Addition of another unit of isopentenyl pyrophosphate (IPP) to farnesyl pyrophosphate by the same process as described in Chapter 6 gives geranylgeranyl pyrophosphate (Figure 7.1), the parent of the diter-penes. Diterpenes are very common in higher plants; 110 are listed as biologically active in the Phytochemical Dictionary (Taylor and Francis, London, 1999 p. 701). They may be acyclic, macrocyclic, or polycyclic.

Diterpenes are less volatile because of their greater molecular mass, and cyclized ones are generally solids, but some diterpenes are found in insect glands where they probably act as pheromones. Uncyclized diterpenes, like isomers of springene (Figure 7.1), simple alcohols such as geranylgeraniol, geranyllinalool, and geranylcitronellol and their esters are particularly found in Hymenoptera, but also in termites. These compounds and uncyclized sesquiterpenes are also found in mammalian scent glands. Geranylgeraniol has been found in the labial glands of male bumblebees, in the Dufour glands of the stingless bee Nannotrigona testaceicornis, the ant Ectatomma ruidum, in the female sex pheromones of click beetles {Agriotes, Sinapus and Melanotus species, Coleoptera, geranylgeraniol

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