Augmentation of Enantioselectivity by Spatial Tuning of Aminocatalyst: Synthesis of 2-Alkyl/aryl-3-nitro-2H-chromenes by Tandem Oxa-Michael-Henry Reaction.

The asymmetric oxa-Michael addition of salicylaldehyde to conjugated nitroalkenes often suffers from poor reactivity and selectivity and a long reaction time. Because of the formation of an iminium ion with aminocatalyst, the nucleophilicity of the phenolic hydroxy group in salicylaldehyde reduces further to make the oxa-Michael reaction reversible. Here, we report a structurally simple and easily accessible l-proline derived aminocatalyst, phenyl l-prolinamide, for asymmetric tandem oxa-Michael-Henry reaction of salicylaldehyde with conjugated nitroalkene to give 2-alkyl/aryl-3-nitro-2H-chromenes in excellent enantioselectivity within a short reaction time.