Synthesis of substituted chiral chromans via organocatalytic kinetic resolution of racemic 3-nitro-2-aryl-2H-chromenes with ketones catalyzed by pyrrolidinyl-camphor-derived organocatalysts

Efficient kinetic resolution of racemic 2-aryl-3-nitro-2H-chromenes (1a-l) has been explored with the pyrrolidinyl-camphor derivative 2b as a bifunctional organocatalyst under neat conditions in the presence of AcOH at 0 °C. In general, the organocatalytic asymmetric Michael addition of ketones proceeded smoothly to give the functionalized Michael adducts (3a-n) with good-to-high diastereo- and enantioselectivities (up to 92:8 dr, 93% ee, and 47% yield). The less reactive chromenes (S)-1a-h, k, l and (R)-1i-j were recovered in high chemical yields and moderate optical purity (up to 42% chemical yield and 72% ee).