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geranylgeraniol geranyllinalool

Figure 7.1 The formation of geranylgeranyl pyrophosphate, the parent of the diterpenes and some simple uncyclized diterpenes found in insects geranyllinalool

Figure 7.1 The formation of geranylgeranyl pyrophosphate, the parent of the diterpenes and some simple uncyclized diterpenes found in insects

Elateridae) and even in a mammalian secretion (the collared peccary, Tayassu tajacu). Geranylcitronellol (Figure 7.1) has also been found in male bumblebees, and springene, first discovered in facial glands of the South African springbok has been found in Dufour glands of ants, stingless bees and a parasitic braconid wasp. The corresponding aldehydes geranylgeranial and geranylcitronellal are also bumblebee substances. The use of a homomevalonate is also observed. The first acyclic homoditerpene from the parasitic wasp Habrobracon hebetor has recently been isolated.

Examples of cyclized diterpenes in insects are also known. The cem-brene structure (Figure 7.2) seems much used by insects. Neocembrene (or cembrene A) is the trail pheromone of some termites, and it, or an isomer of it, is also the sex pheromone of some sandflies, and is the queen pheromone of the ant Monomorium pharaonis. Related structures, crematofuran and isocrematofuran have been found in the defensive or offensive secretions from the Dufour glands of the Brazilian ant Crematogaster brevispinosa (Figure 7.2). These are quite different from the compounds from some European Crematogaster (see Figure 3.34).

Figure 7.2 Cembrene, a diterpene from termites and ants and two derivatives of it from the offensive secretion of Crematogaster ants

7.1.1 Termites

The star performers on the diterpene stage are the soldier caste of some termites. Termites (Isoptera) have developed many means of defense against predators, both physical and chemical. The Nasutitermitinae are the largest, most advanced, and most widely distributed subfamily of higher termites. They have a sterile soldier caste with large frontal glands, and heads modified into a turret shape for discharging the frontal gland secretion. Many of these termites produce a gluey secretion of terpenes. In addition to the monoterpenes, which act as a 'solvent' for the glue, and some sesquiterpenes in the secretion, are a mixture of acyclic and cyclic diterpenes. The mixture oxidizes in the air to produce the sticky secretion which traps ants or other invaders. In Reticulitermes species geranyllinalool is the principal component. The cyclic diterpenes are hydrocarbons of the cembrene and cubitane type and oxygenated bi-, tri-, and tetracyclic diterpenes (Figure 7.3). The tetrapropionate derivative of a trinervitene with an extra methyl group has been reported, without comment, from Hospitalitermes umbrinus. This is the first example of a cyclized homoditerpene, made with a homomevalonate starter unit.

Thirty-nine diterpenes from frontal glands of termites are listed by Wheeler and Duffield in Handbook of Natural Pesticides Vol. IVB, Pheromones (ed. Morgan and Mandava, CRC Press, Boca Raton, 1985, pp. 159), more have been isolated since. It is clear these are insect-produced via the isoprenoid route. When the soldiers of Nasutitermes octopilis were injected with 14C-labelled acetic acid or mevalonolactone, the 14C was incorporated into tri- and tetra-cyclic diterpenes (Prestwich, Jones and Collins, Insect Biochemistry, 1981,11, 331).

In spite of the large number of diterpenes identified from termites, and the interest they have aroused among structural and synthetic chemists, no further biosynthetic studies appear to have been carried out since the work of Prestwich et al. in 1981.

cubitene cembrene seconervitene type cembrene seconervitene type trinervitene type cubitene trinervitene type seconervitene kempane type

3a-hydroxy-7,16-secotrinervita-7,11,15(17)-triene

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