Phase‐Transfer‐Catalyzed Asymmetric Synthesis of Chiral N ‐Substituted Pyrazoles by Aza‐Michael Reaction

An efficient cinchona‐based phase‐transfer‐catalyzed asymmetric aza‐Michael reaction of pyrazole with various α,β‐unsaturated ketones has been developed for the preparation of chiral N ‐substituted pyrazoles. This reaction provided the desired products in good yields (up to 99 %) with excellent enantioselectivities (87–94 % ee ). In addition, the reaction of 3‐methyl‐1 H ‐pyrazole, an unsymmetrically substituted pyrazole, was treated with α,β‐unsaturated ketones to afford the corresponding chiral N ‐substituted pyrazoles in good yields (up to 99 %) with excellent enantioselectivities (90–92 % ee ) and regioisomeric ratios of up to 9:1.