FIGURE 1.

Mechanistic scheme for PaLAS and PaIso. The proposed mechanisms of both PaLAS and PaIso begin with the formation of an isopimar-15-en-8-yl carbocation from geranylgeranyl diphosphate. The mechanisms diverge at this intermediate. For PaIso, loss of a proton results in the formation of isopimaradiene (route b). For PaLAS, intramolecular proton abstraction and the subsequent Wagner-Meerwein 1,2-methyl shift result in formation of an abieta-8(14)-en-13-yl carbocation (route a). Previously, loss of a proton from this intermediate was thought to immediately give rise to the hydrocarbons abietadiene, levopimaradiene, neoabietadiene, and palustradiene. However, the data presented in this work suggest that the abieta-8(14)-en-13-yl carbocation is quenched by water to form 13-hydroxy-8(14)-abietene, a tertiary allylic alcohol. The apparent products abietadiene, levopimaradiene, neoabietadiene, and palustradiene previously reported are consequently the result of thermally induced dehydration of 13-hydroxy-8(14)-abietene.