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 ScIII-based one is both strongly endo-selective and strongly enantioselective. Several catalysts derived from (4′R,5′R)-trans-diphenyl-pybox (9) give up to 50% yields of exo-4 and, at least with PrIII and LaIII, 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 ScIII to LaIII. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)