Abstract
The Mg-catalyzed enantioselective synthesis of 2,5-cis-disubstituted pyrrolidines 3 is reported via a dynamic kinetic asymmetric [3+2] annulation of racemic cyclopropanes 1 with (E)-imines 2. Extensive optimization of the imine N-protecting group showed that the 2-methoxybenzyl substituent gave the best yield and stereoselectivity. Electron-rich and -neutral aryl imines provided high yields and enantioselectivities of pyrrolidines 3. Electron-poor aryl, alkenyl, and aliphatic imines were not competent coupling partners. Electron-rich donor groups (R¹) are also required. Based on mechanistic studies and stereochemical observations, reaction of the imine through the E-geometry via an unusual diaxial transition state is suggested.
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