Enantioselective 1,3‐Dipolar Cycloaddition of Nitrile Imines to α‐Substituted and α,β‐Disubstituted α,β‐Unsaturated Carbonyl Substrates: A Method for Synthesizing Dihydropyrazoles Bearing a Chiral Quaternary Center

Dihydropyrazoles bearing a chiral quaternary center at the 5‐position have been prepared by enantioselective 1,3‐dipolar cycloaddition of nitrile imines to α‐substituted‐ and α,β‐disubstituted‐α,β‐unsaturated carbonyl substrates. Use of α,β‐unsaturated carbonyl substrates with a 1‐benzyl‐5,5‐dimethylpyrazolidin‐3‐one auxiliary in conjunction with MgI 2 and a bisoxazoline ligand derived from (1 R ,2 S )‐(+)‐ cis ‐1‐amino‐2‐indanol 6 proved optimal to obtain chiral dihydropyrazoles with high enantioselectivity (up to 99 % ee ).