Normal-labdanes

Figure 1. Structures of Diterpenes 18. Normal-Labdane-8,13-Epoxide... 

Cyclization of the linear diterpene precursor usually proceeds along two alternative parallel routes (Figure 2 [p. 392]), one leading to the bicyclic normal labdane (normal-labdane) absolute stereochemistry and the other resulting in its antipodal, enantiomeric (e/iMabdane) absolute stereochemistry. As a consequence of these two parallel biosynthetic routes, diterpenes lack the homogeneous absolute stereochemistry of the triterpenes. 

Among the normal-labdanes, the simplest rearrangement product, the relhaniane skeleton (Figure 3, No. 28), results from the concerted migration of C-9a-H to... 

Steps proposed for the biogenesis of rearranged ewMabdanes appear to correspond to the mechanisms indicated for the normal-labdanes, differing only in the opposite orientivity of the migrating groups. 

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