4,5,9,10-tetrahydro-4-methyl-2-phenyl-9,10-epoxy-3H,10aH-cyclobuta[a]benzo[2,3,4-de]isoquinoline-3,5-dione.

In the title compound, C(21)H(15)NO(3), which is one of the photoreaction products of N-methyl-1,8-naphthalenedicarboximide with phenylacetylene, the cyclobutene and epoxy rings are trans to each other across the cyclohexene ring of the tetralin moiety. The dihedral angle between the mean planes of the cyclobutene and cyclohexene rings is 112.80 (2) degrees, while the latter makes a dihedral angle of 103.70 (9) degrees with the epoxy ring. The crystal structure is stabilized by C-H...O intermolecular interactions.