Photochemistry of 2-pyridonemixtures can be selective and thereby lead to useful quantities of [4 + 4] cycloaddition cross products. The selectiveintermolecular reaction described here employs an excess of 4-methoxy-2-pyridone (6), which does not photodimerize but will undergo [4 + 4] cycloaddition with 2-pyridones without a 4-methoxy group such as N-butyl-2-pyridone 12. Isolated yields of the trans