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 a,b-unsaturated aldehydes catalyzed by readily available a,adiarylprolinol salts are reported, providing a facile route to the synthesis of various chiral bipyrazolidin-3-one derivatives under mild conditions. The organocatalyst 1 g 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.Keywords: azomethine imines; bipyrazolidin-3-one; diarylprolinols; dipolar cycloaddition; organocatalysis; unsaturated aldehydes Recently, small organic molecules-catalyzed enantioselective transformations have attracted increasing attention.[1] Particularly, asymmetric amine catalysis has been the subject of intensive research since the discovery of the proline-catalyzed direct asymmetric intermolecular aldol reaction. Numerous 1,2-or 1,4-addition reactions with excellent enantioselectivities have been reported in the last five years.[2] On the other hand, asymmetric cycloaddition reactions have also been actively explored in the amine catalysis. The generation of active iminium ions, which are thought to lower the energy of the dienophile LUMO, generally plays a key role in the successful cycloaddition reactions. Nevertheless, the current examples are mostly limited to the common DielsAlder reactions.[3]The catalytic asymmetric [3 + 2] dipolar cycloaddition reaction provides one of the most powerful strategies for the enantioselective construction of fivemembered heterocycles. While considerable progress has been made in the metal-catalyzed asymmetric synthesis over the past decades, [4] the application of chiral organocatalysts in [3 + 2] cycloaddition reactions remains in its infancy since MacMillan's pioneering works in 2000.[5] Furthermore, only nitrones have been tested as the successful dipoles to date.[6] Based on our successful experience on the asymmetric vinylogous Michael addition of a,a-dicyanoolefins to a,bunsaturated aldehydes catalyzed by chiral a,a-diarylprolinol salts, [7] here we present the first asymmetric [3 + 2] cycloaddition reaction of azomethine imines with a,b-unsaturated aldehydes.Azomethine imines such as 2, prepared by the condensation of pyrazolidin-3-one with aldehydes, are stable and easily handled compounds. Their potential application as dipoles in cycloadditions with alkenes or alkynes has been established a long time ago. [8] Several pyrazolidin-3-one derivatives exhibit biological activities, and the importance of this type of compounds has risen significantly in the last two decades. One of the examples is the cycloaddition product LY 186826, which exhibits high anti-bacterial activity. [9] On the other hand, although a variety of asymmetric cycloadditions of chiral azomethine imines have been reported, [10] the catalytic enantioselective [...
