Labda-8 pyrophosphate

Fig. 22.8. Cyclization of GGPP to labda-8(17),13-dien-15-yl pyrophosphate (modified from West, 1981 used with permission of the copyright owner, Wiley, New York).

The cyclization of a labdadienyl pyrophosphate to the tricyclic diterpene, pimara-8(9), 15-diene (34) appears to be straightforward (West, 1981) (Fig. 22.12). The precursor role of labdadienyl pyrophosphates for tricyclic diterpenes has been confirmed by the conversion of labda-8(17),13-dien-15-yl pyrophosphate (35) to e f-sandaracopimara-8(14),15-diene (36) in cell-free extracts from castor bean seedlings. 

Biosynthesis. The starting compound is geranyl pyrophosphate (see Terpenes). Acyclic D. are formed by hydrolysis of the pyrophosphate residne (e.g. phytol). Geranylgeranylpyrophosphate is probably converted easily to geranyl linalool, which is then converted to bi- and tricyclic compoimds (Rg.). In a few of the cyclic D-, e. g. abietic acid, there is a migration of the substituents. The gibberellins are derived from the labda-diene type of D. 

Copalyl pyrophosphate (235-OPP) has been demonstrated to be a key branching point in the biosynthesis of tricyclic and tetracyclic diterpenes 24, 300, 308). It, therefore, seems likely that A -labda-dienyl pyrophosphate (244-OPP) is the precursor to the corresponding enantiomeric diterpenes as shown in the biogenetic scheme below (Scheme 24). Beyond the existence of both enantiomeric series of these tricyclic diterpenes, there is a further stereochemical diversity which results from the cyclization of (235-OPP) and (244-OPP) to sets of stereoisomers epimeric at the new quaternary carbon at C13, e.g., pimaric acid (246) and sandarocopimaradiene (247). 

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