A branched pathway that produces the precursors to four structurally distinct types of polycyclic diterpenes. The pathway starts with the cyclization of geranylgeranyl diphosphate into ent-copalyl diphosphate and syn-copalyl diphosphate. The catalytic conversion by diterpene cyclases of these two compounds produces the four diterpene hydrocarbons which are precursors to the four structurally distinct classes of diterpene phytoalexins.