Regioselective Asymmetric Formal (3+2) Cycloadditions of Nitrone Ylides from Isatins and Enals

A highly regio-, diastereo- and enantioselective formal (3+2) cycloaddition reaction of nitrone ylides from isatins and ,-unsaturated aldehydes was developed via iminium catalysis in the presence of an additional base, furnishing a spectrum of 1-hydroxy-3,2-pyrrolidinylspirooxindole frameworks. Interestingly, the regioselectivity could be finely switched in the reactions between nitrone ylides and crotonaldehyde by adding catalytic amounts of lithium perchlorate and copper(II) bromide.