Photochemical transformation of a 1,2-dihydropyridin-3-one: an original tandem retro-[4+2]/[2+2] cycloaddition process

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 adduct.