Enantioselective Intramolecular Hetero Diels-Alder Reactions of 1-Oxa-1,3-butadienes with a New Chiral Lewis Acid¹

Abstract

The intramolecular hetero Diels-Alder reaction of 1-oxa-1,3-butadienes 3a-g, obtained in situ by a Knoevenagel condensation of aromatic aldehydes 1a-g and N,N′-dimethylbarbituric acid (2), was studied in the presence of a new chiral titanium Lewis acid (5) which has 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose as a ligand. Depending on the reaction conditions and the substituents on the aromatic ring system, the cis-fused tetracycles 4a-g were formed with ee-values up to 88%. This is the first example of an enantioselective intramolecular hetero Diels-Alder reaction.