Natural amino acids as chiral auxiliaries in asymmetric Diels-Alder reactions

To test whether a-amino acids can be efficient chiral auxiliaries in asymmetric Diels-Alder reactions, the chiral dienophile N-acryloyl-L-phenylalanine methyl ester was made to react with cyclopentadiene. Diastereofacial selectivity is to a great extent dependent on the Lewis acid used to promote the reaction; whereas moderate excesses of the (1 R, 2R)-cycloadduct were achieved with AlC13, the reaction did not show diastereofacial selectivity when titanium catalysts were used. Moreover, with T~('PI-(I)4a transesterification reaction, which leads to the isopropyl ester and competes with the Diels-Alder reaction, was observed. In spite of the structural similarity of this dienophile to the acrylate of (S)-ethyl lactate, it behaved very differently in the titanium-catalyzed reactions.This research was made possible by the generous financial support of the Comisión Asesora de Investigación Cientifica y Técnica (project number PB85-0335). Maria P. Bueno would like to express her gratitude to the Diputación General de Aragón and Paula Charro to the Instituto de Estudios Riojanos for a grant.Peer reviewe