Labdadienyl pyrophosphate

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. 

Momilactones arise from GGPP via 9,pH-labdadienyl pyrophosphate (66) and 9-3H-pimara-7,15-diene (67), whereas oryzalexins arise via copalyl pyrophosphate (35) and sandaracopimaridiene (36). Both groups of diterpenes appear to be synthesized in the cytosol. Only kaurene is found in tissue not attacked by fungi. Chitin proved to be the best elicitor of momilactones in cell culture (West et al., 1990). Chitinase probably breaks down the chitin to active fragments (West et al., 1990). Both types of phytoalexins from rice are probably important in disease resistance, but the relative importance of each has not been assessed (West et al., 1990). 

In the formation of labdanes cyclization is initiated by the addition of a proton to the terminal double bond (see the cyclization of squalene, D 6.4). Depending on the cyclase involved (-f-)- or (— )-labdadienyl pyrophosphates are formed. 

Fig. 111. Synthesis of cyclic diterpenoids from (+)-labdadienyl pyrophosphate...

Fig. 112. Formation of gibberellins from (—)-labdadienyl pyrophosphate 1 Kaurene synthase 2 monooxygenases...

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