Methacrylanilides, ArN(CH3)COC(CH2LG)=CH2, with allylic leaving groups (LG = BocAla, PhCOf, PhCH2CO2, Ph0) undergo photochemical electrocyclic ring closure to produce a zwitterionic intermediate. Further reaction of the intermediate results in expulsion of the leaving group to give an u-methylene lactam as the major product. In addition, a lactam product that retains the leaving group is formed via a 1,5-H shift in the intermediate. Elimination of the leaving group is generally preferred, even for LG = Ph0, although in benzene as the solvent the lactam retaining the phenolate group becomes the sole photoproduct. The electrocyclic ring closure occurs in the singlet excited-state for the para-COPh-substituted anilide derivative and is not quenched by 0.15 M piperylene or 0.01 M sodium 2-naphthalenesulfonate (2-NPS) as triplet quenchers. Comparable concentrations of 2-NPS strongly quench the transient absorption of the triplet excited state observed at 450-700 nm according to laser flash photolysis experiments. In aqueous media, quantum yields for total products are insensitive to leaving group ability, and ~J?~0~(para-CO2CH3) = 0.04-0.06 at 310 nm and `t~0~(para-COPh) = 0.08-0.1 at 365 nm, for which (IT)~s~ = 0.15. [ABSTRACT FROM AUTHOR]