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

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Abstract
To test whether α-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 (1R,2R)-cycloadduct were achieved with AlCl3, the reaction did not show diastereofacial selectivity when titanium catalysts were used. Moreover, with Ti(iPrO)4 a 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.