4-naphthyl-substituted bis(oxazoline): a new, easily recoverable and efficient chiral ligand in asymmetric catalysis of the Diels-Alder reaction

2,2-Bis{2-[(4R)-(2'-naphthyl)-1,3-oxazolinyl]}propane and its (4S) enantiomer were prepared starting from 2-vinylnaphthalene, following the Sharpless protocol, and were found to be optically pure (ee greater than or equal to 99.5%) by hplc analysis. These new bis(oxazolines) are efficient chiral ligands in the asymmetric catalysis of the Diels-Alder reaction of N-alkenoyl-oxazolidin-2-one derivatives since endo cycloadducts are obtained with up to 94% ee. Different Lewis acids were tested, the best one being the Mg(II) cation derived from the corresponding triflate. A remarkable feature of such new chiral ligands is the low solubility in polar solvents, so that the chiral ligand can be easily recovered by a simple filtration. The recovered ligand can be directly reused in other asymmetric syntheses, without any further purification and, overall, without any drop in the induced enantioselectivities. (C) 1998 Elsevier Science Ltd. All rights reserved.