A highly enantioselective catalytic protocol for the desymmetrization of meso-aziridines via ring-opening with pyrazoles is reported using an N,N'-dioxide-Mg(OTf) 2 complex as the catalyst. The corresponding trans-a-pyrazole-substituted amines were obtained in good yields and enantioselectivities (up to 99% yield and 94% ee) under mild reaction conditions. Moreover, a remarkably high asymmetric amplification was observed in the catalytic system.Enantioselective desymmetrization is a powerful strategy for the construction of complex molecules with multiple stereocentres from relatively simple meso starting materials. [1] In particular, the enantioselective desymmetrization of meso-aziridines has captured much attention for providing access to important chiral 1,2-difunctionalized molecules with vicinal stereocenters by ring opening with nucleophiles in one single step. [2] Over the past decades, many nucleophiles such as trimethylsilyl azide, [3] trimethylsilyl cyanide, [4] amines, [5] alcohols, [6] halogen, [7] etc. [8] have been demonstrated successfully for desymmetrization of meso-aziridines. As a pioneer report, Jacobsen and co-workers described the highly enantioselective ring opening of meso-aziridines, which reacted with trimethylsilyl azide to give 1,2-azidoamines in good yields with 83-94% ee under the catalysis of the chiral chromium (III)-salicylimine complex. [9] Shibasaki and co-workers subsequently provided 1,2-amidonitriles in good yields with 81-93% ee in the presence of a catalyst prepared in situ from Gd(OiPr) 3 and the chiral ligand derived from glucose in 2005. [10] Follow-ing these early reports, many excellent researches have been furnished in desymmetrization of mesoaziridines. Recently, Wang's group realized the enantioselective dearomatization reaction and [3 + 2]-cycloaddition of meso-aziridines via an in situ generated magnesium catalyst. [11] In spite of these impressive advances, there are few reports on the direct catalytic asymmetric ring-opening of meso-aziridines with pyrazoles, [12] although pyrazole motif also exists in a number of small molecules that possess diverse chemical, biological, and pharmaceutical activities. [13]