Azomethine ylides can be generated from ring opening of aziridines.  In accordance with the Woodward–Hoffmann rules, the thermal four-electron ring opening proceeds via a conrotatory process, whereas the photochemical reaction is disrotatory.
In this ring opening reaction, there is an issue of torquoselectivity. Electronegative substituents prefer to rotate outwards, to the same side as the R substituent on the nitrogen, whereas electropositive substituents prefer to rotate inwards.
Note that with aziridines, ring opening can result in a different 1,3-dipole, in which a C–N bond (rather than the C–C bond) breaks.