Diastereoselective Diels–Alder reactions. The role of the catalyst

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 gave stereochemical results opposite to those obtained with one equivalent of EtAlCl2. Energy minimizations of proposed complexes of these Lewis acids with the chiral dienophile at the RHF/3–21G level suggest that the aluminum is tetrahedrally complexed with Et2AlCl, but bound in a trigonal bipyramid with EtAlCl2. These complexes expose the diastereotopic faces of the dienophile to reaction with diene.