Diastereoselective Diels-Alder reactions. The role of the catalyst. 2

Abstract

The Diels-Alder reaction between (R)-(-)-methyl (Z)-3-(4,5-dihydro-2- phenyl-4-oxazolyl)-2-propenoate 1 and cyclopentadiene in the presence of one equivalent of Et2AlCl or BF3 gave stereochemical results opposite to those obtained with one equivalent of EtAlCl2, SnCl 4, or TiCl4. Energy minimizations of proposed complexes of these Lewis acids with the chiral dienophile at the RHF/3-21G level suggest that whereas the former coordinate singly to the nitrogen, the latter complex to both the nitrogen and the carbonyl oxygen of the substrate. These different complexes expose diastereotopic faces of the dienophile to reaction with the diene.