An unprecedented organocatalytic cis-selective aziridination of α,β-unsaturated aldehydes is described. The strategy utilizes postreaction isomerization as a key step. Our study demonstrates that by a proper choice of structural features of reagents and the catalyst the outcome of well-established reactions can be altered, leading to the formation of normally inaccessible diastereomers. The usefulness of the developed approach has been confirmed in the syn-selective formal aminohydroxylation of α,β-unsaturated aldehydes.