A Mild and Efficient Asymmetric Hetero‐Diels–Alder Reaction of the Brassard Diene with Aldehydes

This paper describes the successful development of the hetero‐Diels–Alder (HDA) reaction of the Brassard diene with aldehydes in the presence of a series of titanium(IV) tridentate Schiff‐base complexes under mild conditions. The influence of the substituent of the chiral Schiff‐base ligands on the enantioselectivities of the reaction was studied. It was found that ligand L13 is a highly enantioselective ligand for the Ti‐catalyzed HDA reaction, affording 6‐substituted 4‐ethoxy‐5,6‐dihydropyran‐2‐one in up to 99 % ee. The mechanism of the HDA reaction between the Brassard diene and benzaldehyde in the presence of the (Schiff base)TiIV complex was investigated. The results indicate that the reaction pathway is influenced by reaction temperature: at higher temperature (0 °C), the reaction is mostly a Diels–Alder process, whereas at lower temperature (–78 °C) it is a Mukaiyama aldol process. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)