans-diphenyl-pybox (9) give up to 50% yields of exo-4 and, at least with Pr-III and La-III, induce excellent ee values in both endo and exo adducts. The (4'R,5'S)-cis-diphenyl-pybox (8) is a novel and valuable ligand, since opposite enantiomers can be obtained with good selectivity simply by changing the cation from Sc-III to La-III. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).