Simple sesquiterpenes are common substances in insects, but they also make sesquiterpenes with one, two or three extra carbon atoms, by using one, two or three molecules of homomevalonate in place of mevalonate. Labelling experiments by Schooley, Judy, Bergot, Hall and Siddall (Proceedings of the National Academy of Sciences, USA, 1973, 70, 2921), have shown that homomevalonate starts with a propionic acid molecule as shown in Figure 6.23. In some insects it is known that the propionate is derived from isoleucine or valine only. The intermediate 3-methylenepentyl pyrophosphate in Figure 6.23 can be isomerized to two other structures. From these are obtained by reaction with IPP or DMAPP, compounds with either a terminal ethyl branch, an internal ethyl branch or two vicinal methyl branches (Figure 6.23). All three possibilities are found among insect compounds but the one on the left seems more common.

Such sesquiterpene homologues seem plentiful, particularly in ants. The three farnesene homologues from the Dufour glands of Myrmica ants are an example (Figure 6.24). Their purpose there is unknown. In the Dufour gland of the ant Manica rubida nine isomers and homologues of farnesene were found. Their function is again unknown.

Faranal (Figure 6.25) is the trail pheromone of Pharaoh's ant Mono-morium pharaonis, a tropical species that has become a pest inside warm buildings in temperate climates. It contains two homomevalonate

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