A Highly Enantioselective Hetero-Diels−Alder Reaction of Aldehydes with Danishefsky's Diene Catalyzed by Chiral Titanium(IV) 5,5‘,6,6‘,7,7‘,8,8‘-Octahydro-1,1‘-bi-2-naphthol Complexes

The catalytic effect of chiral Lewis acids on the hetero-Diels−Alder reaction between aldehydes
and Danishefsky's diene (1) has been investigated. A variety of combinations of different ligands
and Lewis acids have been examined as catalysts for the hetero-Diels−Alder reaction between
benzaldehyde and 1, and it has been found that the readily accessible Ti(IV)−H8-BINOL (TiHBOL)
complex is a very effective catalyst for the reaction, leading to products with very high
enantioselectivity (up to 99% ee) and yield (92%). The hetero-Diels−Alder reaction of other aldehydes
with 1 under the catalysis of TiHBOL is a general reaction which proceeds well with very high
enantioselectivity and isolated yield for various aldehydes at 0 °C to room temperature. Based on
the experimental results, the proposed mechanism of the hetero-Diels−Alder reaction and the
dihedral angle effects of ligands are discussed.