Catalyzes the conversion of geranylgeranyl diphosphate to the phytoalexin precursor syn-copalyl diphosphate. (2E,6E,10E)-geranylgeranyl diphosphate = 9alpha-copalyl diphosphate By UV irradiation. The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity, presumably through binding to Mg(2+). Phytoalexins are diterpenoid secondary metabolites involved in the defense mechanism of the plant and produced in response to attack (by a pathogen, elicitor or UV irradiation).