The UV irradiation of N-benzyl-2-phenyl-1,2-dihydropyridin-3-one furnished trans-1-benzyl-4-phenyl-3-vinylazetidin-2-one,a structural isomer,as the main product. A novel tandem mechanism involving a[4+2] photocycloreversion followed by a Staudinger cycloaddition reaction is proposed,and is supported with the trapping of the purported vinylketene intermediate by other imines.This process predominates in the presence of ethylene,precluding the formation of an intermolecular [2+2] cyclobutane add uct.