The cleavage of methyl pimarate 8,(14) α-epoxide (4) was studied with a variety of acidic reagents. The major products observed were the corresponding Δ7,8-14α-alcohol and dienes. Participation of the vinyl group was not detected.Consideration of the stereoelectronic requirements for cyclisation of olefins with carbonium ions at C-8 of the pimarane skeleton suggest that the pimaranes are not the biogenetic precursors of the tetracyclic diterpenes. A modification of the traditional route is presented as a working hypothesis.