The scattering amplitude for photoproduction of a large invariant mass diphoton in the generalized Bjorken regime has a very peculiar and interesting analytical structure. The leading twist leading order amplitude is proportional to valence quark generalized parton distributions taken at the border value $x=\pm \xi$. Cross section estimates show that this process is measurable at JLab energies. The angular asymmetry triggered by a linearly polarized photon beam is large.