Hybrid-Type Squaramide-Fused Amino Alcohol Organocatalysts for Enantioselective Diels–Alder Reactions of 3-Hydroxy-2-Pyridones with Maleimides

The hybrid-type squaramide-fused amino alcohol (SFAA) containing both a Brønsted basic site and a hydrogen-bonding site in the molecule showed high catalytic activity as an organocatalyst in the enantioselective Diels–Alder (DA) reactions of 3-hydroxy-2-pyridones with maleimides to afford chiral endo-4-hydroxy-2-azabicyclo[2.2.2]octanes (4-hydroxyisoquinuclidines) with excellent chemical yields and enantioselectivities (up to 95 % yield and 98 % ee). In particular, the use of 4-brominated 2-pyridones afforded the corresponding chiral 4-hydroxyisoquinuclidines with both excellent chemical yields and enantioselectivities (95 %, 98 % ee) with an opposite absolute stereochemistry in comparison with the chiral endo-DA adducts obtained from the reactions performed with common 3-hydroxy-2-pyridones. The DA adducts obtained could be used as synthetic precursors for several natural products that have a broad spectrum of fascinating biologically activities.