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W. SANDERMANN
however, that the heptane was inactive. It does not, therefore, have a
terpenoid origin (26).
Approximately one-third of all diterpenes including cyclized compounds such as dextropimaric acid have the primary carbon skeleton
of geranylgeraniol. In other cases, new diterpene skeletons have arisen
by means of migration of methyl groups, contraction and expansion of
(xxv)
(xxm)
FIG. 4. Rearrangements of the carbon skeleton in the biogenesis of diterpenes.