Chiral Phosphoric Acid-Governed Anti-Diastereoselective and Enantioselective Hetero-Diels-Alder Reaction of Glyoxylate

摘要

Hetero-Diels-Alder reactions between dienes and carbonyl compounds have served as an extremely valuable method for the preparation of dihydropyrans. The development of catalytic enantio- and diastereoselective variants is an area of considerable importance. Previous studies mainly have demonstrated the use of chiral Lewis acid catalysts to control high levels of stereose-lectivity (eq 1). It is noteworthy that vicinal substituents of the dihydropyran were established exclusively with syn selectivity. Presumably, the syn selectivity observed in previous reports is due to the following intrinsic nature of the chiral Lewis acid catalyst: (i) dominant secondary π-orbital interactions and (ii) the steric demand of the chiral Lewis acid catalyst, as the diene could approach the aldehyde with an endo alignment to avoid the steric repulsion between the incoming diene and the catalyst.