MODELS FOR THE USE OF ALPHA-AMINO-ACIDS AS CHIRAL AUXILIARIES IN ASYMMETRIC DIELS-ALDER REACTIONS

Three different models, depending on the Lewis acid used as a catalyst and the alpha-amino acid used as a chiral auxiliary, account for the diastereofacial selectivity obtained in reactions of cyclopentadiene with N-acryloyl derivatives of L-proline, L-phenylalanine, L-alanine, and N-methyl-L-alanine esters. For alpha-amino acids without an NH group and aluminum catalysts, a reactive conformer with antiplanar enoate conformation similar to those postulated for acrylates with only one center capable of coordination is proposed. These dienophiles form a chelate complex, where the acryloyl moiety is in syn conformation, with TiCl4. For alpha-amino acids with an NH group, a reactive intermediate with an antiplanar disposition in the acryloyl moiety and the conformation of the amino ester fixed by an intramolecular hydrogen bond accounts for the results with both kinds of catalyst. These models seem to have general application for the use of alpha-amino acids as chiral auxiliaries in asymmetric Diels-Alder reactions.