Organocatalytic and stereoselective [3+2] cycloadditions of azomethine imines with α,β-unsaturated aldehydes

Bipyrazolidin-3-one derivatives are biologically significant compounds and their importance has increased in the past decades. In this paper, the first stereoselective [3 + 2] dipolar cycloadditions of azomethine imines with alpha,beta-unsaturated aldehydes catalyzed by readily available alpha,alpha-diarylprolinol salts are reported, providing a facile route to the synthesis of various chiral bipyrazolidin-3-one derivatives under mild conditions. The organocatalyst 1g with strongly electron-withdrawing groups exhibited the best stereoselectivity (exo:endo up to 98:2, for exo product up to 97% ee), in the combination with trifluoroacetic acid.