Many of the compounds from plants, important in food, flavours, perfumes and colour, belong to the group called terpenes, made up of five-carbon fragments. Ruzicka in 1922 proposed that the basic building block of the terpenes is an isoprene unit (Figure 6.1), and later Robinson proposed that these isoprene units are joined head-to-tail. The terpenes are sub-divided into groups by their number of isoprene units. Monoter-penes (C10 compounds) contain two isoprene units, sesquiterpenes (C]5 compounds) contain three isoprenes, diterpenes (C20) four units, triter-penes (C30) six units and tetraterpenes (C40) eight units. There are a smaller number of five-isoprene-unit compounds called sesterterpenes. Some simple examples of terpenes are shown in Figure 6.1, with their structures dissected into isoprene units. Although mainly plant substances, terpenes are frequently found in insects, as pheromones, defensive secretions and a hormone. Our understanding of terpenoid biosynthesis was greatly helped by the efforts of Cornforth, Popjak, Lynen and others on the biosynthesis of cholesterol in the 1950s and 60s.

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