Through site-directed mutagenesis targeted at identification of the catalytic base in the rice (Oryza sativa) syn-copalyl diphosphate synthase OsCPS4, changes to a single residue (H501) were found to induce rearrangement rather than immediate deprotonation of the initially formed bicycle, leading to production of the novel compound syn-halimadienyl diphosphate. These mutational results are combined with quantum chemical calculations to provide insight into the underlying reaction mechanism.