Asymmetric Umpolung (3+2) Cycloadditions of Iminoesters with α,β-Unsaturated-2-acyl Imidazoles for the Synthesis of Substituted Pyrrolidines.

An enantioselective (3+2) cycloaddition reaction of iminoesters occurring with opposite regioselectivity is reported. This reaction provides chiral polysubstituted pyrrolidines with high enantioselectivities (up to 97%), diastereoselectivities (>20:1), and yields (up to 99%). Interestingly, changing the alpha-substituents of the iminoesters from an aryl to an aliphatic (benzyl) group or hydrogen resulted in the formation of normal (3+2) cycloaddition products, also in excellent enantioselectivities.